Millions of individuals are living full and active lives thanks to joint replacement. Recent attempts have been made to improve bearing surfaces and address problems such as increased friction and joint wear.

The current state of alternate bearings—ceramic-on-ceramic (CoC), metal-on-metal (MoM), and highly cross-linked polyethylene—for total hip arthroplasty was discussed by a panel of international experts during the 2012 AAOS Annual Meeting. Henrik Malchau, MD, of Boston’s Massachusetts General Hospital moderated the “Worldwide Perspectives on Alternate Bearings” symposium.

All bearings fail
Wear debris–related failure is associated with MoM implants, according to William L. Walter, MD, PhD, of Sydney, Australia. He conducted a retrieval analysis on 55 CoC total hip arthroplasty (THA) and 17 MoM THA patients with similar demographics.

“We did not see any wear debris–related failures in the CoC cohort. The median rate of wear was greater for MoM than for CoC, but it is the outliers in MoM that tend to be much greater than CoC in terms of wear debris. We believe this is due to a difference in biologic activity where metals are much more toxic to tissues than ceramics,” he said.

Dr. Walter added that although posterior edge loading is common in CoC, it is clinically benign. On the other hand, anterosuperior edge loading is less common, but produces more wear and may be associated with problematic squeaking.

“All bearings can and do fail,” added John Skinner, FRCS, of London, United Kingdom. “But the problems that we began seeing with metal bearings—necrosis and tissue damage—were something new. We’re now looking at who to revise and how to follow-up on MoM patients.”

Symptoms such as pain and limp are very important in diagnosing adverse reactions to MoM implants; therefore, follow-up must be clinical, according to Dr. Skinner. Blood tests that measure metal ions level are also helpful, as is cross-sectional imaging.

“Some surgeons are reluctant to perform hip resurfacing procedures due to all the concern about MoM implants, yet these procedures can still produce excellent results,” he said. “However, I don’t believe we should be performing large-diameter MoM total hip replacements.”

Edge loading has been suggested to increase wear in MoM THA, and abnormal periprosthetic soft-tissue masses (pseudotumors) have been associated with elevated metal ion levels in serum and hip aspirate. These associations led to two studies by Young-Min Kwon, MD, PhD, of Massachusetts General Hospital. The first study quantified in vivo wear of implants revised for pseudotumors and implants revised for other reasons (control group); the follow-up study investigated in vivo edge loading during functional activities in patients with pseudotumors.

Results showed that the MoM implants revised due to pseudotumor had significantly greater linear wear rates. Patients with pseudotumors also had elevated in vivo metal ion concentrations, confirming that pseudotumors are associated with increased wear at the MoM articulation. In addition, edge loading in MoM THA patients with pseudotumors occurred with significantly longer duration and greater magnitude of force impulse compared with patients with a well-functioning MoM implant.

This supports the hypothesis that edge loading is an important mechanism leading to localized high wear, with subsequent elevation of metal ion levels in MoM patients with pseudotumors. Dr. Kwon also noted that, although the acetabular component orientation appears to be an important factor in edge loading, the etiology of edge loading is likely to be multifactorial.

Polyethylene bearing surfaces
Highly cross-linked polyethylene THA cups show reduced wear at 10 years follow-up compared to conventional polyethylene cups, according to radiostereometric analysis (RSA) study data presented by Johan N. Kärrholm, MD, of Sweden. RSA involves taking two radiographs from different directions at the same time, thus creating a “stereo” image that enables surgeons to measure precisely how the body and the implant are interacting.

The study involved 60 osteoarthritic patients (30 females, 30 males; median age: 55 years) who were randomized to receive either highly cross-linked or conventional polyethylene cups. All implants had cemented stems and 28-mm cobalt-chromium heads. RSA examinations, radiographs, dual-energy x-ray absorptiometry (DXA) scans, and clinical examinations were performed at regular intervals for up to 10 years. Measurements of proximal wear and proximal migration of the cup and distal stem migration measurements were compared in the 47 patients (21 highly cross-linked, 26 control) who were followed for the entire study period.

