Implant frameworks misfit: the influence of screw tightening forces

Objectives: The present in vitro study was designed to compare the differences in the vertical misfit of implant-supported frameworks using three different forces for tightening the bridge locking screws: fastening by hand until first resistance, and using torque drivers with 10 and 20Ncm. Methods: The investigation was conducted based on the results given by 09 six-unit nickel-chromium (02 abutments/ 04 pontics) screw-retained implant supported frameworks. The structures were exposed to simulated porcelain firings. The marginal misfit measurements were made using a traveling measuring microscope at selected screw tightening forces: fastening by hand until first resistance, and using torque drivers with 10 and 20Ncm. The results were submitted to1-Way ANOVA with repeated measures on one factor, and post hoc pairwise comparisons using Tukey test (5%). Results: The mean marginal misfit of the frameworks, fastening the screws by hand until first resistance, was 41.56mm (SD±12.45mm). The use of torque driver devices caused a significant reduction in marginal opening (p<0.05). With the lowest torque available (10Ncm), the mean marginal discrepancy at the abutment-framework interface was reduced an average of 52% to a mean marginal opening of 19.71mm (SD±2.97mm). After the use of the 20Ncm torque driver, the mean marginal discrepancy of the frameworks was reduced an average of 69% to a mean marginal opening of 12.82mm (SD±4.0mm). Comparing the use of torque drivers with 10 and 20 Ncm torque, the means are not significantly different from one another. Conclusion: The seating force has an important effect on the vertical misfit measurements, once it may considerably narrow the vertical misfit gaps at the abutment-framework interface, thus leading to a misjudgment of the real marginal situation.