Guzaitis, K.L., Knoernschild, K.L., Viana, M.A.

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 25Ncm 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=0.05). 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 one implant and screw from each group was evaluated with SEM.

Results: Pairwise comparisons showed that nine or fewer insertion cycles resulted in significantly greater mean reverse torque (20.9±0.5Ncm; p<0.01). After 19, 29, or 39 cycles, the second, reference screw achieved significantly greater reverse torque than the minimum recorded values (p<0.05). 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 maximise screw reverse torque and maintain preload when an abutment is definitively placed.

J Prosthet Dent 2011; 106 (3): 159-169.