The Pelvis and Pelvic Ring

Pelvis and Sacroiliac Joints

• The sacroiliac joints mark the transition between the axial skeleton and the lower appendicular skeleton.
• The SI joint is located just anterior to the posterior superior iliac spine of the ilium.

Structure of the Sacroiliac Joints

• The joint consists of a relatively rigid articulation between the auricular surface of the sacrum and the matching auricular surface of the iliac bone.
• The articular surface of the joint has a semicircular, boomerang shape, with the open angle of the "boomerang" facing posteriorly.
• Articular cartilage covers both bony auricular surfaces of the joint, with the sacral surface composed of hyaline cartilage and the iliac surface composed of fibrocartilage.
• The thickness of the sacral cartilage is greater than that of the iliac cartilage.

Characteristics of the Sacroiliac Joints

• A mature sacroiliac joint possesses numerous, reciprocally contoured elevations and depressions, etched within the subchondral bone and articular cartilage.
• These roughened areas increase the coefficient of friction within the joint, enhancing the resistance against vertical shear between the joint surfaces.
• The SI joint is a hybrid joint, with one part being a synovial joint (the lower part) and the other a fibrous joint (the upper part).
• The joint is held in place by tough Interrosseous S-I ligaments, which help limit anterior motion.

Functions of the Sacroiliac Joints

• The SIJs permit a small amount of motion that varies among individuals.
• The main functions of the SI joint are:
  • Weight Distribution: weight-bearing function is essential for maintaining balance and stability.
  • Shock Absorption: reduces the impact of physical activities on the spine and other joints.
  • Limiting Mobility: allows slight movements, its primary role is to provide stability to the pelvis and lower back.

Ligaments That Stabilize the SI Joint

• Primary ligaments:
  • Interosseus ligament (stabilizes SIJ)
  • Anterior sacrioiliac ligament
  • Posterior sacroiliac ligaments
    • Short posterior sacroiliac ligament
    • Long posterior sacroiliac ligament
• Secondary ligaments:
  • Indirectly associated with the SI joint
    • Sacrotuberous ligament
    • Sacrospinous ligament
    • Iliolumbar ligament (stabilizer of the lumbosacral joint)

Additional Notes

• The sacroiliac ligaments as a whole extend from the iliac crests to attach to the tubercles of the first four sacral vertebrae.
• The Interosseus ligament consists of a set of very dense and short fibers that fill most of the gap along the posterior and superior margins of the joint.
• The Interosseus ligament is the strongest ligament of the sacroiliac joint, rigidly binding the sacrum with the ilium.