10. Endodontics – its objectives and goals. Endodontic instruments: classification, variety, purpose, rules of usage. ISO standards. Anatomical and topographical features of the tooth cavity and root canals of the maxilla and mandible teeth.

10. Endodontics – its objectives and goals. Endodontic instruments: classification, variety, purpose, rules of usage. ISO standards. Anatomical and topographical features of the tooth cavity and root canals of the maxilla and mandible teeth.

 

The word “endodontology” is derived from the Greek  language and can be translated as “the knowledge of what is inside the tooth”. Thus, endodontology concerns structures and processes within the pulp chamber.

Endodontics it’s the science that study anatomy, pathology and treatment of tooth cavity and root canals.

The goal of endodontic therapy is to relieve pain, control infection, and preserve the tooth so that it may functioormally during mastication. Endodontic treatment is normally preferred to extraction.

Historically, therefore, the main task of endodontic treatment has been to cure toothache due to inflammatory lesions in the pulp (pulpitis) and the periapical tissue (apical periodontitis).

Under normal, physiological conditions the pulp is well protected from injury and injurious elements in the oral cavity by the outer hard tissue encasement of the tooth and an intact periodontium. When the integrity of these tissue barriers is breached for any reason, micro-organisms and the substances they produce may gain access to the pulp and adversely affect its healthy condition. The most common microbial challenge of the pulp derives from caries. Even in its early stages substances from caries-causing bacteria may enter the pulp along the exposed dentinal tubules. Like any connective tissue, the pulp responds to this with inflammation. Inflammation has an important aim to neutralize and eliminate the noxious agents. It also organizes subsequent repair of the damaged tissue. Thus, the pulp may react in a manner that allows it to sustain the irritation and remain in a functional state. Yet, when caries has extended to the vicinity of the pulp, the response may take a destructive course and result in severe pain and death (necrosis) of the tissue.

The dentist’s knowledge of normal pulp shape, size, and depth beneath the enamel is important for proper teeth preparing, those that have deep decay.

When the dentist determines that the tooth can be restored without the need to remove the pulp, he or she must prepare the tooth in such a way to avoid disturbing or injuring the pulpal tissues. This is accomplished through knowledge of the shape of the pulp chamber and canals and a careful evaluation of the patient’s radiographs to determine the location of the pulp relative to the decay and external surface of the tooth.

The shape of pulp cavities and configuration of pulp canals

The pulp cavity is the cavity in the inner portion of the tooth containing the nerves and blood supply to the tooth. It is divided into the pulp chamber (more coronal) and the root canals (in the roots).

Pulp chamber and pulp horns

The pulp chamber is the most occlusal or incisal portion of the pulp cavity. There is one pulp chamber in each tooth. It may be located partly in the crown of anterior teeth, but in posterior teeth, it is mostly in the cervical part of the root. Its walls are the innermost surface of the dentin.

Fig. 1 Parts of a pulp cavity. The pulp cavity of this mandibular second molar is made up

of a coronal pulp chamber with pulp horns and two root (pulp) canals.

 

Each pulp chamber has a roof at its incisal or occlusal border often with projections called pulp horns, and the pulp chambers of multi-rooted teeth have a floor at the cervical portion with an opening (orifice) for each root canal (Fig. 8-1). The number of pulp horns found within each cusped tooth (molars, premolars, and canines) is normally one horn per functional cusp, and in young incisors, it is three (one horn in each of the three facial lobes, which is the same as one lobe per mamelon). An exception is one type of maxillary lateral incisor (called a peg lateral with an incisal edge that somewhat resembles one cusp) that has only one pulp horn. Refer to Table 8-1 for a summary of the number of pulp horns related to the number of cusps normally found within different tooth types.

 ROOT CANALS (PULP CANALS)

Although root shape in cross-section is variable, there are seven general configurations: round, oval, long oval, bowling pin, kidney bean, ribbon, and hourglass. Shape and location of canals are governed by root shape (in cross-section). Different shapes may appear at any level in a single root. For example, a root may be hourglass-shaped in cross-section at the cervical third, taper to a deep oval in the middle third, and blend to oval in the apical third. The number and shape of canals in each level will vary accordingly. It is important to note that a canal is seldom round at any level. To assume that it is may result in improper canal preparation.

 

Root canals (pulp canals) are the portions of the pulp cavity located within the root(s) of a tooth. Root canals connect to the pulp chamber through canal orifices on the  floor of the pulp chamber, and pulp canals open to the outside of the tooth through openings called apical foramina (singular foramen) most commonly located at or near the  root apex. The shape and number of root canals in any one root have been divided into four major anatomic configurations or types. The type I configuration has one canal, whereas types II, III, and IV have either two canals or one canal that is spilt into two for part of the root. The four canal types are defined as follows:

Type I—one canal extends from the pulp chamber to the apex.

Type II—two separate canals leave the pulp chamber, but they join short of the apex to form one canal apically and one apical foramen.

Type III—two separate canals leave the pulp chamber and remain separate, exiting the root apically as two separate apical foramina.

Type IV—one canal leaves the pulp chamber but divides in the apical third of the root into two separate canals with two separate apical foramina. Accessory (or lateral) canals also occur, located most commonly in the apical third of the root and, in maxillary and mandibular molars, are common in the furcation area

Fig. 2 Types of canal configurations occurring in one root.

