Structural Analysis and Experimental Testing of External Fixator System under Axial Compression

This work presents a segment of research results of mechanical stability of the Sarafix external fixation system, applied to a tibia, in the case of an unstable fracture. The research has been conducted using structural analysis of one of the Sarafix fixator configurations by application of the finite element method (FEM) and experimental testing. 3D geometrical and FEM model of fixator configuration have been formed, whence a structural analysis has been performed using CATIA V5 software system. Structural analysis and experimental testing of Sarafix fixator have been performed under axial compression. Values of bone segments displacements at the point of load and fracture gap have been analyzed based on which values of axial construct stiffness and fracture gap stiffness have been determined. Verification of the results obtained from a structural analysis through experimental testing has been carried out by comparing values of an appropriate component of displacement at the point of load. 1. INTRODUCTION External fixator systems are medical devices for stabilization of bone fractures, and their compliance aims at producing an interfragmentary motion that promotes rapid and successful healing. The aim of the fixation technique is anatomical reduction and immobilization of the bone segments, as well as maintenance of this anatomical stabilization concept throughout the treatment by external stiffening of the fracture gap separating the bone segments. This aim is achieved by an external frame that is connected percutaneously to the bone segments by pins or wires. Directions and intensity of interfragmentary motions have the crucial importance on rapid bone healing [3]. It is possible to control interfragmentary 3D displacement of the fracture gap using FEM model. Optimal mechanical environment, which promotes bone healing, has not been completely defined yet. Both excessively high and excessively low interfragmentary motions were shown to have adverse effects on bone fracture healing. Moreover, interfragmentary displacements parallel to the fracture surfaces were shown to result in pseudo-arthrosis instead of fracture healing. This work presents results of structural and experimental analysis of one of the most used configuration of Sarafix external fixator system in the case of an unstable tibial fracture (configuration C, type 4+4). Fixator Sarafix presents unilateral biplanar external fixator of high flexibility, enabling its application on complete human skeleton. Unstable fractures at the middle of tibia have been analyzed with fracture gap of 20 and 50 mm (severe extensive injuries with a considerable defect of bone structure).