Sacroiliac Joints (SIJ)
Sacroiliac Joints (SIJ)
Sacroiliac Joints (SIJ) are critical joints connecting the spine and pelvis, formed between the sacrum and the ilium of the pelvis. It plays a vital role in load transfer, stability, and shock absorption in the human body.
Anatomy
- The Sacroiliac Joint (SIJ) is a large, irregular synovial joint composed of the sacrum and the two innominate bones (each formed by the fusion of the ilium, ischium, and pubis).
- The joint surfaces are irregular and interlock to provide stability. The sacral side is covered with hyaline cartilage, while the iliac side has fibrocartilage.
- The upper one-third of the joint is a syndesmosis (fibrous joint), and the lower two-thirds are synovial, with only the lower third lined by synovium.
- It is surrounded by a strong fibrous joint capsule and reinforced by powerful ligaments:
- Interosseous sacroiliac ligament (strongest, posteriorly located)
- Posterior sacroiliac ligament (covers interosseous ligament)
- Anterior sacroiliac ligament (thinner, anterior side)
- Additional support from sacrotuberous and sacrospinous ligaments.
- The joint is supported by surrounding muscles and fascia, including pelvic floor muscles (levator ani, coccygeus) and large muscles of the lower back and hips.
Function
- The primary function of the SIJ is to transfer and distribute forces between the upper body (axial skeleton) and the lower extremities during standing, walking, and running.
- It acts as a shock absorber for the spine, attenuating forces from the lower limbs.
- The joint converts torque generated by the lower extremities into the rest of the body, facilitating efficient load transfer.
- It provides stability through its irregular surfaces and strong ligamentous support, limiting excessive motion.
- Movements at the SIJ are minimal and include:
- Nutation and counternutation (small tilting movements in the frontal plane)
- Torsion (rotation around an oblique axis in the transverse plane)
- Movement increases during pregnancy due to ligamentous laxity and typically decreases with age as the joint may fuse.
Clinical Relevance
- The SIJ is a common source of lower back and pelvic pain due to inflammation (sacroiliitis), injury, or degenerative changes.
- Its complex anatomy and limited mobility make diagnosis and treatment challenging.
- Understanding its biomechanics is essential for managing conditions like spondyloarthropathies and SIJ dysfunction.
Summary Table
| Aspect | Details |
|---|---|
| Location | Between sacrum and ilium of pelvis |
| Joint Type | Synovial (lower two-thirds), syndesmosis (upper one-third) |
| Cartilage | Sacral side: hyaline cartilage; Iliac side: fibrocartilage |
| Ligaments | Interosseous, posterior, anterior sacroiliac ligaments; sacrotuberous and sacrospinous ligaments |
| Function | Load transfer from spine to lower limbs; shock absorption; stability |
| Movement | Minimal; nutation, counternutation, torsion; increased during pregnancy |
| Support | Strong ligaments and surrounding muscles including pelvic floor muscles |
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References
- Vleeming A, Schuenke MD, Masi AT, et al. The sacroiliac joint: an overview of its anatomy, function and potential clinical implications. J Anat. 2012;221(6):537-567. https://doi.org/10.1111/j.1469-7580.2012.01564.x
- StatPearls: Wong M, Sinkler MA, Kiel J. Anatomy, Abdomen and Pelvis, Sacroiliac Joint. 2020. https://www.ncbi.nlm.nih.gov/books/NBK507801/
- Physio-pedia: Sacroiliac Joint. https://www.physio-pedia.com/Sacroiliac_Joint
- Kenhub: Sacroiliac Joint Anatomy and Function. https://www.kenhub.com/en/library/anatomy/sacroiliac-joint
- Radiopaedia: Sacroiliac Joint. https://radiopaedia.org/articles/sacroiliac-joint
- IJSSurgery: Biomechanics of the Sacroiliac Joint. https://www.ijssurgery.com/content/14/s1/S3
- Spine-Health: Sacroiliac Joint Anatomy. https://www.spine-health.com/conditions/spine-anatomy/sacroiliac-joint-anatomy
- PainTEQ: Exploring SI Joint Anatomy and Its Function. https://www.painteq.com/exploring-si-joint-anatomy-and-its-function
The sacroiliac joint is a vital, highly stable joint that functions as a key load-transferring structure between the spine and lower limbs, combining limited mobility with strong ligamentous support to maintain balance and absorb forces during movement.
