Disruption (break) or 'avulsion' (pulling off) of the anterior cruciate ligament is diagnosed from the 'history', the 'examination' and 'special investigations'.
History of ACL injury
Patients who have suffered an anterior cruciate ligament injury (ACL injury) tend to report similar stories.
- The injury to the ACL often occurs while cutting and running to that side during a fast non-contact activity, or from landing from a twisting jump on that leg while the body still has momentum and continues to twist. A contact injury like a fouled tackle is much less common.
- The person, and even those close by, may have heard a loud 'POP' after which 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, and the person may report that they visited the emergency department and had blood syringed out of the knee.
- X-rays taken at the same time tend to show no bony damage.
- Once the early symptoms settle the patient may continue to experience uncomfortable feelings of instability of the knee and find that they do not have the confidence to return to the original sporting level.
Other structures are frequently damaged at the same time as the cruciate ligament injury, for example there may be associated tears of the meniscus and the collateral ligaments. Even if there has been no other damage at the time of the initial cruciate injury, the subsequent instability of the knee joint may result in later damage to the meniscus.
Examination for ACL injury
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.
The Lachman Test also assesses laxity of the anterior cruciate ligament.
The patient lies on his/her back on the examination couch. The examiner stands alongside the knee, facing the patient, and gently takes the knee into both hands, with the upper hand holding the thigh firmly just above the knee, and the lower hand holding the lower leg firmly with the fingers behind the leg and the thumb on the tibial tuberosity (the bony lump just below the kneecap).
The knee is bent to 20-30 degrees.
The lower leg is pulled forwards while the thigh is held firmly in position, and the examiner feels for any forward movement of the lower leg.
The pivot shift test assesses anterolateral rotational laxity. 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'. Traditionally this was simply considered to be a test for deficiency of the anterior cruciate ligament, but marked pivot shift is now thought to be indicative of additional deficiency of the anterolateral ligament.
The pivot shift. Lane CG, Warren R and Pearle AD. J Am Acad Orthop Surg. 2008 Dec;16(12):679-88.
Special investigations for ACL injury
Special investigations might include -
- KT2000 test - this is an instrumented test, like a the anterior drawer test but with the KT2000 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.