 

Under endodontic intervention one should understand any interference with the purpose of treatment, carried out through the cavity of the tooth.

Under root canal treatment one should understand odonto-surgical intervention inside the tooth in order of its preservation with the subsequent restoration of its form and function with the help of therapeutic or prosthetic methods.

In endodontic practice the knowledge of topographical anatomy of tooth cavities of different teeth’ groups are necessary. The root canal is divided into crown, middle and apical parts. The crown part is the widest and adjacent directly to canal orifices. Most canals are flattened mesio-distally, but become more rounded in the apical 1/3. Lateral canals are branches of the main canal and occur in 17-30% of teeth. In the apical part near the dentinal-cement border root canal ends with a constriction (physiological apical hole), which is usually placed at a distance of 0.5-1.0 mm from the radiological and anatomical apexes.

Some authors identify anatomical apical hole – a place of transition the dentin into cement, and physiological hole – the border between pulp and periodontal ligament, placed 1 mm away from the X-ray hole.

 

These distances ⇑ with age due to deposition of secondary cementum. Root-filling to the constriction provides a natural stop to instrumentation, thus the working length should be established 1-2 mm away from the radiographic apex.

ANATOMICAL AND TOPOGRAPHICAL FEATURES OF THE TOOTH CAVITY AND ROOT CANALS

The dentist who conducts endodontic manipulations before the start of treatment should identify options for topographic and anatomical structure of the tooth.

The illustrations in the following pictures depict the size, shape, and location of the pulp space within each tooth, as well as the more common morphologic variations. Based on this knowledge of the shape of the pulp and its spatial relationship to the crown and root, the correct outline form for access preparation is presented from the occlusal, lingual, and proximal views.

From these illustrations, the following features can be observed:

1. The location of access on posterior teeth relative to occlusal landmarks such as marginal ridges and cusp tips

2. The size and appearance of the access on anterior teeth as viewed from the incisal surface

3. The approximate size of the access opening

4. The location of canal orifices and their positions relative to occlusal landmarks and to each other

5. The canal curvatures and the location of the apical foramina

6. The configuration of the chamber and cervical portion of the canals after straight-line access preparation

7. The root curvatures that are most common

Each illustration gives the following information:

 

 

MAXILLARY RIGHT CENTRAL INCISOR

 

The maxillary central incisor has one root and one canal. In young individuals, the prominent pulp horns present require a triangular-outline form to ensure tissue and obturation materials are removed, which might cause coronal discoloration. While the canal is centered in the root at the cementoenamel junction (CEJ) and when viewing the tooth from a mesial to distal orientation, it is evident that the crown is not directly in line with the long axis of the root. For this reason the establishment of the outline form and initial penetration into enamel are made with the bur perpendicular to the lingual surface  of  the  tooth.  This  outline  form  is  made  in  the  middle-third  of  the  lingual  surface.  After penetration to the depth of 2 to 3 mm, the bur is reoriented to coincide with the long axis and lingual orientation of the root. This reduces the risk of a lateral perforation through the facial surface. An additional common error is the failure to remove the lingual shelf, which will result in inadequate access to the entire canal. The canal is located by using a sharp endodontic explorer. In cases where calcification has occurred, long-shanked burs in a slow-speed handpiece can be used. These burs move the head of the handpiece away from the tooth and enhance the ability to see exactly where the bur is placed in the tooth.

MAXILLARY RIGHT LATERAL INCISOR

 

Access for the maxillary lateral incisor is similar to that for the central incisor. A triangular access is indicated in young patients with pulp horns, and as the pulp horns recede, the outline form becomes ovoid.

Figure. A, Lateral incisor with a receded pulp chamber. B, Initial ovoid outline form is initiated. C, Coronal calcification indicated by the

color change. D, The completed access.

MAXILLARY RIGHT CANINE

 

Maxillary canines exhibit one canal in a single root. Generally, pulp horns are absent so the outline form is ovoid in the middle third of the lingual surface. As attrition occurs, the chamber appears to move more incisally because of the loss of structure. In cross-section the pulp will be wide in a faciolingual direction when compared to the mesiodistal dimension.

 

Figure.  A, The apex is obscured by the screws placed during a maxillary surgical advancement. B, Lingual surface. C, Initial access outline

into dentin and D is finalized. E, Apex locator (arrow). F, Working length.

MAXILLARY RIGHT FIRST PREMOLAR

 

The  maxillary  first  and  second  premolars  exhibit  a  similar  coronal  structure  so  the  outline  form  is  similar  for  both  teeth,  is centered  in  the  crown,  and  exhibits  an  ovoid  shape  in  the  faciolingual  direction.  An  important anatomic consideration with these teeth is the mesial concavity at the CEJ. This is an area in which a lateral perforation is likely  to  occur.  When  two  canals  are  present,  the  canal  orifices  are  located  under  the  buccal  and  lingual  cusp  tips  equal distance from a line drawn through the center of the chamber in a mesial to distal direction. The cross-sectional morphology exhibits a kidney bean– or ribbon-shaped configuration. In rare instances when three canals are present, the outline form is triangular with the base to the facial and the apex toward the lingual.

