Sacroiliac joint
Articulatio sacroiliaca
Also known as: SI joint, SIJ
The strong, weight-bearing joint between the auricular surfaces of the sacrum and ilium. It is a compound joint — synovial anteriorly and a syndesmosis posteriorly — permitting only slight movement.
Overview
The ear-shaped auricular surfaces of the sacrum (hyaline cartilage) and ilium (fibrocartilage) interlock by irregular elevations and depressions. The anterior part is a plane synovial joint; behind it, the sacral and iliac tuberosities are bound by the massive interosseous sacroiliac ligament (a syndesmosis), the strongest ligament of the joint. The anterior sacroiliac ligament (thin, part of the capsule) and the posterior sacroiliac ligaments (short and long) complete it. The sacrotuberous and sacrospinous ligaments resist the tendency of the sacral promontory to tip forward (nutation) under body weight, and the iliolumbar ligaments bind L5 to the ilium.
The sacrum is wedged between the ilia like a keystone and suspended from them by the interosseous and posterior ligaments, so that body weight actually tightens the joint. Movement is small (a few degrees of nutation and counter-nutation, ≈1–2 mm translation), increasing in pregnancy when relaxin loosens the pelvic ligaments. With age the joint often becomes partly fibrosed or ankylosed, especially in men.
- Location
Posterior pelvis, between sacrum and ilium (deep to the PSIS dimples).
Anatomy
- Innervation
Dorsal rami of L5–S3 (posteriorly) and ventral rami of L4–S2 / branches of the sacral plexus (anteriorly).
- Articulations
Auricular surface of the sacrum ↔ auricular surface of the ilium.
- Ligaments
Anterior sacroiliac, interosseous sacroiliac (strongest), posterior sacroiliac; accessory: sacrotuberous, sacrospinous, iliolumbar.
Function
- Transmits the weight of the upper body from the vertebral column to the hip bones
- Absorbs shock and allows slight pelvic flexibility (especially during childbirth)
Clinical significance
Sacroiliitis
Inflammation of the SI joints is the earliest feature of ankylosing spondylitis and other HLA-B27 spondyloarthropathies (psoriatic, reactive, enteropathic): inflammatory low-back and buttock pain, morning stiffness improving with exercise; MRI shows bone-marrow oedema; later erosions, sclerosis and fusion.
Pelvic girdle pain in pregnancy
Hormonal ligament laxity and increased load cause SI joint and symphyseal pain, worse on walking, climbing stairs and turning in bed.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.