Biomechanics in fixed partial prosthodontics/ orthodontics training

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Biomechanics in fixed partial prosthodontics/ orthodontics training

  1. 1. BIOMECHANICS IN FIXEDBIOMECHANICS IN FIXED PARTIALPARTIAL PROSTHODONTICSPROSTHODONTICS INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.comwww.indiandentalacademy.com
  2. 2. INTRODUCTION:INTRODUCTION: Fixed Prosthodontics is concerned with theFixed Prosthodontics is concerned with the replacement of large amounts of missing toothreplacement of large amounts of missing tooth structure. The restorative procedures involved canstructure. The restorative procedures involved can have a great effect on the forces transmitted to thehave a great effect on the forces transmitted to the remaining tooth and its supporting structures. Thisremaining tooth and its supporting structures. This potential is greater than in many other treatmentpotential is greater than in many other treatment modalities because of the magnitude of themodalities because of the magnitude of the replaced missing structural form. For example, toreplaced missing structural form. For example, to evaluate the significance of a simple full crown onevaluate the significance of a simple full crown on a mandibular molar tooth in a patient with relativelya mandibular molar tooth in a patient with relatively normal occlusion, a full complement of teeth, andnormal occlusion, a full complement of teeth, and normal bone support,normal bone support, www.indiandentalacademy.comwww.indiandentalacademy.com
  3. 3. we see that the following parameter of form andwe see that the following parameter of form and forces are within the control and responsibility offorces are within the control and responsibility of the operator:the operator: Number and area of occlusal contactsNumber and area of occlusal contacts Inclination and length of cuspsInclination and length of cusps Axial contoursAxial contours Interproximal contactsInterproximal contacts These parameters are related to the surfaceThese parameters are related to the surface contours of the completed restoration.contours of the completed restoration. www.indiandentalacademy.comwww.indiandentalacademy.com
  4. 4. Number and area of occlusal contacts:Number and area of occlusal contacts: The number and area of occlusal contactsThe number and area of occlusal contacts have a profound influence on the distribution ofhave a profound influence on the distribution of occlusal forces. For example, the larger the totalocclusal forces. For example, the larger the total area of contact over which a given occlusal force isarea of contact over which a given occlusal force is applied, the less stress is concentrated at any oneapplied, the less stress is concentrated at any one point. As the total number of occlusal contactspoint. As the total number of occlusal contacts increases in an occlusal scheme, the force isincreases in an occlusal scheme, the force is applied over a greater number of locations, alsoapplied over a greater number of locations, also reducing the localized stress. In addition, the largerreducing the localized stress. In addition, the larger number of contacts results in more cutting ornumber of contacts results in more cutting or grinding surfaces to facilitate mastication.grinding surfaces to facilitate mastication. www.indiandentalacademy.comwww.indiandentalacademy.com
  5. 5. Inclination and length of cusps:Inclination and length of cusps: The angles of inclines of individual cusps present aThe angles of inclines of individual cusps present a dilemma that must be considered. Greaterdilemma that must be considered. Greater chewing efficiency generally is attained withchewing efficiency generally is attained with steeper cusps. However, if these cusps aresteeper cusps. However, if these cusps are allowed to come into contact in excursiveallowed to come into contact in excursive movements of the mandible, they becomemovements of the mandible, they become interferences, which is a deleterious situation.interferences, which is a deleterious situation. The operator has limited control over theThe operator has limited control over the length of the cusps of restorations unless thelength of the cusps of restorations unless the opposing teeth are also being treated. Whereopposing teeth are also being treated. Where possible, however, excessive length should bepossible, however, excessive length should be avoided, because these cusps tend to transmitavoided, because these cusps tend to transmit greater force to the supporting structures due togreater force to the supporting structures due to the longer lever arm involved.the longer lever arm involved.www.indiandentalacademy.comwww.indiandentalacademy.com
  6. 6. Axial contours:Axial contours: When axial contours are considered, theWhen axial contours are considered, the concern is with forces of a much lower magnitudeconcern is with forces of a much lower magnitude and of a less predictable range. The food bolusand of a less predictable range. The food bolus undergoing mastication can apply some smallundergoing mastication can apply some small amount of force to lateral walls as well as a greateramount of force to lateral walls as well as a greater force to the occlusal surface. In addition, low-level,force to the occlusal surface. In addition, low-level, persistent forces from the tongue or lips acting onpersistent forces from the tongue or lips acting on the lateral surfaces of teeth may result in tooththe lateral surfaces of teeth may result in tooth movement. This kind of action is oftenmovement. This kind of action is often compounded when the contour of a restoration orcompounded when the contour of a restoration or a natural tooth tends to trigger or encourage aa natural tooth tends to trigger or encourage a habit pattern that accentuates this effect.habit pattern that accentuates this effect.www.indiandentalacademy.comwww.indiandentalacademy.com
  7. 7. Interproximal contacts: The size and form of interproximal contacts can have a striking effect on the forces applied to the interseptal bone and in particular that area referred to as the gingival col. This latter feature of the periodontal supporting structures has been found to be particularly vulnerable to adverse and prolonged irritation. Any design parameters incorporated into a restoration that reduce forces acting in this region are desirable. The preceding factors in restoration design are concerned primarily with the outer surface of the final restoration. Two other important factors are concerned with the interaction between the restoration and the tooth.www.indiandentalacademy.comwww.indiandentalacademy.com
  8. 8. Tooth-restoration interface:Tooth-restoration interface: The tooth- restoration interface is not asThe tooth- restoration interface is not as important to the final result from a design stand-important to the final result from a design stand- point as those factors previously discussed.point as those factors previously discussed. However, it has a more definitive, all or nothingHowever, it has a more definitive, all or nothing effect. Unless the crown has at least a minimumeffect. Unless the crown has at least a minimum degree of required retention, occlusal forces tenddegree of required retention, occlusal forces tend to dislodge the crown, making the discussion of allto dislodge the crown, making the discussion of all other parameters academic.other parameters academic. www.indiandentalacademy.comwww.indiandentalacademy.com
  9. 9. Tooth-supporting structure interface:Tooth-supporting structure interface: The interface between the tooth and theThe interface between the tooth and the supporting structures is a region of special concernsupporting structures is a region of special concern and one where a sound knowledge of theand one where a sound knowledge of the principles of restorative design must be applied. Itprinciples of restorative design must be applied. It is here that the greatest chance for damage fromis here that the greatest chance for damage from lack of foresight and poor judgment by thelack of foresight and poor judgment by the operator can play a detrimental role in the futureoperator can play a detrimental role in the future health of the patient. The opportunities related tohealth of the patient. The opportunities related to the fabrication of a full crown on a single tooth arethe fabrication of a full crown on a single tooth are great for both improvement of force distributiongreat for both improvement of force distribution and potential damage. It must then follow thatand potential damage. It must then follow that more extensive restorations can present greatlymore extensive restorations can present greatly increased problems or benefits.increased problems or benefits. www.indiandentalacademy.comwww.indiandentalacademy.com
  10. 10. Abutment Evaluation:Abutment Evaluation: Abutment teeth are called upon to withstandAbutment teeth are called upon to withstand the forces normally directed to the missing teeth, inthe forces normally directed to the missing teeth, in addition to those usually applied to the abutments.addition to those usually applied to the abutments. Whenever possible, an abutment should be a vitalWhenever possible, an abutment should be a vital tooth. However, a tooth that has beentooth. However, a tooth that has been endodontically treated and is asymptomatic, withendodontically treated and is asymptomatic, with radiographic evidence of good seal and completeradiographic evidence of good seal and complete obturation of the canal, can be used as anobturation of the canal, can be used as an abutment. The tooth must have some sound,abutment. The tooth must have some sound, surviving coronal tooth structure to insuresurviving coronal tooth structure to insure longevity. However, some compensation can belongevity. However, some compensation can be made through the use of a dowel core, or a pinmade through the use of a dowel core, or a pin retained amalgam or a composite core.retained amalgam or a composite core. www.indiandentalacademy.comwww.indiandentalacademy.com
  11. 11. Teeth that have been pulp capped in the processTeeth that have been pulp capped in the process of preparing the tooth should not be used as anof preparing the tooth should not be used as an FPD abutment unless they are endodonticallyFPD abutment unless they are endodontically treated. The supporting tissues surrounding thetreated. The supporting tissues surrounding the abutment teeth must be healthy and free fromabutment teeth must be healthy and free from inflammation before any prosthesis can beinflammation before any prosthesis can be contemplated. Normally, abutment teeth shouldcontemplated. Normally, abutment teeth should not exhibit any mobility, since they will be carryingnot exhibit any mobility, since they will be carrying an extra load. The roots and the supporting tissuesan extra load. The roots and the supporting tissues should be evaluated for three factors:should be evaluated for three factors: Crown-root ratio.Crown-root ratio. Root configuration.Root configuration. Periodontal ligament area.Periodontal ligament area. www.indiandentalacademy.comwww.indiandentalacademy.com
  12. 12. Crown-root ratio:Crown-root ratio: This ratio is a measure of the length ofThis ratio is a measure of the length of tooth, occlusal to the alveolar crest of bonetooth, occlusal to the alveolar crest of bone compared with the length of root embedded in thecompared with the length of root embedded in the bone. As the level of alveolar bone moves apically,bone. As the level of alveolar bone moves apically, the lever arm of that portion out of bone increases,the lever arm of that portion out of bone increases, and the chances for harmful lateral forces isand the chances for harmful lateral forces is increased. The optimum crown-root ratio for aincreased. The optimum crown-root ratio for a tooth to betooth to be utilized as a FPD abutment is 2:3. A ratio of 1:1 isutilized as a FPD abutment is 2:3. A ratio of 1:1 is the minimum ratio that is acceptable for athe minimum ratio that is acceptable for a prospective abutment under normalprospective abutment under normal circumstances.circumstances. www.indiandentalacademy.comwww.indiandentalacademy.com
