Components of the masticatory system, their characteristic.

Types of occlusion, their characteristic and feature

 

TEMPOROMANDIBULAR JOINT (TMJ) ANATOMY

The temporomandibular joint (TMJ) makes possible the various movements of the mandible. It allows for the up, down, forward, backward, and side to side movements.

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Bones. The temporomandibular joint derives its name from the two bones that form the joint, the temporal bone and the mandible. The condyloid process of the mandible and the mandibular fossa of the temporal bone form the joint. The condyle moves (articulates) inside the fossa and makes the movements of the mandible possible. It provides for –

• Up and down movements (elevation and depression),
• Forward and backward movements (protrusive and retrusive),
• Side-to-side movements(lateral and rotational).

Articular Disc. The articular disc is a thin, biconcave, oval plate made of fibrous tissue, located between the mandibular fossa and the condyle of the mandible and the articular tubercle (posterior tubercle) of the temporal bone. The disc divides the TMJ into upper and lower cavities, each having synovial membranes which secrete synovial fluid to lubricate the joint. The edges of the disc are attached to the capsular ligament and, in front, it is attached to the lateral pterygoid muscle.

Ligaments. A ligament is a tough, fibrous band that connects bones. There are four ligaments that limit the extreme movement of the mandible. These are the-

• Capsular ligament,
• The lateral (temporomandibular) ligament,
• The sphenomandibular ligament,
• The stylomandibular ligament.

The basic functions of the maxillodental system are associated with different character of movements of the lower jaw. There is a specific phase sequence of these movements in mastication and swallowing in closed and opened dentitions. Voluntary and reflex movements and positions of the lower jaw in respiration, conversation, mimicryare different and are not accompanied by joining of the dentitions.

The movements of the lower jaw are provided by the contracting activity of different groups of the muscles on the basis of the complex combinations of conditional and unconditional reflexes. The pattern of the reflexes (sequence of nerve impulses, which is of specific information value) of the masticatory function is controlled by the centers situated in the brain stem. To estimate the mechanism of the movements of the lower jaw correctly and to determine the character of the dentition relationship, it is necessary to know the specific concepts and terms.

Articulation is a special relationship of the dentitions and jaws during all movements of the lower jaw. Occlusion is joining of the dentitions or group of the teeth of the upper and lower jaws during different masticatory movements of the latter. Occlusion is considered to be a special form of articulation.

In the absence of contact between the dentitions the movements of the lower jaw are directed by the resultant of the contracted muscles and by the contact surfaces of the joint elements. When the dentitions are in the occlusion contact, the character of the displacement of the jaw is determined by the relationship of the masticatory surfaces of the teeth and joint elements.

Depending on the position of the lower jaw regarding the upper one distinguishes: 1) the state of relative physiological rest; 2) central occlusion, or central relationship of the jaws; 3) lateral right occlusions; 4) lateral left occlusions; 5) anterior occlusions.

The state of the relative physiological rest is one of the articulatory positions of the lower jaw with the minimum activity of the masticatory muscles and complete weakening of the mimic musculature. The tone of the elevators and depressors of the lower jaw is equivalent.

It is expedient to consider biomechanics of the lower jaw during the intake of food and to define concretely the relationship of the dentitions and temporomandibular joint elements in the diagnosis. At first visual and olfactory analyzers, the apparatus of memory begin to act. The starting mechanism of the salivary gland activity and muscular apparatus begin to work on the basis of the food analysis, i.e. the selection of optimum activity plan takes place. The secretion of saliva causes the need for its swallowing. In this case because of the contracting activity of the muscles the lower jaw is moved from the state of physiological rest to the central occlusion position, after which a man swallows. Joining of the dentitions in swallowing is accompanied by substantial increase in the tone of the masticatory musculature and specific force of compression of the jaws. In a number of cases the patients experience pain in joining of the jaws and swallowing. These pains may develop in the periodontium of separate teeth, separate groups of the muscles or the joint. Pains in the muscles and joint can be explained by their involvement in the inflammatory process.

The second phase is opening of the mouth and insertion of the food in the mouth. It is characteristic that at this moment the selected optimum version of action begins to operate depending on the visual analysis of the food character and size of the food lump. Thus, in opening of the mouth special displacement of the entire lower jaw occurs. The displacement of the joint elements depends on the movement amplitude: rotation of the heads around the transverse axis without displacement of the intra-articular disk predominates during the insignificant lowering of the lower jaw to the level of the physiological rest, in further lowering of the jaw the head, continuing rotational movement together with the disk, begins to displace along the slope of the articular tuber, i.e. forward and downward. If the articular head is closely adjacent to the articular disk by its anterior surface at the beginning of opening of the mouth, then it touches the disk by the upper or upper-posterior surface during a maximally possible opening of the mouth. Such combined movement – rotational and forward– is a characteristic property of the temporomandibular joint. Further forward movement of the head in the norm is restrained by the ligamentous apparatus and capsule of the joint.

In accordance with the described displacement of the head in the opening of the mouth there is opening of the dentitions without sliding of the lower anterior teeth along the palatine surface of the upper incisors – the lower jaw is displaced downward and somewhat back. The relationship of the dentitions with the orthognathic occlusion changes in the following sequence: the incisors in the vertical plane are located on the same level before the physiological rest; then together with the jaw y they slightly move toward the back. The chin seems to move back, and the cutting surfaces of the incisors of the lower jaw somewhat tilt forward at the moment of joining forward movements of the articular head to the rotational ones, i.e. in the large opening of the mouth, and again they return to the vertical plane passing across the line of joining with the incisors of the upper jaw in the central occlusion relationship. The relationships of the elements of the temporomandibular joint and dentitions are described while considering the movements of the lower jaw laterally.

During the study of displacement of the lower jaw in opening of the mouth the linearity of displacement is clearly determined in the norm from the front – the line between the central incisors of the lower jaw does not deviate from the similar line on the upper jaw. The smoothness of displacement of the lower jaw is evidence of the absence of pathologic processes both in the joint and in the muscular system.

Pathologic processes different in specific character and degree of development significantly change the character of the movements of the lower jaw in opening of the mouth. So, in arthritis opening of the mouth is not only painful, but also it is considerably limited, displacement of the jaw develops gradually or it is zigzag. In chronic subluxation, strain of the joint ligaments, is frequently accompanied by hyperfunction of the sublingual muscles, on the contrary, the amplitude of displacement is increased. In myositis of the external pterygoid muscles, destructive changes in the intra-articular disks, arthroses displacement of the jaw is painless, limited, the smoothness of displacement is disturbed – movements are jerky, with deviations from the central line. Frequently the movements are accompanied by the crunch, click, rustling at different moments of lowering the jaw.

Biting off of the food independent on properties and size of the food lump is achieved due to the contraction of the muscles, which raise the lower jaw, and further synchronous contractions of medial pterygoid muscle. At the first moment of the jaw raising the latter somewhat protrudes it forward regarding the upper incisors till the moment of contact with the food. Overcoming resistance of the food lump (mediate occlusion), the jaw rises upwards to the contact of the cutting surfaces of the incisors of the lower and upper jaws.

In marginal anterior occlusion position of the lower jaw in dependence on the individual peculiarities of the dentition morphology central, lateral and sometimes canine teeth may be in the contact. In the contact of the cutting surfaces of the incisors there are no contacts between the second and third molars or there are point contacts in the region of the masticatory teeth. This relationship was called the three-point contact of Bonville. The presence of contact depends on the degree of incisor overlap, expressiveness of the tubers of the masticatory teeth, degree of the manifestation of Shpee curve, the degree of tilting inclination of the upper anterior teeth, articular way (the articulatory five of Ganau).

The presence of three-point contact contributes to the distribution of pressure on entire anterior group of the teeth and a number of the masticatory ones. It should be emphasized that at the same tone of the musculature the masticatory pressure in the group of the anterior teeth is 2-3 times less than in the region of posterior (masticatory) ones. I. S. Rubinov (1970) established that “…in intact dentitions force of teeth compression in the region of molars was equal to 80 kg, and in the region of the anterior teeth – 40 kg. The tone of the masticatory muscles in these cases was equal to 180 kg”. He explains the divergence of indices by “anatomical-topographical peculiarities of location of the anterior and lateral teeth regarding the articular heads of the lower jaw and places of attachment of the masticatory muscles”. From our point of view, this divergence should be explained based on the positions of the theory of functional systems (TFS). The information specific for two cases in question, comes from three sources – periodontium, muscles and joint.

I. S. Rubinov who described the scheme of the functional masticatory component and established parodontomuscular and gingivomuscular reflex did not take into consideration parodontomuscular – articulatory reflex. In this component the receptor apparatus of the periodontium and ligaments of the temporomandibular joint is most reactive in the physiological norm. Periodontium- muscular reflex appears to be of the greatest value in the perception of the afferent stimuli and regulation of the functional activity. The proof of this thesis is in the comparison of investigation data of I. S. Rubinov and V. Yu. Kurlyandskiy. According to the data of V.Yu. Kurlyandskiy (1977), force of compression reaches 60-75 kg after splinting of the entire group of the anterior teeth.

Further displacement of the lower jaw for biting off of the piece of food is accomplished due to the reduction, first of all, of the posterior bundle of the temporal muscle. During this movement the lower anterior teeth slide on the palatine surface of the upper ones contributing to cutting of the food. The way, which the lower incisors pass from the marginal joining of the cutting surfaces to joining in the central occlusion is  called sagittal incisor way, and the angle, formed during the intersection of the plane of tilt of the  occlusion surfaces of the upper incisors with the occlusion plane is called the angle of the sagittal incisor way. The way itself depends on overlapping by the upper teeth of the lower ones and vertical size of the upper incisors. The synchronous contraction of the muscles, which raise the lower jaw, causes the determinate direction of the resultants in every muscle and the entire group, and also the displacement of the jaw. Tilting of the occlusion surface of the incisors directs this movement. The form of occlusion plays an important role in the displacement of the jaw from the position of biting off into the central occlusion. Additional lateral and antero-posterior movements are possible in biting off for separating the food lump and tearing off action of the hand, which leads to the additional load on the periodontium of the anterior teeth, especially upper ones.

These displacements occur within 3 mm and are accomplished from the position of the lower jaw in the sagittal occlusion. Consequently, during the sagittal movement the complex of lateral movements is possible. Depending on the position of anterior teeth in the dental arch, occlusion contacts in the lateral displacement can be on the central, lateral incisor and the canine tooth or on the separate areas of the canine tooth and incisor.

After biting off the food lump step by step is moved to the canine teeth, premolars and molars under the action of the contracted muscles of the tongue. This displacement is accomplished during the vertical displacement of the lower jaw from the position of the central occlusion, through the mediate occlusion again in the central one. Gradually the food lump is separated into the parts – the phase of crushing and grinding of food. The food lumps are moved from the molars to premolars and vice versa.

The lateral (transverse) movements of the lower jaw are accomplished due to the alternating contraction of the lateral pterygoid muscle. The contraction of the muscle on the left leads to displacement of the jaw to the right, and the right one – to the left side. During these movements the tone of the masticatory, medial pterygoid and temporoparietal muscles increases. The side, to which displacement of the lower jaw occurred, is called a working side, the side, opposite to a working one is called balancing, or nonworking.

In the lateral occlusion displacement of the lower jaw the character of the occlusion contacts, the displacement ways of the articular heads, contraction of the muscles on the working and balancing sides are different. The articular head on the working side, while making the rotary movement around the vertical axis, remains in the fossa. In the rotary movement the external pole of the head is displaced toward the back and it can exert pressure on the tissues, which are located behind the joint. The internal pole of the head is moved along the distal slope of the articular tubercle, which causes the nonuniformity of pressure on the disk.

