The Meniscus: Clinical Anatomy, Structure, Function, and Injuries

What is the clinical anatomy of the meniscus?

The meniscus, a pair of C-shaped (medial) and O-shaped (lateral) fibrocartilaginous structures within the knee joint, is critical for load distribution, shock absorption, joint stability, and lubrication, with its unique anatomical features dictating its function and susceptibility to injury.

Introduction to the Meniscus

The human knee joint, a complex hinge joint, relies on a sophisticated interplay of bones, ligaments, tendons, and cartilage for its remarkable range of motion and stability. Central to its function are the menisci (plural of meniscus), two crescent-shaped pads of fibrocartilage positioned between the femoral condyles and the tibial plateau. These vital structures act as essential intermediaries, optimizing the mechanical environment of the knee during movement and weight-bearing activities. Understanding their precise anatomy is fundamental to appreciating their biomechanical roles and the clinical implications of their injury.

Macroscopic Anatomy: Structure and Shape

The knee joint contains two distinct menisci, each with unique characteristics and attachments:

• Medial Meniscus: This meniscus is semilunar or C-shaped, significantly larger and wider posteriorly than anteriorly. It is firmly attached to the medial collateral ligament (MCL) and the joint capsule, which limits its mobility. Its peripheral attachment also includes the deep fibers of the MCL.
• Lateral Meniscus: In contrast, the lateral meniscus is more circular or O-shaped, covering a larger proportion of the lateral tibial plateau. It is less firmly attached to the joint capsule and has no direct attachment to the lateral collateral ligament (LCL), making it more mobile than the medial meniscus. This increased mobility provides some protection against injury during rotational forces.
• Meniscal Horns: Both menisci possess anterior and posterior horns, which are the tapered ends that attach to the intercondylar region of the tibial plateau via strong ligamentous attachments. The posterior horn of the medial meniscus is particularly vulnerable to degenerative tears.
• Attachments:
  • Coronary Ligaments: These are loose attachments that connect the peripheral edges of both menisci to the tibial plateau, allowing for some movement.
  • Transverse Meniscal Ligament (Intermeniscal Ligament): This small ligament connects the anterior horns of the medial and lateral menisci, providing some stabilization.
  • Meniscofemoral Ligaments: Primarily associated with the lateral meniscus, these include the Ligament of Wrisberg (posterior to the posterior cruciate ligament, PCL) and the Ligament of Humphry (anterior to the PCL). They connect the posterior horn of the lateral meniscus to the medial femoral condyle, influencing its movement during knee flexion and extension.

Microscopic Anatomy: Tissue Composition

At a microscopic level, the menisci are primarily composed of fibrocartilage, a specialized connective tissue that combines the resilience of cartilage with the tensile strength of fibrous tissue.

• Collagen Fibers: The dominant component is Type I collagen, arranged in a highly organized manner. The majority of collagen fibers are oriented circumferentially (parallel to the long axis of the meniscus), forming the main structural framework that resists hoop stresses during load bearing. A smaller number of radial fibers act as "tie-rods," resisting longitudinal splitting and holding the circumferential fibers together.
• Cells: The cellular component consists primarily of fibrochondrocytes, cells that exhibit characteristics of both fibroblasts (producing collagen) and chondrocytes (producing extracellular matrix).
• Extracellular Matrix: Beyond collagen, the matrix contains a relatively small amount of proteoglycans (e.g., aggrecan) and glycoproteins (e.g., fibronectin, elastin), which contribute to the tissue's viscoelastic properties and ability to withstand compressive forces.

Vascularity and Innervation

The blood supply and nerve distribution to the menisci are critical determinants of their healing potential and the sensation of pain.

