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Biomechanics in fixed partial prosthodontics /certified fixed orthodontic courses by Indian dental academy


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Biomechanics in fixed partial prosthodontics /certified fixed orthodontic courses by Indian dental academy

  2. 2. INTRODUCTION: Fixed Prosthodontics is concerned with the replacement of large amounts of missing tooth structure. The restorative procedures involved can have a great effect on the forces transmitted to the remaining tooth and its supporting structures. This potential is greater than in many other treatment modalities because of the magnitude of the replaced missing structural form. For example, to evaluate the significance of a simple full crown on a mandibular molar tooth in a patient with relatively normal occlusion, a full complement of teeth, and normal bone support,
  3. 3. we see that the following parameter of form and forces are within the control and responsibility of the operator: Number and area of occlusal contacts Inclination and length of cusps Axial contours Interproximal contacts These parameters are related to the surface contours of the completed restoration.
  4. 4. Number and area of occlusal contacts: The number and area of occlusal contacts have a profound influence on the distribution of occlusal forces. For example, the larger the total area of contact over which a given occlusal force is applied, the less stress is concentrated at any one point. As the total number of occlusal contacts increases in an occlusal scheme, the force is applied over a greater number of locations, also reducing the localized stress. In addition, the larger number of contacts results in more cutting or grinding surfaces to facilitate mastication.
  5. 5. Inclination and length of cusps: The angles of inclines of individual cusps present a dilemma that must be considered. Greater chewing efficiency generally is attained with steeper cusps. However, if these cusps are allowed to come into contact in excursive movements of the mandible, they become interferences, which is a deleterious situation. The operator has limited control over the length of the cusps of restorations unless the opposing teeth are also being treated. Where possible, however, excessive length should be avoided, because these cusps tend to transmit greater force to the supporting structures due to the longer lever arm involved.
  6. 6. Axial contours: When axial contours are considered, the concern is with forces of a much lower magnitude and of a less predictable range. The food bolus undergoing mastication can apply some small amount of force to lateral walls as well as a greater force to the occlusal surface. In addition, low-level, persistent forces from the tongue or lips acting on the lateral surfaces of teeth may result in tooth movement. This kind of action is often compounded when the contour of a restoration or a natural tooth tends to trigger or encourage a habit pattern that accentuates this effect.
  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.
  8. 8. Tooth-restoration interface: The tooth- restoration interface is not as important to the final result from a design standpoint as those factors previously discussed. However, it has a more definitive, all or nothing effect. Unless the crown has at least a minimum degree of required retention, occlusal forces tend to dislodge the crown, making the discussion of all other parameters academic.
  9. 9. Tooth-supporting structure interface: The interface between the tooth and the supporting structures is a region of special concern and one where a sound knowledge of the principles of restorative design must be applied. It is here that the greatest chance for damage from lack of foresight and poor judgment by the operator can play a detrimental role in the future health of the patient. The opportunities related to the fabrication of a full crown on a single tooth are great for both improvement of force distribution and potential damage. It must then follow that more extensive restorations can present greatly increased problems or benefits.
  10. 10. Abutment Evaluation: Abutment teeth are called upon to withstand the forces normally directed to the missing teeth, in addition to those usually applied to the abutments. Whenever possible, an abutment should be a vital tooth. However, a tooth that has been endodontically treated and is asymptomatic, with radiographic evidence of good seal and complete obturation of the canal, can be used as an abutment. The tooth must have some sound, surviving coronal tooth structure to insure longevity. However, some compensation can be made through the use of a dowel core, or a pin retained amalgam or a composite core.
  11. 11. Teeth that have been pulp capped in the process of preparing the tooth should not be used as an FPD abutment unless they are endodontically treated. The supporting tissues surrounding the abutment teeth must be healthy and free from inflammation before any prosthesis can be contemplated. Normally, abutment teeth should not exhibit any mobility, since they will be carrying an extra load. The roots and the supporting tissues should be evaluated for three factors: Crown-root ratio. Root configuration. Periodontal ligament area.
  12. 12. Crown-root ratio: This ratio is a measure of the length of tooth, occlusal to the alveolar crest of bone compared with the length of root embedded in the bone. As the level of alveolar bone moves apically, the lever arm of that portion out of bone increases, and the chances for harmful lateral forces is increased. The optimum crown-root ratio for a tooth to be utilized as a FPD abutment is 2:3. A ratio of 1:1 is the minimum ratio that is acceptable for a prospective abutment under normal circumstances.
  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.
  14. 14. For the same reasons, an abutment tooth with a less than desirable crown-root ratio is more likely to successfully support FPD if the opposing occlusion is composed of mobile, periodontally involved teeth than if the teeth are periodontally sound.
  15. 15. Root configuration: This is an important point in the assessment of an abutment’s suitability from a periodontal standpoint. Roots that are broader labiolingually than they are mesiodistally are preferable to roots that are round in cross-section.
  16. 16. Multirooted posterior teeth with widely separated roots will offer better periodontal support than roots that converge, fuse, or generally present a conical configuration. The tooth with conical roots can be used as an abutment for a short span FPD if all other factors are optimal. A single rooted tooth with evidence of irregular configuration or with some curvature in the apical third of the root is preferable to the tooth that has a nearly perfect taper.
  17. 17. Periodontal ligament area / Ante’s law: When the normal complement of roots is not available to distribute the forces of mastication, pathologic stress concentrations may result in the periodontal ligament and supporting bone. This condition is the most fundamental problem with which the prosthodontist must contend each time a fixed prosthesis is designed to replace a missing tooth.
  18. 18. Here we must take into accounts Ante’s law, which states, “The total periodontal membrane area of the abutment teeth should equal or exceed that of the teeth to be replaced’’. The essential feature of this clinical guideline is that the actual area of the interface between tooth and supporting structures must be of a certain minimal amount to withstand and resist the forces that will now be transmitted to those supporting structures. Realistic determination of the area of good, healthy, periodontal ligament available on a potential FPD abutment is not an easy matter.
  19. 19. Maxillary  Tooth Area mm2 Mandibular 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
  20. 20. The combined root surface area of the second premolar and the second molar (A2p+A2m) is greater than that of the first molar being replaced (A1m).
  21. 21. The combined root surface area of the first premolar and the second molar abutment (A1p+A2m) is approximately equal to that of the teeth being replaced (A2p+A1m)
  22. 22. The combined root surface area of the canine and the second molar (Ac+A2m) is exceeded by that of the teeth being replaced (A1p+A2p+A1m)
  23. 23. Also of extreme importance is the actual area of contact between the periodontal structures and the root in question as it relates to the normal amount of contact area for that particular tooth in that particular mouth. One can assume that in a given case, without bone loss, the area of this contact is optimal. Therefore, any loss of bone support compared with the optimal situation decreases the chances of this root being an adequate fixed partial denture prosthesis abutment. If one looks at the problem in this manner, it becomes apparent that a root that appears adequate in one situation may be inadequate in another. The planning and design of a restoration of this type must have the benefit of sound clinical judgement and knowledge of basic biomechanical principles.
  24. 24. Table , which compares the root surface areas of 16 teeth, may aid in visualizing root areas. It is interesting to note that the addition of abutment roots will provide a wide variation of additional support. The addition of such support is not necessarily proportional to the number of abutments supporting prosthesis. There fore, the prosthodontist should not expect a proportional increase in stress-bearing ability, particularly when the amount of periodontal ligament is reduced. The area of the normal periodontal ligament (PDL) for teeth to be replaced by pontics should be less than the actual PDL area of the existing abutment candidates.
  25. 25. The values given in the table are averages for the various teeth in the mouth under ideal conditions. Two problems are evident when one attempts to arrive at useful interpretation of such data. Degree and nature of bone loss In clinical practice, the decision making process in which root surface area information is to be used does not always involve ideal situations. More often than not, there has been some bone loss; thus, a tooth with a moderate amount of bone loss may be still a better candidate for use as a FPD abutment than another tooth with no bone loss at all.
  26. 26. To determine as to which of the teeth in question has adequate support for the anticipated loads has, there are but few aids on which the prosthodontist can rely, none of them capable of giving all the information necessary to make a decision. These aids are Radiographs Periodontal probing, and Mobility tests.
  27. 27. The radiograph is unquestionably the most useful tool at the disposal of the prosthodontist in making a determination of the integrity of the remaining periodontal supporting structures as related to the loads anticipated. The opportunity to make radiographs from different projection angles should not be overlooked, since the primary areas that can be visualized on radiographs are the mesial and distal surfaces. Any chance for seeing even a little of the facial or lingual surfaces should be taken advantage of.
