Investigation of the biomechanical properties of circular frames for the foot

Circular frames are used for the treatment of complex foot injuries, including those with bone and soft tissue loss and deformities requiring gradual correction. The most frequently used foot frames are a Charcot frame, a Butt frame and a Mitre frame. The aim of this study was to provide data on the biomechanical properties of these frames under a simulated axial load. This will provide surgeons with information to assist in selecting the most appropriate frame for a given circumstance and will facilitate further biomechanics research in this developing area. Four frame fixation model geometries were constructed using Orthofix TL-HEX components. 3D foot models for use in the study were manufactured using 3D printing. The Charcot frame showed the least stiffness throughout the range of load applied with a stiffness increasing from 31 N/mm at 50 N load to 93 N/mm at a load of 700 N, whereas the Butt frame generally produced the greatest construct stiffness increasing from 47 N/mm at 50 N load to 122 N/mm at a load of 700 N. The Mitre frame had the greatest initial stiffness of 53 N/mm at 50 N but it only increased to 105 N/mm at 700 N. The Charcot frame presents the lowest resistance to axial loading of all the common constructs, which has implications for its use in patients with higher body mass index and peripheral neuropathy. The results of this study have important clinical implications, specifically with regards weightbearing instructions given to patients living with a frame.