Abstract:
Disclosed is a lead for percutaneous insertion into an epidural space of a spinal canal, which includes an elongated lead body having opposed proximal and distal end portions. At least one electrode for stimulating a patient is operatively associated with the distal end portion of the lead body. Structure for conducting signals extends through the lead body to connect the electrode to a connecting structure operatively associated with the proximal end portion of the lead body. The connecting structure is capable of engaging a signal generator such that signals can be conducted from a signal generator to the electrode. The distal end portion of the lead body is adapted for movement between a first state, in which the distal end portion has a generally linear configuration, and a second state, in which the distal end portion has an undulating configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The subject application claims the benefit of commonly-owned, co-pending U.S. Provisional Patent Application Ser. No. 60/602,191, filed on Aug. 17, 2004, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a lead for electrically stimulating a spinal cord and more particularly to an apparatus and method for fixing or otherwise securing such a lead in the epidural space of a spinal column to inhibit lateral lead migration.  
         [0004]     2. Background of the Related Art  
         [0005]     The basic process by which humans perceive pain begins with the generation of pain signals by nocioreceptors. These pain sensors, which are located throughout the body at the extremities of peripheral nerve fibers, generate pain signals in response to stimuli such as increased pressure, elevated temperature, or chemical alterations. The pain signals generated by the nocioreceptors are transmitted along the peripheral nerve fibers to the spinal cord, from which the peripheral nerve fibers emanate. Once pain signals reach the spinal cord, they propagate along the spinal cord to the brain where the signals are processed and perceived as pain.  
         [0006]     The transmission of pain signals is enabled by the multitude of neurons that make up the peripheral nerve fibers and the spinal cord (as well as the brain). Each neuron contains mobile ions that rearrange within the neuron in response to a pain signal to create a potential drop across the neuron. In this way, a pain signal gives rise to an electrical impulse that travels across the neuron. This electrical impulse cannot, however, travel to neighboring neurons, as the neurons making up the nerves and spinal cord are not in electrical contact with one another. Instead, as an electrical impulse representing pain travels across a neuron, the neuron releases a chemical that travels to and reacts with adjacent neurons, causing those neurons to establish the pain-indicating potential drop. In this way, pain signals propagate as an alternating series of electrical impulses (along neurons) and chemical reactions (between neurons).  
         [0007]     In many cases, pain results from discrete causes, such as disease, inflammation, or traumatic injury to tissues, which can be identified and treated. This type of pain is referred to as “acute” pain, and is treated by treating the condition causing the pain, with the pain subsiding as the underlying condition is cured. In other cases, pain persists indefinitely (either in a continuous or intermittent manner) despite the completion of the healing process. Such “chronic” pain can happen, for example, when the body is subject to a degenerative condition, such as arthritis, that cannot be healed. Damaged nerves can also cause chronic pain, by generating pain signals even in the absence of a real stimulus or tissue damage. In some rare instances, initially acute pain can become chronic. In any event, chronic pain is associated with a condition that is relatively immune to medical treatment. As such, it is necessary to continually treat the pain independently of any condition that may have given rise to the pain.  
         [0008]     One of the most historically common treatments of chronic pain was through medication. As mentioned, the transmission of pain signals to the brain involves a series of alternating electrical impulses and chemical reactions. Medications can be used to disrupt the chemical reactions and “block” pain impulses from reaching the brain. Common medications utilized in blocking pain impulses include morphine and other opioid drugs. However, while such treatment is generally effective in relieving pain, continued use of a morphine-like drug can lead to patient sedation, and has the potential to cause addiction. Further, patients receiving morphine also face the problem of morphine tolerance, meaning that, over time, they require increasingly higher doses of the drug to achieve the same level of pain relief.  
         [0009]     Relatively recently, it has been found that establishing an electric field around the spinal cord can serve to effectively reduce or alleviate pain. The electric field interacts with the electrical portion of the pain signal and thereby blocks the transmission of pain impulses along the spinal cord, creating an impaired sensation of the body known as parasthesia. In practice, an electric field is established in the vicinity of the spinal cord by surgically implanting a signal generator and running an electrical lead from the generator to a location adjacent to the spinal cord. This electrical lead is known as a neurological epidural lead. While the implantation of a neurological epidural lead is inappropriate for the temporary treatment required for acute pain due to its invasive nature, the procedure has found use in the continuous treatment of chronic pain.  
