Determination of the Neutral Axis in Total Ankle Replacement

Abstract

Major failures in ankle replacements include loosening, constraint, malalignment, fatigue, and impingement that are due to engineering design and implantation problems. The objective of this study is to propose a new process of implantation that includes customizing and determining specific size of ankle prosthesis using patient’s specific geometric in order to maintain its natural biomechanics. The process started from a set of two dimensional (2D) Computerized Tomography (CT) data of the ankle joint, which was converted to three dimensional (3D) finite element (FE) model using Mimics 8.1. The 3D FE model was used in finite element analysis (FEA) to determine the optimal solution (the minimal stress acting on the ankle complex) using ANSYS 8.1. The new ankle design was implanted into the 3D FE model of the ankle for further analysis of the optimal solution an ankle joint to determine the specific size for this ankle joint. In this study, the 3D model was created from 180 slices of 2D CT scans of an arthritic ankle. Finite element analysis of pre and post implanted ankles are conducted to determine the solution. The result is optimized with a minimal stress of 24.365 MPa acting on the ankle in the neutral position. The 3D FE data are then translated into CNC language to generate a physiologically accurate model of the ankle from a wax block. A new ankle prosthesis designed by one of the authors is used in this study for optimization of the stress in the joint. Based on the optimized result, the proper size of the implant was determined and manufactured. A sample of the prosthesis was then pre-implanted into the cut-out wax model of the patient’s ankle for pre-implantation prior to the actual operation. This process helped not only determine the proper size of the implant, but also allowed surgeons to plan the surgical process before entering the operation room.