Replacing of partial defects of dentition with bugel prostheses. Indications and contraindications for this type of prosthesis. Choosing abutment teeth. Parallelometry.
Construction of the modern removable denture.
Every removable denture has itself constructive features, defined position and in size of defect, amount of remain teeth, condition of their hard tissues and parodontium, condition of covered a prosthetic field mucosa, safety of alveolar process, expression of hard palatum and other anatomical features.
Despite a variety of existing constructions there are can find partsrepeated in all types of removable dentures. It is necessary to carry to them: basis, fixed mechanisms(clasps) and artificial teeth. In the arch denture (except basis, fixed elements)has also an arch and it processes.
Laminardenture consists of leaned to alveolar process basis, the body of jaw, artificial teeth which restore an integrity of dentition, and claspsor other mechanical attachments which fixing a denture oatural teeth.
The main feature of lamellar dentures with fixedclasps is their locating on the non chewing pressure tissues. That’s why it can’t to reach such size like at bridge prothesis when the chewing pressure transferred by most natural path (through periodontium of teeth). Hence, the functional valueor chewing effectively of removable laminarnon-leaning dentures is much less then in bridges. If it at bridges is 90-95%, than at laminar is 25-30% and not much more than 40%. And the main case ofsuch difference is perception of pressure by non-specialized tissue.
The pressure of basis on the tissues which are under prosthetic field and not fitted for its perception causes their atrophy. There is a raised desquamation of an epithelium. It breaks endurance of a mucosa to external borings, therefore in it often there is a chronic inflammatory process. The part of denture which adjacentto the necks of teeth and gingival margin causes a nearly-neck caries and gingivitis with formation of a dento-gingival pathologicalpocket. Fixing denture clasps are constantly slipping on a surface of a crown of a tooth owing to denture immersingin mucosa at a load and homingsin a starting position at it removing. Owing to this the enamel is traumatizing, appear a hyperesthesia, quite oftenand caries.
But removable dentures have an advantage before a fixed bridges in it’s hygiene.
Considering disadvantages of both dentures which remove defects of dentitionsit’s evolution move at the path of removability saving and reduction of denture basis, uniform distribution ofchewing pressure between mucosa ofdenture area and teeth at simultaneous augmentation of fastnessand functional value of denture. If reduce the basis removable lameinar denture than specific pressure will be enlarged (pressureon unit of mucosa of denture area). How it can be compensative? Redistribute the part of a load from mucosa to teeth. This tendency has led to occurrenceto leaning denture.
The questions about construction of basis and other elements of partial removable dentures were given a lot of attention during all period of dentistry development. Along with difficult constructions of a leaningdentures are offered also simple.
In ancient times people were tried to restore the lost teeth. The material for making dentures were been the natural teeth which loss before and processed in appropriate path and teeth of different animals. Such teeth became attached by thread or golden wire to the adjacent natural teeth and, of course, they functional values were been small. The most ancient of found teeth were made from gold. These works cause our surprise as expediency of the device and durability of manufacturing. Etruscans were able to fix the loosened teeth and replace the loosed. Romans borrowed much at Etruscan and indenture technique followed by its way. Throughout many centuries the prosthetics of teeth remainsalmost at the same level, as in the ancient time. And only in begin of XVIII century dental prosthetics were begun to passfrom handicraftsmen to dentists.
One of the first scientists which open the new ways in dental prosthetics was Pyer Foshar. In 1728 he for the first time has published the book «the Treatise about a teeth» in which in detail described various waysof manufacturing and strengthening of artificial teeth. The material for his dentures were been thebull’s or elephant’s bone. Separate teeth Foshar were strengthened for the adjacent by means of the waxed silk thread. The full denture on the upper jaw he strengthened by means of springs. Foshar were made also and a pin teeth by the naturalor cut them from the bone. All this dentures were made by himselfmanually. Further Foshar covered the teeth by a golden plate, because the artificial teeth were turned yellowat the time. Then on this plate he begins to put a layer of white enamel like the natural teeth color. It has suggested on idea the latest inventors to the manufacturing dentures fully from porcelain.
In 1756 Wilhelm Pfaff begins to take out casts from jawsby the wax and on them to cast plaster models. It has served as thebig jerk to denture development, because before the dentures were sharpen «on the eye».
The developer of the porcelain teeth is the druggist from Sent-Germen Dushato. In 1774 he made fully denture with all teeth from fusible porcelain.
Later the French surgeon Fuku has replaced fragile fusible porcelain on refractory. After this the Parisiandoctor Fontsy in 1808 begins to manufacturing separate front teeth from refractory porcelain and also has supplied them behind with a small hooks from platinum (crampons). All this lifted prosthetics on higher step.
In 1834 the master of dentistry and obstetrics Joseff Hall published his book «Popularity administrateabout importance themes of dentistry». In this work he was first which describe «Clasps dental prosthetics». Being based on its this report consider as a founder of modern partial removable dentures. He has research a construction ofdenture on the hooks which was covering the teeth and, thanks toelasticity of material, the denture can be frequentative to put in andto put out. This removable partial denture was opposed non removable.
With dentistry development has increased as well number of produced dentures. In this connection the problem of a cheap material becomes the most actual. Cutting dentures from the bone was required a big expense of the time, but the stamped gold plates was too expensive.
That’s why the scientists try to find the cheapest material for manufacturing the dentures.
The new era in prosthodontics development was an introduction in dentistry the rubber.Vulcanized rubber into the dentistry practice for the first time has been entered in the fifties XIX century by American Putram, and in 1854Nink for the first time has made of it f denture. Since thus the denture basis begin to manufacturing from the rubber, fixing in itporcelain teeth. Such denture was a not just esthetic, but has the most functional quality. By it can be good to chew a food. It was a perfect basis material in these times, and rubber dentures taken a wide distribution.
Simultaneous with the dentures from the rubber in this period was manufactured a basis plates which fixed in the mouth by the clasps.
In 1916 Kummer has offered the bugel denture construction.
Most authors was try to improve already available claspsor to invent new.
