Anatomical variations of occipital bone impressions for dural venous sinuses around the torcular Herophili, with special reference to the consideration of clinical significance.

Venous blood flow through the cerebral dural sinus is variable and clinically significant. It has been investigated by cadaver dissection or radiology; however, we thought that osteology might be informative. A total of 160 dried skulls were macroscopically examined for impressions on the inner surface of the occipital bone in order to interpret the sinus flow around the torcular Herophili. The continuity between the grooves for the superior sagittal sinus (SSS) and the transverse sinuses was categorized into four types. Confluence type was noted in 56 specimens (35%), in which SSS drained into a common pool of venous sinuses. Bifurcation type was noted in 22 cases (14%), in which SSS was divided to drain into the bilateral transverse sinuses. Right dominant type was the most frequent one with 66 cases (41%), in which SSS drained only into the right transverse sinus. Left dominant type was the least frequent one with 16 cases (10%), in which SSS drained to the left, in a mirror image to the right dominant type. Clinical significance is discussed with our preliminary trial for the optimization of the inner skull surface and venous flow using computed tomography and magnetic resonance imaging, and demonstration of cerebrovascular disease.