Total ankle arthroplasty (TAA) is an alternative to ankle arthrodesis, replacing the degenerated joint with a mechanical motion-preserving alternative. Implant loosening remains a primary cause of TAA revision, and has been associated with wear-mediated osteolysis. Differing implant designs have a major influence on the wear performance of joint replacements. Providing a range of implant sizes allows surgeons a greater intra-operative choice for varying patient anatomy and potential to minimise wear. Minimal pre-clinical testing exists in the literature that investigates the effect of implant size on the wear behaviour. The aim of this study therefore was to investigate the effect of two different implant sizes on the wear performance of a TAA. Six ‘medium’ and six ‘extra small’ BOX® (MatOrtho Ltd, UK) TAA implants, of the same conceptual design and polyethylene insert thickness, were tested in a modified 6 station pneumatic knee simulator. 5 million cycles (Mc) of wear simulation were completed for each implant size, under kinematics aiming to replicate an ankle gait cycle (Figure 1) [1]. The simulator used had six degrees of freedom, of which four were controlled. The maximum axial load was 3150N, equivalent to 4.5 times body weight of a 70kg individual. The flexion profile ranged from −15° plantarflexion to 15° dorsiflexion. Rotation about the tibial component ranged from −2.3° of internal rotation to 8° external rotation, and anterior/posterior (AP) displacement ranged from 3.1 mm anterior to −0.9 mm posterior displacement. The lubricant used was 25% bovine serum supplemented with 0.04% sodium azide to prevent bacterial degradation. The wear of the TAA polyethylene inserts were determined gravimetrically after each Mc, with unloaded soak controls used to compensate for the uptake of moisture by the polyethylene.Background
Materials & Methods
