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Lessons from the Hip: Where Did MoM Go Wrong?

By: Terry Stanton

Terry Stanton

The Orthopaedic Research Society devoted a full day yesterday to some of the most high-profile topics in orthopaedic surgery. Panelists shared their perspectives and knowledge on several unresolved, controversial issues, including reports of problems with metal-on-metal (MoM) hip devices, questions about sponsored research and researcher conflicts involving recombinant human bone morphogenetic protein–2 (rhBMP-2), and the risks, benefits, and some questionable assumptions surrounding bisphosphonates and atypical femur fractures.

In reflecting on the various complications and failures reported for MoM hip replacement and resurfacing components, Kevin J. Bozic, MD, MBA, of the University of California–San Francisco, asked “What went wrong? How did it go from such a promising technology that would benefit our patients to the current situation? In retrospect, it looks like we missed a lot of the warning signs; now we need to look forward and think about how we can avoid such problems.”

According to Dr. Bozic, the following four broad categories of policy and practice for the development and use of medical devices were at the heart of the MoM issue:

  • The U.S. Food and Drug Administration’s (FDA) regulatory process for device approval and postmarket surveillance
  • Direct-to-consumer advertising
  • The way new technology is introduced into practice, often with less than comprehensive training for surgeons
  • Surgeon-industry relationships

According to Markus A. Wimmer, PhD, of Rush University Medical Center, strong theoretical and experimental evidence suggested that larger-diameter MoM bearings improve fluid film lubrication. However, he noted, “sensitivity of theory to certain implant design parameters had been overlooked. Adverse conditions—such as malalignment—could have been preclinically investigated. The tribology [the interaction of surfaces] of MoM joints is not fully understood but it is more than just a matter of film lubrication.”

The younger-patient solution
Young-Min Kwon, MD, PhD, of Harvard University, reviewed the way by which less-than-comprehensive clinical case series led to support of widespread adoption of MoM hip resurfacing implants. Introduced as “the solution” for younger, active patients with hip osteoarthritis, resurfacing offered potential advantages: increased stability by virtue of the larger articulation, less wear, and more bone available for revision surgery.

In 2002, Britain’s National Institute for Clinical Excellence (NICE) recommended MoM resurfacing for patients likely to outlive a conventional total hip replacement. This recommendation, however, was based on rather limited evidence; of eight observational studies, three were from manufacturers, and only four were published. Average follow-up was 2 years. No studies had a control group and no studies provided justification of sample size or included data on the outcome of revisions.

In 2011, the revision rate for MoM resurfacing was three times that for other bearing surfaces. A review found that no MoM devices met the full 10-year NICE benchmark of 90 percent survival, and only 43 percent met the 3-year benchmark of 97 percent survival. Revision rates at 3 years varied dramatically among products, ranging from 1.9 percent to 10 percent.

“Minor changes in implant design can have a dramatic influence on performance,” Dr. Kwon said. Important cup design parameters include coverage angle, clearance, geometry of the cup edge, and dome geometry. Implants with a reduced angle subtended by the acetabular component provide less coverage and can incur edge loading. Cups with increased thickness at the dome versus the rim can shift the center of rotation and increase the risk of edge loading. Larger-lip radii reduce edge-loading stress but also decrease coverage.

Dr. Kwon said that the introduction of MoM resurfacing fell short of the optimal “stepwise” process for assessing new innovations, not even fulfilling the first clinical step of prospective randomized studies, which should be followed by multicenter studies and register studies.

Early warnings
Stuart B. Goodman, MD, PhD
, of Stanford University, looked back at some of the early warning signs that led to safety concerns about MoM devices. He noted that early warning signs—such as unpredictable fixation and low survival rates—could be seen just 5 to 7 years after the introduction of some systems.

Registry studies show the failure rates for metal-on-polyethylene (MoP) bearings for total hip replacement (THR) at 9 years are not high. But “MoM implants have the highest incidence of loosening and osteolysis,” Dr. Goodman said. Additionally, MoM bearings with larger head sizes had a “very high revision rate” compared with smaller head sizes, the exact opposite of the resurfacing experience. Issues specific to MoM resurfacing included increased problems in female or obese patients, with smaller-sized implants, and in bilateral cases.

