sentences:  These early designs used metal-on-polyethylene bearings, and had poor results compared to THR at the time.It has since been observed that these poor results were strongly tied with polyethylene wear debris associated with the use of air-sterilised ultra high molecular weight polyethylene (UHMWPE) at the time.A more modern style of Hip Resurfacing emerged in the 1990s, using cobalt-chrome bearings.