Surgery: sensitive drill warns before it damages nerves

Empa – The Swiss Federal Laboratories for Materials Science and Technology- is developing a special drill that can autonomously stimulate nerves when used closely to sensitive nerves. This means surgeons no longer must alternate between drilling and probing the nerves. We asked Dr. Kerstin Thorwarth what led to the creation of the project and wondered how the drill is constructed.

Dr. Thorwarth, you have developed a smart drill that can stimulate the nearby facial nerves during cochlear implant surgery and shuts down automatically before it is at risk of damaging them. What prompted the creation of the project?

Kerstin Thorwarth: A team led by Stefan Weber of the University of Bern’s "ARTORG Center for Biomedical Engineering Research" developed a robotic, minimally invasive approach to cochlear implant surgery. Rather than creating an opening in the skull bone to provide access to the nerves for the surgeon, a robot merely drills a narrow channel from the outer ear to the cochlea by using a previously created 3D model. To ensure the nerves remain undamaged, the drill is pulled out periodically. A probe is subsequently inserted to stimulate the nerves via electric current. This fascinating surgical method shortens the hospital stay significantly, speeds up the healing process, and reduces the risk of nerve injury. We wondered if we could combine the probe with the drill and use it in tandem. Not only would this eliminate the need for repeated removal of the latter during the surgery, but it could also stimulate the nerves continuously in the process. This would speed up the surgery further and make it even safer.

The drill is conductive thanks to a special surface coating. Which coating did you apply and what was the reasoning behind it?

Thorwarth: The drill itself is made of metal and is thus conductive. Previous studies indicated that the probe should feature several conductive areas. The nerve can also be localized via the targeted application of an electrical signal to different areas of the probe or the drill, respectively. This way, you can distinguish whether you move towards the nerve ("unsafe") or drill past the nerve ("safe"). If you only had one electrode on the probe or on the drill, you would not be able to distinguish between the two scenarios.

Another requirement is for the drill to keep only small sections conductive. That’s why the drill must be electrically insulated, using (Si3N4) silicon nitride, for example. A multilayer of conductive titanium nitride (TiN) ceramic defines the electrically conductive areas, which can also be controlled separately. Along with conductivity and insulation, another coating requirement is that is must not be destroyed during the drilling process. Both materials are well-known hard coatings, which are already being used to extend the service life of drilling and cutting tools. Next to the functional coating requirements, biocompatibility is another important criterion, as you could otherwise not obtain approval for the drill and its use on humans.

Does the drill maintain its properties over its life cycle?

Thorwarth: Actually, the hard coating will extend the drill’s service life. Its durability has been tested in corrosion tests and with the help of artificial bone. A subsequent clinical trial requires additional research and investments.

Can the drill be used in other settings?

Thorwarth: The surgical method lends itself to all settings that require drilling or milling close to nerves. Spinal surgery is a great example of this.