Knee Menisci: Understanding Their Crucial Biomechanical Functions

What is the function of the meniscus biomechanics?

The menisci are crucial fibrocartilaginous structures within the knee joint, primarily functioning to enhance joint stability, distribute compressive loads, absorb shock, and facilitate joint lubrication, thereby protecting the articular cartilage and optimizing knee mechanics.

Anatomy and Position within the Knee

The knee joint, a complex hinge joint, relies on several structures for its intricate movements and stability. Among the most vital are the menisci. There are two menisci in each knee:

• Medial Meniscus: Located on the inner side of the knee, it is C-shaped and more firmly attached, making it less mobile and more prone to injury.
• Lateral Meniscus: Situated on the outer side of the knee, it is more O-shaped and less firmly attached, granting it greater mobility.

Both menisci are crescent-shaped wedges of fibrocartilage, a tough, flexible tissue that combines the strength of fibrous tissue with the resilience of cartilage. They sit atop the tibial plateau, acting as crucial interfaces between the rounded femoral condyles and the relatively flat tibial plateau.

Primary Biomechanical Functions

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

Load Transmission and Distribution

One of the most significant functions of the menisci is to increase the contact area between the femoral condyles and the tibial plateau. Without the menisci, the rounded femur would articulate with a relatively flat tibia over a very small area, leading to highly concentrated stress.

• By increasing the contact area, the menisci effectively distribute the compressive forces across a larger surface, significantly reducing the peak stresses on the articular cartilage of both the femur and tibia. This protective mechanism is vital, as excessive stress accelerates articular cartilage degeneration and increases the risk of osteoarthritis.
• During activities like walking, running, or jumping, the menisci can bear up to 50% of the compressive load in extension and up to 85% in 90 degrees of flexion.

Shock Absorption

The menisci act as natural shock absorbers within the knee. When impact forces are transmitted through the leg, such as during landing from a jump or running, the menisci deform and compress, dissipating the energy.

• This dampening effect helps to reduce the magnitude of forces transmitted to the underlying bone and articular cartilage, protecting these structures from damage due to repetitive microtrauma or acute high-impact events.

Joint Stability and Congruity

While ligaments are the primary stabilizers of the knee, the menisci contribute significantly to joint stability by enhancing congruity between the femur and tibia.

• Their wedge shape helps to deepen the shallow tibial plateau, creating a more conforming socket for the femoral condyles. This "chock block" or "wedge" effect prevents excessive anterior-posterior translation (sliding) and medial-lateral movement of the femur on the tibia.
• They also contribute to rotational stability, guiding the rolling and gliding movements of the femur during knee flexion and extension.

Joint Lubrication and Nutrition

The menisci play a role in facilitating the distribution of synovial fluid within the joint.

• As the knee moves and the menisci deform, they help to spread the synovial fluid across the articular surfaces. Synovial fluid is crucial for lubricating the joint, reducing friction during movement, and providing nutrients to the avascular articular cartilage.
• This mechanism helps maintain a low-friction environment, essential for smooth, pain-free movement and the long-term health of the articular cartilage.

Proprioception

Recent research indicates that the menisci also contain mechanoreceptors (nerve endings sensitive to mechanical stimuli like pressure and stretch).

• These receptors contribute to proprioception, the body's sense of its position and movement in space. This sensory input provides feedback to the nervous system, which helps in coordinating muscle activity for joint control and stability, particularly during dynamic movements.

Meniscal Movement and Adaptation

The menisci are not static structures; they move and deform in response to knee motion and loading.

• During flexion, both menisci are pushed posteriorly by the femoral condyles.
• During extension, they move anteriorly.
• The lateral meniscus moves more than the medial meniscus due to its looser attachments, accommodating the rotational movements of the knee (e.g., screw-home mechanism).

This dynamic adaptation allows them to continuously optimize the contact area and distribute loads effectively throughout the full range of motion.

Clinical Implications of Meniscal Dysfunction

Given their critical biomechanical roles, injury or degeneration of the menisci can have profound consequences for knee health.

• Meniscal tears compromise their ability to distribute load and absorb shock, leading to increased stress on the articular cartilage.
• Meniscectomy (surgical removal of part or all of the meniscus) significantly reduces the contact area, leading to a dramatic increase in contact stress on the articular cartilage. This is a major risk factor for the premature development of osteoarthritis in the knee. Studies show that removal of even a small portion of the meniscus can increase contact stresses by over 200%.

Understanding the intricate biomechanics of the menisci underscores their vital role in maintaining knee joint health, stability, and longevity. Protecting these structures through proper movement mechanics and rehabilitation is paramount for long-term knee function.