Horizontal serial sections of human temporal bones were utilized to study the extension of the sinus tympani and its relationship with neighboring structures. The variation in size and shape was found to be significant in the superior and inferior portions. The sinus tympani usually was the deepest at the vertical level of round window membrane; the wall of the sinus tympani approximated the facial nerve in the superior portion and stayed away from it inferiorly. A heavy compact bony zone usually existed posterior to the sinus tympani. This finding and the narrow space between the facial nerve and posterior semicircular canal illustrates the limitation of surgical approach to the sinus tympani via retro-facial cells.
Saito R, Igarashi M, Alford BR, Guilford FR. Anatomical Measurement of the Sinus Tympani. Arch Otolaryngol. 1971;94(5):418–425. doi:10.1001/archotol.1971.00770070664006