Evaluating the effect of the protector cap for castable implant abutments on reverse tightening values

Screw loosening is the most common mechanical complication with implant prostheses. How the alteration of implant-to-abutment connection surfaces that occurs during laboratory procedures affects screw loosening is unclear.

The purpose of this in vitro study was to compare the reverse tightening value (RTV) differences between custom castable abutments before casting, after casting in a conventional manner, and after casting with custom protector caps and pegs.

Thirty implants with a standard-diameter conical connection (NobelReplace Conical Connection 4.3×13 mm; Nobel Biocare AG) and 30 premachined 4.3-mm GoldAdapt abutments (GoldAdapt; Nobel Biocare AG) were selected for this study. Specimens were divided into 3 groups (n=10): the uncast custom castable abutment group (UCCA) in which abutments were new and not cast; the unprotected custom castable abutment group (UPCCA) in which abutments were cast and devested with airborne-particle abrasion; and the protected custom castable abutment group (PCCA) in which abutments were cast by using protector caps and pegs made by milling zirconia and then devested with airborne-particle abrasion. All abutments in each group were tightened to 35 Ncm with a calibrated digital tightening device. After 10 minutes, all screws were retightened to 35 Ncm. At 3 hours, each screw was loosened, and the value at which the initial loosening occurred was documented as the RTV. The results were statistically analyzed with 1-way ANOVA to explore differences, and post hoc tests with Tukey adjustment were used for multiple comparisons.

Among the tested groups, the mean RTV ranged from 19.89 Ncm to 27.19 Ncm: UCCA 27.19 Ncm, UPCCA 19.89 Ncm, and PCCA 24.24 Ncm. A significant difference was found among the tested groups (P<.05).

Casting procedures, especially devestment with airborne-particle abrasion, affected implant-abutment connections and the seat site of the screw. Protecting the implant connection site and the seat site of the abutment screw with protector caps and pegs prevented a significant loss of the RTV.