Configuration design and correction ability evaluation of a novel external fixator for foot and ankle deformity treated by U osteotomy

As a basic osteotomy technique, U osteotomy can be applied for certain complex foot and ankle deformities. Gradual correction cases using Ilizarov apparatus and Taylor Spatial Frame have been reported. This paper proposes a novel parallel distraction apparatus for U osteotomy (PDA-Uos) to supplement the correction equipment for surgeon. Designed with an adjustable structure, the PDA-Uos can adopt different assembly shapes (joint connection points). However, the influence of the change in assembly shape on interference-free workspace and self-structural performance of the external fixator have received little attention. To address this issue and enhance the selection of the most suitable assembly shape for patient, an algorithm to obtain the interference-free workspace of different assembly shapes is proposed based on the inverse position solution of the PDA-Uos. Additionally, correction ability indices are defined according to the requirements of accurately controlled motion and high structural stiffness of the external fixator along the correction path. The results of simulation cases indicate that the interference-free workspace and the correction ability vary according to the assembly shape and thus the proposed method can be used to select an assembly shape with sufficient workspace and the best correction ability before performing correction for a given patient.