Absolutely new construction of basis for dentures was gives E.E. Gaffner (1937, 1942). He has suggested making crowns with nearly-neck prominences for clasps. For this author hasespecially constructed planimetric forceps and also has offereda methodic of making of these prominences. Such prominencesare interfering the shift of clasps in a direction to the root and by that take up a part of chewing pressure. These crowns are founded a wide using in the dental prosthetics. They are easy makes and especiallyvaluable in the partial removable denture.
Interested method of fixing dentures on the roots has given Arnold (1962). Fixation of denture by his method was goes by theway of fixing about the root of tooth the spherical buttonwhich casted from the metal and cemented in the root.
In 1952-1961 Kemeny has offered to fixing the partial removable dentures with using the dento-alveolar clasps(img. 357, 409). This clasps was manufactured from the basis plastic, and in frontal site — from the transparent plastic. The dentures with these fixating elements Kemeny has named retential. Additional coverage of vestibular surface of alveolar process was supplya good stabilization of denture, promotes reductionof vertical loading and amortization the horizontal pushes. At choose the constructions of partial removable denture before the dentist the first of all arise a question about the way of its strengthening in the oral cavity. Effective way of fastening is one of the conditions which provide a good functionalquality of the denture. For fixation the partial removable dentures are resort to the adhesion help, anatomical retention, and artificial mechanic devices: clasps, pelots (one of the types of non-clasps fixation), telescopic crowns and other.
B.E. Lemberk in 1940 has described manufacturing of denture, occupyingaverage position between bugel and lamellardentures. The similar construction of partial removable dentures for upper jaw, which have advantages about thearch dentures, offered in 1951 I.V. Ithighyn. These dentures have all main details of arch denture, except an arch. She has replaced by the plastic intersection, connecting the denture’s parts and placed on a margin of average and back third of tooth.
Principal components ofpartial removable lamellar denture are the basis (plastic or metal), artificial teeth by other materials and every possible mechanical devices for fixation.
The basis of denture. The basis (basic) of removable lamellar denture is the plate from plastic or metal on which fixating the artificial teeth and devicesfor holding it in the mouth. The basis of denture was placedon the alveolar process of lower jaw, and on the upperbesides also and on the palatum. Chewing pressure from the artificial teeth is transferred through it on the mucosa ofdenture field.
With the basis of denture is bounded the series of negative phenomenon. He has causes the disturbance of the tactile, gustatory and thermoesthesia, covering the hard palatum. Simultaneouslycan be observed the disturbance of speech, self-cleaning of the oral cavity mucosa, its irritation, sometimes occurrence the vomitive reflex. In places of adhere the denture to the natural teeth can arising gingivitis with formation ofpathological pockets.
For uniform distribution of chewing pressureon subject tissues the basis of removable denture should to possess sufficient durability, elasticity and minimal plasticity. From hygienic reasons the basis should be manufactured from the material which little adsorbs the components of the oral fluid and foodstuff, easy cleaning by the usual agents which intended for tooth care. The greatestdistribution has received special basis plastics. Its basis is a polymers of an acrylic group which difference by entered co-polymers and excipients. Also has next basis plastic: etacryl, phtorax, bak-rel, acrel, acroneel, transparent plastic.
But the last list of basis plastics not always allow to made the removable dentures of sufficient durability, especially in the hard clinical cases. Also using the metal alloys: stainless steel for the basis removable dentures manufacturing by the method of stamping and cobalt-chromium alloy (CHA) — for casted alloy. The precious metal alloys, ext. on the golden basis, isn’t using for this purpose, because they isn’t have much durability like the non-precious metals, very expensive, and the main — it’s have too many weight.
Thickness of the plastic basis (in average) – 2 mm, that is it is peer to a thickness of the plate of the basis wax. Metal basis at the mostly durability has a smaller thickness — from 0,2 to 0,6 mm. By this case, and also by better heat conductivity, the patients are better carry metal basis, quickly adapting to it at the case of exact conformity of an internal basis relief to the mucosa relief whichis reached by precision moulding of cobalt-chromiumalloy on the fire-resistant models and it’s very hard to receive the metal basis by stamping. The domestic industry does not deliver preparations from sheet stainless steeland presses for manufacturing thestamped metal basises, that’s why it’s not using in our time. Ubiquitous introduction of high-pitched fusion of alloysand precision moulding on a compensation fire-resistance models allow to use these more progressive constructions.
Plastic basises of a removable dentures have various shades of a pink color which is defined by color of an initial powder (etacryll, acrell, phtorax) or by amount of entered stain (acronyll). It is publishing also transparent basis plastic which usingin persons with allergy on the stain for manufacturing dento-alveolar clasps with purpose of taking the esthetic effect.
Size of the denture basis depend from quantity of saved teeth, degree of atrophy of alveolar process, expressions of the crestof the hard palatum, torus presence on it, pliabilityof the mucosa and others. If safeties of teeth lower, than basis is bigger. The good terms for denture fixation(high alveolar process, expressed crestof the hard palatum or increase of quantity of clasps) allow decreasing the denture basis.
Last few years the clasp prosthesis or also sometimes it is called bugle, is in the lead position (among removable prosthesis). So what is it? Clasp denture is based like arc, and in contrast to other prosthesis, it uses for support not only gum, but also patient’s teeth. Thanks to it, bugle is more sensitive to gum, also there is considered, that clasp prosthesis can supply masticatory load more naturally. Thereby clasp denture wins traditional laminar removable dental prosthesis. In addition, we can to add that clasp denture looks compact, is comfortable in using and long-lived. In dentistry is used clasp denture with telescopic fixation system and bugles with micro-locked fastening (clasp denture with klammers and locks).
Clasp prosthesis with klammers is made to teeth, that’s why is fixed the best way on it, and thanks to elastic, it doesn’t spoil enamel. The one minus is the metal details are visible during smile.
The second way of clasp fixation is with help of locks, which are fixed on abutment teeth, it’s more durable. The most important in lock fastening is its invisible, and that it is hided in clasp denture. Everything is supplied by high modern technologies – high-precision casting and special materials, and of course high skilled specialists. Clearly, that there is most desirable is last variant, though you have pay more, as the making process of clasp denture is hard and difficult!
