April 2011, Volume 7, Issue 4
Published by AEGIS Communications
Fixed Partial Dentures
Options for the dentist and the patient.
By Bruce W. Small, DMD, MAGD
Fixed partial dentures (FPDs) are "dental prostheses that are luted, screwed, or mechanically attached or otherwise securely retained to natural teeth, tooth roots, and/or dental implant abutments."1 During the past decades, many types of FPDs or "bridges" have been used to replace missing teeth. With the introduction and widespread use of osseointegrated implants, many missing teeth are now being replaced in this manner rather than with FPDs. Dental bridges can, of course, still be used successfully, and this article will briefly review the many methods of bridge construction and relate them to their applicability and current acceptance of the practicing dentist and the treated patient. These will include: cast-gold, stress-broken bridges; resin-bonded, etched retainers; porcelain-fused-to-metal (PFM) bridges; and all-ceramic bridges, including zirconia.
Cast-Gold and Stress-Broken Bridges
The oldest fixed method of replacing missing teeth is using cast gold. Depending on the case, the retaining abutments can be pinledges,2,3 inlays4,5 (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12), onlays, seven-eights crowns,6 or full-cast crowns sometimes covered with composite or porcelain7 for esthetics (see Figure 5, Figure 6 and Figure 7). Cast gold remains the "standard of care" to which all other materials are compared.8 Advantages of using high-noble gold are longevity, biocompatibility, accuracy of fit, marginal integrity, strength, and versatility. The only disadvantage in some cases is esthetics, but with proper planning, preparation, and laboratory support, cast-gold bridges can be made more esthetic.
Also, it is common for cast-gold bridges to be stress broken to allow for micro-movement of the abutment teeth during function and bruxing. When bridges are placed and are under function, stresses may develop in the connectors.9 This stress can lead to fracture and loosening of the bridge and pain for the patient. By constructing bridges with a non-rigid connector, this stress can be decreased.10
Resin-Bonded, Etched Partial Dentures
In 1973, Rochette11 first published on a wing-like bridge, which had perforations in the wings that were then bonded to replace a missing tooth. Composite resin would come through the small holes and added mechanical retention to the bonding to enamel. These early bonded bridges were used by many dentists to replace anterior and posterior teeth,12 but did not have long-term success. In fact, in 1977 Howe and Denehy13 stated, "dentists should consider it a temporary restoration." The original metal used was a ceramic non-precious alloy 1 mm to 1.5 mm thick with feathered margins and without preparation of the teeth, which was a primary advantage of these early resin-bonded bridges.
Livaditis and Thompson14,15 improved on the Rochette bridge by chemically etching the metal framework using 3.5% nitric and 18% hydrochloric acid, thereby increasing the bond strength to enamel and dentin. This type of retainer was called the "Maryland Bridge" (Figure 13 and Figure 14). Barack16 and others have described various preparation techniques, including an inlay-type rest and proximal grooves, which have improved the longevity of these resin-etched bridges. One 11-year clinical study involving 127 restorations showed a success rate of 92.9%.17 Various attempts were made to increase the resistance form of the preparations by adding an occlusal rest, the establishment of guide planes through axial reduction, and a proximal extension to the facial surface to resist lingual displacement.17
Alternative materials have been used to fabricate these winged-type bridges, such as composite and fired ceramic,18 including a combination of the two with a porcelain veneer on the facial of the composite pontic. Shillinburg et al19 said that these bridges are "probably best suited for use as a long-term provisional restoration or immediate replacement of a missing tooth." Simonsen20 suggested their use in gerodontics, where less of a long-term successful prognosis may be accepted.
The most common type of bridge used today by practicing dentists is the PFM bridge. The alloys used are most commonly a noble metal combination of palladium and silver or high palladium. Other metals used include a high noble of gold-platinum, or gold-palladium, or gold-palladium and silver. A classification system of the American Dental Association based on the noble metal content divides the classes into high noble (60% with at least 40% gold), noble (25%), and predominantly base metal (less than 25% noble).
The dental literature continues to support their use due to their predictable strength and reasonable esthetics.21 Due to advancements in dental porcelains, new and improved firing methods, as well as the improved abilities of many porcelain technicians, the esthetics of PFM crowns and bridges has improved drastically during the last 30 years and, in some cases, approaches that of all-ceramic crowns (Figure 15 and Figure 16).
Because of its inherent strength, PFM can be used to span more than one edentulous site, be used on natural teeth as well as implants (although preferably not together—see below), and be used with stress breakers when indicated.
