After the August 2010 recall of the DePuy ASR hip prostheses, many consumers are asking if metal on metal hip implants are still used today.

The answer is yes and no. Metal on metal hip implants are used very sparingly in the U.S. today and only for situations where it is a better choice to resurface the hip rather than replace it.

What is the Difference Between Hip Replacement and Hip Resurfacing?

Hip replacement is when the top of the femur is amputated and replaced with a prosthetic femoral head. The head is attached to a stem that is implanted in the central cavity of the long thigh bone with or without cement. Prior to metal on metal versions, the head was metal but rotated within a polyethylene cup which eventually wore out and needed to be replaced.

Hip resurfacing puts a metal cap on the naturally existing femoral head, according to Versus Arthritis. A metal acetabular cup is still used sparingly in this situation in this country and with lots of prior patient evaluation. There have to date been far fewer problems with hip resurfacing and the use of these components. This has been purportedly linked to a lessened surface area for frictional shedding to occur.

What is “Frictional Shedding”?

This is at the heart of the issue that has caused so much patient suffering. Frictional shedding (also known as “fretting”) is one of the basic problems that has posed a challenge to mechanical engineers throughout history. When one piece of material – in this case, metal – moves while in contact with another (again, metal), it invariably sheds microscopic bits from the surfaces. This is the primary reason your car’s gasoline or diesel-powered motors have oil in their crankcases, which is circulated throughout the valves, pistons, connecting rods, etc.

This is what the designers of metal on metal hip replacements failed to fully consider. Manufacturers of these medical devices initially thought that the use of metals such as chromium, cobalt and even titanium would make for a more durable, longer-lasting product. They believed that the body’s own synovial fluids (a natural lubricant produced by the cartilage) would be sufficient to keep the device operating smoothly.

This was not the case.

Are Metal on Metal Hips Always a Danger?

Versus Arthritis claims that for a certain percentage of people, these ions are benign and capable of being filtered out by the kidneys and removed via the body’s urine output. For others, however, the concentration of metal ions becomes so concentrated in the hip tissues that they become damaged and the body does everything it can to rid itself of them.

The Arthritis Foundation claims that scientists that have researched this issue suspect that the mesenchymal stem cells (MSCs) are affected by localized debris. The MSCs are located in bone marrow and can be converted into different types of cells by the body as needed, some of which are osteoblasts. Osteoblasts are bone-building and with impaired functionality, they stand at odds with their purpose of creating new bone.

The reduction of new bone growth combined with damage and even death of connective tissue, muscles, and nerve supply gradually allows for loosening of the prosthesis. The area may swell and become painful and there is a loss of range of motion. In addition to its effects on the joint, this metal poisoning can contribute to certain cancers and heart disease.

What are Symptoms of Metal Poisoning?

Metal poisoning, also known as metallosis, can cause a wide range of health problems, including bone loss, inflammation, kidney and thyroid dysfunction, heart disease (cardiomyopathy) and cognitive impairment. Outwardly, metallosis can manifest any or all of the following symptoms:

• skin rash and irritation
• unusual hip or groin pain
• loss of sensation
• reduced range of motion

Hip arthroplasty patients who experience these problems must usually undergo revision surgery.

How Many Metal on Metal Hips Are Affected?

The removal of metal on metal implants from the market was prompted in part by fear surrounding the ASR and other cobalt and chromium hip systems that have received a lot of press. One surgeon has referred to the ASR recall as “one of the biggest disasters in orthopaedic history,” according to BMJ.

According to the BMJ, DePuy had sold 93,000 ASR prostheses at the time of the recall. The manufacturer had publicly denied that the device had been the cause of chronic pain in many and disability in a few.

The ASR recall and subsequent recalls led to a wave of litigation in federal court and a global settlement of $2.5 billion. Johnson & Johnson, DePuy’s parent company continued to deny wrongdoing.

Metal on metal hip devices like the ASR were primarily composed of cobalt and chromium. According to the Arthritis Foundation, the ball of the prosthetic rotated directly within the metal lining of the acetabulum and produced metal ions—tiny bits of debris which either settled in the tissues surrounding the implant or entered the bloodstream.