• Sebastian Saba, DDS, Cert. Pros. •
The success of any prosthetic design depends on proper management of the occlusion. The clinical variables influencing occlusal stability must be determined and considered in the design of the final prosthesis. This paper outlines some of these variables.
MeSH Key Words: biomechanics; dental prosthesis design; dental prosthesis, implant-supported; stress, mechanical
© J Can Dent Assoc 2001; 67(9):522-6
This article has been peer reviewed.
Dental implant-supported restorations may develop complications for different reasons: some biological in origin1,2 (Fig. 1) and others mechanical. The prosthetic design should respect the biomechanical factors that can contribute to prosthetic complications. Occlusal stability is achieved when the variables contributing to failure are identified and corrected or compensated for in the final prosthetic design.
The most significant factor affecting stability is occlusal loading. Excess loading may lead to loosening of abutment screws3,4 and, if undetected, to possible fracture. Overloading may also damage the implant5 (Fig. 2) and superstructure and lead to loss of osseointegration.6 Overloading may occur if the implant prosthesis is designed with inadequate implant-fixture support under normal occlusal loading. The key is to place a sufficient number of implants to support the prosthesis.7 The conventional ratio of implant to prosthetic unit is 1:1 (Fig. 3). However, for posterior restorations, the ratio may vary. Variable bone quality or lack of bone width may require 2 implants per unit molar replaced.8 Two implants can be placed in narrower ridges and will provide greater antirotational and occlusal support and an increased surface area for osseointegration. Two implants positioned off angle will also provide counter support and reduce stress on the angled abutment screws.9
If the ridge height is diminished, the use of a standard-diameter short implant (<10 mm) is not usually recommended in posterior restorations (Fig. 4). A wide-diameter implant (Fig. 5) may provide adequate surface area for osseointegration and provide an alternative for support.10 Ridge diameter, bone height and quality will be determining variables. The width of the proposed restoration will also dictate the amount of support required. The wide-diameter implant provides a larger abutment screw connection (for strength)11 and a wider implant table for occlusal support. The wide-diameter implant has gained popularity in cases where the edentulous area does not provide space for 2 standard-diameter implants, and a single standard-diameter implant has been determined to be inadequate for support.
Abnormal occlusal forces, such as those caused by bruxism or clenching, may also contribute to prosthetic complications.12 These habits are not a contraindication for implant dentistry, but must be diagnosed and compensated for in the final prosthetic design. The use of adjunctive protective guards is mandatory.
The stability of existing teeth must also be confirmed before placement of any fixed partial implant-supported prosthesis. Any mobility in the existing dentition must be diagnosed and corrected. Clinical mobility of existing dentition will result in added occlusal strain on the implant-supported prosthesis. The presence of any interocclusal interferences must also be corrected. Frequently these are detected too late and compromise the occlusal design of the new prosthesis. Stable centric contacts, good excursive guidance of choice and sound periodontal support is required to achieve a stable occlusion.
Occlusal Design and Guidance
Occlusal design in partial fixed-implant-supported prosthetics is based on conventional restorative principles. The key is to provide proper anterior excursive guidance. Minimize any lateral forces on any implant-supported prosthesis, especially in the posterior area (where lateral forces are greater).13 For anterior fixed partial prosthetics, this may be difficult. The occlusion on any anterior implant-supported prosthesis should obtain guidance from the existing anterior or posterior dentition (anterior disclusion, canine guidance or group function occlusal philosophies) which provide proprioceptive feedback, helping to control the intensity of lateral forces.
For complete arch fixed prostheses, the occlusal design is much more complicated and controversial. Occlusal guidance will depend on implant size, number, location, angulation, quality of bone, characteristics of opposing dentition, parafunctional history and occlusal characteristics.
The provisional stage of implant therapy is critical in diagnosing the static and dynamic variables of occlusion.14 A fixed detachable provisional model will help determine occlusal habits that are not readily identifiable otherwise. These can be corrected and compensated for in the final prosthesis. The provisional stage will also be a testing ground for your occlusal hypothesis.15 Abutment selection, length, contour of the restoration and size of the occlusal table will all influence the occlusal design.
Not all patients can be treated with the same type of restoration or design. In certain cases, a screw-retained prosthesis may be preferred; in others, a cemented prosthesis may be appropriate. Variables such as esthetics, occlusion, angulation of implants, mechanism of retrievability or implant location will guide the design of the prothesis.
The key to a stable implant/prosthesis relationship is to achieve a passive fit16 of the framework during try-in. A non-passive fit will create stresses17 in the connecting and abutment screws and on the implant.18 This can lead to premature screw failure, damage to the prosthesis and complications of osseointegration. A positive correlation exists between the discrepancy of fit and stress in the prosthesis.19 Proper seating of abutments or impression copings before impressions will minimize clinical and laboratory complications20 (Fig. 6). Laboratory technique should minimize casting shrinkage and inaccuracies, and a non-passive framework try-in technique should achieve a stable and passive fit.21
The cantilever prosthesis has been used in prosthodontics with guarded success for many years. This design has had a resurgence in implant dentistry.22 Frequently it is not possible to achieve an implant-to-prosthetic-unit ratio of 1:1 for anatomical reasons. In posterior sextant implant-supported restorations, a distal cantilever prosthesis is common. The lack of quality and quantity of bone in the posterior sextants has created the need for this design. Cantilevers must be used with caution23 (Fig. 7). The weakest link in the cantilever design is the location and size of the pontic and the intensity of occluding masticatory forces.24 These forces tend to be greatest in distally located pontic cantilevers.25 A mesial cantilever is favoured over a distal cantilever for this reason (Figs. 8 and 9). A narrow occlusal table is recommended for the pontic.
An overcontoured anterior or posterior restoration will also act as a cantilever and increase stress within the framework during loading (Figs. 10 and 11). The abutment selection should compensate for minor irregularities in implant angulation to help compensate for occlusal factors. A wider occlusal table will increase stress on the abutment screws. Severe angulation problems may be a contraindication for a fixed-type of implant-supported prosthesis.
A significant improvement in abutment-implant stability has been achieved with preloading or torquing of components. Hand torquing has been shown to be unreliable,26 but mechanical torquing has proven to be predictable and has significantly reduced loosening of implant components. The torque wrench is now the standard for insertion and tightening of implant components. Several abutment systems available today clearly indicate the amount of torque that is required for proper stabilization.
Occlusion has been an important variable in the success or failure of most prosthodontic reconstructions. With natural teeth, a certain degree of flexibility permits compensation for any occlusal irregularities. Implant dentistry is not as forgiving. The status of the occlusion must be properly diagnosed, corrected or compensated for, and properly integrated into the design of the definitive restoration. The occlusion must be more rigorously evaluated with implant-supported prosthodontics adjacent to natural dentition.
Acknowledgement: Dr. François Gagnon for his contribution to this manuscript.
Dr. Saba is a prosthodontist in private practice in Montreal, Quebec.
Correspondance to: Dr. Sebastian Saba, 240-3550 ch. de la Côte-des-Neiges, Montreal, QC H3H 1V4. E-mail: firstname.lastname@example.org.
The author has no declared financial interests.
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