Cochlear implants: safe procedure thanks to surgical robots

Interview with Prof. Stefan Weber, D. Eng., ARTORG Center for Biomedical Engineering Research, University of Bern


For many years, cochlear implants have restored a sense of hearing in people with certain types of hearing loss. For surgeons, the implantation requires a precise attention to detail under the microscope. The results for the patients improve significantly with a more precise placement of the electrode array. The use of a surgical robot can increase the accuracy of the procedure.
Image: Man with auburn hair and blue shirt - Prof. Stefan Weber; Copyright: private

Prof. Stefan Weber; ©private

In this interview with, Prof. Stefan Weber talks about the world’s first robot-assisted cochlear implantation, how the safety of the drilling process in the submillimeter range is ensured and discusses other possible applications of this surgical technique.

Prof. Weber, how do conventional cochlear implantations work?

Prof. Stefan Weber: First, the surgeon performs a mastoidectomy to create access to the middle ear. That is to say, the petrous bone, the skull bone behind the ear, is milled to create a funnel-shaped opening. The facial nerve and chorda tympani nerve run just one centimeter beneath the surface in this area. A narrow tunnel is then drilled between these two nerves. Surgeons have about two millimeters to do this. Right behind it is the middle ear cavity where you can see the cochlear wall. It needs to be opened to obtain access to the cochlea through which the silicon electrode of the implant can be inserted.

What are the challenges with this procedure?

Weber: The most difficult part is to optimally place the electrode in the cochlea. That means it needs to be inserted deep enough but not too deep to ensure optimal implant performance. The surgeon monitors the operating field through a microscope and works based on what he sees. However, the surgeon also relies on his experience when it comes to the location of both nerves for example.

Image: Black-and-white picture, with some structures of the human body highlighted in color; Copyright: ARTORG Center for Biomedical Engineering Research, University of Bern

RCI trajectory for entry into the cochlea at the preplanned angle. yellow: facial nerve, orange: chorda tympani, light blue: cochlea, dark blue: posterior wall of the external auditory canal, purple: ossicles, green: planned trajectory, grey: robot drill bit; ©ARTORG, University of Bern

How does the robot-assisted procedure you developed work by comparison?

Weber: The overall concept of the intervention is comparable to the conventional procedure, except in this case, the surgeon is supported by the robot during individual phases. Prior to the procedure, you determine where the drilling should take place based on CT scans. At the beginning, the robot drills a straight, tiny tunnel through the petrous bone in a minimally invasive procedure. When the cochlea is opened, it is crucial to trigger as few disturbances and turbulences as possible in its interior. With the help of measuring devices, the electrode array is subsequently positioned in the cochlea.

The idea of performing cochlear implantation with the help of a robot has been around for a while. This is a direct result of the size and scale in which surgeons have to maneuver in this area and the human tactile perception limits to insert electrodes. At this level, a robot is simply better equipped to measure and assess forces and movements and is, therefore, able to reach specific sites with enormous accuracy.

Image: Surgical robot with a camera system over the phantom of a patient; Copyright: ARTORG Center for Biomedical Engineering Research, Universität Bern

Robot aligned with entry trajectory on phantom head; ©ARTORG Center for Biomedical Engineering Research, University of Bern

The first surgery with this method was conducted in July of 2016. What was the result of it?

Weber: The first procedure went really well. We successfully implanted the device in a patient. Primarily, the intervention served as validation for the method since this was truly the first robot-assisted cochlear implantation in the world. So far, we have treated five patients within the scope of the study.

Having said that, the primary objective of this project was not to make this surgical intervention faster or more accurate. Our main goal was to show that we are able to achieve better results for patients by using a robot-assisted approach – that being in terms of the audiological parameters that are relevant to patients later on. I believe we are on a great trajectory, even if the results are so far statistically irrelevant due to the limited number of patients.

Image: Graphic showing the cochlear implant in the skull; Copyright: ARTORG Center for Biomedical Engineering Research, Universität Bern

RCI drill trajectory across the mastoid bone; ©ARTORG Center for Biomedical Engineering Research, University of Bern

You have integrated three different safety systems into the procedure. Can you briefly explain how they work?

Weber: Surgical robots are still under the visual control of the surgeon. They work based on an optical navigation system that measures the position of the robot in any given space in relation to the patient before it starts to mill or drill. Using these measurements, the surgeon always determines whether the robot should be turned on. The challenge as it pertains to the cochlea is that the surgery has to be far more accurate than is the case with orthopedic interventions, for example, to prevent injuries of the facial nerve and chorda tympani nerve. The drill requires a drilling accuracy to tenths of a millimeter. This is something the surgeon is unable to detect from the outside.

This is why we first use high-precision optical tracking, which can detect and control the robot’s position in relation to the patient with an accuracy of up to 50 microns.

Secondly, we measure the cutting forces in drilling and correlate them with the bone density we derive from the CT data. Cutting force in drilling increases with higher bone density. With the help of bone densitometry, we are able to determine the position of the drill. We compare this data with the optical position measurement.

We use neuromonitoring as a third measure. To do this, we send tiny current pulses from an electrode into the cutting path. Via surface electrodes, we are then able to measure how the facial muscles respond to this stimulation in the face of the patient. This allows us to deduce whether the robot might be drilling too closely to the facial nerve.

Should one of these methods indicate that the robot is drilling in the wrong spot, the surgeon is able to stop the surgery at any time. He subsequently has to conventionally complete the surgery.

Image: Surgeon in the OR is using the robot during cochlear implantation; Copyright: Gianni Pauciello/HNO-Klinik Inselspital Bern

A surgeon in the OR is using the robot during cochlear implantation; ©Gianni Pauciello/ENT clinic, Bern University Hospital

Are there other possible applications for this surgical technique?

Weber: One option is to use it to administer medication at a specific location in the inner ear. For instance, active ingredients that boost the regeneration or regrowth of hair cells in the cochlea are conceivable. These are the cells that convert the mechanical movement of the inner ear fluid into electrical signals, resulting in hearing.

A second possible option is the treatment of balance problems with implants that are being inserted into the vestibular ducts of the organ of equilibrium. Our procedure could also facilitate a minimally invasive implant in this scenario.

Image: Man with glasses and beard - Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.