Subject-specific kinematic analysis of a total ankle arthroplasty implant.

Total Ankle Arthroplasty (TAA) is a well-established surgical treatment for end-stage ankle arthritis, yet functional outcomes often remain inferior to those of the native joint. This may be because restoring the full physiological range of motion would likely require a custom-made surgery, in which both the prosthetic design and component positioning are defined on the basis of a patient-specific kinematic analysis. The aim of this study is to propose a subject-specific kinematic analysis of an ankle joint implanted with the FAR prosthesis and to investigate the compatibility between prosthesis-induced motion and the physiological motion of the healthy joint. A CT-based 3D model and a 4-bar linkage model were used to estimate the physiological path of the instantaneous centre of rotation during passive plantar-dorsiflexion, while the kinematic constraints of the FAR prosthesis were simultaneously analysed. A novel methodology was proposed, based on an analogy between the FAR implant design and a planetary gear mechanism, to reproduce the physiological kinematics of the natural joint with high fidelity. The results, in terms of overall plantar-dorsiflexion range of motion and ligament behaviour, suggested that the analysed TAA design was unable to fully reproduce the physiological kinematics of the healthy joint model for the subject and virtual surgery considered. Comparison with clinical data demonstrated general agreement in terms of range of motion and meniscal bearing displacement. Overall, the proposed methodology provides a framework for systematically analysing subject-specific ankle joint kinematics after implantation and suggests promising directions for future research.