- Email: [email protected]
Twenty years of progress in implant prosthodontics
Twenty years of progress in implant prosthodontics Thomas D. Taylor, DDS, MSD,a and John R. Agar, DDS, MAb School of Dental Medicine, University of Connecticut, Farmington, Conn. Prosthodontic focus on the restoration of osseointegrated dental implants has evolved dramatically in the last 20 years. Many of the original guiding principles for restoring implants have changed and/or disappeared altogether, and new ideas have taken their place. It is appropriate that this evolution be examined with a 2-fold focus. First, the art and science of prosthodontics as it relates to dental implants today is the result of very real and important lessons learned during the past 2 decades. Second, an appreciation of the history of implant prosthodontics as it relates to osseointegration gives insight into the future direction of research and clinical exploration aimed at continually improving the state of the art and, ultimately, the quality of care provided to patients. This article reviews what the authors consider the most important aspects of the evolution of osseointegrated implants. Pertinent literature was selected for citation, but the discussion does not represent a comprehensive review of the literature. (J Prosthet Dent 2002;88:89-95.)
F
or practical purposes and for the purposes of this article, the modern era of dental implant therapy in North American began with the May 1982 Toronto Conference on Osseointegration in Clinical Dentistry. The conference showcased Professor Per Ingvar Brånemark and the concept of osseointegration. It was organized by George Zarb and supported financially by the Ontario Ministry of Health, the University of Toronto, and the Swedish company Bofors Nobelpharma. At this conference, educational leaders from North American dental schools in the specialties of oral and maxillofacial surgery and prosthodontics were exposed to the scientific background of osseointegration and to the clinical success that had been achieved in Sweden and in early clinical trials in Toronto.1,2 The conference was a watershed event in prosthodontics. For the first time in North America, dental implants were shown to be predictably successful on a long-term basis. Before the Toronto conference, dental implants were not widely accepted as alternatives to more traditional prosthodontic procedures.3 The early investigations of Brånemark that eventually led to the dental application of osseointegration focused on wound healing and rheology in bone and soft tissue. The use of titanium implants (referred to as “fixtures” by Brånemark) to support dental prostheses was first described in a study on dogs.4 The first human patient received implants in Brånemark’s Gothenburg clinic in 1965. For the next decade, the application of osseointegration as a foundation for dental prostheses was carefully documented and reported. Other investigators, including Schroeder in Switzerland and Schulte in Presented at the 66th annual meeting of the Pacific Coast Society of Prosthodontics, Olympic Valley, Calif., June 22, 2001. a Professor and Head, Department of Prosthodontics and Operative Dentistry. b Associate Professor and Director, Graduate Prosthodontics, Department of Prosthodontics and Operative Dentistry. JULY 2002
Germany, were experimenting independently with titanium endosseous dental implants during the same time period.5,6 The 1970s saw the accumulation of sufficient data relative to the predictability of osseointegration to move it from experimental to routine clinical use in European centers. The University of Toronto was the first North American center to use osseointegrated dental implants in clinical trials.2 The replication of Swedish success in Toronto led to the 1982 Toronto conference and, shortly thereafter, to the widespread use of osseointegrated dental implants in North America.
OSSEOINTEGRATED IMPLANTS IN THE EDENTULOUS MANDIBLE The initial application of osseointegrated dental implants and the early success documented in clinical trials were based primarily on treatment of the edentulous mandible. Indeed, early training in the use of osseointegration was limited to treatment of the edentulous mandible. The prosthodontic aspects of the treatment protocol introduced at the Toronto conference and at subsequent training courses in many sites were unique and had no parallel in conventional prosthodontics at that time. The use of implant-retained and supported, screw-retained, cantilevered, full-arch fixed prostheses to replace the conventional mandibular complete denture was an innovation that challenged basic prosthodontic principles. The original concept “ad modem Brånemark” as it applied to the restoration of the edentulous mandible called for the placement of 4 to 6 standard 3.75-mm titanium implants in the edentulous mandible anterior to the mental foramina. After a 3- to 4-month period of submerged healing, the implants were exposed and restored with screw-retained fixed prostheses that included posterior cantilever extensions. The rationale for placing implants exclusively in the anterior mandible was based on several considerations. First, by limiting imTHE JOURNAL OF PROSTHETIC DENTISTRY 89
THE JOURNAL OF PROSTHETIC DENTISTRY
plant placement to sites anterior to the mental foramina, implants of substantial length could be used, allowing for bicortical anchorage or stabilization of the implants. Bicortical stabilization was believed to improve the likelihood of successful osseointegration and subsequent functional loading of the implants. Anterior positioning also precluded the possibility that implant placement might violate the integrity of the inferior alveolar nerve; the overall risk of complications was thereby reduced. A third reason for limiting implant placement to the anterior mandible was concern that mandibular flexure might in some way affect the successful integration and function of implants placed in the posterior mandible. Mandibular flexure predominantly occurs posteriorly in the body and ramus of the mandible. It was believed that avoiding placement of implants in this more dynamic area of the mandible would limit potential complications. Whether all of the reasons for use of the anterior implant/posterior cantilever concept were based on scientifically valid considerations is beside the point. The design concept worked very well, and many clinicians still use it with well-documented success. The fixed cantilever prosthesis incorporates a cast or, more recently, a laser-welded metal framework and denture teeth retained to the framework with polymethyl methacrylate (acrylic) resin. The use of denture teeth and acrylic resin in combination with a fixed metal framework effectively merged classic removable prosthodontic materials and concepts with those of fixed prosthodontics and led to the popular (but inappropriate) moniker “hybrid prosthesis.” When faced with lessthan-ideal implant position, the flexibility, ease of repair, and reduced cost (compared to metal-ceramic full-arch prostheses) of this design have made it extremely popular. The cantilever design for full-arch fixed prostheses led to concern about the potential failure of implants and mechanical components of the prostheses because of the torsional loading involved in the design. Numerous authors made empirical recommendations for the length of cantilever extensions and the number of teeth that could be cantilevered posterior to the terminal implant, and many speculated about the preferred type of occlusal scheme.7-13 A new style of denture tooth was even designed for use with the fixed-detachable prosthesis.
OSSEOINTEGRATED IMPLANTS IN THE EDENTULOUS MAXILLA Although the fixed cantilever prosthesis was an extremely effective solution for the edentulous mandible, the outcomes of its use in the maxilla were much less predictable. Most difficulties related to the unique anatomic aspects of the edentulous maxilla. As it resorbs, the edentulous maxilla becomes smaller. The replace90
TAYLOR AND AGAR
ment of missing maxillary teeth often involves placement of the artificial teeth at a substantial distance horizontally and vertically from the location of the supporting bone and implants. Lack of lip support, air escape, esthetic and speech difficulties, and oral hygiene access are among the frequently cited problems with maxillary implants.14-16 The fact that osseointegration is less predictable in the maxilla than in the mandible adds to the complexity of treating the edentulous maxilla with dental implants. Many clinicians have advocated the use of patient-removable, overdenture-type prostheses for treatment of the edentulous maxilla rather than the fixed or fixeddetachable prostheses used successfully in the mandible.17 Among the reasons cited for this preference are improved control of esthetics and lip support, better phonetics, enhanced access for oral hygiene procedures, ease of modification, and reduced cost of fabrication. Enhancements in surgical techniques—including immediate implant placement, ridge augmentation procedures, and sinus lift procedures— continue to improve restoration of the edentulous maxilla with dental implants. It must be stated, however, that as of this writing, most experienced clinicians agree that consistent, predictable success in this arena has yet to be attained.
SUBMERGED VERSUS NONSUBMERGED HEALING In the teachings of Brånemark, submerged, undisturbed healing was a prerequisite for successful osseointegration.18 The work of Schroeder et al5 indicated otherwise. The protocol advocated by Ledermann et al19 included nonsubmerged implant placement and immediate loading of implants in the anterior mandible. Four 1-piece implants were placed in the interforaminal area, an impression was made, and a splinted bar was fabricated. The bar was seated on the newly placed implants, and an overdenture was placed. The success of this implant restoration method was substantiated with the use of both hollow-cylinder implants and solidscrew implants (Swiss screw).20 Several investigators have reported promising results with implants placed in immediate extraction sites, both submerged and nonsubmerged, and with single-tooth implants placed in immediate-loading situations.21-23 Modifications to implant shape and surface characteristics have prompted clinicians to take greater latitude in shortening healing times and restoring implants sooner than previously believed possible.