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The advantages of lock fastening is:
- Higher precision than with klammers;
- Higher esthetic quality of prosthesis and shorter period of patient’s adaptation to such prosthesis;
- Availability of standard interchangeable compound parts;
- The prosthesis is long-lived (in average is 7-10 years).
- Ability to change matrixes and repeated activation;
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1 – Metal-porcelain dental bridge with attachments. |
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1 – Metal-porcelain crowns which will be cemented. |
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1 – Removable part of construction. |
Opt for clasp prosthesis depends on different factors, and the defined factor is price. That’s why laminar dental prosthesis is cheaper, than follows classic clasp denture with klammers and then high-tech clasp denture with attachments (locks).
An important clinical challenge for dentists, orthopedists is to determine the indications and contraindications to the use of clasp dentures. Clasp prosthesis shown in the case of defects of dentition with sufficient number of natural teeth, which can effectively and efficiently distribute chewing pressure between the soft tissues and prosthetic bed. Clasp prosthesis shown to produce if single and bilateral end defects combined defects and included defects wheot to use bridges dentures.
It should be noted that the topography of the defect does not play a major role. Of great importance is its shape and size. Having included defects long, bounded teeth and wisdom teeth are a direct indication for the use of prosthesis. The same applies and included defects of medium size at low clinical crown abutment teeth.
When determining the indications for use clasp prostheses should consider the following factors:
1) the number of teeth in the tooth row should not be less than 6-8 or more to ensure rational distribution of masticatory pressure; important importance is not only the number of teeth, but their placement; not only the number of teeth, but their placement;
2) in the area of periapical tissues supporting the teeth should not be any pathological processes;
3) crown of abutment teeth should be high, with a distinct belt, this requirement is relative, because the form of clinical crowns can change the artificial crown;
4) fissures on the supporting teeth must be well defined – this requirement isalso relative, fissures can be improved through preparation;
5) binding factor that must be considered is the nature of occlusion;
6) state and pliability of mucous membrane areas of toothless alveolar process and parts;
7) on the lower jaw – the depth of placement of the bottom of the mouth;
8) the amount and nature of atrophy of alveolar processes or parts;
9) compulsory consideration of the general condition of the patient.
Contraindications to the use clasp dentures are:
1) high attachment of frenulum of tongue on the lower jaw, it should be placed 1 cm below the necks of the teeth, so there would be place to accommodate arc;
2) low clinical crown in failing to increase their by artificial;
3) the presence of deep bite, especially deep traumatic bite;
4) significant atrophy of alveolar process and parts and flat palate.
When you select abutment teeth for clamp fixation under clasp dentures has matter the shape of and size of the tooth crown, its position in the dental row, character of its inclination stability:
1. Supporting teeth should be high and stable.
2. Supporting teeth should have well expressed anatomical shape, belt and natural fissures.
3. At the tops of roots, selected for support should not be chronic inflammatory processes.
4. Supporting teeth should have some correlation with teeth–antagonists.
The bottom of the mouth should be deep, palate high, with well developed humps on the upper jaw.
Paralelometr is called a device which measures parallel walls of supporting teeth, applies on them clasp lines determines the type and location of elements of clasps, which provides reliable fixation of the prosthesis, free putting and removing it from the oral cavity.
Clinic of Prosthetic Dentistry now has many constructions of paralelometers through which are solved mainly the same type problems, related primarily to the calculation and design of bugel dentures. Uniform classification of types of paralelometers do not exist.
Conventionally, they are divided into three groups:
1) Standard paralelometers intended for common clinical and laboratory work;
2) Special devices designed to meet strictly defined operations, such as intraoral mikroparalelometry;
3) Universal paralelometers with multifunctional purpose.
Surveying of models is normally necessary at two stages of partial denture designing and construction. It can be appreciated that the finished denture must be able to be inserted into the patient’s mouth without difficulty and, once in position, it should be retained securely during function without causing excessive stress to be applied to the oral tissues; to achieve this the most appropriate types of retainers must be positioned correctly.
Both these requirements, the path of insertion and adequate retention, will often require modifications to tooth contour, either by grinding or by the construction of inlays or crowns. Hence, it will be necessary to survey the model obtained from the preliminary alginate impression, so that the modifications necessary to allow a correct path of insertion and adequate retention can be identified before detailed mouth preparation is commenced.
When these preparations have been completed in accordance with the information provided by the first model survey (and, of course, clinical examination), then the final impressions can be taken. When the working models have been obtained from these impressions, then a second survey is done to indicate the detailed position of denture components and the exact angulation of the path of insertion.
Principles of Surveying
Fundamentally, surveying a tooth consists of locating accurately the height of its maximum contour in relation to the plane in which the model is positioned. The majority of natural tooth crowns are bulbous in shape, but the point of maximum bulge may occur anywhere between the occlusal surface and the gingival margin.
If a vertical plane is brought into contact with a convex surface they will contact each other only at the point of maximum convexity, and, if the convex surface is rotated, still in contact with the plane, an imaginary line will be traced at the greatest circumference. If a carbon marking rod is substituted for the vertical plane and a tooth takes the place of the convex surface, then an actual line will be produced at the level of the maximum tooth bulge. This is known as the survey line. The area of the tooth above this line is the non-undercut area and the area below is the undercut area.
The position of this survey line and the location and extent of the undercut and non-undercut areas are determined by holding the tooth in a vertical axis while the carbon marker is passed round it. The principle of surveying has so far been considered in relation to a single tooth. In practice many of the teeth on the model will have to be surveyed and they will not all be in the same vertical axis. Comparable effects are also produced by side-to-side tilting of the model.
When a model is surveyed prior to mouth preparation it is first necessary to decide what is the desirable tilt to employ. This can only be answered when consideration is given to two factors, the path of insertion of the denture and the path of its removal. Although ease and comfort of insertion are important, the successful wearing of the denture depends upon retention during function.
