Cochlear implantation via the middle fossa: surgical and electrode array considerations

Cochlear implantation via the middle cranial fossa may access the entire length of the cochlea without opening the middle ear. Concerns include safety and electrode array design. The objectives of this study were to determine the depth to the superior portion of the basal turn (SPBT) and distances from the facial and greater petrosal nerves, and to describe some ideas about electrode arrays. The study involved operative dissection of 41 bequeathed otitis-free adult crania (82 temporal bones). Mastoid size was assessed by x-ray. Commercially available MED-EL split arrays were inserted. The depth from the floor of the middle cranial fossa to SPBT of the cochlea ranged from 0.5 mm to 4.2 mm. Small mastoid size correlated with shallow depth. Distances from the centre of the SPBT to the labyrinthine portion of the facial nerve, to the geniculate ganglion, and to the greater petrosal nerve ranged from 1.0 mm to 3.0 mm, 2.0 mm to 3.2 mm, and 1.8 mm to 2.8 mm, respectively. More than 75% of electrodes inserted toward the round window extended into the vestibule. Insertions toward the cochlear apex had a median insertion depth of 12 mm (range 6 mm to 18 mm). The middle cranial fossa approach appears safe and allows electrode access to nearly the full length of the cochlea. Electrode arrays specific for this route of implantation are needed, together with a rigorous study comparing outcomes of this route of implantation with traditional implantation through the facial recess.