         [0010]     An example of a typical neurological epidural lead implanted in a spinal canal is shown in  FIGS. 1 and 1   a , generally labeled  10 . Lead  10  has an elongated, substantially linear lead body  12  with opposed proximal  14  and distal  16  end portions, and includes at least two electrodes  18  associated with the distal end portion  16 . The lead  10  is located in the epidural space  70  of the spinal canal  71  (the space between the spinal canal wall  72 , defined by the ligamentum flavum  73 , the vertebrae  74 , and the intervertebral discs  76 , and the spinal cord  75 ), such that the electrodes  18  are located in close proximity to the spinal cord  75 . The proximal end portion  14  of lead body  12  interfaces with a pulse generator (not shown), such as an implantable pulse generator (IPG) located at a separate location within the body of the patient. Conductive wires (not shown) extend through lead body  12  to operatively connect electrodes  18  to the pulse generator. An electrical potential is applied between pairs of the electrodes  18 , and the resulting electric field pervades the spinal column  77  and initiates parasthesia in the patient.  
         [0011]     To place the lead  10  in the epidural space  70 , a needle is percutaneously inserted through the ligamentum flavum  73 . The lead  10  is then passed through the needle and into the epidural space  70 , after which the needle is removed. The lead  10  is then manually guided along the spinal canal  71  to the desired location.  
         [0012]     While treatment involving the use of the above-described lead has proven somewhat effective, recent studies have indicated that ˜25% of patients who undergo this procedure with initially favorable results experience a subsequent deterioration in therapeutic effectiveness. It is believed that this failure in treatment is caused by post-implantation migration of the electrodes, which, even for movements as small as one millimeter, can cause a significant change in the amount and location of parasthesia induced by lead  10 . As such, it is important that the leads remain fixed in place after placement in the epidural space.  
         [0013]     To prevent axial movement of the lead  10 , a stop  40  ( FIG. 1 ) is placed along lead body  12  outside spinal column  77  near the point where lead  10  passes through ligamentum flavum  73 . Stop  40  is sutured to surrounding tissue to prevent lead  10  from moving axially. Transverse movement (i.e., with respect to the long axis of the lead) of the proximal end portion  14  of the lead body  12  is restricted by the surrounding ligamentum flavum  73 .  
         [0014]     Several methods have been described in the prior art for preventing transverse movement of the distal end portion  16  ( FIG. 1 ) of the lead body  12 . First, and most traditionally, the compression of the lead  1  between the spinal cord  75  and the spinal canal wall  72  has been relied upon to secure the lead  10 . This tactic, however, has proven unreliable, and it is now believed that excessive lateral migration of the distal end portion of lead occurs fairly regularly in this arrangement.  
         [0015]     Others have added a protruding structure to the distal end of the lead body. This protruding structure causes the distal end to anchor into the tissue around the distal end, thus preventing the distal end from moving laterally. Because the distal end cannot move laterally, the lead&#39;s electrodes are similarly prevented from moving laterally. An example of this type of lead anchoring system is disclosed in U.S. Pat. No. 5,344,439 to Otten.  
         [0016]     However, the lead anchoring systems, such as in Otten, that rely on protruding structures at the distal end of the lead suffer from a drawback related to the physiology of the spinal column. Referring again to  FIG. 1 , recent research has revealed that the epidural space  70  is not merely the flattened space between the spinal cord  75  and the spinal canal wall  72 . 1  Rather, as shown in  FIG. 1 , the epidural space  70  alternately widens (between vertebra  74 , where the spinal canal wall  72  is mainly defined by the ligamentum flavum  73 ) and narrows (within vertebra  74 , where the spinal cord  75  is in substantial contact with the spinal canal wall  72 ) along the spinal column  77 . Consequently, if a lead with a protruding anchoring fixture at the distal end, such as is shown in Otten, was placed in an epidural space such that the distal end was located in a wider portion of the epidural space, there would be insufficient contact between the anchoring protrusion and the spinal canal to prevent lead from moving laterally.    1 Quinn H. Hogan, “Lumbar Epidural Anatomy, A New Look by Cryomicrotome Section,” in Anesthesiology, vol. 75(5), pp. 767-775 (1991).    
