Lecture 1: An Introduction to Fixed Prosthodontics SDS413 PDF

1 An Introduction to Fixed Prosthodontics The scope of fixed prosthodontics treatment can range from the restoration of a single tooth to the rehabilitation of the entire occlusion. Single teeth can be restored to full function, and improvement in esthetics can be achieved. Missing teeth can be...

1 An Introduction to Fixed Prosthodontics The scope of fixed prosthodontics treatment can range from the restoration of a single tooth to the rehabilitation of the entire occlusion. Single teeth can be restored to full function, and improvement in esthetics can be achieved. Missing teeth can be replaced with fixed prostheses that will improve patient comfort and masticatory ability, maintain the health and integrity of the dental arches, and, in many instances, elevate the patient’s self-image. It is also possible, through the use of fixed restorations, to render an optimal occlusion that improves the orthopedic stability of the temporomandibular joints (TMJs). On the other hand, with improper treatment of the occlusion, it is possible to create disharmony and damage to the stomatognathic system. Terminology A crown is a cemented or permanently affixed extracoronal restoration that covers, or veneers, the outer surface of the clinical crown. It should reproduce the morphology and contours of the damaged coronal portions of a tooth while performing its function. It should also protect the remaining tooth structure from further damage. If it covers the entire clinical crown, the restoration is called a full veneer, full coverage, complete, or just a full crown (Fig 1-1). It may be fabricated entirely of a gold alloy or another untarnishable metal, a ceramic veneer fused to metal, an all-ceramic material, resin and metal, or resin only. If only portions of the clinical crown are veneered, the restoration is called a partial coverage or partial veneer crown (Fig 1-2). Intracoronal restorations are those that fit within the anatomical contours of the clinical crown of a tooth. Inlays may be used as single-tooth restorations for Class II proximo-occlusal or Class V gingival lesions with minimal to moderate extensions. They may be made of gold alloy (Fig 1-3a), a ceramic material (Fig 1-3b), or processed resin. When modified with occlusal coverage, the intracoronal restoration is called an onlay and is useful for restoring more extensively damaged posterior teeth needing wide mesio-occlusodistal (MOD) restorations (Fig 1-4). Another type of cemented restoration that has gained considerable popularity in recent years is the all-ceramic laminate veneer, or facial veneer (Fig 1-5). It is used on anterior teeth that require improved esthetics but are otherwise sound. It consists of a thin layer of dental porcelain or cast ceramic that is bonded to the facial surface of the tooth with an appropriate resin. The fixed partial denture is a prosthetic appliance that is permanently attached to remaining teeth or implants and replaces one or more missing teeth (Fig 1-6). In years past, this type of prosthesis was known as a bridge, a term that has fallen from favor1,2 and is no longer used. A tooth or implant serving as an attachment for a fixed partial denture is called an abutment. The artificial tooth suspended from the abutments is a pontic. The pontic is connected to the fixed partial denture retainers, which are extracoronal restorations that are cemented to or otherwise attached to the abutment teeth or implants. Intracoronal restorations lack the necessary retention and resistance to be used as fixed partial denture retainers. The connectors between the pontic and the retainer may be rigid (ie, solder joints or cast connectors) or nonrigid (ie, precision attachments or stress breakers) if the abutments are teeth. As a rule, only rigid connectors are used with implant abutments. Diagnosis A thorough diagnosis of the patient’s dental condition must first be made, considering both hard and soft tissues. This must be correlated with the individual’s overall physical health and psychologic needs. Using the diagnostic information that has been gathered, it is then possible to formulate a treatment plan based on the patient’s dental needs, mitigated to a variable degree by his or her medical, psychologic, and personal circumstances. Fig 1-1 A full veneer, full coverage, or complete crown covers the entire clinical crown of a tooth. The example shown is a metal-ceramic crown. Fig 1-2 A partial veneer or partial coverage crown covers only portions of the clinical crown. The facial surface is usually left unveneered. Fig 1-3 Inlays are intracoronal restorations with minimal to moderate extensions made of gold alloy (a) or a ceramic material (b). There are five elements to a good diagnostic work-up in preparation for fixed prosthodontic treatment: 1. Health history 2. TMJ and occlusal evaluation 3. Intraoral examination 4. Diagnostic casts 5. Full-mouth radiographs Health history It is important that a good history be taken before the initiation of treatment to determine if any special precautions are necessary. Some elective treatments might be canceled or postponed because of the patient’s physical or emotional health. It may be necessary to premedicate