The natural history of ultra high molecular weight polyethylene.

Degradative changes occurring during the lifetime of ultra high molecular weight polyethylene total joint components made from 2 different starting resins were determined from retrieved tibial inserts. The inserts were manufactured in the Department of Biomechanics of the authors' institution, for intrahospital use by the hospital's surgical staff, allowing the ultra high molecular weight polyethylene properties to be traced from production, to sterilization, to shelf life, and to subsequent retrieval. Changes were found in the density and the infrared spectra. Density was greater after sterilization and at retrieval, when compared with the density of the virgin ultra high molecular weight polyethylene material, with a greater increase near the surfaces than within the bulk of the inserts. The inserts that had the longest life-times demonstrated the greatest increase in density. Density increases are important because they reflect increases in the elastic modulus of the ultra high molecular weight polyethylene. Such increases near the articulating surface of a total joint component would cause significant increases in the stresses associated with wear damage occurring on and near these surfaces. The infrared spectra were consistent with oxidative degradation.