• Vascularity (Blood Supply): The menisci receive their blood supply from branches of the medial and lateral genicular arteries, forming a perimeniscal capillary plexus around their periphery. This supply, however, is not uniform:
  • Red-Red Zone (Outer 1/3): This outermost portion is well-vascularized, meaning it has a good blood supply. Tears in this region have the highest potential for healing.
  • Red-White Zone (Middle 1/3): This transitional zone has a diminishing blood supply. Healing potential is moderate.
  • White-White Zone (Inner 1/3): This innermost portion is largely avascular (lacks blood supply). Tears in this region have very limited healing capacity, often necessitating surgical intervention if symptomatic.
• Innervation (Nerve Supply): Nerve endings, including mechanoreceptors (Ruffini endings, Pacinian corpuscles, Golgi tendon organ-like endings) and nociceptors (free nerve endings), are primarily concentrated in the outer one-third of the menisci and the meniscal horns. This innervation provides the knee with proprioceptive feedback (joint position sense) and pain sensation. The avascular inner two-thirds of the meniscus are largely aneural, which explains why isolated tears in this region may not be painful unless the tear extends to the vascularized and innervated periphery.

Biomechanical Functions of the Meniscus

The unique anatomical structure of the menisci enables them to perform several vital biomechanical functions within the knee joint:

• Load Transmission and Distribution: The menisci act as crucial load sharers, transmitting approximately 50% of the compressive load in extension and up to 85% in 90 degrees of flexion. They convert axial compressive forces into "hoop stresses" within their circumferential collagen fibers, effectively distributing the load over a larger surface area of the articular cartilage. This significantly reduces peak stresses on the tibial plateau and femoral condyles, protecting the articular cartilage from excessive wear and tear.
• Shock Absorption: Due to their viscoelastic properties, the menisci help to absorb and dissipate impact forces during activities like walking, running, and jumping, further protecting the articular cartilage.
• Joint Stability: By deepening the relatively flat tibial plateau, the menisci enhance the congruity between the femoral condyles and the tibia. This "wedging" effect increases the contact area and contributes to the overall stability of the knee joint, particularly against anterior-posterior and rotational forces.
• Joint Lubrication and Nutrition: The menisci assist in spreading synovial fluid across the articular cartilage surfaces during joint movement, facilitating lubrication and nutrient delivery to the avascular articular cartilage.
• Proprioception: The mechanoreceptors within the menisci contribute to proprioception, providing the central nervous system with information about knee joint position and movement, which is essential for motor control and balance.

Clinical Significance: Common Meniscal Injuries

Given their critical roles and the high forces they withstand, the menisci are frequently injured, with significant clinical consequences.

• Types of Meniscal Tears: Meniscal tears can broadly be categorized as:
  • Traumatic Tears: Often seen in younger, active individuals, resulting from acute, forceful twisting injuries with the foot planted (e.g., during sports). These commonly affect the medial meniscus due to its firmer attachment.
  • Degenerative Tears: More common in older individuals, resulting from repetitive microtrauma and the natural aging process, often occurring with minimal or no distinct injury. These are frequently found in the posterior horn of the medial meniscus.
• Morphology of Tears: Tears are classified by their shape and orientation:
  • Longitudinal/Vertical: Runs parallel to the circumferential fibers. A large longitudinal tear can displace to form a "bucket-handle" tear.
  • Radial: Runs perpendicular to the circumferential fibers, often leading to loss of hoop stress function.
  • Horizontal: Splits the meniscus into superior and inferior halves.
  • Flap/Parrot Beak: Irregular tears.
  • Complex: A combination of tear patterns.
• Symptoms: Common symptoms of a meniscal tear include knee pain (often localized to the joint line), swelling (effusion), clicking, popping, or grinding sensations, and mechanical symptoms such as "locking" (inability to fully extend the knee due to a displaced fragment) or "giving way" (instability).
• Diagnosis: Diagnosis typically involves a detailed clinical history, physical examination (e.g., McMurray's test, Apley's compression test), and imaging studies, with Magnetic Resonance Imaging (MRI) being the gold standard for visualizing meniscal tears.

Conclusion

The menisci are far more than simple "shock absorbers" in the knee; they are complex fibrocartilaginous structures with intricate anatomical features that enable them to perform a multitude of essential biomechanical functions. Their unique vascularity dictates their healing potential, while their innervation contributes to vital proprioception. A comprehensive understanding of the clinical anatomy of the meniscus is paramount for healthcare professionals and fitness educators alike, providing the foundation for effective injury prevention, diagnosis, and management strategies to preserve long-term knee health and function.