Abstract:
An implantable lead having at least one electrode contact at or near its distal end prevents undesirable movement of the electrode contact from its initial implant location. One embodiment relates to a spinal cord stimulation (SCS) lead. A first injectable material is injected into the dura space to mechanically position the electrode array with respect to the spinal cord. Conjunctively for use with adhesives, or alternatively for use instead of the adhesives, a balloon may be positioned on the electrode lead array. The balloon is filled with air, liquid or a compliant material. When inflated, the balloon stabilizes the lead with respect to the spinal cord and holds the lead in place. An elastic aspect of the balloon serves as a contained relief valve to limit the pressure the balloon may place on the surrounding tissues when the epidural space is constrained.

Description:
The present application is a Continuation of U.S. application Ser. No. 10/155,146, filed May 24, 2002, now U.S. Pat. No. 7,099,718 issued Aug. 29, 2006; which claims the benefit of U.S. Provisional Application Ser. No. 60/294,283, filed May 29, 2001, which applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a neural stimulation systems, e.g., a spinal cord stimulation system. More particularly, the invention relates to neural stimulation leads that include lead fixation means, i.e., ways to assure that the lead, once implanted, does not move away from its desired implant location. 
     Spinal cord stimulation (SCS) is a well accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an implanted pulse generator, lead wires, and electrodes connected to the lead wires. The pulse generator generates electrical pulses that are delivered to the dorsal column fibers within the spinal cord through the electrodes which are implanted along the dura of the spinal cord. In a typical situation, the attached lead wires exit the spinal cord and are tunneled around the torso of the patient to a sub-cutaneous pocket where the pulse generator is implanted. 
     When an electrical pulse, or sequence of pulses, is applied to a selected electrode, or combination of electrodes, the patient typically experiences a “paresthesia” (usually manifest as a mild tingling sensation) that is therapeutic, i.e., that relieves the pain, or other discomfort, the patient is experiencing, or that otherwise aids the patient in some useful way. 
     Spinal cord and other stimulation systems are known in the art. For example, in U.S. Pat. No. 3,646,940, there is disclosed an implantable electronic stimulator that provides timed sequenced electrical impulses to a plurality of electrodes so that only one electrode has a voltage applied to it at any given time. Thus, the electrical stimuli provided by the apparatus taught in the &#39;940 patent comprise sequential, or non-overlapping, stimuli. 
     In U.S. Pat. No. 3,724,467, an electrode implant is disclosed for the neuro-stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided with a plurality of electrodes formed thereon. The electrodes are connected by leads to an RF receiver, which is also implanted, and which is controlled by an external controller. The implanted RF receiver has no power storage means, and must be coupled to the external controller in order for neuro-stimulation to occur. 
     In U.S. Pat. No. 3,822,708, another type of electrical spinal cord stimulating device is shown. The device has five aligned electrodes which are positioned longitudinally on the spinal cord and transversely to the nerves entering the spinal cord. Current pulses applied to the electrodes are said to block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes are approximately 250 microseconds in width with a repetition rate of from 5 to 200 pulses per second. A patient-operable switch allows the patient to change which electrodes are activated, i.e., which electrodes receive the current stimulus, so that the area between the activated electrodes on the spinal cord can be adjusted, as required, to better block the pain. 
     Other representative patents that show spinal cord stimulation systems or electrodes include U.S. Pat. Nos. 4,338,945; 4,379,462; 4,519,403; 5,121,754; 5,417,719 and 5,501,703, incorporated herein by reference. 
     The &#39;403 patent teaches a balloon lead and inflator. However, the &#39;403 patent does not teach the use of a safety valve to release pressure that might build up, or otherwise be present, when such lead is used. 
