The occipital bone has complex development, ossifying from six centres. The foramen magnum is surrounded by four ossification centres. On each side are the exoccipitals, ventrally located is the basioccipital and dorsally, the supraoccipital centre contains the midline occipital fissure which can sometimes persist antenatally. This pattern of development can therefore give rise to numerous accessory sutures that could be mistaken for fractures especially with plain film evaluation alone.
It is important to know these anatomic variations, mainly on the head trauma image studies in children, where it could be difficult to differentiate non-depressed skull fractures from an accessory suture.
Radiographic differentiation of skull fracture and accessory suture
Accessory sutures usually:
- make a "zig-zag" pattern with interdigitations
- have sclerotic borders similar to major calvarial sutures
- are often present on both sides and are fairly symmetric especially in the parietal bones
- occipital accessory sutures can be complex and multiple but are also frequently bilateral
Simple non-depressed skull fractures usually:
- have sharp lucencies with non-sclerotic edges
- widen as they approach the suture or there is associated diastasis of the adjacent synchondrosis or suture
- can cross suture lines or extend from one major suture to another (in high impact fractures)
- have associated soft tissue swelling or haematoma
- absence does not exclude a fracture