There’s specific criteria used to determine whether a knee replacement is medically necessary. In most cases, the knee joint must be worn or damaged to the point of reduced mobility or significant pain, even when you’re at rest. Doctors will order knee replacement surgery when other interventions, like weight loss, pain medication, cortisone shots or walking assistance devices, are no longer providing relief. The symptoms also must be severely affecting your quality of life, and the benefits of the surgery must outweigh any risks.