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The extent of in vivo induced structural alterations were evaluated in 13 retrieved implants ranging from two weeks' to seven years' implantation. A statistically significant difference in crystallinity was seen between weight-bearing and non-weight-bearing regions of the implants. Further, in the weight-bearing region, crystallinity was higher in components implanted for two or more years than in those implanted for less than one-half year. These results were consistent with our prior in vitro study on the effects of sterilization and aging. Taken together, the data indicate that UHMW polyethylene is not a static material but is continually dynamic changes in vivo.