Knee Joint: Understanding Menisci, Functions, and Injury Management

What is the articular disc in the knee joint?

In the context of the knee, the structures commonly referred to as "articular discs" are precisely termed menisci (singular: meniscus). These are crescent-shaped pads of fibrocartilage that play a crucial role in the knee's function, stability, and longevity.

Anatomical Overview: The Menisci of the Knee Joint

While the term "articular disc" is used in some other joints (like the temporomandibular joint or sternoclavicular joint), in the knee, these vital structures are specifically known as the menisci. There are two distinct menisci in each knee:

• Medial Meniscus: Located on the inner (medial) side of the knee. It is C-shaped and more firmly attached to the tibia and joint capsule, making it less mobile and thus more susceptible to injury.
• Lateral Meniscus: Positioned on the outer (lateral) side of the knee. It is more O-shaped and less firmly attached, allowing for greater mobility and often making it less prone to tears compared to its medial counterpart.

Both menisci sit atop the tibial plateau, nestled between the rounded condyles of the femur above and the relatively flat surface of the tibia below. They are essential components of the tibiofemoral joint, the primary weight-bearing joint of the knee.

Structure and Composition of the Menisci

The menisci are primarily composed of fibrocartilage, a specialized connective tissue that combines the strength of fibrous tissue with the resilience of cartilage. This unique composition allows them to withstand significant compressive and shear forces while maintaining flexibility.

Key structural features include:

• Collagen Fibers: Predominantly Type I collagen, arranged in a circumferential pattern with radial tie fibers, providing exceptional resistance to tensile and compressive stresses.
• Proteoglycans and Water: These components contribute to the menisci's ability to absorb shock and distribute load by attracting water, which helps them deform under pressure and then return to their original shape.
• Vascularity: The blood supply to the menisci is limited. The outer "red-red zone" (peripheral 10-30%) receives direct blood supply from the capsular arteries, allowing for some healing potential. The inner "white-white zone" (central 70-90%) is avascular, meaning tears in this region have very poor healing capacity. The "red-white zone" is an intermediate area.
• Innervation: The outer portions of the menisci are innervated, contributing to proprioception (the body's sense of joint position and movement) and pain perception.

Primary Functions of the Menisci

The menisci perform several critical functions that are vital for optimal knee joint health and performance:

• Shock Absorption and Load Distribution: They act as cushions, absorbing up to 50% of the compressive load in extension and up to 85% in 90 degrees of flexion. By increasing the contact area between the femur and tibia, they distribute forces more evenly across the joint surface, significantly reducing stress on the articular cartilage.
• Joint Stability: The menisci deepen the relatively flat tibial plateau, creating a more congruent fit with the rounded femoral condyles. This "wedging" effect enhances the stability of the tibiofemoral joint, particularly during flexion, extension, and rotational movements.
• Joint Lubrication and Nutrition: By spreading synovial fluid across the articular cartilage surfaces during movement, the menisci contribute to joint lubrication and help nourish the chondrocytes (cartilage cells) within the articular cartilage.
• Proprioception: The nerve endings in the outer meniscal zones provide sensory feedback to the brain regarding knee position and movement, contributing to neuromuscular control and coordination.

Common Meniscal Injuries

Meniscal tears are among the most common knee injuries, affecting athletes and the general population alike.

• Mechanism of Injury:
  • Acute Tears: Often result from a sudden twisting motion of the knee while the foot is planted, or from direct impact. This is common in sports involving pivoting, jumping, and cutting (e.g., soccer, basketball, skiing).
  • Degenerative Tears: Occur gradually over time due to wear and tear, often in older individuals, even from simple activities like squatting or stepping off a curb. These tears are common as part of the aging process and can be associated with osteoarthritis.
• Types of Tears: Meniscal tears can vary widely in their pattern, including longitudinal, radial, horizontal, flap, and "bucket-handle" tears. The type and location of the tear significantly influence symptoms and treatment options.
• Symptoms: Common symptoms include localized pain (often along the joint line), swelling (which may develop hours after injury), stiffness, tenderness, catching or locking of the knee, and a feeling of the knee "giving way."
• Diagnosis: Diagnosis typically involves a physical examination by a healthcare professional, including specific knee maneuvers, and often confirmed with magnetic resonance imaging (MRI).

Management and Rehabilitation

Treatment for meniscal tears depends on several factors, including the type, size, and location of the tear, the patient's age, activity level, and symptoms.

• Non-Surgical Management: For small, stable tears (especially in the vascular "red-red zone") or degenerative tears with mild symptoms, conservative treatment may be effective. This includes:
  • RICE Protocol: Rest, Ice, Compression, Elevation.
  • Pain and Inflammation Management: Over-the-counter or prescription anti-inflammatory medications.
  • Physical Therapy: Focuses on restoring range of motion, strengthening the muscles around the knee (quadriceps, hamstrings, glutes), improving proprioception, and gradually returning to activity.
• Surgical Management: When symptoms persist or the tear is significant (e.g., "bucket-handle" tear, mechanical locking), arthroscopic surgery may be recommended.
  • Meniscal Repair: If the tear is in the vascular zone and suitable for repair, the torn edges are sutured together. This preserves the meniscus, but requires a longer, more protected recovery period.
  • Partial Meniscectomy: If the tear is in the avascular zone or too complex to repair, the damaged portion of the meniscus is surgically removed. This typically allows for a quicker return to activity but can increase the long-term risk of osteoarthritis due to reduced shock absorption.
• Rehabilitation: Post-surgical rehabilitation is crucial for optimal outcomes, focusing on restoring strength, flexibility, balance, and sport-specific movements.

Preserving Meniscal Health

While injuries can be unpredictable, adopting proactive strategies can help maintain meniscal health:

• Maintain a Healthy Weight: Excess body weight increases the load on the knee joints, accelerating wear and tear.
• Strengthen Surrounding Muscles: Strong quadriceps, hamstrings, and gluteal muscles provide dynamic stability and support to the knee joint, reducing stress on the menisci.
• Proper Movement Mechanics: Learn and practice correct form for exercises and daily activities, especially those involving twisting or squatting. Avoid sudden, uncontrolled rotational movements.
• Gradual Progression: When increasing activity levels or starting a new exercise program, do so gradually to allow your tissues to adapt.
• Listen to Your Body: Pay attention to pain or discomfort in your knees. Pushing through sharp pain can exacerbate existing issues or lead to new injuries.
• Appropriate Footwear: Wear supportive shoes that are suitable for your activity and provide adequate cushioning.

Understanding the critical role of the menisci in knee function underscores the importance of protecting these fibrocartilaginous structures to maintain long-term knee health and mobility.