Both upper and lower partial dentures tend to be dislodged by the masticatory process whilst, in addition, upper dentures must also resist the force of gravity. Displacement of dentures during mastication tends to be in a direction at right angles to the occlusal plane. Lateral forces are also developed during mastication but, if the denture is correctly designed and constructed and there is freedom from cuspal interference in the occlusion, these forces should not cause displacement. Consequently, in deciding upon the tilt to be given to the model, prior considerationmust be given to the displacing forces acting at right angles to the occlusal plane (Atkinson).
It could be implied from this statement that it is advisable to survey all models with the occlusal plane at right angles to the carbon survey rod, since by so doing the maximum degree of undercut area in relation to removal atright angles to the occlusal plane will be discovered. Certainly, as Craddock says, ‘If, when surveying without tilt, no undercuts are revealed, a denture cannot be made retentive against vertical removal however the model is tilted’.
Craddock and Bottomley have pointed out that one of the limitations of surveying is the fact that it does not ensure that the path of removal of the denture will always be restricted to the single direction parallel to the surveying rod. They state that if a denture can be removed in only one direction its retention will be greater than if it can be removed in more than one path. For example, if the model has been surveyed with the occlusal plane at right angles to the survey rod it may be found that removal is possible by a rotating path, in addition to the path parallel to the surveying rod. Such displacement may be avoided by giving a small degree of anterior or posterior tilt to the model without lessening the resistance to a displacing force at right angles to the occlusal plane. Conversely, increased resistance to displacement at right angles to the occlusal plane may be provided in free-end saddle dentures by an anterior tilt. This will allow the distal undercuts on the abutment teeth to be engaged by the denture and so increase resistance to vertical displacement; however, no resistance to rotating displacement about an axis located in or near the occlusal rests is provided in this way.
It is recommended that all models are first analyzed and surveyed without any tilt that is with the occlusal plane at right angles to the survey rod. The survey lines resulting will indicate what undercut areas are available to provide retention against vertical displacement. Bearing in mind that this form of dislodgement is likely to be the most common, the majority of dentures will be well retained if the lines resulting from a horizontal survey indicate adequate undercuts for clasp engagement. However, when many teeth are found to be widely divergent or when there are gross differences in abutment tooth angulation to the occlusal plane, additional surveying may be required. Displacement of the saddle can also occur in a rotating manner, the distal part rising even though the mesial part is retained. In such a case an anterior tilt given to the model willenable the operator to find out if any undercut exists on the molar tooth which will help to resist this possible rotating displacement. It is recommended that in such cases additional surveying be carried out with both anterior and posterior tilts as suggested by Atkinson. For the sake of clarity, it may be advisable to use different colored marking rods for the different tilts. The undercut area indicated by the anterior tilt survey line shows the area available for the positioning of a clasp arm that will resist displacement of the denture in a backwards direction. Conversely, the under-cut present with a posterior tilt will show the area available for resistance to forwards removal. If the clasp arm can be placed in some part of the undercut area that is common to all three survey lines then that particular clasp will resist displacement of the denture in all directions. When this is not possible it must be accepted that no resistance to possible displacement in certain directions can be provided by clasping that particular tooth unless some modification of its contour is undertaken.
Shaffer has suggested that the tilt given to the model, and hence the angulation of the path of insertion, should be determined by paralleling the carbon marker to the long axis of the weakest tooth to be clasped. By this means the weaker tooth would be stressed minimally during insertion and removal of the denture.
As a result of the information derived from the various surveys, a decision should be possible regarding the tilt at which final surveying should be done to achieve the best results, not only in terms of retention but also with regard to the path of insertion. It must be remembered that this path of insertion will normally be parallel to the carbon survey rod.
The Model Surveyor
All model surveyors work upon the same basic principles. They consist of a smooth horizontal platform from which projects a rigid vertical arm. Attached to this is a horizontal arm which in some cases can pivot laterally. This, in its turn, supports another vertical arm which is adjustable in an upwards and downwards direction and may be fixed at any required height by a locking screw. The lower end of this arm is designed to hold various tools used in surveying, subsequent model modification, and in clasp positioning.
The analyzing rod is the plane metal rod which may be used for diagnostic purposes in the selection of the tilt at which the model is to be surveyed. The carbon marker is used for the actual tracing of the survey lines on the model. It may be attached either at right angles to, or in the long axis of the movable surveyor arm. For normal surveying the latter type of marker is preferable since contact with the tooth is assured irrespective of its height. The right- angle marker, which should be beveled to a chisel edge at its working end, is useful when two teeth that have only a small-space between them have to be surveyed.
The wax trimmer is used to trim wax that may be inserted into those undercut areas which it has been decided to obliterate for constructional purposes. The undercut gauges are used to measure the extent of the undercuts that are being used for clasp retention; further details regarding their application will be given later in the chapter.
The surveyor is completed by the model table. This consists of a heavy metal base that can slide smoothly on the platform, and a universally adjustable serrated top to which the model is attached. It is tilted to the desired angle by adjusting this top which can be locked into any required position. The model can be fixed to the table by the use of soft plasticine, by three locking screws which engage the sides of the model, or magnetically if a magnetized piece of metal has been cast into the base of the model.
Technique of Surveying
The model, which should be made in hard stone plaster, is attached securely to the model table and the analyzing rod is fixed in the vertical arm. It is advisable to make a preliminary survey of the model with the analyzing rod, since by placing it against the various abutment teeth an assessment can be made of the degree of undercut present on different teeth, the angulation of teeth to the horizontal, and the presence of undercuts in the alveolar ridge areas. With free-end saddle cases it will be possible to decide if the degree of distal undercuts on the abutments is sufficient to indicate that an anterior tilt would be beneficial. This will give a path of denture insertion from behind forward, will allow the denture to engage the undercuts and so give increased resistance to vertical displacement, and will allow the denture to have a more ideal relation to the underlying gingivae. As a result of this preliminary investigation it may be possible to decide if one survey in the horizontal position will be all that is required or whether further surveys in the anterior and posterior tilted positions will be necessary. In any event, whether or not such a decision can be made at this stage, the first survey should be with the model in the horizontal position.
