Transcanal Computed Tomography Views for Transcanal Endoscopic Lateral Skull Base Surgery: Pilot Cadaveric Study

Abstract

Objective Transcanal endoscopic operative approaches provide for a minimally invasive surgical portal to the lateral skull base. Traditional preoperative imaging evaluation involves computed tomography (CT) acquisition in the axial and coronal planes that are not optimized for the transcanal surgical corridor. Herein, we describe a novel CT-based "transcanal view" for preoperative surgical planning and intraoperative navigation. Study Design Present study is a cadaveric imaging study. Methods Cadaveric temporal bones ( n = 6) from three specimens underwent high-resolution CT (0.625 mm slice thickness). Using three-dimensional (3D) Slicer 4.8, reformatted "transcanal" views in the plane of the external auditory canal (EAC) were created. Axial and coronal reformats were used to compare and measure distances between anatomic structures in the plane of the EAC. Results The degree of oblique tilt for transcanal CT reformats was 6.67 ± 1.78 degrees to align the EAC in axial and coronal planes. Anticipated critical landmarks were identified easily using the transcanal view. Mean values were 8.68 ± 0.38 mm for annulus diameter, 9.5 ± 0.93 mm for isthmus diameter, 10.27 ± 0.73 mm for distance between annulus and isthmus, 2.95 ± 0.13 mm for distance between annulus and stapes capitulum, 5.12 ± 0.35 mm for distance between annulus and mastoid facial nerve, and 19.54 ± 1.22 mm for EAC length. Conclusion This study is the first to illustrate a novel "transcanal" CT sequence intended for endoscopic lateral skull base surgery. Future studies may address how incorporation of a transcanal CT reformat may influence surgical decision making.

Keywords: computed tomography; endoscopic; lateral skull base surgery; preoperative planning; transcanal surgery.

Fig. 1

Stepwise 3D reconstruction of orientation of skull from axial to transcanal plane. 3D, three-dimensional; 2D, two-dimensional; A–P, anterior–posterior; L–R, right–left.

Fig. 2

Transcanal CT images through the left temporal bone demonstrating anatomy from lateral to medial ( A–I ). A, anterior; CN VII, cranial nerve VII; I, inferior; P, posterior; S, superior; SCC, semicircular canal.

Fig. 3

Course of facial nerve in the CT scan. Transcanal CT scan sequence from medial to lateral showing the facial nerve course and segments in the left temporal bone of one specimen ( A–E ). The entire course of the facial nerve is shown from an oblique left lateral view ( F ). CN, cranial nerve; CT, computed tomography; IAC, internal auditory canal; SCC, semicircular canal.

Fig. 4

Relatioship of key anatomic structures in subcochlear corridor. The anatomic constraints between the petrous carotid artery, jugular bulb, and basal turn of the cochlea can be appreciated in traditional axial view (A) , transcanal reformatted views (B, C) , and in a virtual 3D TEES view (D) . Images are from a left ear. BT, basal turn of cochlea; ICA, internal carotid artery; JB, jugular bulb; P, promontory; TEES, transcanal endoscopic ear.

Fig. 5

CT image of left ear seen as a single slice medial to the annulus on transcanal reformatted view (left panel), axial plane (center panel), and coronal plane (right panel). Note that the view has been rotated to be consistent with the surgical position. CN VII, cranial nerve VII; CT, computed tomography; SCC, semicircular canal.