When the ACL is torn and the patient is assessed for reconstruction, the surgeon needs to carefully analyse the laxity, which may be increased due to damaged collateral ligaments, meniscus, and/or structures of the posterolateral corner.

It may be a difficult problem to determine the contribution of each to the resultant instability. Patients who are considering revision reconstruction must undergo a comprehensive evaluation by the surgeon that includes observation of gait during walking and analysis of overall limb alignment.


Failure to adequately address the posterolateral corner

The structures of the posterolateral corner play a major role in stabilising the knee. When the posterolateral corner is damaged, the patient may develop an abnormal gait with excessive knee hyperextension (knee bending backward) when weight is placed on the leg. The gait abnormality may be associated with severe quadriceps muscle wasting and pain in the medial joint line due to increased compressive forces as the leg starts to bow (varus malalignment).

The posterolateral corner is the region to the back of the knee on the outer side - the lateral collateral ligament (LCL), the popliteus muscle-tendon-ligament unit, the joint capsule, the fabellofibular ligament and the arcuate ligament.

Isolated injuries to these structures are rare - they are usually combined with a torn ACL, PCL or both.

The anatomy of the posterolateral corner is complex, and the surgeon may either fail to appreciate the contribution of this region to the patient's instability or not be confident to make correct the damage. In failed ACL reconstructions where the posterolateral corner remains incompetent, we have found also that attempts at reconstruction frequently fail because the reconstruction has been 'non-anatomical'; that is, the grafts were not placed and securely fixed in anatomical ligament attachment sites. This non-anatomical group includes the procedures of suture repair, extra-articular iliotibial band augmentation, and biceps tendon routing.


Failure to stabilise existing meniscal damage

When there has been a cruciate ligament injury, the meniscus frequently suffers damage also. This may be as a direct result of the initial injury, or as a consequence of instability associated with cruciate deficiency.

If a meniscus tear has not been stabilised during the initial ACL surgery, the resultant instability will contribute to strain on the ACL graft.