         [0017]     U.S. Pat. No. 4,538,624 to Tarjan and U.S. Pat. No. 4,549,556 to Tarjan et al., disclose methods of anchoring neurological epidural leads. As disclosed by these patents, an extension extends distally beyond the most distal electrode and terminates in an extension end. The lead is introduced percutaneously into the epidural space through a needle, similar to the process described above. The lead is positioned with the electrodes in the desired location, the extension extending within epidural space distally beyond electrodes. The epidural space is then accessed at a location near extension end, and the extension end is manually retrieved and anchored outside the spinal column. While this procedure results in a securely anchored lead, the process of retrieving and anchoring extension end is difficult and requires an additional puncture to and resulting opening in the spinal canal wall. It is desirable to find a way to anchor distal end  6  easily and without having to puncture the spinal canal wall.  
         [0018]     U.S. Pat. No. 5,733,322 to Starkebaum, incorporated herein by reference in its entirety, describes a positive fixation mechanism, including an extension that extends distally beyond the most distal electrode. Implantation is achieved by having the extension placed in a very narrow area of the epidural space. Placement of the extension inside such a narrow area, however, can be very time-consuming and cumbersome.  
         [0019]     In all, it is desirable to have a neurological epidural lead that is easily implanted into an epidural space and is adapted to restrict movement of the lead with respect to the spinal cord.  
       SUMMARY OF THE INVENTION  
       [0020]     The present invention addresses the problems outlined above by providing a novel neurological epidural lead. The novel lead provides a simplified manner for effectively inhibiting lead migration after placement in an epidural space. At the same time, the lead structure allows the lead to be easily directed through the body during lead implantation and placement.  
         [0021]     In one embodiment of the subject invention, a lead for percutaneous insertion into an epidural space of a spinal canal has an elongated lead body with opposed proximal and distal end portions. At least one electrode for stimulating a patient is operatively associated with the distal end portion of the lead body. Conductor means for conducting signals extends through the lead body to connect the electrode to connector means operatively associated with the proximal end portion of the lead body. The connector means is capable of engaging a signal generator such that signals can be conducted from a signal generator to the electrode. The distal end portion of the lead body is adapted for movement between a first state, in which the distal end portion has a generally linear configuration, and a second state, in which the distal end portion has an undulating configuration. The generally linear configuration of the first state facilitates passing the lead through a body and into the epidural space and the undulating configuration of the second state causes the distal end portion of the lead body, once situated within the epidural space, to exert outward force on structures defining the spinal canal, thereby affixing the lead within the spinal canal.  
         [0022]     In a particular embodiment, at least part of the distal end portion of the lead body is formed of a mechanically elastic material and has an undulating configuration. The lead further comprises a substantially linear stiffening member that selectively extends axially through the distal end portion of the lead body to force the distal end portion of the lead body to assume the generally linear configuration of the first state. The distal end portion of the lead body assumes the undulating unloaded configuration of the second state when the stiffening member is retracted. Preferably, the mechanically elastic material is capable of undergoing a solid-state phase transformation.  
         [0023]     The subject invention is also directed to a method for implanting a device for treating pain in a patient. A lead is provided for percutaneous insertion into an epidural space of a spinal canal of the patient. The lead includes an elongated lead body having opposed proximal and distal end portions, wherein the distal end portion of the lead body is adapted for movement between a first state, in which the distal end portion has a generally linear configuration, and a second state, in which the distal end portion has an undulating configuration. A stylet is positioned within the lead body such that the distal end portion of the lead body has the generally linear configuration of the first state. The lead is percutaneously inserted into the epidural space of the patient. The stylet is then retracted such that the distal end portion of the lead body assumes the undulating shape of the second state and contacts structures defining the spinal canal, thereby affixing the lead within the spinal canal.  
