Application of electrical fields to spinal nerve roots, spinal cord, and other nerve bundles for the purpose of chronic pain control has been actively practiced for some time. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue (i.e., spinal nerve roots and spinal cord bundles) can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nerve tissue. Specifically, applying electrical energy to the spinal cord associated with regions of the body afflicted with chronic pain can induce “paresthesia” (a subjective sensation of numbness or tingling) in the afflicted bodily regions. Thereby, paresthesia can effectively mask the transmission of non-acute pain sensations to the brain.
Each exterior region, or each dermatome, of the human body is associated with a particular spinal nerve root at a particular longitudinal spinal position. The head and neck regions are associated with C2-C8, the back regions extends from C2-S3, the central diaphragm is associated with spinal nerve roots between C3 and C5, the upper extremities correspond to C5 and T1, the thoracic wall extends from T1 to T11, the peripheral diaphragm is between T6 and T11, the abdominal wall is associated with T6-L1, lower extremities are located from L2 to S2, and the perineum from L4 to S4. In conventional neurostimulation, when a patient experiences pain in one of these regions, a neurostimulation lead is implanted adjacent to the spinal cord at the corresponding spinal position. By example, to address chronic pain sensations that commonly focus on the lower back and lower extremities using conventional techniques, a specific energy field is typically applied to a region between vertebrae levels T8 and T12. The specific energy field often stimulates a number of nerve fibers and structures of the spinal cord. By applying energy in this manner, the patient commonly experiences paresthesia over a relatively wide region of the patient's body from the lower back to the lower extremities.
Positioning of an applied electrical field relative to a physiological midline is also important. Nerve fibers extend between the brain and a nerve root along the same side of the dorsal column as the peripheral areas the fibers represent. Pain that is concentrated on only one side of the body is “unilateral” in nature. To address unilateral pain, electrical energy is applied to neural structures on the side of a dorsal column that directly corresponds to a side of the body subject to pain. Pain that is present on both sides of a patient is “bilateral.” Accordingly, bilateral pain is addressed through application of electrical energy along both sides of the column and/or along a patient's physiological midline.
Percutaneous leads and laminotomy leads are the two most common types of lead designs that provide conductors to deliver stimulation pulses from an implantable pulse generator (IPG) to distal electrodes adjacent to the pertinent nerve tissue. As shown in FIG. 1A, conventional percutaneous lead 100 includes electrodes 101 that substantially conform to the body of the body portion of the lead. Due to the relatively small profile of percutaneous leads, percutaneous leads are typically positioned above the dura layer through the use of a Touhy-like needle. Specifically, the Touhy-like needle is passed through the skin, between desired vertebrae to open above the dura layer for the insertion of the percutaneous lead.
As shown in FIG. 1B, conventional laminotomy or paddle lead 150 has a paddle configuration and typically possesses a plurality of electrodes 151 (commonly, two, four, eight, or sixteen) arranged in columns. Due to their dimensions and physical characteristics, conventional laminotomy leads require a surgical procedure (a partial laminectomy) for implantation. Multi-column laminotomy leads enable more reliable positioning of a plurality of electrodes as compared to percutaneous leads. Also, laminotomy leads offer a more stable platform that tends to migrate less after implantation and that is capable of being sutured in place. Laminotomy leads also create a uni-directional electrical field and, hence, can be used in a more electrically efficient manner than typical percutaneous leads.
To supply suitable pain-managing electrical energy, multi-programmable IPGs enable the pattern of electrical pulses to be varied across the electrodes of a lead. Specifically, such systems enable electrodes of a connected stimulation lead to be set as an anode (+), as a cathode (−), or to a high-impendence state (OFF). As is well known, negatively charged ions and free electrons flow away from a cathode toward an anode. Consequently, using laminotomy lead 150 of FIG. 1B as an example, a range of very simple to very complex electrical fields can be created by defining different electrodes in various combinations of (+), (−), and OFF. Of course, in any instance, a functional combination must include at least one anode and at least one cathode (although in some cases, the “can” of the IPG can function as an anode).