Secondary Cartilaginous Joints: Examples, Structure, and Clinical Importance

What is an example of a secondary cartilaginous joint?

A prime example of a secondary cartilaginous joint, also known as a symphysis, is the pubic symphysis, where the two halves of the pelvis meet anteriorly. These joints are characterized by a connecting plate of fibrocartilage, often with a thin layer of hyaline cartilage on the articular surfaces of the bones, allowing for limited movement.

Understanding Joint Classification

The human body's skeletal system is a complex framework that provides support, protection, and allows for movement. Joints, or articulations, are the points where two or more bones meet. Kinesiology and anatomy classify joints based on their structure and the degree of movement they permit. Broadly, joints are categorized into three main types:

• Fibrous Joints: These joints are united by dense fibrous connective tissue, typically allowing for very little or no movement (e.g., sutures of the skull).
• Cartilaginous Joints: Bones are united by cartilage, offering more movement than fibrous joints but less than synovial joints. These are further subdivided into primary and secondary types.
• Synovial Joints: The most common and complex type, characterized by a joint capsule, synovial fluid, and articular cartilage, allowing for a wide range of motion (e.g., knee, shoulder).

Delving into Cartilaginous Joints

Cartilaginous joints are unique in that the bones are joined directly by cartilage. This structural arrangement determines their function and the limited mobility they afford.

• Primary Cartilaginous Joints (Synchondroses): These joints are formed by hyaline cartilage and are typically temporary, ossifying with age. They are largely immobile. An example is the epiphyseal plate (growth plate) in long bones, which fuses once growth is complete.
• Secondary Cartilaginous Joints (Symphyses): These joints are more robust and permanent. They are characterized by articular surfaces covered with a thin layer of hyaline cartilage, but the bones themselves are united by a strong, resilient disc of fibrocartilage. This fibrocartilaginous disc provides both strength and a limited degree of flexibility, allowing for slight movement and acting as a shock absorber. Unlike primary cartilaginous joints, symphyses generally persist throughout life.

The Prime Example: The Pubic Symphysis

The pubic symphysis stands out as a classic and functionally significant example of a secondary cartilaginous joint.

• Anatomy and Location: Located at the front of the pelvis, the pubic symphysis connects the superior rami of the left and right pubic bones. It forms a crucial part of the pelvic girdle, which supports the trunk and connects the axial skeleton to the lower limbs.
• Structure of the Pubic Symphysis: Each pubic bone's articular surface is covered by a thin layer of hyaline cartilage. Between these two hyaline cartilage layers lies a thick, interpubic disc composed of fibrocartilage. This disc is typically thicker in females, especially during pregnancy, to allow for greater flexibility. Ligaments, such as the superior and inferior pubic ligaments, reinforce the joint, providing additional stability.
• Function and Significance:
  • Pelvic Stability: The pubic symphysis plays a vital role in maintaining the structural integrity and stability of the pelvic ring.
  • Shock Absorption: The fibrocartilaginous disc acts as a shock absorber, distributing forces across the pelvis during activities like walking, running, and jumping.
  • Limited Movement: While highly stable, the pubic symphysis allows for slight movements, particularly during gait and weight-bearing activities.
  • Childbirth: In females, the pubic symphysis exhibits increased laxity during pregnancy due to hormonal changes (e.g., relaxin). This increased flexibility allows for slight widening of the pelvic outlet, facilitating the passage of the baby during childbirth.

Other Important Secondary Cartilaginous Joints

Beyond the pubic symphysis, several other crucial secondary cartilaginous joints exist in the human body, performing essential roles:

• Intervertebral Discs: These are perhaps the most well-known secondary cartilaginous joints. Located between adjacent vertebral bodies from the axis (C2) to the sacrum, each intervertebral disc consists of an outer annulus fibrosus (fibrocartilage) and an inner nucleus pulposus (gelatinous core). They act as powerful shock absorbers, distribute compressive forces, and allow for the limited flexibility and movement of the vertebral column.
• Manubriosternal Joint: This joint connects the manubrium (upper part) to the body of the sternum. While seemingly rigid, it is a symphysis that allows for slight movement during respiration, contributing to the expansion and contraction of the thoracic cage.

Clinical Relevance and Considerations

The nature of secondary cartilaginous joints makes them susceptible to certain conditions and injuries:

• Degeneration: Like all joints, symphyses can undergo degenerative changes over time, leading to pain and reduced function. Degenerative disc disease in the spine is a common example.
• Inflammation: Conditions such as osteitis pubis (inflammation of the pubic symphysis) are common in athletes due to repetitive stress.
• Trauma: Though robust, severe trauma can lead to dislocation or rupture of these joints, particularly in high-impact accidents.
• Pregnancy-Related Issues: Excessive laxity or instability of the pubic symphysis during or after pregnancy can lead to pelvic girdle pain, a condition known as symphysis pubis dysfunction (SPD).

Conclusion

Secondary cartilaginous joints, or symphyses, are critical articulations characterized by a uniting plate of fibrocartilage. While allowing for only limited movement, this design provides significant strength and shock-absorbing capabilities. The pubic symphysis serves as an excellent and highly illustrative example, demonstrating the vital role these joints play in structural stability, movement, and critical physiological processes like childbirth. Understanding these joints is fundamental for comprehending human biomechanics and addressing various musculoskeletal conditions.