Addressing the issue of metal byproducts, Dr. Goodman recalled that the first warning signs of trouble were patients with “excellent looking radiographs but osteolysis and pain.” In 2005, patients with well-placed, functioning implants exhibited signs of metal sensitivity reactions. He noted that MoM implants produce two to three times more metal particles than MoP implants, and that elevated ion levels can be found in both local tissues and systemically in patients with MoM implants. Issues and hazards for such patients include cytotoxicity, possible DNA damage, hypersensitivity, and the possibility of transplacental migration.

Dr. Goodman noted that adverse tissue reactions (pseudotumors) associated with resurfacing have garnered considerable attention recently, including in the popular press. They involve a large soft-tissue response leading to tissue destruction and need for revision.

Dr. Bozic noted that the two MoM devices that were subsequently recalled had been approved through the FDA’s 510(k) process. This pathway allows devices deemed “substantially equivalent” to predicate devices to be cleared without new testing. Although some have argued that the 510(k) process should be eliminated, Dr. Bozic would prefer to see it “tweaked in a way to make it better,” with more stringent provisions for postmarket surveillance.

Although he questioned the legitimacy of the predicate-based clearances for MoM devices, Dr. Bozic also suggested that more scrutiny in the premarket approval process would not have helped. “Not until they were in use on a much larger scale could we identify the problems that we saw,” he said. “No matter how much scrutiny we put on premarket testing and small clinical testing, we are unlikely to have that information. The critical problem is how to identify higher-risk devices that should go through enhanced postmarket surveillance.”

The impact of advertising
When MoM devices were introduced, Dr. Bozic noted, the direct-to-consumer advertising was considerable, and surgeons felt pressure from patients who requested these devices. “The advertising was not limited to the device industry,” he said. “Most of it came from the surgeon community and hospitals.

“There are important differences between advertising a drug and advertising a device,” he continued. “If something goes wrong with a branded drug, you can often switch to another brand or a generic. If you implant a new device, however, the consequences can be quite dire.”

New devices can be implanted by low-volume surgeons with little or no training who may get their only information from sales representatives. As a result, Dr. Bozic believes that although device companies bear some responsibility, the FDA and educational organizations such as the AAOS also have a role to play in surgeon training.

Addressing surgeon-industry relationships, Dr. Bozic noted that “oftentimes the relationship itself is what drives the innovation.” Before new rules went into effect, device companies retained a slew of surgeons doing consulting work, some of which produced value to the patients but much of which did not. Today, fewer of these pseudo–innovation surgeon design panels exist.

Offering his general recommendations, Dr. Bozic called for “responsible, ethical surgeon-industry collaboration.” He continued, “For market-driven innovation, we should think about patient-centric value creation instead of new gimmicks that will help sell more implants or help us market our practices. We need more transparency and independent evaluations of industry-funded clinical trials.”

During the question-and-answer session, Derek McMinn, the developer of the Birmingham Hip Resurfacing (BHR) system, rose to comment. He contended that good 10-year results for his MoM device had been reported independently. “I don’t think the business of bias from the inventor surgeon stacks up,” he said.

Dr. Bozic responded by noting that the favorable results shown for the BHR in the Australian registry represented the work of about 300 surgeons working in training-rich environments. It may not be possible to generalize those results to the 17,000 surgeons in the United States, “who perform five or six hip replacements a year and may or may not be trained to do these procedures.”

“We’re not denigrating metal-on-metal,” Dr. Goodman said. “We must consider everything—the patient, the limb, the joint, the healthcare team. Everyone talks about industry, the FDA, and the medical societies. But we as surgeons must take responsibility for what we do. We’re going to be living with the outcome and patient for the rest of our professional careers.”

Disclosure information: Dr. Wimmer—National Institutes of Health, Medtronic, Zimmer, Invibio, ATI Allvac, Endolab gmbH; Dr. Kwon—Zimmer, Biomet, Mako Surgical, OMeGa, Orthopaedic Research and Education Foundation; Dr. Goodman—Accelolax, StemCor, Tibion, Clinical Orthopaedics and Related Research; Dr. Bozic—Agency for Healthcare Research and Quality, National Institutes of Health, Robert Wood Johnson Foundation, California HealthCare Foundation, Center for Health Quality and Innovation, BlueCross BlueShield Association, Integrated Healthcare Association, Pacific Business Group on Health, Ingenix, InVivo Link.

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