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The Cruciate Ligaments - A Primer

The torn posterior cruciate ligament (PCL)

Isolated PCL tears cause pain, some swelling and limited range of motion. Instability is not initially a major complaint.

The PCL is much bigger and stronger than the ACL, and is injured much less frequently.

Disruption or 'avulsion' of the posterior cruciate ligament is diagnosed from the 'history', the 'examination' and 'special investigations'.

History of PCL injury

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
  • Isolated 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').

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Examination of the PCL injured knee

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

The sag sign is a simple test of posterior cruciate ligament integrity. 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'. The test is called a 'reverse pivot shift' because the manoeuvre of extending the knee rather than flexing the knee is opposite to that for the more common pivot shift test for an ACL tear.

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Special investigations in PCL injury

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.

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