Effect of misfit and mechanical cycling on screw joint

Objectives: Evaluate the effect of unilateral misfit at different levels on a crown-implant-retention screw system of implant-supported prostheses submitted to mechanical cycling. Methods: Hexed UCLA Co-Cr/plastic sleeve abutments were cast in Co-Cr alloy to fabricate 48 metallic crowns divided into four groups (n=12). In group A (control group) the crowns did not present misfit, but in groups B, C and D the crowns were fabricated with unilateral misfit of 50µm, 100µm and 200µm respectively. The crowns were attached by titanium retention screw with 30N/cm to external hexagonal osseointegrated implants. The replicas were embedded in acrylic resin and submitted to detorque initial evaluation before mechanical cycling. After that, the fatigue testing was performed by an eletromechanic machine calibrated to apply an oblique dynamic load of 130N at a frequency of 2Hz, totalizing 500,000 and 1,000000 cycles. The cycles started at 0. At the end of each period, the retention screw detorque was assessed by an analogic torque gauge. All the retention screws were substituted by a new screw before each cyclic period beginning. Data were evaluated by ANOVA and Tukey test (p<0.05). Results: There was no statistically significant difference in preload decrease among groups (p>0.05) at both cyclic periods. There was no statistically significant difference between the two cyclic periods regardless groups (p>0.05) and in the evaluation of each group (p>0.05). Conclusions: Unilateral misfit did not influence preload decrease. The two periods of mechanical cycling did not influence the preload decrease of groups.

Financial support: FAPESP (grant number 05/56804-4)