Inside a bright, freshly remodeled basement lab at Dartmouth College's Thayer School of Engineering, John Collier slides a small white rectangle of polyethylene plastic toward me. He and I, along with his colleague Michael Mayor, are sitting at a large oak table covered with failed orthopedic implants - shiny cobalt balls and white plastic sockets, titanium stems once thrust into femurs to hold artificial hips in place. Over the last three decades, Collier, an engineer, and Mayor, a surgeon, have collected and tested nearly 9,000 artificial joints to determine how and why they failed. No lab has room for all those parts, so the vast majority of the implant collection is stored off campus in a rented warehouse.
The device in front of me is a component of a knee joint, which closely resembles its more famous polyethylene cousin, the kitchen cutting board. Next, Collier hands me a matching polyethylene part from a second knee joint. Yellowed and disintegrating, it looks oddly like a crumbling rind of aged cheese. The first piece, Collier says, spent 17 years inside someone's knee. The second deteriorated in less than five. "We ask, 'How did it get in this state, and what does that mean for other patients?'" he says.
Collier and Mayor first came together in the early 1970s over a shared interest in then-new devices known as porous-coated implants. An alternative to the brittle bone cement used at the time to hold implants in place, porous-coated surfaces allow human tissue to grow into the implant, creating a living interface between metal and bone. To better understand how porous coatings performed in the body, the pair began studying those removed because of complications, implant failure, or patient death. With that, their "retrieval analysis lab" was born.
A new part arrives in the mail nearly every day. Some 800 surgeons participate, at their own expense, packaging freshly excised implants in ethanol and shipping them to Dartmouth. In a back room that looks like half lab and half wood shop, Collier, Mayor, and a rotating crew of students measure, dismantle, and slice. They test the chemical and mechanical properties of the materials and look for signs of corrosion and wear.
According to the American Academy of Orthopedic Surgeons, more than 700,000 total hip and knee replacements were performed in 2004. The same year, surgeons performed 86,000 revision surgeries to repair or replace an artificial joint. Any number of things can go wrong with an implant. Microscopic specks of plastic or metal can slough off and trigger an immune attack on the implant. Metal can erode, causing parts to become uncomfortably misshapen - a problem Collier demonstrates by displaying a cobalt hip ball worn from a sphere into an irregular ovoid after just two years. According to Collier, "the biggest problem now, as it has been for 27 years, is the new and different ways for polyethylene to fail."
About a decade ago, he and Mayor found that the gamma radiation used to sterilize implants created free radicals that caused polyethylene to oxidize and weaken over time. As it turned out, the crumbling knee joint Collier showed me was a victim of oxidation. To solve the problem, Collier says, manufacturers began irradiating implants in the absence of oxygen.
Manufacturers thought the issue was solved. Collier and Mayor weren't so sure. They continued to study polyethylene, and this year reported that, perhaps not surprisingly, irradiated pieces continue to oxidize in the body (J Bone Joint Surg Am, 89:2023-9, 2007; J Arthroplasty 22:721-31, 2007). "We discovered that the manufacturers' solution to the problem of oxidation was flawed," Collier says. One possible remedy involves heating the material after irradiation, which halts the oxidization process. "But radiation and heating together weaken the material," notes Collier. For the moment, he says, doctors must decide whether individual patients need implants built for strength or for wear resistance.
The pair began the oxidation study five years ago, an opportune time for Mayor, who shortly thereafter received a knee implant of his own. He requested a DePuy Sigma rotating platform knee, a high-tech device that pivots on a cobalt chrome platform to adjust its position below the femur. He specified a unique plastic processing method, too, requesting that the implant be "moderately cross-linked" - that is, sterilized with a midlevel dose of radiation to split the difference between wear resistance and strength. Even with his inside knowledge, he admits, going in for the implant gave him pause. Still, he says, "it gave the colleague I asked to do [the surgery] even more pause."