CHICAGO — Highly cross-linked polyethylene bearing surfaces in total hip arthroplasties have passed their first long-term longitudinal trial with flying colors, according to a blue-ribbon poster presented at the annual meeting of the American College of Orthopaedic Surgeons.
“After a minimum of five years follow-up of patients all done at Massachusetts General Hospital, there were no signs of radiographic osteolysis either on the acetabular or femoral sides, and this is the earliest point where you might normally start to see that with conventional polyethylene,” Charles R. Bragdon, Ph.D., said in an interview.
Electron beam irradiated highly cross-linked polyethylene (HXLPE) is created by taking standard high-density polyethylene and irradiating it, which increases cross-linking and improves wear characteristics. The material then goes through “a melting step which allows the free radicals generated during radiation to extinguish themselves and form more cross-linking. With the free radicals gone, there's no long-term oxidation, so the properties you get do not change over time,” Dr. Bragdon said explained.
HXLPE has been used as total hip arthroplasty bearing surface for nearly 8 years, according to Dr. Bragdon and his colleagues at the Orthopaedic Biomechanics and Biomaterials Laboratory at Massachusetts General Hospital in Boston. Radiostereometric analysis (RSA) has shown little additional femoral head penetration after the early bedding-in period; however, such studies have been performed on relatively small groups of patients, the authors continued. Using standardized measures, the Boston group studied femoral head penetration in 77 primary total hip replacements in 70 patients with HXLPE liners, with either a 28-mm or 32-mm femoral head.
The average total femoral head penetration was calculated based on the total penetration from the initial postoperative film to the longest follow-up film.
“During the first year following surgery, there was about 100 microns of head penetration into the polyethylene,” Dr. Bragdon said. “Thereafter … [there was] no measurable wear of this material at 5 years,” he said. In addition, there was no significant difference in the average penetration rate or the steady state wear rates between the 28-mm and 32-mm groups.
“These results are encouraging for lysis in the long term because we've shown in the past that lysis and wear tend to go hand in hand; we think our study bodes well for 7–15 years' follow-up,” said Dr. Bragdon, adding that continued follow-up will be necessary to evaluate the material's clinical and radiographic durability.
Dr. Bragdon predicted that these more durable replacements will further reduce the average age of both knee and hip recipients. “Cross-linked polyethylene's introduction has been in hip arthroplasty, but now it's also being used in knees because knee components also suffer from wear and osteolysis.”
Osteolysis was seen on x-ray 7 years after placement of a conventional polyethylene liner for total hip arthroplasty (left). No osteolysis was seen 6 years later with a highly cross-linked polyethylene liner (right). Photos courtesy Dr. Meridith Greene
