Biomechanics of the Ilizarov external fixator

The rigidity of the Ilizarov external fixator was evaluated by means of the techniques of Briggs and Chao, and compared to their findings for the Hoffman-Vidal external factor. A typical fibular fixator consisting of 8 crossed K wires and 4 rings with supporting struts was affixed to a fiberglass-filled epoxy bone fracture model. The ensemble was tested with an MTS servohydraulic testing machine in axial loading, bending (A-P, L-M), and torsion. Displacement transducers were placed on the frame and the bone to determine relative motion. The Ilizarov frame was relatively stiff in compression; failure occurred at about 100 kg due to slippage at the wire holders. In bending, it was much less rigid than the Hoffman-Vidal fixator due to bowing of the transverse wires and slippage of the bone along these wires. Stiffness is related to the wire-bone orientation: wires parallel or nearly parallel to the applied force provide little resistance to deformation. In torsion, the laxity in the system is due primarily to wire deflection or wide spacing between adjacent rings. Stiffness in compression and bending increased as a function of wire tension to about 130 kg (further tightness was not possible due to slippage at the wire holder). The Ilizarov fixator is less rigid than other fixators in all loading modes, particularly in axial compression. This may prove to be clinically beneficial as evidenced by increased osteosynthesis. However, the existing system has many sites of laxity. Care must be taken in frame construction to ensure adequate stability and necessary stiffness.