Dislocations commonly occur medially over the wall of the tibial component, but can occur in other directions such as anteriorly, posteriorly or laterally. A medial dislocation should really be considered to be a subluxation as the knee may continue to function well and the patient may not be aware of the problem. A dislocation may reduce spontaneously when the patient is anaesthetised or can occasionally be reduced by manipulation. More commonly, however, the old incision has to be opened and the bearing retrieved under direct vision. A careful exploration is undertaken to identify any potential causes for bearing dislocation, for example, impingement, component loosening, bowstringing of popliteus or ligament injury. A trial bearing is inserted to aid assessment, and the retrieved bearing is inspected for evidence of impingement. These problems are addressed as required, and a new bearing, usually one size thicker, is inserted. Recurrent dislocation is unlikely but can occur.
We have introduced a technique to prevent recurrent medial dislocation (Weston-Simons et al., 2011). The aim is to decrease the gap between the femoral and tibial components through which the bearing dislocates. Two 3.5 mm AO screws about 24 mm in length are inserted under direct vision just medial to the tibial wall one third and two thirds of the way back (Fig. 12.16). They are angled posteriorly and medially. The screws are advanced until their heads sit on top of the wall, thus acting as an extension of the wall. They are tightened so that they do not overhang the wall laterally and therefore will not touch the bearing. A trial reduction, using a bearing that is just tight in extension, must be carried out. The knee is put through a full range of movement and the screw heads are observed to ensure that they do not hit the femoral component or the bearing. If the screws touch the bearing, they should be tightened. If they touch the femoral component they should be removed. A definitive bearing is then inserted.
Weston-Simons et al. (2011) reported on seven cases using this technique. One recurrent dislocation was identified months after the stabilisation of the bearing with screws. During exploration, it was found that the lateral collateral ligament was stretched and slack, presumably because the bearing had been dislocated for an extended period (over 6 months) before the original dislocation was treated. As a result, the tibial component was revised to a fixed bearing component. In all the other cases, this technique has prevented further dislocations. The patients recover quickly and the range of movement is regained rapidly. The clinical outcome scores, both objective as well as functional, are as good as those without bearing dislocation.
