Abstract:
Spinal cord stimulation (SCS) system having a recharging system with self alignment, a system for mapping current fields using a completely wireless system, multiple independent electrode stimulation outsource, and IPG control through software on Smartphone/mobile device and tablet hardware during trial and permanent implants. SCS system can include multiple electrodes, multiple, independently programmable, stimulation channels within an implantable pulse generator (IPG) providing concurrent, but unique stimulation fields. SCS system can include a replenishable power source, rechargeable using transcutaneous power transmissions between antenna coil pairs. An external charger unit, having its own rechargeable battery, can charge the IPG replenishable power source. A real-time clock can provide an auto-run schedule for daily stimulation. A bi-directional telemetry link informs the patient or clinician the status of the system, including the state of charge of the IPG battery. Other processing circuitry in current IPG allows electrode impedance measurements to be made.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 14/173,510 (Publication No. US 2014/02772601 filed on Feb. 5, 2014, which is a non-provisional application that claims priority to provisional application No. 61/792,654 filed on Mar. 15, 2013, which is incorporated in its entirety herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to stimulators using electrical pulses in a medical context, and more particularly, applying electrical pulse stimulators to the spinal cord to control pain. 
     BACKGROUND 
     A Spinal Cord Stimulator (SCS) is used to exert pulsed electrical signals to the spinal cord to control chronic pain. Spinal cord stimulation, in its simplest form, comprises stimulating electrodes implanted in the epidural space, an electrical pulse generator implanted in the lower abdominal area or gluteal region, conducting wires connecting the electrodes to the electrical pulse generator, an electrical pulse generator remote control, and an electrical pulse generator charger. Spinal cord stimulation has notable analgesic properties and, at the present, is used mostly in the treatment of failed back surgery syndrome, complex regional pain syndrome and refractory pain due to ischemia. 
     Electrotherapy of pain by neuro stimulation began shortly after Melzack and Wall proposed the gate control theory in 1965. This theory proposed that nerves carrying painful peripheral stimuli and nerves carrying touch and vibratory sensation both terminate in the dorsal horn (the gate) of the spinal cord. It was hypothesized that input to the dorsal horn of the spinal cord could be manipulated to “close the gate” to the nerves. As an application of the gate control theory, Shealy et al. implanted the first spinal cord stimulator device directly on the dorsal column for the treatment of chronic pain in 1971. 
     Spinal cord stimulation does not eliminate pain. The electrical impulses from the stimulator override the pain messages so that the patient does not feel the pain intensely. In essence, the stimulator masks the pain. A trial implantation is performed before implanting the permanent stimulator. The physician first implants a trial stimulator through the skin (percutaneously) perform stimulations as a trial run. Because a percutaneous trial stimulator tends to move from its original location, it is considered temporary. If the trial is successful, the physician can then implant a permanent stimulator. The permanent stimulator is implanted under the skin of the abdomen, and with the leads inserted under the skin and subcutaneously fed to and inserted into the spinal canal. This placement of the stimulator in the abdomen is a more stable, effective location. The leads, which consist of an array of electrodes, can be percutaneous type or paddle type. Percutaneous electrodes are easier to insert in comparison with paddle type, which are inserted via incision over spinal cord and laminectomy. 
     There are a number of the problems that exist in currently available SCS systems that limit the full benefits of dorsal column stimulation from an effectiveness and patient user friendly perspective. One problem is that current SCS systems are limited to only 16 electrodes with a maximum of 16 independent current sources. Another problem is that current SCS systems have complicated trialing methods that involve multiple gadgets and hardware. Another problem is that patients must carry an independent remote control in order to control the IPG in their daily lives. 
     SUMMARY 
     Disclosed are the following features included within a spinal cord stimulation system: (1) a recharging system with self-alignment, (2) a system for mapping current fields using a completely wireless system, (3) multiple independent electrode stimulation outsource, and (4) IPG control, during trial and permanent implants, through software on generic Smartphone/mobile device and tablet hardware. The SCS system can include multiple electrodes, and multiple, independently programmable, stimulation channels within an implantable pulse generator (IPG) where the channels can provide concurrent, but unique stimulation fields, permitting virtual electrodes to be realized. The SCS system can include a replenishable power source (e.g., rechargeable battery) that may be recharged using transcutaneous power transmissions between antenna coil pairs. An external charger unit, having a rechargeable battery can be used to charge the IPG replenishable power source. A real-time clock can provide an auto-run schedule for daily stimulation. An included bi-directional telemetry link in the system can inform the patient or clinician of the status of the system, including the state of charge of the IPG battery. Other processing circuitry in the IPG allows electrode impedance measurements to be made. Circuitry provided in the external battery charger can provide alignment detection for the coil pairs.  FIG. 1  depicts a SCS system, as described herein, for use during the trial period and the permanent implantation. 
     The SCS system, as disclosed herein, is superior to existing SCS systems in that the SCS system, as disclosed herein, can provide a stimulus to a selected pair or group of a multiplicity of electrodes, e.g., 32 electrodes, grouped into multiple channels, e.g., 6 channels. In an embodiment, each electrode is able to produce a programmable constant output current of at least 12 mA over a range of output voltages that may go as high as 16 volts. In another embodiment, the implant portion of the SCS system includes a rechargeable power source, e.g., one or more rechargeable batteries. The SCS system described herein requires only an occasional recharge, has an implanted portion smaller than existing implant systems, has a self-aligning feature to guide the patient in aligning the charger over the implanted IPG for the most efficient power recharge, has a life of at least 10 years at typical settings, offers a simple connection scheme for detachably connecting a lead system thereto, and is extremely reliable. 
     In an embodiment, each of the electrodes included within the stimulus channels can deliver up to 12.7 mA of current over the entire range of output voltages, and can be combined with other electrodes to deliver current up to a maximum of 20 mA. Additionally, the SCS system provides the ability to stimulate simultaneously on all available electrodes in the SCS system. That is, in operation, each electrode can be grouped with at least one additional electrode to form one channel. The SCS system allows the activation of electrodes to at least 10 channels. In one embodiment, such grouping is achieved by a low impedance switching matrix that allows any electrode contact or the system case (which may be used as a common, or indifferent, electrode) to be connected to any other electrode. In another embodiment, programmable output current DAC&#39;s (digital-to-analog converters) are connected to each electrode node, so that, when enabled, any electrode node can be grouped with any other electrode node that is enabled at the same time, thereby eliminating the need for a low impedance switching matrix. This advantageous feature allows the clinician to provide unique electrical stimulation fields for each current channel, heretofore unavailable with other “multichannel” stimulation systems (which “multi-channel” stimulation systems are really multiplexed single channel stimulation systems). Moreover, this feature, combined with multicontact electrodes arranged in two or three dimensional arrays, allows “virtual electrodes” to be realized, where a “virtual electrode” comprises an electrode that appears to be at a certain physical location, but in actuality is not physically located at the certain physical location. Rather, the “virtual electrode” results from the vector combination of electrical fields from two or more electrodes that are activated simultaneously. 
