Instability in primary total knee arthroplasty.

Instability is one of the most common causes of failure of total knee arthroplasty (TKA). The presentation can vary from pain to frank dislocation with the etiologies just as varied. Instability after TKA can be classified by where the instability occurs in the knee's arc of motion as well as the chronicity of the problem. Acute instability is related to intraoperative injuries or excessive release of important coronal stabilizers such as the medial collateral ligament in extension or the posterolateral corner in flexion. Chronic instability in extension is often related to varus/valgus malalignment. Chronic instability in flexion can be related to an undersized femoral component, excessive tibial slope, or excessive elevation of the joint line affecting the isometry of the collateral ligaments in midflexion. Recurvatum instability is a rare complication that often coincides with extensor mechanism dysfunction or neuromuscular disorders. When addressing instability after TKA, it is critical to determine the root cause of the problem as well as evaluate for other causes of pain such as infection or aseptic loosening. When revision surgery is warranted, it should follow the basic principles of restoring a neutral mechanical alignment, setting the appropriate component rotation, balancing the flexion and extension spaces, and restoring the height of the native joint line.