Compression of A Midfoot Osteotomy Using A Circular External Fixator: What is the Ideal Pin Configuration?

CATEGORY: Midfoot/Forefoot; Diabetes; Trauma INTRODUCTION/PURPOSE: Foot and ankle deformity correction through midfoot osteotomy can be implemented in a wide variety of clinical situations. Use of a circular ring fixator for osteotomy fixation is particularly useful for patients with poor soft tissue envelopes and/or active infection as can occur in diabetes and Charcot arthropathy. The foot pins in a ring fixator can be configured to apply compression at the osteotomy site. However, the optimal pin configuration and force has yet to be determined. The purpose of this study was to quantify compressive forces achieved across midfoot osteotomies with various wire configurations in a circular ring fixator construct. METHODS: Nine through-knee amputation cadavers were stabilized with a standard circular external fixation carbon frame. A midfoot osteotomy through the transverse tarsal joint was performed using an oscillating saw. A 4mm bone wedge was removed from the osteotomy site for placement of a Tekscan pressure sensor. A two-ring frame was applied in the standard fashion and three parallel 1.8mm smooth wires were placed parallel to each other through the foot distal to the osteotomy: wire #1 proximally through the cuneiforms, wire #2 through the base of the metatarsals, wire #3 through the metatarsal shafts. After baseline pressure readings, wires were sequentially attached (wire #1 alone, wire #1-2, wire #1-3) to the ring fixator and tensioned to 90kg. Pressure readings were recorded at the osteotomy site for each sequential wire tensioning both at the hole location where the wire naturally crossed, 'neutral', and again at one hole 'proximal' (e.g., toward the osteotomy). RESULTS: Average compressive load at neutral hole positioning for wire #1 was 382 N. The addition of wire #2 increased the compressive load to 439 N on average. The addition of wire #3 decreased compressive force to 372 N. Similar trends were seen in proximal hole positioning where average compressive force increased following tensioning of wire #2 from 580 N to 600 N but compressive force decreased on wire #3 addition to 425 N. Therefore, tensioning forefoot thin wires in the proximal hole position increases compressive forces by 50% compared to neutral position, and adding a second wire increased compressive force by up to 15% compared to a single forefoot wire. CONCLUSION: In a circular frame midfoot fusion model, the greatest compressive force was achieved with two wires tensioned in the proximal hole position. The addition of a third wire led to force decrease likely due to off-axis forces that may distract the osteotomy site given the difficulty of passing all wires perfectly parallel in all planes. Ideal positioning and tensioning of forefoot pins may optimize construct stability and compression and improve healing of the midfoot osteotomy.