         [0024]     The subject invention is further directed to a lead for percutaneous insertion into an epidural space of a spinal canal. The lead is capable of interfacing with a signal generator and conducting signals from the signal generator to the spinal canal. The lead includes means for altering the shape of the lead between a first configuration and a second configuration. The first configuration of the lead facilitates insertion of the lead into the epidural space, while the second configuration allows the lead, once situated within the epidural space, to exert outward force on structures of the spinal canal, thereby inhibiting movement of the lead within the spinal canal.  
         [0025]     It should be appreciated that the present invention can be implemented and utilized in numerous ways, including without limitation as a process, an apparatus, a system, a device, a method for applications now known and later developed. These and other unique features of the system disclosed herein will become more readily apparent from the following description and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     So that those having ordinary skill in the art to which the present application appertains will more readily understand how to make and use the same, reference may be had to the drawings wherein:  
         [0027]      FIG. 1  is a perspective view of a typical prior art neurological epidural lead implanted in the epidural space of the spinal canal;  
         [0028]      FIG. 1   a  is a side view of a typical prior art neurological epidural lead implanted in the epidural space of the spinal canal, the view taken along line  1   a - 1   a  of  FIG. 1 ;  
         [0029]      FIG. 2  is a perspective view of a neurological epidural lead constructed in accordance with a preferred embodiment of the present invention, wherein the lead body has a two-dimensional undulating configuration;  
         [0030]      FIG. 3  is a side elevational view of the neurological epidural lead of  FIG. 2 , a portion of the lead body being removed to reveal the conductor coil;  
         [0031]      FIG. 4  is an enlarged localized side elevational view in partial cross-section of the neurological epidural lead of  FIG. 3 , particularly illustrating the lumen defined by the conductor coil;  
         [0032]      FIGS. 5   a - 5   c  are a series of side elevational views of a neurological epidural lead constructed in accordance with a preferred embodiment of the present invention, the series illustrating the retraction of a stylet from the lead to allow the lead to move from a straight configuration to an undulating configuration;  
         [0033]      FIG. 6  is a perspective view of a neurological epidural lead constructed in accordance with a preferred embodiment of the present invention, the lead implanted in the epidural space of the spinal canal and having an undulating configuration that causes the lead to be affixed within the epidural space; and  
         [0034]      FIG. 7  is a perspective view of a neurological epidural lead constructed in accordance with another preferred embodiment of the present invention, the lead having a three-dimensional undulating configuration. 
     
    
       [0035]     These and other features of the neurological epidural lead of the subject invention will become more readily apparent to those having ordinary skill in the art from the following description of exemplary embodiments.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0036]     Referring now to the accompanying drawings, wherein like reference numerals identify similar structural features of the present invention, there is illustrated in  FIG. 2 a  neurological epidural lead  100  constructed in accordance with the present invention. The lead  100  includes an elongated lead body  102  having opposed proximal  104  and distal  106  end portions. Several electrodes  108  are secured to the distal end portion  106  of the lead body  102 . Preferably, the lead includes at least two electrodes, although it is possible to utilize a lead with a single electrode. A connector  110  is secured to the proximal end portion  104  of the lead body  102 , and is configured to interface with a pulse generator (not shown). The pulse generator could be an implantable pulse generator (IPG) that is implanted within a patient&#39;s body, or could be a device that remains external to a patient&#39;s body. In a particular embodiment, the connector  110  is a conventional IS-1 type connector, however, those skilled in the art would readily appreciate that other types of connectors could be utilized, such as, for example, IS-4 type connectors, LV-1 type connectors, VS-1 type connectors, and DF-1 type connectors.  
         [0037]     At least part of the distal end portion  106  of the lead body  102  is formed of a mechanically elastic material and has an undulating, substantially sinusoidal unloaded configuration. Preferably, the undulating configuration of the distal end portion  106  of the lead body  102  includes the area where the electrodes  108  are secured. The mechanically elastic material is such that the undulating configuration of the distal end portion  106  of the lead body  102  can be substantially straightened by force and will subsequently return to the undulating shape when the force is removed.  
