Available online 17 November 2022

AbstractStatement of problem

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.

Purpose

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.

Material and methods

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.

Results

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).

Conclusions

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.

Section snippetsMaterial and methods

A pilot study with 6 specimens was conducted to determine the sample size. The statistical analysis revealed a sample size of 10 specimens per group (n=10) that was adequate to achieve a power of 0.8 at a significant threshold (α=.05). Thirty Ø4.3-mm titanium implants with a conical connection (NobelReplace Conical Connection 4.3×13 mm; Nobel Biocare AG) and 30 premachined 4.3-mm engaging conical connection abutments (GoldAdapt; Nobel Biocare AG) were used in the study.28 The abutments were

Results

The mean RTVs for the tested groups ranged from 19.89 Ncm to 27.19 Ncm. The mean RTV was 27.19 Ncm, 19.89 Ncm, and 24.24 Ncm for groups UCCA, UPCCA, and PCCA, respectively (Table 1). A significant difference was found in RTVs among all tested groups (P<.05) (Table 2). In the cast groups, PCCA, which used protector caps and pegs for casting, revealed significantly lower loss of RTV (P<.05) (Fig. 7).

Discussion

Based on the results, the null hypothesis was rejected as significant differences in RTVs were found among the tested groups. Vertical discrepancies at abutment-to-implant and screw-to-abutment screw seat interfaces have been reported to be higher in cast abutments than in premachined abutments.2 In this present study, discrepancies were visualized at the abutment-to-implant interface site in the group that was cast and airborne-particle abraded without protector caps or pegs. The discrepancies

Conclusions

Based on the findings of this in vitro study, the following conclusions were drawn:

1.

RTV was reduced irrespective of the fit of the mating surface contacts.

2.

When protector caps and pegs were used, a significantly lower loss of RTV was found.

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© 2022 by the Editorial Council for The Journal of Prosthetic Dentistry.