Iliac Spines: Key Ligaments, Attachments, and Clinical Importance

What are the ligaments in the iliac spine?

While no major ligaments directly originate from or insert solely into the "iliac spine" itself—a term referring to specific bony prominences of the ilium—these landmarks serve as crucial attachment points for a complex network of ligaments and muscles that stabilize the pelvic girdle and hip joint.

Understanding the Iliac Spines

The iliac spines are distinct bony projections on the ilium, the largest bone of the pelvis. They serve as important anatomical landmarks and attachment sites for various muscles and ligaments, contributing significantly to pelvic stability and lower limb function. The key iliac spines include:

• Anterior Superior Iliac Spine (ASIS): The most prominent point on the front of the ilium, easily palpable.
• Anterior Inferior Iliac Spine (AIIS): Located just below the ASIS.
• Posterior Superior Iliac Spine (PSIS): A palpable bony prominence on the back of the ilium, often marked by "dimples of Venus."
• Posterior Inferior Iliac Spine (PIIS): Located just below the PSIS.

While these spines are primarily known for muscle attachments, their proximity and direct connections to critical ligaments are fundamental to understanding pelvic biomechanics.

Key Ligaments Associated with Iliac Spine Regions

The following ligaments are either directly attached to or are in close anatomical association with the iliac spines, playing vital roles in pelvic and hip stability.

Ligaments of the Anterior Iliac Spines

• Inguinal Ligament:
  • Attachment: Extends from the Anterior Superior Iliac Spine (ASIS) to the pubic tubercle.
  • Function: Forms the inferior border of the anterior abdominal wall, separating the abdomen from the thigh. It acts as a retinaculum for structures passing into the thigh and is a key landmark in the groin region.
• Iliofemoral Ligament (Y-Ligament of Bigelow):
  • Attachment: Originates from the Anterior Inferior Iliac Spine (AIIS) and the rim of the acetabulum, fanning out to insert onto the intertrochanteric line of the femur.
  • Function: Considered the strongest ligament in the human body, it is crucial for stabilizing the hip joint. It prevents excessive hip extension and external rotation, allowing for passive standing with minimal muscular effort.

Ligaments of the Posterior Iliac Spines

The posterior iliac spines (PSIS and PIIS) are intimately involved with the stability of the sacroiliac (SI) joint.

• Posterior Sacroiliac Ligaments (Short and Long):
  • Attachment: These strong ligaments connect the posterior surface of the ilium (near the Posterior Superior Iliac Spine - PSIS) to the sacrum. The short posterior SI ligaments are deep, while the long posterior SI ligaments extend more inferiorly from the PSIS to the sacrum.
  • Function: They are primary stabilizers of the SI joint, resisting excessive movement and maintaining the integrity of the pelvic ring. The long posterior SI ligament, in particular, resists anterior rotation of the sacrum relative to the ilium.
• Iliolumbar Ligament:
  • Attachment: Connects the transverse process of the fifth lumbar vertebra (L5) and sometimes L4 to the posterior part of the iliac crest, near the Posterior Superior Iliac Spine (PSIS).
  • Function: Stabilizes the L5 vertebra on the sacrum and pelvis, limiting movements like flexion, extension, and rotation at the lumbosacral junction.

Broader Pelvic Girdle Ligaments (Contextual Relevance)

While not directly attached to a specific "spine," these ligaments are integral to the overall stability of the pelvic girdle, working in conjunction with the structures mentioned above.

• Sacrotuberous Ligament:
  • Attachment: Extends from the posterior ilium (including the PSIS region), sacrum, and coccyx to the ischial tuberosity.
  • Function: Forms the inferior boundary of the lesser sciatic notch and converts the greater sciatic notch into the greater sciatic foramen. It resists sacral nutation (forward tipping) and helps stabilize the pelvis.
• Sacrospinous Ligament:
  • Attachment: Extends from the lateral border of the sacrum and coccyx to the ischial spine.
  • Function: Located anterior to the sacrotuberous ligament, it converts the greater sciatic notch into the greater sciatic foramen and the lesser sciatic notch into the lesser sciatic foramen. It also resists sacral nutation.
• Interosseous Sacroiliac Ligament:
  • Attachment: A deep, very strong ligament located between the tuberosities of the sacrum and ilium.
  • Function: Forms the primary mechanical link between the sacrum and ilium, resisting separation of the SI joint.

Functional Significance and Clinical Relevance

The ligaments associated with the iliac spines are critical for:

• Pelvic Stability: They form a robust network that maintains the integrity of the pelvic ring, essential for weight-bearing and transmitting forces between the trunk and lower limbs.
• Hip Joint Integrity: The iliofemoral ligament, originating from the AIIS, is a primary stabilizer of the hip, preventing dislocation and excessive movement.
• Spinal Stability: The iliolumbar ligament contributes to the stability of the lumbosacral junction, crucial for healthy spinal mechanics.
• Movement and Posture: The coordinated action of these ligaments allows for efficient transfer of forces during walking, running, and lifting, while also contributing to upright posture.

Dysfunction or injury to these ligaments, such as sprains or laxity, can lead to conditions like sacroiliac joint pain, hip instability, or referred pain patterns, impacting mobility and quality of life. Understanding their precise attachments and functions is therefore vital for diagnosis, rehabilitation, and effective exercise prescription.

Conclusion

While the term "iliac spine" refers to specific bony landmarks, it is essential to appreciate that these points are integral to a complex, interconnected system of ligaments. These ligaments, including the inguinal, iliofemoral, posterior sacroiliac, and iliolumbar ligaments, are fundamental to the stability of the pelvic girdle, the hip joint, and the lumbosacral spine. Their robust nature allows for efficient load transfer and movement, underscoring their critical role in human biomechanics and overall physical function.