The normal intact knee
In full extension, most of the fibres of the ligaments of the normal knee and the posterior capsule are just tight (i.e. at their resting unstretched lengths), and neither the medial nor the lateral joint surfaces can be separated without applying force to stretch the ligaments. With the knee flexed (beyond about 20°) all the fibres of the posterior capsule and the LCL slacken, and distraction of the ar ticular surfaces produces a gap in either compartment. These ‘physiological’ gaps are limited by the compliance of the other three ligaments that span the joint (the MCL and the cruciates), some of whose fibres maintain their resting tension and remain isometric throughout the range of passive flexion (see Chapter 3 and Appendix).
The effect of increasing flexion on the width of these gaps is different in the two compartments.
In the lateral compartment, the gap produced by distraction increases with increasing flexion to about 7 mm at 90° (mean 6.7 ± 1.9 mm) (Fig. 6.1(b)) (Tokuhara et al. 2004).
In the medial compartment, the width of the gap does not alter significantly throughout the range of passive flexion, measuring about 2 mm at 90° (mean 2.1 mm ±1.1 mm) (Fig. 6.1(c)).
Since the LCL and the posterior capsule are demonstrably slack in the flexed knee, the constant width of the medial gap at all angles of flexion (except full extension) implies that the MCL and the cruciates exert a net isometric effect on that compartment throughout that range of movement. After medial unicompartmental replacement, both the stability of the knee and the entrapment of the free bearing depend upon reproducing this isometric mechanism. (The difference between the compliance of the two compartments in flexion explains why bearing dislocation is a problem laterally.)
Figure 6.1 Representative MR scans of the left knee in (a) a neutral position, (b) under passive varus stress, and (c) under passive valgus stress. (Reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Tokuhara Y, Kadoya Y, Nakagawa S, Kobayashi S and Takaoka K. The flexion gap in normal knees. An MRI study. J Bone Joint Surg [Br] 2004; 86-B: 1133–6].)
The knee with anteromedial osteoarthritis
In anteromedial OA, the MCL and the cruciate ligaments are intact and have the same mechanical effects as in the normal joint. However, the posterior capsule tends to be shortened and there is an associated fixed flexion deformity. The effect of this is to close down the medial compartment gap before full extension is reached (Figs. 6.2(a) and 6.2(b)). For this reason, we assess the gap with the knee flexed at 20° to ensure that the posterior capsule is slack (Fig. 6.2(c)). (In anteromedial OA with an intact ACL, the flexion deformity tends to be less than 15°, so 20° flexion should always achieve this.)
In all positions of flexion greater than 20°, the medial condyles can be distracted the same distance as in the normal knee because the gap is limited by the normal MCL and cruciates. Therefore, distraction in flexion above 20° (Fig. 6.2(c)) restores normal alignment of the leg (Fig. 6.2(d)). The medial gap appears wider than normal only because cartilage and bone have been lost from the joint surfaces.
Balancing the ligaments in TKA and OUKA
In TKA, the term ‘ligament balancing’ usually implies elongation of the MCL by medial release to match the length of the LCL so that, at 90° flexion, the medial and lateral gaps are equal (the flexion gap is quadrilateral, Fig. 4.14(e)). As Figures 1.10 and 6.1(b) demonstrate, this is not the physiological state.
Figure 6.2 (a) With the knee extended, as far as possible, the shortened posterior capsule closes down the medial gap before full extension is reached, causing a flexion deformity, and (b) a varus deformity that is not correctable although the MCL is slack (c). At 20° flexion, however, the posterior capsule is slack and (d) an applied valgus force can distract the damaged medial articular surfaces. Because the MCL is of normal length, this corrects the varus deformity.
In OUKA, medial release should never be undertaken. The MCL is of normal length in anteromedial OA; mobility and stability of the joint, alignment of the leg, and entrapment of the bearing all depend upon its integrity. Balancing the ligaments means adjusting the position of the femoral component relative to the femur (by removing bone) so that the medial distraction gap is the same in flexion and extension.
In other words the ‘flexion gap’ and ‘extension gap’ should be equal. As explained above, the extension gap is measured in 20° flexion because the posterior capsule is slack. The flexion gap is measured at 90° to this, at 110° knee flexion. The instrumentation is designed to adjust the extension gap without changing the flexion gap. The flexion gap is established first, then the extension gap is adjusted to match it by milling bone from the inferior femoral condyle.
Because the lateral collateral ligament plays no part in the balancing of ligaments in OUKA, it is not represented on any of the explanatory diagrams
.