We will first describe how the instruments are used to position the component and to balance the ligaments. Reference to the videos may be found helpful.
The tibial component
The extramedullary tibial saw guide is used to direct the transverse saw cut. The saw guide is positioned at the correct height using a system of femoral sizing spoons and G-clamps that reference off the posterior femur with the knee in flexion (Fig. 6.5). It ensures that the appropriate amount of bone is resected from the tibia.
Figure 6.5 Level of tibial saw-cut and of its guiding surface (a) femoral sizing spoon, (b) Oxford G-clamp, (c) EM guide.
The level of the transverse tibial saw cut
We commonly use a 4 mm thick bearing except for small patients, in whom we would use a 3. However, as the operation is easier with a 4 bearing, we would recommend that, when starting, surgeons should aim for a 4. In order to make a space for a 4 mm thick bearing and a tibial component 3 mm thick, the tibial saw cut should be made about 7 mm below the posterior femoral articular surface with the ligament just tight. This will ensure that when the femoral component is appropriately positioned, with its articular surface where the original articular surface was, a 4 bearing will have the appropriate tightness.
To guide the height of the horizontal cut we use a stylus, otherwise known as a spoon (Fig. 6.5(a)), which is linked to the extramedullary tibial saw guide using a G-clamp, (Fig. 6.5(b)). The spoons come in five different sizes, extra small, small, medium, large, and extra large, to match the sizes of the femoral component. We use medium for most patients, small for small women and large for large men. The spoons can be used to confirm the size of femoral component. When inserted, the front of a spoon of appropriate size should be about where the original surface of the condyle was, 3 to 5 mm in front of the eburnated bone (see Fig. 7.8).
Spoons also come in different thicknesses, 1 mm, 2 mm, 3 mm, which are used to ensure that the ligaments are just tight (Fig. 6.6). Usually the 1 spoon is appropriate as it just tightens the ligaments. However, occasionally with a very deep tibial defect or with lax ligaments, a 2 or 3 spoon is necessary. When a thicker spoon is inserted, less tibia is removed so the ligaments will be just tight when the bearing is inserted.
3 and 4 mm G-clamps are available; the 3 should be selected for a 3 bearing and the 4 for a 4 bearing. With the 4 G-clamp, 1 mm more bone is removed than with a 3 mm G-clamp, allowing a 1 mm thicker bearing to be inserted.
Figure 6.6 A spoon of correct thickness (a or b) is selected to just tighten the MCL. With a thicker spoon less tibia is removed.