  13. 13. However, there are situations in which a crown root ratio greater than 1:1 might be considered adequate. If the occlusion opposing a FPD is composed of artificial teeth, occlusal forces will be diminished, with less stress on the abutment teeth. The occlusal forces against prosthesis have been shown to be considerably less than that against natural teeth: 26 lb for removable partial dentures and 56 lb for fixed partial dentures versus 150 lb for natural teeth. www.indiandentalacademy.comwww.indiandentalacademy.com
  14. 14. For the same reasons, an abutment tooth with aFor the same reasons, an abutment tooth with a less than desirable crown-root ratio is more likelyless than desirable crown-root ratio is more likely to successfully support FPD if the opposingto successfully support FPD if the opposing occlusion is composed of mobile, periodontallyocclusion is composed of mobile, periodontally involved teeth than if the teeth are periodontallyinvolved teeth than if the teeth are periodontally sound.sound. www.indiandentalacademy.comwww.indiandentalacademy.com
  15. 15. Root configuration:Root configuration: This is an important point in theThis is an important point in the assessment of an abutment’s suitability from aassessment of an abutment’s suitability from a periodontal standpoint. Roots that are broaderperiodontal standpoint. Roots that are broader labiolingually than they are mesiodistally arelabiolingually than they are mesiodistally are preferable to roots that are round in cross-section.preferable to roots that are round in cross-section. www.indiandentalacademy.comwww.indiandentalacademy.com
  16. 16. Multirooted posterior teeth with widely separatedMultirooted posterior teeth with widely separated roots will offer better periodontal support than rootsroots will offer better periodontal support than roots that converge, fuse, or generally present a conicalthat converge, fuse, or generally present a conical configuration. The tooth with conical roots can beconfiguration. The tooth with conical roots can be used as an abutment for a short span FPD if allused as an abutment for a short span FPD if all other factors are optimal. A single rooted tooth withother factors are optimal. A single rooted tooth with evidence of irregular configuration or with someevidence of irregular configuration or with some curvature in the apical third of the root is preferablecurvature in the apical third of the root is preferable to the tooth that has a nearly perfect taper.to the tooth that has a nearly perfect taper. www.indiandentalacademy.comwww.indiandentalacademy.com
  17. 17. Periodontal ligament area / Ante’s law:Periodontal ligament area / Ante’s law: When the normal complement of roots isWhen the normal complement of roots is not available to distribute the forces of mastication,not available to distribute the forces of mastication, pathologic stress concentrations may result in thepathologic stress concentrations may result in the periodontal ligament and supporting bone. Thisperiodontal ligament and supporting bone. This condition is the most fundamental problem withcondition is the most fundamental problem with which the prosthodontist must contend each time awhich the prosthodontist must contend each time a fixed prosthesis is designed to replace a missingfixed prosthesis is designed to replace a missing tooth.tooth. www.indiandentalacademy.comwww.indiandentalacademy.com
  18. 18. Here weHere we must take into accounts Ante’s law, which states,must take into accounts Ante’s law, which states, “The total periodontal membrane area of the“The total periodontal membrane area of the abutment teeth should equal or exceed that of theabutment teeth should equal or exceed that of the teeth to be replaced’’. Theteeth to be replaced’’. The essential feature of this clinical guideline is that theessential feature of this clinical guideline is that the actual area of the interface between tooth andactual area of the interface between tooth and supporting structures must be of a certain minimalsupporting structures must be of a certain minimal amount to withstand andamount to withstand and resist the forces that will now be transmitted toresist the forces that will now be transmitted to those supporting structures. Realisticthose supporting structures. Realistic determination of the area of good, healthy,determination of the area of good, healthy, periodontal ligament available on a potential FPDperiodontal ligament available on a potential FPD abutment is not an easy matter.abutment is not an easy matter. www.indiandentalacademy.comwww.indiandentalacademy.com
  19. 19.  Maxillary Mandibular Tooth Area mm2 Ranking Area mm2 Ranking Central 139 7 103 8 Lateral 112 8 124 7 Canine 204 3 159 4 First premolar 149 5 135 6 Second Premolar 140 6 135 5 First molar 335 1 352 1 Second molar 272 2 282 2 Third molar 197 4 190 3 www.indiandentalacademy.comwww.indiandentalacademy.com
  20. 20. The combined root surface area of theThe combined root surface area of the second premolar and the second molarsecond premolar and the second molar (A(A2p2p+A+A2m2m) is greater than that of the first) is greater than that of the first molar being replaced (Amolar being replaced (A1m1m).). www.indiandentalacademy.comwww.indiandentalacademy.com
  21. 21. The combined root surface area of theThe combined root surface area of the first premolar and the second molarfirst premolar and the second molar abutment (Aabutment (A1p1p+A+A2m2m) is approximately) is approximately equal to that of the teeth being replacedequal to that of the teeth being replaced (A(A2p2p+A+A1m1m)) www.indiandentalacademy.comwww.indiandentalacademy.com
  22. 22. The combined root surface area of theThe combined root surface area of the canine and the second molar (Acanine and the second molar (Acc+A+A2m2m)) is exceeded by that of the teeth beingis exceeded by that of the teeth being replaced (Areplaced (A1p1p+A+A2p2p+A+A1m1m)) www.indiandentalacademy.comwww.indiandentalacademy.com
  23. 23. Also of extreme importance is the actual area ofAlso of extreme importance is the actual area of contact between the periodontal structures and thecontact between the periodontal structures and the root in question as it relates to the normal amountroot in question as it relates to the normal amount of contact area for that particular tooth in thatof contact area for that particular tooth in that particular mouth. One can assume that in a givenparticular mouth. One can assume that in a given case, without bone loss, the area of this contact iscase, without bone loss, the area of this contact is optimal. Therefore, any loss of bone supportoptimal. Therefore, any loss of bone support compared with the optimal situation decreases thecompared with the optimal situation decreases the chances of this root being an adequate fixedchances of this root being an adequate fixed partial denture prosthesis abutment. If one looks atpartial denture prosthesis abutment. If one looks at the problem in this manner, it becomes apparentthe problem in this manner, it becomes apparent that a root that appears adequate in one situationthat a root that appears adequate in one situation may be inadequate in another. The planning andmay be inadequate in another. The planning and design of a restoration of this type must have thedesign of a restoration of this type must have the benefit of sound clinical judgement and knowledgebenefit of sound clinical judgement and knowledge of basic biomechanical principles.of basic biomechanical principles.www.indiandentalacademy.comwww.indiandentalacademy.com
  24. 24. Table , which compares the root surface areas ofTable , which compares the root surface areas of 16 teeth, may aid in visualizing root areas. It is16 teeth, may aid in visualizing root areas. It is interesting to note that the addition of abutmentinteresting to note that the addition of abutment roots will provide a wide variation of additionalroots will provide a wide variation of additional support. The addition of such support is notsupport. The addition of such support is not necessarily proportional to the number ofnecessarily proportional to the number of abutments supporting prosthesis. There fore, theabutments supporting prosthesis. There fore, the prosthodontist should not expect a proportionalprosthodontist should not expect a proportional increase in stress-bearing ability, particularly whenincrease in stress-bearing ability, particularly when the amount of periodontal ligament is reduced. Thethe amount of periodontal ligament is reduced. The area of the normal periodontal ligament (PDL) forarea of the normal periodontal ligament (PDL) for teeth to be replaced by pontics should be less thanteeth to be replaced by pontics should be less than the actual PDL area of the existing abutmentthe actual PDL area of the existing abutment candidates.candidates. www.indiandentalacademy.comwww.indiandentalacademy.com
  25. 25. The values given in the table are averages for theThe values given in the table are averages for the various teeth in the mouth under ideal conditions.various teeth in the mouth under ideal conditions. Two problems are evident when one attempts toTwo problems are evident when one attempts to arrive at useful interpretation of such data.arrive at useful interpretation of such data. Degree and nature of bone lossDegree and nature of bone loss In clinical practice, the decision makingIn clinical practice, the decision making process in which root surface area information is toprocess in which root surface area information is to be used does not always involve ideal situations.be used does not always involve ideal situations. More often than not, there has been some boneMore often than not, there has been some bone loss; thus, a tooth with a moderate amount of boneloss; thus, a tooth with a moderate amount of bone loss may be still a better candidate for use as aloss may be still a better candidate for use as a FPD abutment than another tooth with no boneFPD abutment than another tooth with no bone loss at all.loss at all. www.indiandentalacademy.comwww.indiandentalacademy.com
  26. 26. To determine as to which of the teeth in questionTo determine as to which of the teeth in question has adequate support for the anticipated loadshas adequate support for the anticipated loads has, there are but few aids on which thehas, there are but few aids on which the prosthodontist can rely, none of them capable ofprosthodontist can rely, none of them capable of giving all the information necessary to make agiving all the information necessary to make a decision.decision. These aids areThese aids are RadiographsRadiographs Periodontal probing, andPeriodontal probing, and Mobility tests.Mobility tests. www.indiandentalacademy.comwww.indiandentalacademy.com
  27. 27. The radiograph is unquestionably the most usefulThe radiograph is unquestionably the most useful tool at the disposal of the prosthodontist in makingtool at the disposal of the prosthodontist in making a determination of the integrity of the remaininga determination of the integrity of the remaining periodontal supporting structures as related to theperiodontal supporting structures as related to the loads anticipated. The opportunity to makeloads anticipated. The opportunity to make radiographs from different projection angles shouldradiographs from different projection angles should not be overlooked, since the primary areas thatnot be overlooked, since the primary areas that can be visualized on radiographs are the mesialcan be visualized on radiographs are the mesial and distal surfaces. Any chance for seeing even aand distal surfaces. Any chance for seeing even a little of the facial or lingual surfaces should belittle of the facial or lingual surfaces should be taken advantagetaken advantage of.of. www.indiandentalacademy.comwww.indiandentalacademy.com
  28. 28. Periodontal probing is the second tool at thePeriodontal probing is the second tool at the disposal at the prosthodontist and should be useddisposal at the prosthodontist and should be used extensively when attempting to determine theextensively when attempting to determine the efficacy of using a given tooth as an abutment.efficacy of using a given tooth as an abutment. Periodontal probing is a particularly important stepPeriodontal probing is a particularly important step as related to the facial and lingual surfaces, sinceas related to the facial and lingual surfaces, since these areas of the tooth to supporting structuresthese areas of the tooth to supporting structures interface cannot be viewed adequately on theinterface cannot be viewed adequately on the radiograph.radiograph. www.indiandentalacademy.comwww.indiandentalacademy.com
