Abstract:
A medical kit and method for treating an ailment, such as chronic pain is provided. The kit comprises first and second medical leads, e.g., stimulation leads. Each lead comprises an elongated body and at least one operative element. The first medical lead comprises a coupling mechanism, such as a slot, and the second medical lead comprises a complementary mechanism, such as a rail, that slidably engages the coupling mechanism of the first medical lead. The method may comprise delivering the first medical lead into a patient&#39;s body, e.g., into the epidural space of the patient, and delivering the second medical lead into the patient&#39;s body by sliding the complementary coupling mechanism of the second medical lead along the coupling mechanism of the first medical lead.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the implantation of stimulation leads within a patient, and in particular, the implantation of electrode leads within a patient&#39;s spine to treat disorders, such as chronic pain. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known to treat chronic pain by electrically stimulating the spinal cord, spinal nerve roots, and other nerve bundles. Although not fully understood, the application of electrical energy to particular regions of the spinal cord induces parasthesia (i.e., a subjective sensation of numbness or tingling) in the afflicted body regions associated with the stimulated spinal regions. This parasthesia effectively masks the transmission of chronic pain sensations from the afflicted body regions to the brain. Since each body region is associated with a particular spinal nerve root, it is important that stimulation be applied at the proper longitudinal position along the spinal cord to provide successful pain management and avoid stimulation of unaffected regions of the body. Also, because nerve fibers extend between the brain and the nerve roots along the same side of the spine as the body regions they control, it is equally important that stimulation be applied at the proper lateral position of the spinal cord. For example, to treat unilateral pain (i.e., pain sensed only on one side of the body), electrical stimulation is applied to the corresponding side of the spinal cord. To treat bilateral pain (i.e., pain sensed on both sides of the body), electrical stimulation is either applied directly to the midline of the spinal cord or applied to both lateral sides of the spinal cord. 
         [0003]    In a typical procedure, one or more stimulation leads are introduced through the patient&#39;s back into the epidural space under fluoroscopy. The specific procedure used to implant the stimulation lead will ultimately depend on the type of stimulation lead used. Currently, there are two types of commercially available stimulation leads: a percutaneous lead and a surgical lead. 
         [0004]    A percutaneous lead comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the spinal cavity above the dura layer. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two percutaneous leads are placed down the respective sides of the midline. 
         [0005]    A surgical lead has a paddle on which multiple electrodes are arranged in independent columns, and is introduced into contact with the affected spinal tissue using a surgical procedure, and specifically, a laminectomy, which involves removal of the laminar vertebral tissue to allow both access to the dura layer and positioning of the lead. 
         [0006]    After the stimulation lead(s) (whether percutaneous or surgical) are placed at the target area of the spinal cord, the lead(s) are anchored in place, and the proximal ends of the lead(s), or alternatively lead extensions, are passed through a tunnel leading to a subcutaneous pocket (typically made in the patient&#39;s abdominal area) where a neurostimulator is implanted. The lead(s) are connected to the neurostimulator, which is then operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. During this procedure, the patient provides verbal feedback regarding the presence of paresthesia over the pain area. Based on this feedback, the lead position(s) may be adjusted and re-anchored if necessary. Any incisions are then closed to fully implant the system. 
         [0007]    Various types of stimulation leads (both percutaneous and surgical), as well as stimulation sources and other components, for performing spinal cord stimulation are commercially available from Medtronic, Inc., located in Minneapolis, Minn., and Advanced Neuromodulation Systems, Inc., located in Plano, Tex. 
         [0008]    The use of surgical leads provides several functional advantages over the use of percutaneous leads. For example, the paddle on a surgical leads has a greater footprint than that of a percutaneous lead. As a result, an implanted surgical lead is less apt to migrate from its optimum position than is an implanted percutaneous lead, thereby providing a more efficacious treatment and minimizing post operative procedures otherwise required to reposition the lead. As another example, the paddle of a surgical lead is insulated on one side. As a result, almost all of the stimulation energy is directed into the targeted neural tissue. The electrodes on the percutaneous leads, however, are entirely circumferentially exposed, so that much of the stimulation energy is directed away from the neural tissue. This ultimately translates into a lack of power efficiency, where percutaneous leads tend to exhaust a stimulator battery supply 25%-50% greater than that exhausted when surgical leads are used. As still another example, the multiple columns of electrodes on a surgical lead are well suited to address both unilateral and bilateral pain, where electrical energy may be administered using either column independently or administered using both columns. 
