The unstable total knee: causes and cures

While there are a number of mechanism of failure of total knee replacement including aseptic loosening, fracture, infection, RSD, etc., perhaps the most common cause of failure relates to instability. Instability may present early or late and the etiology of the instability is quite different depending upon when it is seen.

For example, if one sees an unstable total knee within the first few weeks or months the patient may present with symptoms of catching or giving way with associated unsatisfactory function. The etiology of these events is multifactorial but most commonly relates to malalignment of the components and failure of restoration of the biomechanical axis of the limb. Other causes might include an imbalance of the flexion/extension space, particularly with a loose flexion space resulting in mid flexion instability or rupture of the posterior cruciate ligament or medial collateral ligament. Other causes of early instability relate to the extensor mechanism. It is not unusual in primary total knee replacement to inadequately balance the extensor mechanism such that lateral subluxation of the patella may occur. This is more common in the fixed valgus knee where the quadriceps mechanism is already functionally shortened resulting in difficulty with patellar tracking once it is relocated. Other early causes of an unstable total knee referable to the extensor mechanism also must include patellar fractures. (Table 1). When examining a patient with a history of catching, locking or instability, it is important for the surgeon to diagnose the direction of the instability (varus, valgus or sagittal) and also to determine the underlying cause. Post op varus/valgus instability is most commonly associated with an injury or ruptured medial collateral ligament, conversely, post-operative sagittal plane instability in a cruciate retaining knee probably signals an injury to the posterior cruciate ligament.