Ultrasound-induced fractionation of the intervertebral disk

Abstract

Current surgical treatments for lower back pain, which is strongly associated with degeneration of the intervertebral disk, are highly invasive and have low long-term success rates. The present work thus aims to develop a novel, minimally invasive therapy for disk replacement without the need for surgical incision. Using ex vivo bovine coccygeal spinal segments as an experimental model, two confocally aligned 0.5 MHz HIFU transducers were positioned with their focus inside the disc and used to generate peak rarefactional pressures in the range of 1–12 MPa. Cavitation activity was monitored, characterized, and localized in real time using both a single-element passive cavitation detector and a 2D Passive Acoustic Mapping array. The inertial cavitation threshold in the central portion of the disk, the nucleus pulposus (NP), was first determined both in the absence and in the presence of externally injected cavitation nuclei. HIFU exposure parameters were subsequently optimized to maximize sustained inertial cavitation over 10 min and achieve fractionation of the NP. Following sectioning of treated disks, staining of live and dead cells as well as microscopy under polarized light were used to assess the impact of the treatment on cell viability and collagen structure within the NP, inner annulus and outer annulus.