[Anatomy of locating the internal auditory canal through the middle fossa approach with the assistance of high resolution CT].

OBJECTIVE: To evaluate the feasibility of localizing the internal auditory canal (IAC) and the facial nerve through the root of the zygoma, foramen spinosum and the head of the malleus in middle fossa approach with the assistance of high resolution computerized tomography (HRCT).

METHODS: Eighteen human cadaveric temporal bones were scanned and measured by HRCT. Cadaver specimen were divided into two groups. Group A was studied first through a middle fossa approach to find out the relationship between the HRCT measurements and the anatomic measurements. Then 4 whole human cadaveric heads (8 temporal bones) of group B were dissected using a HRCT oriented middle fossa approach to localize IAC with the root of the zygoma, foramen spinosum and the head of the malleus as landmarks. The two measurement methods were analysed with the Paired-Sample T test, and the difference was thought to be statistical significant when P < 0.05.

RESULTS: In Group A, there were no statistical significant differences between the CT measurements and the anatomic measurements from the head of the malleus to other important anatomic structures. In group B, the operation was guided with CT measurements: the distance between the head of the malleus and the root of the zygoma, and the distance between the head of the malleus and foramen spinosum. Within the range 1.5 mm to 3.7 mm, the head of malleus was correctly localized. In seven out of the eight cases, the root of the zygoma, the head of the malleus and the internal auditory canal were in a straight line, whereas, in one case, there was an angle of 15 degrees between the root of zygoma -head of malleus line and head of malleus -internal auditory canal line.

CONCLUSIONS: HRCT would provide more information on the distance relationship between the head of malleus and the root of the zygoma, foramen spinosum and the internal auditory canal. The head of the malleus could be localized through the root of the zygoma and foramen spinosum with HRCT and therefore the IAC could be exposed with the head of the malleus as a landmark in middle fossa approach when other landmarks were not recognizable.