Beautiful Decay

How do prosthetic joints wear out? MGH researchers count the ways—in an attempt to improve materials.

More than a million people use artificial hip and knee joints designed at the Harris Orthopaedic Biomechanics and Biomaterials Laboratory at Massachusetts General Hospital. At the lab, testing machines run day and night, compressing years of steps into a few months—but however sophisticated those simulations may be, they can’t reproduce the stress endured by an artificial joint in the real world, inside a real body. For that, Harris Lab researchers study artificial joints retrieved from patients; the wear and tear seen in these earlier models, sometimes installed decades ago, inform the new designs.

“Patients differ in their activity levels, the loads applied to their joints, and the composition of the fluids that are absorbed into the joints’ bearing surfaces,” says Shannon Rowell, who manages retrieval research at the lab. “To determine how the joints perform in the long term, we need to look at them after they’ve been implanted in patients. Such studies provide all the unique variables we may not be able to predict or replicate in the lab.”

Here are some extreme examples of prosthetic joints’ beautiful decay.

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A replacement hip joint has two essential features: a spherical head attached to a long stem that fits into the shaft of the femur, and a semispherical lined shell that’s implanted in the pelvis. The ball-and-socket arrangement, shown here in a deteriorated implant retrieved from a 71-year-old woman, allows the leg to swing. (Bruce Peterson for Proto)

The 16 years of service provided by this artificial knee joint, recovered from an 82-year-old man, is evident in its pits, flakes and embedded metal particles. (Bruce Peterson for Proto)

After four years, bone had barely grown into the metal shell of this implant recovered from a 55-year-old man—its surface is essentially unmarked. The loose component caused instability that begot an irregular, oval-shaped wear pattern on the plastic liner inside. Over time the plastic could have worn through. (Bruce Peterson for Proto)

Deterioration is more apparent in the liner of this joint, taken from an 87-year-old woman after she reported hip pain following a fall. The impact dislocated the shell liner, which the femoral implant had already worn through, and the metal-on-metal abrasion that resulted dislodged metal particles that became embedded in the plastic liner. (Bruce Peterson for Proto)

“When we see metal particles embedded in the plastic, we can assume they’re in the surrounding tissue as well,” Rowell explains. “An immune reaction caused by plastic or metal debris can make the implant unstable.” (Bruce Peterson for Proto)

Parts of this socket disintegrated after 17 years. “Such sockets are made of older materials are examples of some of the worst outcomes, with dramatic wear and oxidation,” says Rowell. “Through radiation cross-linking, thermal processing methods and the infusion of stabilizing agents, we’ve made significant improvements over the past 15 years.” (Bruce Peterson for Proto)

After 20 years, this polyethylene hip liner had worn through completely. Yet plastic remains an excellent bearing surface, Rowell notes. “The newest materials have reduced wear rates and improved oxidative stability. This type of damage isn’t something I see coming out of patients with modern polyethylene bearings at this stage.” By infusing its new highly cross-linked polyethylene bearing surfaces with antioxidants, the Harris Lab has been able to slow the degradation. (Bruce Peterson for Proto)

This 26-year-old knee implant is a train wreck of problems: The U-shaped plastic tibial bearing is worn through, and the patellar button is brittle and visibly falling apart. “We’ve improved the material so it doesn’t wear in the same way,” explains Rowell. “Where we were getting 10 or 20 years, now we’re hoping we can push it much further.” (Bruce Peterson for Proto)