The sacroiliac joint, or SI joint, is the joint in the bony pelvis between the sacrum and the ilium of the pelvis, which are joined by strong ligaments. The sacrum supports the spine and is supported, in turn, by an ilium on each side. The joint is a strong, weight-bearing, synovial joint with irregular elevations and depressions that produce interlocking of the two bones. The human body has two sacroiliac joints, one on the left and one on the right, that often match each other but are highly variable from person to person.

Anatomy

The sacroiliac joints are two paired “kidney bean” or L-shaped joints having a small amount of movement that are formed between the articular surfaces of the sacrum and the ilium bones. The two sacroiliac joints move together as a single unit and are considered bicondylar joints (where the two joint surfaces move correlatively together). The joints are covered by two different kinds of cartilage: the sacral surface has hyaline cartilage and the ilial surface has fibrocartilage. The SIJ’s stability is maintained mainly through a combination of some bony structure and very strong intrinsic and extrinsic ligaments.


Articulations of pelvis: Anterior view (left) and posterior view (right)

As the body ages, the characteristics of the sacroiliac joint change. The joint’s surfaces are flat or planar in early life, but as a person starts walking, the sacroiliac joint surfaces develop distinct angular orientations and lose their planar or flat topography. They also develop an elevated ridge along the ilial surface and a depression along the sacral surface. The ridge and corresponding depression, along with the very strong ligaments, increase the sacroiliac joints’ stability and makes dislocations very rare. The fossae lumbales laterales (dimples of Venus) correspond to the superficial topography of the sacroiliac joints.

Ligaments

Five ligaments are important to the sacroiliac joint:

  • Anterior sacroiliac ligament
  • Interosseous sacroiliac ligament
  • The posterior sacroiliac ligament
  • Sacrotuberous ligament
  • Sacrospinous ligament

The anterior sacroiliac ligament is not much of a ligament at all, and in most cases is just a slight thickening of the anterior joint capsule. The anterior ligament is thin and not as well defined as the posterior sacroiliac ligaments.

The posterior sacroiliac (SI) ligaments can be further divided into short (intrinsic) and long (extrinsic). The dorsal interosseous ligaments are very strong ligaments. They are often stronger than bone, such that the pelvis may actually fracture before the ligament tears. The dorsal sacroiliac ligaments include both long and short ligaments. The long dorsal sacroiliac joint ligaments run in an oblique vertical direction, while the short (interosseous) runs perpendicular from just behind the articular surfaces of the sacrum to the ilium and functions to keep the sacroiliac joint from distracting or opening.

The interosseous ligaments are very short and run perpendicular from the iliac surface to the sacrum; they keep the articular surfaces from abducting or opening/distracting. The extrinsic sacroiliac joint ligaments (the sacrotuberous and sacrospinous ligaments) limit the amount the sacrum flexes (or nutates).

The ligaments of the sacroiliac joint become loose during pregnancy due to the hormone relaxin; this loosening allows widening of the pelvic joints during the birthing process, especially the related symphysis pubis.

Physiology

Like most lower extremity joints, one of the functions of the sacroiliac joints is shock absorption for the spine (depending on the amount of available motion at the sacroiliac joint), along with the job of torque conversion allowing the transverse rotations that take place in the lower extremity to be transmitted up the spine. The sacroiliac joints, like all lower extremity joints, provides a “self-locking” mechanism, where the joint occupies or attains its most congruent position, also called the close pack position, that helps with stability during the push-off phase of walking. The joint locks (or rather becomes close packed) on one side as weight is transferred from one leg to the other, and through the pelvis the body weight is transmitted from the sacrum to the hip bone.

The motions of the sacroiliac joint include the following:

  • Anterior innominate tilt of both innominate bones on the sacrum (where the left and right move as a unit)
  • Posterior innominate tilt of both innominate bones on the sacrum (where the left and right move together as a unit)
  • Anterior innominate tilt of one innominate bone while the opposite innominate bone tilts posteriorly on the sacrum (antagonistic innominate tilt) which occurs during gait
  • Sacral flexion (or nutation) Motions of the sacrum occur simultaneous with motion of the ilium so you must be careful in the description of these as isolated motions.
  • Sacral extension (or counter-nutation).

The sacroiliac joints, like all spinal joints (except the atlanto-axial), are bicondylar joints, meaning that movement of one side corresponds to a correlative movement of the other side.

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