Developing a Handheld Fiber-Optic Tissue Sensing Device for Spine Surgery

A critical aspect of spinal fusion procedures is the accuracy of pedicle screw placement. Misplacement can result in severe complications, highlighting the need for precise guidance systems. Current technologies, including computer-assisted navigation, robotics, and intraoperative ultrasonography, offer improvements but face challenges like cost, training requirements, and technical limitations. While diffuse reflectance spectroscopy (DRS), an alternative tissue-sensing method that assesses tissue composition in real time via light interaction, has shown promise, existing systems are expensive and cumbersome. The recent study highlighted here proposed a compact, cost-effective, handheld fiber-optic device using simplified DRS technology. Four wavelength regions within the visible-near-infrared spectral region were used for DRS measurements, those being 695-699 nm, 925-927 nm, 1188 nm, and 1207-1211 nm. This innovation eliminates the need for complex spectrometers and provides real-time audio feedback, aiming to make advanced tissue sensing more accessible to healthcare providers. Spectroscopy spoke to Merle Losch, corresponding author for the paper resulting from this study, about the device and the team’s efforts to produce it.

Your paper (1) introduces a handheld fiber-optic tissue sensing device for real-time bone tissue differentiation during spine surgery using diffuse reflectance spectroscopy (DRS). Is spinal surgery a more prevalent problem today because of increased life expectancy or are there other factors such as lifestyle and diet?
Spine health is influenced by genetics, lifestyle, and diet. The need for spine surgery arises from various factors, including deformities, chronic pain, tumor removal, and traumatic injuries. The demand for spinal fusion is increasing due to aging populations, advancements in surgical techniques, and expanding indications for the procedure. This trend presents significant challenges for healthcare systems worldwide.

Your work is in response to challenges associated with the accurate placement of pedicle screws. Briefly discuss its importance, and the challenges associated with this procedure.
Accurate pedicle screw placement is crucial for spinal fusion success, ensuring stability and minimizing complications such as neurological injury or revision surgery. However, the complex anatomy of the spine and the reliance on tactile feedback make the procedure particularly challenging, especially for less experienced surgeons.

What is currently being done to assure that the placement of these screws is accurate during spinal surgical procedures?
Current methods include intraoperative fluoroscopy, computer-assisted navigation, and robotic guidance. Fluoroscopy, while commonly used, is time-consuming, exposes patients and surgical staff to radiation, and remains dependent on operator experience. Computer- and robot-assisted techniques significantly improve accuracy but require costly equipment and specialized training, so their availability may be limited in low-resource settings.

Briefly summarize your findings that you discuss in your article, and the conclusions you came to after reviewing these findings.
Our study demonstrated the potential of a simple handheld device to improve surgical guidance in spinal fusion. Using two laser diodes, a photodiode, a printed circuit board (PCB), and an Arduino Uno microcontroller, the device emits light at two wavelengths and analyzes tissue signals against a set threshold to provide real-time audio feedback that helps surgeons prevent pedicle screw breaches. Despite initial fiber coupling challenges, our experiments confirmed that the device can reliably emit and collect signals, even with low tissue reflectance. Its compact design, affordability, and real-time feedback make it a promising tool for enhancing pedicle screw placement accuracy and improving patient outcomes.

What were the advantages associated with using handheld spectroscopic equipment in your study?
Our handheld spectroscopy setup allows for real-time intraoperative tissue feedback directly at the surgical site. Its compact design integrates easily with existing navigation or robotic systems without cumbersome setups or cables. Additionally, its affordability makes it accessible even in resource-limited healthcare facilities.

What difficulties did you encounter in your work, specifically sampling and data interpretation challenges?
The main challenge in developing our prototype was designing the electronic circuits and ensuring efficient optical coupling. The electronics included a driver circuit for autonomous current regulation to maintain consistent light output over time, and signal amplification to convert the low photodiode currents into measurable signals, necessary due to low tissue reflectance. While optical coupling efficiency remained an issue in the final prototype, this can be addressed by improving assembly tools and establishing processes for precise butt-coupling of the fiber and diode surfaces.

How did you process the spectroscopic data to obtain the results you were looking for?
Our device uses signals at two distinct wavelengths, which were carefully selected based on literature on spectroscopic data for bone tissues and the availability of suitable laser diodes. A microcontroller processes the signal by calculating the reflectance ratio between these two wavelengths. This ratio is then compared to a preset threshold, providing real-time feedback that enables tissue differentiation.

Were there any factors that might affect the accuracy of your findings?
Several factors could affect the accuracy of our findings. Variability in optical coupling efficiency among prototypes may influence the recorded signal intensity. If the two laser diodes are not coupled with the same efficiency, the calculated reflectance ratio could be biased. Standardizing assembly processes and ensuring reliable calibration can help mitigate this risk.

Additionally, factors such as heterogeneity in tissue samples and variation in optical tissue properties among individuals could impact the ability to differentiate bone tissue during spinal fusion using only two wavelengths. To address this, more comprehensive evaluations, including ex-vivo trials and validation against conventional guidance technologies, are necessary.

How do you imagine the results of your study can/will be more broadly applied?
Our findings pave the way for accessible, real-time surgical guidance tools for spinal fusion. Implementing such a fiber-optic probe in medical environments will require regulatory approval and large-scale manufacturing of sterile probes, which will necessitate cost and complexity reduction, as well as the use of biocompatible components. With further design iterations, our device could be deployed in healthcare facilities with limited access to expensive surgical technologies. It could also serve as an add-on to existing computer-assisted navigation or robotic-assisted systems.

Are there any benefits of using your device above and beyond those associated with screw placement in spinal surgery?
Yes, the device’s real-time tissue differentiation can be applied beyond spinal surgery to other regions like the colon, lungs, and breast. It could assist in detecting malignancies, guiding biopsies, and aiding anesthesia procedures. Its portability and cost-effectiveness make it valuable across various medical fields.

Do you anticipate similar results in using your technique for different surgical procedures?
DRS has been effective in various applications, including cancer detection beyond the spine. The device’s modular design allows for adaptation to other procedures by adjusting the laser wavelengths or modifying the threshold ratio to suit different surgical needs.

Are there any next steps in this research?
The next step involves validating the device for its intended use by testing the prototype on cancellous (spongy bone) and cortical (compact bone) samples. Future prototypes will also need to prioritize enhanced mechanical robustness, as surgeons apply considerable forces to the bone during spine surgery. Additionally, a decision must be made regarding whether the device will be single-use or sterilizable. While single-use devices reduce contamination risks, they raise concerns about environmental sustainability, so (partial) sterilizability is the preferred option.

There are also potential enhancements to our device, including multi-directional sensing, which would provide directional tissue feedback for improved surgical guidance. Another promising direction is integrating machine learning (ML) algorithms for tissue classification, which could increase the speed and reliability of intraoperative feedback.

References

1. Losch, M. S.; Visser, B. E.; Dankelman, J.; Hendriks, B. H. W. A Handheld Fiber-Optic Tissue Sensing Device for Spine Surgery. PLoS One 2024, 19 (12), e0314706. DOI: 10.1371/journal.pone.0314706

Merle S. Losch received her PhD in Mechanical Engineering from TU Delft (The Netherlands) in 2024. Her research at the BITE/MISIT group focused on improving spinal fusion surgery using tissue optics for surgical guidance. Before joining TU Delft in 2020, she earned her BSc and MSc in Mechanical Engineering from RWTH Aachen University (Germany) and a Diplôme d'Ingénieur from École Centrale de Marseille (France). Photo courtesy of Losch.