The short answer from an orthopedic surgeon
Q: I had knee replacement surgery four months ago. Why do I still have pain?
A: Recovery from surgery can take several months, so it’s not unusual to still have soreness in the knee that was replaced. As the intensity of rehabilitation exercises increases, more strain is put on the muscles and joints that have not been used in a period of time. Stick with the routine of exercises that were prescribed by a physical therapist. Though bending and stretching the knee may hurt at first, re-training the body to move normally will help with recovery in the long run.
In addition, postoperative swelling is often a major source of pain for patients. Pain in the knee following the operation can last from six months to as long as one year, but don’t get discouraged. Once fully healed, knee replacement surgery will help provide pain relief and improved function long-term