The Oxford External Fixator: Fixator Stiffness and the Effects of Bone Pin Loosening

Abstract

Simple theoretical deflection analysis and experimental data have been used to assess the stiffness of the Oxford External Fixator for a range of fixator configurations. The loading modes comprised axial compression, bending (in two planes), and torsion. Data are also provided on the effects of clinically loose Schanz bone screws (or pins) on the fixator stiffness. In general: loosening of one bone pin significantly decreases fixator stiffness; loosening of two pins may decrease the stiffness by 50 per cent or more; loosening of one bone pin causes increased loading on the adjacent tight pin, which may lead to loosening of the second pin; the fixator stiffness is maximized by using the shortest practicable bone pin length and the widest practicable spacing between adjacent pins in the same bone segment; short pins and wide spacing result in the lowest bone/pin interface stresses and minimize the possibility of bone pins loosening.