     In embodiments, the SCS system includes an implantable pulse generator (IPG) powered by a rechargeable internal battery, e.g., a rechargeable lithium ion battery providing an output voltage that varies from about 4.1 volts, when fully charged, to about 3.5 volts. 
     Embodiments are comprised of components previously not provided on SCS systems. The components are comprised of a number of different sub-components, as described herein. The SCS system can be comprised of an permanent implantable IPG, an implantable trial IPG, a wireless dongle, an IPG charger, clinical programmer software, patient programmer software, leads (percutaneous and paddle), lead anchors, lead splitters, lead extensions, and accessories.  FIG. 1  depicts the components during trial and permanent implantation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts various components that can be included in a spinal cord stimulation system, according to an embodiment. 
         FIG. 2  depicts an exploded view of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 3  depicts a feed through assembly of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 4  depicts a lead contact system of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 5  depicts a lead contact assembly of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 6  depicts a head unit assembly of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 7  depicts an RF antenna of an implantable pulse generator (IPG) assembly, according to an embodiment. 
         FIG. 8  depicts a percutaneous lead, according to an embodiment. 
         FIG. 9  depicts a paddle lead, according to an embodiment. 
         FIG. 10  depicts a lead extension, according to an embodiment. 
         FIG. 11  depicts a lead splitter, according to an embodiment. 
         FIG. 12  depicts a sleeve anchor, according to an embodiment. 
         FIG. 13  depicts a mechanical locking anchor, according to an embodiment. 
         FIG. 14  illustrates communication via a wireless dongle with a tablet/clinician programmer and smartphone/mobile/patient programmer during trial and/or permanent implantation, according to an embodiment. 
         FIG. 15  depicts a Tuohy needle, according to an embodiment. 
         FIG. 16  depicts a stylet, according to an embodiment. 
         FIG. 17  depicts a passing elevator, according to an embodiment. 
         FIG. 18  depicts a tunneling tool, according to an embodiment. 
         FIG. 19  depicts a torque wrench, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Implantable Pulse Generator (IPG) 
       FIG. 1  illustrates various components that can be included in a SCS system for the trial and the permanent installation periods. The spinal cord stimulator (SCS)  100  is an implantable device used to deliver electrical pulse therapy to the spinal cord in order to treat chronic pain. The implantable components of the system consist of an Implantable Pulse Generator (IPG)  102  and a multitude of stimulation electrodes  130 . The IPG  102  is implanted subcutaneously, no more than 30 mm deep in an area that is comfortable for the patient while the stimulation electrodes  130  are implanted directly in the epidural space. The electrodes  130  are wired to the IPG  102  via leads  140 ,  141  which keep the stimulation pulses isolated from each other in order to deliver the correct therapy to each individual electrode  130 . 
     The therapy delivered consists of electrical pulses with controlled current amplitude ranging from +12.7 to −12.7 mA (current range 0-25.4 mA). These pulses can be programmed in both length and frequency from 10 μS to 2000 μS and 0.5 Hz to 1200 Hz. At any given moment, the sum of the currents sourced from the anodic electrodes  130  must equal the sum of the currents sunk by the cathodic electrodes  130 . In addition, each individual pulse is bi-phasic, meaning that once the initial pulse finishes another pulse of opposite amplitude is generated after a set holdoff period. The electrodes  130  may be grouped into stimulation sets in order to deliver the pulses over a wider area or to target specific areas, but the sum of the currents being sourced at any one given time may not exceed 20 mA. A user can also program different stimulation sets (up to eight) with different parameters in order to target different areas with different therapies. 
       FIG. 2  depicts an exploded view of an IPG  102 . The IPG  102  consists of two major active components  104 ,  106 , a battery  108 , antenna  110 , some support circuitry, and a multitude of output capacitors  112 . The first of the major active components is the microcontroller  104  transceiver  104 . It is responsible for receiving, decoding, and execution both commands and requests from the external remote. If necessary it passes these commands or requests onto the second major component, the ASIC  106 . The ASIC  106  receives the digital data from the microcontroller  104  and performs the entire signal processing to generate the signals necessary for stimulation. These signals are then passed onto the stimulation electrodes  130  in the epidural space. 
     The ASIC  106  is comprised of a digital section and an analog section. The digital section is divided into multiple sections including; Timing Generators, Arbitration Control, Pulse Burst Conditioner, and Electrode Logic. The analog section receives the incoming pulses from the digital section and amplifies them in order to deliver the correct therapy. There are also a multitude of digital register memory elements that each section utilizes, both digital and analog. 
     The digital elements in the ASIC  106  are all made up of standard subsets of digital logic including logic gates, timers, counters, registers, comparators, flip-flips, and decoders. These elements are ideal for processing the stimulation pulses as all of them can function extremely fast—orders of magnitudes faster than the required pulse width. The one drawback is that they must all function at one single voltage, usually 5.0, 3.3, 2.5, or 1.8 volts. Therefore they are not suitable for the final stage in which the pulses are amplified in order to deliver the constant current pulses. 
     The timing generators are the base of each of the stimulation sets. It generates the actual rising and falling edge triggers for each phase of the bi-phasic pulse. It accomplishes this by taking the incoming clock that is fed from the microcontroller  104  and feeding it into a counter. For the purpose of this discussion, assume the counter simply counts these rising clock edges infinitely. The output of the counter is fed into six different comparators. The comparators other input is connected to specific registers that are programmed by the microcontroller  104 . When the count equals the value stored in the register, the comparator asserts a positive signal. 
     The first comparator is connected to the SET signal of a SR flip flop. The SR flip flop stays positive until the RESET signal is asserted, which the second comparator is connected to. The output of the SR flip flop is the first phase of the bi-phasic pulse. Its rising &amp; falling edges are values stored in the registers and programmed by the microcontroller  104 . The third and fourth comparators &amp; registers work in exactly the same way to produce the second phase of the bi-phasic pulse using the second SR flip flop. 
     The fifth comparator is connected the RESET of the final SR-Flip flop in the timing generator. This flip flop is SET by the first comparator, which is the rising edge of the first pulse. The RESET is then triggered by the value the microprocessor programmed into the register connected to the comparator. This allows for a ‘holdoff’ period after the falling edge of the second pulse. The output of this third SR flip flop can be thought of as an envelope of the biphasic pulses indicating when this particular timing generator is active. 
