Secondary Cartilaginous Joints: Examples, Structure, and Function

What are two examples of secondary cartilaginous joint?

Two prominent examples of secondary cartilaginous joints, also known as symphyses, include the pubic symphysis and the intervertebral joints between the bodies of adjacent vertebrae.

Understanding Cartilaginous Joints

Joints, or articulations, are critical connections between bones that facilitate movement or provide stability. Within the intricate classification of joints based on the type of material binding the bones and the presence or absence of a joint cavity, cartilaginous joints stand out. These joints are characterized by the bones being united by cartilage, allowing for limited movement.

There are two primary types of cartilaginous joints:

• Primary Cartilaginous Joints (Synchondroses): These joints are formed when bones are joined by hyaline cartilage. They are typically temporary, ossifying (turning into bone) as the body matures. Examples include the epiphyseal plates (growth plates) in long bones. They are generally immobile (synarthrotic).
• Secondary Cartilaginous Joints (Symphyses): These joints feature a plate of fibrocartilage sandwiched between a thin layer of hyaline cartilage covering the opposing bone surfaces. Unlike primary cartilaginous joints, symphyses are persistent throughout life and allow for slight movement (amphiarthrotic). Their structure provides both strength and flexibility.

Characteristics of Secondary Cartilaginous Joints (Symphyses)

Secondary cartilaginous joints, or symphyses, are critical for providing strong, slightly movable connections in various parts of the axial skeleton. Their defining characteristics include:

• Fibrocartilaginous Disc: A robust pad of fibrocartilage forms the primary connecting material between the bones. This fibrocartilage is highly resistant to compression and tension.
• Hyaline Cartilage Endplates: Each bone surface participating in the joint is covered by a thin layer of hyaline cartilage, which interfaces with the fibrocartilaginous disc.
• Slight Mobility: While not as mobile as synovial joints, symphyses allow for limited movement, which is crucial for their functional roles, such as shock absorption and flexibility.
• Location: They are typically found in the midline of the body.

Example 1: The Pubic Symphysis

The pubic symphysis is a classic example of a secondary cartilaginous joint, located anteriorly between the two pubic bones of the pelvis.

• Anatomical Location: It connects the left and right pubic bones at the front of the pelvis.
• Structure:
  • Each pubic bone surface contributing to the joint is covered by a thin layer of hyaline cartilage.
  • Between these hyaline cartilage layers lies a thick, wedge-shaped fibrocartilaginous disc.
  • Ligaments, such as the superior and inferior pubic ligaments, reinforce the joint, providing additional stability.
• Functional Significance:
  • Stability: It provides robust stability to the anterior pelvis, crucial for weight-bearing and locomotion.
  • Shock Absorption: The fibrocartilaginous disc acts as a shock absorber, cushioning forces transmitted through the pelvis.
  • Limited Movement (especially in females): While generally stable, the pubic symphysis allows for a small degree of movement, particularly notable during pregnancy and childbirth. Hormonal changes (e.g., relaxin) increase the laxity of the ligaments and the fibrocartilage, allowing the pelvis to slightly widen, facilitating the passage of the baby through the birth canal.

Example 2: Intervertebral Joints (Between Vertebral Bodies)

The joints formed between the bodies of adjacent vertebrae in the spinal column are another prime example of secondary cartilaginous joints.

• Anatomical Location: These joints are found throughout the vertebral column, from the axis (C2) down to the sacrum, connecting the anterior aspects of adjacent vertebral bodies.
• Structure:
  • The superior and inferior surfaces of each vertebral body are covered by a thin layer of hyaline cartilage, forming the vertebral endplates.
  • Positioned between these endplates is the intervertebral disc, which is the fibrocartilaginous component. Each intervertebral disc consists of:
    • Annulus Fibrosus: The tough, outer ring composed of concentric lamellae (layers) of fibrocartilage, providing strength and containing the inner nucleus.
    • Nucleus Pulposus: The gel-like, central core, primarily composed of water and proteoglycans, which acts as a hydraulic shock absorber.
• Functional Significance:
  • Shock Absorption: The intervertebral discs are exceptional shock absorbers, protecting the vertebrae and brain from impact forces during activities like walking, running, and jumping.
  • Flexibility and Movement: While each individual intervertebral joint allows only a small amount of movement, the cumulative effect of all these joints provides the spine with its remarkable flexibility, enabling movements such as flexion, extension, lateral flexion, and rotation.
  • Load Bearing: These joints bear significant axial loads, distributing forces evenly across the vertebral column.

Clinical Relevance and Importance

Understanding secondary cartilaginous joints is crucial in exercise science and clinical practice. Conditions like pubic symphysis dysfunction (SPD), common in pregnancy, and intervertebral disc herniation or degenerative disc disease (DDD) highlight the vulnerability and importance of these joints. Proper exercise, posture, and biomechanics are essential for maintaining their health and function, emphasizing the role of core stability and appropriate lifting techniques to protect the spine.

Conclusion

Secondary cartilaginous joints, or symphyses, are robust, slightly movable articulations vital for the stability and limited flexibility of the axial skeleton. The pubic symphysis and the intervertebral joints between vertebral bodies are prime examples, each demonstrating the unique structural composition of hyaline cartilage endplates surrounding a fibrocartilaginous disc, enabling them to effectively absorb shock and facilitate essential, albeit limited, movement. Their proper function is fundamental to human locomotion, posture, and overall musculoskeletal health.