Knee Meniscus: Functions, Anatomy, and Clinical Importance

What is the function of the meniscus fibrocartilage?

The menisci are crucial C-shaped fibrocartilage structures within the knee joint, primarily functioning to absorb shock, distribute load, enhance joint stability, facilitate lubrication, and provide essential proprioceptive feedback, all vital for healthy knee mechanics and long-term joint integrity.

Introduction to the Knee Menisci

The knee joint, one of the largest and most complex joints in the human body, relies on a sophisticated interplay of bones, ligaments, tendons, and cartilage for its remarkable range of motion and load-bearing capacity. Central to this intricate design are the menisci – two crescent-shaped pads of fibrocartilage nestled between the femur (thigh bone) and the tibia (shin bone). Often simply referred to as "the menisci," these structures are distinct from the articular cartilage that covers the ends of the bones, though they work in concert to ensure smooth and efficient knee function.

There are two menisci in each knee:

• Medial Meniscus: Located on the inner (medial) side of the knee, it is larger and more C-shaped, and firmly attached to the medial collateral ligament (MCL). This makes it less mobile and more prone to injury.
• Lateral Meniscus: Situated on the outer (lateral) side, it is smaller, more O-shaped, and less firmly attached, granting it greater mobility.

Composed of approximately 70% water and a dry weight primarily made up of collagen fibers (Type I), proteoglycans, and glycoproteins, the menisci possess unique biomechanical properties that allow them to perform their diverse functions.

Primary Functions Explained

The menisci are far from passive cushions; they are dynamic structures integral to the biomechanical efficiency and longevity of the knee joint. Their primary functions include:

Shock Absorption

The menisci act as the knee's primary shock absorbers. During activities like walking, running, or jumping, significant impact forces are transmitted through the knee. The menisci deform under these loads, effectively dampening and dispersing the forces across a wider area of the tibial plateau. This protective mechanism prevents excessive stress from being concentrated directly onto the articular cartilage of the femur and tibia, thus safeguarding these surfaces from premature wear and tear.

Load Transmission and Distribution

One of the most critical functions of the menisci is to transmit and distribute compressive loads across the knee joint. Without the menisci, the contact area between the rounded femoral condyles and the relatively flat tibial plateau would be small, leading to extremely high-pressure points. The menisci increase the contact area by approximately 50-70%, effectively spreading the weight-bearing forces over a larger surface. This reduction in contact stress is vital for the health and integrity of the articular cartilage, preventing its degeneration and the onset of osteoarthritis. Studies indicate that removal of a meniscus can increase contact stresses by 200-300%, dramatically accelerating cartilage degradation.

Joint Stability

The menisci contribute significantly to knee joint stability, particularly in the tibiofemoral articulation. By deepening the relatively shallow tibial plateau, they create a more congruent fit for the femoral condyles, enhancing the mechanical stability of the joint. They act as wedges, preventing excessive anterior-posterior (front-back) and medial-lateral (side-to-side) translation of the femur on the tibia. This role is especially important during rotational movements and when the knee is in a flexed position, where the collateral and cruciate ligaments might be less taut.

Joint Lubrication and Nutrition

While not their primary role, the menisci also play a part in joint lubrication and nutrition. Their presence helps to spread synovial fluid more evenly across the articular cartilage surfaces during movement. Synovial fluid is crucial for nourishing the avascular articular cartilage and reducing friction, allowing the joint to move smoothly. By aiding in the distribution of this fluid, the menisci indirectly contribute to the overall health and function of the joint.

Proprioception and Sensory Feedback

The menisci are innervated with nerve endings and mechanoreceptors, particularly in their peripheral regions. This grants them a role in proprioception, the body's ability to sense its position and movement in space. These mechanoreceptors provide the brain with crucial sensory feedback regarding joint position, movement speed, and load. This feedback is essential for neuromuscular control, helping to coordinate muscle activity, maintain balance, and protect the knee from potentially injurious movements.

Anatomical Overview of the Menisci

Understanding the anatomy reinforces the functional roles:

• Shape and Attachments: Both menisci are semilunar (C-shaped) wedges, thicker at their periphery and tapering to a thin inner edge. They are anchored to the tibia by anterior and posterior horns and are connected to each other anteriorly by the transverse meniscal ligament.
• Vascularity: The blood supply to the menisci is limited, primarily confined to the outer one-third (the "red zone"). The inner two-thirds (the "white zone") are largely avascular, receiving nutrition via diffusion from the synovial fluid. This limited blood supply significantly impacts their healing potential after injury.
• Composition: The radial orientation of collagen fibers in the deeper layers of the meniscus is crucial for resisting hoop stresses (forces that try to push the meniscus outwards), while the circumferential fibers provide structural integrity.

Clinical Relevance and Injury

Given their multifaceted roles, injury to the menisci can have profound consequences for knee function and long-term joint health. Meniscal tears are common, often resulting from twisting injuries, direct impact, or degenerative changes over time. When a meniscus is torn, its ability to perform its functions is compromised:

• Reduced Shock Absorption: Leads to increased stress on articular cartilage.
• Impaired Load Distribution: Creates high-pressure points, accelerating cartilage degeneration.
• Decreased Stability: Can lead to a feeling of "giving way" or instability in the knee.
• Mechanical Symptoms: Tears can cause pain, swelling, locking, or clicking in the knee.

Preserving meniscal tissue, whether through repair or partial meniscectomy (removal of only the damaged part), is a primary goal in orthopaedic surgery to mitigate the risk of developing early-onset osteoarthritis.

Conclusion: Integral to Knee Health

The meniscus fibrocartilage is far more than just a passive cushion in the knee. Its intricate structure and strategic placement enable it to perform a suite of critical biomechanical functions: absorbing shock, distributing load, enhancing stability, aiding lubrication, and providing proprioceptive feedback. These roles are indispensable for protecting the articular cartilage, maintaining joint integrity, and ensuring the long-term health and functional capacity of the knee. Understanding the vital functions of the menisci underscores the importance of proper knee care, injury prevention, and thoughtful management of meniscal pathology.