The history of SCS devices
From fish to electronics
The history of spinal cord stimulation is part of a larger human story of pain reduction spanning perhaps thousands of years. In the first century CE, Scribonius Largus reported the use of the torpedo fish in treating gout and headache after observing accidental contact with the electrically active fish relieved gout pain . Following the use of various electrostatic friction machines , electrochemical based devices and magnetically derived current apparatus from the 17th to 20th centuries for pain mitigation (and some less appropriate symptoms), the Electreat was patented in 1919 ( Fig. 10.1 ). This device was operated by two standard ‘D’ cell batteries which powered an internal mechanically controlled induction device as a source of pulsing current applied to a roller and/or sponge electrode(s). The Electreat may have been the precursor to the modern TENS unit (transcutaneous electrical nerve stimulation, a term used by Burton and Maurer in 1974 ). Such devices apply current directly to the skin via patch electrodes for pain mitigation. In early SCS, TENS units were used to test patient tolerance to stimulation prior to implantation .
The first implanted dorsal column stimulators
The first human dorsal column stimulator was implanted in 1967 by Shealy and designed by Mortimer , following experimentation in a feline model . The system used a single cathodic electrode sutured through the dura mater while the anodic electrode was placed in the intramuscular space, both electrodes being composed of the material Vitallium ® . Subcutaneous, hypodermic needle-accessed jacks permitted connection of the hand-made stimulator device to the electrodes.
Their second stimulator, implanted 7 months later, was designed by Mortimer and based on Medtronic’s Angiostat and Barostat carotid sinus nerve stimulators . Mortimer’s second device used platinum–iridium electrodes of the same shape as the first stimulator but was RF-coupled thus requiring an external coil for the provision of power ( Fig. 10.2 ). A portable box housed the transmitter electronics, connected to the external coil and contained the stimulation parameter controls (e.g. rate, amplitude and, in the case of the variable frequency transmitter, the rate of frequency change). An account of their first, second and subsequent implants can also be found in .
First commercial SCS devices
The technology behind SCS initially was derived from cardiac pacemakers , which are single source devices. An incomplete look at the SCS business field finds that the first commercially produced SCS device was the Myelostat from Medtronic, based on the Angiostat and Barostat carotid sinus stimulators, made available in 1968 . Avery Laboratories, originally founded to develop phrenic nerve pacing applications, offered their own device in 1972 .
Cordis (purchased by Johnson & Johnson in 1996) introduced the 199A in 1976 . This represented the first totally implantable device, being entirely self-contained in epoxy and powered with a mercury battery. It was a modified cardiac pacemaker with the ability to change externally amplitude and rate, though was first used with movement disorders . In 1980, they developed the 900X–MKI which was the first SCS device to gain The Food and Drug Administration (FDA) approval for pain relief . It is important to note that, prior to 1976, the FDA did not have the complete legal mandate to regulate and require safety and efficacy testing of medical devices .
Medtronic received approval to market their Itrel device in late 1984 . The design of this device continued to leverage the company’s cardiac devices . It was a totally implantable, primary cell, SCS device. Neuromed developed their Quattrode RF system in 1980 . The company was acquired by Quest Medical in 1995, which then changed its name to become Advanced Neuromodulation Systems (ANS) in 1998 which, in turn, was purchased by St Jude Medical (STJ) in 2005. Advanced Bionics (AB), initially a cochlear implant company, whose Pain Division was purchased by Boston Scientific (BSC) in 2004, introduced the first rechargeable, multisource, fully-implantable SCS device with their Precision system . The system has technology similar to the AB Clarion Multi-Strategy cochlear implant with 16 simultaneously active channels (released in 1995) . At the time of this writing, all three companies currently manufacturing SCS systems offer rechargeable devices. Only MDT and STJ offer primary cell devices.
From bipolar to multipolar: the evolution of commercial leads
Connected to the SCS device is an integral part of the therapy delivery system: the lead(s). With their electrode(s) placed over the proper portion(s) of dorsal column, leads are responsible for permitting the application of stimulation to accessible fibers associated with the patient’s pain, as discussed later. The first leads, again borrowed from endocardial pacing technology, eventually addressed the application specific design needs of SCS . As time moved on, leads were designed with more electrodes and varying geometries to mitigate effects of lead migration .
The first leads were of course from those of Mortimer’s first device, a pair of which is shown in Figure 10.3 . They consisted of a single cathode and a single anode, where the cathode was placed endodurally (see for a similar description). During the early stages of the treatment modality, leads were surgically placed. Percutaneous leads were initially used as a minimally invasive screening tool, prior to the surgical implantation of permanent leads .
Moving to a partial look at the commercial realm, in 1978, Medtronic introduced a percutaneously inserted electrode for permanent use. In 1981, the company released a four-electrode percutaneous lead, the PISCES (percutaneously inserted spinal cord electrical stimulation) model 3484 . In 1986, Neuromed made available an eight-electrode RF system, based on their Octrode lead . The work of J. Law, published in 1987, suggested the advantage of multiple rows of electrodes for low-back pain patients . In late 1994, Neuromed received approval to market their Dual Octrode device , a dual lead system representing the first 16-electrode system, just 2 months before Medtronic gained approval for their eight-electrode Mattrix system .