ABUTMENT DESIGN With the expansion of routine implant restoration from the edentulous arch to the partially edentulous arch and to single-tooth replacement, a broader array of restorative abutments and components was necessary VOLUME 88 NUMBER 1
TAYLOR AND AGAR
than when implant treatment was limited to the edentulous mandible. Implant manufacturers have expanded abutment choices substantially, but the use of custom/ laboratory-fabricated abutments remains commonplace. The trend toward impressing the implant and reproducing it in the laboratory rather than using a registration of a prefabricated, stock abutment placed on top of the implant has greatly increased the flexibility of implant prosthodontics and created a new standard for esthetic implant restoration. One of the most important innovations in custom implant abutment design was the UCLA abutment, created by Lewis et al.24,25 The UCLA abutment provides an excellent option for customizing the restoration of implants with an external hexagon connection. The abutment and restorative coping are incorporated into one unit, reducing the number of interfaces and components in the restored implant pillar. The UCLA abutment allows the fabrication of custom abutments for use in difficult situations when space is tight or when implant angulation is less than ideal. The practice of implant-level impression making can be traced directly to the UCLA abutment concept, from which a number of abutment options have evolved.
IMPLANT DESIGN The dental implant marketplace has expanded substantially since 1982, bringing innovation to the industry and increasing the number of available treatment options. One aspect of this innovation relates specifically to the implant body and how it interfaces with bone and soft tissue. Advances in this respect have been dramatic but are beyond the scope of this article. Innovations related to the abutment/prosthetic connection are of primary interest here. Early competition to the external hexagon interface of the Brånemark implant included the Core-Vent implant with its cemented (and bendable) abutment and the IMZ implant, which had a nonmetallic “intermobile element” intended to mimic the resiliency of the periodontal ligament.26 Other early implants included the 1-piece Straumann Type F and Swiss Screw implants (Institut Straumann AG), which incorporated the transmucosal abutment directly into the 1-stage implant, thereby eliminating the possibility of (or the need for) submerged healing. Dissatisfaction with the limitations of externally hexed implants led others to design abutment/implant connections that were internal to the implant body rather than on top of it. Astra, Straumann, Core-Vent, and others introduced internally connected 2-stage implants designed to reduce abutment screw loosening and fracture. In general, such internal connections appear to effectively increase resistance to failure of the nonaxially loaded implant/abutment connection, and they continue to gain popularity. JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
RESTORATIVE/OCCLUSAL MATERIALS As previously noted, when osseointegrated dental implants were introduced in North America, they were used primarily to restore the edentulous mandible. The fixed-detachable complete denture incorporated artificial teeth (fabricated from resin polymers) that had been designed and created to serve in conventional removable prostheses. The extension of denture teeth into osseointegrated rehabilitation met 2 important needs at the time. First and most obviously, resin denture teeth were readily available and their use was commonplace. The incorporation of resin denture teeth into cast metal superstructures supported by implants in the anterior mandible was a natural solution to the new fixed-detachable application. The second benefit was based on the assumption that because of the lack of a periodontal ligament, osseointegrated implants would transmit occlusal forces differently from the occlusal surface to the investing bone. In a theoretical model, Skalak27,28 convincingly demonstrated that resinous occlusal surfaces were indicated to prevent traumatic, shock-type forces from affecting the health and longevity of the osseointegrated interface. In that model, it was postulated that metallic or ceramic occlusal surfaces could transmit substantial impact forces to the osseointegrated interface, putting it at risk for damage and eventual failure. This was the rationale against the use of ceramic occlusal materials for implant-supported prostheses. Not only were resinous occlusal surfaces deemed necessary, but a minimal thickness of resin was advocated to adequately protect the underlying implants and bone. Concern for the traumatic potential of even normal dental (tooth-supported) occlusal schemes and materials led one dentist to create an implant system that incorporated a resilient plastic component into the implant pillar. The IMZ concept introduced by Kirsch and Ackemann26 presumed to provide a protective, forcedampening abutment or “intermobile element” beneath the restoration. This element absorbed occlusal impact and thus spared the surrounding bone. A perceived benefit of the intermobile element concept was that it allowed the use of full ceramic occlusal surfaces on implant-supported restorations. The IMZ implant system was quite popular in the mid 1980s, but its appeal waned when clinicians began to ignore the assumed need to avoid ceramic and metallic occlusal surfaces on implant-supported restorations. Resin denture teeth satisfied the need for prosthetic teeth when the edentulous patient was the predominant recipient of osseointegrated implant restorations. In the mid 1980s, clinicians began to use osseointegrated implants more frequently in other situations. Implant treatment of partial edentulism soon became commonplace in prosthodontic practice, and the need for restorative materials with characteristics beyond what resins 91
THE JOURNAL OF PROSTHETIC DENTISTRY
could offer became obvious. Resin occlusal surfaces proved to be less than optimal in many patient applications. Accelerated wear and eruption or shifting of adjacent teeth, fracture, and/or discoloration became common sequelae of the application of resinous occlusal surfaces in partially edentulous patients. The increased use of metal-ceramic restorations was followed by the introduction of prefabricated ceramic components from implant manufacturers. Improved esthetic outcomes have been the most obvious result of the progressive turn to metal-ceramic and all-ceramic restorations; decreased maintenance problems associated with occlusal material wear also can be assumed. Further applications of materials science and computer technology have led to the introduction of alternative designs and manufacturing processes for implant restorations. Most notably, the Procera system (Nobel Biocare USA, Yorba Linda, Calif.) has successfully replaced traditional lost-wax methods for both all-ceramic and metallic components.29-33 All-ceramic abutment options have increased the possibility of esthetic restoration of dental implants.34 Nonmetallic restorations fabricated from fiber-reinforced composites also have been introduced for implant restoration and appear to hold considerable promise.35
CEMENTED VERSUS SCREWRETAINED IMPLANT RESTORATIONS Mainstream philosophy in the design and restoration of implant prostheses from the 1980s through the early 1990s reflected a strong preference for screw-retained over cement-retained restorations. This preference most likely was the result of how osseointegrated implants and their restorations were introduced. Clinical material coming from European centers seemed to virtually preclude the use of cemented implant restorations. Training courses for early implant systems (Nobelpharma and IMZ) demonstrated only screw-retained restorations. Early exceptions were the U.S.-produced Core-Vent implant and the Swiss-produced Straumann type implants. Nevertheless, the primary focus of implant prosthodontics during the first decade of osseointegration in North America was the use of screw-retained restorations. In 1989 to 1990 the introduction of the Swiss Bonefit implant, later called the ITI implant (Straumann USA, Waltham, Mass.), into the U.S. market brought a challenge to the screw-retained method of prosthesis retention. Initially the ITI system relied exclusively on a cementable abutment system for crown retention. At approximately the same time, the Swedish company Nobel Biocare introduced Cera-One restorative components for single-tooth replacement; these components relied on a cemented crown over a screw-retained abut92
TAYLOR AND AGAR
ment. Shortly thereafter, many implant manufacturers introduced cementable alternatives to screw retention. Although retrievability remains the primary advantage of a screw-retained approach, cemented implant restorations are attractive for several reasons. First, they preclude the interference of screw access holes with esthetic results or with the occlusion of the restoration. Second, cement-based approaches substantially reduce restoration costs. With some systems, restoration with a screw-retained restoration involves nearly 4 times the component cost of a cemented restoration. Third, if no occlusal screw is present, occlusal screw loosening is not a possible complication of implant prosthodontics. Fourth, a cemented restoration is more likely to achieve passive fit than a screw-retained restoration. The argument for increased passivity rests on the assumption that tightening a screw-retained restoration may create substantial strain within the restoration, implant, and investing bone complex. The use of cemented restorations also avoids the clamping effect of multiple screws drawing an ill-fitting prosthesis to place against the supporting implants. Fifth and finally, the implant industry’s desire to extend routine implant use into the general dental practice has stimulated interest in simplifying implant restorative procedures. Cementation of fixed implant restorations more closely follows the procedures routinely performed on natural teeth.
