Decreased knee motion following ACL surgery is expected. Over the next three to nine months, this motion gradually improves to your normal level before surgery. In 5 to 10 out of 100 patients, a long-term decrease in knee motion occurs. This usually happens in patients who had decreased or poor knee motion before the surgery. It can also occur because of scar formation inside the knee. Most of the time, this can be corrected with aggressive therapy and knee bracing, but may require an operation if therapy and bracing are unsuccessful.