The model table should be locked in that position and the analyzing rod exchanged for the carbon maker in the vertical arm.
Holding the model table in one hand and the vertical arm in the other, lines are traced round all the teeth that may be clasped or that have proximal undercuts to be eliminated. In practice this usually means all the teeth with the exception of anteriors when they are all standing. The marker should also indicate the extent of any soft tissue undercuts that must be eliminated beforeconstruction commences. Minor undercuts on the edentulous ridge areas need not be eliminated since they may be helpful in retention. Gross undercuts around upper tuberosities must be removed as also must undercut areas into which a gingivally approaching clasp arm might otherwise be placed. Lingual undercuts that will prevent placing of a lingual bar should also be eliminated.
This surveying should be done with light pressure, so that there is no danger of rubbing the model.
If only one survey is required, as will be the case with the majority of dentures, then the details of design caow be considered. However, if further surveys are to be made with different tilts then they should be done at this stage. When the model has been given an anterior tilt, a second series of lines are traced round the same teeth. To avoid confusion as to which are horizontal survey lines and which are anterior tilt survey lines Atkinson suggests using different colored leads for the different positions.
It has been suggested above that in certain cases three surveys can be carried out using anterior and posterior tilts, but no mention has been made of surveying with lateral tilts. In only one condition is it likely that a lateral tilt survey could be of value and that is when the horizontal survey indicates marked lateral undercut areas on the tuberosities. Tilting the model laterally will mean using a lateral path of insertion for the denture in an effort to engage the undercut mucosal area to assist retention. Whilst insertion of the denture from one side will increase the retention of the denture against vertical displacement on that sideit will usually only serve to decrease the retention against similar forces on the other side. Consequently when considering a lateral path of insertion for this purpose, careful consideration must be given to the position of the lateral tilt survey lines on the opposite side. If, as usually occurs, they are sited low on the teeth or ridge areas, then retention on that side will not be obtained, if a lateral path of insertion is used.
When the final tilt has been decided upon the model table should be locked in that position and the final surveying carried out. Naturally, if the final tilt decided upon is one that has been used previously, for example the horizontal position, then a further tracing of survey lines is not necessary.
When the surveying has been completed consideration can be given to the details of the design. Prior to this stage the practitioner will usually have decided upon the general principles of the design; for example, whether a lingual bar or a lingual plate is to be used, or whether an upper denture is to be of plate or skeleton design. Details regarding clasp design and position, however, cannot be decided until surveying has been carried out. It is after the completion of this surveying, which is done on the study model, that decisions can be made regarding the necessity of any tooth preparation involving modifying tooth contours to produce better undercuts or reducing ‘excessive proximal undercuts that-have been shown to be an embarrassment to the path of insertion. The necessity for rest seat preparation will have been assessed previously by a study of occlusal contacts.
The undercut gauges are not used until the final working model is being surveyed to decide upon the exact positioning of the component parts of the denture.
When a clasp is to be placed on a tooth one of the most difficult problems is to decide how far below the survey line the resilient portion of a retention clasp arm shall be placed. As a general principle it should be appreciated that the extent to which a clasp arm enters the undercut will be dependent upon the degree of retention it is desired to obtain. When multiple clasp arms are available they will not needto enter the undercuts to the same extent as when a denture is dependent for its retention on only two clasp arms.
In relation to this principle the resilience of the clasp arms must be considered. Obviously, the greater the engagement of the undercut the more resilient must be the clasp arm. If this is not so then difficulty will be experienced in fitting the denture initially and the clasp arm may never engage correctly. At a later stage fracture of the arm may occur; this will be a welcome relief for the tooth which will have been stressed excessively each time the denture was inserted or removed.
The degree of resilience will depend upon the shaping of the clasp arm and upon the alloy used. Wrought yellow gold wire makes the most resilient clasp arm and cast chrome alloy the least resilient. Variations in section, taper, and length, however, can produce great variations in resilience.
The last of the surveyor tools, the wax-trimmer, is used during the preparation of the working model for duplication. It is always necessary to duplicate the working model when a metal base is to be cast, but there is muchto be said in favor of duplicating everymodel to which a partial denture is to be made. In the first place, possible damage to the working model is minimized since the work is done on the duplicate; second, the completed denture can be fitted to the working model, thus saving valuable chairside time that might be wasted in making adjustments to the contact points or peripheries; third, check can be made of occlusion and any high spots milled in prior to fitting; fourth, it is easier to eliminate undercuts in wax than in cement,, which is necessary if the working model is to be-used for the final processing. There is no doubt that more accurate laboratory workmanship results if all partial denture construction is done on duplicate models. With modern duplicating materials complete accuracy of reproduction can be guaranteed.
Prior to such duplication all unwanted undercuts should be eliminated by filling them with hard wax. In this way all structures can be paralleled with the path of insertion. To achieve this wax trimmer is fixed to the vertical arm of the surveyor, an excess of wax is placed in the undercuts and the model is placed on the model table at the same tilt at which it was finally surveyed. Wax is now trimmed away until parallelism is secured. The undercutsinto which no wax is placed are those which clasp arms are going to engage. Before duplication it is helpful to build up a strip of wax to the lower border of the clasp arm. This locates the position of the arm on the duplicate model, on which no survey line will be marked. In the case of gingivally approaching arms the wax may be built up to a sloping ledge. Particular attention should be paid to the waxing out of undercut soft tissue areas as well as those around the teeth. Also duplication will be aided if all other undercuts on the model outside the denture-bearing area are eliminated.
It has been suggested that eliminating undercuts to produce parallelism to the path of insertion is not in the best interests of gingival health. It has been found that when saddles can be brought into close approximation to the standing teeth, the reaction of the underlying gingivae is good. If, because of mesial or distal undercuts, the saddle cannot be brought into this close relationship with the gingival margin of the tooth then chronic hypertrophic gingivitis and pocket formation are likely to occur. However, if the saddle is cleared well away from the gingiva, forming a large self-cleansing space, the gingival reaction is better.