RESTORATION ACCURACY AND PASSIVE FIT/MISFIT Concern that restorations supported by endosseous dental implants may play an important role in the longterm stability and success of those implants has led prosthodontic researchers to explore methods to ensure accurate, passive adaptation of restorations to their supporting implants.29,36-46 Increased accuracy in the impression and transfer of implant position in the dental arch to the working cast has been the topic of many contributions to the literature. Similarly, many authors have investigated various methods of increasing accuracy (or decreasing error) in casting and prosthesis fabrication techniques, including computer-aided design– computer-aided manufacturer technology.29,33 Progress has been made in reducing replication and fabrication errors associated with implant-supported prostheses. In spite of the substantial effort and energy spent on reducing the level of misfit in implant prosthodontics, one simple question has largely been ignored in the literature: whether a misfitting prosthesis can negatively affect the survival of dental implants. Many authors and clinicians have assumed that a misfitting prosthesis rigidly attached to multiple dental implants will impart severe strain between the implants, strain that will not dissipate through subsequent orthodontic shifting or relaxation of the implants in the surrounding bone.47-49 VOLUME 88 NUMBER 1
TAYLOR AND AGAR
Strain at a high enough level might lead to breakdown of the surrounding bone and ultimate loss of an implant. Two questions immediately arise from this concern. First, what is an acceptable level of misfit? Nothing ever fits perfectly, and so there must be a clinically acceptable level of misfit that does not adversely affect the health of the implant/bone interface. Second, how does one measure the magnitude of misfit? Although clinicians strive to avoid inaccuracy or misfit in implant restoration, there are no accepted standards for measuring or even identifying misfit in a clinical situation. If the available literature is acknowledged, the answer to these questions may not be terribly important. Few studies have specifically examined the effects of static loading or misfit of implant restorations on osseointegration. To date, authors who have explored this area have been unable to demonstrate negative outcomes of misfit on the bone/implant interface.50-53 Perhaps it should not be surprising that static load may not be detrimental to stable osseointegration when one considers that implants used for long-term orthodontic anchorage and/or distraction osteogenesis suffer no apparent loss of bone contact during the course of loading, even after years of loading.49,54,55 In one study, the only effect of severe static load was increased bone-to-implant contact compared with unloaded control implants.53 If misfit is not the critical issue for long-term maintenance of osseointegration, then what prosthodontic factors (if any) are important? Several well-designed studies have demonstrated that the magnitude and/or direction of occlusal forces have no apparent effect on supporting implants and bone. Extreme implant overload had no effect on the success of osseointegration in animal models in 4 separate studies.56-59 In contrast, Isidor60 demonstrated bone loss around implants subjected to what can only be described as extreme, off-axis, heavy loading of dental implants. The different results from these studies may reflect the as-yet undetermined critical factor(s) that clinicians should recognize when designing and placing implantsupported prostheses. Although the impact of occlusal forces on the bone/implant interface needs to be researched further, clinicians who restore dental implants are aware of the mechanical complications that occur as a result of occlusal loading of implant-supported restorations. Component loosening and fracture are common results of functional loading and overloading.61-65 Similar knowledge gaps exist with respect to factors that affect the long-term success of progressively loaded and immediately loaded implants. Currently there are no well-documented, controlled, prospective studies that have examined the effects of progressive loading on implant survival or bone level changes around implants. Immediate loading of implants is similarly not well understood. At this time, it is safe to say that completely undisturbed healing of the implant/bone interface is JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
not necessary for successful osseointegration to occur, and in some situations, implants placed into immediate full functional load can be expected to survive and function well.19-24 The requisites for predictable osseointegration of immediately loaded implants have yet to be determined. One parallel consideration is whether provisional loading of a tissue-borne prosthesis over an implant during the osseointegration (healing) period will affect the integration of that implant. To date there is no scientific evidence (and no clearly documented subjective clinical evidence) that early failure of a dental implant can be attributed to early loading or “overload” resulting from a tissue-supported interim prosthesis being worn over a recently placed dental implant. Implant failures that occur during the healing period have been related to various surgical problems during placement, including overheating of bone during site preparation, dull osteotomy drills, and lack of primary stability of the implant at the time of placement. Loading an implant through the use of an interim restoration has not been documented as a cause of early implant failure. It is also safe to state that, at this time, there is no scientific evidence that the factors associated with implant restoration (provisional or definitive) have a predictable impact on the survival of the supporting implant. This apparent lack of effect may be deceiving, in that very real determinants of implant success or failure are likely related directly to the prosthodontic aspects of treatment. Unfortunately, those as-yet unidentified determinants are hidden from the view of clinicians intent on providing the best possible care for their patients.
SUMMARY The first 20 years of osseointegration in North America have seen the most dramatic changes in the prosthodontic treatment of patients since the introduction of fluoride (as an anticaries medication) and the high-speed turbine handpiece combined. Prosthodontic treatment planning, treatment principles, and outcomes have been forever changed in this brief period. Similarly, the expectations of clinicians and patients have undergone a shift toward treatment end points with greater function, esthetics, and permanence. Although the initial success of osseointegration has been gratifying, the scientific basis for what is routinely done and what seems to work is largely lacking. Empirical concepts and principles based on the treatment of natural teeth have, by and large, been extrapolated to the treatment of endosseous implants with apparent success. Perhaps the next 20 years will bring answers to the questions of how and why implant-supported restorations function so successfully. It is hoped that the cost of implant therapy will be reduced to allow 93
THE JOURNAL OF PROSTHETIC DENTISTRY
more people to take advantage of this state-of-the-art form of tooth replacement. REFERENCES 1. Adell R, Lekholm U, Rockler B, Brånemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 2. Zarb GA, Schmitt A. Osseointegration and the edentulous predicament. The 10-year-old Toronto study. Br Dent J 1991;170:439-44. 3. Schnitman P, Shulman L. Dental implants: benefit and risk. Proceedings of an NIH-Harvard consensus development conference. US Department of Health and Human Services; 1980. p. 81-1531. 4. Brånemark PI, Adell R, Breine U, Hansson BO, Lindstrom J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 5. Schroeder A, Pohler O, Sutter F. [Tissue reaction to an implant of a titanium hollow cylinder with a titanium surface spray layer.] SSO Schweiz Monatsschr Zahnheilkd 1976;86:713-27. German. 6. Schroeder A, van der Zypen E, Stich H, Sutter F. The reactions of bone, connective tissue, and epithelium to endosteal implants with titaniumsprayed surfaces. J Maxillofac Surg 1981;9:15-25. 7. McAlarney ME, Stavropoulos DN. Determination of cantilever lengthanterior-posterior spread ratio assuming failure criteria to be the compromise of the prosthesis retaining screw-prosthesis joint. Int J Oral Maxillofac Implants 1996;11:331-9. 8. Lundgren D, Falk H, Laurell L. Influence of number and distribution of occlusal cantilever contacts on closing and chewing forces in dentitions with implant-supported fixed prostheses occluding with complete dentures. Int J Oral Maxillofac Implants 1989;4:277-83. 9. Wang S, Hobkirk JA. Load distribution on implants with a cantilevered superstructure: an in vitro pilot study. Implant Dent 1996;5:36-42. 10. Falk H, Laurell L, Lundgren D. Occlusal force pattern in dentitions with mandibular implant-supported fixed cantilever prostheses occluded with complete dentures. Int J Oral Maxillofac Implants 1989;4:55-62. 11. White SN, Caputo AA, Anderkvist T. Effect of cantilever length on stress transfer by implant-supported prostheses. J Prosthet Dent 1994;71:493-9. 12. Rodriguez AM, Aquilino SA, Lund PS. Cantilever and implant biomechanics: a review of the literature, Part 2. J Prosthodont 1994;3:114-8. 13. Falk H, Laurell L, Lundgren D. Occlusal interferences and cantilever joint stress in implant-supported prostheses occluding with complete dentures. Int J Oral Maxillofac Implants 1990;5:70-7. 14. Taylor TD. Fixed implant rehabilitation for the edentulous maxilla. Int J Oral Maxillofac Implants 1991;6:329-37. 15. Desjardins RP. Prosthesis design for osseointegrated implants in the edentulous maxilla. Int J Oral Maxillofac Implants 1992;7:311-20. 16. Keller EE, Van Roekel NB, Desjardins RP, Tolman DD. Prosthetic-surgical reconstruction of the severely resorbed maxilla with iliac bone grafting and tissue-integrated prostheses. Int J Oral Maxillofac Implants 1987;2: 155-65. 17. DeBoer J. Edentulous implants: overdenture versus fixed. J Prosthet Dent 1993;69:386-90. 18. Brånemark P, Zarb G, Albrektsson T, editors. Tissue-integrated prostheses: osseointegration in clinical dentistry. Chicago: Quintessence; 1985. p. 11-76. 19. Ledermann PD, Schroeder A, Sutter F. [Single tooth replacement with the aid of the ITI (International Team fur Implantologie) type F hollow-cylinder implant (late implant).]SSO Schweiz Monatsschr Zahnheilkd 1982;92: 1087-98. German. 20. Babbush CA, Kent JN, Misiek DJ. Titanium plasma-sprayed (TPS) screw implants for the reconstruction of the edentulous mandible. J Oral Maxillofacial Surg 1986;44:274-82. 21. Tarnow DP, Emtiaz S, Classi A. Immediate loading of threaded implants at stage 1 surgery in edentulous arches: ten consecutive case reports with 1to 5-year data. Int J Oral Maxillofac Implants 1997;12:319-24. 22. Schnitman PA, Wohrle PS, Rubenstein JE. Immediate fixed interim prostheses supported by two-stage threaded implants: methodology and results. J Oral Implantol 1990;16:96-105. 23. Schnitman PA, Wohrle PS, Rubenstein JE, DaSilva JD, Wang NH. Ten-year results for Branemark implants immediately loaded with fixed prostheses at implant placement. Int J Oral Maxillofac Implants 1997;12:495-503. 24. Lewis SG, Beumer J 3rd, Perri GR, Hornburg WP. Single tooth implant supported restorations. Int J Oral Maxillofac Implants 1988;3:25-30.
