At four months after surgery, it's not unusual to continue to have soreness in the knee that was operated on. This is a result of the increasing intensity of rehabilitation exercises, which put a tremendous amount of stress on muscles that have not been used for some time. Joint replacement surgery is the last step in treatment for arthritis. By the time a person has surgery, other treatments have failed to help and the condition usually has been going on for a long time. Pain and stiffness probably limited your mobility and your muscles became deconditioned. These muscles will be sore until you restore their tone. And that can take some time.