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.
We have designed, built and tested an anthropomorphic-scale surrogate spinal canal, for use in preliminary evaluations of the performance characteristics of a novel intradural spinal cord stimulator. The surrogate employs a silicone mock spinal cord with semi-major and semi-minor diameters of 10 and 6 mm, respectively, commensurate with those of actual thoracic-level spinal cord. The axial restoring force provided by the 300 µm thick silicone denticulate ligament constructs on the mock cord is ~ 0.32 N mm(-1) over a 1.5 mm range of displacement, which is within a factor of 2 of that measured by others in human cadaver specimens. Examples of testing protocols of prototype intradural stimulators that employ this device are discussed.
We have used finite-element (FE) modeling to investigate the mechanical compliance, positional stability and contact pressures associated with a novel type of spinal cord stimulator that is placed directly on the pial surface of the spinal cord in order to more selectively activate neural structures for relief of intractable pain. The properties used in the model are those of the actual prototype devices employed in recent in vitro and chronic in vivo tests. The agreement between predictions and experimental observations serves to validate our FE approach, which can now be used to further optimize the device's design and performance.
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