Medial Meniscus: Functions, Differences, and Clinical Importance

What is the functional role of the medial meniscus?

The medial meniscus is a crucial C-shaped fibrocartilaginous structure within the knee joint, primarily functioning to enhance joint congruity, distribute compressive loads, absorb shock, and contribute to knee stability.

Understanding the Knee Menisci

The knee joint, a complex hinge joint, relies on several structures for its remarkable mobility and stability. Among these are the menisci – two crescent-shaped pads of fibrocartilage located between the femoral condyles and the tibial plateau. These are the medial meniscus (on the inner side of the knee) and the lateral meniscus (on the outer side). Composed predominantly of collagen fibers, particularly Type I, these structures are essential for the long-term health and optimal function of the knee.

Primary Functions of the Medial Meniscus

The medial meniscus plays several vital roles, directly impacting the biomechanics and longevity of the knee joint:

• Load Transmission and Distribution: This is arguably the most critical function. The menisci effectively increase the contact area between the rounded femoral condyles and the relatively flat tibial plateau. Without the menisci, contact stress would be concentrated over a small area, leading to rapid degeneration of the articular cartilage. The medial meniscus specifically bears a significant portion of the compressive forces across the medial compartment of the knee, distributing these loads more evenly across the tibial plateau, thus reducing peak stresses.
• Shock Absorption: During activities like walking, running, and jumping, the knee experiences considerable impact forces. The menisci act as natural shock absorbers, attenuating these forces and protecting the articular cartilage and subchondral bone from excessive stress and potential damage.
• Joint Stability: By deepening the tibial plateau, the medial meniscus improves the congruence and fit between the femur and tibia. This enhanced congruity limits excessive anterior-posterior translation and rotational movements, contributing significantly to the overall stability of the tibiofemoral joint, particularly when the knee is flexed.
• Lubrication and Nutrition: The menisci facilitate the spreading of synovial fluid across the articular cartilage surfaces. This action aids in joint lubrication, reducing friction during movement, and also assists in the nutrition of the avascular articular cartilage by distributing nutrients within the synovial fluid.
• Proprioception: While not as extensively vascularized or innervated as other joint structures, the periphery of the menisci contains mechanoreceptors. These sensory receptors contribute to proprioception, providing the brain with information about knee position and movement, which is crucial for coordinated motion and balance.

Medial vs. Lateral Meniscus: Key Differences

While both menisci share common functions, there are structural and functional distinctions:

• Shape: The medial meniscus is C-shaped (semicircular), whereas the lateral meniscus is more O-shaped (circular).
• Attachment and Mobility: The medial meniscus is more firmly attached to the tibia via the coronary ligaments and adheres to the deep fibers of the medial collateral ligament (MCL). This strong attachment makes it less mobile and, consequently, more susceptible to tearing during twisting injuries compared to the more mobile lateral meniscus.
• Vascularity: Both menisci are largely avascular, receiving blood supply only from their peripheral attachments (the "red zone"). The inner two-thirds are avascular (the "white zone"), which significantly impacts their healing potential after injury.

Clinical Significance: Injury and Dysfunction

Due to its anatomical attachments and role in load bearing, the medial meniscus is more frequently injured than the lateral meniscus. Tears can occur from:

• Acute trauma: Often involving a twisting motion on a flexed, weight-bearing knee (e.g., sports injuries).
• Degenerative changes: Gradual wear and tear, common in older individuals.

A torn medial meniscus can lead to pain, swelling, clicking, catching, or locking of the knee, and can significantly impair knee function. Furthermore, meniscal damage or removal (meniscectomy) significantly increases the contact stresses on the articular cartilage, accelerating the onset and progression of osteoarthritis in the affected compartment.

Preserving Meniscal Health

Maintaining meniscal health is vital for long-term knee function and joint longevity. Strategies include:

• Proper Movement Mechanics: Ensuring correct form during exercises and daily activities, especially those involving squatting, lunging, or pivoting, minimizes undue stress on the menisci.
• Strength Training: Developing robust musculature around the knee, including the quadriceps, hamstrings, and glutes, enhances dynamic joint stability and helps absorb forces, thereby offloading the menisci.
• Flexibility and Mobility: Maintaining adequate range of motion in the knee and surrounding joints prevents compensatory movements that could strain the menisci.
• Weight Management: Reducing excess body weight significantly decreases the load on the knee joints, lessening the cumulative stress on the menisci.
• Gradual Progression: Incrementally increasing training intensity and volume allows the body, including the menisci, to adapt and strengthen, reducing the risk of overload injuries.