Disruption (break) or 'avulsion' (pulling off) of the anterior cruciate ligament is diagnosed from the 'history', the 'examination' and 'special investigations'.
Examination for anterior cruciate ligament laxity is likely to include -
With the patient in the position above, the surgeon pulls the tibia forward (the 'shinbone') (anterior drawer) and pushes it backwards (posterior drawer). Too much forward movement ('anterior translation') suggests ACL tear; too much backward movement ('posterior translation') suggests PCL tear. It is important that the affected side is compared to the unaffected side, as some people normally have quite lax cruciates.
Similar but a bit more subtle.
The surgeon lifts the leg up and tucks the foot under his arm. Pushing the knee a bit towards the middle, he gently bends the knee joint and a lax cruciate ligament allows a sudden jerky movement in the joint - the 'pivot shift'.
Special investigations might include -
- KT2000 test - this is an instrumented test, like a the anterior drawer test but with an instrument doing the pulling.
- MRI scan - The cruciate ligaments can easily be seen on MRI scan, and a totally disrupted ('exploded') ligament is easily diagnosed. The problem comes when:
- a ligament is not torn but has pulled off its attachment above or below. It is totally incompetent, but may look normal on MRI.
- the sort of 'sheath' in which the cruciate glides is intact, but the ligament within it is totally torn. This may confuse the doctor and appear normal.