Deborah Cohen: “Poisonous” hip implants

Deborah Cohen“Poisonous hip implants ‘putting thousands of British patients at risk’ as medical watchdog launches investigation,” a Daily Mail headline announced today.

It followed on from a front page splash in the Sunday Telegraph which claimed that “metal-on-metal” hip devices are “even more dangerous than previously thought.”

This isn’t a new story—but finally people are waking up to the scale of the problems caused by the device regulatory system in Europe. First there was public outcry over faulty PiP breast implants, and now controversies that have been surrounding metal on metal hips for some time are gathering momentum.

The stories over the weekend described the cobalt and chromium ions that are thrown into the tissues and blood stream leading to “poisoning” when certain metal on metal hip replacements begin to wear. They focused on Depuy’s hip prosthesis, the ASR (a resurfacing device), and the ASR XL (total hip replacement), which were removed from the market in 2010.

Back in May 2011, one senior orthopaedic surgeon told a joint BMJ/Dispatches investigation that the ASR was one of the “biggest disasters in orthopaedic history.”

Metal debris from wear of the implant led to a reaction that destroyed the soft tissues surrounding the joint, leaving some patients with long term disability. Ions of cobalt and chromium—the metals from which the implant was made—were also released into the blood and cerebral spinal fluid in some patients.

Evidence is starting to emerge about the long term effects. A letter to the BMJ by Alaskan surgeon Stephen Tower published earlier this month also reported a case series of patients of whom an “uncharacteristically high proportion” developed progressive, unresolved, or new hip pain, with some showing metal debris around the hip with tissue damage. He also reported patients having cognitive decline, cranial neuropathy, and early cardiomyopathy.

Guidance issued by the US Food and Drug Administration advised doctors to check for systemic effects. This included checking for cardiovascular, neurological, renal, and thyroid signs and symptoms.

Contrary to reports in the press— which claim that experts have not established why the DePuy models have such a high failure rate—this is a situation that could have been prevented. The potential mode of failure was highlighted by Derek McMinn, an orthopaedic surgeon in Birmingham, when the ASR came onto the market in Europe and Australia. The cup component of the prosthesis was particularly shallow leading to accelerated rates of wear.

The problems of the ASR could have been picked up by the regulators in Europe if they had required a trial for approval. Problems with the implant started to emerge only two years after the procedure. As the BMJ reported last year, documents sent to the French Haute Autorité de Santé, showed a two year follow-up study had a 4.9% fracture rate in the ASR resurfacing arm. And the two year survival of the implant was 95.9% (95% confidence interval 93.5% to 99.9%) when only 25.6% of people in the group were women—who typically fared worse with the prosthesis.

Whilst the ASR might be the worst hip prosthesis of the lot, there are concerns over the failure rates of other metal on metal implants—in particular those with a large femoral head component and resurfacing prostheses in smaller patients.

Last year, the National Joint Registry of England and Wales said that the performance of the controversial metal on metal hip prostheses “continues to create cause for concern” in their annual report.

This is clearly troubling for the health service and most importantly, patients.  The stories in the press suggest the MHRA is issuing updated guidance—and if patients are being missed, as reports suggest, it seems like they need to. (Guidance about management of patients with metal on metal hips can be found here and here.)

However, it’s unlikely that we’ve heard the last of the problem—either of metal hips or poor device regulation. And until European and other international bodies realise that device regulation is the poor relative of drug regulation, patients and health services will continue to bear the brunt of failures.

Deborah Cohen is the BMJ investigations editor.