The articular head on the balancing side is displaced downward, forward and somewhat to the middle, passing the specific way, called lateral articular way. This way is determined by the medial and upper walls of the articular fossa, by the slope of the articular tubercle. The angle of deflection of the head to the middle (angle of Benedict) is equal to 15-17°. The complex displacement of the articular head is accompanied by the complex displacement of the articular disk. The synchronization of the displacement of the head and disk is disturbed in a number of diseases; consequently pain sensation develops during the movements of the jaw

The lateral displacement of the lower jaw from the position of the central occlusion is guided by the occlusion surfaces of the teeth of the working side. There are the following guiding functions of the occlusion surfaces: “incisal way”, “group way”. The latter is encountered more rarely. In presence of the “incisal way” all upper and lower posterior teeth and canine teeth on the working side remain in contact at the beginning of the movements of the lower jaw, sliding by the slopes of the buccal tubers along the guiding surfaces of the medial and distal palatine slopes of the buccal tubers of the upper teeth. During further displacement of the jaw in one of the positions of the lateral occlusion the contacts in the region of the lateral teeth are disturbed, remaining in the region of the canine teeth (sometimes incisors), the buccal slopes of the lower canine tooth slide along the palatine (occlusion) surface of the upper canine tooth. Disconnection of the premolars and molars increases in proportion to the displacement of the jaw from the position of the central occlusion and approximation to a position in the extreme lateral occlusion. The degree of disconnection depends on the overlap level of the lower canine tooth by the upper one: the greater the overlap, the greater the disconnection, and the manifestations of the tubers of the posterior teeth: the more expressed tubers, the less are disconnection. The value of disconnection is also influenced by the jaw displacement way from the central occlusion to the extreme lateral occlusion, i.e. when the buccal tubercles of the posterior (masticatory) teeth of the lower jaw are positioned in one plane with the buccal tubercles of the upper one.

In displacement of the lower jaw to the right and to the left both the central incisors and each tubercle of the lower tooth regarding the upper dentition describe the individual way, which is converged at angle to the central sagittal line. This angle is called Gothic; it is equal on the average to 110° and is opened toward the front.

The presence of contacts between all or majority of the teeth of the working side is noted in the group guiding function, cutting edges of the incisors and the vestibular slope of the canine teeth slide along the palatine (occlusion) surfaces of the anterior teeth of the upper jaw. Simultaneously the vestibular slopes of the buccal tubers of the lower molars and premolars move along the palatine slopes of the buccal tubers of the upper lateral teeth, and the buccal slopes of the lingual tubers of the teeth of the lower jaw slide along the palatine surface of the palatine tubers of the teeth of the upper jaw.

As a result of the displacement of the articular head on the nonworking side downward and forward determining the three-dimensional displacement of the jaw, as a rule there are no occlusion contacts with intact dentitions. It is possible to reveal contacts between the lingual slopes of the buccal tubers of the lower teeth with the buccal slopes of the palatine tubers of the upper teeth. If in this case there are simultaneously contacts of the teeth on the working side, then this relationship should be the case of the balanced occlusion, in which the masticatory pressure is distributed most evenly on the dentitions and the elements of the temporal- mandibular joint are loaded more evenly.

One should aim at the creation of such contacts in all forms of prosthesis: in fixed kinds of dentures for prevention of the overload on the abutment teeth, while in removable dentures it is necessary to prevent their dropping on chewing of food.

The character of the movement of the lower jaw in the occlusion contacts depends on two moments: the kind of occlusion and structure of the temporal- mandibular joint. It is accepted to distinguish two groups of occlusion – physiological and abnormal. In the development of the pathologic processes in the dentitions and the bone tissue of the jaws, which lead to a change in the occlusion contacts in this individual, the occlusion is called pathologic.

The orthognathic occlusion is characterized by the following signs. The lower anterior teeth contact with the area of the dental tubercle of the upper teeth by their cutting edges. The upper anterior teeth overlap the lower one by third of their vertical size. The buccal tubers of the upper premolars and molars as if cover the similar teeth of the lower jaw, whose tubers are located in the longitudinal sulcus of the upper one. The anterior buccal tuber of the first upper molar is located in the sulcus between the buccal tubers of the similar tooth of the lower jaw. The similar tooth of the lower jaw and the tooth standing behind (part of its occlusion surface) are the antagonists of each tooth of the upper jaw. Each tooth of the lower jaw occludes with the similar and the tooth of the upper jaw standing in front.

The tubers, which contact with the occlusion surface of the antagonist, in the position of the lower jaw in the central occlusion are called supporting (buccal tubers of the lower jaw teeth and the palatine tubers of the teeth of the upper jaw). Other tubers – buccal at the teeth of the upper jaw and lingual at the teeth of the lower jaw – are called guiding ones.

The relationship of incisors and canine teeth is characterized by the degree of the vertical and horizontal overlap of the lower teeth by the upper ones. Vertical overlap is more frequent within the third of the crowns of the lower incisors. Horizontal overlap is caused by the angulation of the long axis of the upper incisors and is measured by the distance from their cutting edges to the vestibular surface of the lower incisors.

The vertical overlap of the lower incisors more than the crown third is characteristic of the orthognathic bite with deep incisor overlap. The significant tilt of the upper teeth forward in the vertical position of the lower ones is a sign of physiological prognathism, and the simultaneous tilt forward of the upper and lower teeth – of biprognathism.

The straight occlusion is characterized by absence of overlap in the group of the anterior teeth and joining of their cutting surfaces. Joining of the lateral teeth does not differ from their joining in the orthognathic occlusion but, as a rule, this group of the teeth has weakly expressed tubers.

Physiological prognathism differs from the orthognathic occlusion by the large tilt of the cutting surfaces of the frontal group of the upper jaw toward the front, as a result of which the horizontal overlap increases.

In the group of the masticatory teeth occlusion contacts are characterized by joining of the similar buccal tubers, there are cases when the medial- buccal tuber of the first molar of the upper jaw is between the second premolar and the first molar of the lower jaw.

In physiological progenia (prognathism) there is the vestibular displacement of the anterior group of the teeth of the lower jaw, and the cutting surfaces of the anterior teeth of the upper jaw are in the contact with the lingual surface of the lower teeth. Occlusion relationship in the group of the masticatory teeth is characterized by the fact that the medial- buccal tuber of the first molar of the upper jaw contacts with the distal- buccal tuber of the first molar of the lower jaw or is located on the distal slope of this tuber. This relationship characterizes the displacement of the dentition of the lower jaw forward.

In the biprognathic relationship of the dentitions there is a tilt of the anterior teeth of the upper and lower jaws toward the front with retention of the occlusion contacts, characteristic of orthognathia in the group of the masticatory teeth.

In orthognathia, deep incisal overlap and biprognathia the movement of the lower jaw toward the front is combined with the displacement of the jaw downward and guided by the occlusion surfaces of the anterior teeth – anterior guiding component. Simultaneously with contact and displacement of the incisors the articular heads are displaced downward and toward the front, being in contact with the articular disk, which slides along the surface of the articular tubercles. The presence of contact between the joint elements along the entire displacement way allows to speak about the presence of the distal guiding component.

Harmonious interaction between the anterior and distal guiding components ensures uniform load on the anterior teeth and joints. As a rule, in these kinds of occlusion there are no contacts in the region of the masticatory teeth. In a number of cases the distal buccal tubers of the teeth of the lower jaw may come in contact with the medial buccal tubers of the teeth of the upper jaw at some part of the incisor way.

  The masticatory muscles.Masticatory muscles are that group of muscles which promotes displacement of the mandible in various directions by contraction. As the mandible makes the movements in various directions all masticatorymuscles, depending on it, are divided into separate subgroups having a various direction of bunches and differing from each other both by arrangement of points of their attachment, and by character of action.

     The subgroups are the following.

•  A subgroup of the muscles lifting the mandible; the following muscles are related to them: the temporal muscle, masticatorymuscle proper, internal pterygoid muscle.
• A subgroup of the muscles lowering the mandible, they are also called the openers of the oral cavity. They are: the maxillo-hypoglossal muscle, mental-hypoglossal muscle and the anterior belly of the digastric muscle.
•  The third subgroup contains only the external pterygoid muscle whose contraction draws the mandible aside.

THE FIRST SUBGROUP

     The temporal muscleoriginates on scales of the temporal bone where it is located fanlike.

     The anterior bunches of the fibres go vertically and the posterior being almost horizontally and strongly bent. All these bunches converge downward and form the thick sinew passing under the zygomatic arch and attached to the coronoid process of the mandible. The temporal muscle is the largest in all group of the masticatorymuscles. In spite of the fact that separate bunches of the temporal muscle have various directions, resultant force of these bunches in contraction of the muscle pulls the mandible upward and slightly backward.

     The masticatorymuscleproper is a little bit shorter than temporal, though a little thicker and more powerfully than it. It consists of two layers: superficial which bunches of fibres have an oblique direction, and deep, going more steeply. The superficial layer is attached by the sinew at the lower edge of the zygomatic arch, and deep layer is attached directly to the internal surface of the zygomatic arch. The roughness of the external surface of the mandible angle serves as a mobile point of attachment of this muscle. Such character of attachment causes a direction of its action in contraction; in bilateral contraction the muscle lifts the mandible up, and in unilateral contraction it also shifts it outside to the side the contracted muscle.

     The internal pterygoid musclehas the same shape and the same direction, as the masticatory one, with only one difference, it is located on the internal surface of the mandible. It is smaller than the masticatorymuscle. The muscle begins by short but a dense sinew in the fossa of the pterygoid process of the basic bone and a small bunch from the maxilla body and is attached to roughnesses of the internal surface of the mandible angle.

     The internal pterygoid muscle, owing to similarity to themasticatorymuscle carries out a similar role – lifts the mandible upward in bilateral contraction; in unilateral contraction it displaces the mandible inside and to the side opposite to that where there was this contraction.

In joint contraction of the three above described muscles the mandible rises up. Closing of the mouth occurs due to work of not one muscle but all first subgroup, working together in spite of the fact that bunches of separate muscles or even the whole muscles of this group offer counteraction to each other.

THE SECOND SUBGROUP

     Antagonists of all first subgroup is a group of the muscles lowering the mandible. Both points of attachment of this group are mobile and located on the mandible and on the hypoglossal bone. This peculiarity causes extreme mobility of the floor of the oral cavity basically consisting of these muscles.

     The mental – hypoglossal muscle begins from mental spine of the the mandible; it is attached by the other end to the hypoglossal bone and pulls it forward and upward. In a motionless condition of the hypoglossal bone the muscle lowers the mandible.

     The maxillo -hypoglossal musclemakes a basis of the floor of the oral cavity – a diaphragm. It is attached by the narrow edge to the hypoglossal bone, and by wide one to the internal surface of the mandible along the internal oblique line from the third molarup to the middle of the chin on the right and left. Its anterior fibres lay horizontally and a little oblique to the midline of the mouth.

When the hypoglossal bone is motionless, the muscle lowers the mandible downwards, in the motionless mandible it pulls the hypoglossal bone forward and upward.

The bigastric muscle.Its posterior belly begins from the mastoid incisure of the temporal bone and, going forward and downward, it is attached at the hypoglossal bone by the intermediate sinew. The anterior belly originates from this intermediate sinew, and also from the hypoglossal bone and it is attached in the area of the bigastric fossa on the mandible.

     The anterior belly lowers the mandible and pulls it backward, and in motionless mandible it lifts the hypoglossal bone.

THE EXTERNAL PTERYGOID MUSCLE.

     The external pterygoid muscle begins by two heads: upper (smaller) goes from the subtemporal crest and subtemporal surfaces of the big wing of the basic bone, and the lower (big) – from the lateral plate of the pterygoid process of this bone, partly from the maxilla tuber. The former, being attached to the articular capsule, is intertwined by fibres into the interarticular cartilage disk and causes sliding along the posterior slope of the articular tubercle by its contraction, the latter is attached to the articular process neck.

