Knee Menisci: Movement, Function, and Clinical Relevance

Are the Menisci Movable?

Yes, the menisci are indeed movable structures within the knee joint, exhibiting significant anterior-posterior translation and deformation during knee motion. This dynamic mobility is critical for their multifaceted roles in knee function and health.

Introduction

The knee joint, a marvel of biomechanical engineering, relies on a complex interplay of bones, ligaments, tendons, and cartilage to facilitate movement and bear load. Among its most vital components are the menisci – crescent-shaped fibrocartilaginous pads. While often perceived as static shock absorbers, a deeper understanding of knee biomechanics reveals that the menisci are far from immobile. Their ability to move and deform is fundamental to their function, adapting to the varying demands placed upon the joint during activities ranging from walking to high-impact sports.

Anatomy of the Menisci

Located between the femoral condyles and the tibial plateau, there are two menisci in each knee: the medial meniscus and the lateral meniscus.

• Medial Meniscus: C-shaped, larger, and more firmly attached to the joint capsule and the medial collateral ligament (MCL). Its stronger attachments limit its mobility.
• Lateral Meniscus: More O-shaped and smaller, with less firm attachments to the joint capsule and no direct attachment to the lateral collateral ligament (LCL). This allows for greater mobility compared to the medial meniscus.

Both menisci are primarily composed of fibrocartilage, providing a blend of stiffness and elasticity. They are thicker peripherally and taper centrally, forming a concave surface that deepens the relatively flat tibial plateau, enhancing congruence with the convex femoral condyles.

The Dynamic Nature of Meniscal Movement

The movability of the menisci is a sophisticated adaptation, allowing them to optimize load distribution and joint mechanics throughout the knee's range of motion. Their movement is not passive but orchestrated by joint kinematics and muscle activity.

• Anterior-Posterior Translation: As the knee moves from flexion to extension and vice versa, the menisci translate along the tibial plateau.
  • Knee Flexion: Both menisci move posteriorly. The lateral meniscus moves more significantly (approximately 10-12 mm) than the medial meniscus (approximately 5-6 mm) due to its looser attachments and the influence of the popliteus muscle.
  • Knee Extension: Both menisci move anteriorly, returning to their starting positions.
• Deformation and Shape Change: Beyond simple translation, the menisci also deform and change shape in response to compressive and shear forces. This allows them to conform precisely to the changing curvature of the femoral condyles during movement, ensuring optimal contact area.
• Rotational Movement: During tibial rotation, the menisci also exhibit some rotational motion, further adapting their position to maintain congruence. For example, during internal rotation of the tibia, the medial meniscus moves anteriorly while the lateral meniscus moves posteriorly.

This intricate movement is facilitated by specific anatomical attachments and muscle actions:

• Meniscofemoral Ligaments: The anterior and posterior meniscofemoral ligaments connect the lateral meniscus to the femur, influencing its posterior movement during flexion.
• Capsular Attachments: The peripheral attachments of the menisci to the joint capsule allow for controlled gliding.
• Muscle Influence:
  • The semimembranosus muscle has attachments to the posterior aspect of the medial meniscus, pulling it posteriorly during flexion.
  • The popliteus muscle attaches to the lateral meniscus, pulling it posteriorly during flexion and also contributing to external rotation of the tibia, which assists in "unlocking" the knee from full extension.

Functional Significance of Meniscal Movement

The dynamic mobility of the menisci is not merely an anatomical curiosity; it is fundamentally important for the healthy functioning of the knee joint.

• Optimizing Load Distribution: By translating and deforming, the menisci continuously adjust their position to maximize the contact area between the femur and tibia. This effectively distributes compressive loads over a larger surface, reducing peak stresses on the articular cartilage and preventing premature wear and tear.
• Shock Absorption: The menisci act as crucial shock absorbers, particularly during impact activities. Their ability to deform allows them to absorb and dissipate energy, protecting the underlying bone and cartilage.
• Joint Stability: By deepening the tibial plateau, the menisci enhance the congruence of the knee joint, contributing significantly to its stability, especially during weight-bearing activities. Their movement helps maintain this stability throughout the range of motion.
• Joint Lubrication and Nutrition: Meniscal movement aids in the circulation of synovial fluid throughout the joint, facilitating the lubrication of articular surfaces and the transport of nutrients to the avascular cartilage.
• Proprioception: While less understood, the menisci contain mechanoreceptors that contribute to proprioception – the body's sense of its position in space. Their movement likely plays a role in feeding sensory information back to the central nervous system, contributing to neuromuscular control of the knee.

Factors Influencing Meniscal Movement

The precise movement of the menisci is influenced by several factors:

• Knee Angle: As discussed, flexion and extension directly dictate anterior-posterior translation.
• Tibial Rotation: Internal and external rotation of the tibia on the femur also induce specific meniscal movements.
• Muscle Contraction: The active contraction of muscles such as the semimembranosus and popliteus directly pulls the menisci, influencing their position.
• Ligamentous Integrity: The integrity of the cruciate ligaments (ACL and PCL) and collateral ligaments (MCL and LCL) indirectly affects meniscal movement by governing overall knee kinematics.
• Joint Loading: The magnitude and direction of forces applied to the knee influence how the menisci deform and translate to optimize load distribution.

Clinical Relevance and Injuries

Understanding meniscal movability is crucial in clinical practice. Impaired meniscal movement, often due to tears or degeneration, can lead to significant knee pathology.

• Meniscal Tears: When a meniscus tears, its ability to translate and deform properly can be compromised. This can lead to pain, locking, clicking, and a loss of the menisci's protective functions, accelerating articular cartilage degeneration.
• Displaced Fragments: A torn meniscal fragment may displace and interfere with normal joint mechanics, physically blocking movement or causing a "catching" sensation.
• Post-Surgical Implications: Surgical repair or removal (meniscectomy) of a meniscus alters its biomechanical properties and may affect the knee's long-term health, emphasizing the importance of preserving meniscal tissue whenever possible.

Conclusion

In summary, the menisci are not static pads but dynamic, movable structures that are indispensable for the optimal function and long-term health of the knee joint. Their sophisticated ability to translate, deform, and adapt to joint kinematics allows them to effectively distribute loads, absorb shock, enhance stability, and contribute to joint lubrication and proprioception. Recognizing this intricate mobility is fundamental for anyone seeking a comprehensive understanding of knee biomechanics, from fitness enthusiasts and athletes to personal trainers and clinical practitioners.