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Bearing surfaces: Is MOM worth the risk?

By Mary Ann Porucznik

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Implications differ, depending on use and patient

A metal-on-metal (MOM) articulation at the hip joint has a number of benefits, Thomas P. Schmalzried, MD, told members of The Hip Society and the American Association of Hip and Knee Surgeons during the 2011 Specialty Day Program. But, depending on the application and the patient, MOM hip implants also present risks that may outweigh those benefits. And in those situations, alternative bearing surfaces may be more appropriate.

Recently, the downsides of MOM hip implants have garnered attention. “The risks include an adverse local tissue response (ALTR), an identified role of component position leading to increased wear and potentially corrosion, and separate issues related to single-piece (monoblock) sockets,” noted Dr. Schmalzried.

The push to MOM

Hip instability is the main reason for revision hip surgery among Medicare patients, accounting for more than 22 percent of all revision hip surgeries in this group. Because the personal and economic impact of revision hip surgery is significant, efforts to lower the risk of instability have focused on the size of the implant.

“Studies have shown that larger diameters have a lower risk of instability,” said Dr. Schmalzried. “Bearings with a diameter greater than or equal to 36 mm have demonstrated greater range of motion and stability. MOM was largely embraced because of the larger diameters.”

Market trends support this analysis. In 2000, noted Dr. Schmalzried, not a single hip implant sold had a head size of 36 mm or more. By 2003, however, larger head sizes accounted for about 15 percent of sales, and in 2009, nearly 60 percent of the hip implants sold in the U.S. market had a head size of 36 mm or more.

“Larger heads were embraced because they were thought to be forgiving, but that was true just for stability,” said Dr. Schmalzried. “You could have a stable hip that was not well mated.” And that, he noted, could lead to increased wear and higher ion levels.

The issue of component position is a fundamental parameter, he noted, and one that has evolved over the years. Flashing a radiograph on the screen, he said, “It was thought that this component was in satisfactory position. It was thought that you couldn’t measure the lateral opening angle when using metal-on-metal components. Both of those statements are incorrect.”

The risk of ALTR

In discussing current concerns about ALTR, Dr. Schmalzried noted that this is a multifactorial issue. “ALTRs have been reported in anywhere from less than 1 percent to more than 10 percent of patients in studies,” he said. “Our studies indicate a central role of joint pathomechanics. Several pathologies have been identified, including high wear, a lot of wear particles, and a foreign body response, but other cases have variable wear, which is sometimes quite low, an associated immune response, and potentially a pseudotumor.”

Wear particles, metal ions, and corrosion products are among the stimuli that can trigger an ALTR, but Dr. Schmalzried also noted that ALTRs have been associated with a high lateral opening angle, or increased combined anteversion.

Dr. Schmalzried noted that monoblock sockets can be more difficult to properly insert and provide no opportunity for adjunct fixation. If bone ingrowth does not occur, the patient may experience deep groin pain. He also said that very large heads may cause some soft-tissue inflammation and noted the potential for corrosion at the modular taper junction.

As a result of these issues, alternative bearing surfaces have evolved. Cross-linked polyethylene implants are now available with bearing diameters greater than or equal to 36 mm, and multiple studies have reported lower dislocation risks with these implants. Additionally, the U.S. Food and Drug Administration has approved a 28 mm ceramic-on-ceramic implant, and an application is pending for a 36 mm ceramic implant.

But these alternative surfaces also have downsides, including the risk of fracture, potentially higher volumetric wear, and possibly in vivo oxidation.

Hip resurfacing

Because no alternatives to MOM exist for hip resurfacing, however, Dr. Schmalzried next addressed that issue.

“The millennium patient is unaccepting of disability and does not want restrictions or limitations due to the prosthetic joint,” he said. But before agreeing to perform hip resurfacing, the orthopaedist should verify that the patient meets the requirements for this procedure: substrate strength, stability for major motion, low wear, and revisability.

With more than 14 years experience for hip resurfacing procedures, data on survivorship, function, and satisfaction are comparable to data for total hip arthroplasty patients, said Dr. Schmalzried. “The 2010 Australian National Joint Registry showed greater than 96 percent survival for hip resurfacing in males younger than age 65 with osteoarthritis and greater than 96 percent survival for both genders when you have a large bearing diameter, which indicates a surrogate for a large stature patient.”

In summary, Dr. Schmalzried said both patients and surgeons should weigh the risks. “For total hip arthroplasty, the risks outweigh the benefits of MOM for most patients. ALTR has a risk related to component position and alternative bearing surfaces are available in larger sizes. With regard to resurfacing, high survival and function have been demonstrated in young, active patients. At this time, no alternative bearing surface is available for resurfacing, so the benefits outweigh the risks in appropriate patients,” he concluded.

Disclosure information—Dr. Schmalzried: Stryker, Inc.; DePuy, a Johnson & Johnson Company, Orthopedics Today; Orthopaedic Research and Education Foundation.

Mary Ann Porucznik is managing editor, AAOS Now. She can be reached at porucznik@aaos.org

AAOS Now
April 2011 Issue
http://www.aaos.org/news/aaosnow/apr11/clinical8.asp

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