Anatomical landmarks to locate the junction between transverse and sigmoid sinuses in translabyrinthine and retrosigmoid open surgical approaches

Introduction
Hematoma due to dural-sinus damage is a known complication when introducing burr holes in open transcranial surgery. Our objective was to identify safe areas to avoid dural-sinus damage based on anatomical landmarks in translabyrinthine and retrosigmoid open surgical approaches where neuronavigation facilities are not available.

Methods
A descriptive anatomical study was conducted on adult skulls. Distances to transverse and sigmoid sinuses on either side were measured using fixed anatomical landmarks: asterion, inion, margins of suprameatal triangle and superior nuchal line. Measurements were standardized according to the cranial indices (cranial index=anteroposterior diameter/transverse diameter) of each skull.

Results
Thirty-two adult skulls (male:female=22:10) were studied. Mean cranial index, width of transverse and sigmoid sinuses were 0.785±0.045, 9.1±2.3mm and 9.7±1.2mm respectively. Mean vertical distances from asterion and inion to the transverse sinus were 1.1±3.4mm and 14.7±5.9mm respectively. Posterior border of the sigmoid sinus was located 14.7±5.9mm, and 59.9±7.4mm anterior to asterion
and inion respectively. t-tests did not show significant differences of these distances on either sides (p>.05). Pearson's correlations were insignificant between the measurements and the cranial indices (p>.05). Measurements from the suprameatal triangle to the dural-sinuses had the minimum variance. In >95% of the times the sigmoid sinus was located ≤23 mm posterior and ≤7 mm superior to the
suprameatal triangle.

Discussion and conclusions
Suprameatal triangle was a consistent surface landmark to locate dural-sinuses. Dural-sinus damage could be avoided in 95% of the times by placing burr hole at least 7mm superior and 23 mm posterior to the suprameatal triangle.