This is really cool. A girl in Texas got bone cancer (that's not the cool part) and had part of one femur replaced with a metal implant. The problem is her legs would grow, but the implant would not, so they would have to keep replacing it with bigger implants, and she faced maybe 10 more surgeries.
But the implant they used can be lengthened as the leg grows. Every so often, they put her leg in a magnetic donut and stretch the implant magnetically, saving months in the hospital and years of anxiety over operation after operation. What a nice thing to be able to do for a little girl.