     U.S. Pat. No. 5,733,322, also incorporated herein by reference, discloses a positive fixation percutaneous epidural neurostimulation lead that utilizes an extension that extends distally beyond the most distal electrode. The extension is held in place by contact with both the dura and spinal canal wall so that lateral lead migration of the electrodes is minimized. Other electrode fixation techniques are taught, e.g., in U.S. Pat. No. 4,418,697, which describes an adhesive (putty) to fixate electrodes to the skin; and in U.S. Pat. No. 4,282,886, which describes an adhesive adapted to attach an electrode to the epicardium. Both the &#39;697 and the &#39;886 patents are likewise incorporated herein by reference. 
     Disadvantageously, when a neural stimulation lead chronically or temporarily moves, the ramifications to the patient are multifold. For example, an SCS lead that moves up, down or rotates to the side of the spinal cord can result in therapy no longer being adequate to attain the desired paresthesia, thereby rendering the SCS system incapable of performing its intended function. On a temporary basis, the lead can move, which movement may thereafter require patient adjustment of the stimulation energy, e.g., to reduce the stimulation output or increase the stimulation output, and in some instances such adjustment of the stimulation energy may not be possible, thereby again rendering the SCS system ineffective for its intended purpose. 
     Thus, it is seen that maintaining the correct lead position is critical, and an undesirable movement of the lead can render the SCS, or other neural stimulation system, ineffective and useless. Moreover, as the correct lead position is maintained, appropriate safeguards must exist to prevent excessive compression of the spinal cord. What is needed are mechanisms built into the lead design that: (1) acutely fixate the lead to its desired location, e.g., to the dura in the case of an SCS system, to thereby allow scare maturation to occur (which scare maturation will thereafter permanently maintain the lead in its desired location); or (2) permanently fixate the lead in its desired location from the onset of implantation; and (3) limit compression on the spinal cord. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing an implantable neural lead having an electrode array at or near its distal end that prevents undesirable movement of the electrode array from its initial implant location by filling the space that surrounds the lead into which the lead could otherwise move, and/or that adheres, i.e., “glues”, the electrode array to its desired implant location. 
     In the case of an SCS lead, movement of the lead array is predominately caused due to the geometry of the epidural space. That is, the epidural space allows the spinal cord to independently move with respect to the vertebra, disks and ligaments which comprise the structural support of the spine. In one embodiment of the invention, a first injectable material that adheres to the dura is injected into the dura space to mechanically position the electrode array with respect to the spinal cord. This first adhering material is introduced through the lumen of the lead array. An orifice to the lumen, which may be marked with a radio opaque marker, is located on the side of the lead where the attachment is to occur so that the adhering material is emitted from the lead tangential to the lead body at the level of the stimulating electrodes. 
     In a second embodiment, used in conjunction with or as an alternative to the first embodiment, a balloon is positioned on the electrode lead array. Such balloon runs along the axis of the lead at the stimulating electrode level and encompasses a significant portion, e.g., 180 degrees, of the arc when a cross section of the lead is viewed. The balloon is filled with air, liquid or a compliant material. When inflated, the balloon stabilizes the lead with respect to the spinal cord. The pressure of the balloon is monitored or otherwise controlled during the filling process in order to determine at what point the filling process should be discontinued. The balloon may be segmented into several balloon compartments, each having different dilated balloon diameters to account for different geometry between and within a given patient. An elastic aspect of the balloon serves as a contained relief valve to limit the pressure the balloon may place on the surrounding tissues when the epidural space is constrained. 
     In accordance with one aspect of the invention, an implantable lead is thus provided that is fixed to its desired implant location. 
     In accordance with another aspect of the invention, a lumen within the lead, used to allow a removable stylet to help place and position an electrode array at or near a distal end of the lead during the implant process, may also be used as the channel through which the first adhering material is dispensed. Such dispensing may occur either through insertion of a cannula within the lumen (followed by dispensing the first adhering material within the cannula) or direct injection of the first adhering material into the lumen. Additionally, in one embodiment such channel further serves as a fluid inlet (where “fluid” is used here in its broad sense to mean either a liquid or a gas) through which a balloon formed in the lead near or at the location of the electrode array may be inflated to a desired size and/or pressure. In another embodiment, a separate fluid channel may be formed through the lead body through which the balloon may be inflated. In yet another embodiment, the first adhering material also functions as the fluid that inflates the balloon(s). 
