Abutment screw preload verification: Calculation using tightening torque, loosening torque, and screw thread pitch...

Quantifying abutment screw preload is essential in determining the risk of premature screw loosening. Currently, this is complex, requiring sophisticated measuring devices. A simplified methodology for calculating abutment screw preload has recently been proposed, but a comprehensive evaluation of its validity is lacking.

The purpose of this in vitro study was to determine the validity of the dental implant screw preload calculation for accuracy and precision by comparing the calculated method with direct load cell measurements.

Six implant-abutment assemblies underwent preload measurement assessed using a specialized load cell. The implant abutment screw was tightened to 35 Ncm, using an electronic torque device. The screws were then loosened and the reverse torque recorded. Each assembly underwent 10 tightening and reverse torque cycles such that the screw preload was zero. Recorded data, along with screw thread pitch size, were used to calculate preload using the screw equation. Measured versus calculated preload provided percentage differences, linear regression model, Bland-Altman analysis, and concordance correlation coefficients were assessed for validation (α=.05).

Linear regression analysis revealed a significant positive correlation across all groups (R²=.67 to .89, P<.05). The 1:1 identity significantly deviated from 1.0 (P<.05). The Bland-Altman analysis indicated systematic bias from 2.9 to 13.0, with 95% limits of agreement from -43.51 N to +54.71 N. Only implant 2 exhibited a statistically significant mean bias (P=.04). Concordance correlation coefficient (CCC) reached a substantial level for implant 3, while other groups demonstrated moderate agreement (CCC=.69 to .76) CONCLUSIONS: A straightforward calculation based on input torque, reverse torque, and screw geometry was developed and validated. The results indicated high reliability, significant correlation across all implants, and that minor calibration would lead to improvement. The substantial concordance (CCC=.85) and minimal mean bias (2.9 N) validated the technique, which can be considered a robust tool for estimating implant abutment screw preload.