Effect of repeated screw joint closing and opening cycles on implant prosthetic screw reverse torque and implant and screw thread morphology.

STATEMENT OF PROBLEM: Clinicians must know if a new screw can predictably increase reverse torque after multiple screw insertion cycles. PURPOSE: The purpose of this study was (1) to compare the effect of multiple implant prosthetic screw insertion and removal cycles on reverse torque, (2) to determine whether a new screw, after multiple screw insertion cycles, affects reverse torque, and (3) to assess implant and prosthetic screw thread surface morphology with scanning electron microscopy (SEM). MATERIAL AND METHODS: One primary screw was paired with an implant (MT Osseospeed) and inserted to 25 Ncm torque 9, 19, 29, or 39 times (n=10). Primary screw reverse torque values were recorded after each insertion. A second, reference screw was then paired with each implant for a final screw insertion, and reverse torque was measured. Maximum, minimum, median, and mean values (P(max), P(min), P(median), and P(mean)) were identified for primary screws. A 1-way ANOVA and Tukey HSD post hoc analysis assessed the influence of multiple screw insertion cycles on P(max), P(min), P(median), and P(mean) values (α=.05). Confidence intervals were used to test differences between reference (REF) screw data and corresponding DMAX and DMIN (DMAX=P(max)-REF; DMIN=P(min)-REF). The surface topography of an unused implant and screw and of 1 implant and screw from each group was evaluated with SEM. RESULTS: Pairwise comparisons showed that 9 or fewer insertion cycles resulted in significantly greater mean reverse torque (20.9 ± 0.5 Ncm; P<.01). After 19, 29, or 39 cycles, the second, reference screw achieved significantly greater reverse torque than the minimum recorded values (P<.05). Implant thread surface morphology changes occurred primarily during the first 10 insertions. CONCLUSIONS: After 10 screw insertion cycles, a new prosthetic screw should be used with the implant system tested to maximize screw reverse torque and maintain preload when an abutment is definitively placed.