Disruption (break) or 'avulsion' (pulling off) of the anterior cruciate ligament is diagnosed from the 'history', the 'examination' and 'special investigations'.
These are common features of the ACL patient's history (story):
- Injury occurred while
- cutting and running to that side during fast activity (non-contact)
- landing from a twisting jump on that leg, while the body still has momentum and continues to twist (non-contact - torsion)
- fouled tackle (contact) (minority)
- A loud 'POP' is frequently heard
- Generally the person is unable to continue with the activity and has to be assisted home
- The person usually feels that something really bad has happened to the knee
- The knee usually swells up badly after the injury
- The feeling of instability is a major complaint
- Other structures are frequently damaged at the same time
- Even if not damaged at the same time, damage to other structures may occur later due to the instability of the joint. The 'unhappy triad' of O'Donoghue is a combination of cruciate, collateral and meniscus tears.
Examination for anterior cruciate ligament laxity is likely to include -
- Anterior drawer test - 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.
- Lachman test - similar but a bit more subtle.
- Pivot shift test - 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.