After the 6-month follow-up, the proximal penetration increased mainly in the control group, resulting in a mean difference of 0.45 mm at 10 years (P < 0.0005). The mean proximal cup migration in the cross-linked group was 0.1 mm, compared to 0.3 mm in the control group (P = 0.6); the stem subsidence in the cross-linked group was –0.2, compared to –0.5 in the control group (P = 0.001). Based on these results, the researchers concluded that highly cross-linked polyethylene reduced wear, but did not significantly influence cup and stem migration.

In reviewing long-term wear data for highly cross-linked polyethylene as it relates to age and activity level, Charles R. Bragdon, PhD, and his colleagues at Massachusetts General Hospital identified 187 patients with greater than 5-year follow-up and UCLA activity scores. Patients were grouped according to age at the time of surgery (younger or older than 60 years), low (UCLA scores 1-6) or high (UCLA scores 7-10) activity levels, and standard (28 mm to 32 mm) or large (36 mm to 40 mm) femoral head sizes.

The researchers used Martell Hip Analysis to view two-dimensional measurements of the femoral head penetration.

“We found an extremely low femoral head penetration rate regardless of age, activity level, or femoral head size—we could not detect any increase in wear in any of these subgroups,” said Dr. Bragdon. “With metal-on–highly cross-linked polyethylene, it appears that the limiting factor for longevity of these reconstructions will no longer be concerns of wear. However, younger joint replacement patients will continue to challenge the component fixation as well as the modular interfaces of these modern implants.”

“When polyethylene is subjected to radiation for cross-linking to reduce wear resistance, free radicals are formed throughout the material. And when free radicals are present, the material degrades even when just sitting on the shelf before it is implanted in the patient,” said Orhun Muratoglu, PhD, of Massachusetts General Hospital.

Annealing, melting, or doping with antioxidants, such as vitamin E, can be used to stabilize the material. To determine the effectiveness of these methods, Dr. Muratoglu and his colleagues analyzed in vivo oxidation of surgical retrievals that were irradiated and melted (n = 44), irradiated and annealed (n = 14), sequentially irradiated and annealed (n = 27), and irradiated and vitamin E–stabilized (n = 10).

“Our analysis revealed that annealing is not necessarily desirable because it leaves behind residual free radicals, resulting in high levels of in vivo oxidation and ex vivo oxidation,” said Dr. Muratoglu. “Melting results in very low oxidation up to 10 years, but we did see some ex vivo oxidation, which we think is related to lipids absorbed in vivo and/or cyclic loading. In contrast, vitamin E stabilization showed little oxidation; it appears the free-radical scavenging activity of vitamin E is a very effective in preventing both in vivo and ex vivo oxidation.”

The value of arthroplasty registries
“Assessing the outcome of bearing surfaces is difficult,” said Australian Steven Graves, MD. “Registry data is valuable because it provides reliable information on the comparative performance of different bearing surfaces.”

For example, 10-year data from the Australian Orthopaedic Association National Joint Replacement Registry reveal the following:

• The lowest rate of revision occurs when a cross-linked polyethylene bearing is used.
• The highest rate of revision occurs when a large (36 mm or larger) MoM head is used, due to an increased incidence of loosening and metal sensitivity.
• Cross-linked polyethylene has a significantly lower rate of revision at 10 years compared to nonmodified polyethylene, whether the femoral head is metal or ceramic.
• CoC has a higher rate of revision compared to cross-linked polyethylene when a ceramic head of 28 mm or less is used. There is no difference between cross-linked polyethylene and CoC bearings when ceramic heads greater than 28 mm are used.

Disclosures: Dr. Walter—Stryker; DePuy, A Johnson & Johnson Co.; Smith & Nephew; Matortho. Dr. Skinner—London Implant Research Centre for Recalled ASR Implant Analysis; Biomet; DePuy, A Johnson & Johnson Co.; Mathys Ltd.; Smith & Nephew; Zimmer; Stryker; Finsbury. Dr. Kwon—MAKO Surgical Inc. Dr. Kärrholm—Stryker; Link Orthopaedics; RSA Biometica; Umea; Zimmer; Biomet; DePuy, A Johnson & Johnson Co.; Dr. Bragdon—Zimmer. Dr. Graves—no conflicts. Dr. Muratoglu—Biomet; Zimmer; Corin USA; ConforMIS; Renovis; Cambridge Polymer Group; Johnson & Johnson; MAKO Surgical Inc.; DePuy, A Johnson & Johnson Company.

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