Figure.  A, Note the obstructed view of the apical region. B, Maxillary right second premolar. C, The initial outline form prepared into dentin.

D, The chamber and canals are accessed.

 

MAXILLARY RIGHT SECOND PREMOLAR

 

MAXILLARY RIGHT FIRST MOLAR

The maxillary first and second molars have similar access outline forms. The outline form is triangular and located in the mesial half of the tooth, with the base to the facial and the apex toward the lingual. The transverse or oblique ridge is left mostly intact. The external references for canal location serve as a guide in developing the outline form. The  mesiobuccal  canal  orifice  lies  slightly  distal  to  the  mesiobuccal  cusp  tip.  The  distobuccal  canal  orifice  lies  distal  and slightly lingual to the main mesiobuccal canal and is in line with the buccal groove. The lingual or palatal canal orifice generally exhibits  the  largest  orifice  and  lies  slightly  distal  to  the  mesiolingual  cusp  tip.  The  mesiobuccal  root  is  very  broad  in  a buccolingual direction, thus a small second canal is common. The mesiolingual canal orifice (commonly referred to as the MB2canal) is located lingual to the main mesiobuccal canal (MB1 canal) from 1 to 3 mm and is slightly mesial to a line drawn from the mesiobuccal to the lingual or palatal canal. The initial movement of the canal from the chamber is ofteot toward the apex but laterally toward the mesial. Removal of the coronal dentin (cornice) in this area permits exposure of the canal as it begins to move apically and facilitates negotiation (Figures 14-26  and 14-27). In addition, the operating microscope is a valuable aid.

 

Figure 14-26 A, Maxillary left first molar exhibiting calcification. B and C, Initial access and identification of a pulp stone. Color and a thin line

surrounding the periphery identify the hemorrhage. D, The pulp chamber with the stone removed.

Figure 14-27 A, The dashed lines show where dentin must be removed to achieve straight-line access. B,  The  access  completed. C,  The

original canal (a) is modified by Gates-Glidden burs by removing tooth structure at B and C.

MAXILLARY RIGHT SECOND MOLAR

 

 

MANDIBULAR RIGHT CENTRAL AND LATERAL INCISOR

 

The mandibular incisors are narrow in the mesiodistal dimension and broad faciolingually. There may be one canal with an ovoid or ribbon-shaped configuration or there can be two canals. When there are two canals, the facial canal is easier to locate and is generally straighter than the lingual canal, which is often shielded by a lingual shelf. Since the tooth is often tipped facially, the lingual canal is difficult to locate and perforations primarily occur on the facial surface. The  narrow  mesiodistal  dimension  of  these  teeth  makes  access  and  canal  location  difficult.  In  young patients  with mesiodistal pulp horns the outline form is triangular with the base incisally and the apex gingivally. As the pulp recedes over time and the pulp horns disappear, the shape becomes more ovoid. The access is positioned in the middle-thirds of the lingual surface.  Because  of  the  small  size  of  these  teeth  and  the  presence  of  mesiodistal  concavities, access must be precisely positioned. The initial outline form is established into dentin with the bur perpendicular to the lingual surface. When a depth of 2 to 3 mm is reached, the orientation of the bur is reoriented along the long axis of the root. Because the percentage of teeth with two canals is reported to be 25% to 40%,41,42 the lingual surface of the chamber and canal must be diligently explored with a small precurved stainless steel file. A Gates-Glidden drill is used on the lingual to remove the dentin shelf.

 

  

Figure. A, Mandibular lateral incisor. B, Calculation of the estimated depth of access from the middle of the lingual surface to the coronal

extent of the pulp. C, The initial outline form is more oval due to the receded chamber. D, The completed access

 

MANDIBULAR RIGHT CANINE

 

The mandibular canines usually exhibit a long slender crown when compared to the maxillary canine, which is shorter and wider in a mesiodistal direction. The tooth may exhibit one or two roots. The root is broad in a faciolingual dimension and therefore may contain two canals. The outline form is ovoid and positioned in the middle-third of the crown on the lingual surface. On  access  opening  into  the  chamber,  the  lingual  surface  should  be  explored  for  the presence of a lingual canal. As attrition occurs, the access will need to be more incisal, and in severe cases, it may actually include the incisal edge of the tooth.

    

Figure.  A,  Mandibular  canine. B, The initial outline form is established into dentin. C, Exposure of the coronal pulp. D, The completed

access opening.

 

MANDIBULAR RIGHT FIRST PREMOLAR

 

The mandibular premolars appear to be easy teeth to treat, but the anatomy may be complex. One, two, or three roots are possible, and canals often divide deep within the root in these complex morphologic configurations. The crown of the first premolar exhibits a prominent buccal cusp and a vestigial lingual cusp. In addition, there is a lingual constriction. Mesiodistal projections reveal that the chamber and canal orifice are positioned buccally. The access is therefore ovoid in a buccolingual dimension and positioned buccal to the central groove. It extends just short of the buccal cusp tip. The mandibular second premolar has a prominent buccal cusp, but the lingual cusp can be more prominent than with the first premolar. There is also a lingual constriction, so the outline form is ovoid from buccal to lingual and positioned.