It has been suggested, therefore, when possible, that the saddle should be brought into close relationship to the abutting surface of the tooth, or alternatively kept well away to allow the space to be large enough to be self-cleansing. To achieve this latter condition the use of a 25° wax trimmer has been advocated. This would result in a denture fitting accurately at the survey line contact point but possessing a self cleansing channel at the gingival margin. Such a tool could be only used when long saddles were present.
Laboratory stages of the construction of the bar prostheses on the model and out of it.
The bar prosthesis can be made out of the model as the intermediate parts of the bridge soldered prostheses as the framework of metal plaster, cast -in – block constructions. I.e. the wax blank of the future construction is removed from the model, the sprues are fastened to it and the stages of precision casting follow. But in this method deformation of the wax components occurs during removal from the model, during suspended fastening of the sprues. In construction of the intermediate parts, in construction of metal plaster and metal ceramic this deformation is always insignificant, while during the large complex constructions of the bars, this deformation can be essential. Therefore two procedures of construction of the bar prostheses were developed: out of the model as fine elements for casting and on the model – more complex and more expensive procedure.
Let us consider the simpler procedure – construction of the bar prostheses out of the model.
Clinical and laboratory stages of the construction
of the bar prostheses out of the model.
CLINICAL
1. Taking of 2 working and 1 auxiliary impressions.
LABORATORY
1. Casting of the plaster model from the auxiliary impression and 2 super-plaster models by the working impressions.
2. Construction of the bite rim.
CLINICAL
II. Determination of the central occlusion.
LABORATORY
1. Preparation of the model in the surveyor
2. Drawing of the bar prosthesis pattern
3. Plastering of models in the occludator
4. Modeling of the prosthesis framework.
5. Replacement of wax with the metal.
6. Processing of the framework after casting, adjustment to the model.
CLINICAL
III. Fitting of the bar prosthesis framework.
LABORATORY
The final processing of the framework. Covering with protective coatings.
Positioning of the teeth.
CLINICAL
IY. Fitting of the teeth.
LABORATORY
Replacement of wax of the bases with plastic.
Processing and polishing of the prosthesis
CLINICAL
Y. Fixation of the prosthesis in the oral cavity.
YI. Correction of the prosthesis
After appropriate preparation of the model and its marking in the surveyor, modeling of the bar prosthesis framework is made. The model and rubber dams are covered with the thin layer of vaseline so that wax would not adhere to the metal.
Modeling is started with clasps and then the model itself. To remove one-piece wax framework from the model more easily, each modeled part should be preliminarily removed. The standard matrix “Formodent” or ready-made wax blanks are used for modeling.
While modeling of the framework it is necessary to watch on the uniform thickness of the components. The clasp arms are modeled taking into account the mechanical trimming (adjustment) after casting.
The finished modeled bar prosthesis is checked for the removal from the model and cast from the metal.
The trimming of the cast framework on the model requires carefulness and accuracy. The framework adjusted to the model is checked in the oral cavity and the necessary corrections are made.
Before insertion of the artificial teeth, it is necessary to determine the size of the base, whose size depends on the extension of the dentition defects. The more teeth are absent, the larger must be the base. In absence of one or two teeth with presence of the distal support the base size depends on the extension of the absent teeth, configuration of the toothless part of the alveolar process, degree and compliance of the soft tissues as well as the method of connection of the base with clasps.
In absence of the distal support on the upper jaw (1, 2 class according to Kennedy) the prosthesis base must overlapthe tuber maxilla. The area of the base in absence of the remaining support depends on the degree of atrophy of the alveolar process. If it is atrophied, the base area increases. The base size also depends on the degree of compliance of the mucous membrane, if its size is 0.6-1.2 mm, and then the base area must be increased. The base border of the bar prosthesis is a neutral zone – a place of transfer of the fixed mucosa into the mobile. The base must go around the frenulum of the upper and lower lips as well as the fold on the upper jaw behind the premolars. On the lower jaw the base goes around the intermaxillary tuber, but on the side of the oral cavity the base edge must not reach the floor of the oral cavity by 2 mm. The base border on the model is drawn by the pencil. In the bar prosthesis it is desirable that the mucous membrane of the alveolar processes would fit closely the plastic base, but not the metal of the framework. It is associated with simplicity of its construction and possibility of correction of the base, if need be.
The artificial teeth, selected by colour can be porcelain and plastic. Plastic teeth are more frequently used for lightness of construction; however, they are rapidly erased, as a result of which occlusion is lowered and the abutment teeth are overloaded. When teeth are positioned, the base with the teeth is modeled.
After this, the doctor checks position and construction of the bar prosthesis in the clinic. After checking the framework with the base is removed from the model. The wax base is modeled and thoroughly stuck on the model. Plastering is made in a combined manner. Natural teeth, clasps and arches are accurately closed by the plaster roll.
When plastering of the cuvette is finished and plaster is hardened, wax is removed by the boiling water, the cuvette is cooled and packing of plastic begins. The teeth are degreased and lubricated with the tampons moistened in the monomer. Packing is made in the cold cuvette, when plastic matures. If the cuvette was pressed badly and there will be opening between its halves, filled with the layer of plastic, then occlusion will be increased by the thickness of this layer. So packing should be checked. A sheet of cellophane is placed on one half of the cuvette; it is pressed, then the cuvette is opened, excess plastic is cut off and cellophane is removed. The cuvette is again pressed to t complete joining of its edges. The plastic is polymerized according to the instruction.
The cooled cuvette is opened, the prosthesis is pushed out from the cuvette in the special press, thoroughly cleaned from plaster, excess plastic is cut off, and the prosthesis is processed and polished.
At present it is preferable to construct the bar prostheses on the fireproof models.
Clinical and laboratory stages of the construction of the
bar prostheses on the model.
