[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Spine](https://durgical.com/en/anatomy/region/spine/) › SI joint surfaces [Joints and ligaments](https://durgical.com/en/anatomy/system/joints-and-ligaments/) · [Spine](https://durgical.com/en/anatomy/region/spine/) # SI joint surfaces The sacroiliac joint surfaces are ear-shaped, cartilage-covered areas between the sacrum and hip bones that pass the spine's weight to the [pelvis](https://durgical.com/en/anatomy/region/pelvis/). Official name: Sacroiliac joint [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=spine&select=si_joint_cartilage&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The cartilage-covered, ear-shaped joint surfaces between the sacrum and the hip bones. ## What it does Where the weight of the spine passes into the pelvis; the joint moves only a few degrees. ## Key points - Sacroiliitis is often the first imaging sign of axial spondyloarthritis; erosions start on the iliac side, where the cartilage is thinner - Mainly supplied by lateral branches of the dorsal rami (about L5–S3); pain is felt in the buttock and may spread to the thigh, rarely below the knee - The posterior superior iliac [spine](https://durgical.com/en/anatomy/region/spine/) (PSIS), under the dimples above the buttocks, lies at about S2 over the joint ## Key facts **Where** Ear-shaped (auricular) surfaces of the sacrum (S1–S3) and the ilium **Attaches** Sacral side: thicker hyaline cartilage; iliac side: thinner, more fibrous cartilage; held by anterior, interosseous and posterior sacroiliac ligaments **Function** Synovial joint in front, fibrous (syndesmosis) behind; passes body weight from spine to pelvis; only a few degrees of nutation and counternutation ## Relations **Medial** Auricular surface of the [sacrum](https://durgical.com/en/anatomy/sacrum/) (S1–S3), with the sacral foramina and the S1–S2 roots just medial **Lateral** Auricular surface of the ilium, with the iliac tuberosity behind it for the interosseous ligament **In front** Anterior sacroiliac ligament; in front, [psoas major](https://durgical.com/en/anatomy/psoas-major/), the lumbosacral trunk and the [obturator nerve](https://durgical.com/en/anatomy/obturator-nerve/) crossing the pelvic brim, and the internal iliac vessels **Behind** Interosseous and posterior [sacroiliac ligaments](https://durgical.com/en/anatomy/sacroiliac-ligaments/), then the [thoracolumbar fascia](https://durgical.com/en/anatomy/thoracolumbar-fascia/) and [gluteus maximus](https://durgical.com/en/anatomy/gluteus-maximus-muscle/) ## How to find it The joint is deep and faces forward and outward, so it cannot be felt directly. The posterior superior iliac spines are felt under the [skin](https://durgical.com/en/anatomy/skin/) dimples above the buttocks; the joint lies deep and a little below and medial to each one. ## Examination - FABER (Patrick) test: supine, the hip is flexed, abducted and externally rotated (figure 4) and the knee pressed down; pain at the back over the SI region suggests the SI joint, groin pain suggests the hip - Gaenslen test: one hip is held flexed to the [chest](https://durgical.com/en/anatomy/region/chest/) while the other leg hangs extended over the edge of the table; pain in the SI region suggests the SI joint - Thigh thrust, distraction, compression and sacral thrust complete the Laslett cluster: three or more positive tests out of five raise the probability that the SI joint is the pain source - Fortin finger test: the person points with one finger to the pain within about 1 cm below and medial to the PSIS; pointing there consistently suggests the SI joint ## On imaging AP pelvis or angled (Ferguson) view: erosions, sclerosis, joint widening and later fusion. MRI (STIR or fat-suppressed T2) shows bone marrow oedema of active sacroiliitis before radiographs change. CT shows erosions and sacral insufficiency fractures in older people. ## Common problems - SI joint pain: low back and buttock pain, sometimes into the thigh, from strain, a fall, uneven loading, pregnancy or after a lumbar fusion. - Inflammation of the joint (sacroiliitis), for example in ankylosing spondylitis. ## How it is diagnosed Exam with pain provocation tests; a numbing injection into the joint confirms it. X-rays, CT or MRI for arthritis and inflammation. ## Treatment - Physical therapy, a pelvic belt, medicine and joint injections. - Radiofrequency ablation of the nerves, or SI joint fusion surgery for lasting severe pain. ## Clinical notes - Bilateral, symmetric sacroiliitis is typical of ankylosing spondylitis and inflammatory bowel disease; it is often asymmetric in psoriatic and reactive arthritis - Septic sacroiliitis is usually one-sided and can mimic disc or hip disease - Hormonal laxity in pregnancy contributes to pelvic girdle pain - An intra-articular block with local anaesthetic is the reference test for SI joint pain ## Anatomical variants - Accessory sacroiliac joints, small extra contacts between ilium and sacrum outside the main joint, are common (reported in about 8–40% of specimens) ## Development In children the surfaces are flat and mobile; from adolescence they develop interlocking ridges and hollows, and fibrous bridging becomes common in older age. ## Related conditions - [Ankylosing spondylitis](https://durgical.com/en/conditions/ankylosing-spondylitis/) - [Sacroiliac joint pain](https://durgical.com/en/conditions/sacroiliac-joint-pain/) ## Test yourself Think of the answer, then tap the question to see it. ## On which side of the sacroiliac joint surfaces do the erosions of sacroiliitis start, and why? On the iliac side, where the cartilage is thinner. ## How is the FABER (Patrick) test done, and how is it read for the sacroiliac joint? Supine, the hip is flexed, abducted and externally rotated and the knee pressed down; pain at the back over the SI region suggests the SI joint, groin pain the hip. ## What is the Laslett cluster for sacroiliac joint pain? Five provocation tests (distraction, compression, thigh thrust, sacral thrust, Gaenslen); three or more positive raise the probability that the SI joint is the source. ## What is the reference test for sacroiliac joint pain? An intra-articular block with local anaesthetic. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anatomy/sacroiliac-joint/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** articulatio sacroiliaca TA2 1861 **Official Spanish (DeCS)** Articulación sacroiliaca DeCS D012446 **English (TA2)** Sacroiliac joint **Also called** articulus sacrioilicus ## Related - [Sacroiliac ligaments](https://durgical.com/en/anatomy/sacroiliac-ligaments/) - [Hip bone](https://durgical.com/en/anatomy/hip-bone/) - [Sacrum](https://durgical.com/en/anatomy/sacrum/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=12822&filter=ths_termall&q=Articulaci%C3%B3n%20Sacroiliaca); [Anatomy, Abdomen and Pelvis, Sacroiliac Joint](https://www.ncbi.nlm.nih.gov/books/NBK507801/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/sacroiliac-joint/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/sacroiliac-joint/