Torque and angulated retaining screw preload in implant supported prostheses

Abstract

Purpose Angulated screw channel systems provide for screw-retained implant restorations in cases of leass than ideal implant positioning. Theoretical calculations predict a loss in abutment screw preload when the screw is torqued at an angle to the long axis of the implant. This in-vitro study aimed at measuring the loss in preload with a change in the angle of the applied torque and to determine to what value the torque should be increased to minimise the loss in preload for any specific angle.Method A custom built preload measurement device (Southern Implants, Irene, South Africa) was used to measure the preload achieved in the abutment screw (Dynamic Abutment® Solutions, Spain). A manual torque wrench applied torques ranging from 20 to 40 Ncm in 5 Ncm intervals at each angulation interval between 0° and 30° in 5° increments. Each test combination of angle and applied torque was repeated 4 times. Comparison of the preload between angles at a given torque were made by one-way ANOVA. Post-hoc comparisons were done using the Tukey-Kramer adjustment for multiple comparisons. The preload data were modelled using Ordinary Least Squares (OLS) regression. Results For all angles up to 20°, the preload had a positive linear relationship with all torques applied and was close to the value of the applied torque. At angles >20° a statistically significant loss in preload (p < 0.5) occurred for all torque values between 25 and 35 Ncm. At a torque of 20 Ncm, a significant loss in preload (p = 0.023) was seen at 30° whereas at 40 Ncm, a significant loss (p = 0.015) first occurred at 15°. Thus at higher angles a non-linear relationship appeared between preload and angle. The data allowed the use of a non-linear regression model to predict the change in preload as a function of both applied torque and angle. At the recommended torque value of 25 Ncm, a predicted 12.1% and 20.7% loss in preload occurred at angles 25° and 30° respectively. Conclusions In angulated screw channel systems, when the angular correction is >20° a significant loss in preload occurred when applying the recommended torque of 25 Ncm. To minimise the loss in preload a recommended increase in applied torque of 11% (torque to 28Ncm) for angles ranging between 20° and 25° and 24% (torque to 31Ncm) for angles greater than 25° should be applied.