Hip replacement surgery is the most common form of orthopedic surgery. Replacement can be partial (just the ball component of the joint) or total (both the ball and the socket). Hip replacement is generally performed instead of repairing a broken hip or as a means of reducing pain from arthritis in the hip joint.
Although this surgery is popular, it is not without risks.
One risk is deep vein thrombosis (DVT), which is a concern with many types of orthopedic surgery. DVT can lead to a pulmonary embolus, which can be fatal. Blood thinners such as heparin are generally used after orthopedic surgery in order to reduce the risk of DVT, but even such treatment cannot eliminate the risk completely. Patients with metabolic syndrome -- defined as a cluster of symptoms including impaired metabolism of glucose, overweight, high blood pressure, and impaired lipid metabolism -- are at an elevated risk of DVT following hip replacement surgery.
Another risk is toxicity from metal ions that escape into the body from metal replacement hips. One study, whose results were published in the August 2013 issue of the Journal of Orthopaedic Surgery, found an average of 5.6 micrograms of chromium per liter of blood in patients undergoing surgical revision of total hip replacement. This is a matter of concern because of the carcinogenicity of chromium. Additionally, metal ion concentration in the blood is an early sign of hip replacement device failure, as documented in the January 2013 issue of BMJ.
Why do hip replacement devices fail? The etiologies can include osteolysis (bone loss) around the site of the implant, wear and tear of the polyethylene component of the implant, metal fragmentation that causes pain to the patient, and other mechanisms. Although one may expect clinical trials to reveal the likelihood of such adverse events as these and forestall the approval of a risky device, the 510(k) device approval process has not mandated the submission of clinical trial data for all medical devices.
For example, the Articular Surface Replacement implants made by the DePuy division of Johnson & Johnson, which has been recalled and has been documented as failing at a rate of 13%, was cleared for marketing under section 510(k) of the Food, Drug, and Cosmetic Act, after being deemed similar to other devices already on the market. Researchers publishing their findings in the New England Journal of Medicine traced the "510(k) ancestry" of the ASR implant back through several generations of devices, noting the leaps made by the marketer and by the FDA officials who cleared the products in declaring them "substantially equivalent." It remains to be seen how many other devices -- hip replacement implants among them -- will be shown to have significant differences from the predicate devices on whose approval their own marketing clearance is based.