differential excursion. Thompson et al found that the area of least meniscal motion is the posterior medial corner, where the meniscus is constrained by its attachment to the tibial plateau by the meniscotibial portion of the posterior oblique ligament, which has been reported to be more prone to injury.143,165 A reduction in the motion of the posterior horn of the medial meniscus is a potential mechanism for meniscal tears, with a resultant trapping of the fibrocartilage between the femoral condyle and the tibial plateau during full flexion. The greater differential between anterior and posterior horn excursion may place the medial meniscus at a greater risk of injury.165 The differential of anterior horn to posterior horn motion allows the menisci to assume a decreasing radius with flexion, which correlates to the decreased radius of curvature of the posterior femoral condyles.165 This change of radius allows the meniscus to maintain contact with the articulating surface of both the femur and the tibia throughout flexion.
Load Transmission The function of the menisci has been clinically inferred by the degenerative changes that accompany its removal. Fairbank described the increased incidence and predictable degenerative changes of the articular surfaces in completely meniscectomized knees.45 Since this early work, numerous studies have confirmed these findings and have further established the important role of the meniscus as a protective, load-bearing structure. Weightbearing produces axial forces across the knee, which compress the menisci, resulting in hoop (circumferential) stresses.170 Hoop stresses are generated as axial forces and converted to tensile stresses along the circumferential collagen fibers of the meniscus (Figure 8). Firm attachments by the anterior and posterior insertional ligaments prevent the meniscus from extruding peripherally during load bearing.94 Studies by Seedhom and Hargreaves reported that 70% of the load in the lateral compartment and 50% of the load in the medial compartment is transmitted through the menisci.153 The menisci transmit 50% of compressive load through the posterior horns in extension, with 85% transmission at 90 flexion.172 Radin et al demonstrated that these loads are well distributed when the menisci are intact.137 However, removal of the medial meniscus results in a 50% to 70% reduction in femoral condyle contact area and a 100% increase in contact stress.4,50,91 Total lateral meniscectomy results in a 40% to 50% decrease in contact area and increases contact stress in the lateral component to 200% to 300% of normal.18,50,76,91 This significantly increases the load per unit area and may contribute to accelerated articular cartilage damage and degeneration.45,85
Shock Absorption The menisci play a vital role in attenuating the intermittent shock waves generated by impulse loading of the knee with normal gait.94,96,153 Voloshin and Wosk showed that the normal knee has a shock-absorbing capacity about 20% higher than
knees that have undergone meniscectomy.170 As the inability of a joint system to absorb shock has been implicated in the development of osteoarthritis, the meniscus would appear to play an important role in maintaining the health of the knee joint.138
Joint Stability The geometric structure of the menisci provides an important role in maintaining joint congruity and stability.## The superior surface of each meniscus is concave, enabling effective articulation between the convex femoral condyles and flat tibial plateau. When the meniscus is intact, axial loading of the knee has a multidirectional stabilizing function, limiting excess motion in all directions.9 Markolf and colleagues have addressed the effect of meniscectomy on anterior-posterior and rotational knee laxity. Medial meniscectomy in the ACL-intact knee has little effect on anterior-posterior motion, but in the ACL-deficient knee, it results in an increase in anterior-posterior tibial translation of up to 58% at 90o of flexion.109 Shoemaker and Markolf demonstrated that the posterior horn of the medial meniscus is the most important structure resisting an anterior tibial force in the ACL-deficient knee.155 Allen et al showed that the resultant force in the medial meniscus of the ACL-deficient knee increased by 52% in full extension and by 197% at 60 of flexion under a 134-N anterior tibial load.7 The large changes in kinematics due to medial meniscectomy in the ACL-deficient knee confirm the important role of the medial meniscus in knee stability. Recently, Musahl et al reported that the lateral meniscus plays a role in anterior tibial translation during the pivot-shift maneuver.123
Joint Nutrition and Lubrication The menisci may also play a role in the nutrition and lubrication of the knee joint. The mechanics of this lubrication remains unknown; the menisci may compress synovial fluid into the articular cartilage, which reduces frictional forces during weightbearing.13 There is a system of microcanals within the meniscus located close to the blood vessels, which communicates with the synovial cavity; these may provide fluid transport for nutrition and joint lubrication.23,24
Proprioception The perception of joint motion and position (proprioception) is mediated by mechanoreceptors that transduce mechanical deformation into electric neural signals. Mechanoreceptors have been identified in the anterior and posterior horns of the menisci.*** Quick-adapting mechanoreceptors, such as Pacinian corpuscles, are thought to mediate the sensation of joint motion, and slow-adapting receptors, such as Ruffini