MANDIBULAR RIGHT SECOND PREMOLAR

 

 

MANDIBULAR RIGHT FIRST MOLAR

 

The  mandibular  molars  are  similar  in  anatomic  configuration;  however,  there  are  subtle  differences.  The  most  common mandibular first molar configuration is two canals in the mesial root, although three have been reported, and one canal in the distal root. The presence of a second canal in the distal root is 30% to 35%. The roots often exhibit a kidney bean shape in cross-section with the concavity in the furcal region. The most common configuration for the mandibular second molar is two canals in the mesial root and one canal in the distal root. The incidence of four canals is low.

The coronal reference points for canal location in the mandibular molar roots are influenced by the position of the crown on the root and by the lingual tipping of these teeth in the arch. The mesiobuccal canal orifice is located slightly distal to the mesiobuccal cusp tip. The mesiolingual canal orifice is located in the area of the central groove area and slightly distal when compared to the mesiobuccal canal. The distal canal is located near the intersection of the buccal, lingual, and central grooves. When a distobuccal canal is present, the orifice can be found buccal to the main distal canal and often is slightly  more  mesial.  The  mandibular  first  molar  may  even  exhibit  a  distinct  separate  extra  distal  root.  Because  of  these anatomic relationships, the access outline form is rectangular or trapezoidal and positioned in the mesiobuccal portion of the crown.

Figure.  A, The preoperative radiograph of a mandibular first molar. B, The preoperative occlusal anatomy. C, The initial access outline

form. D, The completed access cavity demonstrating the two mesial canals and the single distal canal.

MANDIBULAR RIGHT SECOND MOLAR

 

 

LENGTH DETERMINATION

 

Diagram that shows the length of root canals of mandible and maxilla

Electronic Apex Locators

Apex locators are also used in determining length. Contemporary apex locators are based on the principle that the flow of higher frequencies of alternating current is facilitated in a biologic environment when compared to lower frequencies. Passing two differing frequencies through the canal results in the higher frequency impeding the lower frequency (Figure 14-45). The impedance values  that  change  relative  to  each  other  are  measured  and  converted  to  length  information. At  the  apex,  the impedance values are at their maximum differences. Unlike previous models the impedance apex locator operates accurately in the  presence  of  electrolytes. Apex  locators  are  helpful  in  length  determination  but  must  be  confirmed  with  radiographs. Films/digital images aid in confirming the appropriate length and can identify missed canals. When the file is not centered in the root, a second canal is likely to be present. The apex locator is an electrical device that allows the operator to estimate the canal length, and with practice can be extremely accurate.

Figure 14-45 An impedance apex locator. Note the lip clip (arrow).

 

An apex locator is very helpful in patients with structures or objects that obstruct visualization of the apex, patients that have a gag reflex and cannot tolerate films, and patients with medical problems that prohibit the holding of a film or sensor. The use of apex locators and electric pulp testers in patients with cardiac pacemakers has been questioned. In  a recent study involving 27 patients with either implanted cardiac pacemakers or cardioverter/defibrillators, 2 impedance apex locators and 1 electric pulp tester did not interfere with the functioning of any of the cardiac devices.

 

 

Two popular apex locators; both work using multiple frequencies and are extremely accurate.

Apex locators work by applying an alternating current between two electrodes; one makes contact with the lip or cheek (the ground electrode), the other is attached to a file in the root canal.

 

The tip of the apex locator is kept in contact with the file as it is advanced apically.

The impedance at the apical foramen is approximately equal to that between the periodontal ligament and the oral mucosa; this value is used to calibrate the instrument. The apex locator has a display showing the zero reading that indicates when the file tip is at the apical foramen. Some modern apex locators measure the impedance at two or more frequencies to improve the accuracy of the instrument. Some can be used effectively even in the presence of electrolytes such as sodium hypochlorite and blood, although these are best avoided; the pulp floor should always be dry to prevent short circuiting.

Radiographic

 The working length is defined as the distance from a predetermined coronal reference point (usually the incisal edge in anterior teeth  and  a  cusp  tip  in  posterior  teeth)  to  the  point  that  the  cleaning  and  shaping,  and  obturation  should  terminate.  The reference point must be stable so fracture does not occur between visits. Unsupported cusps that are weakened by caries or restorations should be reduced. The point of termination is empirical, and based on anatomic studies, it should be 1.0 mm from the radiographic apex. This  accounts  for  the  deviation  of  the  foramen  from  the  apex,  and  the  distance  from  the  major diameter of the foramen to the area where a dentinal matrix can be established apically.

