Knee Menisci: Functions of Medial and Lateral Structures

What are the functions of the medial and lateral meniscus in the knee?

The menisci are two crucial fibrocartilaginous structures within the knee joint, primarily functioning to distribute compressive loads, absorb shock, enhance joint stability, lubricate the articular cartilage, and contribute to proprioception.

Introduction to the Menisci

The knee joint, a complex hinge joint, relies on several structures for its intricate movements and stability. Among the most vital are the menisci: the medial meniscus and the lateral meniscus. These crescent-shaped (medial) and more circular (lateral) pads of fibrocartilage are situated between the condyles of the femur (thigh bone) and the tibial plateau (shin bone). Their unique anatomical position and composition allow them to play indispensable roles in knee mechanics and long-term joint health. Unlike the smooth hyaline cartilage covering the bone ends, the menisci are tough, resilient, and somewhat deformable, designed to withstand significant forces.

Primary Functions of the Menisci

The menisci perform a multifaceted role, each function contributing synergistically to the overall health and efficient operation of the knee joint.

• Load Transmission and Distribution: This is arguably the most critical function. The femoral condyles are convex, while the tibial plateau is relatively flat. Without the menisci, the contact area between these bones would be small, leading to highly concentrated stress on the articular cartilage. The menisci effectively increase the contact area by conforming to the shape of the femoral condyles, thereby distributing the compressive loads across a larger surface of the tibial plateau. This significantly reduces the stress per unit area on the articular cartilage, protecting it from premature wear and tear. Studies indicate that the menisci transmit approximately 50% of the compressive load in extension and up to 85% in 90 degrees of knee flexion.

• Shock Absorption: During activities like walking, running, or jumping, the knee joint is subjected to substantial impact forces. The menisci act as natural shock absorbers, deforming under load to dampen these forces. Their viscoelastic properties allow them to compress and expand, dissipating energy and protecting the underlying articular cartilage and subchondral bone from damaging peak stresses.

• Joint Stability: The menisci deepen the relatively shallow tibial plateau, creating a more congruent articulation with the rounded femoral condyles. This "cupping" effect enhances the mechanical stability of the joint, particularly during flexion and extension movements. They also contribute to anteroposterior (front-back) stability, especially when the collateral and cruciate ligaments are compromised. The posterior horn of the menisci, in particular, helps resist anterior translation of the tibia relative to the femur, complementing the function of the anterior cruciate ligament (ACL).

• Joint Lubrication and Nutrition: While not their primary role, the menisci contribute to the lubrication of the knee joint. Their movement and deformation during activity help to spread synovial fluid across the articular cartilage surfaces. This process is crucial for reducing friction within the joint and also aids in the nutrition of the articular cartilage, which is avascular and relies on synovial fluid for nutrient exchange.

• Proprioception: The menisci contain mechanoreceptors and nerve endings, particularly in their peripheral vascularized regions. These specialized nerve cells provide sensory feedback to the brain regarding joint position, movement, and load. This proprioceptive input is vital for neuromuscular control, balance, and coordinating muscle activity around the knee, contributing to dynamic joint stability and preventing injury.

Medial vs. Lateral Meniscus: Subtle Differences

While performing similar primary functions, there are anatomical and biomechanical distinctions between the medial and lateral menisci:

• Shape and Attachment: The medial meniscus is C-shaped and more firmly attached to the tibia and the joint capsule, including a strong attachment to the medial collateral ligament (MCL). This strong anchorage makes it less mobile and, consequently, more susceptible to injury, particularly tears from rotational forces. The lateral meniscus is more O-shaped (nearly a complete circle) and less firmly attached to the tibia, allowing for greater mobility. It has no direct attachment to the lateral collateral ligament (LCL) and is separated from the joint capsule by the popliteus tendon, which further enhances its mobility.

• Load Bearing: Due to its larger surface area and position, the medial meniscus generally bears a slightly greater proportion of the knee's load compared to the lateral meniscus.

Clinical Significance and Injury

Understanding the functions of the menisci underscores their critical importance. Meniscal tears, often resulting from twisting injuries or degenerative changes, can significantly impair knee function. Loss of meniscal tissue, whether through injury or surgical removal (meniscectomy), compromises all their protective functions, leading to increased stress on articular cartilage, reduced shock absorption, and decreased stability. This can accelerate the degenerative process, predisposing the knee to early-onset osteoarthritis.

Conclusion

The medial and lateral menisci are far more than simple pads within the knee. As complex fibrocartilaginous structures, they are indispensable for optimal knee joint function, acting as sophisticated load distributors, shock absorbers, stabilizers, and contributors to joint health and proprioception. Their integrity is paramount for maintaining the long-term health and functional longevity of the knee, highlighting why their preservation is a key objective in orthopedic management.