Table 2.

Patterns of instability and contributing factors

Mediolateral instability

Malpositioning

Ligament imbalance

Inadequate implant selection

Anteroposterior instability (Rare in extension, usually associated with flexion instability)

Traumatism

Polyethylene post breakage

Hyperextension

Extensor mechanism incompetence

Flexion instability

Early form (Usually associated with PCL incompetence, AP and ML instability)

Flexion–extension mismatch

Poor offset restitution: Small femoral component, anterior or extension placement

Excessive tibial posterior slope

Thin polyethylene insert to compensate for thigh extension gap

Displacement of the joint line making the collateral ligaments non functional

Iatrogenic damage of PCL or exhaustive release of posterolateral structures

Inadequate implant selection

Late form (AP, not ML flexion instability)

Late rupture or degeneration of the PCL

Extensor mechanism incompetence

Rotational instability

Rotational instability (Usually associated with flexion instability)

Implant malpositioning

Collateral ligament imbalance