An Indirect Method to Measure Abutment Screw Preload: A Pilot Study Based on Micro-CT Scanning.

Carlos Eduardo E Rezende,Jason Alan Griggs,José Henrique Rubo,Amanda M Mushashe,Yuanyuan Duan,Gisele Maria Correr Nolasco,Ana Flávia Sanches Borges

doi:10.1590/0103-6440201300274

Abstract

This study aimed to measure the preload in different implant platform geometries based on micro-CT images. External hexagon (EH) implants and Morse Tapered (MT) implants (n=5) were used for the preload measurement. The abutment screws were scanned in micro-CT to obtain their virtual models, which were used to record their initial length. The abutments were screwed on the implant with a 20 Ncm torque and the set composed by implant, abutment screw and abutment were taken to the micro-CT scanner to obtain virtual slices of the specimens. These slices allowed the measurement of screw lengths after torque application and based on the screw elongation. Preload values were calculated using the Hooke's Law. The preloads of both groups were compared by independent t-test. Removal torque of each specimen was recorded. To evaluate the accuracy of the micro-CT technique, three rods with known lengths were scanned and the length of their virtual model was measured and compared with the original length. One rod was scanned four times to evaluate the measuring method variation. There was no difference between groups for preload (EH = 461.6 N and MT = 477.4 N), but the EH group showed higher removal torque values (13.8 ± 4.7 against 8.2 ± 3.6 N cm for MT group). The micro-CT technique showed a variability of 0.053% and repeatability showed an error of 0.23 to 0.28%. Within the limitations of this study, there was no difference between external hexagon and Morse taper for preload. The method using micro-CT may be considered for preload calculation.

Highlights

One of the main complications in a single-unit implant supported prosthesis is abutment screw loosening [1,2]
Torque maintenance depends on the preload and this in turn depends on the screw joint stability [4]
The abutments were screwed onto the implants, and these specimens were positioned in a vise-grip for the abutment screw tightening with a digital torque wrench (Instrutherm, São Paulo, SP, Brazil)

Summary

Introduction

One of the main complications in a single-unit implant supported prosthesis is abutment screw loosening [1,2]. This problem occurs due to the reduced torque of the abutment screw [3]. Torque maintenance depends on the preload and this in turn depends on the screw joint stability [4]. In the process of tightening, the screw elongates, bringing the implant and abutment together. This leads to the development of preload that causes a compressive axial force on the system, known as “clamping force”, which maintains the union between the components [5]. The screw has to clamp the joint members together with enough force to prevent separation, slippage and self-loosening when exposed to vibration, shock, and repeated cyclical external loads [4,6]

Objectives

Methods

Conclusion