  28. 28. Periodontal probing is the second tool at the disposal at the prosthodontist and should be used extensively when attempting to determine the efficacy of using a given tooth as an abutment. Periodontal probing is a particularly important step as related to the facial and lingual surfaces, since these areas of the tooth to supporting structures interface cannot be viewed adequately on the radiograph.
  29. 29. Finally, standard clinical tests for mobility should be employed. Any degree of mobility outside the normal accepted physiologic range should be suspected. It means either that the loss of supporting structure, whether or not fully appreciated from the radiographs and probing, is severe enough to alter physically the stability of the tooth or that the occlusion has traumatically loosened the tooth. It is very important to understand which of these processes is at work in a given abutment situations. Occlusal trauma is usually reversible, and given the fact that the prosthodontist is going to construct a restoration on the tooth in question, there is ample opportunity to correct the situation.
  30. 30. A periodontal defect, however, is not always reversible and depending on its severity may require a drastic alteration of the plan of treatment for the tooth in question. The important concept to keep in mind is that the prosthodontist must exercise a certain degree of good judgement in the question of interpretation of the adequacy of supporting structures in a given situations. Many aspects of the final restoration and the chances for success are basically technical and depend on the skill of a particular prosthodontist. Not only good judgment but a conservative approach must be hall marks of the thought processes of the prosthodontist in this all important consideration.
  31. 31. Ante’s law says that in a situation where the preceding values are normal, a prosthesis to replace the maxillary first molar would need abutment teeth with at least 335 mm2 of periodontal membrane. This requirement is theoretically well satisfied in the case because of the total average area of the second molar and second premolar is 412mm2, however, has there been enough loss of bone on these two teeth to result in, for example, a total of only 300mm2, the law would not be satisfied.
  32. 32. Preparations design as related to stress and force distribution A large number of stress distribution studies have been performed on fixed prosthetic restorations, particularly since the 1960s. These studies have provided certain conclusions that are useful in determining the proper design in a given clinical situations. Many of the factors that often cause restorations failure may be eliminated by viewing preparations in the context of force and stress distribution.
  33. 33. Several parameters of preparations design interact with occlusal forces to determine restorative success or failure. The reasons for near parallel walls, grooves, offsets, counter bevels, pins, and other design features must be viewed in light of their functions. Each design features should contribute to retention of the restoration, resistance against displacement, protection of remaining tooth structure, and harmony with the restorative material.
  34. 34. Retention and resistance Looking at these design features one at a time, we must first consider the question of retention and resistance. Retention is the ability of the restoration to withstand forces acting in such a manner as to dislodge the restoration from the preparation along the line of draw. Resistance, on the other hand, is the ability of the preparation and the restoration to resist forces that would tend to dislodge the latter in direction other than the line of draw. In other words, the resistance to vectors of occlusal forces encountered on laterotrusive occlusal movements during either mastication or grinding of the teeth is the important consideration here.
  35. 35. Taper: The relationship of one wall of a preparation to the long axis of that preparation is the inclination of that wall. The axial walls of the preparation must taper slightly to permit the restoration to seat. Theoretically, the more nearly parallel the opposing walls of a preparation, the greater should be the retention. Craig RG, (1969) suggested a taper of 2.5-6.5 degrees as optimum to minimize stress in the cement interface between the preparation and restoration, but there is only a slight increase in stress as taper is increased from 0-15 degrees.
  36. 36. However, at 20 degrees, stress concentration was found to sharply increase. Cement creates a weak bond, largely by mechanical interlocks., between the inner surface of the restoration and the axial wall of the preparation. Therefore, the greater the surface area of a preparation, the greater its retention.
  37. 37. Freedom of displacement: Retention is improved by geometrically limiting the numbers of paths along which a restoration can be removed from the tooth preparation. Maximum retention is achieved when there is only one path.
  38. 38. The walls of a groove that meet the axial wall at an oblique angle do not provide necessary resistance (A). The walls of a groove must be perpendicular to rotating forces to resist displacement (B).
  39. 39. If the buccal and lingual walls of a proximal box forms oblique angle with its pulpal floor, there will not be adequate resistance to rotating forces. The buccal and lingual walls must meet the pulpal wall at angles near 90 degrees so that these walls will be perpendicular to any forces which tend to rotate the restoration.
  40. 40. Length: Occlusogingival length is an important factor in both retention and resistance. Longer preparations will have more surface area and will therefore be more retentive. Because the axial wall occlusal to the finish line interferes with displacement, the length and inclination of that wall become factors in resistance to tipping forces. For the restoration to succeed, the length must be great enough to interfere with the arc of the casting pivoting about a point on the margin on the opposite side of the restoration.
  41. 41.  The preparation with longer walls interferes with the tipping displacement of the restoration better than the short preparation.
  42. 42.  A preparation on a tooth with a smaller diameter resists pivoting movements better than a preparation of equal length on a tooth of larger diameter because smaller teeth will have a short rotational radius for the arc of displacement and the incisal portion of the axial wall will resist displacement.
  43. 43.  Resistance to displacement for a short walled preparation on a large tooth can be improved by placing grooves in the axial walls. In effect, this reduces the rotational radius, and that portion of the walls of the grooves near the occlusal surface of the preparation that will interfere with displacement.
  44. 44. Substitution of internal features: The basic unit of retention for a cemented restoration is two opposing axial walls with a minimal taper. It may not always be possible to use opposing walls for retention: one may have been destroyed previously, or it may be desirable to leave a surface uncovered for a partial veneer restoration. It may also be that the walls are present, but with a greater than desirable inclination. At such times, internal features such as the groove, the box form and the pin hole can be substituted for an axial wall or for each other.
  45. 45.
  46. 46. The forces to be applied by the opposing occlusion in the situations depicted in (a) would dictate a preparation with relatively strict adherence to the basic principles of length and taper.
  47. 47. The lesser angle of the application of force in the situations in (b) would permit the use of a preparation design with less retentive features, as seen in (c). This conclusion is based on the premise that once a crown is cemented on a tooth preparation, the retention will be adequate until such times as a force is applied with proper magnitude and direction to overcome the resistance to that force. The resistance to force referred to here is that which is afforded by the preparation design, the fit of the casting on the preparation, and the cementing medium. If this combination of factors proves inadequate to resist the force, retention will be lost. Conversely, if these factors are adequate for a given application of forces, retention will endure.
  48. 48. Path of insertion: The path of insertion must be an imaginary line along which the restoration will be placed onto or removed from the restoration. It is of special importance when preparing teeth to be fixed partial denture abutments, since the paths of all the abutment preparations must be parallel to each other. The path of insertion must be considered in two dimensions: faciolingually and mesiodistally. The faciolingual orientation of the path can affect the esthetics of metal-ceramic or partial veneer crowns. The mesiodistal inclination of the path must be parallel to the contact areas of the adjacent teeth
  49. 49. Few forces applied to teeth can dislodge a restoration in an occlusal direction. Sticky foods, chewing gum, and the like have been known to remove restorations in the line of draw. However, these situations are relatively rare and usually occur after prior fracturing of the cement film which results in a loss of retention. Most restorations that are properly designed and that fit well generally are not subject to this type of occlusal failure.
  50. 50. The primary function of a crown placed on a tooth is to restore lost contour and function, but we must also consider the necessity for the crown to distribute forces and loads applied to it in a manner that will assure its ability to satisfy the primary purpose over a reasonably long period of time. If a crown were placed on a tooth and expected only to serve this function without being subjected to any external forces, all crowns would look much more similar than they in fact do. However, we known this not to be the case, and therefore find it necessary to logically and predictable adjust the previously mentioned mechanism of retention to meet the needs of a great variety of stresses and strains.
  51. 51. The magnitude and quality of the retentive effect of a restoration should resist the forces that will be applied to it in specific clinical situations. Therefore, a preparation on the same tooth in two different patients could look quite different, yet both could be entirely satisfactory. Furthermore, the degree of retention afforded by fit of one crown might be twice as great as another or stronger cement may be used on one case than on another, and yet both could perfectly adequate for the respective situations.
  52. 52. Protection of tooth structure In the area of protection of the remaining tooth structure, the prosthodontist is faced with even more variables than those of retention and resistance. It is also a fact that serious errors in judgement here would more likely result in loss of the tooth at some future time. Although the most probable failure that would occur if retention is inadequate would be loss of the crown from the tooth, with no real damage to the remaining tooth, the most likely failure in the case of unprotected tooth structure, which obviously is a much more serious type of failure.
  53. 53. Important factors to be considered related to protection of the existing tooth structure are: Amount of enamel supported by adequate viable dentin Amount of adequate viable tooth structure after the preparation for the restoration is made. Accordingly, the first step in any crown preparation should be to remove all old restorative material and new carious lesions. At this point in the procedure, nothing should be left but sound dentin and enamel supported by dentin.