     In bilateral contraction of the external pterygoid muscle the mandible is pushed forward, and in unilateral – it is displaced aside, opposite to which the muscle was contracted.

     Mimic muscles.Of the mimic muscles of the face only the group which is located in the lower part of the face and surrounds the oral fissure plays a primary role in mastication. There is the circular muscle of the mouth in the centre of this group consisting of tissues, incorporated into the upper and lower lip and promoting narrowing and expansion of the oral fissure. This muscle therefore can be named sphincter of the mouth. Fibres of other muscles belonging to this group are intertwined into it, located in the cheek soft tissue thickness and forming walls of the vestibule of the oral cavity These muscles cause rich mimics of the lips and promote performance of various functions of the oral cavity, such as suction, chewing, swallowing, etc. All these muscles have three layers.

      The following muscles lay most superficially:

1) the triangular muscle beginning at the external surface of the mandible backward from the mental aperture and intertwined into the circular muscle at the corner of the mouth; in its contraction it draws it down;

2) the zygomatic muscle beginning on the buccal surface of the zygomatic bone and intertwined into the upper lip at the corner of the mouth; in its contraction it lifts the corner of the mouth up (the antagonist of the first);

3) the square muscle of the upper lip, beginning by three heads (on the external surface of the zygomatic bone, on the frontal process of the maxilla and at the low orbital edges which lower downwards and come to an end in the nasolabial fold; function of this muscle is in raising the upper lip.

     The middle layer is made of the following muscles:

1) the square muscle of the lower lip beginning on the external surface of the mandible and intertwined into the lower lip at the corner of the mouth; in contraction it pulls the lower lip downwards;

2) the canine muscle is lying under the square muscle of the upper lip, it begins in the canine fossa and, being intertwined by fibres into the corner of the mouth, in contraction draws it up.

     The following muscles lay most deeply:

1) the mental muscle beginning at the alveolar edge at the lower incisors and is intertwined into the chin skin; being contracted it extends the lower lip forward;

2) the buccal muscle is incorporated in the cheek thickness forming a lateral wall of the vestibule of the mouth;

3) incisive muscles are attached to the walls of the canine alveoli (on the maxilla and mandibe) and intertwined into the corners of the mouth from different sides, in contraction they work as antagonists.

All listed group of the mimic muscles are innervated by branches ofthe facial and trigeminal nerves. All of them work in common in this or that combination. The bigger amount of muscles are contracted simultaneously, the more richly expressed is the facial expression and more sharply marked participation of these muscles in the process of chewing.

The soft palate

     The muscular layer of the soft palate consists of separate groups of muscles of which only muscles of the uvula come to an end in the palate, and the others, being paired, connect it with other organs.

     They are: 1) the palatoglossal muscle, (lying in the anterior arch and connecting the soft palate with the tongue; 2) the palato-pharyngeal muscle going behind the first and lying in the posterior arch, connecting the soft palate with the larynx; between these two muscles there is the lymphoid tissue called tonsils; 3) two large muscles pulling and lifting the soft palate.

     Bunches of these muscles reach the midline of the soft palate, and sometimes pass through it, interwining with bunches of the same muscles of the opposite side. In contraction of these muscles the soft palate rises to and passes air between the denture and the mucous membrane; it should be taken intoconsideration in prosthesis of the edentulous maxilla.

 Articulation is all possible positions and movements of the lower jaw regarding the maxilla, accomplished by means of the masticatory musculature (A.Ya. Katz). The movement of the lower jaw during the mastication is of the greatest practical value.

Occlusion is any joining of the teeth, a special case of articulation (A.Ya. Katz). The number of occlusions is great. Four occlusions of them are most important in the practical sense: central occlusion, anterior and two lateral (left and right).

It is understandable that occlusion, being the clinical expression of the masticatory movements, is divided into separate phases in accordance with the forms of the masticatory movements. The masticatory movements of the mandible as its general movements are divided into the sagittal, transversal and vertical. Due to this occlusal phases or the phases of dentitions should also be divided into the sagittal (antero-posterior), transversal (lateral) and vertical (central). This coincides with the division of the mastication process into three phases:

1) the phase of gripping and cutting food, which is characterized by slipping of the cutting edge of the lower frontal teeth along the palatine surface of the upper teeth to their marginal joining and vice versa; sagittal movement predominates in this phase and, therefore, sagittal occlusion;

2)the phase of food crushing, which is achieved by the vertical movement of the mandible and is characterized by the maximum contact of the teeth of both jaws; the occlusion of dentitions in this phase is called central and it is the initial and final point of all masticatory movements of the mandible;

3) the phase of food grinding, which is characterized by the alternating movements of the lower jaw to the sides. During the movement of the lower jaw to any side the tubers of the masticatory teeth of the lower jaw will contact with the similar maxilla tubers on this side (buccal with the buccal ones, palatine with the tongue).

     The word “articulation” is borrowed from the anatomy, where it designates joint, articulation; however, many authors put different content in this word. In our dentistry the definition of this term given A.Ya.Katz obtained the widest use – by articulation we understand all possible positions andmovementsof the lower jaw regarding the maxilla, achieved by means of the masticatory musculature.

     This determination of articulation includes not only the masticatory movement of the lower jaw, but also its movements during conversation, yawning, etc. For practical purposes it is most convenient to define articulation as the chain of different versions of changing occlusion. This determination is more specific since it covers only the masticatory movements of the lower jaw whose study is very important for designing special apparatuses reproducing them, i.e. articulators.

Occlusionis joining of dentitions as whole or separate groups of the teeth during the larger or smaller interval of time.

     Thus, occlusion is considered to be a special case of articulation – one of its moments.

     Four basic forms of occlusion are distinguished: central, front and lateral (right and left).

     Central occlusion is characterized by closing of the teeth with a maximum quantity of contacting points.

(central occlusion)

Signs of the central occlusion:

– the center line of the face coincides with the line passing between the central incisors;

– the articular heads are located on the slope of the articular tubers at its base.

– front and side view.

     In this case the simultaneous and uniform contraction of the masticatory and temporal muscles is noted on both sides.

     During the anterior occlusion the movement of the lower jaw forward occurs. This is achieved by the double-sided contraction of the lateral pterygoid muscles.

 Anterior occlusion

Signs of the anterior occlusion:

– the center line of the face coincides with the center line, which passes between the incisors;

– the articular heads in the anterior occlusion are displaced forward and are located at the apex of the articular tubercles.

     Lateral occlusion appears with the displacement of the lower jaw to the right (right occlusion) or to the left (left occlusion).

Signs of the lateral occlusion:

– in displacement of the lower jaw to the right the articular head remains on the side of displacement at the base of the articular tubercle, slightly revolving. On the left side the articular head is located at the apex of the articular tubercle;

– right lateral occlusion is accompanied by the contraction of the lateral pterygoid muscle of the opposite (left) side and, on the contrary, left lateral occlusion – by contraction of the same muscle of the right side.

State of the relative rest of the lower jaw.

     When there is no mastication or conversation the dentitions are usually open since the lower jaw is lowered and opening of 1-6 mm is observed between the front teeth. In hanging-down of the jaw the muscle are somewhat extended, which causes the irritation of proprioreceptors.

 Anterior occlusion  (three-point contact of Bonville).

     This involves the tonic contraction of the muscles, which retains the jaw in the position indicated. At this time different groups of fibers are alternately contracted in the masticatory muscles, which ensure their rest and at the same time allows to be ready to new contraction. Energy consumption of the muscles in the state of relative physiological rest is minimum. The width of opening between the central incisors of the lower jaw in the rest position is individually different. There are data about the fact that it increases with age. Furthermore, the position of the relative rest of the lower jaw is the expedient reflex act (for the periodontist the intermittent masticatory pressure is physiological, whereas constant one would cause its ischemia and development of dystrophy).

The rest position of the lower jaw is a protective congenital reflex. It is initial and final for all its movements.

 

BIOMECHANICS OF THE LOWER JAW

     Biomechanics is a science about the movements of man and animals. It studies movements from the point of view of the laws of mechanics characteristic of all mechanical movements of the material bodies without exception. Biomechanics studies the objective regularities revealed during the investigation. Their knowledge allows to foresee the results of the practical activity, helping to conduct it systematically intending to reach a specific result.

     The study of the movements of the lower jaw allows to obtain idea about their norm as well as determine disturbances and their influence on the activity of the muscles, joints, joining of the teeth and state of the periodontium. The lower jaw participates in many functions: mastication, speech, swallowing, laughter and others, but for orthopedic dentistry its masticatory movements is of the greatest value. Mastication can be accomplished in full value only when the teeth of the lower and upper jaws will come into contact (occlusion). Occlusion is the basic property of the masticatory movements. Other functions (speech, swallowing) are achieved when dentitions are open.

     The lower jaw of man makes movements in three directions:

– vertical (upward and downward), which corresponds to opening and closing of the mouth;

– sagittal (forward and back);

– transversal (to the right and to the left).

     Each movement of the lower jaw is accomplished in the simultaneous slip and rotation of the articular heads.

The vertical movements of the lower jaw.Vertical movements correspond to opening and closing of the mouth and are accomplished thanks to the alternating action of the muscles, which lower and raise the lower jaw.

     Lowering of the lower jaw is achieved by contraction of the digastric (anterior belly), submental – sublingual and maxillary-sublingual muscles.

     In closing of the mouth the raise of the lower jaw is accomplished by contraction of the masticatory, temporal and medial pterygoid muscles.

     In opening of the mouth the articular heads slide on the slope of the articular tubercle downward and forward.

     In maximum opening of the mouth the articular heads are established at the front edge of the articular tubercle. In this case different movements occur in different divisions of the joint. In the upper division there is slip of the disk together with the articular head downward and forward, while in the lower one the articular head revolves in the recess of the lower surface of the disk, which is mobile articular fossa for it.

     In opening of the mouth each tooth of the lower jaw is lowered and, displacing backward, circumscribes a concentric curve with the common center in the articular head. Since the lower jaw is lowered and displaced backward in opening of the mouth, the curves in the space as well as the rotational axis of the articular head will be moved.

     The way passed by the articular head regarding the slope of the articular tubercle is called the articular pathway. The articular pathwaydoes not presentthe correct curve but a broken line consisting of a great number of curves.

     In different phases of movements of the lower jaw the center of rotation will be shifted (by Gizie).

Sagittal movements of the lower jaw.The sagittal movements of the lower jaw are accomplished by double-sided contraction of the lateral pterygoid muscles.

      The movement of the lower jaw forward can be divided into two phases. In the first phase the disk together with the head of the lower jaw slides along the articular surface of the tubercles. In the 2nd phase the slip of head is joined by its hinged movement around the transverse axis, passing through the head.

     The distance, which the articular head passes during the movement of the lower jaw forward, is called the sagittal articular pathway. The sagittal articular pathway is characterized by the specific angle. It is formed by the intersection of the line, which lies along the continuation of the sagittal articular pathway with the occlusion plane. The angle of the articular sagittal pathway, according to the data of Gizie, is on the average equal to 33°.

     The way passed by the lower incisors in protrusion of the lower jaw forward is called sagittal incisor pathway. During the intersection of the line of the sagittal incisor pathway with the occlusal plane the angle is formed, which is called the angle of the sagittal incisor pathway. Its gradient, according to the data of Gizie, is on the average equal to 40-50°.

     The contacts at 3 points are possible during the anterior occlusion:

– the 1st is located on the front teeth;

– two others – on the distal tubers of the third molars.

This phenomenon was called three-point contact of Bonville.

     Transversal movements of the mandible.The lateral movements of the mandible occur as a result of the one-sided contraction of the lateral pterygoid muscle. During the movement to the right the left lateral wing-shaped muscle is contracted, during the displacement to the left – the right one.