CLINICAL
1. Taking of 2 working and 1 auxiliary impressions.
LABORATORY
1. Casting of the plaster model from the auxiliary impression and 2 super-plaster models by the working impressions.
2. Construction of the bite rim.
CLINICAL
II. Determination of the central occlusion.
LABORATORY
3. Preparation of the model in the surveyor
4. Drawing of the bar prosthesis pattern
5. Plastering of models in the occludator
6. Preparation to the duplication of the model
7. Duplication of the model
8. Modeling of the prosthesis framework.
9. Replacement of wax with the metal.
10. Covering of the framework with protective coatings.
11. Positioning of the teeth.
CLINICAL
III. Fitting of the bar prosthesis (positioning of the teeth).
LABORATORY
Replacement of wax of the bases with plastic.
Processing and polishing of the prosthesis
CLINICAL
IY. Fixation of prosthesis in the oral cavity
After the preparation of the bar prosthesis in the surveyor and its preparation to duplication the construction of the fireproof model is started. The fact is that in this method the bar is modeled on the model and during casting it is not removed from it. Usual plaster or superplaster model burns down at the temperature of casting (more than 1000°). Therefore it is necessary to prepare, to copy (to duplicate) the working model from the refractory material.
After the preliminary preparation the model should be moistened, if duplication is made with the aid of the duplicating gel. It is best of all to soak the model in water at the temperature of 38°C for 15-20 min, till disappearance of bubbles on the plaster surface.
Heating to 38°C has 2 reasons:
1. Saturation of the model is more rapid in the warm water.
2. The duplicating gel will not thicken immediately on the cold metallic surfaces, which are present on the crown model during filling with the warm duplicating mass.
The moistening model should be dried by napkin, not by compressed air. Air pressure may contribute to detachment of the glued wax. The prepared model should be fastened on the base of the duplicating cuvette before pouring gel. The classical duplicating cuvette has a base made of the rubber and a frame of aluminum (Fig.5.34). For fixation of the model in the cuvette plasticine-like paste or soft wax is used in the center. The layer of gel around the model must have maximally flat thickness; otherwise there may be distortion of the negative form because of uneven cooling and shrinkage. The cuvette frame is mounted after fixation of the model.
The gels – reversible thermoplastic materials utilized for duplication approximately consist of 70% of water. They are composed of agar-agar and sticky gelatin with the additions of glycerin and mineral substances. Agar-agar is the basic component of the hydro-colloidal impression material. The gel has unstable properties because of its composition.
It is better to melt the gel at 95°C or according to the recommendation of the manufacturer in constant stirring; it preserves fluidity during cooling to the operating temperature of 48-52°C. Before melting it is necessary to cut gel down to pieces in order to avoid partial overheating. It greatly harms the material. Only enameled dishes or dishes of stainless steel are used for melting.
The duplicating gel has the following properties:
1. Precise reproduction of all details, because of its fluidity;
2. Good elasticity, the ability to return to the initial state, if we carefully removal the control model from the mould;
3. Repeated use;
4. Low price.
However, the gels have the drawbacks, which are especially visible in their misapplication:
1. Natural raw material is especially sensitive to heating. During repeated melting the basic substances are lost, so that good properties of the material gradually disappear. This process of decomposition can be slowed down with the addition of the new material. The regeneration of too strongly decomposed gel is impossible.
2. Constant evaporation occurs because of the high amount of water. Good properties of the mass can be preserved only when this loss will be compensated. Therefore the gel should be prepared in the closed apparatuses with the mixer and adjustable temperature with the aid of the thermostat. This is better than use the open dishes and manual mixing – the duplicating gel begins to shrink immediately because of the loss of water, as soon as the control model is taken out from the mould. In using the open flame or electric stove for melting the duplicating mass, the gel must be melted on the water bath.
3. The gel does not have resistance at the moment of break. Only well prepared model can be taken out from the cuvette without damage of the negative.
4. The water, which forms part of the duplicating gel, influences the solidified packing masses. It may result in the change in the form, which cannot be taken into account.
5. It is not possible to obtain precise plaster duplicate with the aid of the gels for duplication, since glycerin in its composition prevents the consolidation of plaster. But new modern materials appear at the market without such negative properties.
6. Shrinkage begins during cooling of still liquid mass from 50°C to 8-10°C in the running water or in the special apparatus. It may be regulated by “directed” cooling.
But the problem remains unsolved when the metal components of crowns and bridges – abutments are duplicated. The explanation is simple: the duplicating gel giving shrinkage during cooling is removed from the smooth metal surface, since metal has another thermal conductivity than plaster.
Before pouring the duplicating mass on the model, it is necessary to check the gel temperature with the aid of the thermometer even in the device for duplication. Liquid mass must slowly flow into one of the openings in the upper part of the cuvette. The jet must not get onto wax components. The slowly rising mass covers and fills in all forms and structures of the model.
The filled cuvette must be cooled in the air, until the mass thickens as jelly. This is easily checked by fingers in the openings for filling. The exposure time is very important so that the thickening of the model would occur. Shrinkage of the gel must be regulated so that the gel would not be scaled from the model. If we begin cooling sharply, then external walls will thicken first and they will begin to absorb internal liquid content to compensation the mass shrinkage. Thus the gel will scale from the model. As a result of these processes the distorted negative is obtained. If the cuvette bottom is made of aluminum, then it transfers heat very rapidly. The model stands on the bottom, the effect of cooling increases, if the base stands on the legs. In this case the cuvette bottom can be cooled more effectively by room air or water. Thus, the gel in the upper part of the cuvette must thicken last, so the cuvette frame must be made of the material, which conducts heat badly for example, plastic.
After the duplicating gel, which filled the form, was cooled during 20-30 min at the room temperature, the cuvette may be placed in the water or on the special cooling device. For complete hardening of the duplicating mass the running water of 8-10°C is sufficient in the subsequent 30-45 min. In this case the cuvette must not be completely immersed in the water. It is better that only two thirds of its height is washed by water. On this phase of cooling the gel must first be cooled in the remote area and harden where the model is located. It is possible to exclude inaccuracies because of the erroneously directed shrinkage by limiting the water level. Then the plaster model is extracted from the gel, and the negative is filled up with fireproof mass.
