A flexible sideways-looking probe for detecting cortical bone proximity: design, manufacturing, and evaluation of a prototype

Abstract

Spinal fusion is a surgical treatment involving stiffening parts of the spinal column to eliminate any relative motion and improve spine stability. A critical part of spinal fusion surgery is the placement of pedicle screws because it is highly complex and can damage the vascular structures and nerves near the vertebrae. To prevent complications, a new anchoring method is developed that strengthens screw contact by following the curved trajectory of the dense cortical shell while avoiding a cortical breach. Diffuse reflectance spectroscopy (DRS) is a technique that could be used in a bone drill to obtain real-time information about the location of the drill within the vertebra. In order to drill the correct trajectory, an optical probe using DRS should determine the parallel closeness of cortical bone to the drill tip. To show the potential of this novel technology for improving spinal fusion surgery, this master thesis introduces and validates a flexible sideways-looking probe using DRS to identify the proximity of cortical bone when measuring along the cortical shell of a vertebra.

In this study, the ideal optical design to detect the proximity of cortical bone is investigated by looking into the effect of fiber angulation on the photon propagation using Monte Carlo simulations and phantom experiments. A conceptual design was created based on the ideal optical design combined with the mechanical requirements to make the probe insertable into a vertebra. As a proof of concept, a prototype was developed and tested on a bone-mimicking phantom and porcine vertebrae.

The conceptual design comprises a flexible needle with two optical fibers attached to a rigid tip with a diameter of 2.9 mm. Two mirrors are positioned in the rigid tip, emitting light 45\degree off-axis and collecting light 90\degree off-axis with a source-detector separation of 2.1 mm. These light directions were chosen because they optimize the reflectance spectrum difference between cancellous and cortical bone due to a longer path length, making detecting cortical bone closeness easier. A prototype was made that demonstrates that the probe can detect the parallel proximity of cortical bone at a depth up to 1.5 mm in an ideal phantom setting. The prototype can distinguish between pure cancellous and cortical bone in porcine vertebrae. Additionally, the prototype can also be inserted into the pedicles and bent along the cortical wall of a vertebra. These positive findings show that the flexible sideways-looking probe created in this study can potentially improve spinal fusion surgery by enabling an increase in cortical bone contact without perforating it. Although proof of concept has been provided, it is essential to improve the design for safety and usability and improve tissue type identification accuracy before continuing to clinical trials.