     In this case the articular head on one side revolves around the axis, which goes almost vertically through the articular process of the mandible. Simultaneously the head of the other side together with the disk slide by means of the articular surface of the tubercle. During the movement of the mandible to the right, the articular head moves downward and forward on the left side, and on the right side it revolves around the vertical axis.

      On the side of the contracted muscle the articular head moves downward forward and somewhat towards the outside. In this case its pathway is located at the angle to the saggital line of the articular pathway. This angle was for the first time described by Benedict and for this reason it was named after him (angle of the lateral articular pathway); on the average it is equal to 17°. The ascending ramus of the lower jaw moves towards the outside on the opposite side, thus standing at the angle to the initial position.

Angle of the saggital articular pathway.(angel of Benet)

 

     The transversal movements are characterized by the specific changes in the occlusion contacts of the teeth. Since the mandible moves both to the right and to the left, the teeth circumscribe the curves, which are intersected at the obtuse angle. The farther from the articular head is the tooth, the more obtuse is the angle.

     Of interest are the changes in interrelations of the masticatory teeth with the lateral excursions of the jaw. There are two sides in lateral movements of the jaw: working and balancing. On the working side the teeth are placed against each other by homonymous tubers, while on the balancing side by the opposite ones, i.e. buccal lower tubers are placed against the palatine ones.

P-working side, Б-balancing side

     In chewing of the food the mandible makes a cycle of movements. Gizie presented the cyclic recurrence of the movements of the mandible in the form of the diagram given below.

 

1. The initial moment of the movement is the position of the central occlusion

2. The jaw is lowered and protrudes forward.

3. The jaw is displaced to the side (lateral movement) and the teeth are closed on the working side by the homonymous tubers, and by opposite ones on the balancing side.

4. The teeth return to the position of the central occlusion and the masticatory cycle is repeated .

The Physiology of Masticatory System

Masticatory system composes of the teeth, the skeletal, and the neuromuscular components. The occlusion of the teeth is the key to oral function. They play the integral part in maintain occlusal harmony, a concept which a skillful practitioner must be trained to recognize them or appreciate their significance. The skeletal components consist of the temporomandibular joints, the mandible, and the maxilla. Finally the neuromuscular components consists of the muscle of mastication and the somatosensory system. The three components worked interactively with each other. Complex occlusal problem often begins with a weak link in one of the three components and later manifests into different components. As described by Henry L Beyron: masticatory system is a unitary system, each part which the teeth, for instance- must be considered in relation to the while. The masticatory system is a functional system, the prime object of which to promote perfect condition functionally rather than morphologically.

To fully understand the concept of natural teeth occlusion, we must analyze the mandibular movements. These series of movement can occur around three axes: the horizontal, the vertical, and the sagittal axis. The movement about the horizontal axis, which occurs in the sagittal plane, can be demonstrated when the retruded mandible produces a purely rotational opening and closing movement around the hinge axis, which extends through both condyles. The movement about the vertical axis, which occurs in the horizontal plane, can be demonstrated when the mandible moves into a lateral excursion. The center for this rotation is a vertical axis extending through the working side condyle. The movement about the sagittal axis, occurs when the mandible moves to one side and cause the condyle on the contralateral side travels forward. As it does, the condyle crosses the eminentia of the glenoid fossa and moves downward simultaneously. Movement of the mandible can be a combination of two or more movements about one or more of the axis. Centric occlusion, the up and down motion, demonstrated a combination of the purely rotational about the hinge axis and also possible the translational movement (gliding movement) in the upper compartment of the joints. The centric relation, is the most duplicatable position, occurs when the condyle is in its most retruded position at the posterior and superior of glenoid fossa. The protrusive movement occurs when the mandible slides forward so that the maxillary and the mandibular teeth are in end to end position. In this movement, the anterior segment of the mandible will travel a path guided by contacts between anterior teeth, thus gives rise to the term anterior guidance. Mandibular working movement occurs when the mandible move to one side; whereas the term nonworking demonstrated the same movement but on the contralateral side. In this type of movement, the condyle on the nonworking side will arc forward and medially, whereas the condyle on the working side will shift laterally and usually slightly posteriorly. This bodily shift of the mandible in the direction of the working side is termed Bennett movement, named after the man first described it. Aull has been demonstrated that the presence of an immediate side shift occured in 86% of the condyles studied. Lundeen and Wirth have shown this median dimension to be approximating 1mm-3mm.

Movement areas of the mandible was measured by Ulf Posselt using the gnatho-thesiometer, a tool to record the positions of the mandible in persons with natural teeth. Earlier investigation have shown the figure of the movement of the anterior part of mandible is rhomboidal in shape. Ulf Posselt sets out to investigate the shape and dimension of the contact area of movement of the anterior measuring point, and of the points on the condyles. Individual variations were studied also in five subjects. He showed that the contact area of movement of the anterior measuring point has the shape of a rhombus and the areas of movement of the condylar points were all different in the five case studied. Furthermore, he found that there are differences between the extreme position and habitual positions. Overall, he found that the shape of the mandibular movement show wide variation at both condyle points and the anterior measuring point.

The determinants of all the mandibular movements described above are the right and left temporomandibular joints in the posterior, the teeth of the maxillary and mandibular arches, and the neuromuscular system.

The temporomandibular joints, as described by Harry Sicher, is a bilateral articulation, right and left joints, though anatomically separted, forming functionally one articulation. There is only one position of the mandible in the dead that imitates the position of the mandible in the living, that is, the position of full occlusion. This centric occlusion is established by intercuspation of the maxillary and mandibular teeth. When the teeth in is CO, there is no bony contact exists at the mandibular articulation of the skull. There is always a space between mandibular condyles and the cranial base at the articular tubercles. Normal physiologic CR position of the jaws may be defined as the stable, comfortable, functional craniomandibular relationship in which the condyles are in their most superior position in intimate contact with the thinnest central bearing area of their respective discs against the distal surface of the articular eminences at any vertical rotational postion of the mandible. CR is a comfortable physiologic work position during mastication and swallowing, provided there are no deflective interferences from the teeth. It is not a rest postion; therefore when the mandible is in CR, considerable electromyographic activity may be observed. When CO = CR, CR is used during mastication and swallowing about 5000 times a day. This position has been found clinically to be the best location for the maximum intercuspation of teeth. Clinically, CR may be defined as the completely retruded position of the mandible with the condyles in their most superior anterior postion at any vertical rotational position of the mandible. As the jaw open, the condyle brace themselves against the postglenoid process as the mandible travels upward and backward. Studies have shown that physiological bone remodelling observed in the condyle has no orientation that could be related to the changed occlusion. However, occlusal forces brought about rebuilding of the bone in the neck of the condyle. Longitudinal studies have shown that changes associated with orthodontic treatment of class II malocclusion are related mainly to altered growth patterns of the alveolar processes rather than to joint changes. It appears that the changes in temporomandibular joint morphology may be the result of pathological rather than physiological processes. Furthermore, there has been striking evidence of periodontal trauma from occlusion and subsequent movement of teeth in all studies of occlusal disharmony and TMJ morphology. The clinical significance of these research findings to the practice of dentistry should be the adaptation of the occlusion to be in harmony with the TMJ rather than hoping for the TMJ to adapt to the occlusion.

The teeth of the maxillary and mandibular arches made up the second determinants of mandibular movement. Tooth morphology is totally genetic and is not specific to race or gender. Teeth occluded together and form “occlusion”. Occlusion refers to the act of closure or the state of being closed. When teeth is in maximum intercuspation, this position is referred to as centric occlusion (CO). As described above, when CR = CO, this is the best clinical situation. In a normal class I relationship, the buccal cusps of the mandibular premolars contact the marginal ridge of the maxillary premolars. The mesiobuccal cusp of the first mandibular molar occludes on the adjacent marginal ridges between the maxillary first molar and second premolars while the distobuccal cusp of the mandibular first molar occludes in the central fossa of the maxillary first molar. In the maxilla, the lingual cusp of maxillary premolars occlude on the marginal ridges or in the distal fossae of the mandibular premolars. There is usually no occlusal contact with the rudimentary lingual cusp of the mandibular first premolar. The mesiolingual cusp of the maxillary first molar occludes in the central fossa of its mandibular mate while the distolingual cusp occludes on the adjacent marginal ridges between the mandibular first and second molars. In good occlusion, all the teeth in the mouth make simultaneous contacts in CR including the anterior teeth. However, the anterior teeth should never contact harder than the posteriors or fremitus may be produced with possible endodontics and periodontal trauma and/or interproximal separation of the teeth. Normally, occlusal contacts on the anterior teeth are not broad, but rather two or three spots per tooth on the incisors and one on each canine. The total tooth contact area has been estimated to be about 4mm2 for the entire mouth. Complete occlusion of the teeth (intercuspal position) approximately 5000 times per day helps to realign and stabilize the craniomandibular relationship into a state of biological equilibrium. Stabilization of the craniomandibular relation in CR is important to comfort, function, and longevity of the dental restoration. The use of properly constructed, adjusted, and maintained maxillary, anterior guided occlusal splint is probably the best way to align and stabilized the craniomandibular relationship prior to treating the occlusion and articulation of the teeth.

Samuel Adam and Helmut Zander described functional tooth contacts in lateral and centric occlusion using miniature radio transmitters which were incorporated in dental bridge work to record the functional tooth contacts on magnetic tape. Three test food were used: bread, lettuce, and peanut, in analyzing the functional tooth contacts in four adults. They found that contact was recorded more often in the intercuspal position than lateral to it. The frequency of contact was least in the early phase of mastication, increased in the middle third of mastication, and was greatest during the last part for both the intercuspal postion and the lateral position. The intercuspal contacts increased in duration as the act of eating progressed and were longest during swallowing: however, the duration of lateral contacts did not increase. Finally, by relating the onset of tooth contact to the envelope of electromyographic activity, it became apparent that lateral contacts occurred before as well as after the intercuspal position was reached. The ratio of these two types of lateral contact was characteristic for each subject and was not altered by the nature of the food being masticated.

Henry L. Beyron investigated the characteristics of functionally optimal occlusion and principles of occlusal rehabilitation. He also described the reaction of the supporting tissues to the load on a single tooth. A tooth should be subjected to a certain occlusal load and should not be permanently without one; that is, it should always have an antagonist. With longitudinal stress, the tooth and the periodontium act as one unit to resist the load; whereas in lateral stress, the forced is taken up regionally and result in bone resorption. In the article it also emphasized that it is not the magnitude but the direction of the force that is decisive. In a functionally optimal occlusion, the teeth should receive the stress consistent with the physiologic requirements for stimulation of the supporting tissues; and, in particular, the direction of stress should coincide as closely as possible with the long axis of the tooth. This is referred to as the principle of axial stress. In viewing of the entire dentition, the total load should be distributed by interproximal tooth contact and by simultaneous occlusal contact in centric position among all teeth and in eccentric postions primarily among the teeth in the engaged tooth segment. Another characteristic of a functionally optimal occlusion is its closure without interference in the centric maxillomandibular relation. The mandible on closing in CR should close into the position of maximal intercuspation (CR=IP). The article also described the need to determined a proper occlusal dimension in a functionally optimal occlusion. There must be a proper interocclusal clearance between the rest postion and the intercuspal position. This space also is referred to by Ramjford as the physiologic resting range which the interocclusal distance average 1.7mm in the clinically determined rest position, and 3.29mm when determined electromyographically on the basis of minimal muscle activity. A functional optimal occlusion also demonstrated free gliding movements in excursion without any interference. The attainment of this objective is faciliated by a flat gliding paths and simultaneous contact in eccentric positions among several teeth in the engaged tooth segment.

