Use of the Middle Cranial Fossa Approach with Extended Posterior Petrosectomy for Resection of Larger Vestibular Schwannomas: A Combined Cadaveric-Clinical Investigation

Outcome Objectives:

1. Describe the optical and mechanical advantages of the middle cranial fossa approach with extended posterior petrosectomy (MCF-EPP) over traditional MCF for exposure of the cerebello-pontine angle (CPA) and brainstem.

2. Discuss surgical outcomes following MCF-EPP in consecutive patients with vestibular schwannomas 12 to 20 mm in size.

Methods: Cadaveric Arm: MCF-EPP and traditional MCF were performed on the right and left ears of 10 cadaveric whole heads, respectively. EPP entailed maximal bone removal from a triangle bordered by the internal auditory canal (IAC), superior semicircular canal (SSC), and posterior fossa plate. Bone was removed along the entire medial face of the SCC leaving only an eggshell of otic capsule. Bone was also removed from the ‘corner’ region bounded by the posterior crus of the SSC and the posterior fossa dura. Traditional MCF entailed creation of 4mm wide troughs anterior and posterior to the IAC. High definition CT images pre- and post- dissection were used to compare the volume and percentage of bone removed in each type of dissection. The optical advantage offered by MCF-EPP for CPA and brainstem exposure was determined by comparing posterior line of site measurements and degrees of variance between right and left ears, respectively

Clinical Arm: Patients with vestibular schwannomas >12 mm in greatest dimension and resected via MCF-EPP were retrospectively reviewed from 2007 to 2015 at a single tertiary center. Tumor size and extra-meatal extent were recorded. Outcomes assessed were completeness of resection, post-operative facial function score (House-Brackmann), and hearing preservation rate.

Results: Cadaveric Arm: Both mean bone volume removed (4.2 mm³ vs. 2.5 mm³; p = 0.03) and percentage removed (53 vs. 33%; p = 0.002) were significantly higher in MCF-EPP. The posterior line of sight angulation following MCF-EPP increased by 2 degrees, which translated to a mean of 5mm additional exposure at the level of the pons.

Clinical Arm: Sixteen patients underwent MCF-EPP for tumors 12 to 20 mm in size with 0– to 4 mm of extra-meatal extension. Complete resection was achieved in 93% (1 near-total). Postoperative House-Brackmann scores of 1 to 2 were noted in 15/16 patients. Of 13 patients with preoperative serviceable hearing, preservation was achieved in 5/13.

Conclusions: MCF-EPP results in significantly more bone removal than traditional MCF and greater CPA exposure. Clinically, the exposure offered by MCF-EPP has made it possible to resect larger tumors up to 20 mm in greatest extent and tumors with greater amounts of extra-meatal extension. Hearing preservation is possible with this approach.