         [0038]     Referring to  FIGS. 3 and 4 , conductors  112  extend axially through the lead body  102 , operatively connecting the connector  110  and the electrodes  108 . The conductors  112  are sheathed in insulating material and arranged in a coil. The coil may consist of a single conductor or may be a multi-filar coil. An example of a suitable multi-filar coil assembly is disclosed in U.S. Patent Application No. 2003/0092303 to Osypka, the disclosure of which is herein incorporated by reference in its entirety. Preferably, respective conductors  112  connect each electrode  108  to connector  110 , although it is also possible to connect multiple electrodes with a single conductor. Through the engagement of the connector  110  by an IPG, the conductors  112  allow signals to pass from the IPG to the electrodes  108 . The conductors  112 , so arranged, define a lumen  114 .  
         [0039]     Referring to  FIGS. 5   a - c , a port  116  allows a stylet  118  to be selectively inserted into and retracted from the lead body  102 . The inserted stylet  118  extends through the lumen  114  defined by the multi-filar coil arrangement of the conductors  112 . The stylet  118  is of sufficient stiffness with respect to the lead body  102  so as to force the distal end portion  106  of the lead body  102  into a substantially straight configuration when inserted through the lead body  102  and into the distal end portion  106 . However, when the stylet  118  is retracted from the distal end portion  106 , the mechanically elastic material composing the undulating part of the distal end portion  106  causes the lead  100  to resume the undulating configuration. The selective insertion and retraction of stylet  118  allows the distal end portion  106  to be selectively moved between a substantially linear configuration (shown in  FIG. 5   a ) and the sinusoidal unloaded configuration (represented in  FIG. 5   c ).  
         [0040]     Referring to  FIGS. 5   a - 5   c  and  6 , in use, lead  100 , with the stylet  118  occupying the distal end portion  106  of the lead body  102 , is inserted percutaneously into the body. With the stylet  118  so inserted, the distal end portion  106  is substantially straight, facilitating navigation of the lead  100  through the body, and, specifically, through the narrow regions of the epidural space  70 . Lead  100  is moved through the body and positioned appropriately in epidural space  70  to allow the electrodes  108  to establish the desired electric field around the spinal cord  75 .  
         [0041]     After the lead  100  is properly positioned, the stylet  118  is withdrawn and distal end portion  106  attempts to assume the undulating shape. However, the undulating configuration is dimensioned to allow the distal end portion  106  of the lead body  102  to contact and exert outward force on the surrounding spinal cord  75  and/or spinal canal wall  72  before reaching the unloaded undulating configuration, thereby stabilizing the position of the lead  100  within the epidural space  70  and preventing lateral migration of the lead  100 . Further, in attempting to assume the undulating configuration, the electrodes  108  in the distal end portion  106  are pressed against the spinal cord  75 , thereby improving the electrical stimulation. Finally, after the lead  100  has been secured in the epidural space  70 , the connector  110  is connected to an IPG (not shown) to complete the procedure.  
         [0042]     In the preferred embodiment of  FIGS. 5   a - 5   c , the stylet  118  extends the length of the lead  100  in occupying the distal end portion  106  of lead body  102 , and is fully retracted from the lead  100  after lead placement. However, another preferred embodiment utilizes a stylet (or other stiffener) that is shorter than the lead and occupies only a distal end portion of the lead body. A flexible guide wire extends from the shorter stylet to an opening in the connector, allowing external manipulation of the stylet via the guide wire. Such a structure allows the lead to move from a straight to an undulating configuration either by partial retraction of the shorter stylet, with the stylet remaining within the lead but not within the distal end portion, or by full retraction. In still another preferred embodiment, the lead body is sufficiently compliant to allow a guide wire alone to act to straighten the lead upon insertion, thereby obviating altogether the need for a stylet. In yet another preferred embodiment, a telescoping stiffener is included in the lead, such that the stiffener may be collapsed to allow the lead to assume an undulating configuration without removing the stiffener from the lead.  