  54. 54. The importance of the principle of adequate removal of carious lesion cannot be over emphasized. An apparent minimally damaged tooth presents a totally different situation after the clean out procedure. The remaining tooth structure must be the primary concern in the design decision making process, not the false appearance of the tooth previous to removal of the unsupported enamel of the occlusal surface.
  55. 55. As is often seen in these situations where the carious lesion was initiated in the occlusal grooves of a posterior tooth, the structure remaining for use in developing a preparations is quite different after all of the unsupported overlying enamel is removed and the carious lesion is excavated.
  56. 56. The second step must be to establish the correct occlusal clearance for the restoration to be constructed on the tooth. By definition, any crown requires occlusal clearance since coverage of this surface is a part of any such restoration except inlays. Therefore, it follows that before any decisions can be made regarding other features of the preparation, the occlusal clearance must be created to properly visualize the parameters with which these decisions must be made.
  57. 57. The next step is to determine whether this remaining dentin and the enamel supported by it will still be available after the axial surfaces are prepared for the type of crown being done on the tooth. This step seems to be one of the more frequently over looked aspects of preparation planning and design. Continuing consideration of the molar seen, one may assume that it requires a full crown.
  58. 58. One can see that after completion of the aforementioned cleanout procedure (left) and occlusal clearance (right) the remaining dentin one the axial walls is minimal to say the least. Only enamel that is supported by adequate sound dentin will provide strength in the resulting preparation. It is the dentin, due to its elasticity, greater toughness, and lesser brittleness, that provides the sound foundation needed for cast restorations, not the enamel.
  59. 59. After reduction of these two axial surfaces there is not adequate dentin remaining to allow development of retention and resistance form and still have sound tooth structure that would not be prone to failure, even under the most minimal loading. Therefore auxiliary intra coronal retention such as pin retained build ups or retentive bases is required.
  60. 60. The essential point to be considered is that the operator must visualize the remaining sound dentin in the context of the final restoration. To carry this principle further, consider the result if the tooth required even greater reduction of the lingual surface to attain a particular line of draw not commensurate with savings as much tooth structure as possible.
  61. 61. One situation in which we often find the remaining tooth structure predisposed to failure is the case of endodontically treated teeth. Let us first consider posterior teeth, which is generally where greater forces are applied to the occlusal surfaces as well as where more controversy seems to exist at the point of preparation and restoration design. As a basic principle, it would seem reasonable to suggest that any posterior tooth that has had endodontic treatment and has had both interproximal surfaces involved with carious lesions or previous restorations should ultimately be restored by casting that provides full occlusal coverage.
  62. 62. Regarding the aforementioned principle that protection of remaining tooth structure is predicated on the presence of sound dentin, it is a fact that in those teeth that have undergone endodontic treatment, there has often been removal of a significant amount of this all important dentin in the area between the pulpal floor of the previous restorations (or carious lesion) and the superior aspect of the pulp chamber.
  63. 63. Depending on the ability of the operator performing the endodontic access opening as well as the presence of anomalies of the position of the chamber, the access opening may significantly reduce the strength of the remaining cusps and their ability to withstand lateral forces. It is clear that a fracture in such a situation is considerably more likely than if the area above the pulp chamber were still sound dentin.
  64. 64. Integrity of the restoration The fixed restoration must be able to withstand forces generated by centric contact, eccentric movements, mastication against hard and soft foods, and mild accidental trauma. The chosen restorative material should not deform permanently or fail under these conditions. In terms of centric contact, it is generally an easier matter to design and execute the restoration in a way that will adequately resist excessive deformation and therefore any possibility of ultimate failure of the material.
  65. 65. Only an occasional error in casting or preparation reduction on the occlusal surface will result in a casting that will later fail due to inadequate amount of material on the occlusal surface. However, often insufficient attention is paid to the problem of potential wear when one decides how much occlusal clearance is needed in a given situation. Normally, it is accepted that about 1 to 1.5mm is adequate for most situations. However, in cases where some wear in anticipated this amount may not be enough clearance and 2 or 2.5 mm might be needed.
  66. 66. Consideration also must be given to the restorations in eccentric movements, because an increasing number of cast metal restorations have become subject to wear due to chronic bruxism. In spite of attempts to control this problem so that wear does not occur on the occlusal surfaces of the teeth, these attempts are not always successful. The operator should take the problem of chronic bruxism into account when designing restorations. Besides wear, there are other considerations. The force, and therefore, the resulting strain, induced in the casting in such a case can be of considerable magnitude.
  67. 67. As a result of wear the occlusal contacts often take on the form of very flat surfaces, no longer possessing the ideal cuspal form that will provide for efficient cutting interaction with the food bolus. The flatter occlusal contacts can cause a decreased effectiveness of the masticating surfaces and a concomitant increase in force needed to properly masticate a bolus of food. It is also possible to have enough wear to result in a perforation through the casting into the cement film and then into the underlying dentin.
  68. 68. Another consideration is the possibility of deformation of the casting due to these forces. The restoration could, and often does, lose retention because the deformation of the casting leads to either adhesive or cohesive failure of the cement layer. Exceeding the yield strength of the casting can result in an open margin and recurrent caries. If the restoration in question is porcelain fused to metal crown or bridge, excessive deformation will result in failure of the porcelain bond or in a fracture of the porcelain at some point. Studies using stress analysis have demonstrated the need for proper framework design, particularly related to the problem of resisting occlusal forces.
  69. 69. Flexure of long span bridges during mastication can be a problem. A very long span may present no real problem in terms of flexure during centric closure with no food in the mouth, because there is distribution of forces among all the occlusal surfaces around the arch. However, when there is a bolus of food interposed between the pontic area of an excessively long span bridge and it’s opposing occlusal surfaces, the effect can be quite different since the occlusal contacts on the opposite side of the arch have not yet come into play. In this case, all of the force is concentrated on the bridge span, inducing a strain that the restoration and / or the abutment teeth may not be capable of withstanding.
  70. 70. In these situations, the case should be designed to provide adequate beam strength in casting. For example: it would be questionable to construct a bridge from the mandibular first premolar to the mandibular third molar when the available inter occlusal distance is only 2 mm. Such a casting is of such a long span that very high flexural stresses would be generated even by the low loads generated during mastication of a bolus of soft food. Then, we must consider the possibility of a rather hard object suddenly finding it ways into the interocclusal space, such as a piece of bone.
  71. 71. The problem in these situations is that the force with which the masticatory muscles are functioning at that moment is the force that was needed and that felt comfortable to the individual for the purpose of chewing the soft food. When suddenly and unexpectedly a small hard object is interposed between the occlusal surfaces, tremendous forces is concentrated at that point. By the time of proprioceptive or pain reflexes of the individual can take over and stop the action of the neuromuscular complex, the damage is often done. The result may be loss of cement bond, porcelain fracture, failure of a build up under a retainer, failure of a connector, and so forth.
  72. 72. When designing dental restorations of any type, we are generally concerned only with resistance to those forces generated within the stomatognathic system. It is not possible to predict damage to our restorations from such causes as automobile accidents, or blows, but there are a limited number of situations where the planning for such eventualities is at least to some extent a possibility. In these instances, the design of restorations can take into account certain type of potential damage and therefore obviate the need for a new restoration, possibly a more extensive one.
  73. 73. One such situation would be that of the epileptic patient who is subject to seizures. These patients often have missing maxillary or mandibular anterior teeth, lost in just such a seizure in which the patient fell. In these cases, it is often wise to modify the usual treatment to avoid use of porcelain fused to metal restoration. Such a restoration is prone to fracture when subject to impact, and if it does not fracture, there is the even more serious risk of fracture of the teeth. In such situations, it is usually wise to consider the use of a more flexible gold alloy with plastic facing. The restoration can be designed and constructed in such a manner that the facing could easily be replaced in the mouth.
  74. 74. This same type of contingency planning can logically be applied to those who are regularly engaged in sports, where it can be predicted that they are likely to repeatedly encounter this type of trauma.
  75. 75. Pontic Selection The pontic is the part of the restoration that replaces the missing tooth. From a biomechanical view point, pontic present some unique problems that must be considered in the design and construction of these restorations. First, there is less bone support for a given number of functioning occlusal surfaces. It is therefore advisable to increase the efficiency of the occlusal surface as a masticatory device.
  76. 76. This task usually involves creating an occlusal form that has the following features: Maximum angle of cusp inclines, Narrow cusp ridges, Sharp cusp tips, A greater number of small occlusal contacts.
