Orthofix Wire Fixator Locking Screw Torque Wrench

Ring fixators depend on tensioned wires to give beam loading (Saleh et al, 1997). Wire placement is critical and often time consuming. The wire is then tensioned to approximately (1200-1400 N) and secured to the clamp with a screw (Lewis et al, 2001). If this is under-torqued the torsion in the wires would be lost; if over-torqued the wire may be damaged and breakage can occur. We investigated an appropriate torque for the wire locking screws and the company produced a specific torque wrench (Fig 1 indicated by arrow) for the purpose, preset to 10N.

Ten surgeons with varying degree of experience torqued locking screw to what they felt was the appropriate degree. All but one turned at least a full turn further when the torque wrench was applied. Illustrations show the effect on standard Orthofix 2mm wire of under-tightening (Fig 2), torqued (Fig 3) and over-tightening (Fig 4). When the torque wrench was applied to the under-torqued wire it was scored as it was drawn through the clamp. A further advantage of the torque wrench is that when the screws are checked in clinic the patients have greater confidence that the wires will not be damaged/broken needing another theatre trip.

The axial fixation resistance of the wire depends on the tightening torque of the screw as per Fig 5.

We commend the use of the torque wrench on the wire locking screw as a simple adjunct.

References:

1. Saleh M, Yang L, Nayagam S.A biomechanical analysis of the Sheffield Hybrid Fixator. J. Bone Joint Surg., 1997; 79B: Orthop. Proceeding Supp. III, 361-362.
2. Lewis DD, Bronson DG, Cross AR, Welch RD, Kubilis PS. Vet Surg. Axial characteristics of circular external skeletal fixator single ring constructs 2001 Jul-Aug;30(4):386-94.