  29. 29. Finally, standard clinical tests for mobility shouldFinally, standard clinical tests for mobility should be employed. Any degree of mobility outside thebe employed. Any degree of mobility outside the normal accepted physiologic range should benormal accepted physiologic range should be suspected. It meanssuspected. It means either that the loss of supporting structure, whethereither that the loss of supporting structure, whether or not fully appreciated from theor not fully appreciated from the radiographs and probing, is severe enough to alterradiographs and probing, is severe enough to alter physically the stability of the tooth or that thephysically the stability of the tooth or that the occlusion has traumatically loosened the tooth. Itocclusion has traumatically loosened the tooth. It is very important to understand which of theseis very important to understand which of these processes is at work in a given abutmentprocesses is at work in a given abutment situations.situations. Occlusal trauma is usually reversible,Occlusal trauma is usually reversible, and given the fact that the prosthodontist is goingand given the fact that the prosthodontist is going to construct a restoration on the tooth in question,to construct a restoration on the tooth in question, there is ample opportunity to correct the situation.there is ample opportunity to correct the situation.www.indiandentalacademy.comwww.indiandentalacademy.com
  30. 30. A periodontal defect, however, is not alwaysA periodontal defect, however, is not always reversible and depending on its severity mayreversible and depending on its severity may require a drastic alteration of the plan of treatmentrequire a drastic alteration of the plan of treatment for the tooth in question. The important concept tofor the tooth in question. The important concept to keep in mind is that the prosthodontist mustkeep in mind is that the prosthodontist must exercise a certain degree of good judgement in theexercise a certain degree of good judgement in the question of interpretation of the adequacy ofquestion of interpretation of the adequacy of supporting structures in a given situations. Manysupporting structures in a given situations. Many aspects of the final restoration and the chances foraspects of the final restoration and the chances for success are basically technical and depend on thesuccess are basically technical and depend on the skill of a particular prosthodontist. Not only goodskill of a particular prosthodontist. Not only good judgment but a conservative approach must bejudgment but a conservative approach must be hall marks of the thought processes of thehall marks of the thought processes of the prosthodontist in this all important consideration.prosthodontist in this all important consideration. www.indiandentalacademy.comwww.indiandentalacademy.com
  31. 31. Ante’s law says that in a situation where theAnte’s law says that in a situation where the preceding values are normal, a prosthesis topreceding values are normal, a prosthesis to replace the maxillary first molar would needreplace the maxillary first molar would need abutment teeth with at least 335 mmabutment teeth with at least 335 mm22 ofof periodontal membrane. This requirement isperiodontal membrane. This requirement is theoretically well satisfied in the case because oftheoretically well satisfied in the case because of the total average area of the second molar andthe total average area of the second molar and second premolar is 412mmsecond premolar is 412mm22 , however, has there, however, has there been enough loss of bone on these two teeth tobeen enough loss of bone on these two teeth to result in, for example, a total of only 300mmresult in, for example, a total of only 300mm22 , the, the law would not be satisfied.law would not be satisfied. www.indiandentalacademy.comwww.indiandentalacademy.com
  32. 32. Preparations design as related to stressPreparations design as related to stress and force distributionand force distribution A large number of stress distribution studiesA large number of stress distribution studies have been performed on fixed prosthetichave been performed on fixed prosthetic restorations, particularly since the 1960s. Theserestorations, particularly since the 1960s. These studies have provided certain conclusions that arestudies have provided certain conclusions that are useful in determining the proper design in a givenuseful in determining the proper design in a given clinical situations. Many of the factors that oftenclinical situations. Many of the factors that often cause restorations failure may be eliminated bycause restorations failure may be eliminated by viewing preparations in the context of force andviewing preparations in the context of force and stress distribution.stress distribution. www.indiandentalacademy.comwww.indiandentalacademy.com
  33. 33. Several parameters of preparations design interactSeveral parameters of preparations design interact with occlusal forces to determine restorativewith occlusal forces to determine restorative success or failure. The reasons for near parallelsuccess or failure. The reasons for near parallel walls, grooves, offsets, counter bevels, pins, andwalls, grooves, offsets, counter bevels, pins, and other design features must be viewed in light ofother design features must be viewed in light of their functions. Each design features shouldtheir functions. Each design features should contribute to retention of the restoration,contribute to retention of the restoration, resistance against displacement, protection ofresistance against displacement, protection of remaining tooth structure, and harmony with theremaining tooth structure, and harmony with the restorative material.restorative material. www.indiandentalacademy.comwww.indiandentalacademy.com
  34. 34. Retention and resistanceRetention and resistance Looking at these design features one at aLooking at these design features one at a time, we must first consider the question oftime, we must first consider the question of retention and resistance. Retention is the ability ofretention and resistance. Retention is the ability of the restoration to withstand forces acting in such athe restoration to withstand forces acting in such a manner as to dislodge the restoration from themanner as to dislodge the restoration from the preparation along the line of draw.preparation along the line of draw. Resistance, onResistance, on the other hand, is the ability of the preparation andthe other hand, is the ability of the preparation and the restoration to resist forces that would tend tothe restoration to resist forces that would tend to dislodge the latter in direction other than the line ofdislodge the latter in direction other than the line of draw. In other words, the resistance to vectors ofdraw. In other words, the resistance to vectors of occlusal forces encountered on laterotrusiveocclusal forces encountered on laterotrusive occlusal movements during either mastication orocclusal movements during either mastication or grinding of the teeth is the important considerationgrinding of the teeth is the important consideration here.here.www.indiandentalacademy.comwww.indiandentalacademy.com
  35. 35. Taper:Taper: The relationship of one wall of aThe relationship of one wall of a preparation to the long axis of that preparation ispreparation to the long axis of that preparation is the inclination of that wall. The axial walls of thethe inclination of that wall. The axial walls of the preparation must taper slightly to permit thepreparation must taper slightly to permit the restoration to seat. Theoretically, the more nearlyrestoration to seat. Theoretically, the more nearly parallel the opposing walls of a preparation, theparallel the opposing walls of a preparation, the greater should be the retention. Craig RG, et.al.greater should be the retention. Craig RG, et.al. (1969) suggested a taper of 2.5-6.5 degrees as(1969) suggested a taper of 2.5-6.5 degrees as optimum to minimize stress in the cement interfaceoptimum to minimize stress in the cement interface between the preparation and restoration, but therebetween the preparation and restoration, but there is only a slight increase in stress as taper isis only a slight increase in stress as taper is increased from 0-15 degrees.increased from 0-15 degrees. www.indiandentalacademy.comwww.indiandentalacademy.com
  36. 36. However, at 20 degrees, stress concentration wasHowever, at 20 degrees, stress concentration was found to sharply increase. Cement creates a weakfound to sharply increase. Cement creates a weak bond, largely by mechanical interlocks., betweenbond, largely by mechanical interlocks., between the inner surface of the restoration and the axialthe inner surface of the restoration and the axial wall of the preparation. Therefore, the greater thewall of the preparation. Therefore, the greater the surface area of a preparation, the greater itssurface area of a preparation, the greater its retention.retention. www.indiandentalacademy.comwww.indiandentalacademy.com
  37. 37. Freedom of displacement:Freedom of displacement: Retention is improved by geometricallyRetention is improved by geometrically limiting the numbers of paths along which alimiting the numbers of paths along which a restoration can be removed from the toothrestoration can be removed from the tooth preparation. Maximum retention is achieved whenpreparation. Maximum retention is achieved when there is only one path.there is only one path. www.indiandentalacademy.comwww.indiandentalacademy.com
  38. 38. The walls of a groove that meet theThe walls of a groove that meet the axial wall at an oblique angle do notaxial wall at an oblique angle do not provide necessary resistance (A). Theprovide necessary resistance (A). The walls of a groove must bewalls of a groove must be perpendicular to rotating forces toperpendicular to rotating forces to resist displacement (B).resist displacement (B). www.indiandentalacademy.comwww.indiandentalacademy.com
  39. 39. If the buccal and lingual walls of a proximalIf the buccal and lingual walls of a proximal box forms oblique angle with its pulpal floor,box forms oblique angle with its pulpal floor, there will not be adequate resistance tothere will not be adequate resistance to rotating forces. The buccal and lingual wallsrotating forces. The buccal and lingual walls must meet the pulpal wall at angles near 90must meet the pulpal wall at angles near 90 degrees so that these walls will bedegrees so that these walls will be perpendicular to any forces which tend toperpendicular to any forces which tend to rotate the restoration.rotate the restoration. www.indiandentalacademy.comwww.indiandentalacademy.com
  40. 40. Length:Length: Occlusogingival length is an importantOcclusogingival length is an important factor in both retention and resistance. Longerfactor in both retention and resistance. Longer preparations will have more surface area and willpreparations will have more surface area and will therefore be more retentive. Because the axial walltherefore be more retentive. Because the axial wall occlusal to the finish line interferes withocclusal to the finish line interferes with displacement, the length and inclination of that walldisplacement, the length and inclination of that wall become factors in resistance to tipping forces. Forbecome factors in resistance to tipping forces. For the restoration to succeed, the length must bethe restoration to succeed, the length must be great enough to interfere with the arc of the castinggreat enough to interfere with the arc of the casting pivoting about a point on the margin on thepivoting about a point on the margin on the opposite side of the restoration.opposite side of the restoration. www.indiandentalacademy.comwww.indiandentalacademy.com
  41. 41.  The preparation with longer walls interferes withThe preparation with longer walls interferes with the tipping displacement of the restoration betterthe tipping displacement of the restoration better than the short preparation.than the short preparation. www.indiandentalacademy.comwww.indiandentalacademy.com