     In accordance with still a further aspect of the invention, the lead includes an implanted pressure release mechanism to ensure that there is not excessive compression placed on the spinal cord by the implantable lead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  illustrates a representative spinal cord stimulation system implanted in a patient; 
         FIG. 2  is a sectional view that depicts the electrode array of the present invention implanted adjacent the spine; 
         FIG. 3  is a first side view of the electrode array of the present invention, with the balloon inflated, as viewed while looking at the back side of the array, i.e., that side opposite from the orifice through which an adhering material may be dispensed; 
         FIG. 3A  is a sectional view of the array shown in  FIG. 3  taken along the line  3 A- 3 A, and illustrates the balloon in both an inflated and deflated condition; 
         FIG. 3B  is a sectional view of the array shown in  FIG. 3  taken along the line  3 B- 3 B, and illustrates the balloon in an inflated condition; 
         FIGS. 4 and 4A  depict an alternative embodiment wherein a series of side lobe balloons are employed with an electrode array; 
         FIGS. 5 and 5A  similarly show an alternative embodiment wherein a series of lateral stabilizing balloons are employed with the electrode array; and 
         FIG. 6  shows another alternative embodiment wherein a spongy, flexible or compressive portion is employed with an electrode array. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
     At the outset, it should be noted that the present invention is directed to the fixation of implantable leads, such as neural stimulation leads or cardiac leads, and more particularly to the fixation of electrodes, or electrode arrays, attached to neural stimulation leads or cardiac leads so that such electrodes, or electrode arrays, remain in a desired position relative to the tissue that is to be stimulated. For purposes of the present invention, the terms “lead” and “electrode” and “electrode array” may thus be used interchangeably, unless the context clearly indicates otherwise. That is, while the narrow purpose of the invention is to fix the electrodes relative to the tissue to be stimulated, in describing such purpose other terminology may be used, such as fixing the lead, or fixing the electrode array. 
     It should further be noted that the principles and teachings of the invention may be used with any kind of neural stimulation lead, or cardiac lead, particularly those that are implanted within a tissue cavity (the presence of which cavity allows undesirable movement of the lead). Thus, while the invention will be described in terms of a spinal cord stimulation (SCS) lead adapted for implantation in the epidural space next to the spine, it is to be understood that such description is intended to be only exemplary and not limiting. The scope of the invention is determined by the claims. 
     Turning first to  FIG. 1 , there is shown a representative SCS system  10  implanted in a patient  11 . The SCS system  10  is used typically to treat chronic pain by applying electrical stimulation pulses to selected locations along the spine. The SCS system  10  includes an Implantable Pulse Generator (IPG)  12  that generates the electrical stimulation pulses used for stimulation. An electrode array  14  at or near the distal end of an implanted stimulation lead  15  is inserted into the epidural space next to the spinal cord  18 . As required, depending upon the location where the IPG  12  is implanted, a lead extension  16  may be used to connect the lead  15  (and hence the electrode array  14 ) to the IPG. The electrical stimulation provided by the IPG  12 , when properly performed, has the effect of blocking sensed pain while not interfering with other nerve functions. The present invention relates to the electrode array  14  and the manner used to fix the location of the electrode array  14  relative to the spinal cord  18 . 
