Knee Cartilage: Types, Functions, and Importance

What part of the knee is cartilage?

Cartilage in the knee joint primarily refers to two distinct types: the smooth, glistening articular (hyaline) cartilage that covers the ends of the bones, and the tough, C-shaped menisci (fibrocartilage), which act as shock absorbers and stabilizers between the thigh and shin bones.

Understanding the Knee Joint: A Brief Overview

The knee is one of the largest and most complex joints in the human body, serving as a critical hinge joint that allows for flexion, extension, and slight rotation. It's formed by the articulation of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). For these bones to move smoothly against each other without friction, and to absorb the significant forces placed upon them during daily activities and exercise, specialized cartilaginous structures are essential.

The Two Primary Forms of Knee Cartilage

Within the knee, cartilage exists in two principal forms, each with unique structural properties and functions:

Articular (Hyaline) Cartilage

• Location: This type of cartilage forms a thin, incredibly smooth, and resilient layer covering the ends of the bones within the joint capsule. Specifically, it caps the distal end of the femur, the proximal end of the tibia, and the posterior surface of the patella where it glides against the femur.
• Function: Its primary role is to provide a low-friction surface for joint movement, allowing the bones to glide effortlessly over one another during activities like walking, running, and squatting. It also helps distribute compressive loads across the joint surface.
• Composition: Articular cartilage is primarily composed of water (65-80%), type II collagen fibers (10-30%), and proteoglycans (5-10%). These components give it its unique viscoelastic properties.
• Characteristics: A key characteristic of articular cartilage is its avascular nature, meaning it has no direct blood supply, and it is largely aneural, meaning it has no nerve endings. This lack of blood supply significantly limits its ability to heal and repair itself after injury or degeneration, making damage to this tissue particularly challenging to manage.

Menisci (Fibrocartilage)

• Location: The menisci are two crescent-shaped pads of fibrocartilage located between the femoral condyles and the tibial plateau. There are two menisci in each knee: the medial meniscus (on the inner side of the knee, more C-shaped) and the lateral meniscus (on the outer side, more O-shaped).
• Function: Unlike articular cartilage, the menisci serve multiple vital functions:
  • Shock Absorption: They act as crucial shock absorbers, dispersing forces across the knee joint by increasing the contact area between the femur and tibia.
  • Load Distribution: They distribute body weight evenly across the joint, preventing concentrated stress on specific areas of the articular cartilage.
  • Joint Stability: They contribute to knee stability by deepening the tibial plateau, creating a more congruent fit for the femoral condyles.
  • Lubrication and Nutrition: They assist in the circulation of synovial fluid, aiding in joint lubrication and nutrient delivery to the articular cartilage.
• Composition: Menisci are primarily composed of type I collagen fibers, which are oriented in a way that allows them to withstand compressive and tensile forces effectively.
• Characteristics: Unlike articular cartilage, the outer one-third of the menisci is vascularized, meaning it has a blood supply, which allows for some capacity for healing if tears occur in this "red zone." The inner two-thirds, the "white zone," are avascular and have limited healing potential.

Why Cartilage is Crucial for Knee Health

The integrity of both articular cartilage and the menisci is paramount for optimal knee function. They collectively ensure:

• Smooth, Pain-Free Movement: By reducing friction between bones.
• Effective Shock Absorption: Protecting the underlying bone from impact forces.
• Even Load Distribution: Preventing premature wear and tear on the joint surfaces.
• Joint Stability: Contributing to the overall mechanical integrity of the knee.

Without healthy cartilage, the knee joint is susceptible to increased friction, bone-on-bone contact, and inefficient force transmission, leading to pain, inflammation, and degenerative conditions.

Common Cartilage Issues and Their Implications

Damage to knee cartilage is a common cause of knee pain and dysfunction, particularly among athletes and the aging population.

• Osteoarthritis: This is the most common form of knee arthritis, characterized by the progressive degeneration and loss of articular (hyaline) cartilage. As the smooth cartilage wears away, bone-on-bone friction increases, leading to pain, stiffness, swelling, and reduced mobility.
• Meniscal Tears: Tears in the menisci can occur acutely due to sudden twisting motions (common in sports) or degeneratively over time due to repeated stress and aging. Symptoms often include pain, swelling, clicking, catching, or locking of the knee.
• Chondromalacia Patellae: This condition involves the softening and breakdown of the articular cartilage on the underside of the patella. It often presents as anterior knee pain, particularly during activities that involve knee bending.

Protecting Your Knee Cartilage

While cartilage has limited healing capacity, several strategies can help preserve its health and mitigate the risk of injury or degeneration:

• Maintain a Healthy Body Weight: Excess body weight significantly increases the load on your knee joints, accelerating cartilage wear.
• Strengthen Supporting Musculature: Develop strong quadriceps, hamstrings, and gluteal muscles. These muscles act as dynamic stabilizers for the knee, absorbing forces and reducing stress on the cartilage.
• Utilize Proper Biomechanics: Pay close attention to your form during exercises and daily movements. Avoid excessive valgus (knee caving inward) or varus (knee bowing outward) stress, and ensure proper alignment during squats, lunges, and jumping.
• Incorporate Low-Impact Activities: Balance high-impact exercises with activities like swimming, cycling, or elliptical training, which provide cardiovascular benefits without excessive joint stress.
• Gradual Progression: When increasing exercise intensity or volume, do so gradually to allow your tissues, including cartilage, to adapt without overload.
• Listen to Your Body: Do not push through sharp or persistent knee pain. Pain is a signal that something is wrong and warrants attention.
• Balanced Nutrition: A diet rich in anti-inflammatory foods, adequate protein, and micronutrients supports overall joint health.

Conclusion: The Unsung Hero of Knee Function

Knee cartilage, in its two distinct forms—articular cartilage and the menisci—is an indispensable component of a healthy, functional knee joint. Understanding its location, structure, and critical roles in shock absorption, friction reduction, and stability empowers you to make informed decisions about your training and lifestyle. By proactively protecting these vital tissues through smart exercise, proper biomechanics, and mindful living, you can significantly enhance your long-term knee health and maintain active, pain-free movement.