     The final comparator of the system is once again connected to a register that stores the frequency values from the microprocessor. Essentially when the count reaches this value it triggers the comparator which is fed back to the counter to reset it to zero and beginning the entire pulse generation cycle again. The ASIC  106  may contain many of these timing generators as each can control anywhere from two to all of the electrodes  130  connected to the IPG  102  at a time. However, when there is more than one timing generator and multiple channels have been actively programmed then there needs to be a mechanism for suppressing a second channel from turning on when another is already active. 
     This brings us to the next circuit block contained in the IPG  102 , the arbitrator. The arbitrator functions by looking at each of the timing generators&#39; envelope signals and makes sure only one can be active at a time. If a second tries to activate then the arbitrator suppresses that signal. 
     It accomplishes this by bringing each of the channel envelope signals into a rising edge detection circuit. Once one is triggered it is fed into the SET pin of an SR flip flop. The output of this SR-flip flop is fed into all of the other rising edge detectors in order to suppress them from triggering. The channel envelope signal is also fed into a falling-edge detector which is then fed into the RESET of the same SR flip flop. The output of the SR flip flops are then connected to switches whose outputs are all tied together that turn on/off that channels particular biphasic pulse train. Therefore the output of this circuit element is a single bi-phasic pulse train and a signal designating which timing generator that particular pulse train is sourced from. Essentially, the circuit looks for a channel to go active. Once it finds one it suppresses all others until that channel becomes inactive. 
     The next section of the circuit works very similarly to the timing generators to create a high speed burst pulse train that is then combined with the stimulation pulse train to create a bursted bi-phasic pulse train if desired. 
     It accomplishes this by taking the incoming clock that is fed from the microcontroller  104  and feeding it into a counter. For the purpose of this discussion, assume the counter simply counts these rising clock edges infinitely. The counter is only active when during a single phase of the bi-phasic signal and begins counting as soon as the rising edge is detected. The output of the counter is fed into a comparator, along with a microcontroller-programmed register, whose output is connected to the reset pin on the counter. Therefore this counter will simply count to a programmed value &amp; reset. This programmed value is the burst frequency. 
     The output of the comparator is then fed into an edge detection circuit and then a flip flop that combines it with the actual stimulation pulse train to create a single phase bursted stimulation pulse. The entire circuit is duplicated for the second phase of the signal resulting in the desired bursted bi-phasic pulse train. The stimulation signal is now ready to be handed over to the electrode logic stage. 
     The electrode logic conditions and directs the bi-phasic signals to the analog section of the ASIC  106 . At this point, the bi-phasic signals contain all of the pertinent timing information, but none of the required amplitude information. The incoming signals include the bi-phasic pulse train and another signal designating which timing generator the current active train came from. Each electrode logic cell has a register for each timing generator that stores this particular electrode&#39;s  130  amplitude values for that timing generator. The electrode logic cell uses the designation signal to determine which register to pull the amplitude values from, e.g. if the third timing generator is passed through the arbitration circuit then the electrode logic would read the value from the third register. 
     Once the value is pulled from the register, it goes through a series of logic gates. The gates first determine that the electrode  130  should be active. If not, they proceed no further and do not activate the analog section of the electrode output, thereby saving precious battery  108  power. Next they determine if this particular electrode  130  is an anode or cathode. If it is deemed to be an anode, the electrode logic passes the amplitude information and the biphasic signal to the positive current (digital to analog converter) DAC in the analog section of the ASIC  106 . If it is deemed to be a cathode, the electrode logic passes the amplitude information and the biphasic signal to the negative current DAC in the analog section of the ASIC  106 . The electrode logic circuit must make these decisions for each phase of the bi-phasic signal as every electrode  130  will switch between being an anode and a cathode. 
     The analog elements in the ASIC  106  are uniquely designed in order to produce the desired signals. The basis of analog IC design is the field effect transistor (FET) and the type of high current multiple output design required in SCS  100  means that the bulk of the silicon in the ASIC  106  will be dedicated to the analog section. 
     The signals from the electrode output are fed into each current DAC when that specific electrode  130  should be activated. Each electrode  130  has a positive and a negative current DAC, triggered by the electrode logic and both are never active at the same time. The job of each current DAC is, when activated, to take the digital value representing a stimulation current amplitude and produce an analog representation of this value to be fed into the output stage. This circuit forms half of the barrier between the digital and analog sections of the ASIC  106 . 
     The digital section of the ASIC  106  is built upon a technology that only allows small voltages to exist. In moving to the analog section, the output of the current DAC (which is a low level analog signal) must be amplified to a higher voltage for use in the analog section. The circuit that performs this task is called a power level shifter. Because this circuit is built upon two different manufacturing technologies and requires high precision analog circuits built upon a digital base, it is extremely difficult to implement. 
     Once the voltages have been converted for usage in the analog portion of the ASIC  106  they are passed on to the output current stages. There are two current sources per electrode output. One will source a positive current and one will sink a negative current, but they will never both be active simultaneously. The current sources themselves are made up of analog elements similar to a Howland current source. There is an input stage, and amplification stage with feedback through a sensing component to maintain the constant current. The input stage takes the analog voltage values from the power level shifter and produces an output pulse designated for the amplifier. The amplifier then creates the pulses of varying voltages but constant current flow. The sources are capable of sourcing or sinking up to 12.7 mA at 0.1 mA resolution into a load of up to 1.2 k Ohms. This translates into range of 15 volts, which will vary depending on the load in order to keep the current constant. 
     The microcontroller  104  to ASIC  106  interface is designed to be as simple as possible with minimal bus ‘chatter’ in order to save battery  108  life. The ASIC  106  will essentially be a collection of registers programmed via a standard I 2 C or SPI bus. Since the ASIC  106  is handling all the power management, there will also be a power good (PG) line between the two chips  104 ,  106  in order to let the microcontroller  104  know when it is safe to power up. The ASIC  106  will also need to use a pin on the microcontroller  104  in order to generate a hardware interrupt in case anything goes awry in the ASIC  106 . The final connection is the time base for all of the stimulation circuitry. The ASIC  106  will require two clocks, one for its internal digital circuitry which will be fed directly from the microcontroller  104  clock output, and one to base all stimulation off of which will need to be synthesized by the microcontroller  104  and fed to the ASIC  106 . All commands and requests to the ASIC  106  will be made over the I 2 C or SPI bus and will involve simply reading a register address or writing to a register. Even when the ASIC  106  generates a hardware interrupt, it will be the responsibility of the microcontroller  104  to poll the ASIC  106  and determine the cause of the interrupt. 
