Sterilization tied to polyethylene wear variations in two generations of implant

Overall, factors for different wear rates pointed to the polyethylene more than anything else.

SAN DIEGO — A recent investigation of two large cohorts that received different generations of the same implant has shown a decrease in polyethylene wear, possibly due to the improved manufacturing and sterilization processes.

In his presentation at the American Academy of Orthopaedic Surgeons (AAOS) 74th Annual Meeting, William L. Griffin, MD, of Charlotte, N.C., said, "Relatively minor manufacturing changes can have a dramatic effect on the survivorship of implants."

He presented a comparative, consecutive, multicenter 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]. The groups had mean follow-ups of 7.8 years and 7 years, respectively.

The implants use an identical tray and polyethylene. However, they differed in sterilization methods, in geometry of the femoral component (the first generation was flatter), and the polyethylene vendor.

The first-generation implants were sterilized using a gamma-irradiation-in-air method, while the second generation was sterilized using gamma irradiation in a vacuum.

Overall, the wear-failure rate for the first-generation knees was 8.3% and 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 dramatic increase in revisions for wear in patients who received the 1991 to the mid-1990s implants."

In 2004, the authors published a study investigating the revisions and found an 8.3% wear-related failure rate for the first-generation prosthesis.

They identified factors they surmised to be implicated in the wear, including shelf age, finishing method, sheet processor, patient age and gender. Because three of these factors were related to the polyethylene, their hypothesis was 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 was marked as a significant factor to wear-related failure.

Griffin noted that, the geometry changes may have improved the results, but the most important factor was the sterilization process.

For more information:
Griffin WL, Fehring TK, Pommeroy DL, et al. Polyethylene sterilization and wear-related failures: A study of first- and second-generation TKAs. #289. Presented at the American Academy of Orthopaedic Surgeons 74th Annual Meeting. Feb. 14-18, 2007. San Diego.
William L. Griffin, MD, orthopedic surgeon specializing in hip and knee replacement at OrthoCarolina, 1915 Randolph Rd., Charlotte, NC 28207; 704-323-2564; He indicated that he is a consultant for and receives research funding and royalties from DePuy Orthopaedics.