Lower risks in laser surgery with optical monitoring

A robotically assisted laser procedure optimized with optical monitoring and precise control of the laser cutting process is intended to make risky surgical interventions on the spinal cord or brain safer. In an interview with MEDICA-tradefair.com, Dr. Achim Lenenbach, head of the Laser Medical Technology and Biophotonics department at Fraunhofer ILT, describes the research work being carried out in this area.

What problems does the robot-assisted laser procedure address?

Dr. Achim Lenenbach: In the case of spinal canal stenosis, the bone grows into the spinal canal and presses on the spinal cord. This causes symptoms of paralysis and pain in those affected. In order to remove the parasitic bone from the spinal canal, the vertebral body is opened with a high-speed milling machine, whereby the surgeon presses the milling head onto the bone with great pressure. At the point of penetration, the milling head may advance uncontrollably into the spinal canal and injure the spinal cord. This often results in severe disabilities such as paraplegia and bladder and bowel incontinence.

We therefore want to use a cutting laser beam to open the vertebral body and superimpose a measuring beam to monitor the laser cutting process. To monitor the cutting process, we use optical coherence tomography (OCT), which can be used to measure the cutting depth during the cutting process and the remaining bone thickness. From a residual bone thickness of 400 micrometers, we can see both the bottom of the kerf (the area of removed material) and the inside of the bone. This allows us to determine the residual thickness of the bone in real time and use it to control the laser cutting process.

How does the system make interventions safer?

Lenenbach: In contrast to a cutting ablation process, there is no mechanical force transmission in the laser cutting process. We use the laser to apply energy to the bone without contact or inertia. The laser radiation is absorbed by the bone tissue, the water in the bone heats up and expands very quickly. This adiabatic expansion occurs explosively and blasts material out of the bone in micrometer dimensions. If we place many nanosecond short laser pulses next to each other very quickly at kilohertz frequencies, this thermomechanical tissue ablation is very efficient, precise and safe.

In Germany alone, around 111,000 spinal canal stenosis operations are performed every year. In around 1.5 percent of operations, the high-speed rotating milling head comes into contact with the nerve tracts in the spinal canal despite the surgeon's caution. The focused laser beam is guided over the bone surface by the surgeon using a handpiece when cutting the bone. In contrast to the milling machine, the laser does not interact mechanically so that the surgeon does not receive any haptic feedback during manual process control. This feedback, which is important for the surgeon, must be generated artificially by a collaborative robot (cobot) to which the handpiece is mechanically connected. It does this by counteracting the feed movement of the handpiece by the surgeon with a force. The robot only releases the feed when the locally processed bone has been removed down to a defined residual thickness. The cobot uses the optical residual thickness measurement of the OCT system for this purpose. It also maintains the correct distance from the bone surface to be processed so that the laser process runs efficiently.

AI plays an important role in generating this haptic feedback. The robot actively supports the surgeon's movements without disturbing them. It has to 'sense' the direction in which the surgeon wants to move the handpiece. The robot specifies a movement corridor to protect risk structures.

What are other areas of application in surgery?

Lenenbach: Another example of an application is the laser craniotome, which can be used to open the skull and for which we also want to use the laser cutting process. An interesting application for this is the implantation of brain pacemakers for deep brain stimulation (DBS) in cases of shaking palsy (tremor). This allows the action potentials in the brain that cause tremor to be suppressed. The laser craniotome is being developed for awake operations because the mechanical drilling of the skullcap while the patient is fully conscious is very psychologically stressful for locally anesthetized patients. The low-vibration and low-noise laser procedure, on the other hand, is almost imperceptible.

Another application is the removal of a tumor in the brain. Due to the high functional density of the tissue, the operation must be performed very carefully and the healthy tissue must be spared. This can be supported by an awake operation in which complex brain functions are tested during the operation. The tumor is then removed up to the functional limit, i.e. until neurological deficits begin to appear. In developing the laser craniotome, we are working together with Dr. Peter Reinacher from the Neurocenter at Freiburg University Hospital, who is developing the laser craniotomy procedure with us and defining the requirements for the laser craniotome from the user's perspective.

What are the next steps in development?

Lenenbach: Firstly, there is the further development of the laser. When combining the laser cutting process with a robot, it is advantageous to guide the laser beam via an optical fiber. At the moment, an articulated mirror arm is used to guide the beam. With the use of fibers, a much more compact and stable beam guidance, as required in the operating room, is possible. We are developing a short-pulse laser that emits short pulses of 100 nanoseconds at a wavelength of three micrometers and can cut bone efficiently.

Another development step is linking the system with preoperative imaging for real-time visualization of the cutting process. With surgical planning and navigation software based on preoperative data, we could visualize the data live during laser craniotomy and spinal surgery and show the surgeon the cutting curve and the current laser focus position in the preoperative planning data, specifying a target corridor.