Morphology and imaging of bony projections on sigmoid sulcus with clinical implication

Morphologically diverse osseous projections were observed on the anterior lip of the sigmoid sulcus in a study conducted on 318 dry unsexed adult human skulls belonging to the Indian race. For convenience of description, this lip was divided into a longer lateral part, a shorter medial part and a junctional angular part having bony attributes characterized into three types, crests (42.9%), plates (22.3%) and bridges (13.5%). The bridges were subtyped into incomplete (11.8%) and complete (1.7%). Crests and plates were present anywhere along this lip, while bridges were confined to the angle. Right and left differences were analyzed, which showed that the above findings were more frequent on the right than the left. Plates showed a higher incidence of variation between right and left sides as compared to crests and bridges. Five cadavers were studied, which showed retinacular bands attached to the projections blending with dura mater over the foramen magnum. It is postulated that these projections are caused by the traction of the fibrous dural bands. Axial CT scan revealed partial and complete osseous bridges spanning the sulcus. These projections are of relevance to surgeons working at the cerebellopontine angle. Their presence has not been reported previously.