The mobile bearing OUKA has worked well in the medial compartment but the results of lateral arthroplasty have been marred by dislocation of the bearing. Dislocation is primarily due to the lax lateral ligament in flexion. Over the years, lateral OUKA has been successively improved but the dislocation rate still remains higher than the medial side.
Initially both the standard Phase 1 and Phase 2 OUKA components were used for lateral and medial unicompartmental replacement and both were implanted through a medial parapatellar approach with patellar dislocation. Due to the high dislocation rate, modifications to implant and technique were introduced. It was observed that the popliteus tendon sometimes wrapped around the bearing postero-laterally, tending to dislocate the bearing into the intercondylar space (Fig. 12.3). Therefore the tendon was routinely divided.
The entrapment of the medial bearing was 3 mm at the back and 5 mm at the front. The reason for the 3 mm entrapment at the back was that this was found to be the largest amount of entrapment that would allow the bearing to be inserted medially. However, on the lateral side, more entrapment could be used because of the lax ligaments. A symmetrical bearing, with entrapment of 5 mm posteriorly as well as anteriorly, was introduced. Unfortunately, despite these changes, the dislocation rate remained high. In 1996, Gunther et al. published the results of 53 Phase 1 and Phase 2 Oxford Knees implanted in Oxford (Gunther et al., 1996). The survival at 8 years was 76% (see Fig. 12.6 ‘early’). The main reason for the poor survival was that the dislocation rate was 10%. The survival with disease progression or implant loosening as failure was similar to that achieved on the medial side. As the dislocation rate was unacceptably high, it was recommended that the mobile bearing should not be used in the lateral side and a fixed bearing device should be used instead. However, if the problem of dislocation could be addressed, the results would be expected to be similar to those achieved on the medial side.
Figure 12.3 Lateral bearing dislocated onto the medial wall of the tibial component.
A radiographic study comparing knees that had and had not dislocated found that those that had dislocated were associated with a high lateral joint line (Robinson et al., 2002). This suggests that the dislocations were caused either by overstuffing the knee with the bearing too tight in extension, or by over-milling the distal femoral condyle and thus making the bearing too tight in flexion. It was therefore decided that the technique should be changed: the femoral component was implanted approximately anatomically to ensure that the distal femur was not over-milled and the bearing thickness was selected in extension so as to ensure the knee was not overstuffed. Usually, in lateral compartment OA, there is relatively little damage to the femoral condyle in full extension and 90° flexion, therefore it is straightforward to implant the femoral component anatomically by using instrumentation to ensure its surface is positioned where the retained articular cartilage was distally in full extension and posteriorly at 90° flexion. This ensures that the knee has the normal tightness in extension and the normal laxity in 90° flexion. In addition, a short lateral parapatellar approach was used without dislocation of the patella. Also, because it was observed that the bearing tended to track medially in extension, the vertical tibial saw cut was internally rotated by making it through the centre of the patellar tendon, and touching the medial side of the lateral femoral condyle. A clinical trial was undertaken using these surgical principles.
For the trial, Phase 3 femoral components were used together with Phase 2 tibial components (which were a more appropriate shape for the lateral tibial plateau) and symmetrical bearings with 5 mm entrapment at the front and back. Sixty-five components were implanted. There were three dislocations giving a 5% dislocation rate and a survival at four years of 98% (see Fig. 12.6 ‘flat’) (Pandit et al., 2010).
This dislocation rate of 5% was still unacceptably high so further modifications were made to the implant. It was observed that, with the Phase 3 components, although normal laxity of the knee was achieved in 90° flexion, in high flexion the knee became very tight and rollback was restricted. It was felt that the reason for this was that in the native knee, the femur tended to drop down on a normal convex tibial plateau (Fig. 12.1(b)), whereas this could not happen with a flat tibial component. It was therefore decided to use a convex tibial component. The surface of the component was spherical to achieve full congruity with a biconcave meniscal bearing in all positions (Fig. 12.4).
Figure 12.4. Diagram of the domed tibial design for lateral arthroplasty with a bi-concave meniscal bearing. The entrapment is increased compared to the flat tibia by distance y1. Both articular surface pairs are spherical.
A study by Baré et al. (2006) demonstrated that the optimal radius for the tibial sphere was about 75 mm. Cadaver studies were undertaken which demonstrated that normal rollback was achieved with the domed tibia (Fig. 12.5) and that the rollback was greater than with the flat tibia. The biconcave bearing (Fig. 12.4) increases the entrapment and, in theory, for a straight AP dislocation it should increase the entrapment from 5 mm to 7 mm.
Figure 12.5 Cadaver study of the domed lateral OUKA with lateral structures removed demonstrating the mobile bearing moving posteriorly in flexion.
A clinical trial was undertaken by the two designer surgeons, using domed components and a modified surgical technique designed for the Phase 3 implants. At eight years in a series of 265 consecutive implants, a total of 13 knees (4.9%) had re-operations (Weston-Simons et al., 2014). Four knees (1.5%) were reoperated for dislocation. Two were primary dislocations giving a primary dislocation rate of 0.8%. The remaining two were secondary to trauma. In both cases, when the knee was explored to reduce the dislocation, ligament damage was seen. Figure 12.6 shows that the survival and dislocation rate has progressively improved during the various developments of the lateral OUKA. With the domed lateral, while the dislocation rate is still higher than on the medial side we believe it is low enough for experienced surgeons to use the device.
