Theoretical assessment of cross sections for cantilevered implant-supported prostheses

Occlusal forces concentrate at the cantilevered section(s) of fixed implant-supported prostheses. Investigators in early clinical applications of these prostheses described fractures of the metal alloy framework at the cantilevered-distal abutment junction. Improved performance was noticed when the framework's cross-sectional area was increased and the metal alloys used had an increased tensile strength and elastic modulus. Long-term service of a cantilevered implant-supported prosthesis is directly dependent on the fatigue life of the metal alloy framework. Two fatigue durability factors that are alterable by the technician/clinician are the framework material and the cross-sectional design of the structure. These factors can increase rigidity and alter the stress distribution through the framework. The importance of cross-sectional design on the rigidity may become more critical in situations with decreased intermaxillary space, especially if lengthened cantilevers are required or patients show discernible parafunctional habits. The theoretical aspects of beam deflection and the cross-sectional design of cantilevered structures under stress were investigated in this study. A subsequent report will provide results from experimental evaluations of designs similar to those assessed in this study.

The cross-sectional designs considered are an L, I, U and a nearly elliptical-shaped configuration. The size of the framework was determined by approximating the total cross-sectional space available for the restoration, minus the space required for artificial teeth in severe restrictions. All theoretical and actual specimens were modeled within these confines. Some allowance for the contours of the inferior surface of the artificial teeth was made to optimize the dimensions of each framework. Calculations enabled comparison of relative deflection and stress characteristics of each design group and predictions of fatigue durability.

The displacements of each cantilever were found to be dependent upon the heights of each framework. As evaluated, the L and elliptical designs experienced larger maximum end deflections than the I- and U-shaped designs. The maximum normal stresses were observed to be less in the I and elliptical sections than in the U and L sections.

The space restrictions for limited intermaxillary conditions reveal that the I cross-sectional design may provide the least displacement and smallest maximum normal stress under conditions in which each framework is subjected to an identical load. Each of the framework cross sections considered, however, is a viable candidate for use. The effectiveness of any particular shape in an intraoral environment cannot be easily assessed from the simple statistical analysis. Limitations of the predictability with statistical analysis as opposed to actual mechanical testing or in vivo use was noted. Less severe space restrictions would probably improve the estimated performance of the other designs.