Abstract
Introduction: The Sheffield Ring Fixator (SRF) uses wires in the metaphysis and screws in the diaphysis for bone stabilisation. It has four 2mm wires tensioned to 1400N in two parallel groups to stabilise the ring in the metaphysis. For maximum stability, these parallel wires should cross at 60 degrees or greater and the position of the crossing should be in the centre of the bone. Fixation stability and clinical outcome may well depend on the accuracy of surgical application.
Aims: To review the consistency with which the SRF frames were applied by a single surgeon.
Materials and Methods: The fixators of 39 patients aged between 6 and 75 years of age (11.5 years mean age in children and 38.7 years mean age in adults) were examined. 7 patients had proximal and distal metaphyseal wires making a total of 46 recording sites. The angle of the wires was calculated using the number of holes between the wire clamps since each hole subtends an arc of 7.5 degrees. The crossing angles were divided into two groups with crossing angles of greater than 60 degrees and less than 60 degrees. The position of the crossing of the wires was determined by creating a cross section of the metaphyseal rings, reducing the size of the ring to 100mm and transposing a cross section of the tibia of the correct size and at the correct level. Scaling down the distances measured between the inner ring and the patient limb, the position of the cross section and consequently of the wire crossing was determined. Using contour lines the tibia was divided into four zones. Zone 1 was central tibia and zone 4 was the tibial cortex. All crossing within zone 1 and 2 were considered satisfactory, and zone 3 and 4 poor.
Results: 67.5% of patients had crossing angles greater than 60 degrees and 32.5 % had crossing angles of less than 60 degrees. 85% of the rings had zone 1 or 2 crossing positions. 6.5% of the rings had subcortical crossing positions and 8.5% of the rings had cortical crossing positions. 8.5% of rings had crossing angles of less than 60 degrees as well as wire crossing positions in zone 3 and 4.
Discussion: In a carefully controlled situation a surgeon’s surgical technique was consistent in 67.5% of the rings, with satisfactory crossing angles and wire crossing positions. Only 8.5 % of the rings had poor crossing angles and crossing positions. Inability to achieve ideals may be due to technical errors or anatomical variations. There were increased infection rates in patients with reduced crossing angles, however the position of the crossing had no apparent effect on infection rates and patient mobility. A further study would be required to separate the relative importance of these two factors on patient complications.
Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.