         [0009]    Although surgical leads are functionally superior to percutaneous leads, there is one major drawback—surgical leads require painful surgery performed by a neurosurgeon, whereas percutaneous leads can be introduced into the epidural space minimally invasively by an anesthesiologist using local anesthesia. 
         [0010]    There, thus, remains a need for a minimally invasive means of introducing stimulation leads within the spine of a patient, while preserving the functional advantages of a surgical lead. 
       SUMMARY OF THE INVENTION 
       [0011]    Although the present inventions should not be so limited in their broadest aspects, they lend themselves well to medical applications, wherein access to a target site must be made through a limited opening, yet the resulting medical platform used to perform a medical procedure at such target site is larger than the access opening. The present inventions lend themselves particularly well to the percutaneous installation and subsequent operation of a stimulation lead assembly within the epidural space of a patient to treat ailments, such as chronic pain. 
         [0012]    In accordance with a first aspect of the present inventions, a stimulation kit comprising first and second tissue stimulation leads is provided. The first stimulation lead comprises a first elongated body, a first stimulation element (e.g., an electrode), and a first coupling mechanism longitudinally extending along at least a portion of the first elongated body. The second stimulation lead comprises a second elongated body, a second stimulation element (e.g., an electrode), and a first complementary coupling mechanism configured to slidably engage the first coupling mechanism, e.g., in a rail and slot arrangement. The stimulation kit may optionally comprise a stimulation source configured to be coupled to the first and second stimulation leads. Optionally, each of the stimulation leads comprises a plurality of stimulation elements in order to provide a more extensive stimulation coverage. 
         [0013]    In one embodiment, the first and second elongated bodies are cylindrically-shaped, although other shapes are possible depending on the particular application. The size of the elongated bodies can be any size that is consistent with the stimulation procedure in which the stimulation leads will be employed. Although, for medical procedures, such as spinal cord stimulation, the greatest cross-sectional dimension of at least one of the elongated bodies is preferably 5 mm or less in order to minimize the size of the opening through which the stimulation leads will be introduced. The elongated bodies can have the same length, or alternatively, one elongated body can be shorter than the other, such that, e.g., the shorter elongated body can be entirely delivered within the patient&#39;s body without any portion extending from the access opening. In one embodiment, the stimulation elements of the respective stimulation leads face the same direction, e.g., to focus the stimulation energy in one direction. 
         [0014]    The stimulation elements may be mounted directly on the elongated bodies, or alternatively, may be mounted to some other element of the stimulation leads. For example, the second stimulation lead may have a flap on which the respective stimulation element is disposed. In this case, the flap may extend along a portion of the complementary coupling mechanism, so that it can be secured by the coupling mechanism of the first stimulation lead when the portion of the complementary coupling mechanism slidably engages the coupling mechanism of the first stimulation lead and released by the coupling mechanism of the first stimulation lead when the portion of the complementary coupling mechanism slidably disengages the coupling mechanism of the first stimulation lead. 
         [0015]    In one embodiment, the distal end of the second elongated body is configured to be in close contact with the first elongated body when engaging each other. Alternatively, the first elongated body is configured to deploy from the first elongated body by slidably disengaging at least a portion of the complementation coupling mechanism from the coupling mechanism of the first stimulation lead. In this case, the distal end of the second elongated body can be pre-curved to provide it with a predefined configuration. Optionally, the second elongated body may be configured to be actively changed from a first geometry to a second geometry after deployment from the first elongated body. For example, the kit may comprise a stylet configured to be introduced through the second elongated body to change the second elongated body from the first geometry to the second geometry. Or the secondary stimulation lead may comprise a pullwire configured to be pulled to change the second elongated body from the first geometry to the second geometry. 
