370 THE JOURNAL OF BONE AND JOINT SURGERY THE RADIATION IMPROVEMENT OF POLYETHYLENE PROSTHESES A PRELIMINARY STUDY

The radiation crosslinking ofhigh-density polyethylene prostheses was investigated over a wide range of doses in the presence and absence of gaseous crosslinking agents. It was found that in the bulk polymer the crosslinkmg pattern Is completely different from the homogeneous crosslinking that occurs in polymer ifims. The presence of crosslinking agents causes highly crosslinked polymer to be formed on the surface while the bulk of the polymer is largely unaffected-which is explained in terms of diffusion phenomena. This surface crosslinking has a profound effect on the mechanical properties ofthe prostheses and restricts cold flow and deformation of the polymer without sacrificing the excellent abrasion-resistance properties of the polyethylene when subjected to high pressures. Based on this research a number of high-density polyethylene knee prostheses have been radiation-crosslinked and the results in vitro appear to be very promising. When high-density polyethylene (HDPE) has been used in total hip replacement for aged and relatively inactive people, the effects of wear and cold flow have proved over the years to be no real problem. Total joint replacement, however, is now being performed i