Clamping of fine Kirschner wires in external fixators

In Ilizarov circular (ring) external fixators, fine Kirschner wires are used to fix the bone to the fixator. Clamping of the wires to the rings with different bolt torques has been studied. However, the relation between the bolt torque and the fixation load applied to the wire was not investigated. In this work, finite element method is used to address this problem. Here, a fully three-dimensional model of the wire fixation assembly was built, with geometric details like threads on the bolt to produce a realistic simulation of the clamping of the wire. Both cannulated and slotted bolt types were studied and values of 0.2, 0.25, 0.3 and 0.45 were used for coefficient of friction. A torque was applied to the nut while the ring section and bolt kept in place. The results for bolt load, nut rotation as well as axial and radial wire deformations were obtained. The results demonstrated a linear relation between the bolt load and the bolt torque. The coefficient of this relationship was shown to be inversely proportional to the coefficient of friction. For all results, the bolt load (N) was approximately 124 times the bolt torque (N m) divided by the friction coefficient. The results highlighted the difference between the cannulated and the slotted bolts in terms of their grip on the wire.