The overall goal of this surgical procedure is to provide a reliable and versatile animal model of cochlear implantation using the guinea pig. This method can help to answer key questions in the field of cochlear implantations such as if pharmaceutical interventions or newly developed electrodes provide beneficial effects on residual hearing preservation or electrodes impedances. The main advantage of the guinea pig is the fast access to the relatively large inner ear which results in a safe, reliable and versatile animal model of cochlear implantation.
For this procedure it's important to have a surgical microscope, a drill, a heating plate and a pulse oximeter positioned around the experimenter for easy handling during the surgery. Set the heating pad to 38 degrees Celsius. Follow sterile technique.
Don a hat, mask and gloves. Then lay out the sterilized surgical tools for the cochlear implantation. Next, prepare a 3.5 centimeter long piece of Teflon insulated gold wire by carefully removing three millimeters of insulation from one end and five millimeters of insulation from the other end.
Next, cut a second length of gold wire that is 2.5 centimeters long and remove five millimeters of insulation from both ends. Then sterilize both wires with alcohol. Next, anesthetize a guinea pig using an anesthetic cocktail as directed in the text protocol.
Then shave its head and position it prone on the heating plate. Next, carefully open the guinea pigs mouth with a small laryngoscope and remove all the food debris from the oral cavity using aspiration. Then carefully insert a stomach tube into the esophagus by slowly pushing to the stomach until resistance is felt.
Check the oxygen saturation levels to be certain the tube is not in the trachea. Next, inject a mixture of physiological saline, 5%glucose and enrofloxacin into the fat pad of the neck using a 23 gauge needle. Prepare the surgical field using alternating scrubs of povidone-iodine and 70%ethanol and then drape the animal.
For an analgesic use lidocaine also redose the animal with anesthetic as needed. Start the surgery with a two to three centimeter skin incision, three to five millimeters posterior to the pinna using a scalpel. To deal with bleeding use bipolar cautery.
Then palpate for the prominence of the auditory bulla and use a number 15 surgical scalpel or surgical scissors to carefully cut the muscles in the retroauricular area. To dissect the muscles from the bulla, gently push aside the muscles and use a retractor to expose the full length of the incision for unhindered access to the bulla. Now use the tip of the number 15 scalpel to make a hole in the bulla.
When the bone is perforated, enlarge the hole as needed to inspect the middle ear structures. Put the animal in the inflected position and inspect the opening. The basal turn of the cochlea and the round window niche must be visible.
Keep an eye on the animals oxygen levels while it's in the inflected position. To proceed, reposition the animal prone and expose the vertex using a rectangular incision. Next, remove the excise skin, dissect the periosteum and clean off the periosteum using a scalpel.
Next, make a small hook on the three millimeter exposed end of the 3.5 centimeter wire. Then guide the other end of the gold wire subcutaneously to the vertex. Through an 18 gauge peripheral venous catheter using a micro forceps.
Using a second pair of micro forceps, carefully position the hooked end of the wire in the middle ear. Then inflect the head of the animal to visualize the area of the round window niche through the bullostomy and hook the gold wire to the bony prominence using micro forceps. Now while maintaining gentle tension on the gold wire, fix it to the cranial verge of the bullostomy using 10 to 15 microliters of cyanoacrylate delivered from a 27 gauge needle.
Baseline measurements can now be taken from the gold wire. Next, to mount the electrode, put the animal in the prone position. And using a one millimeter burr drill, make two holes one millimeter anterior to the lambda suture.
Do not damage the dura. Then implant screws into the holes to harness the electrode connector. Now use an 18 gauge peripheral venous catheter to guide the electrode from the connector to the bulla in a tissue layer as close to the skull as possible.
Then pack 0.5 to 0.7 milliliters of semi fluid dental cement between the screws using a spatula and positioned the electrode connector between the screws. Keep the connector stable in position while the dental cement hardens. With the animal reposition sideways, proceed by carefully drilling the cochleostomy one millimeter from the round window.
Drill using a 0.5 millimeter diamond burr at 5000 rpm. Next, carefully insert the electrode four millimeters into the scala tympani. Then withdraw the electrode and repeat the insertion.
Then apply cyanoacrylate to secure the electrode. A stable implantation will provide more hearing threshold shift. So it is important to stabilize the arm used to insert the electrode against the table to minimize the probability of unwanted jerking movements.
Followed by closing the bullostomy with about 0.3 milliliters of dental cement. And the displacement of dental cement into the bulla needs to be avoided. Next, close the retroauricular incision using 5-0 absorbable sutures.
Then turn the animal to the prone position. Next, use forceps to grab the posterior edge of the rectangular incision at the vertex. And with scissors make a two centimeter subcutaneous tunnel into the neck of the animal.
Through the tunnel, implant the 2.5 centimeter gold wire using forceps. Position the end with less exposed wire at the designated pin of the connector, then solder the connection. Next, solder the round window niche electrode to the appropriate pin of the connector.
After completing the solders, apply more dental cement to fully cover the insulated pins and the electrode. This completes the implantation. The pre and post-operative frequency specific CAP thresholds in the presented animal were almost unchanged in the low frequencies.
Whereas a threshold shift of approximately 25 to 30 decibels was achieved in the high-frequency area starting at eight kilohertz. The foreign body reaction to the implanted electrode can vary significantly. There can be no visible reaction or a large area of the scala tympani might get filled with fibrotic tissue.
It is even possible to fracture the osseous spiral lamina during the CI electrode insertion. In this case, the fracture caused a loss of spiral ganglion cells, fibrosis and osteogenesis in the vestibular duct and loss of the organ of Corti. Such fractures can explain higher than expected threshold shifts in some animals.
Once mastered, this technique can be done in two and a half hours if it's performed properly. If intraoperative hearing measurements are performed, the time required for the total procedure can increase considerably. After watching this video you should have a good understanding of how to perform cochlear implantation in the guinea pig.
Using this animal model, you can address various research questions in this field.
