Abstract
Total ankle replacement (TAR) is a substitute to ankle fusion, replacing the degenerated joint with a mechanical motion-conserving alternative. Compared with hip and knee replacements, TARs remain to be implanted in much smaller numbers, due to the surgical complexity and low mid-to-long term survival rates. TAR manufacturers have recently explored the use of varying implant sizes to improve TAR performance. This would allow surgeons a wider scope for implanting devices for varying patient demographics. Minimal pre-clinical testing has been demonstrated to date, while existing wear simulation standards lack definition. Clinical failure of TARs and limited research into wear testing defined a need for further investigation into the wear performance of TARs to understand the effects of the kinematics on varying implant sizes.
Six medium and six extra small BOX® (MatOrtho) TARs will be tested in a modified knee simulator for 5 million cycles (Mc). The combinations of simulator inputs that mimic natural gait conditions were extracted from ankle kinematic profiles defined in previous literature. The peak axial load will be 3.15 kN, which is equivalent to 4.5 times body weight of a 70kg individual. The flexion profile ranges from 15° plantarflexion to 15° dorsiflexion. Rotation about the tibial component will range from −2.3° of internal rotation to 8° external rotation, while the anterior/posterior displacement will be 7mm anterior to −2mm posterior throughout the gait cycle. The components will be rotated through the simulation stations every Mc to account for inter-station variability. Gravimetric measurements of polyethylene wear will be taken at every Mc stage. A contact profilometer will also be used to measure average surface roughness of each articulating surface pre-and-post simulation.
The development of such methods will be crucial in the ongoing improvement of TARs, and in enhancing clinical functionality, through understanding the envelope of TAR performance.
