Effect of implant number and spatial configuration on reverse torque values of screw-retained restorations after...

Screw loosening has been observed in screw-retained multiunit implant prostheses. The effects of implant number, spatial configuration, and screw position on preload retention remain unclear.

The purpose of this in vitro study was to compare the reverse torque values and torque-loss patterns after mechanical aging in screw-retained prostheses on different numbers of implants and configurations.

Thirty screw-retained, implant-supported cobalt chromium (Remanium Star Co-Cr powder; Dentaurum GmbH & Co KG) framework specimens were fabricated by laser sintering, fastened on multiunit abutments (EL OMNI; Century Technologies S.R.L.), and randomly assigned to 3 groups comprised of 2 parallel implants (15×3.8 mm ΕL; Century Technologies S.R.L.) placed 10-mm apart, 4 parallel implants placed in a straight line with 10 mm spacing, and 4 parallel implants with an anteroposterior spread, with the 2 distal segments in 45-degree angulation. The prosthetic screws (SSR-ASC; Century Technologies S.R.L.) were tightened to 20 Ncm, and initial values were recorded. Mechanical aging followed (1 200 000 cycles, 70 N, 60 mm/second, dry conditions) with vertical loading via a 2-mm-rounded pin. Final reverse torque values and percentage torque loss were calculated relative to 20 Ncm. The primary analysis was the torque loss of the 2 screws adjacent to the load across all groups, and the secondary analysis was the force distribution patterns. Data were analyzed using 2-way repeated-measures ANOVA with post hoc comparisons.

Torque loss was highest at load-adjacent screws, with 2-implant prostheses showing 36.0% and 27.5% loss, straight 4-implant prostheses 23.5%, 13.5%, 8.5%, and 4.5% from the loaded to contralateral screws, and anteroposterior spread 4-implant prostheses 20.0%, 11.0%, 15.0%, and 4.0%. P<.001 indicated a significant loss of RTVs in the 2-implant group.

Based on the findings of this in vitro study, increasing the number of implants improved the reverse torque values for the prosthetic screws. Load-adjacent screws experienced the most significant torque loss, while anteroposterior spread configurations demonstrated localized vulnerability at angled junctions after mechanical aging.