     The wireless interface is based upon the FCCs MedRadio standard operating in the 402-405 Mhz range utilizing up to 10 channels for telemetry. The protocol is envisioned to be very simple once again in order to minimize transmission and maximize battery  108  life. All processing will take place on the user remote/programmer and the only data transmitted is exactly what will be used in the microcontroller  104  to ASIC  106  bus. That is, all of the wireless packets will contain necessary overhead information along with only a register address, data to store in the register, and a command byte instructing the microcontroller  104  what to do with the data. The overhead section of the wireless protocol will contain synchronization bits, start bytes, an address which is synchronized with the IPG&#39;s  102  serial number, and a CRC byte to assure proper transmission. It is essential to keep the packet length as small as possible in order to maintain battery  108  life. Since the IPG  102  cannot listen for packets all the time due to battery  108  life, it cycles on for a duty cycle of less than 0.05% of the time. This time value can be kept small as long as the data packets are also small. The user commands needed to run the system are executed by the entire system using flows. 
     The IPG  102  uses an implantable grade Li ion battery  108  with 215 mAHr with zero volt technology. The voltage of the battery  108  at full capacity is 4.1 V and it supplies current only until it is drained up to 3.3 V which is considered as 100% discharged. The remaining capacity of the battery  108  can be estimated at any time by measuring the voltage across the terminals. The maximum charge rate is 107.5 mA. A Constant Current, Constant Voltage (CCCV) type of regulation can be applied for faster charging of the battery  108 . 
     The internal secondary coil  109  is made up of 30 turns of 30 AWG copper magnet wires. The ID, OD, and the thickness of the coil are 30, 32, and 2 mm, respectively. Inductance L 2  is measured to be 58 uH, a 80 nF capacitor is connected to it to make a series resonance tank at 74 kHz frequency. In the art of induction charging, two types of rectifiers are considered to convert the induced AC into usable DC, either a bridge full wave rectifier or a voltage doubler full wave rectifier. To obtain a higher voltage, the voltage double full wave rectifier is used in this application. The rectifier is built with high speed Schottky diodes to improve its function at high frequencies of the order 100 kH. A Zener diode and also a 5V voltage regulator are used for regulation. This circuit will be able to induce AC voltage, rectify to DC, regulate to 5V and supply 100 mA current to power management IC that charges the internal battery  108  by CCCV regulation. 
     The regulated 5V 100 mA output from the resonance tank is fed to, for example, a Power Management Integrated Circuit (PMIC) MCP73843. This particular chip was specially designed by Microchip to charge a Li ion battery  108  to 4.1 V by CCCV regulation. The fast charge current can be regulated by changing a resistor; it is set to threshold current of 96 mA in this circuit. The chip charges the battery  108  to 4.1V as long as it receives current more than 96 mA. However, if the supply current drops below 96 mA, it stops to charge the battery  108  until the supply is higher than 96 again. For various practical reasons, if the distance between the coils increases, the internal secondary coil  109  receives lesser current than the regulated value, and instead of charging the battery  108  slowly, it pauses the charging completely until it receives more than 96 mA. It is understood to those with skill in the art that other power management chips can be used and the power management chip is not limited to the PMIC MCP738432 chip. 
     All the functions of the IPG  102  are controlled from outside using a hand held remote controller specially designed for this device. Along with the remote control, an additional control is desirable to operate the IPG  102  if the remote control was lost or damaged. For this purpose a Hall effect based magnet switch was incorporated to either turn ON or turn OFF the IPG  102  using an external piece of magnet. Magnet switch acts as a master control for the IPG  102  to turn on or off. A south pole of sufficient strength turns the output on and a north pole of sufficient strength is necessary to turn the output off. The output is latched so that the switch continues to hold the state even after the magnet is removed from its vicinity. 
     The IPG  102  is an active medical implant that generates an electrical signal that stimulates the spinal cord. The signal is carried through a stimulation lead  140  that plugs directly into the IPG  102 . The IPG  102  recharges wirelessly through an induction coil  109 , and communicates via RF radio antenna  110  to change stimulation parameters. The IPG  102  is implanted up to 3 cm below the surface of the skin and is fixed to the fascia by passing two sutures through holes in the epoxy header  114 . The leads  140  are electrically connected to the IPG  102  through a lead contact system  116 , a cylindrical spring-based contact system with inter-contact silicone seals. The leads  140  are secured to the IPG  102  with a set screw  117  that actuates within locking housing  118 . Set screw compression on the lead&#39;s  140  fixation contact is governed by a disposable torque wrench. The wireless recharging is achieved by aligning the exterior induction coil on the charger with the internal induction coil  109  within the IPG  102 . The RF antenna within the remote&#39;s dongle  200  communicates with the RF antenna  110  in the IPG&#39;s  102  epoxy header  114 .  FIG. 2  illustrates an exploded view of the IPG  102  assembly. 
     The IPG  102  is an assembly of a hermetic titanium (6Al-4V) casing  120  which houses the battery  108 , circuitry  104 ,  106 , and charging coil  109 , with an epoxy header  114 , which houses the lead contact assembly  116 , locking housing  118 , and RF antenna  110 . The internal electronics are connected to the components within the epoxy head through a hermetic feedthrough  122 , as shown in  FIG. 3 . The feedthrough  122  is a titanium (6Al-4V) flange with an alumina window and gold trimming. Within the alumina window are thirty-four platinum-iridium (90-10) pins that interface internally with a direct solder to the circuit board, and externally with a series of platinum iridium wires laser-welded to the antenna  110  and lead contacts  126 . The IPG  102  has the ability to interface with 32 electrical contacts  126 , which are arranged in four rows of eight contacts  126 . Thirty two of the feedthrough&#39;s  122  pins  124  will interface with the contacts  126 , while two will interface with the antenna  110 , one to the ground plane and one to the antenna  110  feed. 
       FIGS. 4 and 5  depict a lead contact system  115  and assembly  116 , respectively. The lead contacts  126  consist of an MP35N housing  128  with a platinum-iridium 90-10 spring  129 . Each contact  126  is separated by a silicone seal  127 . At the proximal end of each stack of 8 contacts  126  is a titanium (6Al-4V) cap  125  which acts as a stop for the lead  140 . At the distal end is a titanium (6Al-4V) set screw  119  and block  118  for lead fixation. At the lead entrance point there is a silicone tube  123  which provides strain relief as the lead  140  exits the head unit  114 , and above the set screw  119  is another silicone tube  131  with a small internal canal which allows the torque wrench to enter but does not allow the set screw  119  to back out. In addition to the contacts  126  and antenna  110 , the header  114  also contains a radiopaque titanium (6Al-4V) tag  132  which allows for identification of the device under fluoroscopy. The overmold of the header  114  is Epotek 301, a two-part, biocompatible epoxy.  FIGS. 4, 5, 6, and 7  depict illustrations of lead contact system  115 , lead contact assembly  116 , head unit assembly  114 , and RF antenna  110 , respectively. 
