Before making the horizontal cut remove the shim from the tibial resection guide and insert the slotted zero shim (Fig. 7.12). Also insert a medial collateral ligament (MCL) retractor [19] (sometimes called a Z or curly-whirly retractor). Ensure this retractor lies between the saw and the MCL, protecting the deep fibres of the ligament.
Figure 7.12 The horizontal cut with the Z-retractor used to protect the MCL (a and b), and the slotted shim (a).
Use the 12 mm wide oscillating saw blade, designed for the OUKA, with appropriate markings, to excise the plateau (Fig. 7.13). Ensure the saw blade is guided along the MCL retractor to cut the medial cortex completely without damaging the MCL. Slightly undermine the vertical cut. To cut the posterior cortex, advance the saw cut until the appropriate mark on the saw blade is aligned with the anterior cortex. When the cut is complete, the plateau usually moves. If it cannot be moved, the saw cuts have to be repeated. Remove the slotted shim, lever the plateau up with a broad osteotome to disrupt soft tissue attachments and remove with the knee in extension. If the plateau cannot be removed, soft tissue attachments posteromedially may need to be cut with a knife and occasionally posterior osteophytes need to be removed with an osteotome.
The excised plateau should show the classical lesion of anteromedial osteoarthritis: erosion of cartilage and bone in its mid and anterior parts and preserved cartilage posteriorly. Osteophytes around the edge of the plateau remain attached after its removal (see Fig. 4.4).
Figure 7.13 The excised plateau with the lesion outlined.
Lay templates [1] of the opposite side on the cut surface of the excised plateau to choose the tibial component with the appropriate width ignoring medial osteophytes. If the component of the appropriate width appears short, consider repeating the vertical cut 2 mm further laterally so that a wider and longer component may be used.
Horizontal cut first
Occasionally, using the standard technique, the vertical cut may go too deep posteriorly which weakens the tibia and increases the risk of tibial plateau fracture. An alternative is to do the horizontal cut first and then insert a shim in the cut to prevent the vertical cut from going too deep. The recommended steps are set out below.
Using a diathermy, identify the apex of the medial spine and mark the optimal site of the vertical cut just medial to the apex of the spine and in the direction of the ASIS. Insert the slotted zero tibial saw shim and MCL retractor. Perform the horizontal cut in the standard fashion except that it should be extended laterally so as to undermine the site of the vertical cut by about 5 mm. Remove the MCL retractor and slotted shim then insert a standard zero shim. A protective shim should then be inserted into the horizontal cut and held in place under the vertical cut whilst the vertical cut is being undertaken to prevent it from going too deep. Prototype protective shims are currently being tested and will be available for general use soon. In the interim, a horizontal cut saw blade, a steel rule, or an ‘angel wing’ may be used as a protective shim.
