Sacrum Ligaments: Function, Types, and Clinical Significance

What is the Function of the Sacrum Ligaments?

The sacral ligaments are a complex network of strong, fibrous connective tissues that primarily serve to stabilize the sacrum within the pelvic girdle, facilitate efficient load transfer between the axial skeleton and lower limbs, and limit excessive motion at the sacroiliac joints.

Introduction to the Sacrum and Pelvis

The sacrum is a large, triangular bone located at the base of the spine, forming the posterior wall of the pelvis. It articulates superiorly with the fifth lumbar vertebra (L5) at the lumbosacral joint, laterally with the two iliac bones of the pelvis at the sacroiliac (SI) joints, and inferiorly with the coccyx. This intricate bony arrangement, along with its robust ligamentous and muscular support, creates the pelvic girdle – a critical structure for weight bearing, locomotion, and the protection of internal organs.

Ligaments, as passive stabilizers, are paramount to the integrity and function of the sacrum and the entire pelvic ring. Unlike muscles, which can actively contract and relax, ligaments provide static stability by resisting tensile forces and limiting joint motion, ensuring that the sacrum remains securely positioned and capable of effectively transmitting forces.

Key Sacral Ligaments and Their Functions

The sacrum is supported by several key ligaments, each contributing uniquely to its stability and the biomechanics of the pelvis.

Sacroiliac (SI) Ligaments These are the most direct stabilizers of the sacroiliac joints, connecting the sacrum to the ilium. The SI joints are designed for minimal movement, primarily allowing for slight gliding and rotation (nutation and counternutation) that is crucial for shock absorption and efficient gait.

• Anterior Sacroiliac Ligaments: These are broad, relatively thin bands that reinforce the anterior aspect of the SI joint. They connect the anteromedial surface of the ilium to the anterior surface of the sacrum. Their primary function is to limit excessive external rotation of the ilium relative to the sacrum and provide anterior joint capsule reinforcement.
• Posterior Sacroiliac Ligaments: These are robust and multi-directional, providing significant posterior stability.
  • Short Posterior Sacroiliac Ligaments: These strong, oblique fibers run from the sacral crests to the iliac tuberosity. They are crucial for limiting anterior rotation (nutation) of the sacrum and preventing separation of the posterior SI joint.
  • Long Posterior Sacroiliac Ligaments: Extending from the posterior superior iliac spine (PSIS) to the third and fourth sacral segments, these ligaments play a vital role in limiting counternutation (posterior rotation) of the sacrum relative to the ilium, and also restrict posterior rotation of the ilium.
• Interosseous Sacroiliac Ligaments: Considered the strongest and shortest of the SI ligaments, these deep, dense fibers fill the irregular space between the sacral and iliac tuberosities. They are the primary restraint against separation and excessive translation or rotation at the SI joint, effectively binding the sacrum and ilium together.

Sacrotuberous Ligament This is a strong, broad, and flat ligament that runs from the posterior ilium, sacrum, and coccyx down to the ischial tuberosity. It forms the posteromedial boundary of the lesser sciatic foramen. Its key functions include:

• Preventing Anterior Tilting (Nutation) of the Sacrum: It acts as a tension band, resisting the forward tipping of the sacral apex.
• Restricting Upward Rotation of the Ischial Tuberosity: This helps stabilize the pelvic outlet.
• Providing Stability Against Vertical Shear Forces: It helps transfer weight from the axial skeleton to the lower extremities, particularly during standing and gait.

Sacrospinous Ligament This is a thin, triangular ligament located deep to the sacrotuberous ligament. It extends from the lateral border of the sacrum and coccyx to the ischial spine. Along with the sacrotuberous ligament, it converts the greater sciatic notch into the greater sciatic foramen and the lesser sciatic notch into the lesser sciatic foramen. Its functions are closely intertwined with the sacrotuberous ligament:

• Preventing Excessive Sacral Nutation: It assists in resisting the forward rotation of the sacrum.
• Stabilizing the Sacrum Against Rotational Forces: It contributes to the overall stability of the pelvic ring during various movements.

Iliolumbar Ligaments While not directly connecting to the sacrum, the iliolumbar ligaments are crucial for stabilizing the lumbosacral junction and, by extension, influencing sacral stability. They connect the transverse processes of the fourth and fifth lumbar vertebrae (L4 and L5) to the posterior superior iliac crest. Their primary roles are:

• Stabilizing the L5-S1 Junction: They limit excessive rotation, lateral flexion, and anterior translation of L5 on the sacrum.
• Indirect Pelvic Stability: By stabilizing the lumbar spine's attachment to the pelvis, they contribute to the overall rigidity and integrity of the sacral base.

The Role of Ligaments in Pelvic Stability and Movement

The collective function of the sacral ligaments is multi-faceted:

• Static Stability: They provide the primary passive restraint against excessive movement at the sacroiliac joints, which are inherently designed for limited motion.
• Load Transfer: These ligaments are integral to the efficient transfer of forces between the trunk (via the spine) and the lower extremities. During standing, walking, and lifting, forces are transmitted through the sacrum and its ligamentous attachments, distributing stress and preventing injury.
• Integration with Dynamic Stabilizers: While ligaments offer passive stability, they work in concert with the deep core muscles (e.g., transversus abdominis, multifidus, pelvic floor muscles) and global movers (e.g., gluteal muscles) to create a highly stable yet adaptable pelvic girdle.
• Proprioception: Ligaments contain mechanoreceptors that provide crucial sensory feedback to the nervous system about joint position and movement, contributing to motor control and balance.

Clinical Significance and Injury

Understanding the function of sacral ligaments is vital in clinical practice.

• Sacroiliac Joint Dysfunction (SIJD): Ligamentous laxity or excessive tension can lead to SI joint pain and dysfunction. Conditions like pregnancy, where the hormone relaxin increases ligamentous laxity, can predispose individuals to SIJD.
• Trauma: Direct trauma (e.g., falls, motor vehicle accidents) can sprain or tear these strong ligaments, resulting in significant pain, instability, and functional limitations.
• Chronic Strain: Repetitive micro-trauma or sustained asymmetrical loading can lead to chronic ligamentous strain, contributing to persistent low back and pelvic pain.
• Surgical Considerations: Knowledge of these ligaments is critical for orthopedic and neurosurgeons during spinal and pelvic procedures to maintain or restore stability.

Conclusion

The sacral ligaments are indispensable components of the human musculoskeletal system. Their intricate arrangement and robust nature provide critical passive stability to the sacrum and the entire pelvic girdle. By limiting excessive motion, facilitating efficient load transfer, and contributing to proprioception, these ligaments form the foundational support for upright posture, locomotion, and the overall biomechanical integrity of the lower body. Maintaining their health and understanding their function is paramount for both preventing and addressing musculoskeletal pain and dysfunction.