     Internal to the titanium (6Al-4V) case  120  are the circuit board  105 , battery  108 , charging coil  109 , and internal plastic support frame. The circuit board  105  will be a multi-layered FR-4 board with copper traces and solder mask coating. Non-solder masked areas of the board will be electroless nickel immersion gold. The implantable battery  108 , all surface mount components, ASIC  106 , microcontroller  104 , charging coil  109 , and feedthrough  122  will be soldered to the circuit board  105 . The plastic frame, made of either polycarbonate or ABS, will maintain the battery&#39;s  108  position and provide a snug fit between the circuitry  105  and case  120  to prevent movement. The charging coil  109  is a wound coated copper. 
     Leads 
     The percutaneous stimulation leads  140 , as depicted in  FIG. 8 , are a fully implantable electrical medical accessory to be used in conjunction with the implantable SCS  100 . The primary function of the lead is to carry electrical signals from the IPG  102  to the target stimulation area on the spinal cord. Percutaneous stimulation leads  140  provide circumferential stimulation. The percutaneous stimulation leads  140  must provide a robust, flexible, and bio-compatible electric connection between the IPG  102  and stimulation area. The leads  140  are surgically implanted through a spinal needle, or epidural needle, and are driven through the spinal canal using a steering stylet that passes through the center of the lead  140 . The leads  140  are secured mechanically to the patient using either an anchor or a suture passed through tissue and tied around the body of the lead  140 . The leads  140  are secured at the proximal end with a set-screw  119  on the IPG  102  which applies radial pressure to a blank contact on the distal end of the proximal contacts. 
     The percutaneous stimulation leads  140  consist of a combination of implantable materials. Stimulation electrodes  130  at the distal end and electrical contacts at the proximal end are made of a 90-10 platinum-iridium alloy. This alloy is utilized for its bio-compatibility and electrical conductivity. The electrodes  130  are geometrically cylindrical. The polymeric body of the lead  140  is polyurethane, which is chosen for its bio-compatibility, flexibility, and high lubricity to decrease friction while being passed through tissue. The polyurethane tubing has a multi-lumen cross section, with one center lumen  142  and eight outer lumens  144 . The center lumen  142  acts as a canal to contain the steering stylet during implantation, while the outer lumens  144  provide electrical and mechanical separation between the wires  146  that carry stimulation from the proximal contacts to distal electrodes  130 . These wires  146  are a bundle of MP35N strands with a 28% silver core. The wires  146  are individually coated with ethylene tetrafluoroethylene (ETFE), to provide an additional non-conductive barrier. The wires  146  are laser welded to the contacts and electrodes  130 , creating an electrical connection between respective contacts on the proximal and distal ends. The leads  140  employ a platinum-iridium plug  148 , molded into the distal tip of the center lumen  142  to prevent the tip of the steering stylet from puncturing the distal tip of the lead  140 . Leads  140  are available in a variety of 4 and 8 electrode  130  configurations. These leads  140  have 4 and 8 proximal contacts (+1 fixation contact), respectively. Configurations vary by electrode  130  number, electrode  130  spacing, electrode  130  length, and overall lead  140  length. 
     The paddle stimulation leads  141 , as depicted in  FIG. 9 , are a fully implantable electrical medical accessory to be used in conjunction with the implantable SCS  100 . The primary function of the paddle lead  141  is to carry electrical signals from the IPG  102  to the target stimulation area on the spinal cord. The paddle leads  141  provide uni-direction stimulation across a 2-dimensional array of electrodes  130 , allowing for greater precision in targeting stimulation zones. The paddle stimulation leads  141  must provide a robust, flexible, and bio-compatible electric connection between the IPG  102  and stimulation area. The leads  141  are surgically implanted through a small incision, usually in conjunction with a laminotomy or laminectomy, and are positioned using forceps or a similar surgical tool. The leads  141  are secured mechanically to the patient using either an anchor or a suture passed through tissue and tied around the body of the lead  141 . The leads  141  are secured at the proximal end with a set-screw on the IPG  102  which applies radial pressure to a fixation contact on the distal end of the proximal contacts. 
     The paddle stimulation leads  141  consist of a combination of implantable materials. Stimulation electrodes  130  at the distal end and electrical contacts at the proximal end are made of a 90-10 platinum-iridium alloy. This alloy is utilized for its bio-compatibility and electrical conductivity. The polymeric body of the lead  141  is polyurethane, which is chosen for its bio-compatibility, flexibility, and high lubricity to decrease friction while being passed through tissue. The polyurethane tubing has a multi-lumen cross section, with one center lumen  142  and eight outer lumens  144 . The center lumen  142  acts as a canal to contain the steering stylet during implantation, while the outer lumens  144  provide electrical and mechanical separation between the wires  146  that carry stimulation from the proximal contacts to distal electrodes  130 . These wires  146  are a bundle of MP35N strands with a 28% silver core. The wires  146  are individually coated with ethylene tetrafluoroethylene (ETFE), to provide an additional non-conductive barrier. At the distal tip of the paddle leads  141 , there is a 2-dimensional array of flat rectangular electrodes  130 , molded into a flat silicone body  149 . Only one side of the rectangular electrodes  130  is exposed, providing the desired uni-directional stimulation. The wires  146  are laser welded to the contacts and electrodes  130 , creating an electrical connection between respective contacts on the proximal and distal ends. Also molded into the distal silicone paddle is a polyester mesh  147  which adds stability to the molded body  149  while improving aesthetics by covering wire  146  routing. The number of individual  8 -contact leads  141  used for each paddle  141  is governed by the number of electrodes  130 . Electrodes  130  per paddle  141  range from 8 to 32, which are split into between one and four proximal lead  141  ends. Each proximal lead  141  has 8 contacts (+1 fixation contact). Configurations vary by electrode  130  number, electrode  130  spacing, electrode length, and overall lead length. 
