Meniscus: Function, Anatomy, Types, and Clinical Significance in the Knee Joint

What is the function of the meniscus in the knee joint?

The menisci are crucial C-shaped fibrocartilaginous structures within the knee joint that primarily function to absorb shock, distribute load, enhance joint stability, and facilitate lubrication, thereby protecting the articular cartilage and ensuring smooth knee movement.

Anatomy of the Meniscus

The knee joint, a complex hinge joint, houses two vital menisci: the medial (inner) and lateral (outer) menisci. These structures are crescent-shaped, wedges of fibrocartilage positioned between the femoral condyles (thigh bone) and the tibial plateau (shin bone).

• Composition: Unlike the smooth hyaline cartilage that covers the ends of bones, the menisci are composed of tough, elastic fibrocartilage. This composition provides them with their unique ability to withstand significant compressive and shear forces.
• Location: The medial meniscus is located on the inner side of the knee and is more C-shaped, while the lateral meniscus is on the outer side and is more O-shaped or semi-circular.
• Attachment: Both menisci are attached to the tibial plateau by coronary ligaments and to the joint capsule. The medial meniscus also has a strong attachment to the medial collateral ligament (MCL), which contributes to its relative immobility compared to the lateral meniscus.
• Vascularity: The menisci have a limited blood supply, primarily to their outer third (the "red zone"). The inner two-thirds (the "white zone") are largely avascular, receiving nutrients from the synovial fluid. This limited blood supply has significant implications for healing after injury.

Primary Functions of the Meniscus

The menisci perform several indispensable roles that are critical for the long-term health and optimal function of the knee joint.

• Shock Absorption: One of the most critical functions is to act as natural shock absorbers. During weight-bearing activities like walking, running, or jumping, the menisci help to cushion the impact between the femur and tibia, protecting the delicate articular cartilage from excessive stress and wear.
• Load Transmission and Distribution: Without the menisci, the forces across the knee joint would be concentrated over a small area. The menisci effectively increase the contact surface area between the rounded femoral condyles and the relatively flat tibial plateau, distributing compressive loads more evenly across the joint. This reduces peak stresses on the articular cartilage by up to 50%, significantly delaying degenerative changes.
• Joint Stability: The wedge shape of the menisci deepens the shallow tibial plateau, creating a more congruent fit with the rounded femoral condyles. This enhanced congruency contributes significantly to the knee's stability, particularly during rotational movements and at the extremes of flexion and extension, helping to prevent excessive anterior-posterior and medial-lateral translation of the femur on the tibia.
• Joint Lubrication and Nutrition: The menisci assist in spreading synovial fluid throughout the joint during movement. This action is vital for lubricating the articular cartilage, reducing friction, and facilitating the transport of nutrients to the avascular cartilage, thus promoting its health and longevity.
• Proprioception: Although less prominent than their mechanical roles, the menisci contain mechanoreceptors that contribute to proprioception – the body's sense of joint position and movement. This feedback helps the brain control knee movement and muscle activation, contributing to dynamic joint stability.

Types of Menisci

While both menisci share common functions, their anatomical differences lead to some functional distinctions and injury patterns.

• Medial Meniscus:
  • Shape: Larger, more C-shaped.
  • Mobility: Less mobile due to its strong attachment to the medial collateral ligament and joint capsule.
  • Injury Susceptibility: Its reduced mobility makes it more prone to injury, especially during twisting motions when the foot is planted.
• Lateral Meniscus:
  • Shape: Smaller, more O-shaped or semi-circular.
  • Mobility: More mobile due to less extensive capsular attachments, allowing it to move more freely with the femoral condyle.
  • Injury Susceptibility: Its greater mobility generally makes it less susceptible to injury compared to the medial meniscus, though it can still be damaged.

Clinical Significance and Injury

Given their critical roles, injury to the menisci can have profound implications for knee function and long-term joint health. Meniscal tears are common, often resulting from sudden twisting movements, deep squatting, or direct impact, particularly in sports. In older individuals, degenerative tears can occur with minimal trauma due to age-related wear and tear.

• Consequences of Injury: A torn meniscus can lead to pain, swelling, stiffness, clicking, catching, or even locking of the knee.
• Long-Term Impact: Without the protective functions of the meniscus, the articular cartilage is subjected to increased stress and friction, accelerating the development of osteoarthritis. This underscores the importance of proper diagnosis and management of meniscal injuries to preserve knee joint integrity.

Preserving Meniscal Health

Maintaining meniscal health is crucial for long-term knee function.

• Proper Biomechanics: Focus on correct form during exercises and daily activities to minimize undue stress on the knee joint.
• Strength Training: Develop strong quadriceps, hamstrings, and gluteal muscles to provide dynamic stability and support to the knee.
• Flexibility and Mobility: Maintain good range of motion in the knee and surrounding joints to prevent stiffness and improve joint mechanics.
• Weight Management: Keeping a healthy body weight reduces the compressive forces on the knee joints.
• Gradual Progression: Avoid sudden increases in training intensity or volume, allowing the body to adapt progressively.
• Listen to Your Body: Pay attention to pain signals and seek professional advice if knee discomfort persists.

Conclusion

The menisci are unassuming yet indispensable components of the knee joint. Their multifaceted functions in shock absorption, load distribution, joint stability, and lubrication are paramount for the knee's mechanical efficiency and its long-term health. Understanding their critical roles emphasizes the importance of protecting these fibrocartilaginous structures through mindful movement, appropriate training, and proactive care to ensure a lifetime of healthy, functional knees.