         [0016]    The kit may have more than two stimulation leads. For example, the first stimulation lead may comprise another coupling mechanism longitudinally extending along at least a portion of the respective elongated body, in which case, the kit may further comprise a third stimulation lead comprising an elongated body, a stimulation element mounted on the elongated body, and another complementary coupling mechanism configured to slidably engage the other coupling mechanism of the first stimulation lead. 
         [0017]    In one preferred method of using the stimulation kit to treat a disorder (e.g., chronic pain) in a patient, the first stimulation lead is delivered into the epidural space of the patient&#39;s spine, and the second stimulation lead is delivered into the epidural space by sliding the complementary coupling mechanism along the coupling mechanism of the first stimulation lead. Stimulation energy can then be conveyed from the stimulation elements into the neural tissue. 
         [0018]    Preferably, the first and second stimulation leads are delivered through a percutaneous opening within the patient&#39;s skin, thereby minimizing patient discomfort and damage to otherwise healthy tissue. Although delivered in a minimally invasive manner, the larger footprint created by the coupled stimulation leads provides the assembly with more stability and greater coverage area. Thus, although not necessarily limited in its broadest aspects, the advantages of a surgical lead are retained by the present invention, without the disadvantages associated with invasive surgical procedures otherwise required to implant surgical leads. 
         [0019]    In accordance with a second aspect of the present inventions, a method of treating a disorder (e.g., chronic pain) is provided. The method comprises delivering a first stimulation lead into the epidural space of the patient&#39;s spine, and delivering a second stimulation lead into the epidural space by slidably engaging the second stimulation lead along the first stimulation lead. A third stimulation lead can optionally be delivered into the epidural space by slidably engaging the third stimulation lead along the first stimulation lead. In one preferred method, the stimulation leads are delivered into the epidural space through a percutaneous opening. For example, the first stimulation lead can be introduced through a delivery device into the epidural space, and then the second stimulation lead can be delivered along the first stimulation lead. 
         [0020]    In one preferred method, the stimulation leads are coupled to a stimulation source, in which case, the method may further comprise conveying stimulation energy (e.g., electrical energy) from the stimulation source to the stimulation leads to stimulate neural tissue within the patient&#39;s spine. The stimulation energy may be focused into the neural tissue, as opposed to conveying the stimulation energy in all radial directions. In the preferred method, the stimulation leads are implanted within the patient&#39;s spine, e.g., to provide extended relief. 
         [0021]    In accordance with a third aspect of the present inventions, a medical kit is provided. The medical kit is similar to the previously described stimulation kit, with the exception that the medical kit comprises first and second medical leads with respective operative elements that are not limited to stimulation elements, but rather can be any elements that are capable of performing a medical function within a targeted tissue region. 
         [0022]    In accordance with a fourth aspect of the present inventions, a method of performing a medical procedure on a patient is provided. This method is similar to the previously described method, with the exception that it is not limited to stimulation of tissue within the epidural space of the patient. 
         [0023]    In accordance with a fifth aspect of the present inventions, a stimulation kit is provided. The stimulation kit is similar to the previously described stimulation kit, with the exception that it comprises a guide and a stimulation lead. The guide is similar to the first stimulation lead of the previously described stimulation kit, with the exception that it need not have a stimulation element. 