  77. 77. As can be seen in the cusp form on the left would tend to be more efficient. The same magnitude applied to the occlusal surfaces of the teeth by the elevating musculature can cut through the bolus of food easier if the cusps are narrower and sharper because the available force is more concentrated.
  78. 78. The degree to which modifications to the present occlusal scheme of the patient can be accomplished depends on the controlling factors in the occlusion, such as the amount and angle of canine disocclusion, the angle of the eminentia, and the amount of enamel available for adjustment. Long spans and periodontally involved cases are in particular need of efficient occlusal design but only to the point that no lateral or protrusive interferences are introduced.
  79. 79. Any potential gain in efficiency that was attained while causing traumatic occlusion would not be justified. An attempt should not be made to reduce the magnitude of the load on a posterior prosthesis pontic by lightening the occlusal contacts. When the occlusal contacts are lightened, the opposing tooth usually supererupts into the same tight occlusion in centric as exists on the abutment teeth.
  80. 80. Another factor in pontic design is the width of the occlusal table. The faciolingual width of the occlusal table is the portion of the occlusal surface delineated by the occlusal contacts. This width should be made as narrow as possible to create a greater concentration of force where the work is being done. There is limitation to attaining this goal since the width of the occlusal table is dictated by the anatomy of the opposing dentition.
  81. 81. The occlusal table of the pontic may be narrowed if a cast restoration is indicated on the opposing tooth and if it is possible to narrow the distance between the contacts by equilibration. Any adjustment in the design of the pontic that would distribute the force at the site of the work over a smaller total area of the occlusal surface of the pontic would create a better concentration of force.
  82. 82. Over all, faciolingual width is another important feature of pontic design. This width refers to the greatest faciolingual width of the pontic, which is usually gingival to the occlusal table. It should be a basic goal of all posterior pontic design to keep the overall faciolingual width as narrow as possible to promote oral hygiene in the area of the pontic.
  83. 83. The smaller the total surface of the pontic that faces the residual ridge, the better will be the ability of the patient to clean this surface of bacterial plaque. By making the faciolingual width of the pontic as narrow as possible, the operator may also make a greater percentage of the facial and lingual surfaces more vertical and therefore more easily reached by the tooth brush or other aids. Finally, the forces applied by the pontic on the residual ridge must be considered. There have been various schools of thought concerning this aspect of pontic design.
  84. 84. These opinions range from placing the pontic in a very definite positive contact with the ridge tissue to leaving a 2 to 3 mm space between the pontic and the ridge. Virtually all possibilities between these extremes have been advocated. It is easy to understand that the pontic that does not touch the tissue cannot directly apply any force to the ridge. This design is most often accomplished to promote the best possible hygiene. At the other extreme, the pontic that is placed in heavy contact with the ridge tissues is going to apply a direct force on the tissue and then to the underlying bone and its periosteum.
  85. 85. This type of pontic is no longer advocated to any great degree because of its poor oral hygiene potential. Probably the greatest controversy regarding this problem in recent years has been whether to place the pontic in very light contact or just out of contact with the ridge tissue. Although the great majority of bridge pontics placed in light contact with the tissue show no changes in either tissue or bone, occasionally one can see the proliferation of bone. It is advisable to create pontics that do not actually contact the tissue from the stand point of both oral hygiene and transmission of detrimental forces to the residual ridge.
  86. 86. Connectors All fixed bridges must be united by some type of connector that must satisfy certain structural requirements. It must provide enough strength to resist forces of occlusion that cause flexure of the joint, producing stress in the solder, the interface, and the parent casting. Functional forces applied to the pontic result in a more severe stress condition than when the patient closes into centric occlusion without a bolus of food. In centric closure the force is uniformly shared by the pontic and the retainers.
  87. 87. Studies have demonstrated that the greatest chewing action takes place with the bolus over the second premolar and first molar, with the food being masticated primarily over the premolar at first, and then gradually move farther posteriorly as the degradation of the bolus progresses. Since the restorations in question are usually replacing one or more posterior teeth in the area of greatest chewing function, often a force will be applied to the pontics alone by the bolus. The magnitude as well as the concentration of force can be quite great and may exceed the ability of the abutments to adequately resist it, resulting in failure. This failure often takes place at one of the connectors, which is the thinnest and therefore, the weakest link in the restorations.
  88. 88. In anterior bridges it is of paramount importance to design the connectors so that they are esthetic, that is, so they appear as close as possible to a natural embrasure between two separate teeth. To provide adequate thickness of porcelain in the area of facial embrasure, one must provide the clearance at the expense of what might be considered a more ideal amount of metal in the area. It is also necessary to provide natural lingual embrasure form for phonetics, patient comfort and hygiene potential. All of these requirements tend to limit the faciolingual thickness available.
  89. 89. The other variable in the equation is incisogingival dimension of the connector. Again, we are limited by functional, esthetic, and hygiene considerations. It is necessary from an esthetic stand point to create both an incisal and a gingival embrasure that will match the contra lateral ones. The classic exception is the embrasure between the two central incisors. Since there is no contra lateral embrasure, there is a little more freedom.
  90. 90. The height of the gingival tissue in the area between the teeth will limit the incisogingival height of the connector. This excess tissue can often be adjusted using conventional or electrosurgical methods. Connector design in anterior bridges is dependent on esthetic considerations as well as structural ones. Other means should be considered before relying on increased size to solve the problem of strength.
  91. 91. The proper manipulation of the alloys being used in the connector is very important. If the connector is cast, correct spruing is vital to assure that the alloys is cast and cooled correctly for a porosity free joint. The presence of porosity is probably the most common cause of failure in the cast joint. When soldered joints are used, good principles of soldering must be scrupulously adhered to; cleanliness, access, and heat control. Fractures tend to occur in the parent metal, not the solder to parent metal interface or the solder itself, probably because of changes in the parent metal during the soldering procedures. Another factor, which contributes to the ability of the prosthesis connectors to maintain rigidity of the restorations, is the design of the joint contours.
  92. 92. Photo elastic studies have shown that V-shaped embrasures produce high stress concentrations in the connector area, whereas lower concentrations of stress result with U-shaped embrasures. The need for rounded connector design often conflicts with the esthetic requirements in anterior teeth, where sharp, deep embrasures are preferred because they mimic the natural embrasure.
  93. 93. In the case of posterior prosthesis, the situation is a little less troublesome. There are minimal or no esthetic considerations, depending on location. Also, there is usually a greater area available for bridge connectors by virtue of the fact that the proximal surfaces of the teeth are larger than the anterior teeth. Since the strength of the connector is related directly to the cross-sectional configurations, one can readily see that it is easier to attain strength in the posterior part of the mouth.
  94. 94. In most cases, greater strength is required probably, due to the fact that greater biting forces are applied there. It has been shown that the proprioceptive reflex arc is more active and sensitive in the anterior teeth than in the posterior teeth. What this differences means is that when an individuals bites on an unexpectedly hard object in the food with the anterior teeth, the reflex arc tends to cause the muscles to react and to open the jaws quickly before a great deal of force has been applied. This reaction is often accompanied by a fair amount of discomfort in the periodontal tissues of the involved teeth.
  95. 95. In the posterior teeth, however, it is not uncommon to have a patient bite with enough force on the same unexpected hard object in the food to fracture a cusp. As the posterior teeth acted on the object, the proprioceptive reflex arc was not so sensitive, and the muscles continued to apply the force for a longer period. This force, then, is transmitted from the point of contact to the connectors, the weakest portion of the fixed partial prosthesis and most prone to fracture. In addition, the biting forces are simply of greater magnitude on the posterior teeth than on the anterior teeth.
  96. 96. Comparison between inlay and onlay: Some discussion is in order regarding the use of inlays to restore posterior teeth. Both two and three dimensional photoelastic investigations have shown that stress concentrations occur in critical areas of the tooth when a mesiocclusodistal inlay is loaded by occlusal forces or when the cusps of the tooth are loaded in a tooth so restored. According to Fisher DW; high concentrations of stress were found at the faciopulpal and linguopulpal line angles when the inlay was loaded in centric occlusion as well as when the cusps were loaded in a three point occlusal contact scheme.
  97. 97. Higher concentrations of stress were seen on the walls of the isthmus in the models prepared for inlays than in those for onlays. It appears from the results of these studies that it is particularly dangerous from a stress standpoint to restore these teeth with inlays under which a cement base has been placed. The practice of placing a cement base on the pulpal floor is often done in the interest of the creating a more ideal preparation form. This practice should be avoided because of stress concentrations that result under load.