The most thorough study on the occlusal relations and mastication was done by Henry Beyron on the australian aborigines who lived under settlement conditions in central Australia. The purpose of the investigatio is to obtain information on occlusion and mandibular function concerning the anatomic size and shape of dental arch, the intercuspal postion and its relation to CR, the occlusal contacts, and the mandibular movements and its shape and size during mastication (envelope movement). The subjects were arranged in age group: the youngest, middle aged, and the oldest group. Prior to the study, the anatomic examination of the teeth, the dental arch, and also the gingival condition were carried out. Number of teeth, the mesiodistal width of mandibular and maxillary central incisors and lateral incisors were measured. The tooth spacing, width and length of dental arch, the skeletal and dental relationship in overbite and overjet was recorded, also the attrition of the dentition using the location of wear in the tooth to classified (Broca classification) was documented. The subjects also were examined for the intercuspal position and its relation to the CR position. IP were found to achieved without deviation. The result show the mean difference between the intercuspal and the retruded contact positions was nearly the same for all age group: 1.15, 1.11, and 1.19 for the youngest, middle aged, and oldest groups, respectively. The mean is 1.25 +-1.0mm. The distance between the intercuspal and the lateral positions was from 2-3mm. The presence of occlusal contacts was tested using strip of thin .03mm cellophane placed between opposing teeth. 75% of young age group contact in the IP for molars and premolars only. 50% of the middle aged group contact in IP for molars and premolars only, whereas in old age group, contact spread to canines but incisors still was separated by a small space. The only time incisors were contact is when overbite and overjet was zero, i.e. edge to edge occlusion. He also investigae the masticatory movement using cinematographic recording. The lower incisor was chosen as the reference point, as it was possible to follow directly the movement of an incisal angle of lower incisor. A camera was used to recored the movement of masticatory system in the frontal plane at 32 frames per second. No extraoral indicators were used and thus recording in the sagittal plane could not be obtained. The film analysis showed that an opening an closing movement together form a masticatory cycle. He found that the duration of masticatory cycle varied within and between subjects. The enveloped movement of mastication was registered during chewing, began on one side with 2, 3, and 4 cycles, then move to the other side after an opening movement. This pattern was repeated regularly until the bolus was swallowed. The time for one chewing cycle was less than 1 second. The shape and size of the masticatory cycle also was determined and graphically demonstrated. The means for the vertical dimension of the masticatory cycle were 18mm for young, 17mm for middle age, and 15mm for the oldest. Thus there is loss of vertical dimension as one ages. The cranial part of the most of the masticatory cycles coincided for some distance with the path of contact glide obtained from empty movement. Results have shown that the cranial part of the masticatory cycle is performed under cuspal guidance.

The neuromuscular system consists of the masticatory muscles and the somatosensory nervous system. The masticatory muscles consist of the temporalis, the masseter, the medial pterygoid, the lateral pterygoid, the anterior portion of digastric, and the geniohyoid muscle. The temporalis muscle originates from the lateral surface of the skull, extends as far forward as the lateral boder of the supraorbital ridges, and inserts on the coronoid process and along the anterior border of the ascending ramus of the mandible. The temporal muscle is innervated by the three branches of temporal nerve of V3 of trigeminal nerve. It acts as the principal positioner of the mandible during elevation. It is more sensitive to occlusal interference than any other masticatory muscle.

The masseter originates from the zygomatic archs and inserts into the ramus and the body of mandible. Its insertion extends from the region of second molar on the lateral surface of the mandible to the lower one-third of the posterior lateral surface of the ramus. The masseter acts mainly in power comminution (chewing) and its principal function is mandibular elevation.

Medial pterygoid muscle originates from the pterygoid fossa and inserts on the medial surface of the angle of the mandible. Its principal functions are elevation and lateral positioning of the mandible. It is very active in simple protraction. In combined protrusive and lateral movements, the activity of the medial pterygoid dominates that of the temporal muscle.

The lateral pterygoid has two origins: one originates from the outer surface of the lateral pterygoid plate while a smaller, upper head originates from the greater sphenoid wing. Both joins together and inserts on the anterior surface of the neck of the condyle and some at the capsule of the TMJ joint and the anterior aspect of the articular disk. The principle of the lateral pterygoid muscle is protracting the condyle while drawing the disk forward.

The anterior digastric is found near the lower border of the mandible and near the midline. It is innervated by the branch of mylohyoid nerve (V3 of trigeminal). It is assoicated with the depression of the mandible, especially most prominent toward the end of the mandibular depression (end of the opening of mandible).

Finally, two geniohyoid muscles run down and back from the mylohyoid ridges on the lingual side of the mandible and are inserted on the hyoid bone. It functions in depression and retraction of the mandible. It also lift the hyoid bone during swallowing.

In summary, during opening of mandible, the lateral pterygoid acts first and sustained activity. The activity of the anterior digastric follows that of the lateral pterygoid muscles when the opening movements nears completion. During combined protraction and opening, the medial and lateral pterygoid, the masseter, and the temporalis all participated. The temporal and masseter limit the opening of the mandible; thus they become active at the final stage of opening of the mandible. During mandibular closure, the medial, temporal, and masseter are all actived. In forced heavy closure, all the masticatory muscle contracted, along with many of the facial and neck muscle contraction. During lateral mandibular movement, there are the ipsilateral contraction of the posterior and middle fiber of temporalis and the contralateral contraction of the lateral and medial pterygoid muscles and the anterior fibers of the temporal muscle. During horizontal movement, masseter or the temporal act as antagonists. Lateral movement are initiated by the lateral and medial pterygoid muscle. During protraction, the lateral and medial pterygoid acts simultaneously. Retraction of the mandible is accomplished by contraction of the middle and posterior parts of the temporalis and the suprahyoid muscle.

As we can see, in order to achieve the most optimal occlusion, we must determine all the components of the masticatory system and ensure they are in balance. There must not be any interference, with CR as close to CO or with freedom in centric if possible. Only slight 10% of the population achieved the complete harmony between the teeth and the TMJ. Only small group achieve maximum intercuspation in CR. Therefore, in a normal occlusion, there will be a reflex function of neuromuscular system producing the mandibular movement which avoids premature contacts. This guides the mandible into IP with condyle in a less than optimal occlusion. The result is the hypertonicity of nearby muscle, but well within physiologic adaptive limits. If stress and emotional tension increased, the threshold to this adaptability may be lowered and thus this adaptive normal occlusion may manifested into pathologic occlusion. Dawson described the normal occlusion as: firm contacts on all teeth in CR, anterior guidance which harmonizes with patient customary envelope movement, disclusion of posterior teeth when the mandible protrudes, disclusion of posterior teeth on the nonworking side in lateral excursions, and finally absence of interference on the posterior teeth on the working side in lateral excursions. Mastering occlusion is the primary objective that must be achieved in order to restore the function of the dentition to last a lifetime.

Dynamics of the Human Masticatory System

There is a large body of literature describing the dynamics of the human musculoskeletal system (for a review, see, for instance. In this large and mostly well-developed field, the human masticatory system occupies a relatively small place. One of the reasons for this underexposure is probably its relative complexity, which makes it more difficult to analyze than, for instance, the system of the shoulder, arm, hip, knee, or leg.

There are several reasons why masticatory dynamics are difficult to analyze. First, the masticatory system consists of a large number of muscles of various shapes and sizes, making it impossible to determine, unambiguously, how they might cooperate to perform a certain task. Second, they have a complex architecture, and their actions cannot be determined from their overall orientation only. Third, the upper and lower jaws articulate through two very complexly shaped incongruent temporomandibular joints. Any simplification of these joints based on concepts usually used for other joints (like hinges or balls-and-sockets) leads to considerable loss of functionality. Furthermore, the articular surfaces are separated by a cartilaginous articular disc which is able to move more or less freely between these surfaces and are influenced by, and affect, the movements of the jaw.

Apart from the intrinsic complexity of the system, there are several limitations to the collection of experimental data on masticatory function. For instance, some of the masticatory muscles run deep and are partially hidden behind bony structures, which prevents easy access for electromyographic (EMG) measurements. Furthermore, many jaw movements are relatively small, posing stiff challenges to experimental systems designed to record relevant properties adequately.

In summary, there are many factors that impede assessment of the mutual contributions of the relevant active and passive structures to jaw movements. Recently, the application of biomechanical models has provided an adequate experimental framework to explore masticatory dynamics without several of the drawbacks that accompany experiments with human subjects. They are powerful tools for establishing causal relationships in this field and have led to updates of or new formulations on various insights into the function of the masticatory system.

Generally, monographs on jaw movement have been written from a more clinical perspective, and have provided information mainly about the position, and positional changes, of the lower jaw. The dynamic aspects and their consequences were rarely taken into account. Recent developments in these areas have provided new insights. The present review is based upon a selection of studies on jaw movement analysis, from both clinical and basic perspectives. Its purpose is to establish an updated overview of the fundamentals of jaw movement, and it focuses on the contributions of muscles and the influence of passive constraints. It has been written from a biomechanical perspective and with an emphasis on masticatory dynamics. Since relevant topics such as neural control and feedback as well as properties of masticatory motor units have been reviewed relatively recently, they will not be discussed here.

In section 1, the relevant anatomical properties of the human masticatory system are reviewed briefly. They serve as a basis for all aspects of jaw movement and its determinants. Jaw movement is discussed in section 2, where its physics and its properties are reviewed. Interactions among anatomical structures and jaw movement properties are reviewed in section 3. An attempt is made to analyze the causal relationships between the two components. Finally, relevant issues that have not yet been resolved, but are assumed to be of critical importance for jaw movement analysis, are discussed in section 4.

 The Human Masticatory System

The human masticatory system consists of a mandible which is able to move in relationship to the skull and is guided by two temporomandibular joints through contractions of the masticatory muscles. To establish the contribution of each individual structure to jaw movements, one must explore the construction of the joints and the muscular system as well as the mechanical consequences of this construction. The morphology of the human masticatory system will be summarized very briefly. While there is a large quantity of literature in this area, the list of relevant citations in this article is far from complete.

Joints

Morphology

The anatomy of the temporomandibular joint has been described thoroughly. Mandibular movements are guided by its articular surfaces. These surfaces reside on the temporal bone of the skull, involving an articular eminence and a mandibular fossa, and on the roughly ovoid condylar head of the mandible. They are both irregularly shaped, covered with fibrocartilage, and inaccessible for direct measurements in vivo.

The articular surfaces are separated by a cartilaginous articular disc with non-uniform thickness. This disc is able to move together with the mandibular condyle along the articular eminence while simultaneously rotating on the condyle. Disc movements generally run smoothly with respect to the articular surfaces.

The articular disc is connected superiorly to the temporal bone and inferiorly to the mandible by relatively loose fibrous structures. Together, these structures make up the articular capsule. It is reinforced laterally by the temporomandibular ligament, and is the only capsular structure that runs directly between the temporal bone and the mandible. Cadaver material reveals that the articular capsule is slack. There are two accessory ligaments: the sphenomandibular ligament, which runs medially from the mandibular ramus; and the stylomandibular ligament, which attaches to the mandibular angle from behind.

Mechanical consequences

The articular surfaces are highly incongruent, which means that the shapes of the upper and lower surfaces differ considerably. This allows for a large amount of motion at the cost of a lessened joint stability and relatively small areas of joint contact. The articular disc is supposed to reduce joint incongruency and increase joint stability by enlarging the contact area.

A second consequence of the incongruency of the joint, in combination with the slackness of its capsule, is that the movements in the joint are not restricted to rotations about more or less fixed joint axes, as in classic joints. The condyle and temporal bone can be regarded as two separate bodies in space, usually held in appositional contact when the jaw moves. As a consequence, the mandible may be able to move with six degrees of freedom. Theoretically, it may rotate about an axis through, for instance, its incisor point. Therefore, the motion of this point per se bears no relationship to condylar motion. This property is known as kinematic redundancy. Furthermore, if the incisor moves from one point to another its path is not necessarily defined a priori. It may choose to move along a straight path or along a detour. In principle, the number of possible paths is infinite.