patients with certain conditions or to avoid medication for others. It is not within the scope of this book to describe all the conditions that might influence patient treatment. However, there are some whose frequency or threat to the patient’s or office staff’s well- being is significant enough to merit discussion. A history of infectious diseases, such as serum hepatitis, tuberculosis, and human immunodeficiency virus (HIV)/AIDS, must be known so that protection can be provided for other patients as well as office personnel. There are numerous conditions of a noninfectious nature that also can be important to the patient’s well-being. Fig 1-4 An onlay is an intracoronal restoration with an occlusal veneer. Fig 1-5 A laminate veneer is a thin layer of porcelain or cast ceramic that is bonded to the facial surface of a tooth with resin. Fig 1-6 The components of a fixed partial denture. Medications The patient should be asked what medications, prescribed or over-the-counter, are currently being taken and for what purpose.3 It is important to be aware that an estimated 25% of the population is taking some type of herbal product.4 All medications should be identified and their contraindications noted before proceeding with treatment. The patient should be questioned about current medications at each subsequent appointment to ensure that information on the patient’s medication regimen is kept Fig 6-12 An MOD onlay for a maxillary premolar. Metal inlay Teeth with low esthetic requirements and small- to moderatesized lesions can be restored with metal inlay restorations (Fig 6-10). Although usually made of softer gold alloys, metal inlays also can be fabricated of etchable base metal alloys if a bonding effect is desired.11,12 The preparation isthmus should be narrow to minimize stress in the surrounding tooth structure. Premolars should have one intact marginal ridge to preserve structural integrity and minimize the possibility of coronal fracture. The additional bulk of tooth structure found in a molar permits the use of this restoration type in an MOD configuration. The indications for this type of restoration are much the same as those for an amalgam because this restoration only replaces lost tooth structure and will not protect remaining tooth structure. Because of the amount of destruction of tooth structure required by this restoration, it is not recommended for incipient lesions. Ceramic inlay Ceramic inlay restorations are used to restore teeth with smallto moderate-sized lesions that permit a narrow preparation isthmus in an area of the mouth where the esthetic demand is high. Premolars should have one intact marginal ridge, but MOD ceramic inlays can be used in molars (Fig 6-11). This type of restoration can also be etched to enhance bonding, and there is some evidence that the structural integrity of the tooth cusps may be stabilized by bonding.13 The relatively large size of the cavity preparation required for this restoration precludes its use in the treatment of incipient lesions. Mesio-occlusodistal onlay This design can be used for restoring moderately large lesions on premolars and molars with intact facial and lingual surfaces (Fig 6-12). It will accommodate a wide isthmus and up to one missing cusp on a molar. If a cast metal restoration is needed on a premolar with both marginal ridges compromised, it should include occlusal coverage to protect the remaining tooth structure. This restoration also can be considered an extracoronal restoration because of the occlusal coverage that overlays and protects the tooth cusps.  The MOD onlay does not have the necessary resistance to be used as a fixed partial denture retainer. Although ordinarily fabricated of a gold alloy, this restoration design has been used with cast glass and other types of ceramics. Ceramic MOD onlays should be used very cautiously. Without generous occlusal thickness, these restorations are susceptible to fracture. Extracoronal Restorations If insufficient coronal tooth structure exists to retain the restoration within the crown of the tooth, an extracoronal restoration, or crown, is needed. It may also be used where there are extensive areas of defective axial tooth structure or if there is a need to modify contours to refine occlusion or improve esthetics. Partial coverage crown This is a crown that leaves one or more axial surfaces uncovered (Fig 6-13). Therefore, it can be used to restore a tooth with one or more intact axial surfaces with half or more of the coronal tooth structure remaining. It will provide moderate retention and can be used as a retainer for short-span fixed partial dentures. If tooth destruction is not excessive, a partial coverage crown with a minimally extended preparation and carefully finished margins can satisfy moderate esthetic demands in the maxillary arch. All-metal crown The all-metal conventional crown can be used to restore teeth with multiple defective axial surfaces (Fig 6-14). It will provide the maximum retention possible in any given situation, but its use must be restricted to situations where there are no esthetic expectations. This will usually limit it to second molars, some mandibular first molars, and occasionally mandibular second premolars. Because