Methodology for Designing and Evaluating a Total Ankle Replacement Using a Subject-Specific Numerical Model of the Hind Foot
Authors:   , Meyers Nicholaus Jacob Alexander , Siegler Sorin 
Published: July 2021

Arthrodesis has long been the gold standard for treating the severely degraded talocrural joint, but the rigid kinematic limitations it imposes come at the expense of over-stressed adjacent joints and ligamentous structures. This, in turn, can lead to degeneration and secondary osteoarthritis of the subtalar and metatarsal joints, ligament damage, and persistent pain. Furthermore, the reduced functionality of the ankle makes tasks such as running, treading uneven ground, and climbing stairs very difficult. Total ankle arthroplasty is rising as a means of treating primary, end-stage arthritis. The mobility afforded by a replacement over a fusion should limit the stresses placed on neighboring structures and provide enough range of motion to prevent disruption to normal ambulant tasks. Thus far, ankle arthroplasty has shown disappointing results with high failure rates and multiple failure modes. New findings indicate that the passive mechanical characteristics of the ankle joint may be due to the functional morphology of the talar dome. The oversimplification of the vital tibia-talus bearing surface may lead to an altered kinematic response that over-stresses the surrounding joints, deltoid ligament, and the implant itself. This study proposes a method to use experimentally verified models of the hind foot to design total ankle replacements and evaluate their performance against the characteristic response of the physiological ankle. The primary goal of a total ankle replacement should be restoration of physiological kinematics and ligament tension which may reduce the stresses on adjoining foot structures and increase the clinical longevity of the implant. It is often difficult or impossible to directly measure and quantify these responses making implant optimization challenging. Use of verified numerical models of the hind foot allows direct measurement and enumeration of range of motion, kinematic coupling, and ligament tension under applied loads. This can allow iterative design and optimization before expensive cadaveric or clinical testing. The proposed method uses Autodesk Inventor Professional 2012 to generate 3-dimensional models of designed implant bearing surfaces. The implant components are created based on morphological measurements of the bones from the subject-specific, numerical hind foot model. Two sets of implants are created and imported into a rigid-body, dynamic model in ADAMS View 2011. The bearing surfaces are locked to their respective bones and the contact is defined based on ultra-high molecular weight polyethylene and a titanium alloy commonly used for orthopaedic implants. Kinematic coupling and ligament loading are measured directly in the numerical model and the data of the physiological model is compared with the two models containing implant surfaces. The data demonstrates that assumptions commonly used in ankle replacement design may be incorrect. This evaluation indicates that ligaments may play a larger role in kinematic response than previously thought and the morphology of the talar dome may reduce the stress at the bearing surface by maximizing the contact area as the ligaments restrict the position and orientation of the talus.