The Pubic Symphysis: Joint Type, Structure, Function, and Clinical Importance

What type of joint is between pubic bones in the pelvic girdle?

The joint located between the pubic bones in the pelvic girdle is a cartilaginous joint, specifically classified as a symphysis. This type of joint is characterized by a disc of fibrocartilage uniting the two bones, allowing for limited movement.

Understanding the Pubic Symphysis

The pubic symphysis is a crucial anatomical structure forming the anterior connection of the pelvic girdle. It is not a freely movable joint like those found in the limbs but rather a strong, slightly flexible union designed for stability and subtle accommodation.

• Definition and Location: The pubic symphysis is the midline articulation that joins the left and right superior pubic rami. These are the anterior-most bones of the pelvis, forming the front of the pelvic ring.
• Key Structures:
  • Fibrocartilaginous Disc: The primary component of the symphysis is a dense, wedge-shaped disc of fibrocartilage interposed between the articular surfaces of the two pubic bones. This cartilage acts as a shock absorber and allows for minimal movement.
  • Hyaline Cartilage: The surfaces of the pubic bones themselves, where they articulate with the fibrocartilaginous disc, are covered by a thin layer of hyaline cartilage.
  • Ligaments: A robust network of ligaments reinforces the joint, providing significant stability. These include:
    • Superior Pubic Ligament: Runs along the superior aspect of the joint.
    • Inferior (Arcuate) Pubic Ligament: Forms a strong arch along the inferior aspect, providing significant support.
    • Anterior and Posterior Pubic Ligaments: Contribute to the overall integrity of the joint capsule.

Classification of the Pubic Symphysis Joint

Joints are typically classified based on their structure (the type of tissue connecting the bones) and their function (the degree of movement they permit). The pubic symphysis fits specific criteria within both classification systems.

• Structural Classification: Cartilaginous Joint (Symphysis)
  • Cartilaginous Joints: These are joints where the bones are united by cartilage. There are two main types:
    • Synchondroses: United by hyaline cartilage (e.g., epiphyseal plates in growing bones).
    • Symphyses: United by fibrocartilage (e.g., intervertebral discs, pubic symphysis).
  • The presence of the fibrocartilaginous disc between the pubic bones is the defining characteristic that places it into the symphysis category.
• Functional Classification: Amphiarthrosis
  • Joints are functionally classified by their mobility:
    • Synarthrosis: Immovable joints (e.g., sutures of the skull).
    • Amphiarthrosis: Slightly movable joints (e.g., pubic symphysis, intervertebral discs).
    • Diarthrosis: Freely movable joints (synovial joints like the knee or shoulder).
  • While seemingly rigid, the pubic symphysis allows for a small degree of movement, particularly during activities that stress the pelvis or, most notably, during childbirth.
• Why Not Synovial? Unlike synovial joints, the pubic symphysis lacks a joint capsule, a synovial membrane, and synovial fluid, which are hallmarks of freely movable joints. Its design prioritizes stability over extensive range of motion.

Function and Biomechanical Significance

Despite its limited mobility, the pubic symphysis plays several critical roles in the biomechanics of the human body.

• Pelvic Stability: As a key component of the pelvic ring, it contributes significantly to the overall stability and integrity of the pelvis. This stability is essential for weight bearing, locomotion, and transferring forces between the axial skeleton and the lower limbs.
• Shock Absorption: The fibrocartilaginous disc acts as a shock absorber, attenuating forces transmitted through the pelvis during activities like walking, running, and jumping. This helps to protect the surrounding bones and structures from excessive stress.
• Limited Movement: Although slight, the movement permitted by the symphysis is crucial. It allows for minor adjustments in pelvic shape and position, which can be important during various movements and postures.
• Role in Childbirth: During pregnancy, hormonal changes (particularly relaxin) increase the laxity of the ligaments and fibrocartilage of the pubic symphysis. This increased flexibility allows the pelvic outlet to expand slightly, facilitating the passage of the baby during vaginal delivery.

Clinical Considerations and Common Issues

Given its critical role in pelvic stability and its unique biomechanical properties, the pubic symphysis can be a source of pain and dysfunction.

• Pelvic Girdle Pain (PGP): This is a common condition, especially during pregnancy, where pain can arise from the pubic symphysis or the sacroiliac joints. When the pain is specifically localized to the pubic symphysis, it's often referred to as Pubic Symphysis Dysfunction (PSD) or Symphysis Pubis Dysfunction (SPD).
• Causes of Dysfunction:
  • Hormonal Changes: As mentioned, pregnancy hormones can lead to excessive laxity, causing instability and pain.
  • Trauma: Direct impact or injury to the pelvis can damage the symphysis.
  • Repetitive Stress: Certain athletic activities or occupational demands that place asymmetrical or repetitive stress on the pelvis can contribute to inflammation and pain.
  • Biomechanical Imbalances: Dysfunctions in surrounding muscles (e.g., hip adductors, core muscles) or gait abnormalities can alter forces across the joint, leading to irritation.
• Management: Treatment for pubic symphysis pain often involves conservative approaches such as:
  • Physical Therapy: Focusing on pelvic stability exercises, core strengthening, and correcting muscle imbalances.
  • Pain Management: Rest, ice, and anti-inflammatory medications.
  • Supportive Devices: Pelvic support belts can help stabilize the joint, particularly during pregnancy.
  • Activity Modification: Avoiding activities that exacerbate pain.

Conclusion

The joint between the pubic bones in the pelvic girdle is a cartilaginous joint, specifically a symphysis. This amphiarthrotic joint, while allowing only slight movement, is fundamental to the stability, shock absorption, and adaptability of the pelvic girdle. Understanding its structure and function is crucial for comprehending pelvic biomechanics and addressing common conditions like pubic symphysis dysfunction.