Study examines outcomes with highly cross-linked polyethylene

“Highly cross-linked polyethylene has been proven to have significant resistance to wear in total hip arthroplasty (see “Creep and wear in THA” below); in total knee arthroplasty (TKA), it’s controversial,” says Erik P. Severson, MD, of the University of Utah. “In TKA, highly cross-linked polyethylene is generally believed to have less resistance to crack propagation than standard polyethylene.”

And yet, says Dr. Severson, highly cross-linked polyethylene liners may represent a future research direction in TKA. He bases this assertion on the results of a study that compared the radiographic and clinical outcomes of patients who underwent TKA with a highly cross-linked polyethylene liner with outcomes for those who had a standard polyethylene liner. The study, which had a minimum follow-up of 5 years, is one of the first in vivo studies of highly cross-linked polyethylene in TKA, according to Dr. Severson.

Evaluating patients
Highly cross-linked polyethylene became available for use in February 2001. Since then, it has been used on all TKA surgeries performed by Aaron Adam Hofmann, MD, at the University of Utah. In their retrospective review, researchers focused on the first 100 patients who received this implant and the last 100 patients who received a standard polyethylene insert sterilized with gamma radiation in an oxygen-free environment. All patients underwent routine physical exams and radiographs, which formed the basis for calculating a knee score.

By the time of the study, 12 patients had died and 20 were lost to follow-up. The standard group included 82 patients with a minimum follow-up of 66 months. The highly cross-linked group included 88 patients with a minimum follow-up of 60 months. Five patients in the standard group had undergone additional surgery: two for infection, two for instability, and one for patellar subluxation. In the cross-linked group, three patients had had additional surgery: one for instability, one for infection, and one for a loose body.

Both groups had an average knee score of 197. Fifteen patients in the standard group had radiographs with grade I osteolytic lesions. No osteolysis or wear was seen in the highly cross-linked polyethylene group.

Fewer revisions, no osteolysis
From these results, researchers concluded that although both groups had similar clinical findings, patients with liners made of highly cross-linked polyethylene required fewer revision surgeries and showed no osteolytic lesions on radiographs.

“Highly cross-linked polyethylene shows good durability at this intermediate interval,” says Jeffrey Thomas Hodrick, MD, of The University of Utah Orthopaedic Center, principal investigator for the study. “The material should continue to perform well long-term.”

Dr. Severson concurs.

“We found no evidence that highly cross-linked polyethylene in TKA is associated with early failure,” he says. “Because it has shown very good in vivo wear characteristics at 5 years, this study provides a solid impetus for future research.”

Dr. Hodrick was the principal presenter for the 2008 Scientific Exhibit “Comparison of Highly Cross-Linked Polyethylene with Standard Polyethylene in Total Knee Arthroplasty” at the AAOS 75th Annual Meeting. Co-presenters for the exhibit were Dr. Severson, Deborah M. McAlister, MD; Brian Dahl, BS; and Dr. Hofmann. Dr. Hofmann serves as a consultant to Zimmer; the others report no conflicts.

Jennie McKee is a staff writer for AAOS Now. She can be reached at mckee@aaos.org

Creep and wear in THA
A recent study in The Journal of Bone and Joint Surgery—British reports that “highly cross-linked polyethylene has a 60 percent lower rate of wear than standard polyethylene and therefore will probably perform better in the long-term.”

The study, a 3-year prospective, double-blind randomized, controlled trial, involved 54 patients who received hip replacements using liners made either of highly cross-linked polyethylene or ultra-high molecular weight polyethylene (UHMWPE). Radiostereometric analysis was used to measure wear. Three patients were excluded from the final analysis (one moved; two had poor radiographs), leaving 26 patients in the highly cross-linked polyethylene group and 25 patients in the UHMWPE group.

Researchers found a statistically significant difference in the mean total penetration for the highly cross-linked polyethylene group compared to the UHMWPE group (0.35 mm vs 0.45 mm, p = 0.0184). They also found a significant difference in wear rate (p = 0.012) and a marked difference in the pattern of penetration before and after 1 year.

According to the researchers, “The penetration during the first year was greater than that in subsequent years. After the first year the relationship between penetration and time was virtually linear.”

Researchers found two distinct patterns of penetration—linear creep, which occurred primarily in the first year and predominantly in a proximal direction, and wear, which occurred at a constant rate and primarily in an anterior, proximal, and medial direction. Although the amount of penetration due to creep was similar for both types of polyethylene, the mean wear rate was different.

“The use of highly cross-linked polyethylene should therefore decrease both the mechanical complications of wear such as catastrophic failure and impingement, and the biological consequences such as osteolysis and loosening,” they wrote.

A link to “The creep and wear of highly cross-linked polyethylene: A three-year randomized, controlled trial using radiostereometric analysis” (May 2008 issue of The Journal of Bone and Joint Surgery—British) can be found in the online version of this article at www.aaosnow.org

For more information
Interested in learning more about the use of highly cross-linked polyethylene in implants? Don’t miss the supplement published by the Journal of the AAOS (Vol. 16, Supplement 1, 2008), a 128-page publication that contains the proceedings of a symposium titled 2007 AAOS/NIH Osteolysis and Implant Wear: Biological, Biomedical Engineering, and Surgical Principles.

During the symposium, which was co-chaired by Stuart B. Goodman, MD, PhD, and Timothy M. Wright, PhD in November 2007, experts explored how newer bearing surfaces that use highly cross-linked polyethylene have changed the biologic reaction to wear particles. In addition, participants examined other topics, such as how these implants have performed during wear testing and joint simulator studies. To access the supplement online, visit http://www.jaaos.org/