The sacroiliac joints are stabilised by strong ligaments.
The sacroiliac joint (SIJ) (lat. Articulatio sacroiliaca) is the connection between the sacrum bone (Os sacrum) and the ilium bone (Os ilium). Another name is the sacrum and ilium joint. Even though the sacroiliac joint is commonly referred to in the singular, there is a left and right sacroiliac joint.
The sacroiliac joints have limited mobility: Experts refer to these limited movements as nutation and counternutation. For example, the width of the pelvic ring becomes larger during childbirth due to a nutation movement of the sacroiliac joints, which makes giving birth easier.
The sacroiliac joints are stabilised by strong ligaments. During pregnancy, hormones have an effect on the ligaments so that they become more flexible. The spine and pelvis are connected by the sacroiliac joints.
Sacroiliac joint disorders and their treatment
Pain in the sacroiliac joints can come from sitting or standing for long periods of time, missing a step or jerky movements when lifting weights (e.g. a bottle crate). Inflammation, erosion, breaks or disorders are other causes of symptoms.
In most cases, the symptoms are treated non-surgically (conservatively). Those affected can often alleviate and treat the pain as well as prevent new disorders by doing special exercises provided by a physiotherapist.
Source
Dr. med. Klein, Christoph: Orthopädie für Patienten [Orthopaedics for patients]. Verlag Michels -Klein, Remagen 2014, p. 367-373