The natural knee
The presence of the cartilaginous menisci in the knee of humans (and of all other mammals) gives rise to an entirely different regime of contact (Fig. 2.2). Instead of one incongruous interface, two congruous interfaces are created, with much better distribution of load.
Fairbank, in 1948, first deduced that the human meniscus had a load-bearing function and suggested the mechanism of load transmission shown in Figure 2.3 (Fairbank, 1948). The menisci consist mainly of collagen fibres disposed circumferentially to withstand the tensile hoop stresses engendered by load bearing; these stresses are resisted at the anterior and posterior horns by their attachments to the tibia (Bullough et al., 1970). The proportion of load transmitted indirectly by the menisci in human (and animal) joints has been estimated as between 45 and 70% of the applied load (Shrive et al., 1978). The remaining 30 – 55% is carried by the articular cartilage of the femoral and tibial surfaces within the embrace of the meniscus through their direct contact in the middle third of each plateau.
Figure 2.2 Load-sharing function of the meniscus, increasing effective contact area and reducing contact pressure. Loss of a meniscus reduces contact area and increases contact pressure
Figure 2.3 Mechanism of load transmission: the radially outward component of applied pressure is resisted by hoop stresses in the circumferential fibres of the meniscus. (Adapted from and reproduced with permission from Lippincott Williams & Wilkins [Shrive NG, O’Connor JJ and Goodfellow JW. Load-bearing in the knee joint. Clin Orthop 1978; 131: 279–87].)
Mobility of the natural meniscus
Anteroposterior movements of the femoral condyles on the tibia during flexion–extension and axial rotation (Fig. 2.4) have to be accommodated by movements of the menisci. In 1680, Borelli (1989) noticed that ‘they are pulled forward when the knee is extended and backwards in flexion’. Various estimates and measurements of these movements during flexion have been reported: 6 mm medially and 12 mm laterally (Kapandji, 1970); 5.1 mm (SD 0.96) medially and 11.2 mm (SD 2.29) laterally (Thompson et al., 1991); medial anterior horn 7.1 mm (SD 2.49), medial posterior horn 3.9 mm (SD 1.75), lateral anterior horn 9.5 mm (SD 3.96), and lateral posterior horn 5.6 mm (SD 2.76) (Vedi et al., 1999). Freeman and his group suggest that the knee is a medial pivot with no movement medially; however, even Freeman’s data suggests that there is movement of about 8 mm (Fig 2.4).
Figure 2.4 Anteroposterior movements of the femoral condyles on the tibia during flexion–extension and axial rotation, with annotations by Mr Michael Freeman.
Compliance of the natural meniscus
During flexion–extension and axial rotation, the natural meniscus not only changes its position on the tibial plateau, as the movements of the femoral condyle dictate, but also changes shape to fit the various curvatures of the polyradial femoral condyle (Fig. 2.5). In full extension, the large radius of the inferior surface of the condyle forces the limbs of the meniscus apart in an anteroposterior direction. As the knee flexes, and the smaller radius of the posterior condyle is offered, the anteroposterior measurement of the meniscus diminishes appropriately, possibly because divergence of the tibiofemoral contact areas forces the two menisci apart, drawing their anterior and posterior limbs closer together (Fig. 2.6) (Shrive et al., 1978). Changes in the shapes of the menisci are reflected in the differences in anteroposterior movements of the anterior and posterior horns and in the mediolateral movements of the medial and lateral edges of the two menisci observed by Vedi et al. (1999).
Figure 2.5 Magnetic resonance images demonstrating changes in the anteroposterior span of the meniscus during flexion. (Adapted from and reproduced with permission and copyright © of the British Editorial Society of Bone and Joint Surgery [Vedi V, Williams A, Tennant SJ, Spouse E, Hunt DM, Gedroyc WM. Meniscal movement. An in vivo study using dynamic MRI. J Bone Joint Surg [Br] 999; 81-B: 37–41].)
Discussion
The meniscus is an integral part of the tibial articular surface, serving to maximise the contact area without limiting angular and translational movement between the bones. Therefore, load is transmitted at an average pressure that the articular cartilage can withstand. Evidence of the importance of this mechanism is provided by the observation that excision (or dysfunction) of a meniscus results in osteoarthritic degeneration of the remaining cartilage surfaces in the affected compartment (Fairbank, 1948).
Figure 2.6 Shape changes of the meniscus during flexion-extension. Dotted curves outline contact areas in extension (left) and flexion (right) (diagramatic). (Reproduced with permission from Lippincott Williams & Wilkins [from Shrive N G, O’Connor JJ, and Goodfellow JW. Load-bearing in the knee joint. Clin Orthop 1978; 131: 279–87].)