less tooth structure must be removed for its preparation than for crowns with a ceramic component, and because its fabrication is the simplest of any crown, this restoration should remain among those designs considered in planning single-tooth restorations on molars as well as posterior fixed partial dentures. Metal-ceramic crown A metal-ceramic crown also can be used to restore teeth with multiple defective axial surfaces (Fig 6-15). It, too, is capable of providing maximum retention, but it also will meet high esthetic requirements. It can be used as a fixed partial denture retainer where full coverage and a good cosmetic result must be combined. Fig 6-13 A three-quarter crown being seated on a molar. Fig 6-14 An all-metal full crown on a maxillary second molar. Fig 6-15 A metal-ceramic crown on a maxillary premolar. Fig 6-16 An all-ceramic crown on an incisor. Fig 6-17 A ceramic veneer on a maxillary incisor. All-ceramic crown When full coverage and maximum esthetics must be combined, an all-ceramic crown is the treatment of choice (Fig 6-16). All-ceramic crowns are not as resistant to fracture as metal-ceramic crowns, so their use must be restricted to situations likely to produce low to moderate stress. They are usually used for incisors, although cast glass ceramics are also employed in the restoration of posterior teeth. Preparations for this type of restoration on premolars and molars require the removal of large quantities of tooth structure. Ceramic veneer  Because all-ceramic and metal-ceramic crowns require the removal of such large quantities of tooth structure, there has been considerable interest in less destructive alternatives. The ceramic veneer has emerged as a means of producing an esthetic result on otherwise intact anterior teeth that are marred by severe staining or developmental defects restricted to the facial surface of the tooth (Fig 6-17). This restoration also can be used to restore moderate incisal chipping and small proximal lesions. The use of a veneer requires only minimal tooth preparation and therefore offers an alternative to crowns that is attractive to the patient and dentist alike. The features and capabilities of the 12 types of singletooth restorations described in this chapter are shown in Table 6-1. Table 6-1 Features and applications of single-tooth restorations Restoration Longevity Every dentist would like to be able to answer the patient’s question, “How long will my restoration last?” Logical though this question may be, unfortunately it is impossible to answer directly. We cannot predict the life span of a pair of shoes or a television set, and these everyday items are not custom made, nor do they perform their service in a hostile biologic environment, submerged in water. Clinical studies of restoration longevity have produced widely disparate figures. As a general rule, cast restorations will survive in the mouth longer than amalgam restorations, which in turn will last longer than composite resin restorations. 14 A compilation of five studies of 676 patients concluded that amalgam restorations exhibit a 50% failure rate between 5.5 and 11.5 years, with an extrapolated life expectancy of 10 to 14 years.15 Meeuwissen et al16 reported a 10-year survival rate of 58% for amalgam restorations in Dutch military patients; Arthur et al17 reported an 83% survival rate for the same time span in a US military population. Qvist et al18 found that 50% of the amalgam restorations in a group of Danish patients had failed at 7 years. Christensen19 estimated a 14-year longevity for amalgam restorations. In selected populations, amalgam restorations of unspecified types or sizes in one study14 have shown 10-year survival rates as high as 72%. A 15-year survival rate of 72.8% was reported for simple amalgams in another study.20 A survey of 571 fixed prosthodontists, nonspecialist restorative dentists, and dental school faculty projected an average life span of 11.2 years for simple amalgams and 6.1 years for complex amalgams.21 One group of 125 complex amalgams was reported to have a 76% survival rate at 15 years,20 whereas another group of 171 complex amalgam restorations exhibited a 50% survival rate at 11.5 years.22 Composite resin restorations have not been included in many longevity studies. A study of dental school patients that did incorporate them reported a 10-year survival rate of 55.9%.14 Another report, based on a general patient population, described a shorter life span for composite resin restorations, with 50% of them having failed in 6.1 years.23 Mount24 disclosed an overall success rate of 93% for 1,283 glass-ionomer restorations for up to 7 years, with the rate varying from 2% to 36% depending on the class of cavity and the brand of reason for selecting a removable rather than a fixed partial denture. If there has been a severe loss of tissue in the edentulous ridge, a removable partial denture can more easily be used to restore the space both functionally and esthetically. For successful removable partial denture treatment, the patient should demonstrate acceptable oral hygiene and show signs of being a reliable recall candidate. Table 7-1 Types of prostheses used for the replacement of missing teeth Patients of advanced age who are on