  98. 98. It is considerably more defensible in terms of protection of the remaining tooth structure to design the preparation to have the restoration seated on as much solid tooth structure that is perpendicular rather than nearly parallel to the line of draw. The problem with inlays is that this requirement is not satisfied in a great many situations. First, too much of the preparation involves the walls of the isthmus and the walls of the boxes, and too little of the preparation involves the pulpal floor and the floors of the boxes. When this deficiency is aggravated by the placement of a cement base on the pulpal floor, thus rendering it an ineffective vehicle for resisting forces, the inlay becomes a
  99. 99. The onlay, on the other hand, counteracts these shortcomings of the inlay because its increased occlusal coverage more effectively distributes forces to the tooth substructure. Consequently, in teeth where there is a need for an intra coronal restoration of some type, the material and technique of choice will more often be an amalgam for two main reasons. First, in general, less sound tooth structure will need to be removed to accomplish a proper preparation. Less removal of tooth structure will result in a better opportunity for the remaining cusps of the tooth to resist forces applied during contacts that might occur in lateral excursions.
  100. 100. Second, in centric occlusion, the softer (lower modulus) amalgam will tend to deform more and therefore cause less stress concentrations at the walls and line angles of the preparations than in the case of the inlay that is made from a much higher modulus alloy. Clinical experience has shown that the amalgam materials tend to wear, flow, or even fracture under these loads, whereas the inlays tend to cause high stress concentrations to develop with in the tooth structure.
  101. 101. However, if proper application of the principles were followed, there will be some indications for the use of very conservative inlays in vital posterior teeth, though never in non vital posterior teeth. These indications would normally involve two surface defects that would then leave one of the interproximal surfaces and the corresponding marginal ridge intact to afford a greater degree of structure integrity.
  102. 102. According to the basic principle, the best way to provide strength for the tooth is to leave enamel supported by sound dentin wherever possible. When a cast inlay replaces both proximal surfaces, this principle is not satisfied. However, when only a disto-occlusal or mesio-occlusal inlay is done, the remaining natural tooth structure on the other interproximal surface is usually enough to resist the wedging action referred to earlier, because the facial and lingual cusps remain united by the marginal ridge.
  103. 103. There is, however, no justification for using an inlay of any design in a posterior tooth that has had endodontic therapy. In these cases, even a two surface inlay can transmit enough undesirable force to the remaining tooth structure, already compromised, to cause a fracture.
  104. 104. Multiple unit restorations Fixed partial dentures present certain problems that are unique by virtue of the fact that more than one unit is involved. It is possible in these cases to transmit forces to the abutments even when the forces are not directed over these teeth. This indirect force application to the abutments can result in forces on the preparation and the supporting structures that are related in some complex manner to the direction of the original force.
  105. 105. A relatively common situation in which occlusal forces produce very complex structural responses is the situation involving an abutment in the middle of the multiple unit fixed restoration. A typical example would be a restoration replacing the first premolar and the first molar. This restoration configuration results in three separate abutments in locations where reactive forces are not always conductive to maintaining retention. Such a reactive force is particularly critical on an abutment at one end of the bridge that may have questionable retention to begin with. Here the problem arises from the fact that a certain degree of movement always occurs in the periodontal membrane of teeth in function.
  106. 106. Parfitt GJ (1960), have shown that the faciolingual movement ranges between 56-108 µm, and intrusion of 28 µm. Teeth in different segments of the arch move in different directions. Because of the curvature of the arch, the faciolingual movement of an anterior tooth occurs at a considerable angle to the faciolingual movement of molar. These movements of measurable magnitude and in divergent directions can create stresses in a long span prosthesis that will be transferred to the abutments.
  107. 107. Because of the distance through which it occurs, the independent direction and magnitude of movements of the abutment teeth, and the tendency of the prosthesis to flex, stress can be concentrated around the abutment teeth as well as between retainers and abutment preparations.
  108. 108. When a force is applied to the occlusal surface of one of the retainer of a three unit bridge, that abutment tooth tends to be displaced from its original position to a degree related to the magnitude of the force and the resistance of the supporting structures. The direction of movement is determined by the direction of the applied forced dictated by the occlusal anatomy of the teeth involved.
  109. 109. In these situations, the abutment tooth at the opposite end of the restoration will tend to rotate slightly in its socket. Again, the movement is permitted by the flexible nature (low modulus) of the periodontal membrane. By this mechanism, retention is maintained. However, when there is an abutment tooth in the middle of the restoration, a different set of problems occur. When an occlusal load is applied to the retainer on the abutment tooth at one end of such a restorations, the abutment tooth in the center can act as a fulcrum. Tensile forces would then be generated between the retainer and abutment at the other end of the bridge.
  110. 110. This abutment would be required to under go an extrusive movement to react to the force. If the periodontal support for this abutment is sound, such a movement is well resisted by the root. The result of the tensile force would be at the retainer to abutment interface, namely, the cement layer. The end result is frequently a loss of retention on one of the terminal abutments; the abutment with the least retention fails.
  111. 111. Therefore, the goal in planning and constructing restorations where there is more than a single missing tooth to be replaced should be to create a series of one tooth replacement restorations. The principle here is that the span involving the first premolar replacement is a simple three unit bridge cemented to the canine and second premolar, resulting in the advantages described for a one tooth replacement.
  112. 112. Then, the span involving the first molar replacement is another simple three-unit bridge replacing one missing tooth. In this case, the anterior unit is actually a nonrigid connector that will modify the transmission of forces applied to the anterior section that might be detrimental to the abutment-retainer complex of the molar or vice versa. By incorporating various available attachments, the operator may use this basic principle in the planning and construction of restorations involving nearly any combination of missing teeth and abutments.
  113. 113. The non-rigid connector is a broken-stress mechanical union of retainer and pontic, instead of the usual rigid connector. The most commonly used non-rigid design consists of a T-shaped key that is attached to the pontic, and a dove-tail keyway placed within the retainer. Use of non-rigid connector is restricted to a short span FPD replacing one tooth. Prosthesis with non-rigid connectors should not be used if prospective abutment teeth exhibit significant mobility. There must be equal distribution of occlusal forces on all parts of FPD.
  114. 114. The location of the stress breaking device in the five unit pier abutment restoration is important. It usually is placed on the middle abutment, since placement of it on either of the terminal abutments could result in the pontic acting as a lever arm. The keyway of the connector should be placed within the normal distal contours of the pier abutment, and the key should be placed on the mesial side of the distal pontic. The long axes of the posterior teeth usually lean slightly in the mesial direction, and vertically applied occlusal forces produce further movement in this direction.
  115. 115. If the keyway of the connector is placed on the distal side of the pier abutment, mesial movement seats the key into the keyway more solidly. Placement of the keyway on the mesial side, however, causes the key to be unseated during its mesial movements.
  116. 116. Cementation is another factor related to the problem of multiple abutment restorations in which there is an intermediary tooth. For example, a restoration with minimal taper and a large flat occlusal surface on the preparation is more difficult to seat than one with excessive taper and a very small occlusal surface. Because of the flexibility of the PDL, which allow for some small movement to take place in the abutments at each end of the simple three unit bridge, it is easy to see how it is possible to completely seat this type of restorations. However, the hydraulic back pressure of the cement makes it more difficult to seat a solid bridge with more than two abutments.
  117. 117. Valves reported in the literature for acceptable marginal opening range from 25 to more than 200μm.It is possible that while two of the retainers are seated with a marginal discrepancy of 25μm, the middle retainer is seated less completely, say to an opening 300 µm. This could be due to the effect of the hydraulic pressure exerted by the cement acting on an abutment which can be displaced in an apical direction more than the other abutments because of its lessened bone support. Various techniques have become accepted for improving seating of castings. A few examples of these methods are die spacers, venting, stripping of the casting internal surface, and variations in the liquid /powder ratio of the investment.
  118. 118. In this situation, it would seem impossible to completely seat the third retainer, which is often the case. This problem is magnified many times in the design, construction, and cementation of periodontal splints because the supporting structures have been degraded beyond normal limits. The design of this type of restorations must take into account the difficulty to be encountered during cementation. Here, the use of venting or any other means of reducing hydraulic back pressure of the cement must be considered.
  119. 119. There is another consideration with respect to multiple unit restorations with an abutment in the center of the span. The restoration should be designed so that no part of the occlusal surface of the middle retainer leaves tooth structure uncovered and in occlusal function. As can be seen occlusal forces acting directly on the tooth structures of the middle abutment could easily displace this tooth apically out of its retainer while the bridge is held in position by the other two abutments. This situation often results in loss of retention of the middle retainer on its abutments.
  120. 120. The restorations will usually remain in position, held there by the other retainers, while the middle tooth is allowed to slowly but surely succumb to caries. This problem is often seen where prosthesis with pier abutments have been constructed in one solid piece rather than in sections.
