Background:
The mechanical toughness of polyethylene that has been sterilized
by gamma irradiation in air decreases after a long shelf life. The
purpose of the present study is to report the high failure rate
after unicondylar knee replacements performed with polyethylene
bearings that had been sterilized with gamma irradiation in air
and implanted after a shelf life of =4.4 years.
Methods:
Between December 1997 and January 2000, seventy-five unicondylar
knee replacements were performed in sixty-two patients. All patients
were followed both clinically and radiographically. A revision operation
was offered when the patient had pain, swelling, and radiographic
evidence of rapid polyethylene wear. The effect of aging of the
polyethylene during storage was evaluated by dividing the knees
into three groups on the basis of shelf life and comparing them
with regard to the rate of revision and the observed wear of the
polyethylene. Four retrieved components were examined for the presence
of oxidation.
Results:
At a mean of eighteen months after the arthroplasty, thirty knees
had been revised and seven were scheduled for revision. The rate
of polyethylene wear increased as the shelf life increased. There
was a significant inverse linear correlation between the shelf life
of the polyethylene and the time to revision (p < 0.01, r
2
= 0.64). All retrieved components had greater-than-expected wear
with pitting and delamination of the surface. Seven components had
fractured, and ten had both fractured and fragmented. Analysis of
four components confirmed severe oxidation of the polyethylene.
Conclusion:
The present study demonstrated early, severe wear of tibial polyethylene
bearings that had been sterilized by gamma irradiation in air and
stored for =4.4 years. This risk can be minimized by ensuring
that implants have not been sterilized with gamma irradiation in
air and stored for several years.