Background: The zygoma-inion line (ZI line) is an important surface landmark of transverse sinus in suboccipital craniotomy;
however, previous anatomical studies showed variation in relationship between the ZI line and transverse sinus.
Objective: To study anatomical relationship between the ZI line and transverse sinus.
Material and Method: The ZI line was projected to the inner surface of the skull by using transillumination technique.
Anatomical relationship between the projected ZI line and groove of the transverse sinus was investigated in 60 dried Thai
human skulls (120 sides).
Results: The projected ZI line was located exactly on the transverse sinus in 6 of 120 sides (5 %). In 114 of 120 sides (95%), the projected ZI line was positioned below the inferior margin of the transverse sinus. The distance between the projected ZI line and inferior margin of the transverse sinus gradually increased as it moved outward from the midline.
Conclusion: The ZI line is a reliable external landmark for localization of the transverse sinus. Most of the ZI line was located inferior to the transverse sinus about 4 mm with trend to be lower as transverse sinus moved away from the midline.

Keywords: Relationship, Zygoma-inion line, Transverse sinus, Suboccipital craniotomy, Posterior cranial fossa