Muscle architecture, loading and joint replacement of the ankle

Abstract

The use of total ankle replacement (TAR) for treatment of arthritis is rapidly increasing, but survival rates are of major concern. The primary indication for TAR revision is implant loosening, which is linked with inadequate primary stability manifested in higher levels of initial implant-bone micromotion. Finite-element (FE) modelling has been utilised to assess micromotion of arthroplasty implants, but not TAR. Additionally, the biomechanical consequences of TAR malpositioning during surgery – previously linked with higher failure rates – remain unexplored. The aim of this thesis was therefore to apply FE modelling to estimate implant-bone micromotion and peri-implant bone strains of current TAR designs under optimally-positioned and malpositioned cases, and thereby identify fixation features and malpositioning scenarios that place the reconstructed ankle at risk of early loosening. Computational models simulating commonly-used TAR designs (BOX®, Mobility® and Salto®) implanted into the tibia and talus were developed; the loads applied were the contact forces acting in the ankle during gait, as calculated using a previously-validated musculoskeletal model, while implementing muscle-architecture data obtained through dissections of cadaveric legs. Micromotion and strain outcomes were larger for the tibial compared with the talar components, in agreement with previous clinical observations. The tibial Mobility® and talar Salto® components demonstrated the largest micromotion. A gap between the tibia/talus and implant component resulted in a considerable increase in implant-bone micromotion and peri-implant bone strains; the Salto® design was relatively ‘forgiving’ for such malpositioning. It was concluded that better primary stability can be achieved through fixation nearer to the joint line, while relying on more than a single fixation peg, and preserving more of the cortical sidewalls of the bone; incomplete seating of the implant on the bone increases the risk for TAR failure. The models presented in this thesis may assist implant designers and surgeons in improving TAR designs and surgical techniques.