Developing “See-through” Vision for Middle Fossa Surgery, A Surgical Anatomy Study

Background Middle fossa surgery is challenging and reliable surgical landmarks are key to perform accurate and safe surgery. The objective is to describe a roadmap of easy surgical references to develop a “see-through” vision in the middle fossa.

Methods Forty dry skulls (79 sides) from the anatomical board at Mayo Clinic in Rochester, Minnesota, were studied. Measurements were taken with a digital caliper and a protractor. Measurements from the middle aspect of the external acoustic canal (MEAC) to the lateral projections in the skull of foramen ovale, spinosum, greater petrosal nerve hiatus, and petrous apex were performed. The distance between the lateral aspect of the middle fossa and these anatomical references was also measured (Fig. 1). A new method to find the internal acoustic canal (IAC) from the middle fossa was described. The results were statistically analyzed.

Results The anterior aspect of the root of the zygoma was on average 23.5 mm anterior to the MEAC. The petrous apex in a lateral projection was found 16 mm anterior to the MEAC. The most posterior superior aspect of the petrous part of the temporal bone was 24 mm posterior on average to the MEAC. The foramen ovale and spinosum were on average 20 and 14 mm anterior to the MEAC, respectively, and 30 and 25 mm medial to the lateral aspect of the middle fossa, respectively. The EAC was found in the same coronal plane as the IAC in all the specimens. The posterior aspect of the hiatus for the greater petrosal nerve could be found 6 mm anterior to the MEAC level and 25 mm medial on average, and running nearly parallel to the petrous ridge. Drilling just above the supramastoid crest posterior to the EAC would correspond to the supratentorial space in 95% of cases and infratentorial in 5%. Placing the burr hole 1 cm above the supramastoid crest was found to be supratentorial in all cases. The median axis of the IAC projected to the anterior root of the zygoma in 39% of the cases, to the middle aspect in 31% and to the posterior aspect in 29%. To find the IAC though the middle fossa, the first step would be to find the lateral aspect of the trigeminal impression in the petrous ridge. The IAC opening was found 5.5 mm posterolateral to the lateral aspect of the trigeminal impression and 4.5 deep to the petrous ridge on average. The direction of the IAC could be estimated by this point and a projection to the anterior aspect of the root of the zygoma. We would consider safe to start drilling 3 mm posterolateral to the lateral aspect of the trigeminal impression in Kawase's triangle until the medial aspect of the IAC is found.

Conclusion External landmarks are reliable references for middle fossa surgery. Pathologies often displace or distort normal anatomy and detailed knowledge of these relationships is essential in planning and tackling these pathologies. An easy and safe method to predict the location of the IAC in surgery is described.

Publication History

Publication Date:
02 February 2018 (online)