94
TAYLOR AND AGAR
25. Lewis S, Beumer J 3rd, Hornburg W, Moy P. The “UCLA” abutment. Int J Oral Maxillofac Implants 1988;3:183-9. 26. Kirsch A, Ackermann KL. The IMZ osteointegrated implant system. Dent Clin North Am 1989;33:733-91. 27. Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983;49:843-8. 28. Skalak R. Aspects of biomechanical considerations. In: Brånemark P, Zarb G, Albrektsson T, editors. Tissue-integrated prostheses: osseointegration in clinical dentistry. Chicago: Quintessence; 1985. p. 117-28. 29. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. 30. Sjogren G, Andersson M, Bergman M. Laser welding of titanium in dentistry. Acta Odontol Scand 1988;46:247-53. 31. Jemt T, Linden B. Fixed implant-supported prostheses with welded titanium frameworks. Int J Periodontics Restorative Dent 1992;12:177-84. 32. Chai T, Chou CK. Mechanical properties of laser-welded cast titanium joints under different conditions. J Prosthet Dent 1998;79:477-83. 33. Rubenstien JE, Ma T. Comparison of interface relationships between implant components for laser-welded titanium frameworks and standard cast frameworks. Int J Oral Maxillofac Implants 1999;14:491-5. 34. Prestipino V, Ingber A. Esthetic high-strength implant abutments. Part I. J Esthet Dent 1993;5:29-36. 35. Freilich MA, Karmaker AC, Burstone CJ, Goldberg AJ. Development and clinical applications of a light-polymerized fiber-reinforced composite. J Prosthet Dent 1998;80:311-8. 36. Rangert B, Jemt T, Jorneus L. Forces and moments on Brånemark implants. Int J Oral Maxillofac Implants 1989;4:241-7. 37. Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. Int J Oral Maxillofac Implants 1992;7:26-33. 38. Carr AB, Stewart RB. Full-arch implant framework casting accuracy: preliminary in vitro observation for in vivo testing. J Prosthodont 1993;2:2-8. 39. Lie A, Jemt T. Photogrammetric measurements of implant positions. Description of a technique to determine the fit between implants and superstructures. Clin Oral Implants Res 1994;5:30-6. 40. Jemt T, Lie A. Accuracy of implant-supported prostheses in the edentulous jaw: analysis of precision of fit between cast gold-alloy frameworks and master casts by means of a three-dimensional photogrammetric technique. Clin Oral Implants Res 1995;6:172-80. 41. Tan KB, Rubenstein JE, Nicholls JI, Yuodelis RA. Three-dimensional analysis of the casting accuracy of one-piece, osseointegrated implant-retained prostheses. Int J Prosthodont 1993;6:346-63. 42. Aparicio C. A new method to routinely achieve passive fit of ceramometal prostheses over Brånemark osseointegrated implants: a two-year report. Int J Periodontics Restorative Dent 1994;14:404-19. 43. Jemt T. In vivo measurements of precision of fit involving implant-supported prostheses in edentulous jaw. Int J Oral Maxillofac Implants 1996; 11:151-8. 44. Iglesia M, Moreno J. A method aimed at achieving passive fit in implant prostheses: case report. Int J Prosthodont 2001;14:570-4. 45. May KB, Edge MJ, Lang BR, Wang RF. The Periotest method: implantsupported framework precision of fit evaluation. J Prosthodont 1996;5: 206-13. 46. Kan JY, Rungcharassaeng K, Bohsali K, Goodacre CJ, Lang BR. Clinical methods for evaluating implant framework fit. J Prosthet Dent 1999;81:713. 47. Millington ND, Leung T. Inaccurate fit of implant superstructures. Part 1: Stresses generated on the superstructure relative to the size of fit discrepancy. Int J Prosthodont 1995;8:511-6. 48. Jemt T, Lekholm U. Measurements of bone and framework deformations induced by misfit of implant superstructures. A pilot study in rabbits. Clin Oral Implants Res 1998;9:272-80. 49. Roberts WE, Smith RK, Zilberman Y, Mozsary PG, Smith RS. Osseous adaptation to continuous loading of rigid endosseous implants. Am J Orthod 1984;86:95-111. 50. Carr AB, Gerard DA, Larsen PE. The response of bone in primates around unloaded dental implants supporting prostheses with different levels of fit. J Prosthet Dent 1996;76:500-9. 51. Hurzeler MB, Quinones CR, Kohal RJ, Rohde M, Strub JR, Teuscher U, et al. Changes in peri-implant tissues subjected to orthodontic forces and ligature breakdown in monkeys. J Periodontol 1998;69:396-404. 52. Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants 1996;11:620-2.
VOLUME 88 NUMBER 1
TAYLOR AND AGAR
53. Gotfredsen K, Berglundh T, Lindhe J. Bone reactions adjacent to titanium implants subjected to static load. A study in the dog (I). Clin Oral Implants Res 2001;12:1-8. 54. Wehrbein H, Diedrich P. Endosseous titanium implants during and after orthodontic load--an experimental study in the dog. Clin Oral Implants Res 1993;4:76-82. 55. Wehrbein H, Glatzmaier J, Yildirim M. Orthodontic anchorage capacity of short titanium screw implants in the maxilla. An experimental study in the dog. Clin Oral Implants Res 1997;8:131-41. 56. Celletti R, Pameijer CH, Bracchetti G, Donath K, Persichetti G, Visani I. Histologic evaluation of osseointegrated implants restored in nonaxial functional occlusion with preangled abutments. Int J Periodontics Restorative Dent 1995;15:562-73. 57. Ogiso M, Tabata T, Kuo PT, Borgese D. A histologic comparison of the functional loading capacity of an occluded dense apatite implant and the natural dentition. J Prosthet Dent 1994;71:581-8. 58. Asikainen P, Klemetti E, Vuillemin T, Sutter F, Rainio V, Kotilainen R. Titanium implants and lateral forces. An experimental study with sheep. Clin Oral Implants Res 1997;8:465-8. 59. Miyata T, Kobayashi Y, Araki H, Motomura Y, Shin K. The influence of controlled occlusal overload on peri-implant tissue: a histologic study in monkeys. Int J Oral Maxillofac Implants 1998;13:677-83. 60. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res 1996;7:143-52. 61. Kohavi D. Complications in the tissue integrated prostheses components: clinical and mechanical evaluation. J Oral Rehabil 1993;20:413-22.
JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
62. Kallus T, Bessing C. Loose gold screws frequently occur in full-arch fixed prostheses supported by osseointegrated implants after 5 years. Int J Oral Maxillofac Implants 1994;9:169-78. 63. Taylor TD. Prosthodontic problems and limitations associated with osseointegration. J Prosthet Dent 1998;79:74-8. 64. Walton JN, MacEntee MI. Problems with prostheses on implants: a retrospective study. J Prosthet Dent 1994;71:283-8. 65. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: the Toronto study. Part III: Problems and complications encountered. J Prosthet Dent 1990;64:185-94. Reprint requests to: DR THOMAS D. TAYLOR DEPARTMENT OF PROSTHODONTICS AND OPERATIVE DENTISTRY UNIVERSITY OF CONNECTICUT SCHOOL OF DENTAL MEDICINE 263 FARMINGTON AVE FARMINGTON, CT 06030-1370 FAX: (860)679-1370 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 ⫹ 0 10/1/126818
doi:10.1067/mpr.2002.126818
95
F
or practical purposes and for the purposes of this article, the modern era of dental implant therapy in North American began with the May 1982 Toronto Conference on Osseointegration in Clinical Dentistry. The conference showcased Professor Per Ingvar Brånemark and the concept of osseointegration. It was organized by George Zarb and supported financially by the Ontario Ministry of Health, the University of Toronto, and the Swedish company Bofors Nobelpharma. At this conference, educational leaders from North American dental schools in the specialties of oral and maxillofacial surgery and prosthodontics were exposed to the scientific background of osseointegration and to the clinical success that had been achieved in Sweden and in early clinical trials in Toronto.1,2 The conference was a watershed event in prosthodontics. For the first time in North America, dental implants were shown to be predictably successful on a long-term basis. Before the Toronto conference, dental implants were not widely accepted as alternatives to more traditional prosthodontic procedures.3 The early investigations of Brånemark that eventually led to the dental application of osseointegration focused on wound healing and rheology in bone and soft tissue. The use of titanium implants (referred to as “fixtures” by Brånemark) to support dental prostheses was first described in a study on dogs.4 The first human patient received implants in Brånemark’s Gothenburg clinic in 1965. For the next decade, the application of osseointegration as a foundation for dental prostheses was carefully documented and reported. Other investigators, including Schroeder in Switzerland and Schulte in Presented at the 66th annual meeting of the Pacific Coast Society of Prosthodontics, Olympic Valley, Calif., June 22, 2001. a Professor and Head, Department of Prosthodontics and Operative Dentistry. b Associate Professor and Director, Graduate Prosthodontics, Department of Prosthodontics and Operative Dentistry. JULY 2002
Germany, were experimenting independently with titanium endosseous dental implants during the same time period.5,6 The 1970s saw the accumulation of sufficient data relative to the predictability of osseointegration to move it from experimental to routine clinical use in European centers. The University of Toronto was the first North American center to use osseointegrated dental implants in clinical trials.2 The replication of Swedish success in Toronto led to the 1982 Toronto conference and, shortly thereafter, to the widespread use of osseointegrated dental implants in North America.
OSSEOINTEGRATED IMPLANTS IN THE EDENTULOUS MANDIBLE The initial application of osseointegrated dental implants and the early success documented in clinical trials were based primarily on treatment of the edentulous mandible. Indeed, early training in the use of osseointegration was limited to treatment of the edentulous mandible. The prosthodontic aspects of the treatment protocol introduced at the Toronto conference and at subsequent training courses in many sites were unique and had no parallel in conventional prosthodontics at that time. The use of implant-retained and supported, screw-retained, cantilevered, full-arch fixed prostheses to replace the conventional mandibular complete denture was an innovation that challenged basic prosthodontic principles. The original concept “ad modem Brånemark” as it applied to the restoration of the edentulous mandible called for the placement of 4 to 6 standard 3.75-mm titanium implants in the edentulous mandible anterior to the mental foramina. After a 3- to 4-month period of submerged healing, the implants were exposed and restored with screw-retained fixed prostheses that included posterior cantilever extensions. The rationale for placing implants exclusively in the anterior mandible was based on several considerations. First, by limiting imTHE JOURNAL OF PROSTHETIC DENTISTRY 89
THE JOURNAL OF PROSTHETIC DENTISTRY
plant placement to sites anterior to the mental foramina, implants of substantial length could be used, allowing for bicortical anchorage or stabilization of the implants. Bicortical stabilization was believed to improve the likelihood of successful osseointegration and subsequent functional loading of the implants. Anterior positioning also precluded the possibility that implant placement might violate the integrity of the inferior alveolar nerve; the overall risk of complications was thereby reduced. A third reason for limiting implant placement to the anterior mandible was concern that mandibular flexure might in some way affect the successful integration and function of implants placed in the posterior mandible. Mandibular flexure predominantly occurs posteriorly in the body and ramus of the mandible. It was believed that avoiding placement of implants in this more dynamic area of the mandible would limit potential complications. Whether all of the reasons for use of the anterior implant/posterior cantilever concept were based on scientifically valid considerations is beside the point. The design concept worked very well, and many clinicians still use it with well-documented success. The fixed cantilever prosthesis incorporates a cast or, more recently, a laser-welded metal framework and denture teeth retained to the framework with polymethyl methacrylate (acrylic) resin. The use of denture teeth and acrylic resin in combination with a fixed metal framework effectively merged classic removable prosthodontic materials and concepts with those of fixed prosthodontics and led to the popular (but inappropriate) moniker “hybrid prosthesis.” When faced with lessthan-ideal implant position, the flexibility, ease of repair, and reduced cost (compared to metal-ceramic full-arch prostheses) of this design have made it extremely popular. The cantilever design for full-arch fixed prostheses led to concern about the potential failure of implants and mechanical components of the prostheses because of the torsional loading involved in the design. Numerous authors made empirical recommendations for the length of cantilever extensions and the number of teeth that could be cantilevered posterior to the terminal implant, and many speculated about the preferred type of occlusal scheme.7-13 A new style of denture tooth was even designed for use with the fixed-detachable prosthesis.
OSSEOINTEGRATED IMPLANTS IN THE EDENTULOUS MAXILLA Although the fixed cantilever prosthesis was an extremely effective solution for the edentulous mandible, the outcomes of its use in the maxilla were much less predictable. Most difficulties related to the unique anatomic aspects of the edentulous maxilla. As it resorbs, the edentulous maxilla becomes smaller. The replace90
TAYLOR AND AGAR
ment of missing maxillary teeth often involves placement of the artificial teeth at a substantial distance horizontally and vertically from the location of the supporting bone and implants. Lack of lip support, air escape, esthetic and speech difficulties, and oral hygiene access are among the frequently cited problems with maxillary implants.14-16 The fact that osseointegration is less predictable in the maxilla than in the mandible adds to the complexity of treating the edentulous maxilla with dental implants. Many clinicians have advocated the use of patient-removable, overdenture-type prostheses for treatment of the edentulous maxilla rather than the fixed or fixeddetachable prostheses used successfully in the mandible.17 Among the reasons cited for this preference are improved control of esthetics and lip support, better phonetics, enhanced access for oral hygiene procedures, ease of modification, and reduced cost of fabrication. Enhancements in surgical techniques—including immediate implant placement, ridge augmentation procedures, and sinus lift procedures— continue to improve restoration of the edentulous maxilla with dental implants. It must be stated, however, that as of this writing, most experienced clinicians agree that consistent, predictable success in this arena has yet to be attained.
SUBMERGED VERSUS NONSUBMERGED HEALING In the teachings of Brånemark, submerged, undisturbed healing was a prerequisite for successful osseointegration.18 The work of Schroeder et al5 indicated otherwise. The protocol advocated by Ledermann et al19 included nonsubmerged implant placement and immediate loading of implants in the anterior mandible. Four 1-piece implants were placed in the interforaminal area, an impression was made, and a splinted bar was fabricated. The bar was seated on the newly placed implants, and an overdenture was placed. The success of this implant restoration method was substantiated with the use of both hollow-cylinder implants and solidscrew implants (Swiss screw).20 Several investigators have reported promising results with implants placed in immediate extraction sites, both submerged and nonsubmerged, and with single-tooth implants placed in immediate-loading situations.21-23 Modifications to implant shape and surface characteristics have prompted clinicians to take greater latitude in shortening healing times and restoring implants sooner than previously believed possible.
ABUTMENT DESIGN With the expansion of routine implant restoration from the edentulous arch to the partially edentulous arch and to single-tooth replacement, a broader array of restorative abutments and components was necessary VOLUME 88 NUMBER 1
TAYLOR AND AGAR
than when implant treatment was limited to the edentulous mandible. Implant manufacturers have expanded abutment choices substantially, but the use of custom/ laboratory-fabricated abutments remains commonplace. The trend toward impressing the implant and reproducing it in the laboratory rather than using a registration of a prefabricated, stock abutment placed on top of the implant has greatly increased the flexibility of implant prosthodontics and created a new standard for esthetic implant restoration. One of the most important innovations in custom implant abutment design was the UCLA abutment, created by Lewis et al.24,25 The UCLA abutment provides an excellent option for customizing the restoration of implants with an external hexagon connection. The abutment and restorative coping are incorporated into one unit, reducing the number of interfaces and components in the restored implant pillar. The UCLA abutment allows the fabrication of custom abutments for use in difficult situations when space is tight or when implant angulation is less than ideal. The practice of implant-level impression making can be traced directly to the UCLA abutment concept, from which a number of abutment options have evolved.