  42. 42.  A preparation on a tooth with a smaller diameterA preparation on a tooth with a smaller diameter resists pivoting movements better than aresists pivoting movements better than a preparation of equal length on a tooth of largerpreparation of equal length on a tooth of larger diameter because smaller teeth will have a shortdiameter because smaller teeth will have a short rotational radius for the arc of displacement androtational radius for the arc of displacement and the incisal portion of the axial wall will resistthe incisal portion of the axial wall will resist displacementdisplacement.. www.indiandentalacademy.comwww.indiandentalacademy.com
  43. 43.  Resistance to displacement for a short walledResistance to displacement for a short walled preparation on a large tooth can be improved by placingpreparation on a large tooth can be improved by placing grooves in the axial walls. In effect, this reduces thegrooves in the axial walls. In effect, this reduces the rotational radius, and that portion of the walls of therotational radius, and that portion of the walls of the grooves near the occlusal surface of the preparation thatgrooves near the occlusal surface of the preparation that will interfere with displacement.will interfere with displacement. www.indiandentalacademy.comwww.indiandentalacademy.com
  44. 44. Substitution of internal features:Substitution of internal features: The basic unit of retention for a cementedThe basic unit of retention for a cemented restoration is two opposing axial walls with arestoration is two opposing axial walls with a minimal taper. It may not always be possible tominimal taper. It may not always be possible to use opposing walls for retention: one may haveuse opposing walls for retention: one may have been destroyed previously, or it may be desirablebeen destroyed previously, or it may be desirable to leave a surface uncovered for a partial veneerto leave a surface uncovered for a partial veneer restoration. It may also be that the walls arerestoration. It may also be that the walls are present, but with a greater than desirablepresent, but with a greater than desirable inclination. At such times, internal features such asinclination. At such times, internal features such as the groove, the box form and the pin hole can bethe groove, the box form and the pin hole can be substituted for an axial wall or for each other.substituted for an axial wall or for each other. www.indiandentalacademy.comwww.indiandentalacademy.com
  45. 45. www.indiandentalacademy.comwww.indiandentalacademy.com
  46. 46. The forces to be applied by the opposingThe forces to be applied by the opposing occlusion in the situations depicted in (a)occlusion in the situations depicted in (a) would dictate a preparation with relativelywould dictate a preparation with relatively strict adherence to the basic principles ofstrict adherence to the basic principles of length and taper.length and taper. www.indiandentalacademy.comwww.indiandentalacademy.com
  47. 47. The lesser angle of the application of force in theThe lesser angle of the application of force in the situations in (b) would permit the use of asituations in (b) would permit the use of a preparation design with less retentive features, aspreparation design with less retentive features, as seen in (c). This conclusion is based on theseen in (c). This conclusion is based on the premise that once a crown is cemented on a toothpremise that once a crown is cemented on a tooth preparation, the retention will be adequate untilpreparation, the retention will be adequate until such times as a force is applied with propersuch times as a force is applied with proper magnitude and direction to overcome themagnitude and direction to overcome the resistance to that force. The resistance to forceresistance to that force. The resistance to force referred to here is that which is afforded by thereferred to here is that which is afforded by the preparation design, the fit of the casting on thepreparation design, the fit of the casting on the preparation, and the cementing medium. If thispreparation, and the cementing medium. If this combination of factors proves inadequate to resistcombination of factors proves inadequate to resist the force, retention will be lost. Conversely, ifthe force, retention will be lost. Conversely, if these factors are adequate for a given applicationthese factors are adequate for a given application of forces, retention will endure.of forces, retention will endure.www.indiandentalacademy.comwww.indiandentalacademy.com
  48. 48. Path of insertion:Path of insertion: The path of insertion must be an imaginaryThe path of insertion must be an imaginary line along which the restoration will be placed ontoline along which the restoration will be placed onto or removed from the restoration. It is of specialor removed from the restoration. It is of special importance when preparing teeth to be fixed partialimportance when preparing teeth to be fixed partial denture abutments, since the paths of all thedenture abutments, since the paths of all the abutment preparations must be parallel to eachabutment preparations must be parallel to each other. The path of insertion must be considered inother. The path of insertion must be considered in two dimensions: faciolingually and mesiodistally.two dimensions: faciolingually and mesiodistally. The faciolingual orientation of the path can affectThe faciolingual orientation of the path can affect the esthetics of metal-ceramic or partial veneerthe esthetics of metal-ceramic or partial veneer crowns. The mesiodistal inclination of the pathcrowns. The mesiodistal inclination of the path must be parallel to the contact areas of themust be parallel to the contact areas of the adjacent teethadjacent teethwww.indiandentalacademy.comwww.indiandentalacademy.com
  49. 49. Few forces applied to teeth can dislodge aFew forces applied to teeth can dislodge a restoration in an occlusal direction. Sticky foods,restoration in an occlusal direction. Sticky foods, chewing gum, and the like have been known tochewing gum, and the like have been known to remove restorations in the line of draw. However,remove restorations in the line of draw. However, these situations are relatively rare and usuallythese situations are relatively rare and usually occur after prior fracturing of the cement film whichoccur after prior fracturing of the cement film which results in a loss of retention. Most restorationsresults in a loss of retention. Most restorations that are properly designed and that fit wellthat are properly designed and that fit well generally are not subject to this type of occlusalgenerally are not subject to this type of occlusal failure.failure. www.indiandentalacademy.comwww.indiandentalacademy.com
  50. 50. The primary function of a crown placed on a toothThe primary function of a crown placed on a tooth is to restore lost contour and function, but we mustis to restore lost contour and function, but we must also consider the necessity for the crown toalso consider the necessity for the crown to distribute forces and loads applied to it in adistribute forces and loads applied to it in a manner that will assure its ability to satisfy themanner that will assure its ability to satisfy the primary purpose over a reasonably long period ofprimary purpose over a reasonably long period of time. If a crown were placed on a tooth andtime. If a crown were placed on a tooth and expected only to serve this function without beingexpected only to serve this function without being subjected to any external forces, all crowns wouldsubjected to any external forces, all crowns would look much more similar than they in fact do.look much more similar than they in fact do. However, we known this not to be the case, andHowever, we known this not to be the case, and therefore find it necessary to logically andtherefore find it necessary to logically and predictable adjust the previously mentionedpredictable adjust the previously mentioned mechanism of retention to meet the needs of amechanism of retention to meet the needs of a great variety of stresses and strains.great variety of stresses and strains.www.indiandentalacademy.comwww.indiandentalacademy.com
  51. 51. The magnitude and quality of the retentive effect ofThe magnitude and quality of the retentive effect of a restoration should resist the forces that will bea restoration should resist the forces that will be applied to it in specific clinical situations.applied to it in specific clinical situations. Therefore, a preparation on the same tooth in twoTherefore, a preparation on the same tooth in two different patients could look quite different, yetdifferent patients could look quite different, yet both could be entirely satisfactory. Furthermore,both could be entirely satisfactory. Furthermore, the degree of retention afforded by fit of one crownthe degree of retention afforded by fit of one crown might be twice as great as another or strongermight be twice as great as another or stronger cement may be used on one case than on another,cement may be used on one case than on another, and yet both could perfectly adequate for theand yet both could perfectly adequate for the respective situations.respective situations. www.indiandentalacademy.comwww.indiandentalacademy.com
  52. 52. Protection of tooth structureProtection of tooth structure In the area of protection of the remainingIn the area of protection of the remaining tooth structure, the prosthodontist is faced withtooth structure, the prosthodontist is faced with even more variables than those of retention andeven more variables than those of retention and resistance. It is also a fact that serious errors inresistance. It is also a fact that serious errors in judgement here would more likely result in loss ofjudgement here would more likely result in loss of the tooth at some future time. Although the mostthe tooth at some future time. Although the most probable failure that would occur if retention isprobable failure that would occur if retention is inadequate would be loss of the crown from theinadequate would be loss of the crown from the tooth, with no real damage to the remaining tooth,tooth, with no real damage to the remaining tooth, the most likely failure in the case of unprotectedthe most likely failure in the case of unprotected tooth structure, which obviously is a much moretooth structure, which obviously is a much more serious type of failure.serious type of failure. www.indiandentalacademy.comwww.indiandentalacademy.com
  53. 53. Important factors to be considered related toImportant factors to be considered related to protection of the existing tooth structure are:protection of the existing tooth structure are: Amount of enamel supported by adequate viableAmount of enamel supported by adequate viable dentindentin Amount of adequate viable tooth structure after theAmount of adequate viable tooth structure after the preparation for the restoration is made.preparation for the restoration is made. Accordingly, the first step in any crownAccordingly, the first step in any crown preparation should be to remove all old restorativepreparation should be to remove all old restorative material and new carious lesions. At this point inmaterial and new carious lesions. At this point in the procedure, nothing should be left but soundthe procedure, nothing should be left but sound dentin and enamel supported by dentin.dentin and enamel supported by dentin. www.indiandentalacademy.comwww.indiandentalacademy.com
  54. 54. The importance of the principle of adequateThe importance of the principle of adequate removal of carious lesion cannot be overremoval of carious lesion cannot be over emphasized. An apparent minimally damagedemphasized. An apparent minimally damaged tooth presents a totally different situation after thetooth presents a totally different situation after the clean out procedure. The remaining toothclean out procedure. The remaining tooth structure must be the primary concern in thestructure must be the primary concern in the design decision making process, not the falsedesign decision making process, not the false appearance of the tooth previous to removal of theappearance of the tooth previous to removal of the unsupported enamel of the occlusal surface.unsupported enamel of the occlusal surface. www.indiandentalacademy.comwww.indiandentalacademy.com
