Different surgeons define a failed ACL repair in different ways. Failure rates range from 10 to 25 per cent depending on how the failure is defined.
Some experts use joint laxity (looseness) as the main indicator. Tests of joint side-to-side motion show an excess of motion. This test is called the pivot-shift test. There may be forward and back laxity measured by the anterior drawer test.
A special measuring tool called the KT-1000 Arthrometer may be used to test ligament laxity. The KT1000 gives an objective measure of the forward-to-back motions of the tibia (lower leg bone) relative to the femur (thigh bone).
The Arthrometer gives accurate assessment of the integrity of the ACL. More than a 5-millimeter difference between sides suggests ACL instability. If the patient's knee gives way during daily activities or during sports or recreation, the joint is considered unstable and tested for ligament laxity.
Certainly a combination of all these tests can be used to assess ligamentous stability. A positive response to all three tests is a good sign that the ACL repair has failed and is not holding the joint stable.