Biomechanical performance of an ovine model of intradural spinal cord stimulation.

The authors are developing a novel type of spinal cord stimulator, designed to be placed directly on the pial surface of the spinal cord, for more selective activation of target tissues within the dorsal columns. For pre-clinical testing of the device components, an ovine model has been implemented which utilizes the agility and flexibility of a sheep's cervical and upper thoracic regions, thus providing an optimal environment of accelerated stress-cycling on small gauge lead wires implanted along the dorsal spinal columns. The results are presented of representative biomechanical measurements of the angles of rotation and the angular velocities and accelerations associated with the relevant head, neck and upper back motions, and these findings are interpreted in terms of their impact on assessing the robustness of the stimulator implant systems.

MR-based measurement of spinal cord motion during flexion of the spine: implications for intradural spinal cord stimulator systems.

This study develops a means of delivering electrical stimuli directly to the pial surface of the spinal cord for treatment of intractable pain. This intradural implant must remain in direct contact with the cord as it moves within the spinal canal. Therefore, magnetic resonance imaging was used to measure the movement of the spinal cord between neutral and flexed-back positions in a series of volunteers (n = 16). Following flexion of the back, the mean change in the pedicle-to-spinal cord dorsal root entry zone distance at the T10-11 level was (8.5 ± 6.0) mm, i.e. a 71% variation in the range of rostral-caudal movement of the spinal cord across all patients. There will be a large spectrum of spinal cord strains associated with this observed range of rostral-caudal motions, thus calling for suitable axial compliance within the electrode bearing portion of the intradural implant.

Apparatus for simulating dynamic interactions between the spinal cord and soft-coupled intradural implants.

We have designed, built, and tested an apparatus used for investigating the biomechanical response of a novel intradural spinal cord stimulator to the simulated physiological movement of the spinal cord within the thecal sac. In this apparatus, the rostral-caudal displacements of an anthropomorphic spinal cord surrogate can be controlled with a resolution of approximately 0.1% of a target value for up to 10(7) lateral movement cycles occurring at a repetition rate of 2 Hz. Using this system, we have been able to determine that the restoring force of the stimulator's suspension system works in concert with the frictional coupling between the electrode array and the surrogate to overcome the 0.42 µN inertial force associated with the lateral motion of the array. The result is a positional stability of the array on the surrogate (in air) of better than 0.2 mm over ~500,000 movement cycles. Design modifications that might lead to improved physiological performance are discussed.

Pulsatile spinal cord surrogate for intradural neuromodulation studies.

We have designed, built and tested a novel spinal cord surrogate that mimics the low-amplitude cardiac-driven pulsations of the human spinal cord, for use in developing intradural implants to be used in a novel form of neuromodulation for the treatment of intractable pain and motor system dysfunction. The silicone surrogate has an oval cross section, 10 mm major axis × 6 mm minor axis, and incorporates a 3 mm diameter × 3 cm long angioplasty balloon that serves as the pulsation actuator. When pneumatically driven at 1 Hz and 1.5 atmospheres (≈ 1140 mm Hg), the surrogate's diametric pulsation is ≈ 100 µm, which corresponds well to in vivo observations. The applications for this surrogate are presented and discussed.