s-10-00791],[@B41-materials-10-00791],[@B42-materials-10-00791]\], it becomes apparent that radiation sterilized samples will exhibit less wear of unirradiated or EtO/gas plasma sterilized ones, at a short post irradiation time scale and storage conditions that allow crosslinking to prevail to a larger extent than chain scission. On the contrary, after suitable accelerated ageing or longer real time aging, in the shelf or in vivo, the wear and fatigue performances of gamma irradiated samples deteriorate considerably as a consequence of oxidative degradation and negate any short term benefits of crosslinking associated with sterilization methods that use ionizing radiation \[[@B21-materials-10-00791],[@B29-materials-10-00791],[@B32-materials-10-00791],[@B33-materials-10-00791],[@B43-materials-10-00791]\].

This observation prompted researchers in the field to implement strategies to take advantage of the benefit of the crosslinking induced by radiation treatments, yet minimizing the drawback of long-term oxidation.

The first adopted measure was to sterilize UHMWPE with high energy radiation in a low-oxygen environment (vacuum or inert gas, i.e., argon or nitrogen) \[[@B10-materials-10-00791],[@B44-materials-10-00791],[@B45-materials-10-00791],[@B46-materials-10-00791]\]. This practice avoids contact with oxygen during sterilization and, if the liner is wrapped in a suitable barrier packaging, during the following shelf life as well \[[@B11-materials-10-00791],[@B47-materials-10-00791]\]. Unfortunately, it does not prevent contact with the oxygen solubilized into polyethylene before packaging in the low-oxygen environment, nor with that available in vivo \[[@B43-materials-10-00791]\], so that some oxidation has also been observed in these polyethylenes, even if to much lower levels than for those radiation sterilized in air \[[@B11-materials-10-00791],[@B48-materials-10-00791],[@B49-materials-10-00791]\].

4. 1st Generation Highly Crosslinked Polyethylene {#sec4-materials-10-00791}
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