Muscular system

Morphology

From a classic anatomical perspective, the masticatory muscles are divided into elevator and depressor groups. The elevator group consists of the masseter and temporalis muscles, which are located more or less superficially, and the medial pterygoid muscle, which is located more deeply. The muscles of the depressor group are located in the floor of the mouth. This group consists (from superior to inferior) of the geniohyoid, the mylohyoid, and digastric muscles. The geniohyoid and mylohyoid muscles connect the hyoid bone with the body of the mandible. The digastric muscle connects the mastoid process of the skull with the body of the mandible and is attached to the hyoid bone via a fibrous loop which runs around its intermediate tendon. The lateral pterygoid muscle completes the muscular system. It consists of a superior and inferior head running from the mandibular neck in forward and medial directions. Since both heads are considered to have different actions, they cannot be regarded exclusively as elevator or depressor.

The elevator muscles are heavily pennate. They have relatively large physiological cross-sectional areas and are suitable for the generation of large forces. The fibers are short, which limits their capacity for active shortening during contraction. The depressor muscles and the lateral pterygoid have more or less parallel fibers and are therefore able to contract over a longer distance with less force.

Mechanical consequences

The human masticatory system contains more muscles than are apparently necessary to accomplish its tasks. This seems to be unnecessary from a mechanical perspective, but it must be noted that there are also spatial requirements to the construction of the muscular system. For instance, a muscular system that is mechanically optimal probably violates spatial requirements with respect to the adjacent airway and alimentary tract. The muscles can perform almost any task in various ways. Although the system is able to generate cyclic movements controlled by a central pattern generator, its muscles cannot be lumped into a limited number of alternating muscle groups. One of the reasons for this is that they have to adapt constantly to the texture of the food between the teeth. The system is mechanically redundant, which means that there is an infinite number of muscle contraction patterns which can cause the same movement.

It has been demonstrated that various masticatory muscles have the capacity to deploy regionally different portions for different tasks. Such functional heterogeneity, in combination with a relatively large attachment area, may cause the direction of the line of action of such a muscle to vary as well. While there is no a priori evidence of co-activation between different muscle portions, the system is capable of fine-tuning, to a large extent, the orientation of the required muscle force by selective activation of motor units. The relatively large extensive nature of some muscles may also cause spatially distant fibers within a muscle to shorten to various degrees during mandibular movements. This may cause shifts in muscle lines of action which are not caused by the nervous system.

The depressors are directly or indirectly attached to the hyoid bone. When this bone moves downward through action of the infrahyoid muscles during wide jaw-opening, the jaw depressors are stretched, which, in turn, lengthens their possibilities for active shortening. This may help in obtaining wider jaw gapes.

Jaw Movement Basics

Degrees of freedom for jaw movement

In three-dimensional space, a body able to move freely may perform translations and/or rotations. This applies to the lower jaw, although the degrees of the various movements are limited. Translations can be performed along, and rotations about, three independent axes. The translation axes and the rotation axes are not necessarily the same, but usually the three orthogonal axes of some Cartesian systems aligned to anatomical planes are used for this purpose. Translations can be described along axes which are, for instance, anteroposterior or X, mediolateral or Y, and supero-inferior or Z. Rotations can be defined by terms such as azimuth (about the Z-axis), elevation (about the Y-axis), and roll (about the X-axis) or yaw, pitch, and roll. It must be noted that there are many other conventions about sets of axes which are applicable. Independent of the applied set of axes, every movement can be expressed by a unique combination of the six independent fundamental movements (which are known as the six degrees of freedom of movement). The lower jaw does not move freely but is guided by its joints. These structures, therefore, may reduce the number of degrees of freedom (vide infra). Although translations and rotations relative to any of the three independent axes remain possible, they are no longer independent. For instance, if the joints should restrict one degree of freedom, the movement is completely determined by the other five.

 

Six degrees of freedom for jaw movement. Dashed lines: principal axes. a: (linear) accelerations. F: (linear) forces. m: mass. α: angular accelerations. M: torques. I: moments of inertia.

If the movement of a point is studied—for example, the incisor point of the lower jaw or a point representing the condyle—it should be recognized that a point does not have an orientation. Rotations about axes through this point are thus meaningless, and it should be recognized that movements accomplished by rotations about distant axes can also be performed by translations. A point, therefore, can move with, at most, three degrees of freedom. The consequence is that the movement of any point on the jaw can be reconstructed from the movement of the jaw, but not the other way around.

It is not easy to recognize functional aspects of jaw movement from a combination of translations and rotations with respect to pre-defined axes. An alternative way to describe a movement is by a rotation about and a translation along a so-called helical axis or screw axis. A movement can be described by subsequent (six degrees of freedom) small displacements. The orientation and location of the helical axis related to such a displacement provide information as to how it took place, while the moving body translates along and rotates about this axis. Generally, the helical axis is not stationary and may itself undergo displacements during movement. Therefore, each instant of movement is connected to a unique instantaneous helical axis. For sagittal movements, this axis is directly analogous to the instantaneous center of rotation for plane motion. It should be noted that the location and inclination of the helical axis, and the amounts of rotation about and translation along this axis, contain six independent variables according to the six degrees of freedom for movement.

 

Helical axis. (A) Rotations about and translations along a hypothetical helical axis during a non-midline jaw movement (dashed line). (B) Subsequent helical axes during jaw closure.

Instantaneous helical axes thus provide a complete overview of jaw movement. The movement of teeth and condyles can be derived from them. The relative contributions of rotations and translations of the mandibular condyle, for instance, can be determined from the distance to the axis. If, at a certain instant, the condylar movement is characterized primarily by a rotation, the helical axis will be situated close to the joint. If, in contrast, the translation component is dominant, the helical axis will be located at a distant location. These differences were demonstrated for jaw-opening movements performed with different muscle recruitment patterns showing no clear visual differences from normal movements in terms of displacement of teeth and condyles, but great differences in terms of helical axis positions. This emphasizes that, from a clinical perspective as well, this approach can be relevant, for example, to an enhanced possibility of discrimination among different translations of the same type of movement by quantification of the inclination and remoteness of the helical axis. Furthermore, it enables on to discriminate between and among apparently similar movements caused by different muscle contraction patterns.

Physics of jaw movement: Newton’s laws

The dynamics of a moving lower jaw are expressed by its position, its velocity, and its acceleration. According to the six degrees of freedom for movement, each of these three properties also consists of six independent variables. In a Cartesian system, the position is not defined only by the (X, Y, and Z) position of the center of gravity with respect to the origin of this system, but also by the orientation (azimuth, elevation, and roll) of the jaw. The velocity and the acceleration also have three linear and three angular components. For each of the six components, velocity is the (time) derivative of position and acceleration the derivative of velocity.

Every moving body, including the lower jaw, obeys Newton’s laws. Movements are caused by forces acting on the jaw. They may be active muscle forces and also passive (reaction) forces generated by joints, ligaments, and dental elements. The forces also have six components. Each linear force (F_x, F_y, F_z) is accompanied by a moment (angular) or torque (M_azimuth, M_elevation, M_roll). The resultant forces and torques generate accelerations according to Newton’s second law (acceleration equals force divided by mass). This accounts for each degree of freedom, emphasizing the fact that the mass of the jaw also consists of three linear and three angular components. The three linear components of the mass of the lower jaw are equal to the common mass. The three angular masses (moments of inertia) are dependent on the distribution of mass about the axis under consideration and therefore on the shape of the lower jaw and adhering structures. The moment of inertia about an axis is defined as the sum of the mass of each particle multiplied by the distance between this particle and the relevant axis to the power of two. For a lower jaw of about 0.44 kg, the moments of inertia have been estimated as 8.6 kg.cm², 2.9 kg.cm², and 6.1 kg.cm² for I_azimuth (about the z-axis), I_elevation (about the y-axis), and I_roll (about the x-axis), respectively. This means that it requires about three times less muscle torque to accelerate the jaw for open-close movements than for latero-deviations. The accelerations cause changes in (linear and angular) velocity, and the velocities cause changes in (linear and angular) jaw position.

Influence of joints

The degrees of freedom of articulating bones are often reduced due to the construction of the connecting joint. An ideal ball-and-socket joint, for instance, does not allow for translations. Therefore, such a joint allows for movements with a maximum of three degrees of freedom. In contrast, the degrees of freedom in the temporomandibular joint are not reduced by its construction. While its articular capsule is relatively slack and its articulating surfaces incongruent, the mandibular condyle is able to move almost freely in the three-dimensional half-space bounded superiorly by the articular surface of the temporal bone. Articular contact is not necessarily maintained, although the distance along which the condyle is able to move perpendicular to the articular surfaces is relatively short. Furthermore, the articular surfaces are not rigid. The articular cartilage and the articular disc are deformable such that the distance between the bony surfaces will be proportional to the joint load. Consequently, the mandible is also able to move with six degrees of freedom. If the joints are assumed to maintain articular contact all the time, and the joint contact is assumed to be rigid, a translation of the condyle in a direction perpendicular to the articular surface of the temporal bone is restricted, and the number of degrees of freedom for condylar movement is reduced to five. Furthermore, if both joints are assumed to be connected rigidly through the mandibular symphysis, the rotation of the lower jaw about an antero-posterior axis is restricted. In this (simplified) situation, it is able to move with four degrees of freedom.

Influence of muscles

The jaw moves through contractions of the masticatory muscles. Each muscle contraction is associated with a force which is expressed by three independent variables: its magnitude, its point of application, and its orientation. The latter two are determined by the muscle’s line of action, defined by the geometry of the system. Each muscle can produce a translation of the lower jaw along its line of action, and a rotation about an axis perpendicular to it and running through the jaw’s center of gravity. The translation and rotation caused by a muscle are not independent, and they express only one degree of freedom. Therefore, if such a muscle is activated homogeneously, the nervous system is able to influence only one degree of freedom through the magnitude of its force. If the muscle can be activated heterogeneously, and is represented by more than one independent line of action, it can influence more than one degree of freedom. Conversely, if separate muscles or muscle portions cannot be activated independently, then, together, they are able to influence only one degree of freedom. A system of muscles, therefore, is represented by a number of degrees of freedom equal to the number of independent lines of action. The masticatory system contains at least 20 muscle portions which may be activated independently. The number of degrees of freedom of the muscular system, therefore, is considerably larger than the (maximum) six degrees of freedom of jaw movement. This causes a mechanical redundancy in the masticatory system.

 

Force and torque generated by a muscle (arrow) with respect to the center of gravity of the lower jaw.

 

Overview of the masticatory system. Ventro-lateral view. Continuous lines: muscle lines of action. Cross-bar: muscle origin. Circle: muscle insertion. MAS_S: superficial masseter. MAS_P: deep masseter. MPT: medial pterygoid. TEM_A: anterior temporalis. TEM_P: posterior temporalis. LPT_S: superior lateral pterygoid. LPT_I: inferior lateral pterygoid. DIG: digastric. GEH: geniohyoid. MYH: mylohyoid. Dots: position of centers of right and left condyle and incisor point.

Determinants of Jaw Movement

Jaw movements caused by the masticatory muscles are guided by passive structures; thus, both passive and active elements generate forces and torques which accelerate the jaw . Because of the large number of these forces, their changes during jaw movements, and their strong interdependency, it is difficult to separate these influences.

 

Forces acting on the lower jaw in the sagittal plane. Cross-hairs: center of gravity. F_closers: mean force of the jaw-closing muscles. F_openers: mean force of jaw-opening muscles. F_joint: joint force. F_bite: bite force. a: moment arm of the different forces.

