Cantilever rests: An alternative to the unsupported distal cantilever of osseointegrated implant-supported prostheses for the edentulous mandible John

W. McCartney,

DDSa

Department of Veterans Affairs Medical Center, Washington, D.C. Mandibular prostheses supported by implants placed between the mandibular foramina restrict the length of the distally cantilevered components to minimize torque-related stress to the supporting implants. Additional posterior implants used as abutments with rigid attachments better distribute occlusal forces and reduce cantilever torque to the anterior implants. Because of the phenomenon of mandibular flexure, these posterior implants could be subjected to stress-induced microdamage to the bone-implant interface. The use of posterior abutments for without connection reduces the potential hazard of support of the cantilever stress-induced microdamage. (J PROSTHETDENT 1992;68:817-9.)

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n the conventional design of an implant-supported mandibular prosthesis in edentulous patients, placing implants between the mental foramina, with bilateral cantilever extensions of the framework is suggested.l To limit potential torque on the anteriorly placed implants, the suggested length of the distal cantilever has been 12 to 20 mm. Despite verified accurate passive fit of the frameworks and caution to obtain optimal occlusal relationships, fractures of the framework-retaining screws and abutment screws have been reported.2 The opinions or assertions contained herein are the private views of the author and are not to be construed as official or reflecting the views of the Department of the Army or the Department of Veterans Affairs. aFormer Chief, USA Area Dental Laboratory. 10/l/40196

It appears likely that limiting the length of the cantilevers would concentrate the functional and parafunctional forces anteriorly and restrict the available occlusal surface. When a mandibular implant-supported prosthesis opposes a complete denture, the anterior maxilla may be subjected to increased damaging forces, and maxillary denture stability may be compromised by the lack of distal occlusal contacts. If patient requirements dictate a fixed, rigid implantsupported mandibular prosthesis, it is possible to modify the framework design to better distribute functional forces to the maxilla without overloading the anterior mandibular implants. When there is sufficient bone of good quality over the mandibular canal, a single dental implant can be placed bilaterally (Fig. 1). These two implants can provide distal support for the bilateral cantilevers. However, con-

Fig. 1. A single posterior implant is placed bilaterally in addition to the five anterior implants. Fig. 2. Inferior surface of framework with smooth surface and lack of screw access holes in cantilevered sections.

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Fig.

3. Each d&al abutment is fitted with a gold cylinder modified with alloy reinforce-

ment. 4. Framework rests on terminal abutment but is not connected to it. 5. Support provided by the cantilever rests permits more posterior extension of the framework. Fig. 6. Distally extended mandibular prosthesis supported by cantilever rests. Fig. Fig.

nection of these abutments to a rigid prosthesis must accommodate mandibular flexure. Mandibular flexure has been demonstrated during opening and closing movements.3 Functional and parafunctional jaw movements have demonstrated mandibular flexure even when jaw-opening was at a minimum.4 Rigidly connecting the two posterior implants to the anterior framework may subject the bone-implant interface to flexure-related stress-induced microdamage. However, support for the distal cantilevers can reduce torque to the anterior implants without direct connection of the cantilevers to the posterior implants. This torque reduction is accomplished by connection of the framework to the anterior implants only. The framework covers but does not attach to the posterior implants (Fig. 2). Alloy-reinforced gold cylinders (Nobelpharma USA Inc., Chicago, Ill.) are fabri-

cated (Fig. 3) and contoured to passively contact the inferior surface of the cantilevered sections (Fig. 4). The extended framework permits more posterior extension of the prosthesis and the occlusal scheme (Fig. 5). This increased occlusion in the completed prosthesis can provide better distribution of occlusal forces and increase the stability of the opposing denture (Fig. 6). The nonrigid connection should eliminate potential mandibular flexureinduced microdamage to the bone-implant interface of the posterior implants. The cantilever torque to the anterior implants should also be minimized. CONCLUSION When proximity of the mandibular canal and the location, amount, and quality of bone permits placement of bilateral posterior implants, they can provide support for

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cantilevered framework extensions of osseointegrated implant-supported prostheses. REFERENCES 1. Branemark P-I, Zarb GA, Albrektsson T. Tissue integrated prostheses. Osseointegration in clinical dentistry. Chicago: QuintessenceInternational Co, 1985. 2. Zarb GA, Schmitt A. The longitudinal clinical effectivenessof osseointegrated dental implants: the Toronto study. Part III: problems and complications encountered. J PROSTHETDENT 1990;64:185-94.

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3. Goodkind R.J,Heringlake CB. Mandibular flexure in opening and closing movements. J PROSTHETDENT 1973;30:134-8. 4. Oman R, Wise MD. Mandibular flexure associated with muscle force applied in the retruded axis position. J Oral Rehabil 1981;6:299-321. 5. Fischman B. The rotational aspect of mandibular flexure. J PROSTHET DENT 1990;64:483-5. Reprint

requests to:

DR. JOHNW. MCCARTNEY VA MEDICALCENTER CENTRALDENTALLABORATORY (16OL) 50 IRVINGSTREET,NW WASHINGTON, DC 20422

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