Advanced Testing Protocols Simulate Failures and Validate Antioxidant Polyethylene in Ankle Implants.

Abstract

Total ankle replacement (TAR) has become an effective treatment for end-stage ankle osteoarthritis. Multiple factors, including patient characteristics, surgical technique, alignment, and bearing surfaces, influence TAR survivorship. Polyethylene (PE) fatigue is a key consideration in improving outcomes. This study establishes a novel, clinically relevant testing protocol incorporating varus-valgus rotation to simulate polyethylene fatigue failures observed in mobile-bearing total ankle replacements. Using this robust methodology, we evaluated the impact of oxidation and antioxidant stabilization on ultrahigh-molecular-weight polyethylene (UHMWPE) performance in a mobile bearing implant application. A six-degree-of-freedom simulator was used to iteratively adjust loading parameters (1500-3000 N, -4° to +8° flexion-extension, ±5° axial rotation, and ±3° or ±8° varus-valgus rotation at 37 ± 3°C in 20 g/L bovine serum) until clinically observed midline fractures were replicated. Oxidation levels were measured by Fourier-transform infrared spectroscopy per ASTM F2102. This validated loading protocol was then applied to conventional (25 kGy GUR 1020) and vitamin E-stabilized (75 kGy GUR 1020-E) UHMWPE inserts and tested to visible fracture or a 3-million-cycle runout. Post-test fractographic analysis identified crack initiation sites. Conventional aged UHMWPE demonstrated fatigue failure under varus-valgus rotation (OI = 2.59 ± 1.11) but no failure without rotation. Vitamin E-stabilized UHMWPE showed no fatigue failure after 3 million cycles, even under varus-valgus rotation (OI = 0.23 ± 0.02). Fractography revealed fractures originating at the trough and propagating with cyclic loading. Oxidation significantly reduces polyethylene fatigue life, and varus-valgus rotation exacerbates this effect in mobile bearing TAR implants. Antioxidant-stabilized UHMWPE showed promising resistance to fatigue and oxidation. These findings support the role of antioxidant stabilization in improving TAR performance, and the protocols developed here provide a framework for assessing the safety of alternative materials.