Active elements

Although passive structures in the masticatory system may act as constraints for jaw movements and guide the mandible along its path, active masticatory muscles are the prime movers in this system. Therefore, it can be expected that they will be the dominant determinants of jaw motion.

Muscle lines of action

While muscle lines of action differ considerably between muscles, each contributes to masticatory movements in a unique manner. Furthermore, the lines of action depend on the position of the lower jaw with respect to the skull. This causes continuous changes in the interplay of muscle forces and torques.

Investigators have estimated the lines of action of the masticatory muscles, in vitro, by connecting the centers of the attachment areas (centroids) on the skull and the mandible. However, functionally different muscle portions that share attachment areas cannot be discriminated with this approach. This drawback can be overcome by measurement of the orientation of fiber bundles and incorporation of the influence of tendinous sheets. Furthermore, this method facilitates the estimation of fiber length as a function of jaw position. The drawback of all in vitro methods is that results are not necessarily applicable to individual subjects. Therefore, muscle lines of action have been estimated in vivo by a determination of putative muscle attachment points or by fitting the long axis through “Magnetic Resonance Imaging” sections. Although this enables individual characteristics to be incorporated, the estimates of the lines of action remain coarse.

Generally, the contribution of a muscle to jaw movements can be established by the direction of its line of action and the position of this line with respect to the center of gravity of the lower jaw. It accelerates the jaw in the direction of the line of action according to: a = F/m, where a is the linear acceleration vector, F the muscle force vector, and m the mass of the jaw. Also, an angular acceleration about the center of gravity occurs according to: α = M/I, where α is the angular acceleration vector, M the muscle torque vector about the center of gravity, and I the moment of inertia vector. The moment of inertia is dependent on the related axis, whereas the mass is not. The actual movement, then, is determined by the resultant instantaneous linear and angular accelerations initiated by the forces of all active and passive structures. For a single muscle, the ratio between angular and linear acceleration is proportional to the length of the moment arm of the muscle with respect to the center of gravity. Furthermore, it is proportional to the ratio between the mass of the lower jaw and its moment of inertia relative to the axis of the muscle torque. These combined factors determine the effect of muscle contraction and, consequently, the contribution of each single muscle to jaw movement. It must be emphasized that these equations represent simplified dynamics. To avoid excessive complexity, the terms relating to, for instance, inertial coupling, centripetal forces, and coriolis forces have beeeglected.

In a sagittal plane analysis, the lines of action of most jaw-closers are directed upward, and those of the jaw-openers, downward and backward. However, in both cases, each line of action has a similarly directed moment with respect to the sagittal axis through the center of gravity of the lower jaw. Jaw-closers and -openers are able to produce a similarly directed torque about this axis which leads to an angular acceleration in the “negative elevation” (opening) direction. Consequently, almost every muscle pair that is activated symmetrically attempts, aside from its specific action, to perform an opening rotation about the center of gravity. It is through this mechanism that both jaw-closers and -openers, despite their difference in orientation, are able to maintain articular contact while performing unloaded (symmetrical) jaw movements.

Muscle dynamics

The optimum isometric force produced by a muscle F_opt is proportional to its physiological cross-section S (in cm²) and its activation A (in %), as denoted in the equation F_opt = 37 x S x A. Due to the dynamic muscle properties, the instantaneous force of a concentrically contracting muscle is less than F_opt, and an eccentrically contracting muscle may produce an instantaneous force larger than F_opt. Due to these properties, the forces produced by masticatory muscles may change constantly during a movement, even though their activation levels remain constant. The force-length relationship quantifies the property that enables a muscle to produce a force when its sarcomeres are not shortened below, or elongated beyond, certain lengths, and this property has been demonstrated to be an important limiting factor for masticatory muscle force development. For instance, the limited amount of maximum shortening of the fibers of the lateral pterygoid muscles prevent protrusion of the jaw beyond its normal limits. Furthermore, maximum jaw-opening is limited by the maximum shortening of the jaw-openers, which is counteracted by the passive forces of the elevators. The latter effect would be even more dramatic if the instantaneous center of rotation remained close to the joint .

 

Dynamic muscle properties. Total force is the sum of the forces produced by the sarcomeres (F_sarcomeres) The active force (F_active) is dependent on the activation through the activation dynamics, the instantaneous sarcomere length, and contraction velocity. The parallel elastic force (F_passive) is dependent on the instantaneous sarcomere length.

Muscle force is also dependent on the shortening velocity through the force-velocity relationship. For jaw open-close movements, where all jaw-opening or jaw-closing muscles were activated simultaneously, it was demonstrated that the trajectory of movement is not very dependent on the speed of movement. While this trajectory depends on mutual muscle forces, the force-velocity relationship does not considerably affect the mutual ratio of instantaneous muscle forces. Consequently, the possibilities for force production are affected similarly in all contributing muscles. In contrast, the force-velocity relationship does appear to assist in deceleration of the lower jaw after a sudden disappearance of resistance during forceful biting. When such an event occurs, the force of the closing muscles can disappear instantly through the suddenly large shortening velocity, which may be considered profitable when the teeth are near occlusion, and there is little time to activate the jaw openers to decelerate the jaw.

Contribution of muscle action to jaw movements

Jaw movements are performed through co-contraction of various muscles. Electromyographic (EMG) measurement of the masticatory muscles during various jaw movement tasks, therefore, cannot be used to establish the individual contributions of the various muscles to a movement. For instance, it does not provide a means to decide whether two or more active muscles assist each other to perform a certain movement or work against each other to stabilize the system. EMG measurement has been used to detect functional heterogeneity in the activation of different muscle portions. For example, it has been shown that the temporalis muscle shows a gradual heterogeneity in the antero-posterior direction, and that the masseter muscle can be subdivided functionally into a superficial, an anterior deep, and a posterior deep portion.

EMG registrations of masticatory muscles correlate muscle activity with jaw movements or static bite tasks. These correlations, however, do not necessarily reflect causal relationships. For causal relationships between muscle contraction and jaw movement to be established, the influence of the passive constraints must also be taken into account. Generally, this influence can be simplified to a screw displacement axis defined by the movement constraints in the joint. The function of a muscle can then be defined by its moment with respect to this pre-determined axis. This concept was adopted by Grant (1973) and applied to the instantaneous center of rotation during jaw movements. Unfortunately, the method applies to joints influenced by only one degree of freedom by any muscle. Since the temporomandibular joint allows for movements with at least four degrees of freedom, this kind of analysis is irrelevant both for static situations (when the influence of the joint force is omitted) and for dynamic situations (when torques are expressed with respect to the center of gravity. In contrast, masticatory muscle function is not dependent on the location of the center of rotation. Conversely, the location of the instantaneous center of rotation (or instantaneous helical axis for non-midline movements) is dependent on the actions of the masticatory muscles. For instance, it has been demonstrated that jaw-open movements can be performed with different muscle recruitment patterns. Despite the relatively similar appearance of these movements, the trajectories of the instantaneous centers of rotation are very different. If one wishes to estimate the work done by a muscle force during a movement, then its position with respect to the instantaneous helical axis is relevant.

A causal relationship between masticatory muscle contraction and jaw movement can be demonstrated experimentally only when muscles are activated independently. This is not an option in a regular experimental setting, nor is it likely even with intensive training, due to the central organization of motor control in the jaw muscles. An effective method, however, is registration of jaw movements evoked by electrical stimulation of isolated muscles. This can be done with in-dwelling electrodes or by electromagnetic stimulation of selected portions of the motor cortex. The amplitude of such stimulation is necessarily restricted, to protect the experimental subject. Thus, the evoked jaw movements are small and require a very sensitive jaw-tracking device to be recorded. When the masticatory muscles are not fully relaxed, the other muscles may easily disturb the measurements. Even if complete rest can be obtained (for instance, in the unconscious state), the passive muscle forces may be relatively large compared with the evoked force and cannot be ignored. Consequently, direct muscle stimulation has not yet convincingly demonstrated the contributions of individual masticatory muscles to jaw movements.

Though direct experimental methods have failed to disclose the full functional potential of the masticatory muscles, biomechanical modeling approaches can help to develop reliable predictions on this subject. Such models are always simplifications; care must be taken that all relevant properties be included, and correct assumptions regarding model behavior must be made. In one model, for instance, it has been predicted that the jaw-opening muscles have the tendency to dislocate the temporomandibular joint, while in another, it has been suggested that they have the tendency to stabilize the joint. In the former model, torques were computed not with respect to the center of gravity but with respect to the joint, thereby neglecting the law of conservation of angular momentum. In contrast, in the latter model, all relevant properties of (Newtonian) rigid body dynamics were implemented, strengthening the reliability of its predictions. According to these rigid body dynamics, the temporomandibular ligaments play an insignificant role during symmetrical jaw movements. It was demonstrated that activation of all jaw-closing muscles simultaneously leads to natural-looking jaw-closing movements, including a condylar movement similar to that observed experimentally. Conversely, activation of all jaw-opening muscles resulted iormal jaw-opening movements. It is possible that the varying instantaneous center of rotation can be used as a means for the assessment of different muscle recruitment patterns during apparently similar jaw movements.

With the exception of the anterior temporalis and the superficial masseter, the principle that muscles, when activated unilaterally, generate a translation along their line of action and a rotation about the center of gravity has been confirmed. When their torque about the vertical axis through the center of gravity is considered, a contralateral latero-deviation would be expected, but instead, an ipsilateral latero-deviation occurs. This apparent paradox can be resolved if one takes into account that joint loads caused by muscle forces also contribute to jaw movement. Both the anterior temporalis and superficial masseter tend to tilt the contralateral condyle from the articular eminence because of a large ipsilateral joint load. The ipsilateral joint reaction force results in an ipsilaterally directed joint torque about the vertical axis through the center of gravity, which overcomes the muscle torque to cause an ipsilateral latero-deviation.

 

Schematic overview of the possible actions generated by force F_m of the superficial masseter or anterior temporalis muscle viewed in the horizontal plane. (A) Possible rotations: R_g, about center of gravity; R_r, about a vertical axis behind the right joint for a latero-deviation to the right; and R_l, about a vertical axis behind the left joint for a latero-deviation to the left. (B) Influence of joint forces: a_m, moment arm of muscle force; F_rj, right joint force; a_rj, moment arm of right joint force; F_jl, left joint force; and a_jl, moment arm of left joint force.

Passive structures

Passive structures contribute to jaw motion because they have the ability to resist its movements along one or more degrees of freedom. This resistance is expressed by the structure’s ability to generate a reaction force and/or torque. Furthermore, the jaw itself contributes to its movements through its inertial properties. The ratio between linear and angular accelerations effected by a muscle is subtly dependent on the mass and moments of inertia of the jaw, and all structures that are more or less rigidly attached to it. This attachment may include the part of the masticatory muscles attached to the mandible, the tongue, skin, and other soft tissues. It has been demonstrated, however, that the influence of these inertial properties on the final movement is small.

Articular surfaces

When the temporomandibular joints are loaded, reaction forces are transferred to the mandibular condyles. The cartilaginous structures in the joint have very low friction, and their deformation is fairly independent on loading direction, so it is probable that these reaction forces are directed perpendicular to the contacting articular surfaces. In a sagittal plane, they are directed inferiorly and pass posteriorly to the center of gravity of the lower jaw. These forces, therefore, have the tendency to lower the condyle and separate the articular surfaces. The joint reaction forces also apply torques with respect to the center of gravity of the lower jaw. While the line of action of the reaction forces passes posteriorly to this center, the joint reaction torques lead to an angular acceleration about the sagittal axis through the center of gravity, which is bound to produce an elevation movement in the positive direction. Consequently, the reaction forces attempt to perform a closing jaw rotation about this axis.

