The anatomical relation between the Transverse-Sigmoid complex and skull landmarks in a 3D imaging study of CTV

Siriwut Pokanan¹Arnupap Puchaisang¹Chonnawee Chaisawasthomrong²Woottinun Phondet¹Atthaporn Boongird^1*

• ¹Mahidol University Faculty of Medicine Ramathibodi Hospital, Bangkok, Thailand
• ²Ratchaburi Hospital, Ratchaburi, Thailand

Abstract Background: The retrosigmoid craniotomy is one of the most common approaches in neurosurgery, providing access to posterior fossa lesions. Safe and effective surgery in this area requires precise knowledge of the bony landmarks related to the transverse–sigmoid sinus complex. Although various surface landmarks have been proposed, their accuracy and consistency remain uncertain. This study aimed to establish a reliable and reproducible reference line—the asterion–digastric point (DP) line—for identifying the vertical segment of the sigmoid sinus and related structures using three-dimensional computed tomography venography (CTV). Methods: A radiographic analysis was performed on 51 three-dimensional CTV scans obtained from patients older than 15 years at Ramathibodi Hospital between 2021 and 2023. Using volume-rendering techniques, the asterion, digastric point, transverse–sigmoid junction, and vertical segment of the sigmoid sinus were identified. The spatial relationship between these venous structures and the asterion–DP line was measured at proximal, mid, and distal levels, with reference to standardized orientation angles (x-axis: 12°, y-axis: 15°, z-axis: 120°). Results: The asterion–DP line reliably (100%) indicated the location of the vertical segment of the sigmoid sinus bilaterally. The mean distances from this line to the medial border of the sigmoid sinus were 4.16 ± 2.45 mm (proximal), 7.90 ± 2.33 mm (mid), and 3.24 ± 1.64 mm (distal). The mastoid emissary foramen was more frequently present on the right and predominantly situated medial to this line. The occipitomastoid suture consistently lay medially along the asterion–DP line, with mean distances from the DP of 9.36 ± 1.30 mm (right) and 9.48 ± 1.22 mm (left). The right sigmoid sinus was significantly larger than the left (7.50 ± 1.22 mm, P < 0.001). Conclusion: The asterion–DP line provides a consistent and easily identifiable external landmark for locating the vertical segment of the sigmoid sinus. Recognizing this reference line, together with the occipitomastoid suture, can enhance the safety and accuracy of retrosigmoid craniotomy by reducing the risk of venous sinus injury.