Mechanical performance of Ilizarov ring fixators. Influence of the frame configuration on the stiffness and consequences for the clinical application

Using a mechanical testing procedure,various fixator constructs were tested in vitro. In addition,the influence of the passive soft tissue structures on the fixation stiffness was determined. An increased number of Schanz' sc rews or Kirschner wires led to a comparable increase in stiffness than that observed with an increasing screw or wire dia meter. In consequence, large r diameters should be preferred over an additional screw or wire where clin ically applicable. With diaphyseal telescoping rods only the axial stiffnes s decreased. As expected, large ring diameters as well as titanium wires re duced stiffness components. Bracing the outer rings caused a reduction of t he overall stiffness. Asymmetric pre-tensioning of the K-wires resulted in a significant reduction of tension in the neighboring wire. Removal of the soft tissues reduced stiffness to a similar extend as experi enced in a fibula defect situation. The study demonstrates the correlation between design parameters, passive soft tissues and fixation stiffness and presents guidelines for an optimized fixator design.