Ultrasound-guided spinal stereotactic system for intraspinal implants

Abstract

<jats:sec><jats:title>OBJECTIVE</jats:title><jats:p>The overall goal of this study was to develop an image-guided spinal stereotactic setup for intraoperative intraspinal microstimulation (ISMS). System requirements were as follows: 1) ability to place implants in various segments of the spinal cord, targeting the gray matter with a &lt; 0.5-mm error; 2) modularity; and 3) compatibility with standard surgical tools.</jats:p></jats:sec><jats:sec><jats:title>METHODS</jats:title><jats:p>A spine-mounted stereotactic system was developed, optimized, and tested in pigs. The system consists of a platform supporting a micromanipulator with 6 degrees of freedom. It is modular and flexible in design and can be applied to various regions of the spine. An intraoperative ultrasound imaging technique was also developed and assessed for guidance of electrode alignment prior to and after electrode insertion into the spinal cord. Performance of the ultrasound-guided stereotactic system was assessed both in pigs (1 live and 6 fresh cadaveric pigs) and on the bench using four gelatin-based surrogate spinal cords. Pig experiments were conducted to evaluate the performance of ultrasound imaging in aligning the electrode trajectory using three techniques and under two conditions. Benchtop experiments were performed to assess the performance of ultrasound-guided targeting more directly. These experiments were used to quantify the accuracy of electrode alignment as well as assess the accuracy of the implantation depth and the error in spatial targeting within the gray matter of the spinal cord. As proof of concept, an intraoperative ISMS experiment was also conducted in an additional live pig using the stereotactic system, and the resulting movements and electromyographic responses were recorded.</jats:p></jats:sec><jats:sec><jats:title>RESULTS</jats:title><jats:p>The stereotactic system was quick to set up (&lt; 10 minutes) and provided sufficient stability and range of motion to reach the ISMS targets reliably in the pigs. Transverse ultrasound images with the probe angled at 25°–45° provided acceptable contrast between the gray and white matter of the spinal cord. In pigs, the largest electrode alignment error using ultrasound guidance, relative to the minor axis of the spinal cord, was ≤ 3.57° (upper bound of the 95% confidence interval). The targeting error with ultrasound guidance in bench testing for targets 4 mm deep into the surrogate spinal cords was 0.2 ± 0.02 mm (mean ± standard deviation).</jats:p></jats:sec><jats:sec><jats:title>CONCLUSIONS</jats:title><jats:p>The authors developed and evaluated an ultrasound-guided spinal stereotactic system for precise insertion of intraspinal implants. The system is compatible with existing spinal instrumentation. Intraoperative ultrasound imaging of the spinal cord aids in alignment of the implants before insertion and provides feedback during and after implantation. The ability of ultrasound imaging to distinguish between spinal cord gray and white matter also improves confidence in the localization of targets within the gray matter. This system would be suitable for accurate guidance of intraspinal electrodes and drug or cell injections.</jats:p></jats:sec>

Functional organization of motor networks in the lumbosacral spinal cord of non-human primates

“…Responses to intraspinal microstimulation under pentobarbital anesthesia have previously been shown to be representative of those evoked in awake animals 5,6 . The lumbosacral enlargement of the spinal cord was surgically exposed and a stereotactic setup with a micromanipulator was assembled onto the spine 31 . The micromanipulator was used to guide a microelectrode to various locations in the ventral horn of the gray matter.…”

“…Prior to electrode implantation, the dura mater and arachnoid were opened and retracted with sutures placed in the paraspinal muscles. Four pedicle screws (3.5 mm diameter × 25 mm length, Medtronic Inc., Dublin, Ireland) were placed bilaterally in L1 and L3 pedicles, over which a spine-mounted stereotactic setup 31 was assembled. The setup consisted of 2 frames and a platform with a micromanipulator that was used to place the microelectrode into the spinal cord 31 .…”

“…Four pedicle screws (3.5 mm diameter × 25 mm length, Medtronic Inc., Dublin, Ireland) were placed bilaterally in L1 and L3 pedicles, over which a spine-mounted stereotactic setup 31 was assembled. The setup consisted of 2 frames and a platform with a micromanipulator that was used to place the microelectrode into the spinal cord 31 . The frames, which were secured to the pedicle screws, provided mechanical fixation over vertebral levels L1 to L3.…”

Comparative Neuroanatomy of the Lumbosacral Spinal Cord of the Rat, Cat, Pig, Monkey, and Human

“…Knowing the dimensions of targeted regions of the spinal cord is essential for surgical planning and successful delivery of cells, drugs, electrodes or optrodes to their intraparenchymal targets. This information can guide the technological design of the necessary implants [33], [42], [43] and delivery apparatuses [22], [44], [45]. Geometrical dimensions are also important for informed interpretation of cell migration and drug perfusion in preclinical models and their implications for human translation.…”

“…As an example, intraspinal microstimulation (ISMS) [17], [18], [33] is a spinal cord neuroprosthesis for restoring mobility after spinal cord injury that involves the implantation of fine microelectrodes into the ventral horns of the spinal cord to activate functional spinal motor networks. ISMS microelectrodes are typically implanted perpendicularly to the major axis of the cord along the anteroposterior (dorsoventral) axis [22], [42], and target regions within Rexed lamina IX of the spinal cord [35], [46], [47]. Therefore, technological design and surgical implantation of ISMS implants requires species-and segment-specific knowledge about the location and dimensions of the ventral horns.…”