IMPLANT DESIGN The dental implant marketplace has expanded substantially since 1982, bringing innovation to the industry and increasing the number of available treatment options. One aspect of this innovation relates specifically to the implant body and how it interfaces with bone and soft tissue. Advances in this respect have been dramatic but are beyond the scope of this article. Innovations related to the abutment/prosthetic connection are of primary interest here. Early competition to the external hexagon interface of the Brånemark implant included the Core-Vent implant with its cemented (and bendable) abutment and the IMZ implant, which had a nonmetallic “intermobile element” intended to mimic the resiliency of the periodontal ligament.26 Other early implants included the 1-piece Straumann Type F and Swiss Screw implants (Institut Straumann AG), which incorporated the transmucosal abutment directly into the 1-stage implant, thereby eliminating the possibility of (or the need for) submerged healing. Dissatisfaction with the limitations of externally hexed implants led others to design abutment/implant connections that were internal to the implant body rather than on top of it. Astra, Straumann, Core-Vent, and others introduced internally connected 2-stage implants designed to reduce abutment screw loosening and fracture. In general, such internal connections appear to effectively increase resistance to failure of the nonaxially loaded implant/abutment connection, and they continue to gain popularity. JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
RESTORATIVE/OCCLUSAL MATERIALS As previously noted, when osseointegrated dental implants were introduced in North America, they were used primarily to restore the edentulous mandible. The fixed-detachable complete denture incorporated artificial teeth (fabricated from resin polymers) that had been designed and created to serve in conventional removable prostheses. The extension of denture teeth into osseointegrated rehabilitation met 2 important needs at the time. First and most obviously, resin denture teeth were readily available and their use was commonplace. The incorporation of resin denture teeth into cast metal superstructures supported by implants in the anterior mandible was a natural solution to the new fixed-detachable application. The second benefit was based on the assumption that because of the lack of a periodontal ligament, osseointegrated implants would transmit occlusal forces differently from the occlusal surface to the investing bone. In a theoretical model, Skalak27,28 convincingly demonstrated that resinous occlusal surfaces were indicated to prevent traumatic, shock-type forces from affecting the health and longevity of the osseointegrated interface. In that model, it was postulated that metallic or ceramic occlusal surfaces could transmit substantial impact forces to the osseointegrated interface, putting it at risk for damage and eventual failure. This was the rationale against the use of ceramic occlusal materials for implant-supported prostheses. Not only were resinous occlusal surfaces deemed necessary, but a minimal thickness of resin was advocated to adequately protect the underlying implants and bone. Concern for the traumatic potential of even normal dental (tooth-supported) occlusal schemes and materials led one dentist to create an implant system that incorporated a resilient plastic component into the implant pillar. The IMZ concept introduced by Kirsch and Ackemann26 presumed to provide a protective, forcedampening abutment or “intermobile element” beneath the restoration. This element absorbed occlusal impact and thus spared the surrounding bone. A perceived benefit of the intermobile element concept was that it allowed the use of full ceramic occlusal surfaces on implant-supported restorations. The IMZ implant system was quite popular in the mid 1980s, but its appeal waned when clinicians began to ignore the assumed need to avoid ceramic and metallic occlusal surfaces on implant-supported restorations. Resin denture teeth satisfied the need for prosthetic teeth when the edentulous patient was the predominant recipient of osseointegrated implant restorations. In the mid 1980s, clinicians began to use osseointegrated implants more frequently in other situations. Implant treatment of partial edentulism soon became commonplace in prosthodontic practice, and the need for restorative materials with characteristics beyond what resins 91
THE JOURNAL OF PROSTHETIC DENTISTRY
could offer became obvious. Resin occlusal surfaces proved to be less than optimal in many patient applications. Accelerated wear and eruption or shifting of adjacent teeth, fracture, and/or discoloration became common sequelae of the application of resinous occlusal surfaces in partially edentulous patients. The increased use of metal-ceramic restorations was followed by the introduction of prefabricated ceramic components from implant manufacturers. Improved esthetic outcomes have been the most obvious result of the progressive turn to metal-ceramic and all-ceramic restorations; decreased maintenance problems associated with occlusal material wear also can be assumed. Further applications of materials science and computer technology have led to the introduction of alternative designs and manufacturing processes for implant restorations. Most notably, the Procera system (Nobel Biocare USA, Yorba Linda, Calif.) has successfully replaced traditional lost-wax methods for both all-ceramic and metallic components.29-33 All-ceramic abutment options have increased the possibility of esthetic restoration of dental implants.34 Nonmetallic restorations fabricated from fiber-reinforced composites also have been introduced for implant restoration and appear to hold considerable promise.35
CEMENTED VERSUS SCREWRETAINED IMPLANT RESTORATIONS Mainstream philosophy in the design and restoration of implant prostheses from the 1980s through the early 1990s reflected a strong preference for screw-retained over cement-retained restorations. This preference most likely was the result of how osseointegrated implants and their restorations were introduced. Clinical material coming from European centers seemed to virtually preclude the use of cemented implant restorations. Training courses for early implant systems (Nobelpharma and IMZ) demonstrated only screw-retained restorations. Early exceptions were the U.S.-produced Core-Vent implant and the Swiss-produced Straumann type implants. Nevertheless, the primary focus of implant prosthodontics during the first decade of osseointegration in North America was the use of screw-retained restorations. In 1989 to 1990 the introduction of the Swiss Bonefit implant, later called the ITI implant (Straumann USA, Waltham, Mass.), into the U.S. market brought a challenge to the screw-retained method of prosthesis retention. Initially the ITI system relied exclusively on a cementable abutment system for crown retention. At approximately the same time, the Swedish company Nobel Biocare introduced Cera-One restorative components for single-tooth replacement; these components relied on a cemented crown over a screw-retained abut92
TAYLOR AND AGAR
ment. Shortly thereafter, many implant manufacturers introduced cementable alternatives to screw retention. Although retrievability remains the primary advantage of a screw-retained approach, cemented implant restorations are attractive for several reasons. First, they preclude the interference of screw access holes with esthetic results or with the occlusion of the restoration. Second, cement-based approaches substantially reduce restoration costs. With some systems, restoration with a screw-retained restoration involves nearly 4 times the component cost of a cemented restoration. Third, if no occlusal screw is present, occlusal screw loosening is not a possible complication of implant prosthodontics. Fourth, a cemented restoration is more likely to achieve passive fit than a screw-retained restoration. The argument for increased passivity rests on the assumption that tightening a screw-retained restoration may create substantial strain within the restoration, implant, and investing bone complex. The use of cemented restorations also avoids the clamping effect of multiple screws drawing an ill-fitting prosthesis to place against the supporting implants. Fifth and finally, the implant industry’s desire to extend routine implant use into the general dental practice has stimulated interest in simplifying implant restorative procedures. Cementation of fixed implant restorations more closely follows the procedures routinely performed on natural teeth.