Before  access  an  estimated  working  length  is  calculated  by  measuring  the  total  length  of  the  tooth  on  the  diagnostic parallel radiograph or digital image. In cases where the canal is curved, the canal length can be measured by placing a file that has been curved to duplicate the canal morphology against the film. The stop can be adjusted to coincide with the reference point, while the file tip is aligned with the radiographic apex. After adjustment of the stop, the file is straightened and the length measured.  From  a  practical  perspective,  a  calculation  to  the  nearest  0.5  mm  should  be  made.  Then  2.0  millimeters  are subtracted  to  account  for  the  foramen  distance  (1.0  mm)  and  radiographic  image  distortion/magnification  (1.0  mm).  This provides a safety factor so instruments are not placed beyond the apex. Violation of the apex may result in inoculation of the periapical tissues with necrotic tissue, debris, and bacteria54 and lead to extrusion of materials during obturation and a decreased prognosis. After access preparation, a small file is used to explore the canal and establish patency to the estimated working length. The largest file to bind is then inserted to this estimated length because a file that is loose in the canal may be displaced during film exposure or forced beyond the apex if the patient bites down inadvertently. Millimeter markings on the file shaft or rubber stops on the instrument shaft are used for length control. A sterile millimeter ruler or measuring device can be used to adjust the stops on the file. To ensure accurate measurement and length control during canal preparation, the stop must physically contact the coronal reference point. To obtain an accurate measurement, the minimum size of the working length should be a No. 20. With files smaller than No. 20, it is difficult to interpret the location of the file tip on the working length film or digital image. In multirooted teeth, files are placed in all canals before exposing the film.

Angled films are necessary to separate superimposed files and structures (Figure 14-40), to provide an efficient method of determining the working length, and to reduce radiation to the patient. It is imperative that the rubber dam be left in place during working length determination to ensure an aseptic environment and to protect the patient from swallowing or aspirating instruments.

Figure 14-40 A, Parallel preoperative radiograph.

B, The mesial working length film is made correctly.

The apices and file tips are clearly visible. Note the mesiolingual canal (arrow).

 

ENDODONTIC INSTRUMENTS

HAND INSTRUMENTS

Endodontic research has shown how different instruments and materials behave within the confined space of the root canal system during instrumentation. In parallel with advances in hand instruments, come a new preparation techniques.

Instrumentation techniques where apical preparation is carried out at the start of treatment have been superseded by crown-down techniques, in which the coronal element of the root canal is prepared first. The actions with which files can be used have been analysed, and now many modern methods of instrumentation use a balanced force motion as opposed to filing. With the introduction of highly flexible materials such as nickel-titanium alloys it has been possible to produce instruments with tapers that are greater than the original stainless steel hand files, without losing flexibility; such instruments are invaluable for tapering the root canal preparation predictably.

As new instrument systems are produced, new preparation techniques evolve. The morphology of the root canal space is highly variable, however, and one preparation technique cannot be applied to every situation. It is perhaps more useful to develop and understand basic concepts of root canal preparation. These can then be implemented to master the diversity of root canal systems with which the dentist may be faced. This approach also allows the clinician to modify his or her current technique as new instruments are produced. Unfortunately no one system can be a panacea. An understanding of endodontic concepts and a consequent adaptation of techniques with which the clinician is conversant will avoid the unfortunate accumulation of expensive equipment that becomes redundant when it fails to deliver its promises! There are no secrets to effective root canal preparation: just practice, patience and persistence.

ISO STANDARDIZATION

In 1959, a new line of standardized instruments and filling material was introduced to the professional:

1.  A formula for the diameter and taper in each size of instrument and filling material was agreed on.

2.  A formula for a graduated increment in size from one instrument to the next was developed.

3.  A new instrument numbering system based on instrument metric diameter was established.

This  numbering system,  last  revised  in  2002 using numbers from  6  to  140,  is  based  on the diameter of the  instruments in hundredths of a  millimeter at  the beginning  of the  tip  of the  blades, a  point  called  D0 (diameter  1  mm)  and  extending  up  the blades  to the most coronal part of the cutting edge  at D16  (diameter 2- 16  mm in  length). Additional revisions are under way to cover instruments constructed with new materials, designs, and tapers greater than 0.02 mm/mm.

Instruments with a taper greater than the  ISO  (International Standards Organization) standard of 0.02  mm/ mm  have  become  popular:  0.04,  0.06,  0.08,  0.10,  and 0.12.  This  means  that  for  every  millimeter gain  in  the length  of the  cutting  blade,  the  width  (taper)  of the instrument increases  in  size  by 0.04,  0.06,  0.08,  0.10,  or 0.12  of a  millimeter  rather  than  the  ISO  standard  of 0.02  mm/mm. These new instruments allow for greater coronal flaring than the 0.02 instruments. The full extent of the shaft, up to the handle, comes in three lengths: standard: 25 mm; long: 33(28) mm; and short: 21 mm. The long instruments are ofteecessary when treating canines over 25 mm long. Shorter instruments are helpful in second and third molars or in the patient who cannot  open  widely.  Other special lengths  are available. The working length (part of an instrument that are in contact with tooth substance) of both files and reamers are 16 mm. 

Nowadays, endodontic instruments are made universally of nickel- titanium and stainless steel rather than carbon steel, K-type instruments are  produced  using  one of two  techniques. The  more  traditional  is  produced  by  grinding  graduated  sizes  of round wire  into various shapes  such  as  square,  triangular,  or  rhomboid. A second grinding operation properly tapers these pieces. To  give  the  instruments the  spirals  that provide the cutting edges,  the  square or triangular stock  is  then grasped  by a machine that twists  it  counterclockwise  a  programed  number  of times– tight spirals for files,  loose spirals for  reamers.

