Internal Acoustic Meatus Localization During Anterior Petrosectomy: A Novel Technique.

OBJECTIVE:Anterior petrosectomy demands localization of the internal auditory canal (IAC) for safe bone drilling and maximum exposure. Various techniques have been described in the literature, each with shortcomings. We propose a new technique to localize the internal acoustic meatus (IAM) using more consistent anatomical landmarks. METHODS:The study was done in three phases. In phase-I (radiological), computed tomography scan heads of fifty patients (100 sides) were analyzed. Arcuate eminence-Greater Superficial Petrosal Nerve bifurcation angle(Garcia-Ibanez technique), Arcuate eminence-IAC angle(Fisch technique) and a new angle formed between foramen ovale (FO) and foramen spinosum (FS) line, and FS and IAM line (FO-FS-IAM angle) was measured. The mean, standard deviation, and variance were calculated. In phase-II (cadaveric), the FO-FS-IAM angle was measured on five (10 sides) dry skulls. In phase-III (clinical), the IAM was localized using the FO-FS-IAM angle in 13 patients. RESULTS:The mean angle between arcuate eminence and Greater Superficial Petrosal Nerve (Garcia-Ibanez technique) was 126.20 ± 11.63°(range 106-156) with a variance of 135.20. The mean bifurcation angle was 63 ± 5.81°(range 53-78). By the Fisch technique, the mean arcuate-IAM angle was 73.5 ± 11.70°(range 51-105) with a variance of 137.18. By our technique, the mean FO-FS-IAM angle was 94.72 ± 5.89°(range 84-108). The variance was 34.73. The mean FO-FS-IAM angle on dry skulls was identical (95 ± 1.97°) to our radiological measurements. This angle was reproduced reliably in clinical cases for localizing the IAM during anterior petrosectomy. CONCLUSIONS:The FO-FS-IAM angle variance was much lower than the analogous angles measured by Garcia-Ibanez and Fisch techniques, making it a more reliable and effective tool for localizing the IAM.