Stress levels for well-fitting implant superstructures as a function of tightening force levels, tightening sequence, and different operators

Statement of problem. Unfavorable stress distribution and occlusal overload have been reported to result in failures ranging from screw loosening to loss of osseointegration. Purpose. The purpose of this study was to assess the effect of different tightening forces and sequences, with different operators, on stresses generated on an accurately fitting implant superstructure on multiple working casts made with a splinted impression technique. Material and methods. The effects of different tightening forces (10 and 20 Ncm) were assessed with the use of 30 stone casts made from a metal master model with a splinted impression technique. Stresses generated were recorded by 4 strain gauges attached to the superior surface of the master framework. A multiple analysis of variance with repeated measures was performed to test for significant differences among the groups. Results. Tightening force values at 10 Ncm ranged from 150.43 to 256 Ncm. At 20 Ncm, microstrain values ranged from 149.43 to 284.37 Ncm. Microstrain values related to the sequence of tightening ranged from 150.8 to 308.43 Ncm (left to right) and 154.63 to 274.80 Ncm (light to left). For the different operators, microstrain values ranged from 100.13 to 206.07 Ncm. No statistically significant differences among the variables of tightening force, tightening sequence, and operators were found (P>.05). The interaction between groups and strain gauges was also found to be nonsignificant (P>.05). Conclusion. The potential of variable tightening force and tightening sequence to generate unfavorable preload stresses can be minimized through use of the splinted impression technique,which ensures an accurately fitting superstructure.