The soft tissues of the lateral side of the knee are a complex arrangement of static and dynamic stabilisers whose primary function is to provide stability in varus angulation, anteroposterior translation of the tibia and internal/external rotation (Chapter 3).
The stabilising effect of the LCL is very different to that of the MCL. The latter provides stability throughout the full range of movement and therefore dislocation of a mobile bearing is rare (Chapter 9). Conversely, the LCL is tight only in extension. All of its fibres are slack beyond 12° flexion (Chapter 3) and, by 90°, 5 – 10 mm of distraction is possible in the lateral compartment. An MRI study showed that, in flexion under valgus and varus load, the medial compartment opens an average 2 mm whereas the lateral opens 7 mm (see Figs. 1.10 and 6.1) (Tokuhara et al., 2004). Dislocation of the mobile bearing is therefore a potential problem in mobile bearing lateral UKA.
The lateral collateral ligament (LCL) is round and cordlike, usually between 59 and 74 mm in length. It arises from the lateral epicondyle of the femur (from a fovea posterior to the apex of a bony ridge). It inserts distally into the apex of the fibular head as a conjoint tendon with the biceps femoris tendon (Takeda et al., 2015).
Dynamic stability is provided by popliteus, the ileo-tibial tract and the extensor mechanism to keep the lateral articular surfaces in contact during activity.
The medial plateau is larger than the lateral and is slightly concave whereas the lateral plateau is convex (Fig. 12.1). The lateral meniscus is more mobile than the medial meniscus, and its excursion during flexion–extension and axial rotation is greater than that of the medial meniscus. In high flexion (>120°), the lateral femoral condyle rolls over the posterior lip of the lateral plateau onto the posterior surface of the tibia (Fig. 12.1(b)), taking the posterior horn of the meniscus with it. By contrast, the posterior excursion of the medial meniscus is much less and the medial condyle and its meniscus, stay on the upper surface of the tibia (Fig 12.1 (a)).
Figure 12.1 In the normal knee, in high flexion (a) in the medial side the femoral condyle is on top of the tibia; (b) in the lateral side the femoral condyle subluxes off the back of the convex tibial plateau. (Dr Y Kadoya, personal communication).
Iwaki et al. (2000) amongst others has shown much more movement in the lateral compartment (up to 20 mm) compared to medial side (8 mm) although the difference does depend on the loading conditions (Chapter 3) (Freeman & Pinskerova, 2005).
