Even the old injury to the knee can affect the normal mobility of the person. We cannot leave aside thinking that the injury is a mild one and had happened very long ago and that it will not disturb the day to day activities. Some mild and moderate injuries can be treated with physical therapy, exercises, hot and cold therapy. But if the injury is a serious one, you may have to undergo surgery.
If there is a continuous loss of motion, unable to bend or extend the knee to the full length, then it is the sign for immediate surgery requirement. If the person is not able to carry his or her own weight in the knee, or a pop sound heard due to a twist or turn in the knee, then surgery is an inevitable one. If the inflammation and pain do not subside in regular rest and physical therapy, then surgery has to be done after a complete examination of the affected part through X-ray, scan, etc.
Surgery is the most appropriate treatment to get relieved of pain, be it arthroscopic surgery or knee replacement surgery.