Development of a Mechanical Torsion Test to Evaluate Slotted Bolts on a Bolt-Wire-Ring-Nut Setup for Ilizarov External Fixator

Abstract

Abstract The effectiveness of Ilizarov external fixators in osteosynthesis of fractured bones depends on the pre-tension applied to the transosseous wires. The pre-tension on these wires depends on the initial torque applied to the slotted bolts, which occasionally fracture during clinical setup. There is no standard method to determine the risk of failure of these bolts. This study aimed to develop a test to evaluate the mechanical strength of these bolts by imitating a bolt-wire-ring-nut configuration of an Ilizarov fixator. The fracture micromechanisms of the slotted bolts were investigated to understand the mechanical forces involved. This resulted in a reference group (Group III). Then, with these fracture characteristics, a new bolt-wire-ring-nut test was developed to mimic use in an Ilizarov fixator. Eleven slotted bolts were tested (Group II). The new test results were compared with a standardized screw torsion test (Group I) according to ASTM F543, Standard Specification and Test Methods for Metallic Medical Bone Screws. The mechanical strengths of Groups I and II were analyzed statistically. Additionally, the fracture micromechanisms of all groups were investigated. The bolts tested according to ASTM F543 (Group I) showed values with less dispersion compared with those in Group II; however, there were no statistically significant differences between their maximum torques. The bolts submitted to the new test (Group II) showed similar fracture surfaces as those in Group III, distinct from Group I. The results indicate that the new torsion test is more clinically relevant and reliable when checking whether the slotted bolts are liable to fracture during clinical setup.