A Pilot Cadaveric Investigation Recommending the Lesser Petrosal Nerve as an Anatomical Landmark for Middle Fossa Approaches.

Abstract

Objective: Ambiguity of certain reference points like the arcuate eminence and facial hiatus while approaching the middle fossa is not to be underestimated. This investigation proposes the use of the lesser petrosal nerve (LPN) as a novel landmark from which to determine the precise localization of the superior semicircular canal and internal acoustic canal.

Subjects: Twelve adult cadavers (6 males and 6 females) aged 70.70 ± 15.10 years (range: 40-90 yr).

Intervention: This clinical capsule covers the dissection report of 24 temporal bones.

Main outcome measures: The numerical relationship of LPN to adjacent structures at the skull base was determined for middle fossa approaches.

Results: The angles of LPN with the internal acoustic canal, greater superficial petrosal nerve, and superior semicircular canal were found to be 77.58 ± 12.78°, 7.43 ± 3.73°, and 112.01 ± 10.06°, respectively. The distance from the most posterior part of LPN to the head of the malleus was measured as 4.82 ± 1.58 mm.

Conclusion: If the most posterior part of the groove for LPN is traced posteriorly for approximately 5 mm, the head of the malleus is reached by opening the tegmen tympani. Taking the medial margin of the head of the malleus as reference, a surgeon may find the superior semicircular canal, by following it along 110° posterior to the long axis of LPN. In addition, the surgeon may detect the internal acoustic canal, by tracing it along 78° posterior to the long axis of LPN. Our strategy is based on the initial identification of LPN and then access to SSC and IAC by forming a surgical map through LPN.