RESTORATION ACCURACY AND PASSIVE FIT/MISFIT Concern that restorations supported by endosseous dental implants may play an important role in the longterm stability and success of those implants has led prosthodontic researchers to explore methods to ensure accurate, passive adaptation of restorations to their supporting implants.29,36-46 Increased accuracy in the impression and transfer of implant position in the dental arch to the working cast has been the topic of many contributions to the literature. Similarly, many authors have investigated various methods of increasing accuracy (or decreasing error) in casting and prosthesis fabrication techniques, including computer-aided design– computer-aided manufacturer technology.29,33 Progress has been made in reducing replication and fabrication errors associated with implant-supported prostheses. In spite of the substantial effort and energy spent on reducing the level of misfit in implant prosthodontics, one simple question has largely been ignored in the literature: whether a misfitting prosthesis can negatively affect the survival of dental implants. Many authors and clinicians have assumed that a misfitting prosthesis rigidly attached to multiple dental implants will impart severe strain between the implants, strain that will not dissipate through subsequent orthodontic shifting or relaxation of the implants in the surrounding bone.47-49 VOLUME 88 NUMBER 1
TAYLOR AND AGAR
Strain at a high enough level might lead to breakdown of the surrounding bone and ultimate loss of an implant. Two questions immediately arise from this concern. First, what is an acceptable level of misfit? Nothing ever fits perfectly, and so there must be a clinically acceptable level of misfit that does not adversely affect the health of the implant/bone interface. Second, how does one measure the magnitude of misfit? Although clinicians strive to avoid inaccuracy or misfit in implant restoration, there are no accepted standards for measuring or even identifying misfit in a clinical situation. If the available literature is acknowledged, the answer to these questions may not be terribly important. Few studies have specifically examined the effects of static loading or misfit of implant restorations on osseointegration. To date, authors who have explored this area have been unable to demonstrate negative outcomes of misfit on the bone/implant interface.50-53 Perhaps it should not be surprising that static load may not be detrimental to stable osseointegration when one considers that implants used for long-term orthodontic anchorage and/or distraction osteogenesis suffer no apparent loss of bone contact during the course of loading, even after years of loading.49,54,55 In one study, the only effect of severe static load was increased bone-to-implant contact compared with unloaded control implants.53 If misfit is not the critical issue for long-term maintenance of osseointegration, then what prosthodontic factors (if any) are important? Several well-designed studies have demonstrated that the magnitude and/or direction of occlusal forces have no apparent effect on supporting implants and bone. Extreme implant overload had no effect on the success of osseointegration in animal models in 4 separate studies.56-59 In contrast, Isidor60 demonstrated bone loss around implants subjected to what can only be described as extreme, off-axis, heavy loading of dental implants. The different results from these studies may reflect the as-yet undetermined critical factor(s) that clinicians should recognize when designing and placing implantsupported prostheses. Although the impact of occlusal forces on the bone/implant interface needs to be researched further, clinicians who restore dental implants are aware of the mechanical complications that occur as a result of occlusal loading of implant-supported restorations. Component loosening and fracture are common results of functional loading and overloading.61-65 Similar knowledge gaps exist with respect to factors that affect the long-term success of progressively loaded and immediately loaded implants. Currently there are no well-documented, controlled, prospective studies that have examined the effects of progressive loading on implant survival or bone level changes around implants. Immediate loading of implants is similarly not well understood. At this time, it is safe to say that completely undisturbed healing of the implant/bone interface is JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
not necessary for successful osseointegration to occur, and in some situations, implants placed into immediate full functional load can be expected to survive and function well.19-24 The requisites for predictable osseointegration of immediately loaded implants have yet to be determined. One parallel consideration is whether provisional loading of a tissue-borne prosthesis over an implant during the osseointegration (healing) period will affect the integration of that implant. To date there is no scientific evidence (and no clearly documented subjective clinical evidence) that early failure of a dental implant can be attributed to early loading or “overload” resulting from a tissue-supported interim prosthesis being worn over a recently placed dental implant. Implant failures that occur during the healing period have been related to various surgical problems during placement, including overheating of bone during site preparation, dull osteotomy drills, and lack of primary stability of the implant at the time of placement. Loading an implant through the use of an interim restoration has not been documented as a cause of early implant failure. It is also safe to state that, at this time, there is no scientific evidence that the factors associated with implant restoration (provisional or definitive) have a predictable impact on the survival of the supporting implant. This apparent lack of effect may be deceiving, in that very real determinants of implant success or failure are likely related directly to the prosthodontic aspects of treatment. Unfortunately, those as-yet unidentified determinants are hidden from the view of clinicians intent on providing the best possible care for their patients.
SUMMARY The first 20 years of osseointegration in North America have seen the most dramatic changes in the prosthodontic treatment of patients since the introduction of fluoride (as an anticaries medication) and the high-speed turbine handpiece combined. Prosthodontic treatment planning, treatment principles, and outcomes have been forever changed in this brief period. Similarly, the expectations of clinicians and patients have undergone a shift toward treatment end points with greater function, esthetics, and permanence. Although the initial success of osseointegration has been gratifying, the scientific basis for what is routinely done and what seems to work is largely lacking. Empirical concepts and principles based on the treatment of natural teeth have, by and large, been extrapolated to the treatment of endosseous implants with apparent success. Perhaps the next 20 years will bring answers to the questions of how and why implant-supported restorations function so successfully. It is hoped that the cost of implant therapy will be reduced to allow 93
THE JOURNAL OF PROSTHETIC DENTISTRY
more people to take advantage of this state-of-the-art form of tooth replacement. REFERENCES 1. Adell R, Lekholm U, Rockler B, Brånemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 2. Zarb GA, Schmitt A. Osseointegration and the edentulous predicament. The 10-year-old Toronto study. Br Dent J 1991;170:439-44. 3. Schnitman P, Shulman L. Dental implants: benefit and risk. Proceedings of an NIH-Harvard consensus development conference. US Department of Health and Human Services; 1980. p. 81-1531. 4. Brånemark PI, Adell R, Breine U, Hansson BO, Lindstrom J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 5. Schroeder A, Pohler O, Sutter F. [Tissue reaction to an implant of a titanium hollow cylinder with a titanium surface spray layer.] SSO Schweiz Monatsschr Zahnheilkd 1976;86:713-27. German. 6. Schroeder A, van der Zypen E, Stich H, Sutter F. The reactions of bone, connective tissue, and epithelium to endosteal implants with titaniumsprayed surfaces. J Maxillofac Surg 1981;9:15-25. 7. McAlarney ME, Stavropoulos DN. Determination of cantilever lengthanterior-posterior spread ratio assuming failure criteria to be the compromise of the prosthesis retaining screw-prosthesis joint. Int J Oral Maxillofac Implants 1996;11:331-9. 8. Lundgren D, Falk H, Laurell L. Influence of number and distribution of occlusal cantilever contacts on closing and chewing forces in dentitions with implant-supported fixed prostheses occluding with complete dentures. Int J Oral Maxillofac Implants 1989;4:277-83. 9. Wang S, Hobkirk JA. Load distribution on implants with a cantilevered superstructure: an in vitro pilot study. Implant Dent 1996;5:36-42. 10. Falk H, Laurell L, Lundgren D. Occlusal force pattern in dentitions with mandibular implant-supported fixed cantilever prostheses occluded with complete dentures. Int J Oral Maxillofac Implants 1989;4:55-62. 11. White SN, Caputo AA, Anderkvist T. Effect of cantilever length on stress transfer by implant-supported prostheses. J Prosthet Dent 1994;71:493-9. 12. Rodriguez AM, Aquilino SA, Lund PS. Cantilever and implant biomechanics: a review of the literature, Part 2. J Prosthodont 1994;3:114-8. 13. Falk H, Laurell L, Lundgren D. Occlusal interferences and cantilever joint stress in implant-supported prostheses occluding with complete dentures. Int J Oral Maxillofac Implants 1990;5:70-7. 14. Taylor TD. Fixed implant rehabilitation for the edentulous maxilla. Int J Oral Maxillofac Implants 1991;6:329-37. 15. Desjardins RP. Prosthesis design for osseointegrated implants in the edentulous maxilla. Int J Oral Maxillofac Implants 1992;7:311-20. 16. Keller EE, Van Roekel NB, Desjardins RP, Tolman DD. Prosthetic-surgical reconstruction of the severely resorbed maxilla with iliac bone grafting and tissue-integrated prostheses. Int J Oral Maxillofac Implants 1987;2: 155-65. 17. DeBoer J. Edentulous implants: overdenture versus fixed. J Prosthet Dent 1993;69:386-90. 18. Brånemark P, Zarb G, Albrektsson T, editors. Tissue-integrated prostheses: osseointegration in clinical dentistry. Chicago: Quintessence; 1985. p. 11-76. 19. Ledermann PD, Schroeder A, Sutter F. [Single tooth replacement with the aid of the ITI (International Team fur Implantologie) type F hollow-cylinder implant (late implant).]SSO Schweiz Monatsschr Zahnheilkd 1982;92: 1087-98. German. 20. Babbush CA, Kent JN, Misiek DJ. Titanium plasma-sprayed (TPS) screw implants for the reconstruction of the edentulous mandible. J Oral Maxillofacial Surg 1986;44:274-82. 21. Tarnow DP, Emtiaz S, Classi A. Immediate loading of threaded implants at stage 1 surgery in edentulous arches: ten consecutive case reports with 1to 5-year data. Int J Oral Maxillofac Implants 1997;12:319-24. 22. Schnitman PA, Wohrle PS, Rubenstein JE. Immediate fixed interim prostheses supported by two-stage threaded implants: methodology and results. J Oral Implantol 1990;16:96-105. 23. Schnitman PA, Wohrle PS, Rubenstein JE, DaSilva JD, Wang NH. Ten-year results for Branemark implants immediately loaded with fixed prostheses at implant placement. Int J Oral Maxillofac Implants 1997;12:495-503. 24. Lewis SG, Beumer J 3rd, Perri GR, Hornburg WP. Single tooth implant supported restorations. Int J Oral Maxillofac Implants 1988;3:25-30.
