The sacrum and the two innominate bones form a ring. The ring is weak anteriorly at the pubic symphysis and strong posteriorly due to the sacroiliac and iliolumbar ligaments.
In the adult with a rigid pelvis a fracture at one point of the ring is invariably accompanied by a second fracture elsewhere. This is not the case in children whose symphysis and sacroiliac joints are more flexible. This second fracture is however not always visible because it is either reduced immediately, impacted, or through a joint.
Pelvic fractures may be stable or unstable. A fracture is stable as long as the sacroiliac elements are not completely disrupted.
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