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THREADED WIRES IMPROVE THE MECHANICAL PERFORMANCE OF CIRCULAR FIXATORS

Department of Orthopaedic and Traumatic Surgery, Northern General Hospital, Sheffield, UK

Introduction: Circular fixators are widely utilised in orthopaedic surgery. Their biomechanical characteristics have been studied in some detail and it is known that the widest wire crossing angles yield maximum stability. Unfortunately, due to anatomical constraints, mechanically optimum wire crossing angles are seldom achievable, especially in the tibial diaphysis. Narrowed crossing angles are usually accepted thereby compromising sagittal plane bending stability. With a hybrid circular fixator, narrowed crossing angles exacerbate the problem of fracture site shear. It is hypothesised that by minimising slippage at the wire-bone interface by using threaded wires, stability can be maintained even with narrowed crossing angles. The aim of this study is to examine the effect of threaded wires on fracture site shear with a hybrid fixator.

Method: Bone-fixator models were created from nylon rods and the Orthofix Ring Fixator. Constructs with wire crossing angles of 70, 60, 45 and 30 degrees were loaded axially and in four point bending. Each construct was tested four times; the first test was not analysed. The whole fixator was then rebuilt and all tests repeated. Fracture motion (compression, angulation & shear) was measured using a strain gauge intersegmentary motion device and stiffness calculated by linear regression. Smooth & threaded wires were compared by univariate analysis of variance, which makes allowance for variation between individual frames.

Results: Axial stiffness was comparable to previous studies (85–96N/mm) with no difference between wire types. Threaded wires produced a 29% reduction in shear during axial compression (p=0.02). In four-point-bending, angulation stiffness (in the half-pin plane) was directly related to crossing angle and at all angles threaded wires were associated with a significant improvement. The table shows the effect of wire type on shear (in mm) measured in the plane of the half-pins for a 10Nm bending moment. Shear becomes appreciably higher with narrow crossing angles (almost 2mm) but this is effectively controlled by threaded wires.

Discussion: Of all the factors influencing fracture healing, the mechanical environment is one over which the surgeon has most control. It is generally accepted that excessive shear inhibits fracture healing. This study has shown that by using threaded wires in a circular frame, crossing angles can be narrowed without compromising stability. in particular they control undesirable shear motion seen with hybrid frames and narrow crossing angles. The principle is equally applicable to all-wire frames as they invariably are constructed with compromised crossing angles leading to reduced sagittal plane stability.

Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.