Concerns have grown in the medical community that the so-called metal-on-metal hip implants shed metal particles through wear on their ball-and-socket structure. Many patients who have received the devices have had elevated levels of cobalt and chromium particles detected in the bloodstream, leading some doctors to worry about their risk of developing cancer.

Johnson & Johnson and Stryker Corp have recalled several models of all-metal implants because of high failure rates -- meaning the patient needed another surgery to fix a problem, such as dislocation -- and set aside billions of dollars to cover patient claims. Neither company was immediately available to comment. Since the recalls, the use of all-metal implants, which were sold as being more durable, has declined sharply.

"Some basic science and some epidemiological studies have suggested that metal-on-metal hip replacements may be associated with an increased risk of developing cancer after total hip replacement and specifically with an increase in malignant melanoma and haematological, prostate, and renal tract cancers," the UK National Joint Registry (NJR) said in its annual report, which was released this week at the British Orthopaedic Association meeting in Manchester, UK.

The NJR, which collects data on joint replacement surgery and monitors the performance of the implants, said it used UK National Health Service Hospital Statistics data to test whether metal-on-metal hip implants are associated with an increased risk of cancer in the early years after hip replacement, compared with other surfaces, such as metal-on-polyethelene.

The NJR said it found no association seven years after implantation but that the devices should continue to be tracked as many cancers can take longer to manifest themselves.

"Due to the age of the NJR we could only assess the risk for the first seven years after hip replacement and are happy to report that we could not identify an increased risk of developing cancer," the NJR said.

"We must, however, point out that many cancers have prolonged latency after initial exposure to carcinogens and thus long-term follow up is needed to provide a definitive answer," it added.

NO U.S. TRACKING MECHANISM

Hundreds of thousands of people have all-metal hip implants. Since the recall, J&J faces thousands of lawsuits over the failure of its ASR model, which was pulled from the market, and more lawsuits tied to its Pinnacle all-metal device, which is still being used.

The United States has no independent registry tracking the performance of medical devices over time, and health regulators are still discussing how manufacturers should follow up on complaints over the all-metal implants.

The U.S. Food and Drug Administration has been monitoring problems with all-metal hip implants and convened a panel of outside advisers earlier this year to discuss their safety. The panel concluded there was little reason for surgeons to use these devices given the risks.

Last year, the FDA ordered implant manufacturers to conduct follow-up studies in cases in which an implant's failure could have serious consequences. Companies were ordered to take blood samples from patients to measure metal ion levels. That analysis has not been completed.

The NJR said its latest analysis continues to support conclusions it drew last year showing markedly higher failure rate with metal-on-metal implants.

In addition to J&J and Stryker, makers of all-metal hip implants include Smith & Nephew, Zimmer Holdings, Wright Medical and Biomet Inc. (Reporting by Debra Sherman in Chicago; Editing by Michele Gershberg, Carol Bishopric and Steve Orlofsky)

• A Permanent Economic Emergency