Posterior Sacroiliac Ligament: Attachments, Function, and Clinical Relevance

What is the attachment of posterior sacroiliac ligament?

The posterior sacroiliac ligament (PSIL) primarily attaches the sacrum to the ilium, specifically connecting the lateral sacral crest to the posterior superior iliac spine (PSIS) and adjacent iliac crest.

Understanding the Sacroiliac Joint (SIJ)

The sacroiliac joint (SIJ) is a critical anatomical structure located at the base of the spine, connecting the sacrum (a triangular bone formed by the fusion of five vertebrae at the base of the spine) to the ilium (the largest part of the hip bone). This joint plays a pivotal role in transferring forces between the axial skeleton (spine and skull) and the appendicular skeleton (lower limbs). While often perceived as a rigid joint, the SIJ allows for a small degree of movement, essential for shock absorption and efficient locomotion. Its stability is paramount, and this stability is primarily conferred by a robust network of ligaments, with the posterior sacroiliac ligament being a key component.

The Posterior Sacroiliac Ligament (PSIL) in Detail

The posterior sacroiliac ligament (PSIL) is a strong, complex ligamentous structure that forms a significant part of the posterior aspect of the sacroiliac joint capsule. It is often described as comprising several distinct bundles of fibers, contributing to its multi-directional stabilizing capabilities.

Key Attachments:

• Sacral Attachment: The PSIL originates from the lateral sacral crest, primarily at the level of the first, second, and third (and sometimes fourth) sacral segments.
• Iliac Attachment: From the sacrum, the fibers extend laterally and inferiorly to attach to the posterior superior iliac spine (PSIS) and the rough bony surface immediately inferior and medial to it on the adjacent iliac crest.

Fiber Orientation: The PSIL is characterized by its distinct fiber orientation, which contributes to its functional role:

• Short, Oblique Fibers: These fibers run from the lateral sacral crest superiorly to the iliac tuberosity inferiorly, providing strong resistance to anterior and inferior displacement of the sacrum relative to the ilium.
• Long, Horizontal Fibers: These superficial fibers run more horizontally from the PSIS to the lateral sacral crest, offering stability against rotational forces and preventing excessive separation of the posterior iliac bones.

Relationship to Other Structures: The PSIL is intimately associated with other important structures:

• It lies superficial to the interosseous sacroiliac ligament, which is considered the strongest of the SIJ ligaments due to its deep, dense nature.
• It blends inferiorly with the sacrotuberous ligament and sacrospinous ligament, further enhancing the stability of the pelvic girdle.
• Superficially, it is covered by and interweaves with the fibers of the thoracolumbar fascia, a crucial connective tissue structure that links the trunk musculature and contributes to spinal stability.

Functional Significance of the PSIL

The posterior sacroiliac ligament is indispensable for the biomechanical integrity and stability of the SIJ and, by extension, the entire pelvic girdle and lower kinetic chain.

• Limiting Motion: Its primary function is to restrict excessive movement at the SIJ, particularly preventing anterior rotation and inferior translation of the sacrum relative to the ilium. This is crucial during activities involving weight-bearing and locomotion.
• Force Transmission: The PSIL plays a vital role in the efficient transmission of forces between the spine and the lower limbs. It helps to maintain the "form closure" of the joint, where the shape of the articulating surfaces inherently provides stability, and contributes significantly to "force closure," where active muscle contraction and passive ligamentous tension create compression across the joint.
• Proprioception: Like many ligaments, the PSIL contains mechanoreceptors that provide sensory feedback to the central nervous system about joint position and movement. This proprioceptive input is critical for motor control and maintaining posture.
• Clinical Relevance: Due to its significant role in SIJ stability, the PSIL is often implicated in cases of sacroiliac joint dysfunction and pain. Injury, laxity, or inflammation of this ligament can lead to localized pain, referred pain patterns, and impaired functional movement.

Anatomical Context and Associated Structures

Understanding the PSIL's attachments is enhanced by appreciating its surrounding anatomical context:

• Interosseous Sacroiliac Ligament: Located deep to the PSIL, this is the strongest and shortest of the SIJ ligaments, connecting the tuberosities of the sacrum and ilium. It acts as the primary binder of the joint.
• Anterior Sacroiliac Ligament: A thinner, broader ligament located on the anterior aspect of the joint, providing some anterior stability but less robust than its posterior counterparts.
• Muscular Attachments: While not directly attaching to the PSIL, several muscles have fascial connections or close proximity that indirectly influence SIJ stability, including the gluteus maximus, latissimus dorsi, erector spinae, and transversus abdominis. These muscles, through their contribution to the thoracolumbar fascia, create a "force closure" mechanism that compresses the SI joint.

Conclusion: The PSIL as a Key Stabilizer

In summary, the posterior sacroiliac ligament is a robust and multifaceted ligament with crucial attachments spanning from the lateral sacral crest to the posterior superior iliac spine and adjacent iliac crest. Its intricate fiber arrangement, combined with its strong connections to other pelvic ligaments and the thoracolumbar fascia, establishes it as a fundamental stabilizer of the sacroiliac joint. A thorough understanding of its anatomy and function is essential for fitness professionals, clinicians, and anyone seeking to comprehend the biomechanics of the human pelvis and spine.