With the knee in 110° flexion, insert the femoral sizing spoon [23] (of appropriate size based on the pre-operative estimate) and 1 mm thick under the centre of the medial condyle. Its handle should lie approximately parallel with the long axis of the femur. With all retraction removed, assess the ligament tension by twisting the spoon. It should freely twist about 20° in both directions. Usually the 1 mm thick femoral sizing spoon achieves the proper ligament tension; if not, replace it with a thicker sizing spoon until the proper tension is achieved. The optimal size of the femoral component is confirmed by examining the relationship of the front of the spoon and the surface of the eburnated bone. Ideally it should be 3 to 5 mm above the surface approximately where the cartilage surface of the femur would have been before the arthritis. Once the appropriate spoon is inserted, apply a self retaining retractor which locks it in place.
Figure 7.8 (a) The correct femoral sizing spoon in place with a gap of about 5 mm between the spoon and the eburnated bone. (b) Femoral spoon (the grey spoon is the correct size; the pink is too large).
Apply the tibial saw guide assembly [21, 25 & 53], with its shaft parallel with long axis of the tibia in both planes (Fig. 7.9). The ankle yoke [25] should be pointing towards the ipsilateral anterior superior iliac spine (ASIS). The tibial saw guide has 7° of posterior slope built in. The zero shim [20] should be in place.
Figure 7.9 The assembled tibial saw guide in situ.
The femoral sizing spoon, tibial saw guide and G-clamp [27], when used together, will accurately establish the level of bone resection (this can be seen in the video at www.oxfordpartialknee.com). Select either the 3 or the 4 G-clamp and apply to the femoral sizing spoon and to the medial side of the tibial saw guide to ensure access to pin holes in the guide. Although there is an option to adjust the height of the tibial cut using different shims [20], the zero shim must always be used with the G-clamp. In general, a 3 G-clamp is used for Extra Small and Small femurs and 4 for the rest although surgeons starting out with the OUKA should use the 4 G-clamp.
Confirm that the knee is flexed to 110°. Manipulate the upper end of the guide so that its face lies against the exposed bone. Push the guide laterally so its recess accommodates the patellar tendon (Fig. 7.10). Engage the cam on the G-clamp, by pulling the lever downwards, to lock the three components together. Fix the saw guide in place using a headed pin [4] through the central or lateral hole in the tibial saw guide. Unlock the G-clamp and remove along with the femoral sizing spoon.