         [0043]     In a preferred embodiment, the mechanically elastic material composing the undulating part of the distal end portion  106  of the lead body  102  undergoes a solid-state phase change when moving between the undulating configuration and the generally linear configuration. Such a phase change is often accompanied by a shape change in the material, this shape change serving to enhance the magnitude of elastically recoverable deformation, as is well known to those skilled in the art. Materials capable of undergoing such a solid-state phase change are commonly referred to as shape memory materials, some examples being nickel-titanium alloy, copper-zinc-aluminum alloy, and copper-aluminum-nickel alloy. The use of a shape memory material in the distal end portion  106  of the lead body  102  thereby increases the amount of shape change that can be achieved in the lead  100  when moving between the straight and undulating configurations.  
         [0044]     In another preferred embodiment, a solid-state phase change is induced not by mechanical deformation, as described above, but through temperature change. The distal end portion  106  of the lead body  102  is formed, at least in part, of a material having multiple stable solid phases below the melting temperature, the transition from one phase to another requiring only limited diffusion (so-called “diffusionless” phase changes). A temperature change prompts the material to change phases, such phase change (as with the above-described mechanically-induced case) being accompanied by a shape change. In a particular embodiment, a lead can be moved between an undulating and a substantially straight configuration entirely through thermally induced shape change, removing the need for a stylet. In still another preferred embodiment, the material composing at least part of the distal end portion  106  is a piezoelectric material, such as quartz, rather than a phase changing material. In that case, shape change in the distal end portion  106  is induced, at least in part, by the establishment of an electric field, which causes the material to change shape.  
         [0045]     Referring to  FIG. 7 , in another preferred embodiment of the present invention, lead  200  includes an elongated lead body  202  with opposed proximal  204  and distal  206  end portions. The distal end portion  206  has a substantially helical unloaded configuration extending in three dimensions. A connector  210  is secured to the proximal end portion  204  of the lead body  202 , and is configured to interface with a pulse generator (not shown). For some applications and/or physiologies, use of such a three-dimensional configuration for distal end portion  206  is advantageous, allowing for more secure fixation of the lead  200  within a body and/or delivering better therapeutic performance. Along these lines, the present invention is not specifically limited to specific unloaded shapes of the distal end portion, but contemplates any number of two-dimensional and three-dimensional shapes as may be desired for a particular application, whether these shapes involve regular, repeating patterns or irregular configurations. Further, leads can be designed with shapes specifically suited for the particular anatomy of the patient.  
         [0046]     It should also be understood that the foregoing is only illustrative of exemplary and preferred embodiments, as well as principles of the subject invention. Those skilled in the art will readily appreciate that various modifications can be made without departing from the scope and spirit of the invention, as demonstrated below.  
         [0047]     The present invention contemplates a variety of possible arrangements for the conductors in an implantable lead. For example, in another preferred embodiment, the conductors can be replaced by low resistance stranded wires or cables, or by drawn filled tubing (DFT). In a particular embodiment, such DFT extends through multi-lumen tubing in order to connect the connector and the electrodes. An example of such multi-lumen tubing is disclosed in U.S. Patent Application No. 60/622,864 to Osypka, the disclosure of which is herein incorporated by reference in its entirety. Preferably, one of the lumens is left available for receiving a stylet or other stiffening member, which is selectively inserted to effectuate the straightening of the lead. Alternatively, such DFT wires may each be encased in respective insulation tubes.  
         [0048]     In other preferred embodiments, the conductors of the lead serve both to determine the unloaded shape of the lead and to provide the ability for the lead to recover this unloaded shape following deformation. The lead body is then formed of flexible materials such that the lead body generally conforms to the shape of the conductors. For example, the conductors can be arranged in a multi-filar coil and the coil initially deformed into an undulating configuration. The initial deformation can be plastic, such that strain hardening of the conductor material allows subsequent deformations of the coil between the undulating configuration and a forcibly straightened configuration to occur elastically. Alternatively, the coil can be deformed elastically and annealed while maintained in this deformed state, such that the undulating configuration remains after unloading. In a particular embodiment, the conductors are formed of a shape memory material (either mechanical, thermal, or both) that determines or enhances the range of elastic deformation of the conductors.  
         [0049]     While the invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention as defined by the appended claims.