  121. 121. In addition to the increased load placed on the periodontal ligament by a long span fixed partial denture, longer spans are less rigid. Bending or deflection varies directly with the cube of the length and conversely with the cube of the occlusogingival thickness of the pontic. Compared with a fixed partial denture having a single tooth pontic span, a two tooth pontic span will bend 8times as much. A three tooth pontic span bends 27-times as much as a single pontic.
  122. 122.
  123. 123. A pontic with a given occlusogingival dimension will bend 8-times as much if the pontic thickness is halved. Longer pontic spans also have the potential for producing more torquing forces on the fixed partial denture, especially on the weaker abutment. To minimize flexing caused by long and/ or thin spans, pontic designs with a greater occlusogingival dimension should be selected. The prosthesis may also be fabricated of an alloy with higher yield strength, such as nickel chromium.
  124. 124.
  125. 125. Tilted molar abutment Titled abutment teeth are a common problem that must be addressed in construction of fixed partial prostheses. The tooth to be replaced by the restoration frequently has been missing for a long time. Therefore, the tooth distal to the missing one often will have tilted into the space. It is impossible to prepare the abutment teeth for a fixed partial denture along the long axis of the respective teeth and achieve a common path of insertion. There is further complication if the third molar is present. It will usually have drifted and tilted with the second molar.
  126. 126. Because the path of insertion of the fixed partial denture will be dictated by the smaller premolar abutment, it is probable that the path of insertion will be nearly parallel to the former long axis of the molar abutment before it tilted mesially. As a result, the mesial surface of the tipped third molar will encroach upon the path of insertion of the fixed partial denture, thereby preventing it from seating completely.
  127. 127. Some possible solutions to these problems are: Preparation modifications: The design of the preparation could be modified to be in harmony with the line of draw requirements of the other abutment and adjacent teeth while at the same time satisfying all other preparation criteria, such as retention and protection of the pulp. A proximal half crown can be used as a retainer on the distal abutment.
  128. 128. This retainer can be used only if the distal surface itself is untouched by caries or decalcification and if there is very low incidence of proximal caries throughout the mouth. If there is a severe marginal ridge height discrepancy between the distal of the second molar and the mesial of the third molar as a result of tipping, the proximal half crown is contraindicated.
  129. 129. Telescopic crown designs: A two piece restorations is constructed whereby the line of the draw of one component (seated on the tipped tooth preparations) is such that it favors the tooth. The line of the draw of the component is then in harmony with the other abutment preparation.
  130. 130. Broken connectors: In these situations it is desirable to connect units of fixed bridges in some manner that will allow the various components of the prosthesis to be seated separately.
  131. 131. Pre-prosthetic orthodontics. Some degree of uprighting the tooth may be accomplished orthodontically.
  132. 132. Composite resin bonded prosthesis: The most recent innovation in multiple unit restorations is the composite resin bonded prosthesis. Utilization and popularization of this technique is based on the ability to etch certain high modulus, non precious alloys. After etching, the metal can be placed after only a minimum of tooth reduction. To accomplish the goals of this conservative restoration, one must make the metal frame work thin and in-conspicuous which has led to FPD’s with minimal structural integrity.
  133. 133. The essential features of this type of restoration have included: Minimal axial reduction lingually at the height of contour. 1 mm deep occlusal rests inclined toward the center of the abutment teeth. 180-degree proximal wraparounds approximately 0.4mm thick. A distinct path of insertion. For anterior abutments, bonded cingulum rests have been advocated.
  134. 134.
  135. 135. When these composite resin bonded prosthesis are subjected to occlusal loadings, very high complex stresses are generated at the connector areas and extend into the high flexure of the wraparound arms. These high flexural stresses are transmitted to the resin adhesive. During function, the bridge is subjected to a large number of chewing cycles, which may be translated into fatigue failure of the adhesive layer When the thickness is increased, a substantial decrease in the level of stress concentration results. Another means to substantially reduce the level of stresses within the frame work is to include occlusogingival extensions adjacent to the extraction site.
  136. 136. The occlusal rests are also important structural elements in the transmitting of forces from the pontic to the abutment teeth. A similar structural support may be obtained by preparing a ledge on which the occlusogingival extension rests. This support is, in essence, a very minor box preparation. There are pros and cons to both approaches, but one of these two rest concepts should be used.
  137. 137. Structural considerations for the success of this technique should include: Wraparound arms as thick as possible consistent with reasonable tooth contour. Occlusogingival proximal extensions and A sound rest, whether it is on the occlusal surface or in the form of a gingival box.
  138. 138. Canine replacement fixed partial dentures Fixed partial dentures replacing canines can be difficult because the canine often lies outside the interabutment axis. The prospective abutments are the lateral incisors, usually the weakest tooth in the entire arch, and the premolar, the weakest posterior tooth.
  139. 139. A fixed partial denture replacing a maxillary canine is subjected to more stresses than that replacing a mandibular canine, since forces are transmitted outward (labially) on the maxillary arch, against the inside of the curve (its weakest point).
  140. 140. On the mandibular canine, the forces are directed inward (lingually), against the outside of the curve (its strongest point). Any fixed partial denture replacing a canine should be considered a complex a fixed partial denture. No fixed partial denture replacing a canine should replace more than one additional tooth. An edentulous space created by the loss of a canine and any two contiguous teeth is best restored with a removable fixed partial denture.
  141. 141. Cantilever fixed partial dentures A cantilever fixed partial denture is one that has an abutment or abutments at one end only, with the other end of the pontic remaining unattached. This is a potentially destructive design with the lever arm created by the pontic. In a routine three-unit fixed partial denture, force that is applied to the pontic is distributed equally to the abutment teeth. If there is only one pontic and it is near the interabutment axis line, less leverage is applied to the abutment teeth or to the retainers than with a cantilever.
  142. 142. When a cantilever pontic is employed to replace a missing tooth, forces applied to the pontic have an entirely different effect on the abutment tooth. The pontic acts as a lever that tends to be depressed under forces with a strong occlusal vector.
  143. 143. Prospective abutment teeth for cantilever fixed partial dentures should be evaluated with an eye towards lengthy roots with a favorable configuration, long clinical crowns, good crownroot ratios, and healthy periodontium. Generally, cantilever fixed partial dentures should replace only one tooth and have at least two abutments. A cantilever can be used for replacing a maxillary lateral incisor. There should be no occlusal contact in either centric or lateral excursions. The canine must be used as an abutment, and it can serve in the role of solo abutment only if it has a long root and good bone support.
  144. 144. There should be a rest on the mesial of the pontic against a rest seat preparation in an inlay or other metallic restoration on the distal of the central incisor to prevent rotation of the pontic and the abutment. The mesial side of the pontic can be little ‘wrapped around’ the distal portion of the uninvolved central incisor to stabilize the pontic faciolingually. The root configuration of the central incisor does not make it a desirable cantilever abutment.
  145. 145. A cantilever pontic can also be used to replace a missing first premolar. This scheme will best work if occlusal contact is limited to the distal fossa. Full veneer retainers are required on both the second premolar and first molar. These teeth must exhibit excellent bone support. This design is acceptable if the canine is unmarred and if a full veneer restoration is required for the first molar in any event.
  146. 146. Cantilever fixed partial denture can also be used to replace molars when there is no distal abutment present. When used judiciously, it is possible to avoid the insertion of a unilateral removable partial denture. Most commonly, this type of fixed partial denture is used to replace the first molar, although occasionally it is used to replace a second molar to prevent supereruption of opposing teeth.
  147. 147. When pontic is loaded occlusally, the adjacent abutment tends to act as a fulcrum, with a lifting tendency on the farthest retainer. To minimize the leverage effect, the pontic should be kept as small as possible, more nearly representing a premolar than a molar. There should be absolutely no contact in any excursion. The pontic should possess maximum occlusogingival height to ensure a rigid prosthesis.
  148. 148. A posterior cantilever pontic places maximum demands on the retentive capacity of the retainers. Its use, therefore, should be reserved for those situations in which there is adequate clinical crown length on the abutment teeth to permit preparations of maximum length and retention. The success of cantilevers in the restoration of the periodontally compromised dentition is probably due, at least by part, to the fact that periodontally involved abutments do have extremely long clinical crowns. While cantilever fixed partial dentures appears to be a conservative restoration, the potential for damage to the abutment teeth requires that they be used sparingly.
  149. 149. Double abutment Many clinical situations require the use of double abutments in the fixed bridges. The term as used here refers to the use of two adjacent teeth at one or both ends of a fixed prosthesis joined by a solid connector. The usual reasons for use of double abutment are: Increase retention of the restorations as a whole Splint and stabilize periodontally compromised teeth and Increase the area of the supporting PDL and bone.
