A Nimbler Knuckle
These pieces, when paired, form a sleek facsimile of the main knuckle (the metacarpophalangeal joint), which is often gnarled by arthritis. Made of pyrolytic carbon, the same ultra-durable material used in heart valves and nuclear reactor fuel, the prosthesis has an elasticity similar to bone. This is an important feature: If the prosthesis is too soft, like polyethylene, bone might deform it; if it’s too hard, like zircon titanium, it might wear down the bone. 

This knuckle is just one example of more than 30 models that have emerged since the metallic hinge—the first finger joint replacement—which was so simple that it was supposedly designed on a cocktail napkin. The best surgical candidate has only one or two severely arthritic digits. (Too much surgery on one hand can lead to exponential complications.) Yet even one fewer painful joint can provide immeasurable relief—and so be it if, as some patients have reported, the new one squeaks.

A Closer Shave
It used to be that the only surgical option for hip repair was to fuse the femur and pelvis, which impaired the patient’s gait. When total hip replacement came along in the early 1960s, it was lauded as the century’s greatest achievement in orthopedic surgery. The procedure now restores mobility to more than 200,000 patients each year.

But even this advance has had serious limitations: During surgery, the entire head and neck of the femur are replaced, and because the life expectancy of an implant rarely surpasses 20 years, younger patients may eventually need to endure replacement of the replacement. This is both complicated and risky, because little bone remains within which to anchor a new implant.