The Relationship Between Time of Retightening and Preload Loss of Abutment Screws for Two Different Implant Designs: An In Vitro Study
The loosening of an abutment screw is one of the most frequent complications in implant-prosthetic rehabilitation, especially for single-crown cemented prostheses. This complication is due to several mechanical factors including type of connection, abutment-screw geometry, settling effects, and cyclical load. The purpose of the present in vitro study was to compare and associate different times of retightening with reductions in preload losses. We evaluated 40 internal hexagon dental implants and 40 external hexagon dental implants, with their related abutment screws. The implants were embedded in acrylic resin in cylindrical polyvinyl chloride tubes (26 mm diameter, 20 mm height). The abutments were fixed to the implants with screws to an initial torque of 35 Ncm using a digital torque meter with decimal precision. Two different types of connection were randomly divided in 4 subgroups of 10 samples each. One subgroup was used as control. The test groups underwent retightening to the same initial torque at increasing times from initial torque application for tightening of the abutment screws, to their retightening at 2 minutes, 5 minutes, and 10 minutes. The retightening time of 2 minutes shows significantly reduced preload loss. Randomized clinical trials are strongly required to provide clinicians with a beneficial standardized protocol of retightening that can be applied in routine clinical practice.

Photograph of the set-up of the experimental model used in the present study. (a) Implant positioned in the torque application device for tightening torque to be applied to the screw (right). The readings of the digital torque meter (left) were then recorded as the initial tightening torques, and as the retightening torques at the defined times. (b) Close-up side view of the positioning of the implant for torque application to the screw.

Figure 2. Box plot showing the data distribution of the preload loss for the IG2, IG5, IG10 groups and the significance (P < .05) from the 1-way ANOVA analysis. *Values from the 2-minute retightening were significantly different to the other two groups. Figure 3. Box plot showing the data distribution of the preload loss for the EG2, EG5, EG10 groups and the significance (P < .05) from the 1-way ANOVA analysis. *Values from the 2-minute retightening were significantly different from the other two groups. Figure 4. Diagram depicting the Student t tests comparing the preload losses of the EG group vs the IG group. The differences between the two groups were not significant.
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