fixed incomes or have systemic health problems may require special treatment simplification efforts, either to cut down on the amount of appointment time required to restore the mouth or to make the treatment affordable. Cajoling patients of limited means into overinvesting their resources is not in their best interest. A large tongue is a good reason to avoid a removable prosthesis if at all possible, as is a lack of muscular coordination. An unfavorable attitude toward a removable partial denture also makes it a poor choice. Conventional tooth-supported fixed partial denture When a missing tooth is to be replaced, a fixed partial denture is preferred by the majority of patients. The usual configuration for a fixed partial denture uses an abutment tooth on each end of the edentulous space to support the prosthesis. If the abutment teeth are periodontally sound, the edentulous span is short and straight, and the retainers are well designed and executed, the fixed partial denture can be expected to provide a long life of function for the patient. Several factors influence the decisions of whether to fabricate a fixed partial denture, what teeth to use as abutments, and what retainer designs to use (see Table 7-1). There should be no gross soft tissue defect in the edentulous ridge. If there is, it may be possible to augment the ridge with grafts to enable the construction of a fixed prosthesis. This treatment is reserved for patients who are both highly motivated and able to afford this special procedure. If the patient does not meet these criteria, a removable partial denture should be considered. A dry mouth creates a poor environment for any crown. The margins of the retainers will be at great risk from recurrent caries, limiting the life span of the prosthesis. However, an absence of moisture in the mouth also will hinder the success of a removable partial denture. In either case, the patient must be made aware of the high risk involved. The risk may be minimized through home fluoride application and frequent recall, but it cannot be eliminated. Resin-bonded tooth-supported fixed partial denture The resin-bonded fixed partial denture is a conservative restoration that is reserved for use on defect-free abutments in situations where there is a single missing tooth, usually an incisor or premolar. A single molar can be replaced by this type of prosthesis if the patient’s muscles of mastication are not too well developed, thus assuring that a minimum load will be placed on the retainers. The resin-bonded fixed partial denture requires an abutment both mesial and distal to the edentulous space. This prosthesis utilizes a standard pontic form, accommodating an edentulous ridge with moderate resorption and no gross soft tissue defects. Because it requires a shallow preparation that is restricted to enamel, the resin-bonded fixed partial denture is especially useful in younger patients whose immature teeth with large pulps are poor candidates for endodonticfree abutment preparations.  Tilted abutments can be accommodated only if there is enough tooth structure to allow a change in the normal alignment of axial reduction. This is limited by the need to restrict most of the reduction to enamel. Rarely can a mesiodistal difference in abutment inclination greater than 15 degrees be accommodated. There can be little or no difference in the inclination of the abutments faciolingually. The resin-bonded prosthesis cannot be used for replacing missing anterior teeth where there is a deep vertical overlap. Reduction deep into the underlying dentin of the abutment teeth will be required in this situation, so a conventional fixed partial denture should be employed. Although this type of prosthesis has been described for periodontal splints, it should be used with extreme care in those situations. Preparations will demand additional resistance features, such as long, well-defined grooves. Abutment mobility has been shown to be a serious hazard in the successful use of this type of restoration. Implant-supported fixed partial denture Fixed partial dentures supported by implants are ideally suited for use where there are insufficient numbers of abutment teeth or inadequate strength in the abutments to support a conventional fixed partial denture and when patient attitude and/or a combination of intraoral factors make a removable partial denture a poor choice. Implant-supported fixed partial dentures can be employed in the replacement of teeth when there is no distal abutment. Span length is limited only by the availability of alveolar bone with satisfactory density and thickness in a broad, flat ridge configuration that will permit implant placement. A single tooth can be replaced by a single implant, saving defect-free adjacent teeth from the destructive effects of retainer crown preparations. A span length of two to six teeth can be replaced by multiple implants, either as singleunit restorations or as implant-supported fixed partial dentures. In fact, an entire arch can be replaced by an implant-supported complete prosthesis, but that type of restoration lies outside the realm of this discussion. The retainers used for most implant systems require a great degree of abutment alignment precision, as do the