The reversible thermoplastic masses have some disadvantages, namely inaccurate reproduction of the form of the duplicated metal components. It may result in the inexact fit of the fixing elements on the crowns. There are fluid silicone masses for duplication at the market, which compensate many drawbacks in the gels. Two components are mixed up thoroughly in the ratio of volumes according to instructions, and further stages of work are the same as with duplicating gels.
Advantages of silicones:
1. Silicones reproduce form and relief very accurately. The problem of duplicating metal components with the aid of these masses can be considered solved;
2. The model must not be soaked;
3. Approximately in 45 min, beginning with mixing, the negative form is ready for use;
4. A repeated filling is possible, first of all with plaster for the control model;
5. There is no reaction between the material of the form and packing mass.
It is only related to silicones.
The duplicating masses on the base of polyester react in other way. High price of the gel masses and single application are the drawbacks. There may be smallest savings to 25% if we cut silicone having been used and put it round the base of the plaster model in the cuvette before duplication. Savings are also possible with the use of the special dosing device for the silicone materials. It allows to dose precisely a quantity of the mass and mix up the components of silicone evenly without the air inlet.
The fireproof model must maintain the heating temperature up to 1400-1600° without being deformed and without changing.
Usually the fireproof mass consists of the mixture of the refractory thinly grinding stocks, which are mixed up with water. 100-120 g of the powder is necessary for one ceramic model. A precise quantity of attachments can be determined, if the weight of dry plaster model is multiplied by 1.7. A quantity of water per 100 g of the powder depends on the composition of the fireproof mass and as it is indicated in the instruction.
A definite quantity of the powder is poured into the rubber flask. Fill it the measured quantity of water and mix with a spatula. After mixing the mass together with the cup is placed on the vibration table, after that its surface becomes bright.
Pouring of the fireproof mass in the impression is also made on the vibration-table with the subsequent application of vacuum. The cuvette with the impression is placed on the vibration-table, it is turned on. Place the fireproof mass on the edge of impression so that small portions of it flowed and evenly fill cavities; it protects the model from formation of the pores. The entire process of the model casting lasts for 2- 3 min. However, after this method of filling there are still small pores, which do not give smooth surface.
For elimination of the gas bubbles from the molding compound and consolidation of the model, it is placed in the reservoir, from which the air is pumped out. Low vacuum contributes to the suction of the air from the compound. The process of degassing lasts for 4-5 min, after that the vibration table is turned off. The model begins to harden in 10-12 min after filling. It is at this time it is necessary to remove the sprue cup from the cuvette.
The final hardening of the model begins in 40-45 min. After this, the model is released from the duplex mass.
After hardening the models of the fireproof mass are fragile; therefore they should be extracted carefully – cut the duplex mass in parts in order not to injure the model. After the model release from the duplex mass it should have a smooth bright surface without pores and be the precise copy of the original. For strengthening of the fireproof models, they undergo drying in the cabinet drier at a temperature of 200-250° for 30 min. From the dry-air cabinet the model is placed in wax heated to 150° for its fixation for 10 sec.
The impregnation of the models with a fixing agent is achieved in the electrothermal device, which is a bath with 1 liter of capacity with electrical heating and the heat-control device.
After finishing duplication we obtain a fireproof model with the bar prosthesis framework applied onto it. After this, we start modeling.
In modeling the prosthesis framework on the fireproof model, wax should fit the surface of the fireproof model and have sufficient plasticity.
Special matrices on the silicone basis (Formodent) should be used. The shape and size of the prosthesis components should correspond to the construction of the prosthesis, value of the working load and depend on the properties of the poured alloy. The sprue basin is made in the bottom of the fireproof model, which gives the smooth filling of the mould with liquid metal.
In making the sprue system it is necessary to provide the rational arrangement of the sprues in order to avoid the appearance of the incorrect direction of metal jet and its strike against the mould wall. In some parts of the metal casting it is necessary to provide the installation of additional compensating balls from wax; for complete filling of the casting mold the sprues should be of appropriate diameter and length.
For obtaining the wax components of the deepening of the plate, Formodent is covered with casting wax. After thickening the excess of wax is removed from the plate surface by the sharp heated dental spatula. Wax component is easily extracted from the silicone plate during its small bend.
During casting of wax components the silicone form is not lubricated.
Long components are shortened by knife; separate parts are connected with the aid of molten wax.
The preparation of the model of the prosthesis framework begins with the application of the thin plate of heated wax onto the fireproof model and it is pressed out thoroughly, attaining its tight fit to the surface. It cannot be always achieved by application of the plate onto the entire surface of the model. Such difficulties are frequently encountered in the area of the abutment teeth. In such cases thin wax is applied first by separate parts onto the abutment teeth, the edges of the applied wax are fixed by a hot spatula to the least critical places of the model. Then clasps are modeled.
In the clasp installationwe are guided by the guide line bearing in mind that the rigid part of the clasp should be located above this line, and the flexible part –below it.
The arch of the upper prosthesis is modeled from the zoned wax strip of semioval section, while the arches of the lower prosthesis – from the same strip of semicircular section. Moreover, the width of these wax strips must be 1.5 times less than the designed width of the arch. For example, if the width of the upper arch in the final formulation must have6 mm, then the zoned wax strip of 4 mm in width is placed on the base from thin wax. At the junction of this strip with the wax base the edge of its each side are widen by 1.5 mm by fusing elastic model wax. Thus, the arch obtains a three-layered modeling. It is very important as usual volumetric shrinkage of wax makes 0.1-0.15% per each degree of the temperature interval from 8 to 20°C.
In modeling the saddle of the prosthesis framework attention should be first of all paid to the strong joint of the clasp frame with this part and did not prevent
loose fit of the prosthesis by its direction, moving away for a sufficient distance from the neck of the abutment tooth. Saddle parts must have smooth taper to the prosthesis arch without the formation of acute angles and other irregularities, which can cause inconveniences for the tongue and become the retention place for the food remnants.
The modeled prosthesis together with the sprue system is directed to casting and is cast together with the model. After casting the framework does not require fitting and checking in the clinic.
Positioning of the teeth and further stages do not differ from those in construction of the bar prostheses by the method of removal from the model.
