This method is significant because it demonstrates how cochlea can be obtained for research purposes from cadaver temporal bone by following key anatomical structures inside the cochlea. This technique involves heavy surgical drilling. Learning this process might take practice and cause a risk of cochlear trauma.
Additionally, the temporal bone anatomy can be challenging for first timers. The visual demonstration of this method provides technical insight in the drilling and exposure of the anatomical landmarks, which cannot be presented in written or image form as good as in video form. Start by opening the facial recess between the facial nerve and the external ear canal with a two millimeter diamond bur.
Spot the chorda tympani position between the facial nerve and the external ear canal and carefully detach the chorda tympani. After opening the posterior tympanotomy, identify and open the incudostapedial joint. Slice the stapedial tendon with micro scissors to prevent stapes dislocation during the inferior extraction cut.
Next, extract the incus and drill away the buttress. Extract the posterior part of the bony external ear canal with the malleus, tympanic membrane, and skin. Remove the soft tissue, more specifically the remnant of the tympanic membrane, with the rest of the external ear canal skin using suction and a micro dissector.
Drill along the posterior side of the tympanic section of the facial nerve. Create a cutting line at the middle fossa using the superior semi-circular canal as a reference and proceed with it through the tegmen. Open the superior semicircular canal from a superior approach through the middle fossa.
Then, open the lateral semicircular canal and the posterior semicircular canal. Continue the drilling vertically towards the tip of the mastoid, starting from the vestibula, then open the posterior cut inferiorly until below the level of the cochlea's basal turn. Expose the genicular ganglion of the facial nerve with a diamond drill.
Follow the facial nerve to the internal auditory canal and reveal the landmarks. Then, open the labyrinthine section of the facial nerve and follow it to the internal auditory canal. Drill the internal auditory canal.
Drill the space in the anterior parts of the middle ear to open the mandibular joint. Identify the round window, the basal turn of the cochlea, and the canal of the internal carotid artery anterior to the cochlea's basal turn. Look for the internal carotid artery at the front of the middle ear, medial to the Eustachian tubes'opening.
Use scissors to cut the facial nerve. Drill the middle ear floor under the basal turn to create space for the inferior cut. Then, drill anterolaterally to the internal carotid artery to open the mandibular joint.
Once the mandibular joint is opened, locate the internal carotid artery and basal turn, then remove the lateral parts of the temporal bone without cochlear damage. Drill open the tegmen from the level of the middle ear to the sigmoid sinus and continue the cut to the sinus. Drill open the internal carotid artery through the middle ear.
Continue parallel drilling superiorly from the internal carotid artery turn. Remove any excess bone above and below the artery. Next, drill down about one to 1.5 centimeters alongside the basal turn of the cochlea.
Using the medial parts of the carotid artery and internal auditory canal as reference points, form an incision line medial to the cochlea. Execute the drilling from the proximal internal auditory canal to the proximal parts of the horizontal segment of the internal carotid artery. Lastly, link the medial line by exposing the internal auditory canal to the inferior cut.
Finalize the connection with the posterior cut to the medial cut and extract the cochlea. Using the fine diamond bur, gently trim excess bone around the cochlea. At this point, there should be a 1.5 by 1.5 centimeter piece containing the intact cochlea.
Make sure to maintain the orientation using the facial nerve, stapes, and round window. Successful cochlear extraction was performed in 33 of 36 cadavers. A-measure is the largest distance from the round window to the lateral wall, while B-measure is the perpendicular distance of the cochlea.
This technique helps to provide a systematic approach to obtain an intact, whole cochlea. This technique is especially useful for preclinical cochlear implant studies. Additionally, it may be useful for obtaining smaller samples for other purposes in cochlear studies.
for example, for micro CT scans.