retainers for a tooth-supported fixed partial denture. If implants are placed by someone other than the restorative dentist, implant/abutment alignment demands close coordination between surgeon and restorative dentist. The abutments should be positioned so that the occlusal forces will be as nearly vertical to the implants as possible to prevent destructive lateral forces. Implants should be better able than natural teeth to survive in a dry mouth. Implants may be a better choice for fixed partial denture abutments if prospective tooth abutments would require endodontic therapy with or without dowel cores, periodontal surgery, and possibly root resections to support a long-span, complex, and expensive prosthesis. No prosthetic treatment If a patient presents with a long-standing edentulous space into which there has been little or no drifting or elongation of the adjacent or opposing teeth, the question of replacement should be left to the patient’s wishes. If the patient perceives no functional, occlusal, or esthetic impairment, it would be a dubious service to place a prosthesis. This in no way contradicts the recommendation that a missing tooth routinely should be replaced. The teeth adjoining an edentulous space usually move, but they do not always move. When meeting the occasional patient who has beaten the odds, the dentist should recognize it for what it is, congratulate the patient for being fortunate, and tend to his or her other needs. Fig 7-22 The amount of faciolingual movement (in μm) for each tooth in the maxillary arch (based on data by Rudd et al16). The direction of movement, indicated by arrows, varies considerably from the anterior to the posterior segment of the arch. Special Problems Some problem situations occur often enough to deserve mention. Some of the commonly used solutions to the problems are also presented. Pier abutments Rigid connectors (eg, solder joints) between pontics and retainers are the preferred way of fabricating most fixed partial dentures. A fixed partial denture with the pontic rigidly fixed to the retainers provides desirable strength and stability to the prosthesis while minimizing the stresses associated with the restoration. However, a completely rigid restoration is not indicated for all situations requiring a fixed prosthesis. An edentulous space can occur on both sides of a tooth, creating a lone, freestanding pier abutment (Fig 7-21). Physiologic tooth movement, arch position of the abutments, and a disparity in the retentive capacity of the retainers can make a rigid fiveunit fixed partial denture a less-than-ideal treatment plan. Studies in periodontometry have shown that the faciolingual movement ranges from 56 to 108 μm,16 and intrusion is 28 μm.17 Teeth in different segments of the arch move in different directions.18 Because of the curvature of the arch, the faciolingual movement of an anterior tooth occurs at a considerable angle to the faciolingual movement of a molar (Fig 7-22). These movements of measurable magnitude and in divergent directions can create stresses in a long-span prosthesis that will be transferred to the abutments. Because of the distance through which movement 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. It has been theorized that forces are transmitted to the terminal retainers as a result of the middle abutment acting as a fulcrum, causing failure of the weaker retainer.19 However, photoelastic stress analysis and displacement measurement indicate that the prosthesis bends rather than rocks. Standlee and Caputo20 suggest that tension between the terminal retainers and their respective abutments, rather than a pier fulcrum, is the mechanism of failure. Intrusion of the abutments under the loading could lead to failure between any retainer and its respective abutment. The loosened casting will leak around the margin, and caries is likely to become extensive before discovery. The retention on an anterior tooth is usually less than that of a posterior tooth because of its generally smaller dimensions. Because there are limits to increasing a retainer’s capacity to withstand displacing forces, some means must be used to neutralize the effects of those forces. The use of a nonrigid connector has been recommended to reduce this hazard.19 In spite of an apparently close fit, the movement in a nonrigid connector is enough to prevent the transfer of stress from the segment being loaded to the rest of the fixed partial denture (Fig 7-23). The nonrigid connector is a broken-stress mechanical union of retainer and pontic instead of the usual rigid connector. The most commonly used nonrigid design consists of a T-shaped key that is attached to the pontic and a dovetail keyway placed within a retainer. Fig 7-23 A nonrigid connector on the middle abutment isolates force to the segment of the fixed partial denture to which it is applied. (Reprinted from Shillingburg and Fisher19 with permission.) Fig 7-24 If a nonrigid connector is placed on the distal side of the retainer on a middle abutment, movement in a mesial direction will seat the key into the keyway. (Reprinted from Shillingburg and Fisher19 with permission.) Fig 7-25 If a nonrigid connector is placed on the mesial side of the middle abutment, mesially