Now imagine that the repair process of a bridge is undertaken not by engineers and workers but by the bridge material itself. That is what bone does.
Bone senses how it's being used and maintains a dynamic balance.
Bone's constant remodeling of itself involves complex mechanisms using special cells that suck up aging bone, and other special cells that lay down fresh bone. If bone is underused, as happens when astronauts travel in space, or even when a broken limb is in a cast, it thins and weakens. On the other hand, an overloaded bone thickens, strengthening the stressed area, as can be seen in the greater mass of a tennis player's dominant arm. Nature can handle most repairs, but when a healer intervenes to help make sure the healing is straight and firm, both engineering and biology must be applied. Until our era, most improvements in orthopedic care derived from progress in engineering-better braces and casts, the invention 60 years ago of pins and screws that can stabilize fractures, and the development almost 40 years ago of artificial joints like the Instacone.