There are four type of knee instability each characterised by different directional instability based on structures involved.

Anteromedial Instability (AMRI)

Anteromedial instability relates to an unstable to medial knee compartment caused by chronic medical collateral ligament damage/laxity.

The pattern of instability noted is anterior displacement of the medial tibial plateau on the medial femoral condyle. The lateral compartment remains stable unless any ACL ligament involvement.

Anterolateral Instability

Anterolateral instability is associated with ACL ligament injuries. Instability is characterised by a posterior and lateral displacement of the lateral femoral condyle on the tibial plateau.

Posteromedial Instability

Posteromedial instability can be caused by damage to the posterior horn of medial meniscus, superficial and deep fibres of the MCL and the posterior oblique ligament. This type of injury is usually associated with multi-ligamentous injuries.

Posterolateral instability (PLRI)

This pattern of instability is characterised by posterior displacement of the lateral tibial plateau in relation to the lateral femoral condyle. Injury to the posterolateral corner associated with this type of instability. The posterolateral corner consists of the lateral collateral ligament, popliteal ligament and the popliteus tendon complex.