  150. 150. Improvement of the retentive aspects of the restoration would seem to be a reasonable justification for including an extra abutment. This rationale is not always true. As seen, the second premolar has insufficient coronal dentin to provide the necessary retention for use as an abutment. The assumption was made that adding the extra premolar abutment would give the bridge adequate retention of the anterior end. This abutment would allow retention of the second premolar root to reduce future bone loss, which would occur if this tooth were extracted. This latter point would certainly add credibility to the rationale, but at least two other more conservative methods could be considered to render the second premolar a sound abutment.
  151. 151. First, a pin retained intra coronal casting or build up could be made for the second premolar if maintenance of the vitality of this tooth is a prime concern. Second, endodontic therapy and a retentive post and core could be done on the second premolar. The latter method would usually be the method of choice in this situation due to the greater chance of long term success compared with the pin buildup.
  152. 152. Either of these options, particularly the post and core, could obviate the need for double abutting this restoration. The reason being that by correcting the problem involving the second premolar, which is lack of retention, the operator has created a typical three unit prosthesis situation. The preceding example considered the use of double abutment strictly on the basis of lack of retention of the primary abutment choice. A discussion of other reasons for the use of multiple abutments follows. However, before proceeding, it is advisable to consider some of the ramifications of using double abutment as a solution for lack of abutment retention.
  153. 153. During function, the prosthesis often develops a cement failure at the second premolar because of poor retention characteristics. The other units will often be retained adequately. Breakdown of the cement layer of this abutment tooth leads to slow destruction by action of the saliva and its acidic components. Had this same loss of retention occurred in the case of a single unit restoration, it would have simply come away from the preparation and the patient would have sought treatment for an obvious problem. Dislodgement of the restoration does not occur, however, when other retainers of a multiple unit restoration remain in place on their respective abutment teeth.
  154. 154. This problem is difficult to diagnose because “loose” retainer is still held in its correct position in relation to the abutment tooth, though no longer cemented. The patient complains of pain. Since the retainer is still held in its correct position relative to the abutment tooth preparation, no marginal opening can be detected, nor can any looseness or movement. As can be easily seen, diagnosis of the patient’s complaint can be difficult, if not impossible, without removal of the entire restoration.
  155. 155. Due to these problems, it is imperative that precautions be taken in the design and construction of multiple unit restorations to preclude the loss of retention on any abutment. Further it is strongly recommended that the use of double abutments to compensate for lack of retention on one of the abutment teeth of a fixed prosthesis be discouraged. The procedure may be justified from the view point of maintaining bone, but it is less justifiable when considered in the light of resistance to the forces to which the restoration will ultimately be subjected. The alternative of pins or posts will usually be found to be the treatment of choice to permit saving of the root.
  156. 156. Splinting and stabilization of a periodontally compromised tooth can be more valid reasons for the use of double abutments on a fixed bridge. However, a fundamental decision must be made early in the planning of the case; is the mobility the result of a continuing process of periodontitis, or occlusal trauma. If the mobility of the tooth is only the result of occlusal trauma, stabilization of such a tooth in this manner may be perfectly justified, providing that the trauma can be eliminated in the occlusal scheme of the restorations.
  157. 157. When a tooth is subjected to occlusal forces that cannot be controlled, the adjacent tooth might be added to the restoration as a double abutment to provide the needed resistance to lateral forces. A classic example on this situation would be a bridge replacing a missing maxillary canine. In such a case, the lateral occlusal forces generated on the canine pontic are such that the lateral incisor is seldom an adequate abutment due to its short root form. It is then justified to add the central incisor to such design. It has been shown that mobility resulting from occlusal trauma is reversible once the cause for the trauma is removed.
  158. 158. On the other hand, if the lack of bone support is due to periodontal disease, and if the disease is not totally controlled, using this tooth as part of double abutment is contraindicated. In such a situation, bone loss on the affected abutment tooth continues, with the end result being that this tooth eventually becomes simply another pontic in the bridge. Also, pockets become less cleanable after the placement of the restoration due to poorer access, compounding the problem.
  159. 159. Finally, the best justification, for using double abutments is to satisfy Ante’s law. If there are not enough periodontal ligaments for a given number of missing teeth, there is no better solution than to add one or more teeth that do have sound support. When many missing teeth are replaced by a fixed restoration using a limited number of abutments, most of which do not even possess the normal amount of bone support, failure is assured.
  160. 160. One must make decision whether the addition of more abutments in the design of the restoration is more important than satisfying the concomitant requirement for conservatism. There may be no choice if the restoration is to be made at all. If it is not possible to satisfy Ante’s law in this regard, a removable partial denture should be considered so that occlusal forces may be distributed cross arch and to the edentulous ridges.
  161. 161. From the viewpoint of mechanical principles, the advantage of adding a second abutment at one end of a fixed prosthesis is that in so doing, we are better able to distribute the forces that would be applied to the prosthesis. Nothing would be gained if a crown were placed on the added abutment were it not connected rigidly to the remainder of the prosthesis.
  162. 162. When the added tooth is made an integral part of the prosthesis, its periodontal ligaments provide resistance to forces transmitted by the other abutment at this end of the bridge. This shared load-bearing responsibility is the essence of Ante’s law. An additional abutment tooth, or teeth, is used to replace the missing tooth. Other wise, only two abutment teeth would be performing the function of resisting forces applied to three occlusal surfaces.
  163. 163. There is a common problem in replacing all four maxillary incisors with a fixed partial denture and the problem is more pronounced in the arch that is pointed in the anterior. This occurs because the pontics lie outside the interabutment axis line and thus acts as a lever arm, which can produce a torquing movement. In order to offset the torque, additional retention is obtained in the opposite direction of the lever arm and at a distance from the interabutment axis equal to the length of the lever arm.
  164. 164. The first premolars sometimes are used as secondary abutments for a maxillary four-pontic canine to canine fixed partial denture. Because of the tensile forces that will be applied to the premolar retainers, they must have excellent retention.
  165. 165. Fixed versus removable restorations When it is necessary to replace missing teeth, a choice must be made whether to use a fixed or a removable restoration. This decision is best made from the standpoint of force distribution in relation to the ability of the supporting structures to withstand those forces. The number of fixed restorations since the late 1950s has greatly increased due to improvements in high-speed instrumentation, developments in impression materials, and advancements with the porcelainfused-to-metal technique.
  166. 166. prosthodontists are now able to construct rather extensive and complicated multiple unit restorations replacing many missing teeth with relative ease. There has been a trend in recent years towards better esthetics and what the public considers to be better prosthodontistry (i.e. more extensive fixed restorations) However, from the viewpoint of proper distribution of forces in the gnathologic system, particularly lateral forces, more consideration should be given to the use of removable restorations in many of the more extreme cases of multiple missing teeth.
  167. 167. The most important factor in this consideration is that the removable partial denture provides cross arch stabilization, whereas individual fixed partial dentures normally do not. Since many patients have some degree of bone loss, it is often desirable to distribute lateral forces to other teeth in the arch that may have the benefit of more adequate bone support. The bilateral removable partial denture will accomplish this and also aid in the improved distribution of vertical forces in the periodontally compromised patient. Occlusal loads on tipped or isolated teeth can better be directed along their long axes. Again, Ante’s law is relevant.
  168. 168. The total area of remaining good periodontal ligament on the teeth to be used as abutments for the removable partial denture will be increased by including teeth in other areas of the arch. Another advantage of removable prosthesis is the opportunity to distribute functional loads to the gingival tissues and supporting bone of the edentulous ridges and palate. When there is inadequate PDL to accommodate fixed prosthesis, the use of ridges becomes mandatory.
  169. 169. It has been shown that the periodontal ligament cushions the shock of tooth-to-tooth contact by moving as much as 0.05 mm. If it is assumed that the degree to which this cushioning effect is brought into play may be related to the hardness of the occlusal surfaces of the restoration, the removable partial denture may afford yet another advantage. The occlusal surfaces of the replacement pontics on removable partial denture prosthesis are usually made of an acrylic resin rather than gold or porcelain, as in the case of most fixed prosthesis.
  170. 170. It would be reasonable to assume that shock transfer to abutment teeth and their periodontal ligaments will be less than in the case of a fixed restoration. Therefore, a little less periodontal ligament would suffice in such a situation and would adequately withstand occlusal forces. It would seem to follow, then, that many patients might benefit from a design of fixed prostheses that incorporate some type of plastic material on the occlusal surface rather than gold or porcelain.
