Failure for instability in primary total knee arthroplasty

In the literature instability after total knee arthroplasty (TKA) is reported from 10% to 20% of cases being second to infection only. It is rarely reported as itself and the clinical examination and radiographic images should be attentively performed and checked in order to exclude this complication in every painful total knee arthroplasty. Instability can be divided into three different instability patterns: flexion, extension and genu recurvation. Anamnesis has to investigate index surgery, perioperative complications, type, location and onset time of pain. It is always recommended to exclude infection when analysing a painful TKA as first step. Radiographic analysis includes AP, lateral, full-length weight-bearing, and patellar views. Extension instability can be differentiated in symmetric or asymmetric. Asymmetric instability is more frequently observed and can be consequence of a iatrogenic ligament lesion. Flexion instability is difficult to diagnose and normally underestimated or confused and associated to surgical error in PS and CR designs or associated to late PCL rupture in CR designs. Genu recurvatum is very uncommon and is usually associated to underlying pathologies, given this, is the most challenging instability pattern to face in TKA. In conclusion instability is a difficult complication to diagnose and solve and, when suspected, a thorough investigation is necessary.