     The lead extensions  150 , as depicted in  FIG. 10 , are a fully implantable electrical medical accessory to be used in conjunction with the implantable SCS  100  and either percutaneous  140  or paddle  141  leads. The primary function of the lead extension  150  is to increase the overall length of the lead  140 ,  141  by carrying electrical signals from the IPG  102  to the proximal end of the stimulation lead  140 ,  141 . This extends the overall range of the lead  140 ,  141  in cases where the length of the provided leads  140 ,  141  is insufficient for case. The lead extensions  150  must provide a robust, flexible, and bio-compatible electric connection between the IPG  102  and proximal end of the stimulation lead  140 ,  141 . The extensions  150  may be secured mechanically to the patient using either an anchor or a suture passed through tissue and tied around the body of the extension  150 . Extensions  150  are secured at the proximal end with a set-screw  119  on the IPG  102  which applies radial pressure to a fixation contact on the distal end of the proximal contacts of the extension  150 . The stimulation lead  140 ,  141  is secured to the extension  150  in a similar fashion, using a set screw  152  inside the molded tip of extension  150  to apply a radial pressure to the fixation contact at the proximal end of the stimulation lead  140 ,  141 . 
     The lead extension  150  consists of a combination of implantable materials. At the distal tip of the extension  150  is a 1×8 array of implantable electrical contacts  154 , each consisting of MP35 housing  128  and 90-10 platinum-iridium spring. A silicone seal  127  separates each of the housings  128 . At the proximal end of the contacts is a titanium (6Al4V) cap which acts as a stop for the lead, and at the distal tip, a titanium (6Al4V) block and set screw  152  for lead fixation. The electrical contacts at the proximal end are made of a 90-10 platinum-iridium alloy. This alloy is utilized for its bio-compatibility and electrical conductivity. The polymeric body  156  of the lead  150  is polyurethane, which is chosen for its bio-compatibility, flexibility, and high lubricity to decrease friction while being passed through tissue. The polyurethane tubing  158  has a multi-lumen cross section, with one center lumen  142  and eight outer lumens  144 . The center lumen  142  acts as a canal to contain the steering stylet during implantation, while the outer lumens  144  provide electrical and mechanical separation between the wires  146  that carry stimulation from the proximal contacts to distal electrodes. These wires  146  are a bundle of MP35N strands with a 28% silver core. The wires  146  are individually coated with ethylene tetrafluoroethylene (ETFE), to provide an additional non-conductive barrier. Each lead extension  150  has 8 proximal cylindrical contacts (+1 fixation contact). 
     The lead splitter  160 , as depicted in  FIG. 11 , is a fully implantable electrical medical accessory which is used in conjunction with the SCS  100  and typically a pair of 4-contact percutaneous leads  140 . The primary function of the lead splitter  160  is to split a single lead  140  of eight contacts into a pair of 4 contact leads  140 . The splitter  160  carries electrical signals from the IPG  102  to the proximal end of two 4-contact percutaneous stimulation leads  140 . This allows the surgeon access to more stimulation areas by increasing the number of stimulation leads  140  available. The lead splitter  160  must provide a robust, flexible, and bio-compatible electrical connection between the IPG  102  and proximal ends of the stimulation leads  140 . The splitters  160  may be secured mechanically to the patient using either an anchor or a suture passed through tissue and tied around the body of the splitter  160 . Splitters  160  are secured at the proximal end with a set-screw  119  on the IPG  102  which applies radial pressure to a fixation contact on the distal end of the proximal contacts of the splitter  160 . The stimulation leads  140  are secured to the splitter  160  in a similar fashion, using a pair of set screws inside the molded tip of splitter  160  to apply a radial pressure to the fixation contact at the proximal end of each stimulation lead  140 . 
     The lead splitter  160  consists of a combination of implantable materials. At the distal tip of the splitter  160  is a 2×4 array of implantable electrical contacts  162 , with each contact  162  consisting of MP35 housing  128  and 90-10 platinum-iridium spring. A silicone seal  127  separates each of the housings  128 . At the proximal end of each row of contacts  162  is a titanium (6Al4V) cap which acts as a stop for the lead, and at the distal tip, a titanium (6Al4V) block and set screw for lead fixation. The electrical contacts at the proximal end of the splitter  160  are made of a 90-10 platinum-iridium alloy. This alloy is utilized for its bio-compatibility and electrical conductivity. The polymeric body  164  of the lead  160  is polyurethane, which is chosen for its bio-compatibility, flexibility, and high lubricity to decrease friction while being passed through tissue. The polyurethane tubing  166  has a multi-lumen cross section, with one center lumen  142  and eight outer lumens  144 . The center lumen  142  acts as a canal to contain the steering stylet during implantation, while the outer lumens  144  provide electrical and mechanical separation between the wires  146  that carry stimulation from the proximal contacts to distal electrodes  130 . These wires  146  are a bundle of MP35N strands with a 28% silver core. The wires  146  are individually coated with ethylene tetrafluoroethylene (ETFE), to provide an additional non-conductive barrier. Each lead splitter  160  has 8 proximal contacts (+1 fixation contact), and 2 rows of 4 contacts  162  at the distal end. 
     Anchors 
     The lead anchor  170 , as depicted in  FIGS. 12 and 13 , is a fully implantable electrical medical accessory which is used in conjunction with both percutaneous  140  and paddle  141  stimulation leads. The primary function of the lead anchor  170  is to prevent migration of the distal tip of the lead  140 ,  141  by mechanically locking the lead  140 ,  141  to the tissue. There are currently two types of anchors  170 , a simple sleeve  171 , depicted in  FIG. 12 , and a locking mechanism  172 , depicted in  FIG. 13 , and each has a slightly different interface. For the simple sleeve type anchor  171 , the lead  140 ,  141  is passed through the center thru-hole  174  of the anchor  171 , and then a suture is passed around the outside of the anchor  171  and tightened to secure the lead  140 ,  141  within the anchor  171 . The anchor  171  can then be sutured to the fascia. The locking anchor  172  uses a set screw  176  for locking purposes, and a bi-directional disposable torque wrench for locking and unlocking. Tactile and audible feedback is provided for both locking and unlocking. 
     Both anchors  171 ,  172  are molded from implant-grade silicone, but the locking anchor  172  uses an internal titanium assembly for locking. The 3-part mechanism is made of a housing  175 , a locking set screw  176 , and a blocking set screw  177  to prevent the locking set screw from back out. All three components are titanium (6Al4V). The bi-directional torque wrench has a plastic body and stainless steel hex shaft. 
     Wireless Dongle 
     The wireless dongle  200  is the hardware connection to a smartphone/mobile  202  or tablet  204  that allows communication between the trial generator  107  or IPG  102  and the smartphone/mobile device  202  or tablet  204 , as illustrated in  FIG. 14 . During the trial or permanent implant phases, the wireless dongle  200  is connected to the tablet  204  through the tablet  204  specific connection pins and the clinician programmer software on the tablet  204  is used to control the stimulation parameters. The commands from the clinician programmer software are transferred to the wireless dongle  200  which is then transferred from the wireless dongle  200  using RF signals to the trial generator  107  or the IPG  102 . Once the parameters on the clinician programmers have been set, the parameters are saved on the tablet  204  and transferred to the patient programmer software on the smartphone/mobile device  202 . The wireless dongle  200  is composed of an antenna, a microcontroller (having the same specifications as the IPG  102  and trial generator  107 ), and a pin connector to connect with the smartphone/mobile device  202  and the tablet  204 . 