directed movement will unseat the key. (Reprinted from Shillingburg and Fisher19 with permission.) Use of the nonrigid connector is restricted to a short-span fixed partial denture replacing one tooth.21 The magnification of force created by a long span is too destructive to the abutment tooth under the soldered retainer. Prostheses with nonrigid connectors should not be used if prospective abutment teeth exhibit significant mobility. There must be equal distribution of occlusal forces on all parts of the fixed partial denture. A nonrigid fixed partial denture transfers shear stress to supporting bone rather than concentrating it in the connectors. It appears to minimize mesiodistal torquing of the abutments while permitting them to move independently.22 A rigid fixed partial denture distributes the load more evenly than a nonrigid design, making it preferable for teeth with decreased periodontal attachment.23 If the posterior abutment and pontic are either opposed by a removable partial denture or unopposed, and if the three anterior units are opposed by natural teeth, the key and the posterior units that are subjected to little or no occlusal forces may supererupt. The location of the stress-breaking device in the five-unit pier-abutment restoration is important. It is usually placed on the middle abutment because placement 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 a mesial direction, and vertically applied occlusal forces produce further movement in this direction. Nearly 98% of posterior teeth tilt mesially when subjected to occlusal forces.24 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 solidly19 (Fig 7-24). Placement of the keyway on the mesial side, however, causes the key to be unseated during mesial movements20 (Fig 7-25). In time, this could produce a pathologic mobility in the canine or failure of the canine retainer. Fig 7-26 When a mandibular molar tilts mesially, there is a discrepancy between its long axis and that of the premolar. Fig 7-27 This fixed partial denture will not seat because the tooth distal to the fixed partial denture intrudes on the path of insertion (arrow). Fig 7-28 Orthodontic appliance for uprighting a tilted molar: (a) occlusal view; (b) facial view. Tilted molar abutments A problem that occurs with some frequency is a mandibular second molar abutment that has tilted mesially into the space formerly occupied by the first molar. It is impossible to prepare the abutment teeth for a fixed partial denture along the long axes of the respective teeth and achieve a common path of insertion (Fig 7-26). There is further complication if the third molar is present. It usually will have drifted and tilted with the second molar. Because the path of insertion for 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 (Fig 7-27). If the encroachment is slight, the problem can be remedied by restoring or recontouring the mesial surface of the third molar. However, the overtapered second molar preparation must have its retention bolstered by the addition of facial and lingual grooves. If the tilting is severe, more extensive corrective measures are called for. The treatment of choice is the uprighting of the molar by orthodontic treatment. In addition to placing the abutment tooth in a better position for preparation and for distribution of forces under occlusal loading, uprighting the molar also helps to eliminate bony defects along the mesial surface of the root. Uprighting is best accomplished through the use of a fixed appliance.25 Both premolars and the canine are banded and tied to a passive stabilizing wire (Fig 7-28). A helical uprighting spring is inserted into a tube on the banded molar and activated by hooking it over the wire on the anterior segment.25,26 This is frequently followed by the use of an open coil spring to complete the uprighting and bring the tooth into the best possible alignment for fabrication of the fixed restoration. The average treatment time required is 3 months.27  The third molar, if present, is often removed to facilitate the distal movement of the second molar. The second molar will arc occlusally as it moves distally; therefore, it must be watched closely and ground out of occlusion to allow it to continue moving. Immediately upon removal of the appliance, the teeth are prepared, and a provisional fixed partial denture is fabricated to prevent posttreatment relapse.28 Fig 7-29 Fixed partial denture using a proximal half crown as a retainer on a tilted molar abutment. Fig 7-30 Fixed partial denture using a telescope crown and coping as a retainer on a tilted molar abutment. Fig 7-31 A nonrigid connector on the distal aspect of the premolar retainer compensates for the inclination of the tilted molar. If orthodontic correction is not possible, or if it is possible to achieve only a partial correction, a fixed partial denture can still be made. It has been suggested that the long axis of the prospective abutments should converge by no more than 25 to 30 degrees.29 Photoelastic30 and finite element31 stress analyses have shown that a molar that has tipped mesially will actually exhibit less stress in the alveolar bone, along the mesial surface of its mesial root, with a fixed partial denture than without it. There will be an increase in stress along the premolar, however. A proximal half crown sometimes can be used as a retainer on the distal abutment32 (Fig 7-29). This preparation design is simply a three-quarter crown that has been rotated 90 degrees so that the distal surface is uncovered. This retainer can be used only if the distal surface itself is untouched by caries or decalcification and if there is a very low incidence of proximal caries throughout the mouth. The patient must also demonstrate an ability to keep the area exceptionally clean. 