The Painful Joint Prosthesis

Description: Even though prosthetic joints are a Good Thing, they are not a source of unalloyed joy to their recipients. Loosening and infection are two reasons why. There are other causes for a painful prosthetic joint, but these two items are always uppermost in the minds of orthopedists.
Why does a good prosthesis go bad? Largely because they are composed of foreign stuff that acts differently from the original bone. First of all, consider Wolfe's Law of Bone, which is that bone is formed and retained along the lines of stress in that bone. Another way of putting it is: "Form follows function." Look at the trabecular pattern in a calcaneus or a proximal femur -- it's easy to see where the lines of stress are here, because that's where the trabeculae are. Another way to express this rule is: "Use it or lose it." Bone is an amazingly dynamic tissue in this respect. Something as innocuous as spending the night in bed causes a definite but small degree of calcium loss from the bones. If one underuses a body part for a longer period, a striking amount of bone loss can be seen. A good example of this is the disuse osteoporosis seen following casting of an injured extremity. This can easily be seen on follow-up radiographs. In order to see any bone loss whatsoever on plain radiographs, one must first lose 30 - 50 % of one's bone mass. When one considers this, the disuse osteoporosis seen after casting seems even more impressive.