Researchers at the Steadman-Hawkins Research Foundation in Vail, Colorado have been researching this very problem. They noticed some of their ACL patients were just fine for 10 years -- a perfect outcome. Then all of a sudden, they developed arthritis.

They think the problem may be a lack of mobility between the patellar tendon and the tibia (lower leg bone). A condition referred to as patella infera may be part of the problem. With patella infera, there is a permanent shortening of the patellar ligament. The kneecap sits too low in relation to femur (thighbone). The result can be a severely limited range of motion of the knee joint.

Patella infera is a common complication of injury or surgery to the knee joint. It usually doesn't show up until much time has passed after injury and/or surgical repair.

Treatment options include physical therapy to manually release the kneecap and/or surgery to revise the soft tissues around the knee. If the joint degeneration has gone too far for conservative care to be successful, then total knee replacement may be needed.