New practical landmarks to determine sigmoid sinus free zones for suboccipital approaches: an anatomical study

Literature defines the landmarks to identify the courses and locations of the transverse and sigmoid sinuses on the outer surface of the skull and inner surface of the scalp. These natural landmarks may only be helpful after skin incision and are inadequate to determine the length and size of the skin incision. Still, there is a need to identify palpable landmarks easily to determine the ideal location to open the initial burr hole before an operation. Twenty-eight dried adult human skulls and 2 cadavers were evaluated. The zygomatic root, the inion, and the mastoid process were identified on the external, and the grooves for sigmoid and transverse sinuses, on the internal surfaces. The distances between the 3 landmarks and the midpoints, and the shortest distances of the midpoints to the border of the groove for sigmoid sinus and groove for transverse sinus were measured. Statistically significant differences were evaluated for both sides. Based on the measurements, the defined "artificial landmarks" can be considered safe points that involve no vascular structures and may be used to perform the initial burr hole during posterolateral approaches. Identification of the midpoints and palpation of the defined landmarks easily before the operation render the study feasible and practical unlike with natural landmarks. To avoid venous injury, the midpoints of mastoid-inion line and zygomatic root-inion line can be used safely in skin incision during posterior fossa approaches and craniotomy.