A morphometric study of the structures bordering the infra-cochlear corridor - Relevant for endoscopic/microscopic ear surgery

Abstract

Application of surgical endoscope, used alone or in combination with the surgical microscope, for the operative management of ear and temporal bone conditions may allow improved access and clearance of disease. Preservation of normal structures may also be improved. As the use of this tool is increasing, the need for better understanding of the anatomy of the ear is becoming evident. This is particularly so for endoscopic surgery aiming at removal of lesions involving the infra-cochlear corridor and/or petrous apex. Human temporal bone-derived labyrinth casts (molds), originally made for endolymphatic duct and sac analysis which genuinely represent the membranous labyrinth and its adjacent soft tissues, were morphometrically analyzed in terms of the anatomic relations between structures in and around the infra-cochlear corridor. The distance between the petrous carotid artery (PCA) and the basal turn of the cochlea, the distance between PCA and infra-cochlear vein (ICV)/cochlear aqueduct (CA), and the distance between the lower surface of basal cochlear turn and the point where the carotid artery and jugular vein (JV) meet close to the jugular foramen, were measured to be around 1.3 mm, 6 mm and 8 mm respectively, thus constituting an approximate 6 × 8 mm² infra-cochlear corridor. This analysis and further study with larger samples might be helpful for operation via this corridor led to the petrous apex where cholesterol granuloma, cholesteatoma and other lesions are not uncommon.

Keywords: Anatomical study; Ear endoscopy; Infra-cochlear corridor; Petrous apex; Surgery.

Fig. 1

In A, the distance between the basal cochlear turn and the point where the PCA and jugular vein (JV) approach (a–b). This distance in A looks apparently bigger than the one in B, although both have a shorter a-b distance than average because of the high position of the jugular bulbs that render the cochlear aqueduct/infra-cochlear vein (CA/ICV) invisible. Fn, facial nerve; Co, cochlear basal turn; IAM, internal acoustic meatus; PCV, petrous carotid artery; JV, jugular vein.

Fig. 2

The lateral and superior view of silicon temporal bone casts. In A, the distance between the PCA and the CA/ICV (c–d) and the one between the PCA and basal turn of the cochlea (e–f) are included in our analysis. Note the CA/ICV forms a short curve (short white arrow in A), as it exits the scala tympani anterior to the round window (* in A) membrane (at a distance around 0.3 mm). This curved segment, also the thinnest part, passes parallel to the IAM, and becomes straighter and thicker as it turns down and ends at the jugular foramen. In B, one can see the upper side of the PCA, cochlear turns and the petrous apex that is divided by the content in the internal acoustic meatus (IAM) into anterior and posterior parts (*). Fn, facial nerve; Co, cochlear basal turn; IAM, internal acoustic meatus; PCV, petrous coratid artery; CA/ICV, cochlear aqueduct/infra-cochlear vein.

Fig. 3

A chart shows measurement of the three distances in 17 temporal bone casts (silicone and plastic). The series 1 represents the distance between lowest margin of the basal cochlear turn and the bifurcation point of PCA and the jugular vein; series 2 represents the shortest distance between the medial aspect of PCA and the infra-cochlear vein/cochlear aqueduct; series 3 represents the shortest distance between the medial aspect of the PCA and the anterior cochlear margin. The mean value of series 1 is 7.976 mm, range from 4.5 to 11.5 mm; the mean value of series 2 is 6.053 mm, range 5–8 mm; the mean value of series 3 is 1.31 mm, range 0.3–1.6 mm. * high jugular bulb.