The PCL is much bigger and stronger than the ACL, and is injured much less frequently. When the ACL is also torn the PCL component is also often clinically missed.
Disruption or 'avulsion' of the posterior cruciate ligament is diagnosed from the 'history', the 'examination' and 'special investigations'.
The history may involve -
- Injury occurred while -
- falling onto the shin with a bent knee and foot pointed
- dashboard injury in a car
- hyperextension (leg bent backwards)
- excessive hyperflexion (forced bending)
- Often other ligaments are torn at the same time and the symptoms related to them usually dominate the history
- Isolate PCL tears cause pain, some swelling and limited range of motion. Instability is not initially a major complaint - but with time the patient becomes convinced that all is not well inside the knee.
- Note that in 50% of cases the ligament does not actually tear through its substance, but rather it gets pulled off its bony attachment above or below ('avulsion').
Examination for anterior cruciate ligament laxity are likely to include -
- Posterior 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). Backward movement ('posterior translation') suggests PCL tear. This test is highly sensitive for isolated PCL tears.
- Sag sign - With the patient lying on his/her back and the hips and knees bent to 90 degrees, the examiner holds both heels. With PCL tears the affected leg 'sags' below the good leg.
- Reverse pivot shift test - the surgeon lifts the leg up and tucks the foot under his arm with the foot turned out and some inwards force on the side of the knee. As the leg is straightened a lax posterior cruciate ligament allows a sudden jerky movement in the joint - the 'pivot shift'. It is called a 'reverse pivot shift' because the manoevre is done in an opposite way to that for the more common ACL tear.
Special investigations might include -
- KT2000 test - this is an instrumented test, like 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.