  55. 55. As is often seen in these situations where theAs is often seen in these situations where the carious lesion was initiated in the occlusal groovescarious lesion was initiated in the occlusal grooves of a posterior tooth, the structure remaining for useof a posterior tooth, the structure remaining for use in developing a preparations is quite different afterin developing a preparations is quite different after all of the unsupported overlying enamel is removedall of the unsupported overlying enamel is removed and the carious lesion is excavated.and the carious lesion is excavated. www.indiandentalacademy.comwww.indiandentalacademy.com
  56. 56. The second step must be to establish the correctThe second step must be to establish the correct occlusal clearance for the restoration to beocclusal clearance for the restoration to be constructed on the tooth. By definition, any crownconstructed on the tooth. By definition, any crown requires occlusal clearance since coverage of thisrequires occlusal clearance since coverage of this surface is a part of any such restoration exceptsurface is a part of any such restoration except inlays. Therefore, it follows that before anyinlays. Therefore, it follows that before any decisions can be made regarding other features ofdecisions can be made regarding other features of the preparation, the occlusal clearance must bethe preparation, the occlusal clearance must be created to properly visualize the parameters withcreated to properly visualize the parameters with which these decisions must be made.which these decisions must be made. www.indiandentalacademy.comwww.indiandentalacademy.com
  57. 57. The next step is to determine whether thisThe next step is to determine whether this remaining dentin and the enamel supported by itremaining dentin and the enamel supported by it will still be available after the axial surfaces arewill still be available after the axial surfaces are prepared for the type of crown being done on theprepared for the type of crown being done on the tooth. This step seems to be one of the moretooth. This step seems to be one of the more frequently over looked aspects of preparationfrequently over looked aspects of preparation planning and design. Continuing consideration ofplanning and design. Continuing consideration of the molar seen, one may assume that it requires athe molar seen, one may assume that it requires a full crown.full crown. www.indiandentalacademy.comwww.indiandentalacademy.com
  58. 58. One can see that after completion ofOne can see that after completion of the aforementioned cleanoutthe aforementioned cleanout procedure (left) and occlusal clearanceprocedure (left) and occlusal clearance (right) the remaining dentin one the(right) the remaining dentin one the axial walls is minimal to say the least.axial walls is minimal to say the least. Only enamel that is supported byOnly enamel that is supported by adequate sound dentin will provideadequate sound dentin will provide strength in the resulting preparation. Itstrength in the resulting preparation. It is the dentin, due to its elasticity,is the dentin, due to its elasticity, greater toughness, and lessergreater toughness, and lesser brittleness, that provides the soundbrittleness, that provides the sound foundation needed for castfoundation needed for cast restorations, not the enamel.restorations, not the enamel.www.indiandentalacademy.comwww.indiandentalacademy.com
  59. 59. After reduction of these two axial surfaces there isAfter reduction of these two axial surfaces there is not adequate dentin remaining to allownot adequate dentin remaining to allow development of retention and resistance form anddevelopment of retention and resistance form and still have sound tooth structure that would not bestill have sound tooth structure that would not be prone to failure, even under the most minimalprone to failure, even under the most minimal loading. Therefore auxiliary intra coronal retentionloading. Therefore auxiliary intra coronal retention such as pin retained build ups or retentive bases issuch as pin retained build ups or retentive bases is required.required. www.indiandentalacademy.comwww.indiandentalacademy.com
  60. 60. The essential point to be considered is that theThe essential point to be considered is that the operator must visualize the remaining soundoperator must visualize the remaining sound dentin in the context of the final restoration. Todentin in the context of the final restoration. To carry this principle further, consider the result if thecarry this principle further, consider the result if the tooth required even greater reduction of the lingualtooth required even greater reduction of the lingual surface to attain a particular line of draw notsurface to attain a particular line of draw not commensurate with savings as much toothcommensurate with savings as much tooth structure as possible.structure as possible. www.indiandentalacademy.comwww.indiandentalacademy.com
  61. 61. One situation in which we often find the remainingOne situation in which we often find the remaining tooth structure predisposed to failure is the case oftooth structure predisposed to failure is the case of endodontically treated teeth. Let us first considerendodontically treated teeth. Let us first consider posterior teeth, which is generally where greaterposterior teeth, which is generally where greater forces are applied to the occlusal surfaces as wellforces are applied to the occlusal surfaces as well as where more controversy seems to exist at theas where more controversy seems to exist at the point of preparation and restoration design. As apoint of preparation and restoration design. As a basic principle, it would seem reasonable tobasic principle, it would seem reasonable to suggest that any posterior tooth that has hadsuggest that any posterior tooth that has had endodontic treatment and has had bothendodontic treatment and has had both interproximal surfaces involved with cariousinterproximal surfaces involved with carious lesions or previous restorations should ultimatelylesions or previous restorations should ultimately be restored by casting that provides full occlusalbe restored by casting that provides full occlusal coverage.coverage.www.indiandentalacademy.comwww.indiandentalacademy.com
  62. 62. Regarding the aforementioned principle thatRegarding the aforementioned principle that protection of remaining tooth structure isprotection of remaining tooth structure is predicated on the presence of sound dentin, it is apredicated on the presence of sound dentin, it is a fact that in those teeth that have undergonefact that in those teeth that have undergone endodontic treatment, there has often beenendodontic treatment, there has often been removal of a significant amount of this all importantremoval of a significant amount of this all important dentin in the area between the pulpal floor of thedentin in the area between the pulpal floor of the previous restorations (or carious lesion) and theprevious restorations (or carious lesion) and the superior aspect of the pulp chamber.superior aspect of the pulp chamber. www.indiandentalacademy.comwww.indiandentalacademy.com
  63. 63. Depending on the ability of the operator performingDepending on the ability of the operator performing the endodontic access opening as well as thethe endodontic access opening as well as the presence of anomalies of the position of thepresence of anomalies of the position of the chamber, the access opening may significantlychamber, the access opening may significantly reduce the strength of the remaining cusps andreduce the strength of the remaining cusps and their ability to withstand lateral forces. It is cleartheir ability to withstand lateral forces. It is clear that a fracture in such a situation is considerablythat a fracture in such a situation is considerably more likely than if the area above the pulpmore likely than if the area above the pulp chamber were still sound dentin.chamber were still sound dentin. www.indiandentalacademy.comwww.indiandentalacademy.com
  64. 64. Integrity of the restorationIntegrity of the restoration The fixed restoration must be able toThe fixed restoration must be able to withstand forces generated by centric contact,withstand forces generated by centric contact, eccentric movements, mastication against hardeccentric movements, mastication against hard and soft foods, and mild accidental trauma. Theand soft foods, and mild accidental trauma. The chosen restorative material should not deformchosen restorative material should not deform permanently or fail under these conditions.permanently or fail under these conditions. In terms of centric contact, it is generally anIn terms of centric contact, it is generally an easier matter to design and execute the restorationeasier matter to design and execute the restoration in a way that will adequately resist excessivein a way that will adequately resist excessive deformation and therefore any possibility ofdeformation and therefore any possibility of ultimate failure of the material.ultimate failure of the material. www.indiandentalacademy.comwww.indiandentalacademy.com
  65. 65. Only an occasional error in casting or preparationOnly an occasional error in casting or preparation reduction on the occlusal surface will result in areduction on the occlusal surface will result in a casting that will later fail due to inadequate amountcasting that will later fail due to inadequate amount of material on the occlusal surface. However,of material on the occlusal surface. However, often insufficient attention is paid to the problem ofoften insufficient attention is paid to the problem of potential wear when one decides how muchpotential wear when one decides how much occlusal clearance is needed in a given situation.occlusal clearance is needed in a given situation. Normally, it is accepted that about 1 to 1.5mm isNormally, it is accepted that about 1 to 1.5mm is adequate for most situations. However, in casesadequate for most situations. However, in cases where some wear in anticipated this amount maywhere some wear in anticipated this amount may not be enough clearance and 2 or 2.5 mm mightnot be enough clearance and 2 or 2.5 mm might be needed.be needed. www.indiandentalacademy.comwww.indiandentalacademy.com
  66. 66. Consideration also must be given to theConsideration also must be given to the restorations in eccentric movements, because anrestorations in eccentric movements, because an increasing number of cast metal restorations haveincreasing number of cast metal restorations have become subject to wear due to chronic bruxism. Inbecome subject to wear due to chronic bruxism. In spite of attempts to control this problem so thatspite of attempts to control this problem so that wear does not occur on the occlusal surfaces ofwear does not occur on the occlusal surfaces of the teeth, these attempts are not alwaysthe teeth, these attempts are not always successful. The operator should take the problemsuccessful. The operator should take the problem of chronic bruxism into account when designingof chronic bruxism into account when designing restorations. Besides wear, there are otherrestorations. Besides wear, there are other considerations. The force, and therefore, theconsiderations. The force, and therefore, the resulting strain, induced in the casting in such aresulting strain, induced in the casting in such a case can be of considerable magnitude.case can be of considerable magnitude. www.indiandentalacademy.comwww.indiandentalacademy.com
  67. 67. As a result of wear the occlusal contacts often takeAs a result of wear the occlusal contacts often take on the form of very flat surfaces, no longeron the form of very flat surfaces, no longer possessing the ideal cuspal form that will providepossessing the ideal cuspal form that will provide for efficient cutting interaction with the food bolus.for efficient cutting interaction with the food bolus. The flatter occlusal contacts can cause aThe flatter occlusal contacts can cause a decreased effectiveness of the masticatingdecreased effectiveness of the masticating surfaces and a concomitant increase in forcesurfaces and a concomitant increase in force needed to properly masticate a bolus of food. It isneeded to properly masticate a bolus of food. It is also possible to have enough wear to result in aalso possible to have enough wear to result in a perforation through the casting into the cement filmperforation through the casting into the cement film and then into the underlying dentin.and then into the underlying dentin. www.indiandentalacademy.comwww.indiandentalacademy.com
