I have read that people who have knee and hip replacements may have to have revision surgeries. What do they mean by that?

When a knee or hip has been replaced, there may be some issues that need to be corrected later on. In some cases, the hardware itself may break down or wear out, in other cases, the bones around the hardware may be causing some problems.

A revision surgery may be done if there is an infection in the joint, if the bones are wearing down or if the joint needs to be repaired or replaced.