Sacroiliac Ligaments

1. Overview

The sacroiliac (SI) ligaments are a group of strong, fibrous structures that stabilize the sacroiliac joints—the articulations between the sacrum and the ilium of the pelvis. These ligaments are essential for maintaining pelvic integrity, distributing weight from the upper body to the lower limbs, and limiting excessive motion at the SI joint, which is mostly an immobile or slightly mobile joint.

2. Location

The sacroiliac ligaments are located in the posterior pelvis, surrounding the sacroiliac joints:

• Connect: The sacrum (S1–S4 vertebrae) to the ilium of each hip bone.

• Span: From the lateral sacral surface to the ilial auricular surface and iliac tuberosity.

• Lie posterior to: The sacroiliac joint capsule and anterior pelvic organs.

3. Structure

There are several distinct sacroiliac ligaments, classified into anterior, interosseous, and posterior groups:

• Anterior sacroiliac ligament:

  • A thin, broad band located on the anterior surface of the joint capsule.

  • Connects the anterior sacral ala to the ilium's auricular surface.

• Interosseous sacroiliac ligament:

  • The strongest and deepest of the SI ligaments.

  • Occupies the space between the sacrum and the ilium posterior to the joint.

  • Provides the primary stabilization of the SI joint.

• Posterior sacroiliac ligament:

  • Located on the dorsal surface of the sacrum and ilium.

  • Subdivided into short (horizontal) and long (oblique) portions.

  • Attaches from the sacral tubercles to the posterior superior iliac spine (PSIS) and iliac crest.

These ligaments are composed of densely packed collagen fibers capable of resisting high tensile loads and shear forces.

4. Function

The sacroiliac ligaments provide essential structural and mechanical functions:

• Stabilize the sacroiliac joint: Limit anterior and inferior movement of the sacrum relative to the ilium.

• Distribute mechanical load: Transfer weight from the spine to the pelvic girdle and lower limbs.

• Resist shear forces: Prevent displacement during twisting, lifting, or shifting motions.

• Anchor pelvic structures: Support the connection between the axial and appendicular skeletons.

5. Physiological role(s)

Though passive in nature, sacroiliac ligaments contribute to:

• Postural integrity: Maintain pelvic alignment during upright stance, walking, and load-bearing.

• Shock absorption: Act as tension-bearing structures during spine and hip motion.

• Pelvic movement regulation: Control minute joint glides (nutation and counternutation) needed for locomotion and childbirth.

• Proprioception: Contain mechanoreceptors that help the nervous system regulate posture and movement.

6. Clinical Significance

Dysfunction or injury to the sacroiliac ligaments can lead to pain and mobility issues:

• Sacroiliac joint dysfunction:

  • May result from ligamentous laxity, trauma, leg-length discrepancy, or poor biomechanics.

  • Presents as lower back or buttock pain, sometimes radiating to the thigh.

• Sacroiliitis:

  • Inflammation of the SI joint, often involving the ligaments; associated with conditions like ankylosing spondylitis and other spondyloarthropathies.

• Ligament strain or tear:

  • Can occur from sudden rotational forces, lifting injuries, or falls.

  • May cause acute localized pain, swelling, and tenderness over the SI joint.

• Pregnancy-related pelvic pain:

  • Hormonal relaxation of the sacroiliac ligaments increases joint mobility, which can result in pain and instability.

• Injection or surgical intervention:

  • SI joint injections target inflammation involving these ligaments.

  • Fusion surgery may be considered for chronic, treatment-resistant SI joint instability.