       FIG. 2  schematically illustrates a sectional view of the epidural space  20  adjacent the spinal cord  18  into which the implantable lead  15 , and more particularly the electrode array  14  at the distal end of the lead  15 , of the present invention is implanted. As seen in  FIG. 2 , the electrode array  14  typically includes a multiplicity of spaced-apart electrode contacts  24 . Such electrode contacts  24  may reside along one side of the array  14 , or may be bands that encircle a body  15 A of the lead  15 . Each electrode contact  24  is electrically connected to a respective wire  26  that is embedded, or otherwise located within, the body  15 A of the lead  15 . For the example shown in  FIG. 2 , the wires  26  are helically wound, thereby allowing the lead  15  to readily flex and bend. The inside of the helically wound wires  26  defines a lumen  22  that passes longitudinally through the body of the lead. A proximal end of the wires  26  (not shown in  FIG. 2 ) connects to the IPG  12  either directly or through an extension lead  16  ( FIG. 1 ). It is to be understood that a lead having helically-wound wires  26  therein represents only one way in which an implantable neural stimulation lead may be formed. Any method known in the art, or yet to be developed in the art, for electrically connecting an electrode contact  24  to an implantable pulse generator may be used with the present invention. 
     The body  15 A of the lead  15  may be made from any suitable biocompatible material, such as silicone or Silastic or other compliant material, as is known in the art, or as will yet be developed in the art. The body  15 A of the lead  15  may also be referred to as a flexible carrier on which the spaced-apart electrode contacts  24  are carried. 
     At least one orifice  27  is formed in the body  15 A of the array  14  near the electrode contacts  24 . The location of the orifice  27  along the lead body  15 A may be marked with a radio opaque marker  29 . It is through this orifice  27  that a first adhering material  30 , e.g., an adhesive, may be dispensed through the lumen  22  of the lead  15  to the area where the electrode array  14  is to be attached, e.g., adhered or glued, to the spinal cord  18 . (This adhering material  30  is represented in  FIG. 2  by closely spaced small dots.) 
     Rather than dispensing the adhesive through a lumen  22  that passes through the lead body, it is also possible, in accordance with an alternative embodiment of the invention, to provide a cannula, or similar tubing, that passes along one side of the lead body, e.g., parallel to the flexible carrier, with an opening or orifice near the electrode contacts. 
     It should be noted that the adhesive may have properties that cause it to adhere to the body tissue and to (1) the electrode contacts  24 , or (2) insulative material separating the electrode contacts, or (3) both the electrode contacts  24  and the insulative materials separating the electrode contacts. In some embodiments of an implantable lead, in the region near the electrode contacts  24 , the electrode contacts are separated by insulative material, which is non-conductive. Thus, for purposes of maintaining or holding the position of the electrode contacts near or against the tissue to be stimulated, it matters little whether the adhesive bonds with the electrode contacts, and/or with the insulative material that separates or insulates the electrode contacts from each other, or other insulative material, in the vicinity of the electrode contacts. However, as discussed below, it is generally preferable that the adhesive not bond with a compliant material, i.e., that it not bond with silicone or other material from which the lead body  15 A is made. Thus, in most instances, except when an insulative or other material is used near the electrodes that is different from silicone, or other material from which the lead body is made, the adhesive preferably bonds with the electrode contacts and with the surrounding tissue. 
     The adhering material  30  has properties that allow it to be injected through the lumen  22  and orifice  27  of the lead  15 . Additionally, the material  30  should have properties that cause it to adhere to the dura  19  (an outer layer that surrounds the spinal cord  18 ) and the electrode contacts  24 , but which do not cause it to adhere to the dura space lining or a compliant material (e.g., the silicone or other material from which the lead body  15 A is made). By way of example, the first adhering material  30  may be selected from the group that includes cyanoacrylate, fibrin, reconstituted collagen, polyethylene glycol, and polyacrylamide. 
