Menisci: Shapes, Functions, and Clinical Relevance

What shapes are menisci?

Menisci are crescent-shaped, semi-circular, or C-shaped fibrocartilaginous structures primarily found in the knee joint, acting as crucial shock absorbers and stabilizers, with distinct shapes tailored to their specific locations and functions.

Introduction to Menisci

Menisci, plural for meniscus, are specialized C-shaped or O-shaped pads of fibrocartilage that reside within certain synovial joints. Their primary roles include distributing load, absorbing shock, aiding joint stability, and facilitating smooth articulation between bones. While menisci are most famously associated with the knee joint, similar fibrocartilaginous discs or meniscoids can be found in other joints like the temporomandibular joint (TMJ), acromioclavicular (AC) joint, and sternoclavicular joint, each adapted to the unique biomechanics of its respective location.

The Unique Shapes of the Knee Menisci

Within the human knee, there are two distinct menisci: the medial meniscus and the lateral meniscus. Their differing shapes are critical adaptations to the contours of the femoral condyles and the tibial plateau, optimizing their function and interaction during joint movement.

• Medial Meniscus:

  • The medial meniscus is characteristically C-shaped or semi-circular. It is larger and broader than the lateral meniscus.
  • Its open end faces laterally, providing a larger surface area to articulate with the medial femoral condyle.
  • It is more firmly attached to the tibia and the joint capsule, particularly via the deep fibers of the medial collateral ligament (MCL). This stronger attachment makes it less mobile and, consequently, more susceptible to tears (especially in rotational injuries) compared to the lateral meniscus.

• Lateral Meniscus:

  • The lateral meniscus is more O-shaped or nearly circular, resembling a closed ring.
  • It covers a greater proportion of the articular surface of the lateral tibial plateau than the medial meniscus does on its side.
  • It is less firmly attached to the joint capsule and has no direct attachment to the lateral collateral ligament (LCL), allowing for greater mobility. This increased freedom of movement helps it accommodate the greater excursion of the lateral femoral condyle during knee flexion and extension, making it generally less prone to injury than the medial meniscus.

Functional Significance of Meniscal Shape

The specific shapes of the menisci are integral to their multifaceted roles in knee joint function:

• Load Distribution and Shock Absorption: The wedge-like cross-sectional shape and the overall C/O shapes allow menisci to effectively increase the contact area between the femoral condyles and the tibial plateau. This distributes compressive forces over a wider area, reducing stress on the articular cartilage and acting as crucial shock absorbers during activities like walking, running, and jumping.
• Joint Stability: By deepening the relatively flat tibial plateau, the menisci enhance the congruence between the femur and tibia, providing static and dynamic stability to the knee joint. Their shapes help guide the movement of the femoral condyles during flexion and extension.
• Lubrication and Nutrition: Menisci aid in the distribution of synovial fluid across the articular cartilage, contributing to joint lubrication and the nourishment of the chondrocytes within the cartilage.
• Proprioception: Nerve endings within the menisci, particularly in their peripheral vascularized zones, contribute to proprioception – the body's sense of joint position and movement. This feedback is vital for coordinated movement and balance.

Anatomical Considerations and Clinical Relevance

The shape of each meniscus, combined with its unique attachments and vascular supply, directly impacts its vulnerability to injury and its capacity for healing.

• Attachment Points: Both menisci are anchored to the tibial plateau by anterior and posterior horns, and to the joint capsule by coronary ligaments. The medial meniscus's firm attachment to the MCL is a key reason for its higher incidence of tears. The lateral meniscus's greater mobility is partly due to the popliteus tendon inserting into its posterior aspect, pulling it posteriorly during knee flexion.
• Vascularity: The outer (red) zone of the menisci is vascularized, meaning it receives blood supply and has a better capacity for healing. The inner (white) zone is avascular and receives nutrition primarily from synovial fluid, making tears in this region less likely to heal spontaneously. The overall shape influences the proportion of each zone.

Beyond the Knee: Menisci in Other Joints

While the knee menisci are the most prominent examples, similar fibrocartilaginous structures exist in other joints, fulfilling analogous roles:

• Temporomandibular Joint (TMJ): An articular disc, often described as biconcave or oval, separates the temporal bone and the mandible. It allows for complex movements (hinging and gliding) and distributes forces during chewing.
• Acromioclavicular (AC) Joint: A small fibrocartilaginous disc or meniscus can sometimes be found here, helping to improve congruence between the acromion and clavicle.
• Sternoclavicular (SC) Joint: This joint frequently contains an articular disc that helps absorb shock and allows for a wide range of motion of the clavicle.
• Wrist (Triangular Fibrocartilage Complex - TFCC): While not a classic meniscus, the TFCC in the wrist serves similar functions, providing stability, distributing load, and enabling smooth movement between the ulna and carpal bones.

Conclusion

The shapes of menisci, particularly the distinct C-shape of the medial meniscus and the more O-shape of the lateral meniscus in the knee, are not arbitrary. They are precise anatomical adaptations that optimize the biomechanical function of the joint. Understanding these shapes, their attachments, and their functional roles is fundamental for comprehending knee mechanics, assessing injury patterns, and guiding rehabilitation strategies for these vital structures.