The  cutting  blades  that  are  produced  have  the  sharp edges  of either  the  square  or  the  triangle.  In any instrument, these edges are known as the “rake” of the blade. The  more  acute  the  angle  of the  rake,  the sharper  the  blade.  There  is  approximately  twice  the number  of spirals  on  a  file  than  on  a  reamer  of a corresponding size  (Figure  4 A  and B). The second and newer manufacturing method is  to grind  the  spirals  into  the  tapered  wire  rather  than twist  the wire to produce the cutting blades. Grinding is usually necessary for nickel-titanium instruments. Because of their super-elasticity, they cannot be twisted. Originally, the cross-section of the K-file was square and the reamer triangular.  However, manufacturers have started using many configurations  to achieve better cutting and/or  flexibility.  Cross-section is now the prerogative of individual companies.

K-Style Modification

K-style endodontic instruments came into a series of modifications beginning in the 1980s. Not wholly satisfied  with  the  characteristics  of  their  K- style instrument ,  the  Kerr  Manufacturing  Company  in 1982  introduced  a  new  instrument  design  that  they termed  the  K-Flex  File, a departure from  the square and triangular configurations  (Figure 4 C).

The cross-section of the K-Flex is rhombus or diamond shaped.  The  spirals  or flutes  are  produced  by the  same  twisting  procedure  used  to  produce  the cutting  edge  of the  standard  K-type  files;  however, this  new  cross-section  presents significant changes  in instrument  flexibility  and  cutting characteristics.  The cutting edges  of the high  flutes  are formed  by  the two acute  angles  of the  rhombus  and  present  increased sharpness and  cutting  efficiency.  The  alternating low flutes  formed by the obtuse angles of the rhombus are meant  to  act  as  an  auger,  providing  more  area  for increased  debris  removal.  The  decreased  contact  by the  instrument with  the canal  walls  provides a  space reservoir  that, with  proper irrigation,  further  reduces the danger of compacting dentinal filings  in  the canal.

REAMERS

The clinician should understand the importance of differentiating endodontic files and reamers from burs. Burs are used for boring holes in solid materials such as gold, enamel, and dentin.  Files, by definition, are used by rasping.  Reamers, on the other hand, are instruments that  ream  (twisting)-specifically,  a  sharp-edged  tool for  enlarging  or tapering holes  (see  Figure  6 B).  Endodontic  reamers  cut  by  being  tightly  inserted  into the  canal,  twisted  clockwise  one quarter- to  one  half-turn  to  engage  their  blades  into  the  dentin,  and  then withdrawn- penetration, rotation, and  retraction.  The cut is made during retraction.  The process is then repeated, penetrating deeper and deeper into the canal. When working length is reached, the next size instrument is used, and so on.

 

FILES

The tighter spiral  of a file  (see  Figure 4 A) establishes  a cutting angle (rake) that achieves  its primary action on withdrawal, although it  will  cut in the  push  motion as well . The cutting action of the file can be effected in either a filing (rasping) or a reaming (drilling) motion. In  a  filing  motion,  the  instrument  is  placed  into  the canal  at  the desired  length,  pressure  is  exerted  against the  canal  wall,  and  while  this  pressure  is  maintained, the rake  of the flutes  rasps the wall  as  the  instrument is withdrawn  without turning. The file has no need to contact all walls simultaneously. For example, the entire length and  circumference  of large-diameter  canals  can  be filed  by  inserting  the  instrument  to  the  desired working  distance and  filing  circumferentially around all  of the walls.

To use a file in a reaming action, the motion is the same as for a reamer – penetration, rotation, and retraction. The file tends to set in the dentin more readily than the reamer and must therefore be treated more cautiously. Withdrawing the file cuts away the engaged dentin. To summarize  the basic action of files  and reamers, it  may  be  stated  that  either  files  or  reamers  may  be used  to  ream  out  a  round,  tapered  apical  cavity  but that  files  are  also  used  as  push-pull  instruments  to enlarge  by rasping certain curved canals as  well  as  the ovoid  portion  of large  canals.  In  addition,  copious irrigation  and  constant  cleansing  of the  instrument are  necessary  to  clean the  flutes  and  prevent  packing debris at or through the  apical  foramen.

HEDSTROM FILES

H-type  files  are  made  by cutting  the  spiraling  flutes into  the  shaft  of a  piece  of round,  tapered,  stainless steel  wire.  Actually, the machine used is similar to  a screw-cutting  machine.  This  accounts  for  the  resemblance  between  the  Hedstrom  configuration  and  a wood screw  (Figure 7 A). It is impossible to ream  or  drill  with  this  instrument.  To  do so  locks  the  flutes  into the  dentin  much as  a screw  is  locked  in  wood.  To continue the drilling action would fracture the instrument.  Furthermore, the file is  impossible  to  withdraw once it  is  locked  in the dentin  and  can  be  withdrawn  only by backing off until  the  flutes  are  free. 

Hedstrom files cut in one direction only – retraction.  Hedstrom files are not to be used in a torqueing action.

 

 

Hand Files and Reamers

The main difference between  K-file and  K-reamer is the angle of the flutes along the length of the shaft. The flutes on a K-file are about twice as horizontally oriented as the flutes on a K-reamer. Since this is the case, the number of flutes on a K-file will be about twice the number of flutes that are present on a K-reamer. Reamers have fewer turns per unit length than the equivalent-sized file.

For the last forty years root canal instruments have been produced to international standards.

