SI

Question 1

What forms the SI joints

Answer 1

SI joints are atypical diarthrodial joints, forming the articulation between the ventral aspect of the ilium and the transverse process of the first sacral vertebra (=sacral wing)

Joint consists of flat synovial articulation with hyaline cartilage on the sacral auricular surface and fibrocartilage on the iliac auricular surface

Joint space is located deep to the iliac wing, between the auricular surface of the iliac wing and the auricular surface of the first transverse process of the sacrum (sacral wing)

Question 2

Which 3 sets of ligaments stableise the SIj region

Answer 2

1. Ventral sacroiliac ligament: lies along the ventral margin of the auricular surfaces of the ilium and sacrum and its fasciculi extend mediolaterally (Fig 2).
2. Interosseous sacroiliac ligament: made of several strong fasciculi separated by fat. It joins the dorsal surface of the sacral wing and the cranioventral surface of the iliac wing (Fig 2).
3. Dorsal sacroiliac ligament: strong and no direct anatomical relationship with the SI joint space. It inserts on the tuber sacrale of the ilium and is divided in 2 parts. The dorsal (or funicular) part extends caudally and attaches on the spinous processes of the sacrum (median sacral crest). The ventral (or membranous) part extends ventrally to insert on the transverse processes of the sacrum (lateral sacral crest)

These 3 SI ligaments build a strong sling to the sacrum and give stability to the SIj, allowing only 1-2° of flexion/extension

Gluteal muscles, erector spinae mm and psoas mm also strongly contribute to stabilise the SIj

Question 3

Proposed grading system by Tallaj et al 2020 (EVE) for transrectal US of SIjs

Answer 3