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AAOS Now

Published 1/1/2010
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Mary Ann Porucznik

Newer THA bearing surfaces fail to show early advantages for elderly

Study compares effectiveness of bearing surfaces in Medicare population

Metal-on-polyethylene (M-PE) may be the gold standard in hip replacement bearing surfaces, but newer designs have promised better outcomes, particularly with regard to reducing the risk of complications and reoperation. According to the results of a study presented at the American Association of Hip and Knee Surgeons (AAHKS) 2009 annual meeting, these new designs fail to deliver on that promise, at least in the U.S. Medicare population.

“We know that bearing surface failure is a common cause of long-term issues with hip replacement so newer—and more costly—bearing surfaces, including metal-metal (M-M) and ceramic-ceramic (C-C), have been developed,” explained Thomas P. Vail, MD. “Interestingly, these newer bearing surfaces are being used in all age categories, which is important as it relates to this study.”

Researchers hoped to answer the following questions:

  • What are the specific indications for new bearing technology?
  • Can the benefits of new hip bearings be demonstrated in terms of lower revision rates and complication rates?
  • Do hard-on-hard bearings have value in older patients?

Coding provides a clue
With the introduction of International Classification of Diseases-9-Clinical Modification [ICD-9-CM] optional procedure codes specific to bearing surfaces, researchers were able to identify what type of bearing surface was used in Medicare total hip arthroplasty (THA) patients.

Using the 2005–2007 100 percent Medicare inpatient claim files, the research team performed a matched cohort analysis to compare complication and revision THA risks, adjusted for medical comorbidities, race, socioeconomic status, and hospital factors. About 40 percent of the claim files included bearing surface coding, enabling the researchers to establish the matched cohorts.

“The 100 percent sample from Medicare is a large data set that allows very powerful analysis because it includes a large number of demographic and clinical variables. It includes only inpatient encounters, but it does have unique patient identifiers, so researchers can identify complications and revisions by patient during a specific period,” explained Dr. Vail. “Based on the overall data set for Medicare patients (2005–2007), M-PE bearing surfaces were used in about 64 percent of hip replacements; M-M bearing surfaces were used in 30 percent of cases; and C-C bearing surfaces were used in 5 percent of cases (Fig. 1).”

The use of M-PE bearing surfaces increased as the patient’s age increased (Fig. 2). The researchers then developed a matched cohort analysis for the three different bearing types, matching patients by age, gender, and census region. Finally, they adjusted the data for medical comorbidities, race, socioeconomic status, and hospital factors, and constructed a hazards model to compare the relative risk of complications (deep venous thrombosis [DVT], dislocation, infection, and mechanical loosening) and revision THA among the cohorts within the first 2 years.

Differences are minor
“Each bearing surface has its own unique complications,” noted Dr. Vail. “For C-C bearings, the risk is ceramic fracture; for M-M bearings, it’s exposure to metal ions; and for M-PE bearings, wear and fracture are among the risks.”

After controlling for patient and hospital factors, M-M bearings were associated with a significantly higher risk of periprosthetic joint infection (hazard ratio 3.03; confidence interval = 1.02–9.09) when compared with C-C bearings. Researchers found no other significant differences among cohorts in the adjusted risk of revision THA or any other complication, including dislocation, mechanical loosening, or periprosthetic fracture, although M-M bearings showed a trend toward higher overall rates of revision and DVT than C-C bearings.

“We found that Medicare THA patients with hard-on-hard (M-M and C-C) bearings had a similar risk of complications and revision THA compared to patients who had M-PE bearings during the first 2 years following primary THA,” said Dr. Vail.

“Information from the Australian registry indicates that differences among the bearing surfaces may occur later, in terms of long-term performance,” he continued. “But looking at the demographics of older patients and their life expectancies, we have to ask: If there’s not an early advantage, will this Medicare patient ever see an advantage from receiving a higher-priced, hard-on-hard bearing?”

Importantly, researchers found no difference in the risk of dislocation among bearing cohorts. The higher risk of dislocation in elderly patients is often cited as a reason for using larger-diameter M-M bearings in these patients, which theoretically could reduce the risk of dislocation. “That leads to the question of whether the option for a larger head size with M-PE bearings has changed the equation,” said Dr. Vail.

Based on this study, hard-on-hard bearings do not demonstrate short-term value in Medicare patients.

“If you look at the pricing of these devices over the last decade or so, hard bearings have been priced at a premium, going along with the promise of better outcomes,” he concluded. “This study speaks to the need for broader diligence with coding, and more robust data sets, so we can demonstrate that if we are spending more money, we are getting more value.”

Dr. Vail’s co-authors include Kevin J. Bozic, MD, MBA; Kevin Ong, PhD; Edmund Lau, MS; Steven M. Kurtz, PhD; Harry E. Rubash, MD; and Daniel J. Berry, MD. Dr. Vail disclosed ties to DePuy, Smith & Nephew, and Zimmer.

Disclosure information: Dr. Bozic—United Health Care; Center for Medicare & Medicaid Services; Pacific Business Group on Health; Integrated Health Care Association; Dr. Ong—Kyphon Inc.; Stryker; Dr. Kurtz—Medtronic; Synthes; Johnson & Johnson; Stryker; DePuy Spine; Archus Orthopaedics; Zimmer; Biomet; DePuy, A Johnson & Johnson Company; Encore Medical; Exactech, Inc; Medtronic; National Institutes of Health (NIAMS & NICHD); Invibio; Kyphon Inc.; Active Implants; Stelkast; LDR Spine; Dr. Rubash—Zimmer; Dr. Berry—Depuy, Zimmer, Stryker, ODC, Wright, Medtronic, Smith Nephew, DJO, Acumed, SBI, Ascencion, Tournier; Mr. Lau—no conflicts.

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