94
TAYLOR AND AGAR
25. Lewis S, Beumer J 3rd, Hornburg W, Moy P. The “UCLA” abutment. Int J Oral Maxillofac Implants 1988;3:183-9. 26. Kirsch A, Ackermann KL. The IMZ osteointegrated implant system. Dent Clin North Am 1989;33:733-91. 27. Skalak R. Biomechanical considerations in osseointegrated prostheses. J Prosthet Dent 1983;49:843-8. 28. Skalak R. Aspects of biomechanical considerations. In: Brånemark P, Zarb G, Albrektsson T, editors. Tissue-integrated prostheses: osseointegration in clinical dentistry. Chicago: Quintessence; 1985. p. 117-28. 29. Riedy SJ, Lang BR, Lang BE. Fit of implant frameworks fabricated by different techniques. J Prosthet Dent 1997;78:596-604. 30. Sjogren G, Andersson M, Bergman M. Laser welding of titanium in dentistry. Acta Odontol Scand 1988;46:247-53. 31. Jemt T, Linden B. Fixed implant-supported prostheses with welded titanium frameworks. Int J Periodontics Restorative Dent 1992;12:177-84. 32. Chai T, Chou CK. Mechanical properties of laser-welded cast titanium joints under different conditions. J Prosthet Dent 1998;79:477-83. 33. Rubenstien JE, Ma T. Comparison of interface relationships between implant components for laser-welded titanium frameworks and standard cast frameworks. Int J Oral Maxillofac Implants 1999;14:491-5. 34. Prestipino V, Ingber A. Esthetic high-strength implant abutments. Part I. J Esthet Dent 1993;5:29-36. 35. Freilich MA, Karmaker AC, Burstone CJ, Goldberg AJ. Development and clinical applications of a light-polymerized fiber-reinforced composite. J Prosthet Dent 1998;80:311-8. 36. Rangert B, Jemt T, Jorneus L. Forces and moments on Brånemark implants. Int J Oral Maxillofac Implants 1989;4:241-7. 37. Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. Int J Oral Maxillofac Implants 1992;7:26-33. 38. Carr AB, Stewart RB. Full-arch implant framework casting accuracy: preliminary in vitro observation for in vivo testing. J Prosthodont 1993;2:2-8. 39. Lie A, Jemt T. Photogrammetric measurements of implant positions. Description of a technique to determine the fit between implants and superstructures. Clin Oral Implants Res 1994;5:30-6. 40. Jemt T, Lie A. Accuracy of implant-supported prostheses in the edentulous jaw: analysis of precision of fit between cast gold-alloy frameworks and master casts by means of a three-dimensional photogrammetric technique. Clin Oral Implants Res 1995;6:172-80. 41. Tan KB, Rubenstein JE, Nicholls JI, Yuodelis RA. Three-dimensional analysis of the casting accuracy of one-piece, osseointegrated implant-retained prostheses. Int J Prosthodont 1993;6:346-63. 42. Aparicio C. A new method to routinely achieve passive fit of ceramometal prostheses over Brånemark osseointegrated implants: a two-year report. Int J Periodontics Restorative Dent 1994;14:404-19. 43. Jemt T. In vivo measurements of precision of fit involving implant-supported prostheses in edentulous jaw. Int J Oral Maxillofac Implants 1996; 11:151-8. 44. Iglesia M, Moreno J. A method aimed at achieving passive fit in implant prostheses: case report. Int J Prosthodont 2001;14:570-4. 45. May KB, Edge MJ, Lang BR, Wang RF. The Periotest method: implantsupported framework precision of fit evaluation. J Prosthodont 1996;5: 206-13. 46. Kan JY, Rungcharassaeng K, Bohsali K, Goodacre CJ, Lang BR. Clinical methods for evaluating implant framework fit. J Prosthet Dent 1999;81:713. 47. Millington ND, Leung T. Inaccurate fit of implant superstructures. Part 1: Stresses generated on the superstructure relative to the size of fit discrepancy. Int J Prosthodont 1995;8:511-6. 48. Jemt T, Lekholm U. Measurements of bone and framework deformations induced by misfit of implant superstructures. A pilot study in rabbits. Clin Oral Implants Res 1998;9:272-80. 49. Roberts WE, Smith RK, Zilberman Y, Mozsary PG, Smith RS. Osseous adaptation to continuous loading of rigid endosseous implants. Am J Orthod 1984;86:95-111. 50. Carr AB, Gerard DA, Larsen PE. The response of bone in primates around unloaded dental implants supporting prostheses with different levels of fit. J Prosthet Dent 1996;76:500-9. 51. Hurzeler MB, Quinones CR, Kohal RJ, Rohde M, Strub JR, Teuscher U, et al. Changes in peri-implant tissues subjected to orthodontic forces and ligature breakdown in monkeys. J Periodontol 1998;69:396-404. 52. Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants 1996;11:620-2.
VOLUME 88 NUMBER 1
TAYLOR AND AGAR
53. Gotfredsen K, Berglundh T, Lindhe J. Bone reactions adjacent to titanium implants subjected to static load. A study in the dog (I). Clin Oral Implants Res 2001;12:1-8. 54. Wehrbein H, Diedrich P. Endosseous titanium implants during and after orthodontic load--an experimental study in the dog. Clin Oral Implants Res 1993;4:76-82. 55. Wehrbein H, Glatzmaier J, Yildirim M. Orthodontic anchorage capacity of short titanium screw implants in the maxilla. An experimental study in the dog. Clin Oral Implants Res 1997;8:131-41. 56. Celletti R, Pameijer CH, Bracchetti G, Donath K, Persichetti G, Visani I. Histologic evaluation of osseointegrated implants restored in nonaxial functional occlusion with preangled abutments. Int J Periodontics Restorative Dent 1995;15:562-73. 57. Ogiso M, Tabata T, Kuo PT, Borgese D. A histologic comparison of the functional loading capacity of an occluded dense apatite implant and the natural dentition. J Prosthet Dent 1994;71:581-8. 58. Asikainen P, Klemetti E, Vuillemin T, Sutter F, Rainio V, Kotilainen R. Titanium implants and lateral forces. An experimental study with sheep. Clin Oral Implants Res 1997;8:465-8. 59. Miyata T, Kobayashi Y, Araki H, Motomura Y, Shin K. The influence of controlled occlusal overload on peri-implant tissue: a histologic study in monkeys. Int J Oral Maxillofac Implants 1998;13:677-83. 60. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res 1996;7:143-52. 61. Kohavi D. Complications in the tissue integrated prostheses components: clinical and mechanical evaluation. J Oral Rehabil 1993;20:413-22.
JULY 2002
THE JOURNAL OF PROSTHETIC DENTISTRY
62. Kallus T, Bessing C. Loose gold screws frequently occur in full-arch fixed prostheses supported by osseointegrated implants after 5 years. Int J Oral Maxillofac Implants 1994;9:169-78. 63. Taylor TD. Prosthodontic problems and limitations associated with osseointegration. J Prosthet Dent 1998;79:74-8. 64. Walton JN, MacEntee MI. Problems with prostheses on implants: a retrospective study. J Prosthet Dent 1994;71:283-8. 65. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: the Toronto study. Part III: Problems and complications encountered. J Prosthet Dent 1990;64:185-94. Reprint requests to: DR THOMAS D. TAYLOR DEPARTMENT OF PROSTHODONTICS AND OPERATIVE DENTISTRY UNIVERSITY OF CONNECTICUT SCHOOL OF DENTAL MEDICINE 263 FARMINGTON AVE FARMINGTON, CT 06030-1370 FAX: (860)679-1370 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 ⫹ 0 10/1/126818
doi:10.1067/mpr.2002.126818
95