The Design Flaw Behind J&J's $2.5-Billion Artificial-Hip Settlement

Johnson & Johnson and its DePuy unit stands to pay at least $2.5 billion to settle thousands of lawsuits related to its metal-on-metal hip implants in a recent proposed settlement. In total, the company would shell out $250,000 per individual to roughly 8000 patients in the United States. On top of that, J&J would establish a fund valued at nearly $500 million to cover medical injuries ranging from artificial-hip replacement procedures to heart attacks.

Earlier in the year, an internal trial exposed a design flaw related to the Articular Surface Replacement (ASR) metal-on-metal hip implant that caused it to generate large amounts of metallic debris. The device, which designers expected to last 15 years or so, was voluntarily recalled in August of 2010.

The metal-on-metal implant is so-named because of the construction of the head and the cup's lining, which are made of cobalt chrome. The implant was designed to have minimal friction between the ball and socket joint as the surfaces within the joint were exceptionally smooth. In addition, designers expected a thin layer of bodily fluid to fill the joint, minimizing friction further.

Metal-on-metal is meant to last longer than the more traditional metal-ball-in-plastic-cup hip implant designs, which can wear out and require replacement within a patient's lifetime.

The ASR implant, however, did not perform as intended, and earlier this year, it was revealed that the implant failed a test in 2007 that pit the device against other Johnson & Johnson hip implants. Even more striking, an internal document from 2011 revealed that the company expected the ASR to fail in 40% of recipients five years after implantation.

The problem was related to a design flaw. An internal groove on the cup provided space for an orthopedic surgeon to use a tool to position the implant within a patient's hip. The groove, however, limited the surface area of the cup, making it more likely that the ball would rub up against the edge, generating significant friction and metallic debris in the process.

James Anderson, a DePuy engineer, testified in court earlier this year that a small change to the geometry "could represent a large improvement for many patients." Eliminating the groove would lead to roughly three times less wear in Anderson's estimation. 

Medical device companies such as J&J obviously want to make implantation as easy as possible for the surgeons, especially because the danger of infection is a major problem for orthopedic implants. The ASR fault appears to be that it may have been too accommodating, resulting in a device that was actually dangerous for patients.

Internal documents from J&J division DePuy revealed that the product failed when tested against the company's 36-mm Ultamet device. The 63-mm ASR was then tested against two other joints: a 28-mm CoCrMo Ultima metal-on-metal bearing and a 39-mm cast CoCrMO ASR metal-on-metal bearing, both of which had a "good clinical history." According to the tests, the ASR outperformed those products in terms of wear resistance.

An investigation by the BMJ (British Medical Journal) on the other hand recounts how experts were questioning the design's comprise with surgeon technique soon after it came on the European market in 2003:

David Langton, a surgical researcher at the University Hospital of North Tees and Newcastle University, gave a 2008 presentation at the British Orthopaedic Association conference where he also cited the shallowness of the ASR cup as an issue, saying that it was causing increased wear at the cup's edge, according to the BMJ.

Medical device companies such as J&J obviously want to make implantation as easy as possible for the surgeons, especially because the danger of infection is a major problem for orthopedic implants. The ASR fault appears to be that it may have been too accommodating in this regard, resulting in a device that was actually dangerous for patients.

The BMJ also raised a host of concerns that the ARS's story raised about the European and U.S. medtech regulatory systems, not to mention the cozy relationship between researching surgeons and companies such as J&J.

To be fair, metal-on-metal hip implants to be having problems in general. At the time of writing, there appears to be no effective way to avoid the generation of all metallic debris over time, which can be toxic. Other metal-on-metal hip implant makers, like Biomet, are facing similar problems. Biomet is facing litigation related to its M2a Magnum hip implants, for example. But the shallowness of the ASR's cup design seemed to compound the problem, leading to high friction along the edge of the cup.

Brian Buntz and Chris Newmarker are the editors of MPMN. Follow them on Twitter at @brian_buntz and Google+, and @newmarker and Google+.