  68. 68. Another consideration is the possibility ofAnother consideration is the possibility of deformation of the casting due to these forces.deformation of the casting due to these forces. The restoration could, and often does, loseThe restoration could, and often does, lose retention because the deformation of the castingretention because the deformation of the casting leads to either adhesive or cohesive failure of theleads to either adhesive or cohesive failure of the cement layer. Exceeding the yield strength of thecement layer. Exceeding the yield strength of the casting can result in an open margin and recurrentcasting can result in an open margin and recurrent caries. If the restoration in question is porcelaincaries. If the restoration in question is porcelain fused to metal crown or bridge, excessivefused to metal crown or bridge, excessive deformation will result in failure of the porcelaindeformation will result in failure of the porcelain bond or in a fracture of the porcelain at somebond or in a fracture of the porcelain at some point. Studies using stress analysis havepoint. Studies using stress analysis have demonstrated the need for proper frameworkdemonstrated the need for proper framework design, particularly related to the problem ofdesign, particularly related to the problem of resisting occlusal forces.resisting occlusal forces.www.indiandentalacademy.comwww.indiandentalacademy.com
  69. 69. Flexure of long span bridges during masticationFlexure of long span bridges during mastication can be a problem. A very long span may presentcan be a problem. A very long span may present no real problem in terms of flexure during centricno real problem in terms of flexure during centric closure with no food in the mouth, because there isclosure with no food in the mouth, because there is distribution of forces among all the occlusaldistribution of forces among all the occlusal surfaces around the arch. However, when there issurfaces around the arch. However, when there is a bolus of food interposed between the pontic areaa bolus of food interposed between the pontic area of an excessively long span bridge and it’sof an excessively long span bridge and it’s opposing occlusal surfaces, the effect can be quiteopposing occlusal surfaces, the effect can be quite different since the occlusal contacts on thedifferent since the occlusal contacts on the opposite side of the arch have not yet come intoopposite side of the arch have not yet come into play. In this case, all of the force is concentratedplay. In this case, all of the force is concentrated on the bridge span, inducing a strain that theon the bridge span, inducing a strain that the restoration and / or the abutment teeth may not berestoration and / or the abutment teeth may not be capable of withstanding.capable of withstanding.www.indiandentalacademy.comwww.indiandentalacademy.com
  70. 70. In these situations, the case should be designed toIn these situations, the case should be designed to provide adequate beam strength in casting. Forprovide adequate beam strength in casting. For example: it would be questionable to construct aexample: it would be questionable to construct a bridge from the mandibular first premolar to thebridge from the mandibular first premolar to the mandibular third molar when the available intermandibular third molar when the available inter occlusal distance is only 2 mm. Such a casting isocclusal distance is only 2 mm. Such a casting is of such a long span that very high flexural stressesof such a long span that very high flexural stresses would be generated even by the low loadswould be generated even by the low loads generated during mastication of a bolus of softgenerated during mastication of a bolus of soft food. Then, we must consider the possibility of afood. Then, we must consider the possibility of a rather hard object suddenly finding it ways into therather hard object suddenly finding it ways into the interocclusal space, such as a piece of bone.interocclusal space, such as a piece of bone. www.indiandentalacademy.comwww.indiandentalacademy.com
  71. 71. The problem in these situations is that the forceThe problem in these situations is that the force with which the masticatory muscles are functioningwith which the masticatory muscles are functioning at that moment is the force that was needed andat that moment is the force that was needed and that felt comfortable to the individual for thethat felt comfortable to the individual for the purpose of chewing the soft food. When suddenlypurpose of chewing the soft food. When suddenly and unexpectedly a small hard object is interposedand unexpectedly a small hard object is interposed between the occlusal surfaces, tremendous forcesbetween the occlusal surfaces, tremendous forces is concentrated at that point. By the time ofis concentrated at that point. By the time of proprioceptive or pain reflexes of the individual canproprioceptive or pain reflexes of the individual can take over and stop the action of the neuromusculartake over and stop the action of the neuromuscular complex, the damage is often done. The resultcomplex, the damage is often done. The result may be loss of cement bond, porcelain fracture,may be loss of cement bond, porcelain fracture, failure of a build up under a retainer, failure of afailure of a build up under a retainer, failure of a connector, and so forth.connector, and so forth. www.indiandentalacademy.comwww.indiandentalacademy.com
  72. 72. When designing dental restorations of any type,When designing dental restorations of any type, we are generally concerned only with resistance towe are generally concerned only with resistance to those forces generated within the stomatognathicthose forces generated within the stomatognathic system. It is not possible to predict damage to oursystem. It is not possible to predict damage to our restorations from such causes as automobilerestorations from such causes as automobile accidents, or blows, but there are a limited numberaccidents, or blows, but there are a limited number of situations where the planning for suchof situations where the planning for such eventualities is at least to some extent aeventualities is at least to some extent a possibility. In these instances, the design ofpossibility. In these instances, the design of restorations can take into account certain type ofrestorations can take into account certain type of potential damage and therefore obviate the needpotential damage and therefore obviate the need for a new restoration, possibly a more extensivefor a new restoration, possibly a more extensive one.one. www.indiandentalacademy.comwww.indiandentalacademy.com
  73. 73. One such situation would be that of the epilepticOne such situation would be that of the epileptic patient who is subject to seizures. These patientspatient who is subject to seizures. These patients often have missing maxillary or mandibularoften have missing maxillary or mandibular anterior teeth, lost in just such a seizure in whichanterior teeth, lost in just such a seizure in which the patient fell. In these cases, it is often wise tothe patient fell. In these cases, it is often wise to modify the usual treatment to avoid use ofmodify the usual treatment to avoid use of porcelain fused to metal restoration. Such aporcelain fused to metal restoration. Such a restoration is prone to fracture when subject torestoration is prone to fracture when subject to impact, and if it does not fracture, there is the evenimpact, and if it does not fracture, there is the even more serious risk of fracture of the teeth. In suchmore serious risk of fracture of the teeth. In such situations, it is usually wise to consider the use ofsituations, it is usually wise to consider the use of a more flexible gold alloy with plastic facing. Thea more flexible gold alloy with plastic facing. The restoration can be designed and constructed inrestoration can be designed and constructed in such a manner that the facing could easily besuch a manner that the facing could easily be replaced in the mouth.replaced in the mouth.www.indiandentalacademy.comwww.indiandentalacademy.com
  74. 74. This same type of contingency planning canThis same type of contingency planning can logically be applied to those who are regularlylogically be applied to those who are regularly engaged in sports, where it can be predicted thatengaged in sports, where it can be predicted that they are likely to repeatedly encounter this type ofthey are likely to repeatedly encounter this type of trauma.trauma. www.indiandentalacademy.comwww.indiandentalacademy.com
  75. 75. Pontic SelectionPontic Selection The pontic is the part of the restoration thatThe pontic is the part of the restoration that replaces the missing tooth. From a biomechanicalreplaces the missing tooth. From a biomechanical view point, pontic present some unique problemsview point, pontic present some unique problems that must be considered in the design andthat must be considered in the design and construction of these restorations. First, there isconstruction of these restorations. First, there is less bone support for a given number ofless bone support for a given number of functioning occlusal surfaces. It is thereforefunctioning occlusal surfaces. It is therefore advisable to increase the efficiency of the occlusaladvisable to increase the efficiency of the occlusal surface as a masticatory device.surface as a masticatory device. www.indiandentalacademy.comwww.indiandentalacademy.com
  76. 76. This task usually involves creating an occlusalThis task usually involves creating an occlusal form that has the following features:form that has the following features: Maximum angle of cusp inclines,Maximum angle of cusp inclines, Narrow cusp ridges,Narrow cusp ridges, Sharp cusp tips,Sharp cusp tips, A greater number of small occlusal contacts.A greater number of small occlusal contacts. www.indiandentalacademy.comwww.indiandentalacademy.com
  77. 77. As can be seen in the cusp form on the left wouldAs can be seen in the cusp form on the left would tend to be more efficient. The same magnitudetend to be more efficient. The same magnitude applied to the occlusal surfaces of the teeth by theapplied to the occlusal surfaces of the teeth by the elevating musculature can cut through the bolus ofelevating musculature can cut through the bolus of food easier if the cusps are narrower and sharperfood easier if the cusps are narrower and sharper because the available force is more concentrated.because the available force is more concentrated. www.indiandentalacademy.comwww.indiandentalacademy.com
  78. 78. The degree to which modifications to the presentThe degree to which modifications to the present occlusal scheme of the patient can beocclusal scheme of the patient can be accomplished depends on the controlling factors inaccomplished depends on the controlling factors in the occlusion, such as the amount and angle ofthe occlusion, such as the amount and angle of canine disocclusion, the angle of the eminentia,canine disocclusion, the angle of the eminentia, and the amount of enamel available forand the amount of enamel available for adjustment. Long spans and periodontallyadjustment. Long spans and periodontally involved cases are in particular need of efficientinvolved cases are in particular need of efficient occlusal design but only to the point that no lateralocclusal design but only to the point that no lateral or protrusive interferences are introduced.or protrusive interferences are introduced. www.indiandentalacademy.comwww.indiandentalacademy.com
