Manipulation can result in relocation. On a few occasions reduction has occurred, more or less spontaneously, under anaesthesia. However, arthrotomy is almost always required to remove the bearing and to determine the cause of its displacement. The bearing can usually be retrieved through a small anterior incision, even if it is in the back of the joint, but an additional posterior arthrotomy has sometimes been needed. We are aware of two cases in which the bearing, which could not be retrieved from the back of the knee, was left in the knee and did not cause problems (Tibrewal et al, 2014). The femoral component was dislodged on one occasion while retrieving the bearing and was successfully re-cemented.
Primary dislocation
When both the metal components are found to be securely fixed to the bones, other causes of dislocation need to be sought.
Any bone or cement that might impinge on the bearing is removed. Retained posterior femoral osteophytes can be removed with the posterior osteophyte chisel. An anatomical bearing, usually one size thicker, is inserted (see Fig. 6.14). It is important not to over tighten the ligaments.
If there is recurrent dislocation, MCL damage or a serious mismatch between the 110° and 20° flexion gaps, TKA should be performed. Since the introduction of a fixed-bearing tibial plateau to articulate with the OUKA femoral component, some surgeons have converted to this in cases where instability of the mobile bearing is the only defect in the arthroplasty. However, it should be noted that Australian Orthopaedic Association National Joint Replacement Registry data demonstrate that revisions of failed UKA to another UKA have generally been less successful than revisions from UKA to TKA (Hang et al, 2010). Recurrent dislocation is rare and should be treated by conversion to TKA.
Secondary dislocation
This is dealt with in the section below on loosening of a fixed component.
Traumatic dislocation
The few patients in which this has occurred have been successfully managed by either closed reduction of the displaced bearing or open insertion of a new bearing.