         [0024]    In accordance with a sixth aspect of the present inventions, a medical kit is provided. The medical kit is similar to the previously described medical kit, with the exception that it comprises a guide and a medical lead. The guide is similar to the first medical lead of the previously described medical kit, with the exception that it need not have an operative element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The drawings illustrate the design and utility of preferred embodiment(s) of the invention, in which similar elements are referred to by common reference numerals. In order to better appreciate the advantages and objects of the invention, reference should be made to the accompanying drawings that illustrate the preferred embodiment(s). The drawings, however, depict the embodiment(s) of the invention, and should not be taken as limiting its scope. With this caveat, the embodiment(s) of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
           [0026]      FIG. 1  is a plan view of a modular stimulation lead kit arranged in accordance with a preferred embodiment of the present invention; 
           [0027]      FIG. 2  is a cutaway top view of a stimulation lead assembly formed from the kit of  FIG. 1 ; 
           [0028]      FIG. 3  is a cutaway perspective view of a primary stimulation lead used in the kit of  FIG. 1 ; 
           [0029]      FIG. 4  is a cutaway perspective view of a secondary stimulation lead used in the kit of  FIG. 1 ; 
           [0030]      FIG. 5  is a cross-sectional view of the stimulation lead assembly of  FIG. 2 , taken along the line  5 - 5 ; 
           [0031]      FIG. 6  is a cutaway view of an alternative stimulation lead assembly that can be formed from the kit of  FIG. 1 ; 
           [0032]      FIG. 7  is a cross-sectional view of the stimulation lead assembly of  FIG. 6 , taken along the line  7 - 7 ; 
           [0033]      FIGS. 8A-8D  are various views illustrating the installation of the kit of  FIG. 1  into a patient&#39;s spine; 
           [0034]      FIG. 9  is a plan view of another modular stimulation lead kit arranged in accordance with another preferred embodiment of the present invention; 
           [0035]      FIG. 10  is a cutaway top view of a stimulation lead assembly formed from the kit of  FIG. 9 ; 
           [0036]      FIG. 11  is a cross-sectional view of the secondary stimulation lead of  FIG. 10 , taken along the line  11 - 11 ; 
           [0037]      FIGS. 12A-12B  are various views illustrating the installation of the kit of  FIG. 9  into a patient&#39;s spine; 
           [0038]      FIG. 13  is a partially cutaway top view of the distal end of an alternative primary stimulation lead that can be used in kit of  FIG. 1 ; 
           [0039]      FIG. 14   a  is a cutaway top view of an alternative stimulation lead assembly that can be formed from the kit of  FIG. 1  when the primary stimulation lead of  FIG. 13  is used, wherein the secondary stimulation leads are shown in a normally curved geometry that converges towards the primary stimulation lead; 
           [0040]      FIG. 15   a  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   a , taken along the line  15   a - 15   a;    
           [0041]      FIG. 14   b  is a cutaway top view of an alternative stimulation lead assembly that can be formed from the kit of  FIG. 1  when the primary stimulation lead of  FIG. 13  is used, wherein the secondary stimulation leads can be placed into a curved geometry that converges towards the primary stimulation lead when a stylet is introduced; 
           [0042]      FIG. 15   b  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   b , taken along the line  15   b - 15   b;    
           [0043]      FIG. 14   c  is a cutaway top view of another alternative stimulation lead assembly that can be formed from the kit of  FIG. 1  when the primary stimulation lead of  FIG. 13  is used, wherein the secondary stimulation leads can be placed into a curved geometry that converges towards the primary stimulation lead when a pullwire is tensioned; 
           [0044]      FIG. 15   c  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   c , taken along the line  15   c - 15   c;    
           [0045]      FIG. 14   d  is a cutaway top view of still another alternative stimulation lead assembly that can be formed from the kit of  FIG. 1  when the primary stimulation lead of  FIG. 3  is used, wherein the secondary stimulation leads can be placed into a curved geometry that bows away from the primary stimulation lead; 
           [0046]      FIG. 15   d  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   d , taken along the line  15   d - 15   d;    
           [0047]      FIG. 15   e  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   d , taken along the line  15   e - 15   e;    
           [0048]      FIG. 15   f  is a cross-sectional view of the stimulation lead assembly of  FIG. 14   d , taken along the line  15   f - 15   f;    
           [0049]      FIG. 16  is a cutaway top view of an alternative stimulation lead assembly; 
           [0050]      FIG. 17  is a cross-sectional view of a portion of the stimulation lead assembly of  FIG. 16 , particularly showing an electrode flap of a secondary stimulation lead constrained by the primary stimulation lead; and 
           [0051]      FIG. 18  is a cross-sectional view of a portion of the stimulation lead assembly of  FIG. 16 , particularly showing the electrode flap of the secondary stimulation lead released by the primary stimulation lead. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]    Referring now to  FIG. 1 , a modular stimulation lead kit  100  arranged in accordance with one preferred embodiment of the present invention is shown. In its simplest form, the stimulation kit  100  generally comprises a primary stimulation lead  102  and two secondary stimulation leads  104 , which are configured to be percutaneously delivered and implanted into the epidural space of a patient&#39;s spine, an implantable electrical stimulation source  106  configured for delivering stimulation energy to the stimulation leads  102 / 104 , and an optional extension lead  108  configured for connecting the stimulation leads  102 / 104  to the remotely implanted stimulation source  106 . As will be described in further detail below, the secondary stimulation leads  104  can be attached to the primary stimulation lead  102  to form a modularized stimulation lead assembly  110 , as illustrated in  FIG. 2 . 