  79. 79. Any potential gain in efficiency that was attainedAny potential gain in efficiency that was attained while causing traumatic occlusion would not bewhile causing traumatic occlusion would not be justified. An attempt should not be made to reducejustified. An attempt should not be made to reduce the magnitude of the load on a posterior prosthesisthe magnitude of the load on a posterior prosthesis pontic by lightening the occlusal contacts. Whenpontic by lightening the occlusal contacts. When the occlusal contacts are lightened, the opposingthe occlusal contacts are lightened, the opposing tooth usually supererupts into the same tighttooth usually supererupts into the same tight occlusion in centric as exists on the abutmentocclusion in centric as exists on the abutment teeth.teeth. www.indiandentalacademy.comwww.indiandentalacademy.com
  80. 80. Another factor in pontic design is the width of theAnother factor in pontic design is the width of the occlusal table. The faciolingual width of theocclusal table. The faciolingual width of the occlusal table is the portion of the occlusal surfaceocclusal table is the portion of the occlusal surface delineated by the occlusal contacts. This widthdelineated by the occlusal contacts. This width should be made as narrow as possible to create ashould be made as narrow as possible to create a greater concentration of force where the work isgreater concentration of force where the work is being done. There is limitation to attaining thisbeing done. There is limitation to attaining this goal since the width of the occlusal table isgoal since the width of the occlusal table is dictated by the anatomy of the opposing dentition.dictated by the anatomy of the opposing dentition. www.indiandentalacademy.comwww.indiandentalacademy.com
  81. 81. The occlusal table of the pontic may be narrowed ifThe occlusal table of the pontic may be narrowed if a cast restoration is indicated on the opposinga cast restoration is indicated on the opposing tooth and if it is possible to narrow the distancetooth and if it is possible to narrow the distance between the contacts by equilibration. Anybetween the contacts by equilibration. Any adjustment in the design of the pontic that wouldadjustment in the design of the pontic that would distribute the force at the site of the work over adistribute the force at the site of the work over a smaller total area of the occlusal surface of thesmaller total area of the occlusal surface of the pontic would create a better concentration of force.pontic would create a better concentration of force. www.indiandentalacademy.comwww.indiandentalacademy.com
  82. 82. Over all, faciolingual width is another importantOver all, faciolingual width is another important feature of pontic design. This width refers to thefeature of pontic design. This width refers to the greatest faciolingual width of the pontic, which isgreatest faciolingual width of the pontic, which is usually gingival to the occlusal table. It should beusually gingival to the occlusal table. It should be a basic goal of all posterior pontic design to keepa basic goal of all posterior pontic design to keep the overall faciolingual width as narrow as possiblethe overall faciolingual width as narrow as possible to promote oral hygiene in the area of the pontic.to promote oral hygiene in the area of the pontic. www.indiandentalacademy.comwww.indiandentalacademy.com
  83. 83. The smaller the total surface of the pontic thatThe smaller the total surface of the pontic that faces the residual ridge, the better will be thefaces the residual ridge, the better will be the ability of the patient to clean this surface ofability of the patient to clean this surface of bacterial plaque. By making the faciolingual widthbacterial plaque. By making the faciolingual width of the pontic as narrow as possible, the operatorof the pontic as narrow as possible, the operator may also make a greater percentage of the facialmay also make a greater percentage of the facial and lingual surfaces more vertical and thereforeand lingual surfaces more vertical and therefore more easily reached by the tooth brush or othermore easily reached by the tooth brush or other aids.aids. Finally, the forces applied by the pontic onFinally, the forces applied by the pontic on the residual ridge must be considered. There havethe residual ridge must be considered. There have been various schools of thought concerning thisbeen various schools of thought concerning this aspect of pontic design.aspect of pontic design. www.indiandentalacademy.comwww.indiandentalacademy.com
  84. 84. These opinions range from placing the pontic in aThese opinions range from placing the pontic in a very definite positive contact with the ridge tissuevery definite positive contact with the ridge tissue to leaving a 2 to 3 mm space between the ponticto leaving a 2 to 3 mm space between the pontic and the ridge. Virtually all possibilities betweenand the ridge. Virtually all possibilities between these extremes have been advocated. It is easy tothese extremes have been advocated. It is easy to understand that the pontic that does not touch theunderstand that the pontic that does not touch the tissue cannot directly apply any force to the ridge.tissue cannot directly apply any force to the ridge. This design is most often accomplished to promoteThis design is most often accomplished to promote the best possible hygiene. At the other extreme,the best possible hygiene. At the other extreme, the pontic that is placed in heavy contact with thethe pontic that is placed in heavy contact with the ridge tissues is going to apply a direct force on theridge tissues is going to apply a direct force on the tissue and then to the underlying bone and itstissue and then to the underlying bone and its periosteum.periosteum. www.indiandentalacademy.comwww.indiandentalacademy.com
  85. 85. This type of pontic is no longer advocated to anyThis type of pontic is no longer advocated to any great degree because of its poor oral hygienegreat degree because of its poor oral hygiene potential. Probably the greatest controversypotential. Probably the greatest controversy regarding this problem in recent years has beenregarding this problem in recent years has been whether to place the pontic in very light contact orwhether to place the pontic in very light contact or just out of contact with the ridge tissue. Althoughjust out of contact with the ridge tissue. Although the great majority of bridge pontics placed in lightthe great majority of bridge pontics placed in light contact with the tissue show no changes in eithercontact with the tissue show no changes in either tissue or bone, occasionally one can see thetissue or bone, occasionally one can see the proliferation of bone. It is advisable to createproliferation of bone. It is advisable to create pontics that do not actually contact the tissue frompontics that do not actually contact the tissue from the stand point of both oral hygiene andthe stand point of both oral hygiene and transmission of detrimental forces to the residualtransmission of detrimental forces to the residual ridge.ridge. www.indiandentalacademy.comwww.indiandentalacademy.com
  86. 86. ConnectorsConnectors All fixed bridges must be united by someAll fixed bridges must be united by some type of connector that must satisfy certaintype of connector that must satisfy certain structural requirements. It must provide enoughstructural requirements. It must provide enough strength to resist forces of occlusion that causestrength to resist forces of occlusion that cause flexure of the joint, producing stress in the solder,flexure of the joint, producing stress in the solder, the interface, and the parent casting. Functionalthe interface, and the parent casting. Functional forces applied to the pontic result in a more severeforces applied to the pontic result in a more severe stress condition than when the patient closes intostress condition than when the patient closes into centric occlusion without a bolus of food. In centriccentric occlusion without a bolus of food. In centric closure the force is uniformly shared by the ponticclosure the force is uniformly shared by the pontic and the retainers.and the retainers. www.indiandentalacademy.comwww.indiandentalacademy.com
  87. 87. Studies have demonstrated that the greatestStudies have demonstrated that the greatest chewing action takes place with the bolus over thechewing action takes place with the bolus over the second premolar and first molar, with the foodsecond premolar and first molar, with the food being masticated primarily over the premolar atbeing masticated primarily over the premolar at first, and then gradually move farther posteriorly asfirst, and then gradually move farther posteriorly as the degradation of the bolus progresses. Since thethe degradation of the bolus progresses. Since the restorations in question are usually replacing onerestorations in question are usually replacing one or more posterior teeth in the area of greatestor more posterior teeth in the area of greatest chewing function, often a force will be applied tochewing function, often a force will be applied to the pontics alone by the bolus. The magnitude asthe pontics alone by the bolus. The magnitude as well as the concentration of force can be quitewell as the concentration of force can be quite great and may exceed the ability of the abutmentsgreat and may exceed the ability of the abutments to adequately resist it, resulting in failure. Thisto adequately resist it, resulting in failure. This failure often takes place at one of the connectors,failure often takes place at one of the connectors, which is the thinnest and therefore, the weakestwhich is the thinnest and therefore, the weakest link in the restorations.link in the restorations.www.indiandentalacademy.comwww.indiandentalacademy.com
  88. 88. In anterior bridges it is of paramount importance toIn anterior bridges it is of paramount importance to design the connectors so that they are esthetic,design the connectors so that they are esthetic, that is, so they appear as close as possible to athat is, so they appear as close as possible to a natural embrasure between two separate teeth. Tonatural embrasure between two separate teeth. To provide adequate thickness of porcelain in theprovide adequate thickness of porcelain in the area of facial embrasure, one must provide thearea of facial embrasure, one must provide the clearance at the expense of what might beclearance at the expense of what might be considered a more ideal amount of metal in theconsidered a more ideal amount of metal in the area. It is also necessary to provide natural lingualarea. It is also necessary to provide natural lingual embrasure form for phonetics, patient comfort andembrasure form for phonetics, patient comfort and hygiene potential. All of these requirements tendhygiene potential. All of these requirements tend to limit the faciolingual thickness available.to limit the faciolingual thickness available. www.indiandentalacademy.comwww.indiandentalacademy.com
  89. 89. The other variable in the equation is incisogingivalThe other variable in the equation is incisogingival dimension of the connector. Again, we are limiteddimension of the connector. Again, we are limited by functional, esthetic, and hygieneby functional, esthetic, and hygiene considerations. It is necessary from an estheticconsiderations. It is necessary from an esthetic stand point to create both an incisal and a gingivalstand point to create both an incisal and a gingival embrasure that will match the contra lateral ones.embrasure that will match the contra lateral ones. The classic exception is the embrasure betweenThe classic exception is the embrasure between the two central incisors. Since there is no contrathe two central incisors. Since there is no contra lateral embrasure, there is a little more freedom.lateral embrasure, there is a little more freedom. www.indiandentalacademy.comwww.indiandentalacademy.com
  90. 90. The height of the gingival tissue in the areaThe height of the gingival tissue in the area between the teeth will limit the incisogingival heightbetween the teeth will limit the incisogingival height of the connector. This excess tissue can often beof the connector. This excess tissue can often be adjusted using conventional or electrosurgicaladjusted using conventional or electrosurgical methods. Connector design in anterior bridges ismethods. Connector design in anterior bridges is dependent on esthetic considerations as well asdependent on esthetic considerations as well as structural ones. Other means should bestructural ones. Other means should be considered before relying on increased size toconsidered before relying on increased size to solve the problem of strength.solve the problem of strength. www.indiandentalacademy.comwww.indiandentalacademy.com
