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Meniscal Function

While the term “shock absorber” has often been used to describe the meniscus, the menisci actually serve many functions in the human knee. Additional functions are theorized to be load bearing, lubrication, and proprioception. The meniscal function of load bearing may be clinically inferred by the degenerative changes that accompany meniscectomy.

Fairbank described radiographic changes following meniscectomy, which included narrowing of the joint space, flattening of the femoral condyle, and the formation of osteophytes. These changes were attributed to the loss of the weight bearing function of the meniscus. Biomechanical studies have demonstrated that at least 50% of the compressive load of the knee joint is transmitted through the meniscus in extension, and approximately 85% of the load is transmitted in 90 degrees of flexion.

In the totally meniscectomized knee, the contact area is reduced approximately 50%. This significantly increases the load per unit area and results in articular damage and degeneration. Partial meniscectomy has also been shown to significantly increase contact pressures. In an experimental study, resection of as little as 15% to 34% of the meniscus increased tibiofemoral contact pressures by more than 350%. Thus, even partial meniscectomy can affect the ability of the meniscus to function in load transmission across the knee.

Another proposed function of the meniscus is that of shock absorption. By examining the compressive load-deformation response of the normal and meniscectomized knee, it has been suggested that the viscoelastic menisci may function to attenuate the intermittent shock waves generated by impulse loading of the knee during gait.

Studies have shown that the normal knee has a shock-absorbing capacity about 20% higher than knees that have undergone meniscectomy . As the inability of a joint system to absorb shock has been implicated in the development of osteoarthritis, the shock absorption mechanism would appear to play a role in maintaining the health of the knee joint.

In addition to the role of the meniscus in load transmission and shock absorption, the menisci are thought to contribute to knee joint stability. Although medial meniscectomy alone does not significantly increase anterior-posterior joint stability, several studies have shown that medial meniscectomy in association with ACL insufficiency significantly increases the anterior laxity of the knee . However, lateral meniscectomy, alone or in association with ACL insufficiency, has not been shown to increase knee joint laxity.

Because the menisci serve to increase the congruity between the condyles of the femur and tibia, they contribute significantly to overall joint conformity. It has been suggested that this function assists in the overall lubrication of the articular surfaces of the knee joint. Posterior translation of the menisci (lateral greater than medial) during knee flexion has been demonstrated in magnetic resonance imaging (MRI) studies. Additionally, the anterior and posterior translation of the menisci during flexion and extension is hypothesized to protect the articular surfaces from injury.

Finally, the menisci have been suggested as proprioceptive structures providing a feedback mechanism for joint position sense. Neural elements have been identified within the meniscal tissue. While the anterior and posterior horns of the menisci appear to be the most richly innervated, myelinated and unmyelinated nerve fibers have been identified within the peripheral body of the meniscus.

These nerve fibers originate in the highly innervated perimeniscal tissue and radiate into the peripheral third of the meniscus. Many of these fibers accompany the vascular network of the meniscus; however, some neural elements are not exclusively paravascular in position, suggesting a function other than vasomotor or vasosensory.

Studies of human specimens have identified three morphologically distinct mechanoreceptors within the medial meniscus: Ruffini endings, Golgi tendon organs, and Pacinian corpuscles. These neural elements were found in greatest concentration in the horns of the meniscus, particularly the posterior horn. The presence of these neuroreceptors in the meniscus has led to the hypothesis that the menisci may serve an important afferent role in the sensory feedback mechanism of the knee. During extremes of knee flexion and extension, the horns of the menisci become more taut.

This increase in tension would activate the mechanoreceptors located in the meniscal horns and provide the central nervous system with information regarding joint position. This may, in turn, contribute to a reflex arc that stimulates protective or postural muscular reflexes. It is theorized that the greatest concentration of neural elements found in the anterior and posterior horns of the meniscus reflects the need for afferent feedback at the extremes of flexion and extension.

In summary, the proposed functions of the menisci include load bearing, shock absorption, joint stability, lubrication, and proprioception. Loss of the meniscus, partially or totally, significantly alters these functions and predisposes the joint to degenerative changes.

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