This is fundamentally different to the medial philosophy (Chapters 6 and 7). On the lateral side, ligament balance is impossible because the LCL is slack in flexion. The aim is to place the femoral component anatomically. The cartilage in lateral OA is relatively preserved at 0° and 90° flexion (Gulati et al., 2009). The technique therefore aims to position the implant flush to the condyle surfaces at 0° and 90° flexion. This is achieved using the standard femoral sawing jig, referencing from the posterior cartilage for AP position and by milling with a 4 spigot for distal positioning.
Femoral drill guide
Operator disorientation is quite common with this step of the procedure and mal- positioning of the femoral component can occur. The aim is to position the 6 mm hole in the centre of the condyle parallel to the mechanical axis and flexed about 5°. Positioning of the IM rod is critical. The entry hole is made about 1 cm above and 0.5 cm lateral to the lateral border of the notch (Fig. 12.14). The IM rod is then inserted. A line is drawn down the centre of the condyle to aid anatomical positioning.
Figure 12.14 Entry hole for left femur.
We recommend that the Microplasty femoral drill guide is used with the lateral adapter. However, if this is not available, the Phase 3 lateral guide can be positioned parallel to the IM rod using the extramedullary rod as an extra guide.
An appropriate sized femoral component is selected based on gender and height. This is usually one size smaller than that used for the medial condyle. The medial tibial spoons can be used to help estimate the femoral component size. The appropriate femoral drill guide is set to fill the flexion gap. The lateral adapter is applied and clamped in place. This flexes the femoral component about 5° relative to the IM rod. The femoral drill guide is inserted in the knee and linked to the IM rod. The guide is adjusted so the 6 mm hole is central or slightly lateral on the femoral condyle. It is important that the femoral component does not overhang anterolaterally. It should therefore be confirmed that the femoral drill guide does not overhang anterolaterally. If it does, it should be internally rotated or moved slightly medially. When the drill guide is satisfactorily positioned, the small and large drill holes are made.
Femoral condyle preparation
The posterior saw guide is used in the standard manner to remove posterior femoral bone, thereby accurately restoring the joint line and positioning the femoral component anatomically in the AP direction. The meniscus is then removed. The next step is to place the femoral component anatomically which is usually achieved by milling using the 4 spigot. Baseline milling is therefore undertaken using 0 spigot. The gaps are then measured to ensure the flexion gap is at least 4 mm larger than the extension gap, which is nearly always the case. The distal femur is then milled with 4 spigot.
There are two caveats:
1. If the flexion gap is less than 4 mm larger than the extension gap after primary milling, then a smaller spigot should be used. The surgeon may need to recut the tibia.
2. If the flexion gap is very loose after the second milling, then a 5 spigot can be used but never more.