  91. 91. The proper manipulation of the alloys being usedThe proper manipulation of the alloys being used in the connector is very important. If the connectorin the connector is very important. If the connector is cast, correct spruing is vital to assure that theis cast, correct spruing is vital to assure that the alloys is cast and cooled correctly for a porosityalloys is cast and cooled correctly for a porosity free joint. The presence of porosity is probably thefree joint. The presence of porosity is probably the most common cause of failure in the cast joint.most common cause of failure in the cast joint. When soldered joints are used, good principles ofWhen soldered joints are used, good principles of soldering must be scrupulously adhered to;soldering must be scrupulously adhered to; cleanliness, access, and heat control. Fracturescleanliness, access, and heat control. Fractures tend to occur in the parent metal, not the solder totend to occur in the parent metal, not the solder to parent metal interface or the solder itself, probablyparent metal interface or the solder itself, probably because of changes in the parent metal during thebecause of changes in the parent metal during the soldering procedures. Another factor, whichsoldering procedures. Another factor, which contributes to the ability of the prosthesiscontributes to the ability of the prosthesis connectors to maintain rigidity of the restorations,connectors to maintain rigidity of the restorations, is the design of the joint contours.is the design of the joint contours.www.indiandentalacademy.comwww.indiandentalacademy.com
  92. 92. Photo elastic studies have shown that V-shapedPhoto elastic studies have shown that V-shaped embrasures produce high stress concentrations inembrasures produce high stress concentrations in the connector area, whereas lower concentrationsthe connector area, whereas lower concentrations of stress result with U-shaped embrasures. Theof stress result with U-shaped embrasures. The need for rounded connector design often conflictsneed for rounded connector design often conflicts with the esthetic requirements in anterior teeth,with the esthetic requirements in anterior teeth, where sharp, deep embrasures are preferredwhere sharp, deep embrasures are preferred because they mimic the natural embrasure.because they mimic the natural embrasure. www.indiandentalacademy.comwww.indiandentalacademy.com
  93. 93. In the case of posterior prosthesis, the situation isIn the case of posterior prosthesis, the situation is a little less troublesome. There are minimal or noa little less troublesome. There are minimal or no esthetic considerations, depending on location.esthetic considerations, depending on location. Also, there is usually a greater area available forAlso, there is usually a greater area available for bridge connectors by virtue of the fact that thebridge connectors by virtue of the fact that the proximal surfaces of the teeth are larger than theproximal surfaces of the teeth are larger than the anterior teeth. Since the strength of the connectoranterior teeth. Since the strength of the connector is related directly to the cross-sectionalis related directly to the cross-sectional configurations, one can readily see that it is easierconfigurations, one can readily see that it is easier to attain strength in the posterior part of the mouth.to attain strength in the posterior part of the mouth. www.indiandentalacademy.comwww.indiandentalacademy.com
  94. 94. In most cases, greater strength is requiredIn most cases, greater strength is required probably, due to the fact that greater biting forcesprobably, due to the fact that greater biting forces are applied there. It has been shown that theare applied there. It has been shown that the proprioceptive reflex arc is more active andproprioceptive reflex arc is more active and sensitive in the anterior teeth than in the posteriorsensitive in the anterior teeth than in the posterior teeth. What this differences means is that whenteeth. What this differences means is that when an individuals bites on an unexpectedly hardan individuals bites on an unexpectedly hard object in the food with the anterior teeth, the reflexobject in the food with the anterior teeth, the reflex arc tends to cause the muscles to react and toarc tends to cause the muscles to react and to open the jaws quickly before a great deal of forceopen the jaws quickly before a great deal of force has been applied. This reaction is oftenhas been applied. This reaction is often accompanied by a fair amount of discomfort in theaccompanied by a fair amount of discomfort in the periodontal tissues of the involved teeth.periodontal tissues of the involved teeth. www.indiandentalacademy.comwww.indiandentalacademy.com
  95. 95. In the posterior teeth, however, it is not uncommonIn the posterior teeth, however, it is not uncommon to have a patient bite with enough force on theto have a patient bite with enough force on the same unexpected hard object in the food tosame unexpected hard object in the food to fracture a cusp. As the posterior teeth acted onfracture a cusp. As the posterior teeth acted on the object, the proprioceptive reflex arc was not sothe object, the proprioceptive reflex arc was not so sensitive, and the muscles continued to apply thesensitive, and the muscles continued to apply the force for a longer period. This force, then, isforce for a longer period. This force, then, is transmitted from the point of contact to thetransmitted from the point of contact to the connectors, the weakest portion of the fixed partialconnectors, the weakest portion of the fixed partial prosthesis and most prone to fracture. In addition,prosthesis and most prone to fracture. In addition, the biting forces are simply of greater magnitudethe biting forces are simply of greater magnitude on the posterior teeth than on the anterior teeth.on the posterior teeth than on the anterior teeth. www.indiandentalacademy.comwww.indiandentalacademy.com
  96. 96. Comparison between inlay and onlay:Comparison between inlay and onlay: Some discussion is in order regarding theSome discussion is in order regarding the use of inlays to restore posterior teeth. Both twouse of inlays to restore posterior teeth. Both two and three dimensional photoelastic investigationsand three dimensional photoelastic investigations have shown that stress concentrations occur inhave shown that stress concentrations occur in critical areas of the tooth when a mesiocclusodistalcritical areas of the tooth when a mesiocclusodistal inlay is loaded by occlusal forces or when theinlay is loaded by occlusal forces or when the cusps of the tooth are loaded in a tooth socusps of the tooth are loaded in a tooth so restored. According to Fisher DW et.al; highrestored. According to Fisher DW et.al; high concentrations of stress were found at theconcentrations of stress were found at the faciopulpal and linguopulpal line angles when thefaciopulpal and linguopulpal line angles when the inlay was loaded in centric occlusion as well asinlay was loaded in centric occlusion as well as when the cusps were loaded in a three pointwhen the cusps were loaded in a three point occlusal contact scheme.occlusal contact scheme.www.indiandentalacademy.comwww.indiandentalacademy.com
  97. 97. Higher concentrations of stress were seen on theHigher concentrations of stress were seen on the walls of the isthmus in the models prepared forwalls of the isthmus in the models prepared for inlays than in those for onlays. It appears from theinlays than in those for onlays. It appears from the results of these studies that it is particularlyresults of these studies that it is particularly dangerous from a stress standpoint to restoredangerous from a stress standpoint to restore these teeth with inlays under which a cement basethese teeth with inlays under which a cement base has been placed. The practice of placing a cementhas been placed. The practice of placing a cement base on the pulpal floor is often done in thebase on the pulpal floor is often done in the interest of the creating a more ideal preparationinterest of the creating a more ideal preparation form. This practice should be avoided because ofform. This practice should be avoided because of stress concentrations that result under load.stress concentrations that result under load. www.indiandentalacademy.comwww.indiandentalacademy.com
  98. 98. It is considerably more defensible in terms ofIt is considerably more defensible in terms of protection of the remaining tooth structure toprotection of the remaining tooth structure to design the preparation to have the restorationdesign the preparation to have the restoration seated on as much solid tooth structure that isseated on as much solid tooth structure that is perpendicular rather than nearly parallel to the lineperpendicular rather than nearly parallel to the line of draw. The problem with inlays is that thisof draw. The problem with inlays is that this requirement is not satisfied in a great manyrequirement is not satisfied in a great many situations. First, too much of the preparationsituations. First, too much of the preparation involves the walls of the isthmus and the walls ofinvolves the walls of the isthmus and the walls of the boxes, and too little of the preparation involvesthe boxes, and too little of the preparation involves the pulpal floor and the floors of the boxes. Whenthe pulpal floor and the floors of the boxes. When this deficiency is aggravated by the placement of athis deficiency is aggravated by the placement of a cement base on the pulpal floor, thus rendering itcement base on the pulpal floor, thus rendering it an ineffective vehicle for resisting forces, the inlayan ineffective vehicle for resisting forces, the inlay becomes a wedge.becomes a wedge.www.indiandentalacademy.comwww.indiandentalacademy.com
  99. 99. The onlay, on the other hand, counteracts theseThe onlay, on the other hand, counteracts these shortcomings of the inlay because its increasedshortcomings of the inlay because its increased occlusal coverage more effectively distributesocclusal coverage more effectively distributes forces to the tooth substructure. Consequently, inforces to the tooth substructure. Consequently, in teeth where there is a need for an intra coronalteeth where there is a need for an intra coronal restoration of some type, the material andrestoration of some type, the material and technique of choice will more often be an amalgamtechnique of choice will more often be an amalgam for two main reasons. First, in general, less soundfor two main reasons. First, in general, less sound tooth structure will need to be removed totooth structure will need to be removed to accomplish a proper preparation. Less removal ofaccomplish a proper preparation. Less removal of tooth structure will result in a better opportunity fortooth structure will result in a better opportunity for the remaining cusps of the tooth to resist forcesthe remaining cusps of the tooth to resist forces applied during contacts that might occur in lateralapplied during contacts that might occur in lateral excursions.excursions. www.indiandentalacademy.comwww.indiandentalacademy.com
  100. 100. Second, in centric occlusion, the softer (lowerSecond, in centric occlusion, the softer (lower modulus) amalgam will tend to deform more andmodulus) amalgam will tend to deform more and therefore cause less stress concentrations at thetherefore cause less stress concentrations at the walls and line angles of the preparations than inwalls and line angles of the preparations than in the case of the inlay that is made from a muchthe case of the inlay that is made from a much higher modulus alloy. Clinical experience hashigher modulus alloy. Clinical experience has shown that the amalgam materials tend to wear,shown that the amalgam materials tend to wear, flow, or even fracture under these loads, whereasflow, or even fracture under these loads, whereas the inlays tend to cause high stress concentrationsthe inlays tend to cause high stress concentrations to develop with in the tooth structure.to develop with in the tooth structure. www.indiandentalacademy.comwww.indiandentalacademy.com

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