Heat pressing the surface ol'polyethylene inserts gave them a smooth

shiny imish. '1'he heat-pressed /.one extended approximately 1  mm into

-^frhc--po!-yethy.JeiKV   'his junction corresponded to the Jevel of maximal

shear stress and resulted in dramatic delaminalion of the Ihinaer implants

2-Stcrili/ation:

In vitro studies have linked gamma radiation sterilization in air and subsequent   oxidation to  decreasing mechanical  properties  and rapid

polyethylene wear (78).

Research has shown that gamma radiation sterilization degraded ultrahigh molecular weight polyethylene (UHMWPE) by breaking the polymer chains ana forming free radicals (79). •

Reaction of the free radicals with oxygen is known as oxidation, a self-propagating process reported to continue degrading UHMWPE for an extended time after irradiation (79).

Recent laboratory results have demonstrated that the property changes which accompany oxidation increase the susceptibility of UHMWPE to the cyclic stresses of fatigue-related wear mechanisms (79).

in addition to increasing the susceptibility of"UHMWPE to fatigue-related wear mechanisms, gamma irradiation-induced oxidation increases the stiffness of UHMWPE. For the relatively non-conforming nature of total knee designs, increasing UHMWPE stiffness results in higher contact stresses for the same supported weight (79).

Currently, orthopaedic surgeons can choose from conventional ultra-high molecular weight polyethylene components sterilized with two fundamentally different methods (80).

pr

The first approach is sterilization without radiation, which includes surfaces treatment such as ethylene oxide and gas plasma sterilization.