My wife is a former semipro athlete. She was a track and field star in high school and earned a place on the Stanford University team,  51  taking second in the Pac-10 championship in the heptathlon. She made it  52  the Olympic trials before retiring, at the age of 23, due to bad knees.

But even in  53 , man, could she run. In the early days of our romance, I introduced her to Ultimate Frisbee, a fledgling sport  54  a co-ed team of soccer and lacrosse refugees chase a disc around a field. Where the rest of us had two gears—jogging and running—my wife had something like four or five. She also had a 30-inch vertical and could outleap anyone guarding her.

But soon she began spending more of each game with ice  55  around her swollen knees. Before long, her physician advised her to find a form of exercise that didn't involve running. The meniscus in each knee, after two surgeries and nearly a decade of seven-event training, is in tatters. Between now and the end of her fife, she can only hope to fight a rearguard action against pain.

Each year in the U.S., 340 000 people like my wife  56  knee replacements, and thousands more require meniscal repair or ACL surgery. We need new knees.

Bioprinting could provide them. Researchers can now print body parts, such as skin and ears. Tissue engineering has even  57  a prototype knee meniscus. Full-fledged organs could be next. Automating their manufacture could  58  waiting on a donor list. Consider Sarah Murnaghan, a 10-year old suffering from end-stage cystic fibrosis. She had finally found a lung donor after  59  for months without hope. Many patients never find a donor. I think it's clear that this is about more than just knee joints.

But knees are what I think of. My wife's Olympic dreams are over. But our two-year old daughter is already unimaginably  60  and fast. Bioprinting could give my wife—and many parents—the upgrade necessary to keep up.