Knee Cartilage: Understanding Its Types, Functions, and Health

What is cartilage in the knee?

Cartilage in the knee is a specialized, resilient connective tissue that provides a smooth, low-friction surface for joint movement, absorbs shock, and distributes load across the joint, enabling pain-free and efficient locomotion.

Introduction to Knee Cartilage

The knee joint, a complex hinge joint, relies heavily on its cartilaginous structures for optimal function. These tissues are critical for allowing the femur (thigh bone) and tibia (shin bone) to articulate smoothly, providing the necessary cushioning and stability for everyday activities and athletic endeavors. Without healthy cartilage, the bones would grind directly against each other, leading to pain, inflammation, and significant joint degradation.

Types of Cartilage in the Knee

The knee joint contains two primary types of cartilage, each with distinct structures and functions:

• Articular (Hyaline) Cartilage: This is the smooth, white, glistening tissue that covers the ends of the bones within the knee joint – specifically, the femoral condyles (ends of the thigh bone), the top of the tibial plateau (shin bone), and the posterior surface of the patella (kneecap).

  • Function: Its primary role is to provide an incredibly low-friction surface, allowing the bones to glide effortlessly over each other during movement. It also helps to distribute compressive forces across the joint surface during weight-bearing activities.
  • Composition: Articular cartilage is primarily composed of water (65-80%), collagen fibers (primarily Type II), and proteoglycans (large protein-sugar molecules like aggrecan). These components form a strong yet resilient matrix.
  • Properties: It is remarkably smooth, with a coefficient of friction lower than ice on ice, and possesses viscoelastic properties, meaning it can deform under stress and return to its original shape.

• Meniscus (Fibrocartilage): The menisci (plural for meniscus) are two C-shaped, wedge-like pieces of fibrocartilage located between the femur and tibia on each side of the knee: the medial meniscus (on the inner side) and the lateral meniscus (on the outer side).

  • Function: Menisci serve several crucial roles, including:
    • Shock Absorption: They act as cushions, absorbing up to 50% of the compressive load on the knee.
    • Load Distribution: They help distribute weight evenly across the joint surface, increasing the contact area between the bones and reducing stress on the articular cartilage.
    • Joint Stability: Their wedge shape deepens the shallow tibial plateau, improving the congruity (fit) between the femur and tibia and enhancing joint stability.
    • Lubrication and Nutrition: They assist in the circulation of synovial fluid, which lubricates the joint and provides nutrients to the articular cartilage.
  • Shape and Characteristics: The medial meniscus is typically larger and more C-shaped, while the lateral meniscus is more circular. Both are thicker on their outer edges and taper down towards the inner edge. Unlike articular cartilage, menisci have a limited blood supply, particularly in their inner regions, which impacts their healing potential.

Composition and Structure of Cartilage

Both types of knee cartilage, while distinct, share fundamental structural components:

• Chondrocytes: These are the only cells found within cartilage. They are responsible for synthesizing and maintaining the extracellular matrix (ECM). Chondrocytes reside in small spaces called lacunae within the matrix.
• Extracellular Matrix (ECM): This non-cellular component is what gives cartilage its unique properties. It consists of:
  • Collagen Fibers: Primarily Type II collagen in articular cartilage and Type I collagen in fibrocartilage (meniscus), providing tensile strength and structural integrity.
  • Proteoglycans: Large molecules that attract and retain water, giving cartilage its resilience and ability to withstand compression. Aggrecan is the most abundant proteoglycan in articular cartilage.
  • Water: Constitutes a large percentage of cartilage volume and is crucial for its shock-absorbing and load-distributing capabilities.

A key characteristic of cartilage is its avascular nature, meaning it lacks a direct blood supply. It receives its nutrients primarily through diffusion from the synovial fluid during joint movement. This lack of blood vessels explains why cartilage has a very limited capacity for self-repair after injury or degeneration.

Primary Functions of Knee Cartilage

The collective functions of articular cartilage and menisci are vital for healthy knee joint mechanics:

• Smooth Movement and Reduced Friction: Articular cartilage provides an exceptionally slick surface, allowing the femoral and tibial surfaces to glide over each other with minimal resistance.
• Shock Absorption: Both articular cartilage and menisci deform under load, cushioning the impact of movements like walking, running, and jumping, protecting the underlying bone.
• Load Distribution: They spread the forces applied to the joint over a wider surface area, reducing the peak pressure on any single point and preventing localized damage to the subchondral bone.
• Joint Stability: The menisci, in particular, enhance the fit between the bones, contributing significantly to the overall stability of the knee joint.

Common Issues and Injuries Affecting Knee Cartilage

Given its critical role and limited healing capacity, cartilage is susceptible to various issues:

• Osteoarthritis (OA): This is the most common form of arthritis, characterized by the progressive degeneration and breakdown of articular cartilage. As cartilage wears away, bones can rub directly against each other, leading to pain, stiffness, swelling, and reduced range of motion.
• Acute Tears (e.g., Meniscus Tears): Often resulting from twisting injuries or direct impact, meniscus tears are common in sports. Symptoms include pain, swelling, locking or catching of the knee, and difficulty straightening the leg.
• Chondromalacia Patellae: This refers to the softening and breakdown of the articular cartilage on the underside of the patella (kneecap). It typically causes pain around or under the kneecap, especially with activities like climbing stairs, squatting, or prolonged sitting.
• Cartilage Defects: Localized areas of cartilage damage can occur due to trauma, repetitive stress, or osteochondritis dissecans (a condition where a piece of cartilage and underlying bone separates). These defects can lead to pain, swelling, and mechanical symptoms.

Maintaining Knee Cartilage Health

While cartilage has limited repair capabilities, proactive measures can significantly support its health and slow degeneration:

• Balanced Exercise:
  • Low-Impact Activities: Engage in activities like swimming, cycling, elliptical training, and walking, which minimize stress on the knee joint while promoting joint lubrication and cartilage nutrition.
  • Strength Training: Focus on strengthening the muscles around the knee (quadriceps, hamstrings, glutes, calves) to provide better support and stability, reducing direct stress on the cartilage.
  • Flexibility and Mobility: Maintain good range of motion in the knee and surrounding joints to prevent undue stress during movement.
• Proper Nutrition: A diet rich in anti-inflammatory foods (fruits, vegetables, omega-3 fatty acids) can help reduce systemic inflammation that may impact joint health. Some individuals also consider supplements like glucosamine and chondroitin, though their efficacy is debated.
• Weight Management: Maintaining a healthy body weight significantly reduces the load and stress on your knee joints, thereby preserving cartilage integrity.
• Avoiding Overuse and Improper Form: Be mindful of repetitive high-impact activities and ensure proper biomechanics during exercise and daily movements to prevent excessive wear and tear.
• Listen to Your Body: Pay attention to pain signals. Persistent knee pain, swelling, or stiffness should not be ignored.

When to Seek Medical Attention

If you experience persistent knee pain, swelling, stiffness, a popping or clicking sensation, or a feeling of instability that interferes with your daily activities or exercise, it is important to consult a healthcare professional. Early diagnosis and appropriate management can help preserve joint function and improve long-term outcomes for cartilage health.