CHAPTER 27 Principles of Occlusion in Implantology
The occlusal scheme design for implant-supported prostheses often is discussed only briefly in the implantology literature. Yet, as with all phases of prosthodontics, occlusion is a critical factor in the success or failure of implant-supported restorations. Tremendous efforts have been made to design complex and intricate implant infrastructures for these prostheses. However, it is equally or even more important to design restorations that use occlusal forces constructively, not destructively.
First, the patient controls the occlusal forces on natural teeth in response to proprioception. Because natural periodontal ligament–mediated proprioception does not occur with implants, its protective influence is lost in determining mandibular velocity and displacement. This means that occlusal forces on implant-supported prostheses must be, if anything, more carefully designed and exactly accomplished than with tooth-supported prostheses.
Second, no single occlusal scheme can be used for every type of restoration. The purpose and mechanism of complete denture occlusion are different from those of a single-crown restoration. For implant-supported prostheses, the occlusal scheme must suit the purpose and location of the restoration.
Third, two arches are involved in every occlusion. Whether the implant-supported prosthesis occludes with natural dentition, tooth-, or tissue-supported prostheses or another implant-supported prosthesis is critical, because each of these prostheses exerts a different range of forces in both functional and resting movements.
Finally, the natural resorption pattern of edentulous alveolar bone is almost never parallel to the ideal occlusal plane of the dentition. In determining abutment height and position, not only adequacy of interarch space but also orientation of the plane of occlusion must be considered, because a disrupted or incorrectly oriented plane of occlusion makes achieving the optimum occlusal scheme impossible (Fig. 27-1).
In all phases of prosthetic rehabilitation, the occlusal scheme is designed to meet the requirements of the oral situation. When an implant-supported prosthesis replaces a single tooth in an otherwise intact dental arch, the occlusion is designed like that of a tooth-supported single restoration. This generally is agreed to include lighter than normal tripodal centric occlusal contacts, to prevent supereruption of the opposing natural tooth. Because teeth have a periodontal ligament, they are microscopically compressible. Centric stops are taken by having the patient close hard in centric. The adjacent periodontal ligaments must be compressed to prevent hyperocclusion of the implant restoration. The implant-supported prosthetic crown should follow the guidance of natural teeth in working relationships, similar to other fixed restorations bounded by intact natural teeth (Fig. 27-2).
FIGURE 27-2. A, B, Implant-supported prosthetic crowns bounded by natural teeth should follow their guidance in working relationships, similar to other fixed restorations. The canine guidance found in these arches is maintained after restoration of the implants.
Canine disclusion is used in cases in which natural canine teeth are present and periodontally strong. Where canine teeth are replaced with implant-supported prostheses that occlude with natural teeth or other implant-supported prostheses, group function occlusion supported by natural teeth or other implant-supported prostheses is preferable, to avoid loading a single supporting implant during disclusion. Group function occlusion provides occlusal contact in centric position and in working movements but complete disclusion in balancing positions (Fig. 27-3).
FIGURE 27-3. A, B, For canine area implant-supported crowns, group function occlusion supported by natural teeth or implant-supported prostheses is preferable to canine guidance to avoid loading a single supporting implant during disclusion.
By far the most controversial design for implant-supported prosthesis occlusion is in the totally edentulous mandible opposing the edentulous maxilla. Osseointegrated, subperiosteal, staple, and other implant types originally were designed to treat patients who use maxillary dentures with little difficulty but have unfavorable hard and soft tissue configurations for retention and stability in the mandibular arch. This is still probably the oral situation most frequently treated with implant dentistry.
The occlusal scheme requirements are similar between the case of maxillary complete denture or mandibular implant-supported prosthesis and the combination of the maxillary complete denture and mandibular distal extension removable partial denture. Both situations provide a maxillary denture that rests on movable and depressible tissue and a mandibular arch with dentition, either natural or artificial, essentially fixed into place.
At first glance, the principles of complete dentures would seem to apply to implant-supported prosthesis restorations; however, the quality of prosthesis support in the two cases is different. In conventional complete denture cases, both dentures are free to move on the tissues, and balanced occlusion (dynamic occlusal contacts throughout the arch) usually is accomplished, at least in part, by denture base movement on resilient tissue. This is especially true of flat plane cases, which have no cuspal inclines to compensate for Christensen’s phenomenon (the disclusion of posterior teeth in protrusive position). In dynamic occlusion with complete dentures, each base moves slightly, and the compressive force delivered to tissue is relatively low. Tissue proprioception is a limiting factor, because the patient patterns the movements to seat, rather than unseat, the mandibular denture.
In the complete denture or implant-supported prosthesis, only the maxillary denture moves in function. Soft tissue resiliency and denture movement cannot be relied upon to help balance dynamic occlusal contacts without excessive compressive and tensile forces under the denture base. Occlusal forces generated against the maxillary denture by the implant-supported prosthesis are similar in magnitude to those for cantilever restorations using natural teeth. These forces are many times greater than would be possible with even a well-supported mandibular complete denture, and tissue proprioception of the mandibular denture base is no longer significant. An occlusal scheme that requires movement of the maxillary restoration could predispose the patient to combination syndrome, as has been documented in patients using maxillary denture restorations that provide only anterior tooth contacts in protrusive position.