Improvements in polyethylene wear seen between two generations of same knee implant

Change in shape may be a factor along with changes in sterilization methods, manufacturing and packaging.

SAN DIEGO — A recent investigation of two large groups of patients who received different generations of the same total knee arthroplasty implant has shown a decrease in polyethylene wear, according to a surgeon speaking here. The decreased wear may be attributed to improved manufacturing and sterilization processes, he said.

"Relatively minor manufacturing changes can have a dramatic effect on the survivorship of implants," said William L. Griffin, MD, of Charlotte, N.C. He presented results of the study at the American Academy of Orthopaedic Surgeons annual meeting.

Griffin and colleagues conducted a comparative, consecutive, multi-centered study of 1,183 patients who received a first generation PFC modular total knee and 1,287 patients who received a second generation PFC Sigma (both, DePuy Orthopaedics). Follow-up averaged 7.8 years for first-generation patients and 7 years for second-generation patients.

Both implants use identical trays and polyethylene; the differences include a change in femoral component geometry — the first generation was flatter — and a change in polyethylene vendors. Additionally, there was a change in the sterilization methods used; the first generation implants were sterilized using gamma irradiation in air while the second generation is sterilized using gamma irradiation in a vacuum.

Overall the wear-failure rate for the first generation knees was 8.3% compared to 1.1% for the second generation.

"Historically, the first-generation design of pre-1991 implants in a published series showed no evidence of osteolysis," Griffin said. "In our hands in Charlotte, we noted a direct increase in revisions for wear in patients who received the 1991 to the mid-'90s implants."

In 2004, the authors published a study investigating revisions and found an 8.3% wear rate for the first generation knee. They identified significant factors they surmised to be implicated, including shelf age, finishing method, sheet processor, patient age and gender. Because three of the factors were related directly to the polyethylene, they hypothesized that the wear was due to premature oxidation.

In this second leg of the investigation, the authors compared the results of that study with results from the second-generation implants. In that study, only patient age emerged as a significant factor influencing wear-related failure.

Other results: At 10 years, the Kaplan-Meier survivorship was 87% for the first generation and 97% for the second generation.

"While there were changes in the geometry that may have improved our results, the single most important factor was the sterilization process and subsequent decrease in the oxidation of the polyethylene," Griffin noted.

For more information:
Griffin WL, Fehring TK, Pmmeroy DL, et al. Polyethylene sterilization and wear-related failures: A study of first and second generation TKAs. Paper #289 presented at the 74th Annual Meeting of the American Academy of Orthopaedic Surgeons. February 13-18, 2007. San Diego.
Dr. Griffin has noted that he receives research-funding, royalties and is a consultant for DePuy Orthopaedics.