     Charger 
     The IPG  102  has a rechargeable Lithium ion battery  108  to power its activities. An external induction type charger  210  ( FIG. 1 ) is necessary to recharge the included battery  108  inside the IPG  102  wirelessly. The charger  210  consists of a rechargeable battery, a primary coil of wire and a printed circuit board (PCB) for the electronics—all packaged into a housing. When switched on, this charger  210  produces magnetic field and induces voltage into the secondary coil  109  in the implant. The induced voltage is then rectified and then used to charge the battery  108  inside the IPG  102 . To maximize the coupling between the coils, both internal and external coils are combined with capacitors to make them resonate at a particular common frequency. The coil acting as an inductor L forms an LC resonance tank. The charger uses a Class-E amplifier topology to produce the alternating current in the primary coil around the resonant frequency. Below are the charger  210  features; 
     Charges IPG  102  wirelessly 
     Charges up to a maximum depth of 30 mm 
     Integrated alignment sensor helps align the charger with IPG  102  for higher power transfer efficiency 
     Alignment sensor gives an audible and visual feedback to the user 
     Compact and Portable 
     A protected type of cylindrical Li ion battery is used as the charger  210  battery. A Class-E of the topologies of the power amplifiers has been the most preferred type of amplifier for induction chargers, especially for implantable electronic medical devices. It&#39;s relatively high theoretical efficiency made it the most favorable choice for devices where high efficiency power transfer is necessary. A 0.1 ohm high wattage resistor is used in series to sense the current through this circuit. 
     The primary coil L 1  is made by 60 turns of Litz wire type 100/44-100 strands of 44 AWG each. The Litz wire solves the problem of skin effect and keeps its impedance low at high frequencies. Inductance of this coil was initially set at 181 uH, but backing it with a Ferrite plate increases the inductance to 229.7 uH. The attached ferrite plate focuses the produced magnetic field towards the direction of the implant. Such a setup helps the secondary coil receive more magnetic fields and aids it to induce higher power. 
     When the switch is ON, the resonance is at frequency 
             f   =     1     2   ⁢   π   ⁢       L   ⁢           ⁢   1   ⁢   C   ⁢           ⁢   2                 
When the switch is OFF, it shifts to
 
             f   =     1     2   ⁢   π   ⁢       L   ⁢           ⁢   1   ⁢       C   ⁢           ⁢   1   ⁢   C   ⁢           ⁢   2         C   ⁢           ⁢   1     +     C   ⁢           ⁢   2                       
In a continuous operation the resonance frequency will be in the range
 
     
       
         
           
             
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             f 
             &lt; 
             
               1 
               
                 2 
                 ⁢ 
                 π 
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                     L 
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                     1 
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                         C 
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                         1 
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                         C 
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                         2 
                       
                       
                         
                           C 
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     To make the ON and OFF resonance frequencies closer, a relatively larger value of C1 can be chosen by a simple criteria as follows 
     C1=nC2; a value of n=4 was used in the example above; in most cases 3&lt;n&lt;10. 
     The voltages in these Class-E amplifiers typically go up to the order of 300 VAC. Capacitors selected must be able to withstand these high voltages, sustain high currents and still maintain low Effective Series Resistance (ESR). Higher ESRs result in unnecessary power losses in the form of heat. The circuit is connected to the battery through an inductor which acts as a choke. The choke helps to smoothen the supply to the circuit. The N Channel MOSFET acts as a switch in this Class-E power amplifier. A FET with low ON resistance and with high drain current I d  is desirable. 
     In summary, the circuit is able to recharge the IPG  102  battery  108  from 0 to 100% in 2 Hr 45 Min with distance between the coils being 29 mm. The primary coil and the Class-E amplifier draws DC current of 0.866 A to achieve this task. To improve the efficiency of the circuit, a feedback closed loop control is implemented to reduce the losses. The losses are minimum when the MOSFET is switched ON and when the voltage on its drain side is close to zero. 
     The controller takes the outputs from operational amplifiers, checks if they meet the criteria, then it triggers the driver to switch ON the MOSFET for next cycle. The controller needs to use a delay timer, an OR gate and a 555 timer in monostable configuration to condition the signal for driver. When the device is switched ON, the circuit does not start to function right away as there will be no active feedback loop. The feedback becomes active only if the circuit starts to function. To solve this riddle, an initial external trigger is applied to jump start the system. 
     Alignment Sensor 
     The efficiency of the power transfer between the external charger  210  and the internal IPG  102  will be maximum only when they are properly aligned. An alignment sensor is absolutely necessary to ensure a proper alignment. This is a part of the external circuit design. The first design is based on the principle called reflected impedance. When the external is brought closer to the internal, the impedance of the both circuits change. The sensing is based on measuring the reflected impedance and test whether it crosses the threshold. A beeper is used to give an audible feedback to the patient; an LED is used for visual feedback. 
     When the impedance of the circuit changes, the current passing through it also changes. A high power 0.1 ohm resistor is used in the series of the circuit to monitor the change in current. The voltage drop across the resistor is amplified 40 times and then compared to a fixed threshold value using a operational amplifier voltage comparator. The output was fed to a timer chip which in turn activates the beeper and LED to give feedback to the user. 
     This circuit was successfully implemented in the lab on the table top version. The circuit was able to sense the alignment up to a distance of 30 mm. The current fluctuation in the circuit depends on more factors than reflected impedance alone and the circuit is sensitive to other parameters of the circuit as well. To reduce the sensitivity related to other parameters, one option is to eliminate interference of all the other factors and improve the functionality of the reflected impedance sensor—which is very challenging to implement within the limited space available for circuitry. Another option is to use a dedicated sensor chip to measure the reflected impedance. 
     A second design uses sensors designed for proximity detector or metal detectors for alignment sensing. Chips designed to detect metal bodies by the effect of Eddy currents on the HF losses of a coil can be used for this application. The TDE0160 is an example of such a chip. 
     The external charger is designed to work at 75 to 80 kHz, whereas the proximity sensor was designed for 1 MHz. The sensor circuit is designed to be compatible with the rest of the external and is fine tuned to detect the internal IPG  102  from a distance of 30 mm. 
