Anatomy of the Sacroiliac Joint

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The sacroiliac joints are located in the pelvis, linking the left and right iliac bones to the sacrum (lowest part of the spine above the tailbone). The SI joint allows some motion of the sacrum with respect to the iliac bones.  This motion allows the spine and pelvis to change position during normal activities.  The SI joints also provide a stable base that allows efficient transfer of weight and force between your torso and upper extremities through the pelvis to the legs. The SI joint complex is an essential component for shock absorption and this helps to prevent impact forces during activity from reaching the spine.

The posterior part of the pelvis consists of the two sacroiliac joints, the sacrum, and the posterior part of the two iliac bones.  The anterior part of the pelvis, where the two pelvic bones come together, is called the symphysis pubis. The surfaces of the sacroiliac joint that are in contact with each other are ear shaped and contain small irregular ridges and depressions. The sacral surface is covered with a thick layer of hyaline cartilage and the iliac surface is covered with a much thinner layer of hyaline cartilage.

The picture on the left shows the thick hyaline cartilage covering the surface of the sacrum (tailbone).  The articular cartilage of the sacroiliac joint is enclosed within a joint capsule (a firm fibrous covering) containing a liquid called synovial fluid.  The SI joints are supported and stabilized by a network of ligaments and muscles. The normal sacroiliac joint has only a small amount of motion, approximately 2 mm of gliding movement in any direction and 1-4 degrees of rotational movement. The sacroiliac ligaments in women are less stiff than in men. The additional flexibility allows the pelvis to widen during childbirth.








Joint Structures Above and Below the Sacroiliac Joint

The sacrum connects with the lumbar spine (the vertebrae and discs of the lower back).  The iliac bones connect with the femurs at the hip joints. The lumbar spine is more mobile than the SI joint and its lowest motion segment, the L5-S1 intervertebral disc, connects to the sacrum.  There are also left and right facet joints that link the L5 vertebra to the sacrum.  The sacrum, the ilium and the lumbar vertebrae are linked with ligaments, muscles, and connective tissue called fascia.  Studies have shown that when motion is limited in the lumbar spine due to natural causes or a surgery called a lumbar fusion, that there may be an increased motion, increased  wear and tear and increased forces at the SI joint 1, 2, 3.   Continued or new onset low back pain after lumbar fusion may be coming from the SI joint in 35-43% of cases. 3, 4, 5

The hip joints are ball and socket type joints that link the femur bones to the iliac bones.  The hip joint consists of a socket located on the front and lower part of each side of the pelvis (ilium) called the acetabulum. The ball at the top of the femur (thigh bone) sits in the socket and this makes up the hip joint. Maintaining normal mobility in the hip joint and surrounding structures may help to avoid excess strain on the SI joint.


Functional Stability of the SI joint

Stability of the sacroiliac joint is dependent on three mechanisms that allow proper accommodation of forces across the joint: Form Closure, Force Closure and Motor Control. Form closure describes the passive stability of the joint due to the shape of the sacrum and the integrity of the supporting ligaments 6, 7, 8. Force closure describes the active stability of the joint due to the external dynamic forces created by contraction of the stabilizing muscles that cross the joint and their fascial and ligamentous attachments 6, 7, 8. Motor Control   describes the coordinated action of the nervous system and the deep and superficial muscles that stabilize the joint 9.10.11

How does the SI joint cause pain?

The SI joint is a synovial joint, it has free nerve endings and other pain fibers that can transmit pain signals if the joint degenerates, does not move properly, or does not properly accommodate the forces that cross the joint. The SI joint has been long known to cause pain in the lower back, buttock on the painful side and at times in the groin region or along the back and side of the thigh. Like any other joint in the body, the SI joint can become painful due to repetitive daily stressors (wear and tear) from muscle strength or length imbalances, repetitive strain, inflammatory processes such as psoriatic arthritis or from trauma to the joint itself or to the soft tissue structures that support the joint.

How common are SI joint problems?

It is reported in the clinical literature that up to 25% of low back pain is caused by the SI joint 12, 13.  Women may be at increased risk for SI joint problems because of their broader pelvises, greater curvature of lumbar spine, and shorter limb lengths.  Women have more elastin in the collagen that makes up their ligaments leading to increased flexibility of ligaments and this flexibility may further increase due to secretion of a hormone during pregnancy called relaxin.

Changes during pregnancy in the length and strength of the musculature that supports the SI joint may also contribute to this increased risk of SI joint pain.  . Changes in the weight distribution in the body may alter the body’s center of gravity which may alter the forces on the sacroiliac joint. Health care professionals may ask the patient if they have a history of pregnancy, lumbar fusion surgery, or any conditions such as arthritis in the hip and lumbar spine which may limit motion in the joints above and below the pelvis.


  1. Ha K-Y, Lee J-S, Kim K-W. Degeneration of sacroiliac joint after instrumented lumbar or lumbosacral fusion: a prospective cohort study over five-year follow-up. Spine. 2008;33:1192–8.
  2. Ivanov AA, Kiapour A, Ebraheim NA, Goel V. Lumbar fusion leads to increases in angular motion and stress across sacroiliac joint: A finite element study. Spine. 2009;34:E162–9.
  3. Maigne JY, Planchon CA. Sacroiliac joint pain after lumbar fusion: A study with anesthetic blocks. Eur Spine J. 2005;14(7):654-8.
  4. DePalma MJ, Ketchum JM, Saullo TR. Etiology of Chronic Low Back Pain in Patients Having Undergone Lumbar Fusion. Pain Med. 2011;12(5):732-9.
  5. Liliang P-C, Lu K, Liang C-L, Tsai Y-D, Wang K-W, Chen H-J. Sacroiliac joint pain after lumbar and lumbosacral fusion: findings using dual sacroiliac joint blocks. Pain Med. 2011;12(4):565-70.
  6. Lee DG, Vleeming A. Impaired load transfer through the pelvic girdle- a new model of altered neutral zone function. In: Proceedings from the 3rd interdisciplinary world congress on low back and pelvic pain.  Vienna, Austria. 1998
  7. Vleeming A, Stoeckart R, Volkens ACW, Anijders CJ. Relationship between form and function of the sacroiliac joint Part 1: Clinical Anatomical Aspects. Spine. 1990 Feb;15(2):130-2.
  8. Vleeming A, Stoeckart R, Volkens ACW, Anijders CJ. Relationship between form and function of the sacroiliac joint Part 2: Biomechanical Aspects. Spine. 1990 Feb;15(2):133-6.
  9. Snijders CJ, Vleeming A, Stoeckart R. Transfer of lumbosacral load to iliac bones and legs. Part 2: Loading of the sacroiliac joints when lifting in a stooped posture. Clin Biomech (Bristol, Avon). 1993;8:295-301.
  10. Hodges PW, Richardson CA. Inefficient muscular stabilization of the lumbar spine associated with low back pain. A motor control evaluation of transversus abdominis. Spine 1996;21(22):2640-50.
  11. Richardson CA, Snijders CJ, Hides JA, Damen L, Pas MS, Storm J. The relationship between the transversely oriented abdominal muscles, sacroiliac joint mechanics and low back pain. Spine. 2002;27(4):399-405.
  12. Bernard TN, Kirkaldy-Willis WH. Recognizing specific characteristics of nonspecific low back pain. Clin Orthop Relat Res. 1987 Apr;(217):266–80.
  13. Cohen SP. Sacroiliac Joint Pain: A Comprehensive Review of Anatomy, Diagnosis, and Treatment. Anesth Analg 2005;101:1440-53.


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