There are specifications for dimensions, fracture resistance, stiffness and colour coding of endodontic files and reamers.

Material

Endodontic files and reamers are mainly manufactured from stainless steel, although carbon steel, titanium and nickel-titanium are also used. The different materials give the instruments different properties, which in turn affect the way in which they should be used.

Tip Design

The tip of the instrument can have various shapes. Originally instrument tips were sharp and had a cutting action; but non-cutting (bullet-shaped) tips are now available that allow the instrument to slide along the outer curvature of a root canal, allowing preparation to be centred on the original canal curvature.

There is a non-linear increase in the diameter of tip sizes between consecutive instruments.

To address this, one manufacturer produced intermediate sizes between 10 and 30. Golden Mediums (Maillefer) are available in ISO sizes 12, 17, 22 and 27. Another method of reducing the uneven ‘jumps’ in diameter between sizes has been to produce instruments with a uniform increase in diameter between consecutively sized instruments. Series 29 (Dentsply, Weybridge, Surrey, UK) instruments have a 29% increase in tip size between instruments.

 

Restoring Force

This is the force produced by a file when it resists bending. The restoring force for a nickel-titanium instrument is 3-4 times less than that for an equivalent-sized stainless steel file. The restoring force increases with file diameter: i.e. files get stiffer as their diameter increases.

Taper

The standards for instruments specify that hand files have a 0.02 taper; i.e. the diameter of the instrument increases by 0.02 mm per mm along the length of the instrument. However because nickel-titanium is superelastic, it has been possible to create flexible instruments with larger tapers such as 0.04, 0.06, 0.08, 0.10 and 0.12.

Instrument Design

Endodontic files can be twisted from square, rhomboid or triangular stainless steel blanks, or machined. The standardized length of a working part of a file or a reamer blade is 16 mm. Reamers normally have fewer flutes/blades per unit length than an equivalent file, and are intended for use in a rotary action. Nickel-titanium instruments need to be machined using computer-assisted manufacturing (CAM), as the material is super-elastic and cannot be twisted. Modern manufacturing methods allow complex cross-sectional shapes to be milled.

REAMING

Reamers are instruments designed to enlarge or taper preexisting spaces.  Traditionally,  endodontic  reamers were  thought to  cut  by  being inserted  into  the  canal, twisted  clockwise one-quarter to one-half turn to engage  their  blades  into  the  dentin,  and  then  withdrawn,  that  is: penetration,  rotation,  and  retraction.  The cut is made during retraction. The  process  is  then  repeated,  penetrating  deeper  and  deeper into  the  canal. When WL (working length) is reached,  the  next  size instrument is  used,  and so on.  Reaming is  a method that  produces a  round,  tapered  preparation, and  this is  used  only  in  perfectly  straight  canals. In such  a situation, reamers can be rotated one-half turn before retracting. In a slightly curved canal, a reamer should be rotated only one-quarter turn.

FILING

The  tighter spiral of a file  establishes a rake  angle  that achieves  its  primary  cutting  action  on  withdrawal, although  it  will  cut  in  the  push  motion  as  well. The cutting  action  of the  file  can  be  effected  in  either  a filing  or  a  reaming  motion.  In  a  filing  motion,  the instrument  is  placed  into  the  canal  at  the  desired length,  pressure  is  exerted  against  the canal wall,  and while this pressure is  maintained, the rake of the flutes rasps  the wall  as  the  instrument is withdrawn  without turning.  The file has no need to contact all walls simultaneously. For example,  the  entire  length  and  circumference of large-diameter canals can be  filed  by inserting  the  instrument  to  the  desired  working  distance and  filing  circumferentially  around  all  of the  walls. When  using a file  in  a reaming  action,  the motion  is obviously  the  same  as  for  a  reamer.  Withdrawing the file then cuts the engaged dentin. Filing is very efficiently done with Hedstrom files while K-files are the most popular instruments. On the other hand, the  often  advocated  technique  of circumferential filing  has  been  shown to  leave  significant canal  areas unprepared.  Finally,  hand versions  of current NiTi rotary instruments,  such  as  ProSystem GT (Dentsply Tulsa  Dental,  Tulsa,  OK)  and  ProTaper  (Dentsply Maillefer), may be  used  in  filing  and reaming action.

According to ISO endodontic instruments are classified:

–    Hand instruments: files (K and H), barbed broaches, spreader and plugger (vertical and lateral gutta percha condensers ).

–    Rotary instruments: H-files and K-reamers for slow handpiece, lentulo spiral filler/rotary paste filler.

–    Rotary instruments: Gates Glidden drills, PeesoReamer drills.

–    Pins: gutta percha pins, silver pins.

But its more convenient to use classification by Curson(1996) that is based on clinical usage of endodontic instruments.