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. A telescope crown and coping can also be used as a retainer on the distal abutment.33 A full crown preparation with heavy reduction is made to follow the long axis of the tilted molar. An inner coping is made to fit the tooth preparation, and the proximal half crown that will serve as the retainer for the fixed partial denture is fitted over the coping (Fig 7-30). This restoration allows for total coverage of the clinical crown while compensating for the discrepancy between the paths of insertion of the abutments. The marginal adaptation for this restoration is provided by the coping. The nonrigid connector is another solution to the problem of the tilted fixed partial denture abutment (Fig 7-31). A full crown preparation is done on the molar, with its path of insertion parallel with the long axis of that tilted tooth. A box form is placed in the distal surface of the premolar to accommodate a keyway in the distal of the premolar crown. It is tempting to place the connector on the mesial aspect of the tipped molar, but this could lead to even greater tipping of the tooth. A nonrigid connector for the tipped molar abutment is most useful when the molar exhibits a marked lingual as well as mesial inclination. Preparing a tooth with a combined mesial and lingual inclination as an abutment for a routine fixed partial denture can lead to a drastically over-tapered preparation with no retention. Because telescope crowns and nonrigid connectors both require tooth preparations that are more destructive than normal, the selection of one of these would be influenced by the nature of previous destruction of the prospective abutment teeth. The presence of a dowel core or a disto-occlusal amalgam on the premolar, for example, would favor placement of a nonrigid connector on that tooth, while extensive facial and/or lingual restorations on the tilted molar would call for the use of a telescope crown. Fig 7-32 A fixed partial denture replacing a maxillary canine is subjected to more damaging stresses than that replacing a mandibular canine because the forces are directed outward and the pontic lies farther outside the interabutment axis.v Fig 7-33 A fixed partial denture replacing a mandibular canine has a more favorable prognosis than that replacing a maxillary canine because the forces are directed inward and the pontic will be closer to the interabutment axis. 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 incisor, usually the weakest tooth in the entire arch, and the first premolar, the weakest posterior tooth. A fixed partial denture replacing a maxillary canine is subjected to more stresses than that replacing a mandibular canine because forces are transmitted outward (labially) on the maxillary arch, against the inside of the curve (its weakest point) (Fig 7-32). On the mandibular canine, the forces are directed inward (lingually), against the outside of the curve (its strongest point) (Fig 7-33). Any fixed partial denture replacing a canine should be considered a complex 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 partial denture. 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, and it is frequently misused. In the routine three-unit fixed partial denture, force that is applied to the pontic is distributed equally to the abutment teeth (Fig 7-34). If there is only one pontic and it is near the interabutment axis line, less leverage is applied to the abutment teeth and to the retainers than with a cantilever. 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 (Fig 7-35). Prospective abutment teeth for cantilever fixed partial dentures should be evaluated with an eye toward lengthy roots with a favorable configuration, long clinical crowns, good crown-root ratios, and healthy periodontium.34 Generally, cantilever fixed partial dentures should replace only one tooth and have at least two abutments.35,36 A cantilever can be used for replacing a maxillary lateral incisor (Fig 7-36). There should be no occlusal contact on the pontic in either centric or lateral excursions.37 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. There should be a rest on the mesial of the pontic against a rest preparation in an inlay or other metallic restoration on the distal of the central incisor to prevent rotation of the pontic and abutment. The mesial aspect of the pontic can be slightly wrapped around the distal portion of the uninvolved central incisor to stabilize the pontic faciolingually.37 The root configuration of a central incisor makes it an undesirable cantilever abutment. Fig 7-34 Forces applied to the pontic of a routine fixed partial denture are transmitted to both abutment teeth.