Vertical cut
The bearing tends to move more medially with extension so the vertical cut has to be internally rotated to prevent impingement between the bearing and the wall. This is difficult to achieve if the saw cut is made lateral to the patellar tendon, therefore the cut is made through the centre of the patellar tendon (Fig.12.12). The position of the saw cut cannot be seen and is done by feel. It should rest against the medial side of the lateral condyle, and should be directed towards the ipsilateral ASIS. The cut is advanced to the depth of saw with a 7° slope similar to that used on the medial side. There should be no risk of tibial plateau fracture because the cut is made over the tibial shaft.
Figure 12.12 Vertical cut.
Horizontal cut
In general, more bone is removed from the lateral tibial plateau than during medial OUKA because the domed tibial component is thicker than the flat. The extramedullary sawing jig is applied in the same manner as for the medial side with the slope set at 7°. The jig is placed parallel to the tibial crest. The cut is made just below the top of Gerdy’s tubercle, usually 2 – 3 mm below the defect. The Z-retractor (curly-whirly retractor) is used to protect the soft tissues.
Figure 12.13 Horizontal cut.
An assessment is made to ensure that an adequate resection has been undertaken by inserting an appropriate tibial template and a 3 mm feeler gauge. The knee is then brought into full extension to assess the ligament tension. If the gap in extension is too tight, then the tibia is recut 2 mm distally by removing the shim from the sawing jig and repeating the horizontal cut. The final bearing thickness is selected in full extension so that the gap in full extension with the tibial template in place is an estimate of the final bearing thickness.
