Knee Meniscus: Anatomy, Biomechanics, and Clinical Significance

What is the Physiology of the Meniscus?

The meniscus, a crucial fibrocartilaginous structure within the knee joint, plays a multifaceted physiological role, primarily centered on optimizing load distribution, enhancing joint stability, and facilitating smooth articulation between the femur and tibia.

Anatomical Overview of the Menisci

The human knee joint houses two menisci: the medial meniscus and the lateral meniscus. These crescent-shaped structures are strategically positioned between the femoral condyles and the tibial plateau.

• Medial Meniscus: This is a C-shaped structure, larger and more firmly attached to the tibia and the joint capsule (via the deep medial collateral ligament). Its robust attachments limit its mobility, making it more susceptible to injury.
• Lateral Meniscus: More O-shaped or circular, the lateral meniscus is smaller and less firmly attached to the joint capsule, allowing for greater mobility. It has a unique connection to the popliteus muscle via the popliteal hiatus, which helps retract it during knee flexion.
• Attachments: Both menisci are anchored to the tibial plateau by anterior and posterior horns. They are connected to each other anteriorly by the transverse meniscal ligament (intermeniscal ligament).

Histological Composition and Structure

The meniscus is composed of fibrocartilage, a specialized connective tissue that combines the strength of fibrous tissue with the resilience of cartilage. This unique composition is fundamental to its physiological functions.

• Collagen Fibers: The predominant structural component, primarily Type I collagen, accounts for 75% of the dry weight. These fibers are arranged in a highly organized manner:
  • Circumferential (longitudinal) fibers: Form the bulk of the tissue, running parallel to the periphery. These fibers resist hoop stresses, preventing the meniscus from extruding radially under compressive loads.
  • Radial (tie) fibers: Interconnect the circumferential fibers, increasing the tissue's resistance to longitudinal splitting and providing structural integrity.
• Proteoglycans and Glycoproteins: These molecules, including aggrecan and decorin, attract and retain water within the extracellular matrix, contributing to the meniscus's viscoelastic properties and its ability to absorb shock.
• Cellular Components: The primary cells are fibrochondrocytes, which are responsible for synthesizing and maintaining the extracellular matrix. These cells exhibit characteristics of both fibroblasts and chondrocytes.
• Vascularity: The blood supply to the meniscus is limited. The outer 10-30% (the "red zone") receives direct blood supply from the geniculate arteries, making this region capable of healing. The inner two-thirds (the "white zone") is largely avascular, relying on diffusion from synovial fluid for nutrition, which significantly impairs its healing capacity.
• Innervation: Nerve endings are concentrated in the outer one-third of the meniscus, particularly in the horns. This innervation provides the brain with proprioceptive information about knee joint position and movement, as well as pain sensation in the event of injury.

Biomechanical Functions of the Menisci

The unique anatomy and histology of the menisci enable them to perform several critical biomechanical functions within the knee joint.

• Load Transmission and Distribution: This is arguably the most vital function. The menisci increase the contact area between the rounded femoral condyles and the relatively flat tibial plateau by approximately 2-3 times. This dramatically reduces the contact stress on the articular cartilage, protecting it from excessive wear and tear. Without menisci, peak contact stresses would increase by 200-300%.
• Shock Absorption: The viscoelastic properties of the fibrocartilage allow the menisci to absorb and dissipate impact forces during activities like walking, running, and jumping, preventing direct bone-on-bone collision and protecting the articular cartilage and subchondral bone.
• Joint Stability: By deepening the shallow tibial plateau and acting as dynamic wedges, the menisci enhance the congruity of the knee joint. They contribute significantly to secondary stability, particularly in limiting anterior-posterior translation of the tibia relative to the femur and resisting varus/valgus forces.
• Joint Lubrication and Nutrition: The menisci help spread synovial fluid across the articular cartilage surfaces during joint movement, facilitating lubrication and nutrient distribution to the avascular articular cartilage.
• Proprioception: The rich innervation in the meniscal horns provides crucial sensory feedback regarding joint position, movement, and load. This proprioceptive input contributes to neuromuscular control and dynamic joint stability.

Physiological Adaptations and Responses

The meniscus is a dynamic structure that responds to mechanical loads and physiological changes.

• Response to Load: Under physiological loading, the meniscal matrix undergoes deformation, which helps distribute stress. Chronic excessive loads or acute traumatic forces can lead to micro-damage or macroscopic tears. Conversely, prolonged immobilization can lead to atrophy and weakening of the meniscal tissue.
• Healing Potential: Due to the limited vascularity of the inner zones, the healing capacity of a meniscal tear is highly dependent on its location. Tears in the outer "red zone" have a greater potential to heal spontaneously or with surgical repair, whereas tears in the avascular "white zone" typically do not heal and may require surgical removal (meniscectomy).
• Degeneration: With aging, the meniscus undergoes degenerative changes, including reduced cellularity, decreased water content, and altered collagen organization. These changes make the meniscus more brittle and susceptible to tears, even with minor trauma.
• Role in Osteoarthritis: Meniscal damage or loss significantly alters knee joint mechanics, leading to increased contact stresses on the articular cartilage. This accelerated wear and tear is a major contributing factor to the progression of knee osteoarthritis.

Clinical Significance for Movement and Health

Understanding meniscal physiology is paramount in the fields of exercise science, rehabilitation, and orthopedic medicine. Injuries to the meniscus, such as tears, can severely compromise the knee's biomechanical function, leading to pain, swelling, instability, and a heightened risk of developing early-onset osteoarthritis. Exercise professionals must appreciate the menisci's role in load management and joint protection when designing training programs, especially for individuals with a history of knee injury or those engaging in high-impact activities. Rehabilitation strategies post-meniscal injury or surgery focus on restoring joint mechanics, strengthening surrounding musculature, and gradually reintroducing load to protect the integrity of the knee joint and its vital fibrocartilaginous shock absorbers.