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.