Robotic End Effector for Ultrasound-Guided Facet Joint Injections

Abstract

Facet joint inflammations contribute to low back pain with a prevalence of 75% and are a major burden for the healthcare system, especially in societies with an aging demographic. Facet joint injections are a common diagnostic and therapeutic tool for the treatment of facet joint inflammations. The fluoroscopy imaging technique is conventionally used to navigate the needle into the facet joint gap, but it exploits doctor and patient to ionizing radiation. Research with robotic facet joint injections has provided a similar accuracy for the needle placement using the innocuous ultrasound imaging technique. With an actuated angle of the needle guide however, ultrasound coupling can be maintained for a wider range of facet joint angles. Furthermore, a repeatable mount for different shapes of ultrasound probes and a user interface integrated into the end effector can speed up and simplify the clinical workflow. An anatomical study of the facet joints is performed using the computer tomography scans of four patients, which acts as a guideline for the design of a remote center of motion, parallelogram mechanism to change the needle guide angle. A crank mechanism for the ultrasound probe mount exploits sloped surfaces to attach different ultrasound probes in a deterministic fashion. The user interface consists of an LED ring and three buttons in close proximity to a handle which is integrated into the end effector housing. The end effector is built using mostly rapid prototyping and off-shelf parts. Three perpendicular point Lasers are used to measure the repeatability and stability of the ultrasound probe mount in 3D space. The needle guide mechanism is equipped with optical markers and filmed with a smartphone camera to relate the motor angle to the needle angle and position. The small angular and positional standard deviations for repeated ultrasound probe attachments are calculated to be sigma_y = [0.36°; 0.34°; 0.37°]*10-4 and sigma_o = [0.49 mm; 0.51 mm; 0.13 mm] respectively. Thus, saving previous calibrations for the same specific ultrasound probe could be a promising method to save time and maintain accuracy. The change in position dx = (3.318 ± 0.468) mm when a force F = 450 N is applied, suggests that the frame of the ultrasound probe mount needs to be stiffer. Due to friction, play and elasticity in the parallelogram mechanism, a non-linear relation between the motor angle and the needle angle and position can be detected. Nonetheless, the new end effector allows for the control of the needle angle while maintaining ultrasound visibility and can be used as a tool for further research on robotic, ultrasound-guided facet joint injections.