Resistance to loosening of intentionally shortened screws used to solve the unsuccessful removal of fractured prosthetic screws

Abstract

Statement of problem: Fractured prosthetic implant screws cannot be removed in all patients, ultimately leading to the removal of the implant. Whether an intentionally shortened prosthetic implant screw (SPIS) can provide adequate retention is unclear.

Purpose: The purpose of this in vitro study was to evaluate the resistance to loosening of SPISs engaging the remaining coronal internal threads as a possible solution to maintaining both implant and restoration.

Material and methods: Fifty grade V titanium SPISs were used to tighten 50 titanium transepithelial abutments on implants to 30 Ncm. The specimens were distributed into 5 groups (n=10) according to the conditions under which the screws were secured to manufacturer-recommended preload: dry (D), moistened in saliva (AS), moistened in chlorhexidine (CLHX), wrapped in polytetrafluoroethylene tape (PTFE), and resin cemented (RE). All groups were subjected to a cyclic loading test (240 000 cycles). The reverse torque value (RTV) of the SPIS was registered twice: 24 hours after initial tightening (T1); and after retightening and the cyclic loading test (T2). The resultant RTV was compared with the 30-Ncm tightening torque to assess torque loosening. The Kruskal-Wallis and Mann-Whitney tests were used for the comparisons between groups and the Wilcoxon test for the intragroup comparisons (α=.05 with Bonferroni correction).

Results: At T1, all groups found lower mean±standard deviation RTVs than the reference tightening torque (30 Ncm) (D 24.82±2.34 Ncm, AS 25.56±2.89 Ncm, PTFE 26.02±2.26 Ncm, CLHX 26.26±1.82 Ncm), except the resin-cemented group, which increased its RTV (RE 44.01±19.94 Ncm). At T2, all the groups found lower RTVs than the reference tightening torque, and the torque values at T1 (D 19.81±6.59 Ncm, CLHX 18.98±6.36 Ncm, AS 21.28±7.32 Ncm), with the exception of PTFE (24.07±3.41 Ncm) and RE (41.47±21.68 Ncm), where RTV was similar to that recorded at T1. At T1, significant differences were found among the groups (P=.024). At T2, after cyclic loading, the RE group found the highest RTV, reporting significant differences with the D and CHLX groups (P<.05) and statistically similar to the AS group (P=.068).

Conclusions: PTFE-wrapped screws found similar RTVs after the fatigue test than dry, moistened with saliva, and moistened with chlorhexidine screws. Resin-cemented shortened prosthetic implant screws were found to be the most resistant to loosening after cyclic loading.