  171. 171. The potential for abrasion is the primary factor obviating such an idealistic solution to the problem. When the acrylic teeth on a removable partial denture wear, they can easily be replaced without great expense or extensive clinical procedures. This would not be the case if a fixed restoration and the plastic occlusal surfaces needed replacement.
  172. 172. When the remaining attachment apparatus is inadequate for distribution of occlusal loads, the removable prosthesis has a decided advantage over a fixed prosthesis. The abutment tooth, periodontal ligament, and bone can accommodate only a certain level of stress concentration. The removable partial denture prosthesis can help by transferring excessive stresses to the tissues of the edentulous ridge – fixed partial denture prosthesis cannot. At this point the objective is only to decide which type of prosthodontic replacement to use.
  173. 173. The significance of providing biomechanically advantageous stress distribution in occlusal loading becomes more evident when one considers the amount of time the teeth are in contact without the presence of food. It must be pointed out that tooth contact, with and without food interposed between the occlusal surfaces, presents different situations. When food is present, forces are distributed over a broader area and are generally assumed to be of lesser magnitude since some of the energy is absorbed in doing work, namely, masticating the food. However, when no food is present, the tooth contact is made directly with another tooth surface.
  174. 174. Therefore, forces are concentrated at the point of contact, and all the energy is expended on abrasive action or traumatic loading of the supporting structures. Graf has indicated that the average person exerts deglutition forces on the teeth 25 times each hour during the day and 10 times each hour during the night while asleep. During these 16 and 8 hour periods, respectively, the person will make tooth to tooth contact (thus applying force to the occlusal surfaces and supporting tissues) of 8 minutes each day. Added to this time are 30 seconds of contact during swallowing of masticated food.
  175. 175. This amount of time may seem to be rather small, but during these periods of tooth to tooth contact, the magnitude of this force is rather high. Also, this force is exerted between two very hard surfaces. To emphasize the significance of the point, consider that the tooth enamel is the hardest tissue in the human body and further that now where in the body do two hard surfaces come together in this fashion without the protection of cartilage. The effects of this force are cumulative over long periods of time. For example, in 1 month, the average patient will have the teeth in heavy occlusal contact for about 4 hours.
  176. 176. In light of the facts that in the oral environment we have other debilitating factors at work such as the presence of periodontal disease, it is the best interest of the patient to avoid loading these various structures beyond the normal limits. Tooth to tooth contact will damage the occlusal surfaces, exacerbate pre-existing periodontal disease processes, or both when it is excessive, as in bruxism. Another factor that should be taken into account in this matter is the periodontal maintenance of case. The removable partial dentures can be used, if designed properly, as a periodontal splint, even though the tooth or teeth requiring the splinting are not in the same area of the arch as the teeth being replaced.
  177. 177. Obviously, the fixed partial denture cannot provide this benefit, at least not one that is properly designed. It is true that in theory some of the same splinting effect could be provided by roundhouse, or full, arch fixed bridge, but the other disadvantage of such a restoration more than outweigh the possible benefits. In addition, the oral hygiene difficulty in the area of the missing teeth, particularly surfaces of the abutment teeth facing the pontic areas, is greater in the case of a removable restoration. In actual fact, for all of its advantages, the fixed restoration will nearly always leave the patient with a greater problem of access for cleaning than if the teeth had never been replaced.
  178. 178. Review of literature Aydinlik E, Dayangac B, Celik E. (1983), investigated the effect of splinting on abutment tooth movement and concluded that a significant decrease in the magnitude of movement resulted when the abutment teeth were splinted.
  179. 179. Revah A, Rehany A, Zalkind M, Stern N. (1985) discussed the problem of achieving a common path of insertion for a fixed partial denture when a tilted posterior abutment is involved and concluded that the problem can usually be solved by well planned tooth preparation in conjunction at times with intentional endodontic therapy. When tooth preparation alone cannot solve the problem, the mechanical solutions of the locked attachment and the telescopic retainer are available and must be considered.
  180. 180. Ziada HM, Orr JF, Benington IC. (1989), analyzed the stresses induced in a pier retainer of an anterior resin-bonded fixed partial denture and concluded that the use of pier abutments should be avoided and it is more favorable to use 3-unit resin-bonded fixed partial dentures.
  181. 181. Farah JW, Craig RG, Meroueh KA. (1989), used a two-dimensional finite element model of a mandibular quadrant to examine differences in magnitude of the principal stresses from the placement of three- and four-unit bridges. No significant differences in magnitude were observed between the three- and four-unit bridge. From a stress standpoint the bridges resulted in more uniform stress distribution around the abutments and an increase in the tensile stress distal to the abutments. Such findings support the placement of a fixed bridge to maintain bone in an edentulous area.
  182. 182. Awadalla HA, Azarbal M, Ismail YH, elIbiari W. (1992), constructed a three-dimensional mathematical model representing a three-unit cantilever fixed partial denture and its supporting mandibular structures. The results showed that a cantilever pontic creates considerable compressive stress on the abutment nearest to the pontic and produces tensile stress on the abutment farthest from the pontic.
  183. 183. Kerschbaum T, Haastert B, Marinello CP. (1996), concluded that rebonded resin-bonded fixed partial dentures developed a greater risk of debonding. The risk of failure for refabricated fixed partial dentures was similar to that of the originally inserted resin-bonded fixed partial dentures. There were no signs of greater caries incidence after multiple recementation procedures.
  184. 184. el-Mowafy OM. (1998), described an uncomplicated clinical procedure to enhance the retention of posterior resin-bonded fixed partial dentures which involves some modifications to the preparation and casting design and requires slightly more time and attention at the cementation stage of the prosthetic treatment.
  185. 185. Yang HS, Lang LA, Felton DA. (1999), analyzed the stress levels in the teeth and supporting structures of a fixed prosthesis and ascertained how the addition of multiple abutments in a fixed prosthesis modifies the stresses and their deflection and concluded that Increasing the number of the splinted abutment did not compensate for the mechanical problems of a long-span fixed partial denture sufficiently.
  186. 186. Nishimura RD, Ochiai KT, Caputo AA, Jeong CM. ( 1999), examined stress transfer patterns with variable implant support and simulated natural teeth through rigid and nonrigid connection under simulated functional loads and concluded that lower stresses apical to the tooth or implant occurred with forces applied further from the supporting abutment. Although the least stress was observed when using a nonrigid connector, the rigid connector in particular situations caused only slightly higher stresses in the supporting structure.
  187. 187. The rigid connector demonstrated more widespread stress transfer in the 2 implantsupported restoration. Recommendations for selection of connector design should be based on sound clinical periodontal health of a tooth and the support provided by implants.
  188. 188. Botelho M. (2000), concludes that the 2-unit prosthesis is successful and adds value to the clinical use of resin-bonded fixed partial dentures because the single-abutment prosthesis is even simpler and more cost effective than fixed-fixed designs. However, there is no evidence-based information relating to design principles for abutment preparation and framework design for the single-abutment, single-retainer prosthesis
  189. 189. Botelho MG, Nor LC, Kwong HW, Kuen BS (2000), evaluated the clinical retention and abutment movement of 2-unit cantilevered resinbonded fixed partial dentures (FPD) and concluded that two-unit cantilevered resin-bonded FPDs are successful in the short term, but further research is required to determine if they offer a viable alternative to fixed-fixed resin-bonded FPD designs.
  190. 190. Koutayas SO, Kern M, Ferraresso F, Strub JR. (2002), evaluated the influence of the framework design on the fracture strength of allceramic resin-bonded fixed partial dentures (RBFPD) in the mandibular incisor region and concluded that the clinical application of cantilevered all-ceramic RBFPDs in the mandible may be an alternative to all-ceramic RBFPDs with two retainers.
  191. 191. Hood JA, Farah JW, Craig RG. Investigated the stresses induced in the supporting bone by a tilted molar tooth under load and following conclusions were reached: 1. Altering the angle of the load applied to the unsupported molar from 0 (axial) to 30 degrees resulted in a fourfold increase in compressive stress in the supporting bone mesial to the tooth. 2. Increasing the load from 30 to 90 pounds while maintaining a 30 degree angle of application resulted in a linear increase in the shear stress on the supporting bone mesial to the tooth.
  192. 192. 3. Following the placement of a fixed partial denture, the induced stress at a point on the mesial aspect of the molar tooth, subjected to a 60 pound load at 30 degrees to the long axis, was reduced from 241 to 43 p.s.i. 4. The introduction of a fixed partial denture resulted in a decrease in the compressive stress in the bone adjacent to the apex of the mesial root of the molar from 481 to 174 p.s.i. 5. A distributed 120 pound load applied over the length of the fixed partial denture compared against individual tooth loadings of 60 pounds revealed that placement of the fixed partial denture favored the tilted molar at the expense of the premolar.