Study On The Microsurgical Anatomy Of The Modified Presigmoid Medial Translabyrinthine Approach

Part 1: The Relationship between Surgical Anatomy of the Posterolateral Cranial Base and Petrosal ApproachObjective: To study the surgical anatomy of the posterolateral cranial base that is related to petrosal approach and to investigate the cranial bony anatomy relationships to underlying structures and the significances for surgical planning and approach. Methods: We inspected 20 dry human skulls and dissected 17 cadaver heads via different surgical approaches and exposed transverse sinus, sigmoid sinus, superior petrosal sinus, sinodural angle and other structures, and measured the distances and the angles between those structures. Results: The middle and lateral segments of transverse sinus in the right sides were wider than that in the left sides. The skull bone above the inferior curve of the sigmoid sinuses was thicker markedly than that above the upper curve and vertical segment. The transverse sinus was found to lay along the axis of a line connecting the squamosal-parietomastoid suture junction with the external occipital protuberance, and the sigmoid sinus was found to lay along the axis of a line connecting the squamosal-parietomastoid suture junction with the mastoid tip. The "intersinus angle", which consisted of the two lines, was 102 ± 17.3 degrees on the left side and 100 ± 15.5 degrees on the right side. The average distances from the mastoid emissary foramen to the posterior margin of the sigmoid sinus were 12.0 ± 2.2 mm on the left side, and 12.3 ± 2.5mm on the right side. Conclusion: The discrepancy of the transversesinus widths between two sides should be considered when a craniotomy is performed on the right side. We suggest a new method of placing the burr holes during a presigmoid approach: Hole 1, anterior and superior to the squamosal-parietomastoid suture junction; Hole 2, below the line from the squamosal-parietomastoid suture junction to the external occipital protuberance and its anterior border along the posterior margin of the mastoid; Hole 3 and Hole 4, 2~3cm medial to the asterion and astride of the line from the squamosal-parietomastoid suture junction to the external occipital protuberance; Hole 5, superior to the middle point of the zygomatic arch. The junction of the transverse sinus and the sigmoid sinus must be exposed in the retrosigmoid approach and the cranial opening was located inferior and posterior to the squamosal-parietomastoid suture junction.