es ranging from 33 to 1000 kGy, remelted and EtO sterilized, demonstrated an 87% wear reduction with increase in the radiation dose from 33 to 95 kGy, when tested for 5 million cycles in a hip simulator; again, the wear decreased to undetectable levels at a radiation dose higher than 200 kGy \[[@B41-materials-10-00791]\]. In the same work, by observing the trade-off between the reduction in tensile properties and the increase in wear resistance with increasing the radiation dose, the authors concluded that approximately 100 kGy represents the most appropriate dose to reduce the wear below the threshold for clinically significant effects, simultaneously preserving sufficient tensile properties.

Kurtz et al. \[[@B61-materials-10-00791]\] summarized the wear test results of seven independent hip simulator studies, all using the same hip simulator design and comparable conditions, of a conventional, gamma inert sterilized polyethylene (30 kGy gamma radiation, in nitrogen: N2-Vac™ Stryker Howmedica Osteonics, Mahwah, NJ, USA) vs. a highly crosslinked and annealed UHMWPE (75 kGy gamma radiation, annealing at 130 °C, 30 kGy gamma radiation in nitrogen for sterilization purpose, Crossfire™, Stryker Howmedica Osteonics, Mahwah, NJ, USA). Counterfaces included CoCr, alumina, and Zirconia ceramic femoral heads with sizes ranging between 28 and 36 mm. The linear wear rate was calculated in the first 2 to 3 million cycles of the test to facilitate comparison between studies. Despite a relatively broad distribution in wear rates, which may be partly explained by variations in liner design, femoral head material, and femoral head size, the survey demonstrated a 92% reduction in median wear rate for the highly crosslinked and annealed UHMWPE, relative to the conventional, gamma inert sterilized polyethylene.

4.4. Does the Counterface Matter with Crosslinked Polyethylene? {#sec4dot4-materials-10-00791}
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The influence of the type of counterface on the wear beha