Anatomy of Sacroiliac Joint Ligaments

Sacroiliac Joint

• Located between the auricular surfaces of the sacrum and ilium
• Synovial joint with joint capsule and synovial membrane
• Some resources describe SI as part synovial on its anterior side and in part a syndesmosis on its posterior side
• Functions to control and distribute forces from the upper body into the legs
• Acts as a shock absorber for the spine and controls transmission of forces from the lower body into the spine
• Allows spinal and thigh movements, and changes in posture or position
• Supports the weight of the upper body
• Facilitates pregnancy and childbirth in women by widening and becoming more mobile

Ligaments within SI Joint

• Three main ligaments:
  • Anterior Sacro Iliac ligament
  • Posterior Sacro Iliac ligament
  • Interosseous ligament
• Two accessory ligaments:
  • Sacrospinous ligament
  • Sacrotuberous ligament
• Anterior Sacro Iliac ligament:
  • Blends with fibers of the capsule
  • Rich innervation of nociceptive fibers, may be a significant source of pain
• Posterior Sacroiliac ligament:
  • Overlies the interosseous sacroiliac ligament
  • Contributes most to SIJ mobility
  • Undergoes tension during transmission of forces from the legs to the upper body and vice versa
• Interosseous Ligament:
  • Strongest ligament in the body
  • Consists of short, strong bands that detach from the bone
  • Blends with fibers of the capsule
  • Prevents forward and downward movement of the sacrum

Sacrococcygeal Joint

• Articulation between the apex of the sacrum and the base of the coccyx
• A symphysis, the two bones lined by hyaline cartilage and connected by a fibrous disc
• Only slight mobility; flexion-extension to increase the anteroposterior diameter of the pelvis during labor and defecation

Ligaments of the Sacrococcygeal Joint

• Anterior Sacrococcygeal ligament:
  • Extends from the anterior surface of the sacrum to the anterior surface of the apex of the coccyx
• Lateral Sacrococcygeal ligament:
  • Attaches to the inferolateral angle of the sacrum and the transverse processes of the coccyx
• Superficial Posterior Sacrococcygeal ligament:
  • Arises from the margin of the sacral hiatus and attaches to the dorsal surface of the coccyx
• Deep Posterior Sacrococcygeal ligament:
  • Spans from the dorsal surface of the S5 to the dorsal surface of the coccyx

Coccyx Anatomy

• Bony Landmark:
  • Base (oval)
  • Apex (rounded)
  • Anterior surface
  • Posterior surface
  • Lateral surface
  • Base
  • Cornua
  • Rudiments of Spinous process
  • The Cornua of Co1 articulates with the Sacral cornua
  • Anterior: pelvic surface, concave
  • Lateral: thin edges remnant of TVP
  • Apex: posterior, convex
  • 4 rows of tubercles (rudiments of Spinous process)

Sacrum

• Five sacral vertebrae fused to form a triangular or wedge-shaped structure
• Base sits at an oblique angle forming the lumbosacral angle that measures 30 degrees +/- 10 in 95% of cases studied
• Lumbosacral Articular facets/surface orients posteriorly and articulates with Inferior facets of L5
• Sacral apex (formed by the S5 vertebra) articulates with the coccyx
• Sacral Alar (wings) articulates with Ilium
• Sacral tuberosities articulate with the iliac tuberosity

Sacrum Anterior Surface

• Transverse ridges:
  • Middle portion has 4 transverse ridges corresponding to 5 sacral bodies
• Anterior Sacral foramina:
  • Anterior sacral foramina 4 on each side
  • First 4 sacral nerves exit here, arteries enter here

Sacrum Posterior Surface

• Convex, narrower, highly irregular
• Sacral canal runs down the center of the sacrum and represents the end of the vertebral canal