Surgical landmarks for accessing the Cochlear Apex

Access to the cochlear apex has so far been of minor importance in otosurgery. However, new concepts of cochlear implantation and tinnitus suppression using extracochlear electrical stimulation require electrode placement at or in the apex via a direct apical approach. The present study examines the relation between the cochleariform process, tensor tympani muscle and apex cochleae in 34 temporal bone specimens. For this purpose, experimental CTs with a resolution of 99 µm were prepared and the relevant structures of the middle and inner ear were segmented in the 3D data set. A coordinate system based on the A and B vectors was then drawn. The distances between proc. cochleariformis and apex as well as between apex and canalis m. tensor tympani were recorded. In addition, it was recorded whether the projection of the modiolus intersected the m. tensor tympani. A very stable positional relationship was found between the apex and proc. cochleariformis with a median distance of 4.3 mm (SD 0.72 mm) and between the apex and canalis m. tensor tympani of 1.5 mm (SD 0.49 mm). The apex was within the projection of the modiolus onto the tensor canal in 33 of 34 temporal bones, i.e., surgically below the easily recognizable canal. Processus cochleariformis and tensor canal are therefore well suited for direct access to the apex cochleae. For this purpose, it is recommended to dissect from the cochleariform process along and below the tensor canal medially until the apex appears through the thinned bone. The incus usually has to be removed for this procedure.