The cartilaginous temporomandibular joint disc and the cartilage lining of the articular surfaces are deformable, with a finite and non-linear elasticity. This elasticity causes the joint reaction force to be dependent on the deformation. The more both bony articular surfaces move toward each other, the more the cartilaginous structures between these surfaces are compressed, and the larger the magnitude of the reaction force. Consequently, even if articular contact is maintained during static and dynamic situations, some movement perpendicular to the articular surfaces occurs. This is supported indirectly by the finding that the condylar path during an unloaded opening lies superior to the path during an unloaded closing movement. The consequence of this observation is that joint loading during (unloaded) jaw opening is most likely larger than that during unloaded jaw closing. Furthermore, it supports the suggestion that the joints are loaded by the torque of the jaw-opening muscles.

The movement range of the condyle is not limited to any great extent by the articular surfaces of the skull. Only when the joints are compressed in an upward or obliquely backward direction by manipulation can the fossa restrain jaw movements in the region of the condyles. The articular surfaces do not restrict protrusive and medio-lateral translations or rotations about any of the three axes. If the mandible performs a latero-deviation, the contralateral condyle has to move forward relative to the ipsilateral one, due to the interconnection of both condyles. During this movement, it is forced to move downward along the articular eminence. Consequently, this movement includes not only an azimuth rotation, but also a roll rotation. While these rotations are interdependent, they nevertheless imply only one degree of freedom.

 

Rotations about vertical and horizontal axes during a latero-deviation movement.

Articular capsule and ligaments

The bony parts of the temporomandibular joint are connected by an articular capsule composed of relatively loose collagenous fiber bundles organized in a more or less parallel fashion. On the lateral side, it is reinforced by a temporomandibular ligament. The fibers are able to withstand some stretching. They deform according to their (non-linear) elastic properties and, in doing so, generate tensile forces. These forces may decelerate the attached condyle when it moves away from the articular surface of the temporal bone. A similar mechanical function can be attributed to the accessory ligaments, though these are very thin and their function is probably negligible.

For quite a while, the temporomandibular ligaments have been considered a dominant constraint for condylar movement and, therefore, for jaw movement. This has been illustrated by the construction of more or less fixed axes for mandibular rotations, especially in the final phase of jaw closure, and the idea has been applied widely in the areas of prosthetic dentistry  and temporomandibular dysfunction rehabilitation. Presently, there is a considerable amount of evidence that does not support the presence of a fixed hinge axis iormal functional movements. Another dominant role for the temporomandibular ligaments has been proposed for symmetrical jaw-opening movements. Here, it has been assumed that the temporomandibular ligament is always taut and forces the condyle to slide down the articular eminence. However, if the temporomandibular ligament is taut, it cannot allow for habitual latero-deviation movements unless there is a mechanism for tightening the ligaments during midline movements and for slackening them during non-midline movements. It is very unlikely that such a mechanism exists. These considerations favor a less dominant role for the temporomandibular ligaments in controlling habitual jaw movements.

The temporomandibular ligament may play a dominant role in preventing the mandibular condyle from moving beyond the limits of the articular surface of the temporal bone. It has been shown that the masticatory muscles could pull the condyles a few mm beyond these limits if the temporomandibular ligaments were not present. Due to the interconnection of left and right condyles through the mandibular symphysis, the two ligaments are sufficient to limit both antero-posterior and medio-lateral condylar movements, during protrusive and retrusive mandibular movements, and during latero-deviations, respectively. Furthermore, when the mandible reaches these positions, the contracting masticatory muscles have almost reached the length where they become insufficient for force production. The forces applied to the ligaments will then probably not be large.

Teeth and food

The direct influence of teeth and food on jaw movements is due to the reaction forces occurring when the upper and lower teeth come into direct contact with each other or with a bolus of food in between. Through the interplay of muscle and joint forces, these reaction forces will be predominantly directed downward and will be accompanied by an opening torque with respect to the center of gravity, thus causing a joint loading. There is also an indirect influence, since the central nervous system is able to detect forces on the dental elements through mechanoreceptors in the periodontal ligament. This system, therefore, is able to adapt muscle activation as required by the presence of food. Furthermore, the nervous system is able to react even faster through reflexes.

The reaction forces are due to the resistance to deformation by the underlying structures. The dental elements are very hard, and should they come into contact with each other with some velocity, even a small deformation could result in a very large reaction force. Fortunately, the dentition is connected to the mandible via a deformable, collagenous periodontium. When a tooth of the lower dentition comes into contact with its upper neighbor with a certain velocity, it undergoes little deformation but is pushed into its socket. Due to the elasticity of the periodontium, a reaction force occurs which becomes larger as the tooth is pushed further into its socket. This reaction force acts on the mandible and causes deceleration until the movement stops. If food is compressed between the teeth, the reaction force increases more slowly, while the bolus itself is deformable.

The direct influence of the teeth on jaw movements is reflected by the superior portion of the Posselt envelope of incisal point motion, but the dentition can also have an indirect influence on jaw movements. It has been demonstrated that subjects with malocclusion have a more irregular chewing pattern thaormally found. It is not clear whether these aberrant patterns are due to tooth contacts themselves or to external factors.

During mastication, food is compressed and/or fractured between the dentition to reduce the particle size and facilitate swallowing. This compression and fracturing take place in the slow-closing phase of the masticatory cycle. It is apparent that the movement in this phase will be dependent on the mechanical properties of the food. For tough foods, the compression will be slower than for soft foods. Notably, the muscles are able to generate larger forces when contracting slowly. The peak velocity that follows the fracturing of hard, brittle food is much greater than that for soft food. This peak velocity, however, is less than might be expected, possibly due to a decrease in muscle force as a consequence of the force-velocity relationship. Also, the size of the food affects mandibular movements, since the mandible has to be opened wider for larger pieces of food to be chewed. In the frontal plane, it has been observed that subjects chewing hard food tended to perform larger lateral excursions than when chewing soft food .

Impact loads on the dentition may have consequences for the joints, since they transfer to the joints via the mandible. A healthy periodontium partially absorbs impact loads, and thus, it may prevent peak loads on the joints. This property does not exist if the dentition is connected with the mandible through an artificial implant. The mandible itself, however, is deformable, so it is possible that the transfer of impact loads of the teeth to the joints may be reduced by its elasticity. At present, there are no quantitative data on this subject.

Muscles

When inactive, the masticatory muscles generate passive forces which are dependent on the instantaneous length of their sarcomeres. When the sarcomeres are at or below optimum length, estimated at 2.73 μm, they are negligible, but increase exponentially if they are stretched beyond this length. Apart from these passive forces, muscle stretch can, indirectly, cause reflexes, because it is detected by muscle receptors.

The passive forces of the jaw-closing muscles are believed to decelerate the jaw at the end of jaw opening during mastication and become significant when the jaw nears its maximum opening. It has been proposed that they are a determinant of maximum jaw opening. Mathematical models applied to the study of the passive forces of the masticatory muscles have been unable to open the jaw more than about 3 cm, whereas an opening of 6 cm is frequently observed in vivo. Therefore, the quantitative nature of these predictions is disputable. Due to the proposed exponential relationship between the passive muscle forces and their sarcomere lengths, small errors in the constants that determine this relationship may lead to relatively large errors in the projected passive forces. As long as there are no accurate quantitative data on the relationship between sarcomere length and passive force of the human masticatory muscles, this issue remains uncertain.

Interplay of passive and active structures

The most dominant determinants for jaw movement are the forces generated by active muscles. Passive forces may modulate jaw movements, but become dominant as the jaw reaches its movement boundaries. Axes of rotation of the jaw during free jaw movements are not primarily related to passive structures of the masticatory system, but are determined by muscle use. Nevertheless, in clinical practice, axes of rotation which were assumed to be connected to the temporomandibular joint have been measured and applied successfully for the diagnosis and treatment planning of masticatory dysfunction. This indicates that conclusions drawn from studies which were performed to search for practical solutions to a clinical problem cannot be automatically extrapolated to other problems in particular or to jaw movements in general.

The influence of the passive constraints appears to be more dominant as jaw movement deviates from the midline. Dynamic biomechanical analysis has demonstrated that the masticatory muscles are capable of maintaining the integrity of the masticatory system, in most cases, without the need for an articular capsule with ligaments to maintain articular apposition. In contrast, they appear to play a role in reducing the medio-lateral movements of the mandibular condyle during latero-deviation. If the joints are loaded asymmetrically, the influence of their reaction forces on jaw movement has to be considered. When a muscle is activated unilaterally, the condylar reaction forces may produce a reverse movement compared with the one expected from the muscle’s line of action (vide ante). In practice, however, the muscles contract as groups rather than in isolation. For both midline and non-midline jaw movements, dynamic muscle properties should be taken into account, since they limit the force-producing capacities of the muscles, thereby restricting jaw movement possibilities.

Final Remarks

Jaw movement analysis has evolved from early observation to experiments designed to formulate and validate or falsify testable hypotheses. In particular, the availability of dynamic biomechanical modeling methods has accelerated our understanding of jaw movements and the masticatory system. It is now possible to predict the actions of the different muscles in this complex system by applying Newton’s laws. It has become clear that the masticatory muscles not only control jaw movements, but also maintain the physical integrity of the masticatory system. However, during relatively large medio-lateral excursions, the muscles may fail to keep the articular components in apposition, at which time the articular ligaments may be presumed to perform this role.

Although these developments have improved our knowledge of the working of the human masticatory system, the persistence of some outdated theories is striking. A prominent example is related to the articulation, where there is a need to find a simple, reliable method for describing jaw movements near dental occlusion in anatomically different patients. Often, researchers and clinicians have attempted to use hinge axes to describe this movement. However, it has been known, since the end of the 19th century, that such axes are non-existent during habitual jaw movements. The concept of a hinge axis may have been revived by the demonstration that rotary jaw movement near occlusion can be accomplished through manipulation on cadaverous material. This concept has survived in clinical practice, where, despite a lack of a scientific basis, it has been applied successfully for a long period.

While, for instance, with the use of dynamic biomechanical models, hypotheses regarding muscle actions in the functioning masticatory system are being validated, joint load predictions have not yet been verified satisfactorily. This is a consequence of the fact that direct measurement of temporomandibular joint loading without disturbing articular integrity has remained impossible. Unfortunately, this parameter is considered as a major influence on the development of wear and degeneration of the cartilaginous and bony structures of this joint. Insight on temporomandibular joint loading is, therefore, still limited to model predictions, and the reliability of these predictions is directly related to the assumptions and parameters built in such models. These include joint morphology and the material properties of its deformable structures that contribute to load distribution. Furthermore, muscle tensions applied during joint loading are required. In each subject, these parameters may be different, leading to the need for in vivo measurement. Progress has been made in reconstructing the relevant muscle lines of action  and bony parts of the joint in vivo, but, to date, no reliable method is available to create reconstructions of the cartilaginous tissues in the joints. For a complete overview of the applied muscle tensions to be acquired, their physiological cross-sections, architecture, and degree of activation must be estimated. Physiological cross-sections have been estimated from anatomic cross-sections and muscle activation from EMG recordings, but it is questionable whether such methods can be applied routinely to all muscles involved. Masticatory muscle architecture has been studied in vitro, but the influence of individual variations on model predictions and the possibility of applying relevant corrections have not been established.

A start has been made on assessing the dynamic material properties of the cartilaginous structures in the human temporomandibular joint, but the nature and influence of individual variations are subject to speculation. Consequently, although much is known qualitatively, quantification of joint forces that incorporate individual variation still cannot be performed unambiguously.

The ultimate limiting factor for reliable masticatory function analysis incorporating biological variation is the lack of knowledge about masticatory muscle recruitment patterns. The mechanical redundancy of the masticatory system prevents their unambiguous prediction. The forces generated by the active muscles are the most dominant determinants of jaw movement and joint loading. The search for a rational way to predict muscle recruitment patterns remains a dominant challenge in the field of jaw movement analysis.