         [0053]    It should be noted that although the kit  100  illustrated in  FIG. 1  is described herein as being used in spinal cord stimulation (SCS) for the treatment of chronic pain, the kit  100 , or a modification of the kit  100 , can be used in an SCS procedure to treat other ailments, or can used in other applications other than SCS procedures, such as peripheral nervous system stimulation, sacral root stimulation, and brain tissue stimulation, including cortical and deep brain stimulation. In the latter case, the stimulation leads  102 / 104  can be delivered through a miniature cranial burr hole into the brain tissue. 
         [0054]    The primary stimulation lead  102  comprises an elongated sheath body  112  having a proximal end  114  and a distal end  116 . The sheath body  112  is composed of a suitably flexible material (such as polyurethane, silicone, etc.), which may either be resilient or non-resilient, and may be formed via an extrusion process or by any other suitable means. The distal end  116  of the sheath body  112  is soft and tapered to prevent injury to nerve roots that exit the spinal cord when delivered into the epidural space of the patient&#39;s spine. In the illustrated embodiment, the sheath body  112  is cylindrically-shaped and sized to fit through a Touhy-like needle (not shown). In this case, the diameter of the sheath body  112  is preferably less than 5 mm to allow it to be percutaneously introduced through a needle. More preferably, the diameter of the sheath body  112  is within the range of 1 mm to 3 mm, so that the primary stimulation lead  102 , along with the secondary stimulation leads  104  described below, can comfortably fit within the epidural space of the patient. The sheath body  112  may have other cross-sectional geometries, such as elliptical, rectangular, triangular, etc. If rectangular, the width of the primary stimulation lead  102  can be up to 5 mm, since the width of an epidural space is greater than its height. The sheath body  112  may have an optional lumen (not shown) for receiving a stylet (not shown) that axially stiffens the sheath body  112  to facilitate percutaneous introduction of the primary stimulation lead  102  within the epidural space of the patient&#39;s spine, as will be described in further detail below. 
         [0055]    The primary stimulation lead  102  further comprises a plurality of terminals  118  (in this case, three) mounted on the proximal end  114  of the sheath body  112 , and a plurality of stimulation elements, and in particular electrodes  120  (in this case, three), mounted on the distal end  116  of the sheath body  112 . The terminals  118  are formed of ring-shaped elements composed of a suitable biocompatible metallic material, such as platinum, platinum/iridium, stainless steel, gold, or combinations or alloys of these materials, and can be mounted to the sheath body  112  in an interference fit arrangement. 
         [0056]    In the illustrated embodiment, the electrodes  120  are formed on one circumferential side of the sheath body  112  (shown best in  FIG. 3 ) in order to focus stimulation energy in one direction, thereby maximizing energy efficiency. The electrodes  120  can be formed onto the sheath body  112  using known deposition processes, such as sputtering, vapor deposition, ion beam deposition, electroplating over a deposited seed layer, or a combination of these processes. Alternatively, the electrodes  120  can be formed onto the sheath body  112  as a thin sheet or foil of electrically conductive metal affixed to the wall of the sheath body  112 . The electrodes  120  can be composed of the same electrically conductive and biocompatible material as the terminals  118 , e.g., platinum, platinum/iridium, stainless steel, gold, or combinations or alloys of these materials. 
         [0057]    The primary stimulation lead  102  further comprises a plurality of conductors  122  (shown in  FIG. 3 ) extending through the sheath body  112  and connecting each electrode  120  with a respective terminal  118 . The conductors  122  are composed of a suitably electrically conductive material that exhibits the desired mechanical properties of low resistance, corrosion resistance, flexibility, and strength. 
