Pelvic Cartilage: The Pubic Symphysis, Sacroiliac Joints, and Their Importance

What is the cartilage between the pelvic bones?

The primary cartilage connecting the two halves of the pelvic bones anteriorly is the pubic symphysis, a fibrocartilaginous joint that provides stability while allowing for slight movement. Posteriorly, the sacroiliac joints, though primarily synovial, also involve cartilage surfaces.

Understanding the Pelvic Girdle

The human pelvis is a robust, ring-like bony structure located at the base of the spine, serving as a critical anatomical bridge between the axial skeleton (spine and skull) and the lower appendicular skeleton (legs). It is formed by the fusion of several bones: the sacrum and coccyx posteriorly, and two large, irregularly shaped hip bones (also known as innominate bones or coxal bones) laterally and anteriorly. Each hip bone is itself a fusion of three distinct bones: the ilium (the large, flaring upper part), the ischium (the lower, posterior part you sit on), and the pubis (the anterior, lower part). The connections between these bones are facilitated by specific types of joints, some of which are cartilaginous.

The Pubic Symphysis: The Key Cartilaginous Connection

The most direct answer to the question lies at the front of the pelvis.

Location and Structure: The pubic symphysis is a midline cartilaginous joint located anteriorly, connecting the left and right pubic bones. It is classified as a secondary cartilaginous joint, also known as a symphysis. Unlike primary cartilaginous joints (synchondroses) which are typically temporary, symphyses are permanent and allow for limited movement.

Composition: The joint consists of a disc of fibrocartilage sandwiched between the articular surfaces of the two pubic bones, which are themselves covered by a thin layer of hyaline cartilage. Fibrocartilage is particularly suited for this role due to its high tensile strength and ability to resist compression, making it excellent for weight-bearing and shock absorption.

Ligamentous Support: Strong ligaments reinforce the pubic symphysis, providing crucial stability:

• 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.

Function: The pubic symphysis plays a vital role in:

• Pelvic Stability: It helps to maintain the integrity of the pelvic ring, distributing forces evenly across the pelvis.
• Shock Absorption: Its fibrocartilaginous nature allows it to absorb impact forces transmitted up from the lower limbs or down from the spine.
• Limited Movement: While highly stable, the pubic symphysis allows for slight, subtle movements. This minimal flexibility becomes particularly significant during childbirth, where hormonal changes (like relaxin) can increase the laxity of the joint, allowing for slight widening of the pelvic outlet to facilitate passage of the baby.

Other Pelvic Joints and Their Cartilage

While the pubic symphysis is the primary cartilaginous connection, it's important to acknowledge other joints within the pelvic girdle that also involve cartilage.

Sacroiliac (SI) Joints:

• Location: These are two large, complex joints located posteriorly, connecting the sacrum (the triangular bone at the base of the spine) with the ilium of each hip bone.
• Type of Joint: The SI joints are classified as synovial joints, meaning they have a joint capsule, synovial fluid, and articular cartilage. However, their movement is highly restricted by powerful ligaments, making them functionally more akin to fibrous joints.
• Cartilage: The articular surfaces of the sacrum and ilium within the SI joint are covered by cartilage. The sacral surface is typically covered by hyaline cartilage, while the iliac surface is covered by fibrocartilage. This combination contributes to the joint's ability to withstand significant loads and provide stability.
• Function: The SI joints are crucial for transferring the weight of the upper body to the lower limbs, acting as major shock absorbers and providing stability during standing, walking, and running.

Why is This Cartilage Important for Movement and Health?

The cartilaginous structures within the pelvis are fundamental to its overall function and our ability to move efficiently and without pain.

• Load Bearing and Force Transmission: The pubic symphysis and SI joints, with their robust cartilaginous components, are designed to withstand immense compressive and shear forces. They efficiently transmit weight from the spine to the legs and absorb ground reaction forces.
• Dynamic Stability: While stable, the inherent slight flexibility of these cartilaginous joints allows for necessary micro-movements that accommodate changes in posture, gait, and internal physiological demands (e.g., pregnancy).
• Prevention of Bone-on-Bone Friction: The smooth, resilient cartilage surfaces prevent direct bone-on-bone contact, reducing friction and protecting the underlying bone from wear and tear.
• Clinical Relevance: Dysfunction or degeneration of these cartilaginous structures can lead to significant pain and mobility issues.
  • Pubic Symphysis Dysfunction (PSD): Often seen during pregnancy, involving excessive laxity and pain in the pubic symphysis.
  • Sacroiliac Joint Dysfunction (SIJD): Can cause lower back, buttock, and leg pain due to either too much or too little movement in the SI joint.
  • Osteoarthritis: While less common in the pubic symphysis compared to other joints, degeneration of the cartilage can occur in both the pubic symphysis and SI joints, leading to pain and stiffness.

Maintaining Pelvic Health

Preserving the health and integrity of the cartilage within the pelvic girdle is essential for lifelong mobility and pain-free movement.

• Strengthen Core and Pelvic Floor: Strong core muscles (abdominals, obliques, back extensors) and a well-functioning pelvic floor provide critical support and stability to the entire pelvic region, reducing undue stress on the joints.
• Practice Good Biomechanics: Proper lifting techniques, maintaining good posture, and using correct form during exercise minimize asymmetric loading and excessive forces on the pelvic joints.
• Balanced Movement: A combination of strength training and mobility work helps maintain optimal joint health. Avoid over-stretching or activities that cause pain, especially if you have known joint hypermobility.
• Listen to Your Body: Persistent pain in the pelvic region, especially during movement or weight-bearing activities, warrants consultation with a healthcare professional, such as a physical therapist, orthopedist, or kinesiologist. Early intervention can help manage conditions and prevent further degeneration.

Understanding the role of the cartilage between the pelvic bones highlights the intricate design of the human body and the importance of supporting these structures through informed movement and lifestyle choices.