     Programmer 
     The Clinician Programmer is an application that is installed on a tablet  204 . It is used by the clinician to set the stimulation parameters on the trial generator  107  or IPG  102  during trial and permanent implantation in the operating room. The clinician programmer is capable of saving multiple settings for multiple patients and can be used to adjust the stimulation parameters outside of the operations room. It is capable of changing the stimulation parameters though the RF wireless dongle  200  when the trial generator  107  or IPG  102  in the patient is within the RF range. In addition, it is also capable of setting or changing the stimulation parameters on the trial generator  107  and/or the IPG  102  through the internet when both the tablet  204  and the Patient Programmers on a smartphone/mobile device  202  both have access to the internet. 
     The Patient Programmer is an application that is installed on a smartphone/mobile device  202 . It is used by the patient to set the stimulation parameters on the trial generator  107  or IPG  102  after trial and permanent implantation outside the operating room. The clinician programmer is capable of saving multiple settings for multiple patients and can be transferred to the Patient Programmer wirelessly when the Clinician Programmer tablet  204  and the Patient Programmer smartphone/mobile device  202  are within wireless range such as Bluetooth from each other. In the scenario where the Clinician Programmer tablet  204  and the Patient Programmer smartphone/mobile device  202  are out of wireless range from each other, the data can be transferred through the internet where both devices  202 ,  204  have wireless access such as Wi-Fi. The Patient Programmer is capable of changing the stimulation parameters on the trial generator  107  or IPG  102  though the RF wireless dongle  200  when the trial generator  107  or IPG in the patient is within the RF range. However, the Patient Programmer has limitations to changing the stimulation parameters. 
     Tuohy Needle 
     The tuohy needle  240 , as depicted in  FIG. 15 , is used in conjunction with a saline-loaded syringe for loss-of-resistance needle placement, and percutaneous stimulation leads  140 , for lead  140  placement into the spinal canal. The tuohy epidural needle  240  is inserted slowly into the spinal canal using a loss-of-resistance technique to gauge needle  240  depth. Once inserted to the appropriate depth, the percutaneous stimulation lead  140  is passed through the needle  240  and into the spinal canal. 
     The epidural needle  240  is a non-coring  14 G stainless steel spinal needle  240  and will be available in lengths of 5″ (127 mm) and 6″ (152.4). The distal tip  242  of the needle  240  has a slight curve to direct the stimulation lead  140  into the spinal canal. The proximal end  246  is a standard Leur-Lock connection  248 . 
     Stylet 
     The stylet  250 , as depicted in  FIG. 16 , is used to drive the tip of a percutaneous stimulation lead  140  to the desired stimulation zone by adding rigidity and steerability. The stylet  250  wire  252  passes through the center lumen  142  of the percutaneous lead  140  and stops at the blocking plug at the distal tip of the lead  140 . The tip of the stylet  250  comes with both straight and curved tips. A small handle  254  is used at the proximal end of the stylet  250  to rotate the stylet  250  within the center lumen  142  to assist with driving. This handle  254  can be removed and reattached allowing anchors  170  to pass over the lead  140  while the stylet  250  is still in place. The stylet  250  wire  252  is a PTFE coated stainless steel wire and the handle  254  is plastic. 
     Passing Elevator 
     The passing elevator  260 , as depicted in  FIG. 17 , is used prior to paddle lead  141  placement to clear out tissue in the spinal canal and help the surgeon size the lead to the anatomy. The passing elevator  260  provides a flexible paddle-shaped tip  262  to clear the spinal canal of obstructions. The flexible tip is attached to a surgical handle  264 . 
     The passing elevator  260  is a one-piece disposable plastic instrument made of a flexible high strength material with high lubricity. The flexibility allows the instrument to easily conform to the angle of the spinal canal and the lubricity allows the instrument to easily pass through tissue. 
     Tunneling Tool 
     The tunneling tool  270 , as depicted in  FIG. 18 , is used to provide a subcutaneous canal to pass stimulation leads  140  from the entrance point into the spinal canal to the IPG implantation site. The tunneling tool  270  is a long skewer-shaped tool with a ringlet handle  272  at the proximal end  274 . The tool  270  is covered by a plastic sheath  276  with a tapered tip  278  which allows the tool  270  to easily pass through tissue. Once the IPG  102  implantation zone is bridge to the lead  140  entrance point into the spinal canal, the inner core  275  is removed, leaving the sheath  276  behind. The leads  140  can then be passed through the sheath  276  to the IPG  102  implantation site. The tunneling tool  270  is often bent to assist in steering through the tissue. 
     The tunneling tool  270  is made of a 304 stainless steel core with a fluorinated ethylene propylene (FEP) sheath  276 . The 304 stainless steel is used for its strength and ductility during bending, and the sheath  276  is used for its strength and lubricity. 
     Torque Wrench 
     The torque wrench  280 , as depicted in  FIG. 19 , is used in conjunction with the IPG  102 , lead extension  150  and lead splitter  160  to tighten the internal set screw  119 , which provides a radial force against the fixation contact of the stimulation leads  140 ,  141 , preventing the leads  140 ,  141  from detaching. The torque wrench  280  is also used to lock and unlock the anchor  170 . The torque wrench  280  is a small, disposable, medical instrument that is used in every SCS  100  case. The torque wrench  280  provides audible and tactile feedback to the surgeon that the lead  140 ,  141  is secured to the IPG  102 , extension  150 , or splitter  160 , or that the anchor  170  is in the locked or unlocked position. 
     The torque wrench  280  is a 0.9 mm stainless steel hex shaft  282  assembled with a plastic body  284 . The wrench&#39;s  280  torque rating is bi-directional, primarily to provide feedback that the anchor  170  is either locked or unlocked. The torque rating allows firm fixation of the set screws  119 ,  152  against the stimulation leads  140 ,  141  without over-tightening. 
     Trial Patch 
     The trial patch is used in conjunction with the trialing pulse generator  107  to provide a clean, ergonomic protective cover of the stimulation lead  140 ,  141  entrance point in the spinal canal. The patch is also intended to cover and contain the trial generator  107 . The patch is a large, adhesive bandage that is applied to the patient post-operatively during the trialing stage. The patch completely covers the leads  140 ,  141  and generator  107 , and fixates to the patient with anti-microbial adhesive. 
     The patch is a watertight, 150 mm×250 mm anti-microbial adhesive patch. The watertight patch allows patients to shower during the trialing period, and the anti-microbial adhesive decreases the risk of infection. The patch will be made of polyethylene, silicone, urethane, acrylate, and rayon. 
     Magnetic Switch 
     The Magnetic switch is a magnet the size of a coin that, when placed near the IPG  102 , can switch it on or off. The direction the magnet is facing the IPG  102  determines if the magnetic switch is switching the IPG  102  on or off.