Cartilage: Its Role in Cartilaginous and Synovial Joints

What are the joints between cartilage?

While cartilage itself does not form joints between cartilage, it is the primary connective tissue forming a specific class of joints known as cartilaginous joints, and it also plays a critical role as an articular surface within the highly mobile synovial joints.

Understanding Joint Classification

To fully grasp the role of cartilage in articulation, it's essential to understand how joints are classified. A joint, or articulation, is any place where two or more bones meet. Joints are typically classified in two ways:

• Structural Classification: Based on the type of connective tissue that binds the bones together (fibrous, cartilaginous, or synovial).
• Functional Classification: Based on the degree of movement allowed at the joint (synarthrosis - immovable, amphiarthrosis - slightly movable, diarthrosis - freely movable).

Cartilage is fundamental to two of these structural categories, directly forming one type of joint and serving as a crucial component in another.

Cartilaginous Joints (Amphiarthroses)

Cartilaginous joints are a type of structural joint where the bones are entirely united by cartilage (either hyaline cartilage or fibrocartilage), allowing for limited movement. Functionally, these are classified as amphiarthroses. These joints are strong and designed for stability with some flexibility.

There are two primary types of cartilaginous joints:

• Synchondroses: These joints feature bones connected by hyaline cartilage. They are often temporary structures that eventually ossify (turn into bone) with age.

  • Examples:
    • Epiphyseal plates (growth plates): Found between the diaphysis (shaft) and epiphysis (end) of long bones in growing children. These allow for longitudinal bone growth and typically ossify into a synostosis (bony joint) in adulthood.
    • First sternocostal joint: The joint between the first rib and the sternum (breastbone). All other sternocostal joints are synovial.

• Symphyses: In these joints, bones are united by a pad of fibrocartilage. Fibrocartilage is remarkably strong and provides excellent shock absorption, allowing for slight movement. These joints are typically found in the axial skeleton.

  • Examples:
    • Pubic symphysis: The joint connecting the two halves of the pelvis anteriorly. It allows for slight movement, particularly important during childbirth.
    • Intervertebral discs: These thick pads of fibrocartilage are located between the bodies of adjacent vertebrae along the spinal column. Each disc allows for a small degree of movement, but collectively, they enable significant flexibility of the spine while also acting as crucial shock absorbers.

The Role of Cartilage in Synovial Joints (Diarthroses)

While not "joints between cartilage," synovial joints are the most common and movable type of joint in the body, and cartilage plays an indispensable role within them. Synovial joints are characterized by a joint cavity filled with synovial fluid, and their articulating bone surfaces are covered by articular cartilage.

• Articular Cartilage: This is a layer of smooth, slippery hyaline cartilage that covers the ends of the bones within the joint capsule. Its primary functions are:

  • Reducing Friction: Providing a low-friction surface, allowing bones to glide smoothly past each other during movement.
  • Shock Absorption: Distributing forces evenly across the joint surface, protecting the underlying bone from impact.
  • Load Bearing: Helping to transmit compressive loads across the joint.

• Menisci and Articular Discs: In some synovial joints (e.g., the knee, temporomandibular joint), pads of fibrocartilage called menisci (in the knee) or articular discs are present. These structures further enhance joint stability, improve the fit between articulating surfaces, and provide additional shock absorption.

Why Cartilage Health Matters for Joint Function

The integrity of cartilage is paramount for healthy joint function. Given its limited capacity for self-repair, damage or degradation of cartilage can lead to significant pain, reduced mobility, and conditions like osteoarthritis. Understanding the specific types of cartilage and their roles in different joint classifications is crucial for comprehending joint mechanics, injury mechanisms, and rehabilitation strategies in exercise science and kinesiology.

Conclusion: Cartilage as a Keystone of Articulation

In summary, while the phrase "joints between cartilage" might initially seem to suggest cartilage connecting to itself, the reality is that cartilage is the defining tissue of cartilaginous joints (synchondroses and symphyses) and a vital component, as articular cartilage, within the highly mobile synovial joints. Its unique properties—ranging from the temporary growth-plate function of hyaline cartilage to the robust shock absorption of fibrocartilage—make it a keystone structure in the human musculoskeletal system, enabling movement, providing stability, and absorbing the forces of daily life.