This complication was introduced into surgery with the invention of mobile-bearing knee arthroplasty (Goodfellow et al, 1974). In the NJR, the incidence is reported as 1.2 revisions for dislocation/subluxation per 1000 component years (95% CI 1.05 – 1.37) for mobile bearing UKA. This dislocation rate may be low because surgeons may not consider dislocation to be a failure. In a meta-analysis of published or presented studies on Phase 3 OUKA, it is 0.73% (34 studies, total number of knees 10,125 and 74 dislocations at a mean follow up of 54 months [range 24-138 months]). In our 1000 Phase 3 cohort, the dislocation rate was 0.5%. Most dislocations occur early (Pandit et al, 2011).
Causes
Primary dislocations are usually caused by a combination of distraction of the joint and displacement of the bearing due to impingement. They are the most common type of dislocation. They occur early and are usually due to surgical error.
The following mistakes all increase the risk of dislocation (the mechanism of entrapment is explained in detail in Chapter 6).
1. Failure to remove osteophytes from the back of the femoral condyle causing impingement in flexion, stretching of the ligaments and anterior displacement of the bearing, particularly in patients who achieve high degrees of flexion.
2. Inequality of the 110° and 20° flexion gaps or MCL damage.
3. Retained cement protruding above the tibial plateau surface.
4. Femoral component (and therefore the bearing) sited too far from the lateral wall of the tibial component so that the bearing is free to rotate through 90°. This is unlikely to be a problem with anatomic bearings, which are now routinely used (see Fig. 6.14).
5. A bearing that is much too thin relative to gap width may, theoretically, dislocate, and beginners, fearful of dislocation, tend to insert the thickest bearing possible, but this is a mistake. ‘Overstuffing’ the knee should be avoided as it increases the risk of dislocation, pain and delayed recovery.
Secondary dislocation is the result of loss of entrapment from loosening (and subsidence) of the metal components. Spontaneous elongation of ligaments over time does not seem to occur unless there is impingement, when forced flexion or extension may stretch ligaments. Secondary dislocation is rare.
Traumatic dislocation has occasionally been encountered when a normally functioning OUKA has been forced into an extreme posture and the MCL has been momentarily stretched or damaged.
Diagnosis
Dislocation occurs when the knee is unloaded or at the moment when load is re-applied, for example rising from a chair or getting out of bed. It is usually a dramatic event and the patient seeks urgent advice, but dislocation can occur relatively silently. Walking may be resumed with the bearing displaced; the weight is borne (painlessly) through the opposed metal components.
Radiographs demonstrate the site of the displaced bearing, and may suggest its cause (e.g. osteophytes, retained cement, or displacement of a metal component).
Figure 11.6 (a) Anterior and (b) posterior dislocation of the bearing.
Since the anterior rim of the bearing is higher than its posterior rim, posterior dislocation requires more distraction of the joint than anterior dislocation (5 mm compared with 3 mm). Therefore, the displaced bearing is most commonly found in the anterior joint space, and occasionally in the suprapatellar pouch (Fig. 11.6(a)). Displacement into the posterior joint space (Fig. 11.6(b)) suggests that the bearing has rotated through 90° (point (2) above), from which position it is as easy for it to dislocate backwards as forwards as the entrapment has decreased to 2 mm. Introduction of the anatomical bearing has significantly reduced the incidence of posterior dislocation, as the long lateral side prevents bearing rotation. Occasionally, the bearing is found to be tilted into the intercondylar space where it may stabilise in a subluxed position (see Chapter 12).