PFM bridges can be used to cantilever a pontic in selected cases. A recent review of the dental literature regarding cantilever FPDs was published and reported that, "the aggregate proportion of cantilever FPDs successful (free of all defects) at ten years was 63%."22 A missing maxillary lateral incisor is a good choice if a cantilever has to be employed, because in most cases the occlusal forces on the lateral pontic can be reduced in centric occlusion as well as all excursions. Two abutments are recommended when constructing a cantilevered bridge whenever possible in order to prevent dislodgement.23
Similar to any other restoration with porcelain, such as PFM bridges, there always remains a risk of porcelain fracture. This can occur as a result of laboratory error, restoration design, adjustment of the porcelain with a handpiece at try-in or insertion (which can cause micro-fractures), and the patient’s occlusion and/or bruxing. To help prevent this problem, the metal framework of the bridge must have a passive fit without any binding or movement of the abutment teeth. Larger bridge frameworks should be assembled in the mouth by connecting copings after a passive fit is achieved and then soldered in the laboratory.24 As the length of a single-piece FPD casting increases, so does its inaccuracy.25 Savion et al26 reported that bridges with a "pier" abutment, such as a five-unit with a abutment in the middle, is not a favorable situation.
Recent reports have been published regarding the use of computer-aided design and computer-aided manufacturing (CAD/CAM) of the metal frameworks for PFM bridges.27,28 These were pilot studies, but they did show that it was possible to accomplish. This procedure is now being done more routinely using zirconia as the substrate (see below). When a large one-piece bridge is being constructed, it is recommended that a cement be used that is strong enough to maintain the bridge in the mouth but will also allow removal by the dentist, such as a polycarboxylate-type cement when using a PFM bridge. The ability to remove a bridge is sometimes important, especially after fracture of the porcelain veneering material. When more adhesive cements are employed, such as resin cements, the removal may be more challenging.
Screw-Retained vs Cemented Implants and Bridges
Bridges have been placed on osseointegrated implants since their introduction in the early 1980s. In those days, all bridges were screw-retained with channels that allowed the operator to reach the screw attaching the abutment to the implant fixture. The successful long-term prognosis of screw retention has been documented by the Brånemark group,29 which introduced osseointegrated implants in the early 1980s. Since then, there have been many articles published on the advantages and disadvantages of either screw retention or cementing bridges directly to the titanium or other alloy abutments. Part of the reason for dentists beginning to cement bridges on implants was esthetics—because with screw-retained restorations, the access holes must be filled and many times can be noticed by esthetically driven patients. Chee and Jivraj30 reported on these two attachment options and discussed esthetics, retrievability, retention implant placement, passivity, provisionals, occlusion, immediate loading, impression procedures, and long-term treatment planning. All of these factors are case-dependent but a few deserve discussion.
Retrievability is a primary factor in using screw-retained prostheses on implants. In cases of porcelain fracture or other reasons to remove the implant-retained fixed prosthesis, the access channels could be opened and the screws removed. Depending on the cement used, a cemented case may not be as easy to remove, particularly when using resin cements and abutments with undercuts. Certain cements for natural teeth may form a chemical bond between the tooth and the abutment, making it more difficult to remove the case, but this does not occur with titanium abutments.31 Using the proper cement will allow the bridge to be removed.
Passivity is an important factor in implant bridges. Misch states, "A truly passive screw-retained dental restoration is virtually impossible to fabricate."32 Karl et al,33 in an in vivo study of four groups of five-unit bridges on implants, reported that there were none that revealed a truly passive fit. When a case is screwed down and it does not seat passively on the abutments, the tightening of the screws to 30 Ncm may cause distortion of the frame and fractures of the porcelain. When cementing the same case, there are not the same torquing forces, and therefore less bending and possible breakage (Figure 17 and Figure 18).
In cases of fewer and shorter abutments, a screwed-in case may be the treatment of choice rather than cemented for a longer prognosis. Depending on the available surface area, the abutment height, surface finish, and type of cement used are all factors that need to be considered even before the implants are placed.34 If the inter-arch distance is limited, screw retention is recommended because of the difficulty in creating adequate abutment height for cementation.
Occlusion is one of the most important factors in the success of any dental restoration. In the failing case in Figure 19, the implant access openings are not in ideal locations for an adequate occlusion. When implants are not placed ideally in a screw-retained case, an ideal cusp-to-fossa relationship can be difficult. The restorative material placed to cover the access openings may wear at a different rate than the surrounding porcelain, allowing occlusal imbalance, fracturing of the ceramic material, TMJ problems, and possible loosening of the restoration. In a cemented case, non-ideal implant placement can be hidden to an extent and better occlusion can be achieved.
Prior to implant placement, an attempt should be made to decide the retention method of the case. Cone beam 3-dimensional viewing of the patient’s osseous structures as well as the sinuses is a good starting point in making this important decision. Occlusion, available inter-arch space, retrievability, and the type of implant-retained FPD planned are also important factors that must be evaluated before fixture placement. A surgical stent of some kind can be useful, but during surgery a planned location may not be ideal because of a lack of bone density, infection, or approximation to other teeth or sinuses. If a screw-retained case is planned, the ideal location of the implants is under the center of the occlusal table of the crowns rather than on the marginal ridges or buccal and lingual cusps.
