Fatigue Strength of Highly Crosslinked Polyethylene in Total Ankle Arthroplasty

CATEGORY: Ankle INTRODUCTION/PURPOSE: Highly crosslinked polyethylene (HXLPE) was developed for its superior wear properties in comparison to conventional polyethylene (CPE). The higher dose irradiation required for HXLPE may also cause embrittlement, which reduces fatigue resistance and leads to surface cracking or fracture of the polyethylene bearing. Concern over fatigue resistance has prevented widespread adoption of HXLPE for use in in total ankle arthroplasty (TAA). The aim of this study was to determine whether HXLPE has sufficient fatigue strength for total ankle arthroplasty under simulated physiologically relevant motion profiles and loading in the ankle. METHODS: A bicondylar, semi-constrained HXLPE TAA design was subjected to 10 million cycles (Mc) of fatigue testing under loading conditions representative of a walking gait. Kinetics and kinematics of gait were incorporated into a computational model (Dassault Systemes / SIMULIA, Johnston, RI), for prediction of peak stresses on the HXLPE insert. Based on predicted peak stresses, worst case component size and loading configuration were identified. Ten samples were tested on a closed loop servohydraulic test frame (MTS Systems Corp., Minneapolis, MN) for 10Mc. Testing was conducted to a peak load of 5600 N (1259lbs), representing approximately 5 times body weight for a 240 lb individual. Following testing, all samples were evaluated for evidence of polyethylene fracture or surface cracking. RESULTS: Peak stresses in the HXLPE insert occurred during heel off, closely corresponding to both peak axial force and dorsiflexion during gait. The smallest sized component had the highest polyethylene insert stresses, whereas larger sized components had more material to bear the same load, resulting in up to a 30% decrease in stress. All 10 specimens completed 10Mc of testing at 5 times body weight without fracture or surface cracking of the polyethylene insert. CONCLUSION: HXLPE has sufficient fatigue strength to withstand 10Mc of loading at 5 times body weight at the point of peak stresses during gait in total ankle arthroplasty, and therefore, may be mechanically strong enough to withstand the demands of the ankle. Further clinical evidence is necessary to determine if these results translate to adequate fatigue strength with clinical use of HXLPE.