Abstract
Background
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.
Materials & Methods
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.
Results
There were no significant differences (P = 0.872) in the mean wear rates (± 95% confidence limits) between the medium (11.00 ± 3.06 mm3/Mc) and extra small (10.64 ± 4.61 mm3/Mc) implant sizes (Figure 2). An observation of insert surfaces showed clear signs of abrasive wear and burnishing (Figure 3). There was evidence of polyethylene transfer and scratching on the tibial components, while talar components displayed fine linear scratching in similar directions for both implant sizes.
Conclusions
The wear rates of both implant sizes are comparable to the wear rate (13.30 ± 2.50 mm3/Mc) of a previous wear study, which was conducted on ‘medium-sized’ Corin Zenith TAAs, under the same simulator conditions for 2 Mc [1]. The wear rates for both implant sizes are substantially lower than the wear of four ‘small-sized’ BOX® ankles (18.60 ± 12.80 mm3/Mc) for 2Mc [2]. The considerable difference in wear rates may be due to the lower forces, higher AP and deionised water as the test lubricant [2], which does not replicate the features of the natural synovial fluid and produce tribological artefact. The results from this study suggest that under the same kinematic and kinetic conditions, the wear rates are unaffected by a change in TAA implant size.
