Mechanical performance of Ilizarov circular external fixators in comparison with other external fixators

The fundamental difference of the Ilizarov fixator is the type of pins used for bone fixation, i.e., Kirschner wires 1.5 mm and 1.8 mm in diameter, pretensioned from 50 to 130 kg before being affixed to the rings that are connected and fixed by threaded rods. The mechanical characteristics of external fixators may influence the biologic environment at the fracture site and ultimately decide the outcome of a surgical procedure. Thus, knowledge of the mechanical properties of the Ilizarov fixator is essential to a surgeon using it in clinical practice. The main objective of this study was to quantitate the mechanical behavior of the standard Ilizarov fixator under different loading conditions and fixator frame/wire configurations. The fixator was found to have a nonlinear stiffness behavior under axial compression. The nonlinearity in axial stiffness also varied with wire pretension. Such characteristics, however, were not as pronounced under torsion and bending loads within the test range studied. Besides the wire pretension, the most important factor affecting the structural stiffness of the Ilizarov device was the diameter of the wire. Offset bone position provided greater stiffness in loads up to 45 kg in axial compression, in torsion up to 5 degrees of rotation, and in the final loading range under bending. Fixators with wires crossing at 45 degrees had significantly greater stiffness in torsion as compared with 90 degrees crossing wires, but the opposite was true in axial compression. Torsional stiffness increased significantly under coupled axial compression applied through the bone ends. All four-point bending tests demonstrated two distinct stiffness curves that were probably due to slippage of the bone model on the wires. This information should help to understand the mechanical behavior of the Ilizarov device and thereby improve its clinical performance.