In immediate load cases—particularly full arches—it is recommended to use a screw-retained, implant-supported FPD because of the potential stress on the newly placed implants.30 Gentle tightening of the screws will allow adequate retention during the integration period, and then a final decision can be made on retentive mechanisms after complete healing. If cases are cemented immediately, the forces necessary to remove the bridge may cause loosening of the implants.
Another consideration in the retentive mechanisms of implant bridges is the possibility of additional work in the future. If there is a chance of redesigning a case or connecting an existing implant-supported bridge to additional adjacent implants, a screw-retained restoration may be easier to remove and replace after additional restorative procedures.
Connection of Implants and Natural Teeth
One of the unusual occurrences that have been noticed by restorative dentists is the intrusion of natural teeth when they are connected to implants with a bridge. This was first seen in the early 1990s, and various methods were suggested to prevent its occurrence. One of these was the use of a stress breaker to, theoretically, allow the movement of the natural tooth because of its periodontal ligament (Figure 20 and Figure 21). Weinberg35 published in 1994 that "a non-rigid attachment is recommended between a tooth-supported prostheses and an implant-supported prostheses when they are combined."
One of the theories that have been reported by Pesun36 was the idea that when a natural tooth was connected to an implant, it lost the normal stimulation of the periodontal ligament, which then produced atrophy of the ligament and intrusion of the teeth. More recent literature37,38 supports the rigid connection of implants to teeth. This research supports the conclusion that if a strong cement is used on the bridge, it may not completely prevent but will minimize the intrusion phenomena.
During the past 3 decades, clinical dentists have been placing non-metallic bridges made of all ceramic materials (Figure 22, Figure 23 and Figure 24). They have been constructed of many different types of materials, including: feldspathic porcelain, leucite-reinforced glass ceramic, aluminum oxide, lithium disilicate, and, most recently, zirconium oxide.39,40 The feldspathic bridges were not very strong (Figure 25) and were prone to fracture in both the anterior and posterior of the mouth. It was not until pressed leucite-reinforced ceramic began to be used for anterior bridges that any degree of success was attained.
Other stronger ceramics have been tried as frameworks, such as aluminum oxide, and a newer material called lithium disilicate (Figure 26 and Figure 27) (pressed or CAD/CAM), but studies have shown that these had a higher failure rate than when zirconia was used. Zirconia crowns and bridges are the newest addition to the clinician’s choices for a non-metallic restoration (Figure 28 , Figure 29 and Figure 30). Sailer4 recently completed a study of zirconia bridges of 57 three- to five-unit posterior bridges in 45 subjects where CAD/CAM frameworks were veneered with porcelain and cemented with resin cement. At the 5-year recall, only 33 of the bridges were left and 12 of those needed to be replaced. Marginal gaps were found in almost 60% of the cases with secondary caries. Only 3% of the frameworks fractured, but 15.2% of the bridges had fracturing of the surface porcelain.
Chipping of the veneered porcelain has been a common problem with zirconia frameworks. Swain42 has reported that the unstable chipping of the veneering porcelain could be caused by the difference in the thermo-elastic properties between the zirconia frameworks and the veneering material. Two-year results in an ongoing clinical study by Clinician’s Report43 comparing PFM bridges with those made from a zirconia framework revealed "external ceramic fractures were five times more prevalent with ceramic formulations used on zirconia versus those used on metal." This study also showed that 48% of the bridges had chips, 45% had surface degradation, 7% had cracks, and 1% had delamination. Unlike PFM, there has been little research done with the zirconia/porcelain combination. Many articles are now appearing in various journals studying all aspects of zirconia crowns and bridges, such as: bond strengths to different types of zirconia,44 the effect of various surface treatments—including hydrochloric acid—on the bond strength between the zirconia and the veneering porcelain,45,46 as well as preparation design, which has been recommended by Beyer et al47 to be a shoulder.
The elusive search for a strong, long-lasting, esthetic, biocompatible dental material used to support one or more pontics has been a goal of dental researchers for many years. In most cases, these frameworks have to be veneered with a porcelain material and have to have compatible thermo-elastic properties. Studies of PFM bridges have been shown to have significantly higher fracture strengths than all-ceramic bridges,43 and will continue to do so for the immediate future. Zirconia is the latest material being used but, as reported above, it does not have the same prognosis as metal bridges. Time will tell if the dense zirconia material will become the state of the art of non-metallic bridges.
Based on the research underway, the advancements of CAD/CAM technology, and the desire of many for a non-metallic restorative material, someday there will be a combination of framework and veneering ceramic material that will approach the strength and longevity of gold or other metal bonded to porcelain for the replacement of missing teeth.
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About the Author
Bruce W. Small, DMD, MAGD
Lawrenceville, New Jersey