Cyclic loading of implant-supported prostheses: Changes in component fit over time

Statement of problem. Dental literature suggests that an implant-supported prosthesis must exhibit a passive fit to prevent implant fracture, component breakage, and screw loosening. From a practical standpoint, passive fit is impossible to achieve; instead, minimal misfit may be the clinical goal. To date no specific range of misfit (below which problems are minimal and above which catastrophic failure occurs) has been established. Purpose. The purpose of this study was to determine whether the fit of an implant-supported prosthesis changes through cyclic loading and to quanti, the amount of change between the gold cylinder and implant abutment over time. Materials and methods. Fifteen implant-supported frameworks were fabricated with conventional casting techniques and were cyclically loaded under 3 different loading conditions. Five frameworks were loaded on the anterior portion of the framework, 5 were loaded on the left unilateral posterior cantilever, and 5 were loaded bilaterally on the posterior cantilevers with a servohydraulic testing machine. A cyclical load of 200 N was applied to each framework for up to 200,000 cycles. Linear measurements were made in micrometers of the gap between the prosthetic cylinder and the implant-supported abutment at 4 predetermined reference points. These measurements were recorded before the application of the cyclical load, after 50,000 cycles, and after 200,000 cycles. A repeated measures of variance model was fit separately to the data for each load location (P<.05). Results. There was a significant (P=.024) decrease in gap dimensions at individual reference points and a significant (P=.031) decrease in the average gap when the load was applied to the anterior portion of the framework. When the load was applied unilaterally or bilaterally on the posterior cantilever, significant gap closure was not observed (P=.33 and P=.35, respectively). Conclusion. Within the limitations of this study, the fit between the prosthetic superstructure and the implant-supported abutment changed when simulated functional loading of the anterior portion of the prosthesis was performed. Simulated functional loading applied unilaterally or bilaterally to the posterior cantilever portion of the prosthesis did not result in changes in the measured gap sizes.