Diagnosis and management of patients with instability of the knee.

Instability of a total knee arthroplasty is a fundamentally different problem from instability of the knee without an arthroplasty. Revision surgery to correct the inciting forces will usually be necessary, and ligament reconstruction alone is usually unsuccessful. Because it may be the presenting complaint for any of the usual conditions that require revision arthroplasty, instability as reported by a patient should be considered a symptom that requires detailed evaluation rather than immediate surgery. Evaluation should be systematic and comprehensive, meaning that the same algorithm or system should be applied to all knee arthroplasties, and all diagnostic entities should be considered. There are several common types of instability, each requiring a different surgical strategy. Any dysfunction of the extensor mechanism, including pain inhibition (even from the ipsilateral hip) may result in buckling. Structural recurvatum, often originating from relative quadriceps weakness, may require arthrodesis if extensor function is completely absent. Varus or valgus instability will require stabilization in the form of constrained implants, with or without ligament releases, advancements, or substitution. Realignment will almost always be advantageous. Flexion instability is invariably linked to flexion gaps that are larger or more lax than the extension gap, requiring revision with attention to gap balance, and in many cases, some degree of mechanically constrained devices. Arthritic knee joints in obese patients and those with severe angular deformity or fixed flexion contractures are at particular risk for instability after total knee arthroplasty. Instability that becomes apparent intraoperatively is a challenging condition, particularly when there is no immediate recourse to using constrained implants.