– diagnostic instruments: root needles(Miller needles)

– instruments for removing the soft teeth’ tissues: barbed broaches

– instruments for passing, enlargement and shaping of root canals (K-reamers, K- files, H-files)

The main endodontic instruments and their usage

Functions and precautions

– Finger instruments

– Disposed of in the sharps’ container

-Used to remove the intact pulp

– ‘Barbs’ on the broach snag the pulp to facilitate removal

– They need to be used cautiously as they can bind and break in the canal

Varieties

Available in different sizes and widths

 

 

Function, features and precautions

–    To enlarge the coronal third of the canal during endodontic treatment

–     Small flame-shaped cutting instrument used in the conventional handpiece

–     Different sizes – coded by rings or coloured  bands on shank

–    Are slightly flexible and will follow the canal shape but can perforate the canal if used too deeply

–     Dispose of in sharps’ container

–    Should be used only in the straight sections of  the canal

 

Function, features and precautions

–           To remove gutta percha during post preparation

–            Small flame-shaped cutting instrument used in the conventional handpiece

–            Different sizes – coded by rings or coloured bands on shank

–            Peeso reamers are not flexible or adaptable, if not used with care can perforate canal

–            Dispose of in sharps’ container

Hand instruments for root canal passing, enlargement and shaping

Reamers Rarely used or indicated. Disadvantages of reamers include their inflexibility with ⇑ size, which can result in a wider canal being cut apically. Have now been replaced by files. 

Files. These are used either with a longitudinal rasping or a rotary action (e.g. clockwise direction for ¼ – ½ of a turn).

The main types of file available are:

K-type–file. Made by twisting a square metal blank.

Hedstroem file: are machined from a tapered cylindrical block. In cross-section they have the appearance of a series of intersecting cones. More aggressive than K-file. Must never be used with a rotary action as liable to fracture. Movements in the root canal are scraping, meaning, removal of roughness of the root canal walls. 

K-flex file. Similar to K-file but made by twisting a rhomboid shape blank  alternating blades with acute and obtuse angles. More flexible than K-file but becomes blunt more quickly.

Flex-o-file. Looks similar to a K-type-file but is made from a triangular blank of a more flexible steel. The file also has a blunt tip, which means that it is unlikely to create a false canal. This file is more flexible than K-types and is now becoming a popular replacement.

Gretaer taper. (GT) Hand files made from nickel titanium (NiTi). They have increasing tapers (0.06-0.12) with matched GP cones.

Files have traditionally been made from steel, but newer varieties made of NiTi are gaining popularity as they are much more flexible. Larger-sized files can be sterilized and re-used more than the smaller sizes. All instruments should be examined regularly, and discarded if there are any signs of damage. It is good practice to dispose of smaller files after one use. In future files may all be single use (disposable).

NiTi rotary instruments (Profile). reduce creation of blocks, ledges, transportations and perforations by remaining centred within the natural path of the canal. Useful  for curved canals but ⇑ risk of fracture. Prior to use check for patency/ glide-path up to the full working length with a size 10 file.  

Endodontic K- files are also called: Root canal hand files

Function, features and precaution

• Finger instrument

• Colour coded by size. The 6 colours used most often are: size 15 (white); 20 (yellow); 25 (red); 30 (blue); 35 (green); 40 (black). Also available in size 6 (pink), 8 (grey) and 10 (purple)

• Operator gradually increases the size of the file to smooth, shape and enlarge canal

• The larger the number of the file, the larger the diameter of the working end

• Disposed of in the sharps’ container

Varieties

• Different lengths: 21mm, 25mm and 30mm

• Hedstroem files, Flexofiles®

 

 

Function, features and directions for use

• Used to clean and shape the canals

• Used with endodontic handpiece and motor

• NiTi is flexible and instruments follow the canal outline very well

• Several varieties of systems with different sequences of instruments are used

• Important to follow the manufacturer’s recommended speeds and instructions for use

Varieties

Different lengths: 21mm and 25mm

 

Function and features

• Small flexible instrument used to place materials into the canal

• Fits into the conventional handpiece

• Use with caution as it can be easily broken

• Different sizes available

 

 

The main purposes of root canal treatment are:

– removal of pulp;

– removal of infected dentine from the inner wall of the root canal;

– enlargement and shaping of root canal for its adequate sealing.

The procedure of root canal treatment consists of such stages:

 – disclosure of the tooth cavity;

 – disclosure of the root canal orifices;

 – the root canal passing;

 – the root canal enlargement;

 – the root canal shaping.

 

Manipulations of root canal treatment (RCT) are carried out manually or with the help of rotary instruments by several treatment methods, the most widespread  among them are:

– apical-crown – envisage treatment from the apical hole to canal orifices with gradually increasing of  instrument diameter( e.g. from №10 -№ 40)

– crown-apical – envisage root canal treatment that starts from canal orifices to apical hole with a gradual decrease in instrument diameter(e.g. from №40 –№ 10)

– hybrid method of treatment

Description of root canal treatment techniques

Step-back technique. The apical part of the root canal is prepared first and the canal is then flared from apex to crown. Blockage of canals may occur using this technique, and irrigation can be difficult.

Crown-down technique. This (along with several others) prepares the coronal part of the canal before the apical part. This has advantages and is the preferred technique.

Balanced force technique. This involves using blunt-tipped files with an anticlockwise rotation whilst applying an apically directed force. It requires practise to master but is particularly useful when preparing the apical part of severely curved canals.

Anticurvature filing. This was developed to minimize the possibility of creating a ‘strip’ perforation on the inner walls of curved root canals. It is used in conjunction with other techniques or preparation, and the essential principle is the direction of most force away from the curvature.