Is the entire fundus of the internal auditory canal visible during the middle fossa approach for acoustic neuroma?

HYPOTHESIS: To determine the degree to which the fundus of the internal auditory canal (IAC) can be visualized during the middle fossa approach (MFA). BACKGROUND: Conventional wisdom states that the MFA provides excellent access to the IAC from the porus acusticus to the fundus. On the basis of observations derived from a substantial surgical experience, it became obvious that a variable fraction of the fundus lies obscure from the surgeon's line of sight during the MFA because of (1) the overhand of the transverse crest and/or (2) the immobility of the facial nerve at its entry into the fallopian canal. METHODS: Intraoperative measurements were performed in ten cases to determine the typical angle of view to the fundus of the IAC in the MFA. This angle of view was projected onto coronal computed tomography scans of 40 temporal bones. Measurements of the IAC were made to determine the amount of fundus that could not be directly visualized during a MF exposure. RESULTS: On the basis of a surgical line of sight, the fraction of the inferior compartment of the canal that could not be directly visualized because of overhand of the transverse crest ranged from 14% to 34% (median 25%). CONCLUSIONS: Complete resection of IAC tumors involving the fundus via the MFA requires some degree of blind dissection. Specialized tools and techniques are required to minimize the risk of neural injury during this indirect dissection. Inspection of the fundus with either mirror or endoscope is often necessary to exclude the possibility of retained tumor fragments.

Extradural temporal lobe retraction in the middle fossa approach to the internal auditory canal: biomechanical analysis.

HYPOTHESIS: The middle fossa (MF) approach is undergoing a marked resurgence in vestibular schwannoma surgery as a hearing conservation technique. It is widely recognized that the extradural temporal lobe retractors used in this procedure, despite their cleverness of design, could be improved. METHODS: To identify the characteristics of an ideal MF retractor, a systematic analysis of the safety and functionality of four commonly used retractors (House-Urban, Fisch, Garcia-Ibanez, and UCSF) in a human anatomical model was conducted. Intensity of temporal lobe compression, width of exposure, angle of visualization, obstruction to instrument access, ergonomic convenience of use, and adaptability to other subtemporal procedures (e.g. lesions of Meckel's cave and cavernous sinus) were quantified. RESULTS: Because the intracranial portions of the retractors are similar, the force transmitted to the brain differed little among the four retractors. Numerous differences were noted in the ergonomics of use and versatility of the various designs. CONCLUSIONS: The optimal MF retractor would incorporate the best features of each of the existing systems: the integral suction of the Garcia-Ibanez, the bone contour-following design of the Fisch retractor base, the unobtrusiveness and adaptability of the UCSF, and the three-plane adjustability of the vintage House-Urban. Evolution of an "ideal" MF retractor requires further technical refinements and the development of an experimental model of extradural brain retraction to assess the optimal strategy for obtaining exposure while minimizing the risk for temporal lobe injury.