     It is thus seen that the present invention addresses the use of an adhesive to fix the actual electrode itself to the tissue. The preferred embodiment contemplates injecting the adhesive through the lumen  22  and orifice  27  of the lead  15 . However, other techniques may also be used, particularly for other types of lead and electrode configurations. For example, the adhesive may be placed prior to, during or after the implantation of the electrode(s). The adhesive is advantageously formulated to be gentle to the tissue, but assures that the electrode(s) remain in the desired proper location and orientation because the fixation is on the electrode itself. The adhesive may be used in conjunction with traditional methods, e.g., fixation at a distance. The adhesive may also be made soft or hard, permanent or temporary, and can be made to be biocompatible. The adhesive may be synthetic, e.g., cyanoacrylate, or it may be a substance that is natural to the body, e.g., fibrin glue. 
     Returning to  FIG. 2 , it is further seen in  FIG. 2 , the electrode array  14  also includes a balloon  40  that, when inflated, fills the remaining epidural space  20  surrounding the electrode array  14  and lead  15 . Further details relating to the balloon  40  are discussed below in conjunction with the description of  FIGS. 3-5 . While both the balloon  40  and adhesive  30  are illustrated as being used in connection with the same lead  15  in  FIG. 2 , such conjunctive use is only exemplary. In some instances, only the adhesive  30  need be used to fix the electrode contacts to a desired location. In other instances, only the balloon  40  need be used. 
     Typically, after removal of the stylet from the lumen  22 , the adhering material  30  is injected into the lumen  22 . As it is injected, it fills the distal end of the lumen first, and then exits out the orifice  27  into the space adjacent the spinal cord  18  where the adhesion occurs between the dura  19  and electrode contacts  24 . Because the volume of the lumen at the distal end of the lead up to the location of the orifice  27  is known, or can be readily calculated (i.e., the orifice  27  is a known distance D from the tip of the electrode array  14 , and the distance d 1  from the tip to the beginning of the lumen  22  is also known or can be readily estimated; thus, the approximate volume of the lumen  22  up to the location of the orifice  27  can be determined), a desired amount (volume) of material  30  to be injected through the lumen  22  can be determined. 
     After the proper amount, e.g., volume, of adhesive or other adhering material  30  has been injected into the lumen  22 , a second material, e.g., a gas, such as air, or a liquid which does not mix with the first material  30 , may also be injected into the lumen  22 , forcing the first adhering material  30  to the distal end of the lumen, and through the first orifice  27 , as desired. This second material may then pass through a second orifice  28  in order to inflate the balloon  40  to a desired pressure. Alternatively, where the first material  30  does not readily adhere to compliant materials, such as the materials from which the body  15 A of the lead  15  is made, and from which the balloon  40  is made, the first material  30  may, in addition to being injected through the orifice  27  in order to adhere the dura  19  to the electrode contacts  24 , also be used as the substance that inflates the balloon  40  to a desired pressure. As a still further means of inflating the balloon, the balloon  40  may be inflated to a desired pressure and/or volume by injecting an inflating material, e.g., a saline solution, through a separate channel  25  apart from the lumen  22 , as shown in  FIG. 3B , discussed below. 
     Turning next to  FIGS. 3 ,  3 A and  3 B, further details associated with one embodiment of the electrode array  14  of the present invention are illustrated.  FIG. 3  is a first side view of the electrode array  14 , with the balloon  40  inflated, as viewed while looking at the back side of the array, i.e., that side opposite from the orifice  27  through which the adhering material  30  is dispensed.  FIG. 3A  is a sectional view of the array shown in  FIG. 3  taken along the line  3 A- 3 A, and illustrates the balloon  40  in both an inflated and deflated condition.  FIG. 3B  is a sectional view of the array shown in  FIG. 3  taken along the line  3 B- 3 B, and illustrates the balloon  40  in an inflated condition. As seen in  FIGS. 3 ,  3 A and  3 B, the orifice  27  through which the adhering material is dispensed is located near the electrode contacts  24 . The balloon  40  inflates around the portion of the electrode array  14  where the electrode contacts  24  are located. As seen best in the cross-sectional view of the lead  15  shown in  FIG. 3A , the balloon  40  preferably encircles the lead  14  over an angle α. The angle α will typically range from about 20 to 180 degrees, but may, in some embodiments, range from about 20 to 270 degrees. 