At the time of this writing, all three companies currently manufacturing SCS systems (MDT, STJ and BSC) offer 16-electrode systems with varying lead and electrode geometries in both percutaneous and surgically implanted paddle leads. Currently, a greater number of electrodes is not available. The various leads include electrodes as small as 3 mm to as large as 6 mm with spacing as tight as 1 mm to as wide as 12 mm. Currently, only MDT and STJ offer tripole paddle leads: the Specify 5-6-5 (model 39565) and Lamitrode (models 8, 8C and 16C) series, respectively. A tripole configuration may penetrate deeper into the dorsal columns , possibly dependent on the electrical capabilities of the system .
The number of electrodes is but one means of assessing the targeting ability of a lead or how well it mitigates lead migration effects. For example, the number of simultaneously active electrodes and the number of sources both provide field superposition in independent ways. Having more than one channel allows for independent sets of stimulation parameters to be applied rapidly in sequence, which can provide the cognitive perception of simultaneity. It is also a means to emulate simultaneously active electrodes. All of these features are part of a suite of tools used to recruit the specific fibers associated with the patient’s pain, each discussed later.
Targeting fibers spatially
The need for placing SCS leads over the area(s) of the dorsal columns that will place paresthesia in the patient’s pain area has partially driven lead design to move from Mortimer’s first lead having a two electrode approach (a single cathode and a single anode) to today’s 16-electrode leads. Additional electrodes can also mitigate loss of paresthesia due to migration. From a biomedical engineering design standpoint, coupled with the electrical capabilities of the system, the following characteristics of the lead(s) all work together to provide the clinician a level of control and selectivity in placing paresthesia:
- 1.
the number of electrodes
- 2.
the electrode geometry
- 3.
the relative positions of the electrodes (electrode spacing).
Addressing the first point, for a given electrode geometry and spacing, as the number of electrodes increases, the available population of neurons that can be recruited into paresthesia increases. This is simply due to there being more accessible points of stimulation delivery over the dorsal columns. However, both the geometry and relative positions of the electrodes play an important factor in determining the recruitable population. As noted above, these factors may not be to the exclusion of the electrical capabilities of the pulse generator. In fact, as will be discussed later, therapeutic potential of a lead can be dependent on the ability of the pulse generator.
Next, looking at the second point, as the size of an electrode increases its contact area and depth with tissue increases accordingly. Thus, a larger electrode can permit access to a larger neuron population than a smaller electrode. However, when considering electrode size one must consider both charge density and spatial resolution. As the contact area with the tissue increases, the charge density on the electrode decreases (when the stimulation current is kept constant). With a larger electrode then, the stimulation current may need to be quite high to ensure that the charge density is adequate to provide any useful therapy. Such a case would require a high output device with the requisite design limitations. Conversely, a smaller electrode has a larger charge density on its surface. This limits the available stimulation current for therapy as exposure to excessive charge density is well understood to cause physiological damage .
Spatially, a smaller electrode permits a more confined and therefore selective population of neurons to be recruited thus allowing paresthesia to be more defined. However, a small electrode may not provide access to the entirety of the population of neurons necessary to recruit for complete paresthesia coverage. This suggests that a larger number of smaller electrodes is favorable to either fewer electrodes, larger electrodes or some combination of the two.
Finally, relative electrode position is an important design factor. If electrodes are spaced far apart, then their activating functions may not overlap to provide a cumulative effect which would effectively fill in the gaps in recruitable neurons between electrodes. This is demonstrated in Figure 10.4 .
Thus, an electrode with a size that does not challenge charge density safety or stimulation efficacy, placed in an array of similar electrodes utilizing a spacing permitting activating functions to overlap constructively, can give the clinician access to both large swaths of fibers for broad paresthesia coverage and provide spatial resolution to target small fiber populations for paresthesia selectivity.
Electronics for spinal cord stimulation
Next, the various aspects of the design of spinal cord stimulator pulse generators will be presented. These include the pulse delivery type (constant voltage or constant current), the pulsewidth, the pulse rate, the number of pulse sources, the power source (RF, primary cell or rechargeable), implantable pulse generator (IPG) efficiency concerns, telemetry design needs, clinically specific application design and reliability issues. All of these aspects work in concert with each other and the clinician to provide clinical results.
Pulse delivery types
The pulses delivered by an IPG can either be constant voltage or constant current. The implementation of each has both design implications and physiological implications. However, prior to the discussion of pulse types, it is important to recapitulate how the membrane potential of a neuron is determined.
Equation 10.1 shows the well-known Goldman-Hodgkin-Katz equation . It illustrates how the concentration gradients of important ions with respect to the inside and outside of a cell dictate the membrane potential. When the local membrane potential meets or exceeds threshold, the voltage-gated sodium channels open. If enough of these channels open and remain open long enough, an action potential is generated which may be conducted via saltatory conduction along the axon.
V r e s t = R T F ln P K [ K + ] o u t + P N a [ N a + ] o u t + P C l [ C l − ] i n P K [ K + ] i n + P N a [ N a + ] i n + P C l [ C l − ] o u t
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