The physical relationship between anatomical landmarks such as the tympanic portion of the facial nerve and the orientation of the middle turn of the cochlea are sufficiently constant to provide guidance to surgeons performing cochlear implantation in the ossified cochlea.
Placing an implant in an ossified cochlea is technically difficult. The surgeon needs to drill two tunnels through the promontory bone to insert electrode arrays in the region of the basal and middle turns of the cochlea. However, few studies describe the orientation of these tunnels or how to use anatomical landmarks to guide the surgeon during their creation. This problem is particularly true for the superior (middle turn) tunnel.
Twenty human temporal bones from adult cadavers were analyzed. They were dissected with exposure of all middle ear structures, followed by the removal of the tympanic ring, tympanic membrane, malleus, and incus. We measured the angle between the tympanic segment of the fallopian canal and a line passing through the lower portion of the middle turn of the cochlea.
The measured angles varied from 123 and 152 degrees with an average of 133.85 degrees and the standard deviation of ±6.83 degrees.
The tunnel for the middle turn should be created with an inclination of about 134 degrees in relation to the tympanic segment of the facial nerve.