There are significant individual variations in the polyethylene (PE) wear of Charnley total hip arthroplasty (THA) in published studies. This could be in part related to variations in hip joint kinematics with abnormal heating at the metal/PE interface. The objectives of our hip simulator experiment were: (1) to measure PE wear as a function of hip kinematics and temperature variations at the interface; (2) to compare ultra-high molecular weight polyethylene (UHMWPE) to latest generation highly cross-linked PE (XLPE).
Our hypothesis was that PE wear is correlated with temperature increases at the interface and thereby hip joint kinematics.
Material and methods
A simulator study was performed with four UHMWPE cups (Initiale™, Amplitude, Valence, France) and two XLPE cups (X3, Stryker, Kalamazoo, Michigan, USA) subjected to 5 million cycles each. The temperature at the femoral head/cup interface was measured every 500 cycles and implant dimensions were measured every 1 million cycles.
The average temperature was 42°C for 1Hz and 50°C for 1.5Hz, no matter the type of PE tested. There was a large difference between UHMWPE and XLPE in their roughness, but no temperature variations or wear effects. Femoral head penetration after the first 1 million cycles was 0.18mm for the XLPE and 0.075mm UHMWPE on average. Between 1 and 5 million cycles, the penetration was less than 0.1mm per million cycles, with XLPE being similar to UHMPWE.
Our study found a significant temperature increase at the bearing interface as a function of frequency. But there was no correlation between temperature variations and PE degradation. However, shear stresses were under-estimated because our simulator could not reproduce abduction and adduction movements. Our hypothesis was not confirmed because PE deformation was not correlated to temperature variations. XLPE was not better than UHMWPE in the particular conditions of this study. Simulator studies are limited because of the lack of standards on cycling and the simulator bath.
Level of evidence
III–prospective case-control study in vitro.Le texte complet de cet article est disponible en PDF.
Keywords : Polyethylene, Wear, Temperature, Total hip arthroplasty