Robert L. Barrack, MD

AAOS Now

Published 5/1/2015
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Maureen Leahy

The Search for Arthroplasty Longevity

Experts examine pros and cons of CoC, MoM, and cross-linked polyethylene

Total hip arthroplasty (THA) may be the most effective surgical procedure ever developed, yet it has had problems from the beginning,” said David G. Lewallen, MD, during the AAOS and the Orthopaedic Research Society joint symposium “Articulations in Total Joint Replacement: Have We Lost Our Bearings?” at the 2015 AAOS Annual Meeting in Las Vegas.

“During the 1970s and 1980s, arthroplasty failures were attributed to cement technique and cement disease, resulting in a trend toward cementless fixation. However, initial cementless fixation fell short of expectations and we continued to see problems,” he said.

It was then that the synergy between wear and loosening became very clear, with osteolysis emerging as the major factor in the failure of long-term THA and total knee arthroplasty (TKA), according to Dr. Lewallen. “This was a new radiographic finding; we were experiencing a new phenomenon of well-fixed implants with gigantic lytic defects,” he said.

These and other problems related to implant wear—thought at the time to be associated with standard polyethylene materials—prompted interest in alternative bearing surfaces for joint replacement implants. Over the last two decades, three improved bearing surfaces have been introduced: ceramic on ceramic (CoC), metal on metal (MoM), and highly cross-linked polyethylene, each with advantages and disadvantages.

The good and bad of CoC
“Ceramic bearings aren’t really new—they’ve been around for nearly 40 years in France and for about 20 years in the United States,” said Robert L. Barrack, MD. “However, almost every aspect of the manufacturing process for these bearings has been substantially improved during the last 20 years, leading to a renewed interest in their use.”

The major benefits of CoC bearings are extremely low wear and bioinertness, meaning that incidences of osteolysis are almost unheard of, according to Dr. Barrack. “Unlike with MoM and polyethylene bearings, osteolysis is very rare with CoC bearings, particularly in younger patients. And that’s very important because younger patients are the fastest growing segment of the THA market,” he said.

Another advantage of CoC bearings—particularly the newer hybrid ceramic bearing—is that the material properties enable use of a larger femoral head size for improved range of motion and stability, Dr. Barrack noted. “In this respect, CoC bearings may offer a clinical advantage in young patients with smaller stature, particularly females, who have a higher dislocation rate and in whom a larger head is desirable,” he said.

Dr. Barrack cautioned, however, that although newer-generation CoC bearings are much better than earlier generations with respect to fracture risk and intraoperative options, problems still persist. “There is less margin for error in component placement with CoC bearings and more risk of edge loading, which may be associated with debris generation and squeaking,” he said.

Dr. Lewallen added, “Hard-on-hard bearings, including CoC, make the target smaller, requiring surgeons to be much more precise in positioning implants. That is a principle of hard-on-hard articulation that we won’t be able to avoid anytime soon.”

What happened to MoM?
MoM bearings were among the first bearing surfaces to be used in THA. However, their use was largely abandoned in the 1970s due to successes with ultra-high molecular weight polyethylene bearings. In the 1980s, improvements in manufacturing and metallurgy, as well as the ability to use larger diameter femoral head components, prompted a renewed interest in MoM bearings.

“Use of MoM bearings exploded across the U.S. market, accounting for up to 40 percent of implantations in peak years,” Dr. Lewallen said. “And then came the bad news.”

Patients with MoM hips started exhibiting signs of metal sensitivity, and MoM bearings were found to be associated with elevated metal ion levels and cobalt toxicity. According to Dr. Lewallen, the real problems include adverse local tissue reactions—either from metal wear or corrosion of the metal head and trunnion—as well as possible systemic effects.

“We have not seen the end of this yet. It’s a still a major problem with more than 1 million patients at risk worldwide,” he said.

Is cross linking the key?
“When cross-linked polyethylene bearing surfaces were introduced, we were all concerned about failures due to change in material properties,” William A. Jiranek, MD, said. “We were worried about smaller wear particles—could those be more bioreactive and lead to more osteolysis?”

According to arthroplasty registry data, the answer is no.

“Significant trends in several arthroplasty registries clearly indicate that the use of highly cross-linked polyethylene has decreased revision rates and osteolysis in THA, with relatively few cases of material failure,” Dr. Jiranek said. “There is also evidence indicating that cross-linked polyethylene bearing surfaces, through the use of larger femoral heads, reduce the risk of dislocation.”

With respect to TKA, Dr. Jiranek found no demonstrated difference in clinical survival and outcomes between highly cross-linked polyethylene and conventional polyethylene bearings at early follow-up, based on registry and meta-analysis data. “This indicates that the different material properties of cross-linked polyethylene have not led to an early failure rate; however, the long-term effect on wear and osteolysis remains to be demonstrated,” he explained.

“The problem of implant wear has been with us from the very beginning and it’s not going away,” Dr. Lewallen concluded. “But I’m optimistic about the future because of the tools that we have and the 10-year data coming out on highly cross-linked polyethylene products. Moving forward, I think we will have a durable procedure, even for our youngest and most active patients.”

“Articulations in Total Joint Replacement: Have We Lost Our Bearings?” was moderated by William M. Mihalko, MD, PhD, and Stuart B. Goodman, MD. Additional presenters included A. S. Greenwald, DPhil Oxon; Craig J. Della Valle, MD; Young-Min Kwon, MD, PhD; Steven M. Kurtz, PhD; and Timothy M. Wright, PhD.

The presenters’ disclosure information, including potential conflicts of interest, can be viewed at www.aaos.org/disclosure

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org