Sacroiliac Joint: Anatomy, Ligaments, and Stability

What are the Ligaments in the Sacroiliac Joint Anatomy?

The sacroiliac (SI) joint, a critical link between the spine and pelvis, is stabilized by an intricate network of powerful ligaments, including intrinsic ligaments like the anterior, interosseous, and posterior sacroiliac ligaments, and extrinsic ligaments such as the sacrotuberous and sacrospinous ligaments.

Introduction to the Sacroiliac Joint

The sacroiliac (SI) joint is a pair of synovial joints located where the sacrum (the triangular bone at the base of the spine) articulates with the ilium (the largest bone of the pelvis). Despite being synovial, the SI joint possesses very limited mobility, primarily functioning to transfer weight and forces between the upper body and lower extremities. Its stability is paramount for efficient gait, posture, and overall biomechanical integrity of the lumbopelvic region. This stability is predominantly conferred by a robust system of ligaments.

The Role of Ligaments in SI Joint Stability

Ligaments are strong, fibrous connective tissues that connect bones to other bones, providing stability to joints and limiting excessive movement. In the SI joint, ligaments play a crucial role in:

• Load Transfer: Facilitating the efficient transfer of forces from the spine to the lower limbs and vice versa.
• Joint Integrity: Holding the sacrum and ilium together, preventing separation and displacement.
• Movement Control: Restricting abnormal or excessive motion within the joint, which is vital given its weight-bearing function.

The ligaments of the SI joint are broadly categorized into two groups: intrinsic (or true) ligaments, which are directly associated with the joint capsule, and extrinsic (or accessory) ligaments, which provide additional support from a distance.

Intrinsic Sacroiliac Ligaments

These ligaments form an integral part of the SI joint capsule, providing direct and robust stabilization.

• Anterior Sacroiliac Ligament:

  • This is a relatively thin but broad ligament that covers the anterior (front) aspect of the SI joint.
  • It connects the anterior surface of the sacrum to the ilium.
  • While less robust than the posterior ligaments, it provides some anterior stability and resists anterior displacement of the sacrum. Its fibers are often continuous with the anterior longitudinal ligament of the spine.

• Interosseous Sacroiliac Ligament:

  • Considered the strongest and most important intrinsic ligament of the SI joint.
  • It occupies the deep, rough area between the sacrum and ilium, filling the space superior to the auricular surfaces of the joint.
  • Composed of numerous short, strong fibers, it forms a powerful syndesmosis (fibrous joint) that binds the sacrum and ilium tightly together.
  • Its primary function is to resist separation of the joint surfaces and prevent excessive anterior and inferior movement of the sacrum relative to the ilium.

• Posterior Sacroiliac Ligaments:

  • These are a complex and extremely strong group of ligaments located on the posterior (back) aspect of the joint. They are typically divided into short and long components.
  • Short Posterior Sacroiliac Ligaments:
    • These strong, obliquely oriented fibers run from the first and second transverse tubercles of the sacrum to the iliac tuberosity.
    • They are positioned closer to the joint line and contribute significantly to resisting anterior and inferior sacral displacement.
  • Long Posterior Sacroiliac Ligaments:
    • These run more vertically from the third and fourth lateral sacral crests to the posterior superior iliac spine (PSIS) and the inner lip of the iliac crest.
    • They play a crucial role in preventing excessive nutation (anterior tilt) of the sacrum and resisting external rotation of the ilium. They are often implicated in SI joint pain due to their strategic position and susceptibility to tension.

Extrinsic Sacroiliac Ligaments (Accessory Ligaments)

These ligaments are not directly part of the joint capsule but provide significant indirect support to the SI joint by connecting the sacrum or ilium to other pelvic structures.

• Sacrotuberous Ligament:

  • This is a large, strong, fan-shaped ligament that originates from the posterior superior iliac spine (PSIS), posterior inferior iliac spine (PIIS), and the lateral margins of the sacrum and coccyx.
  • It inserts onto the ischial tuberosity (the bony prominence you sit on).
  • Functionally, it resists sacral nutation and posterior rotation of the ilium, providing a crucial link between the sacrum and the lower pelvis. It also serves as an attachment point for certain gluteal and hamstring muscles.

• Sacrospinous Ligament:

  • A thinner, triangular ligament located deep to the sacrotuberous ligament.
  • It runs from the lateral border of the sacrum and coccyx to the ischial spine.
  • Like the sacrotuberous ligament, it primarily resists sacral nutation (anterior tilt of the sacrum) and helps stabilize the pelvis. It also helps to convert the greater sciatic notch into the greater sciatic foramen.

Functional Significance and Clinical Relevance

The intricate arrangement of these ligaments ensures that the SI joint, despite its limited movement, can withstand immense compressive and shearing forces inherent in human locomotion and weight-bearing. Dysfunction or injury to these ligaments, often due to trauma, repetitive stress, or degenerative changes, can lead to SI joint pain and instability. Understanding their anatomy is critical for clinicians and fitness professionals in assessing and managing conditions related to the lumbopelvic region.

Conclusion

The ligaments of the sacroiliac joint are foundational to the stability and function of the entire lumbopelvic complex. From the primary stabilizing force of the interosseous ligament to the robust support provided by the anterior, posterior, sacrotuberous, and sacrospinous ligaments, each plays a vital role in maintaining joint integrity, facilitating efficient load transfer, and preventing excessive motion. A comprehensive understanding of these structures is essential for anyone seeking to optimize human movement, prevent injury, or address pain in the lower back and pelvis.