Aspects of the biomechanics of Ilizarov external fixation.
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