The aim of the final step of preparation is to select the bearing thickness in extension to restore the ligament tension and leg alignment to normal. Currently a one-peg femoral component should be used. A two-peg component with rounded anterior edges is available and can be used.
The surgeon must ensure that all impingement is prevented. Any posterior femoral osteophytes are removed with the chisel and slotted anti-impingement guide. The anterior mill must not be used on the lateral side as this will remove too much bone and the patella can jam in the defect, causing the knee to lock in flexion. The surgeon should do a trial reduction with an appropriate size of bearing to confirm that the bearing does not impinge on bone anteriorly. If there is anterior impingement, a small amount of bone should be removed from in front of the femoral component to prevent this. Care must be taken not to remove too much anterior femoral bone due to the risk of the patella jamming in the defect.
It is also important to ensure the bearing does not jam against the wall in full extension. If a thin dissector put between the bearing and the wall cannot easily be removed in full extension, the vertical cut will need to be repeated 2 mm more medially. If during trial reduction the bearing subluxes forward in high flexion and there is no posterior impingement, then the popliteus tendon should be divided. This may help prevent dislocation.
Final tibial preparation
The final tibial preparation is similar to that of the medial side.
The template must be flush with the posterior cortex and should not overhang in the region of Gerdy’s tubercle. The cemented keel cut saw is used to prepare the keel slot. The cement pick should be used to clear out the bottom of the keel slot.
Careful cementation is then undertaken. Care must be taken to remove all retained cement. Definitive bearing assessment is now undertaken. A bearing that is just gripped in full extension is ideal. This will be loose in flexion. If it is difficult to insert the bearing from the front, it may be inserted antero-laterally. Routine closure is undertaken. Closure of the capsule tends to tighten the flexion gap.
Fixed bearing
In view of the high dislocation rate, our current recommendation for surgeons is to use the fixed bearing components, at least when beginning with lateral UKA. The Fixed Lateral Oxford (FLO) prosthesis (see Fig. 12.15) has been recently introduced and can be used on the lateral side with the same instrumentation (Microplasty and adapter) and the same surgical technique.
The wear of the FLO is expected to be similar to other fixed bearing UKA but an order of magnitude greater than the mobile UKA.