Metal On Metal Hip Replacements & Contaminations

The wear and corrosion of metal-on-metal hip replacement products creates metal ion particles so small they can’t be seen under a microscope. When carried in the blood stream throughout the body, the particles are believed to create inflammation in the bone, ligaments, tendons and muscles surrounding the hip. That inflammation can cause the tissue inside the hip joint to die and loss of bone.

Manufacturers of metal-on-metal hip devices have played down the risk associated with the release of this metal debris. One manufacturer, Zimmer Holdings, Inc. issued a statement saying that available data suggests that the levels of ions released from its metal-on-metal hip devices are the same as the levels released from its competitors’ metal-on-metal systems and that they are not associated with any significant risks to patients.

There are several choices of implants from which the surgeon can choose, but the type that presents the greatest problems is what’s called the metal-on-metal hip replacement. Metal-on-metal hip implants, many of which contain a cobalt chromium alloy, first were used in 1955 because they were thought to be more durable than the other types of implants being used at that time. They did not receive approval for use from the Food and Drug Administration (FDA) until 1999.

Ion Release Source of Metal On Metal Hip Replacement Problems

When the bearing surfaces of the hip become damaged because of disease or injury, an individual feels pain and loses range of motion. To relieve these conditions, an orthopedic surgeon can replace the damaged parts of the hip joint with synthetic parts.

Medical researchers say there are some defective hip replacement devices that release more ion particles than others through routine normal wear and tear. An interdisciplinary team of physicians and engineers from the United States and Germany received a grant from the National Institute of Arthritis and Musculoskeletal and Skin Diseases to study metal-on-metal hip replacement devices.

These scientists analyzed metal joint components that were removed in revision surgeries, procedures that replace hip devices implanted in patients. The focus of the study was tribology, which examines friction, lubrication and wear. Two of the members of the research team discovered previously that a lubricating layer is created on metallic joints as a result of friction.

They found this layer was not the result of proteins in the body that adhered to the surface of the implant. Instead, the layer is made in part of graphitic carbon, a solid lubricant that is used in industrial applications. If the layer had been proteins, it would work like the natural lubrication in joints. Instead the graphitic carbon flakes off, releasing metal ions.

Metal-on-Metal Failure Rates A Serious Issue

The durability of metal-on-metal hip replacement devices is significantly less than anticipated. In the first eight months of 2011, the FDA received more than 5,000 reports from patients who had revision surgery because their all-metal hip device failed after only a few years. This was more filings about hip device failure than the total of complaints the agency had received in the previous four years (2007-10).

An accurate total of all failed implants reported to the FDA is difficult because the agency has multiple mechanisms by which medical device failures can be reported. The New York Times conducted its own analysis from 2007 to June 2011for several implants. Among its findings, the newspaper noted that the FDA had received approximately 1,600 complaints against the Zimmer Durom Cup and another large number of complaints about the DePuy ASR hip replacement.

A spokesman for Zimmer Holdings told The Times the newspaper’s assessment of the number of complaints filed against its device was “in the ballpark” of the company’s own evaluation of the FDA’s filings.

FDA Approval Method Evaluated

Unlike the rigorous requirements for approving a drug to be marketed in the United States, the FDA approves Class II medical devices like hip replacement prostheses through a less taxing procedure known as the 501(k). The manufacturer of the device is not required to test the hip replacement on patients and provide the agency with safety and efficacy data. However, because of an increasing number of complaints filed against hip replacement prostheses, the agency requested in May 2011that 22 manufacturers study how frequently the devices fail and what are the adverse reactions experienced by patients.

Sources:

1. Liao, Y., et al. (2011). Graphitic Tribological Layers in Metal-on-Metal Hip Replacements. Science 334 (6063) 1687-1690 doi: 10.1126/science.1213902
2. Meier, Barry. (2011, March 3). Concerns Over ‘Metal on Metal’ Hip Implants. The New York Times, Retrieved from http://www.nytimes.com/2010/03/04/health/04metalhip.html
3. Meier, Barry and Roberts, Janet. (2011, August 22). Hip Implant Complaints Surge, Even as the Dangers Are Studied. The New York Times. Retrieved from http://www.nytimes.com/2011/08/23/business/complaints-soar-on-hip-implants-as-dangers-are-studied.html?_r=1&pagewanted=all
4. Hip Replacement Surgery. (n.d.). Retrieved December 27, 2011, from http://bonesmart.org/joint-replacement-surgery/hip