The original Ilizarov frame is a form of circular external fixation in which bone fragments are supported by tensioned fine wires; the wires give the frame a nonlinear axial stiffness which is one of its key qualities. However, as the wires deform plastically in response to loads imposed by functional weight bearing, the stiffness of frame gradually decreases with time. To circumvent this problem the modified Ilizarov frame was conceived in which half pins rather than wires are used for bone support. As fractures managed with Ilizarov fixation tend to unite with little radiographic evidence, monitoring the progression of fracture healing is difficult. The study described in this dissertation had three primary objectives. The first was to investigate the significance of the plastic deformation which occurs in the tensioned fine wires to the long term performance of the original frame. The second was to investigate the biomechanics of the modified frame. The third objective was to conduct a in-vivo feasibility study on the use of fracture axial stiffness measurements as method of monitoring the progression of fracture healing. Plastic deformation of the wires in the original frame readily occurs at moderate load levels because stress concentrations arise at the wire-clamp and wire-bone interfaces. The reduction in frame stiffness is typically 20-30%; re-tensioning only temporarily restores the original frame stiffness. In contrast to the original frame, the modified frame displays a linear stiffness and, as the half pins act as cantilevers, shearing of the bone ends can occur under axial loading. The in-vivo study showed that the technique of relative stiffness measurement, which has been successfully applied to uniaxial fixators, is not directly applicable to Ilizarov fixation. However, it was noted that the standardd eviation of repeatm easurementsd ecreasedw ith the progressiono f healing. It is suggestedt hat this may arise as a result of decreasedm icromovement at the fracture site and might provide a means of monitoring fracture healing itself