     For the embodiment of the invention shown in  FIGS. 3 ,  3 A and  3 B, the balloon  40  is inflated through a channel  25  and orifice  28 ′ that is separate from the lead lumen  22  and orifice  27 . When the balloon  40  is inflated, it is preferred to use some means for monitoring the injected material pressure so as to determine an appropriate point at which the inflation should terminate. Such means may be simply injecting a known volume of inflating fluid. Alternatively, a pressure gauge may be used in conjunction with the inflating process so that the pressure of the inflating fluid can be accurately measured. 
     Also, for the embodiment of the invention shown in  FIGS. 3 ,  3 A and  3 B, the balloon  40  includes sections  42  that have different elastic properties than the main balloon portion, e.g., are made so as to have a thicker wall than the main balloon portion. Thus, these balloon sections  42  require more pressure to inflate. As such, the balloon portions  42  function as a contained relief valve to limit the pressure the balloon  40  may place on the surrounding tissue when the epidural space is constrained (which may occur, for example, when the patient bends his back or spine). Advantageously, limiting the pressure that the balloon  40  can impart to the surrounding tissue through use of such contained relief values  42  helps assure that the patient will not experience any undue discomfort as he or she moves about. In this way, excessive compression on the spinal cord is avoided. 
     Next, with reference to  FIGS. 4 and 4A , an alternative embodiment of the invention is illustrated wherein a sequence of lobe balloons  41  are employed with an electrode array  14 . Such embodiment includes a sequence of three lobe balloons  41  for each electrode contact  14 . Such number of lobe balloons in each sequence is only exemplary. Further, while the embodiment shown in  FIGS. 4 and 4A  has one sequence of lobe balloons associated with each electrode contact, it is also possible to have a sequence of lobe balloons associated with only selected electrode contacts, e.g., a most distal electrode contact, a most proximal electrode contact, and/or a middle electrode contact. Each lobe balloon  41  includes a pressure relief portion  43 . 
     Further, with reference to  FIGS. 5 and 5A , yet another embodiment of the invention is depicted wherein a series of lateral stabilizing balloons  40 ′ are employed with the electrode array  14 . Each stabilizing balloon  40 ′ includes a pressure relief portion  42 ′. The number of lateral stabilizing balloons  40 ′ will typically be at least two, and could be as many as four or five, or more, depending upon the length of the electrode array  14 . 
     In addition to the pressure relief portions  42  ( FIGS. 3A ,  3 B),  42 ′ ( FIG. 5) and 43  ( FIG. 4 ), which form part of the main balloon portion but with a thicker wall, other types of pressure relief mechanisms may be employed by the invention. For example, other highly compliant expandable positioning systems, such as foam, sponge or other lattice materials may be employed by the invention. Indeed, the invention contemplates any pressure relief mechanism built into the lead that ensures excessive compression of the spine is avoided. Thus, as illustrated in  FIG. 6 , a spongy, flexible and/or compressible portion  50  is formed on the lead  15 , opposite the electrodes  24 . While the spongy, flexible and/or compressible portion  50  may be an elongate member, as shown in  FIG. 6 , it should also be understood that such portion(s)  50  may be at the same location and using the same or similar patterns, as are employed by the balloon portions previously described. 
     The portion  50  is made from a silastic spongy material that is both flexible and compressible. A suitable flexible and compressible material that may be used for the portion  50  is, e.g., polyurethane, or equivalents. The portion  50  may be fabricated as: (1) an open cell material that is collapsed through application of a vacuum on the lumen and/or implant insertion, with the vacuum being removed after insertion, allowing the open cell material to open from its collapsed state; or (2) a balloon filled with a foam, which foam is flexible and compressible. It is to be emphasized that any material that is compatible with body tissue and sufficiently flexible and compressible may be used for the portion  50 , including materials that may not yet be developed. 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.