         [0058]    Like the primary stimulation lead  102 , each secondary stimulation lead  104  comprises an elongated sheath body  132  having a proximal end  134  and a distal end  136 , a plurality of terminals  138  (in this case, four) mounted to the proximal end  134  of the sheath body  132 , a plurality of electrodes  140  (in this case, four) mounted to the distal end  136  of the sheath body  132 , and a plurality of conductors  142  (shown in  FIG. 4 ) extending through the sheath body  132  and respectively connecting the electrodes  120  to the terminals  118 . The sheath bodies  132  of the secondary stimulation leads  104  are similar to the sheath body  112  of the primary stimulation lead  102 , with the exception that the distal ends  136  are tapered in only one direction. In this manner, the stimulation lead assembly  110 , as illustrated in  FIG. 2 , forms a lower profile distal end to facilitate placement of the assembly  110  within the epidural space of the patient&#39;s spine. Like the sheath body  112  of the primary stimulation lead  102 , the sheath bodies  132  of the secondary stimulation leads  104  may each have an optional lumen (not shown) for receiving a stylet (not shown) to facilitate percutaneous introduction of the secondary stimulation lead  104  within the epidural space of the patient&#39;s spine, as will be described in further detail below. 
         [0059]    The terminals  118  and electrodes  120  of the secondary stimulation leads  104  are similar to the terminals  118  and electrodes  120  of the primary stimulation lead  102 , with the exception that there are four sets of terminals  118  and electrodes  120  instead of three. Notably, the electrodes  120  of the secondary stimulation leads  104  face the same direction as the electrodes  140  of the primary stimulation leads  102 , so that the entire stimulation lead assembly  110  is capable of focusing electrical energy in a single direction, as shown in  FIG. 3 . Also, as illustrated in  FIG. 3 , the electrodes  120 / 140  are arranged on the respective sheath bodies  112 / 132 , such that the electrodes  140  of the secondary stimulation leads  104  are offset from the electrodes  120  of the primary stimulation lead  102  in the longitudinal direction, thereby preventing accidental shorting between adjacent electrodes when the assembly  110  is formed. 
         [0060]    Further details regarding the structure and composition of standard percutaneous stimulation leads are disclosed in U.S. Pat. No. 6,216,045, which is expressly incorporated herein by reference. 
         [0061]    The primary stimulation lead  102  and the respective secondary stimulation leads  104  are configured to slidably engage each other to form the lead assembly  110  illustrated in  FIG. 2 . In particular, referring to  FIGS. 3-5 , the primary stimulation lead  102  comprises a pair of circumferentially opposed slots  150  extending along the length of the sheath body  112 . The slots  150  can be formed in the sheath body  112  using any one of a variety of manners, but in the illustrated embodiment, the slots  150  are formed during the extrusion process. Alternatively, the slots  150  can be formed by embedding, or otherwise mounting, discrete slotted members (not shown) along the sheath body  112 . In contrast, each of the secondary stimulation leads  104  comprises a rail  152  extending along the sheath body  132 . Like the slots  150 , the rail  152  can be formed on the sheath body  132  using any one of a variety of manners, such as forming the rail  152  during the extrusion process. Alternatively, the rail  152  can be formed of a discrete member (not shown) that is bonded, or otherwise mounted, to the sheath body  132 . In other embodiments, the primary stimulation lead  102  may have a pair of circumferentially opposed rails extending along its sheath body  112 , while the secondary stimulation leads  104  may have slots  150  extending along their sheath bodies  132 . In any event, the rails  152  and slots  150  are sized to snuggly engage each other in a sliding relationship, as best shown in  FIG. 5 . 
         [0062]    Thus, it can be appreciated that the secondary stimulation leads  104  can be coupled to the primary stimulation lead  102  by sliding the rails  152  of the respective secondary stimulation leads  104  along the respective slots  150  of the primary stimulation lead  102 , thereby forming the stimulation assembly  110  illustrated in  FIG. 2 . The opposing slots  150  of the primary stimulation lead  102  and the rails  152  of the secondary stimulation leads  104  are circumferentially offset ninety degrees from the centers of the respective electrodes  120 . In this manner, all of the electrodes  120 , which generally face in the same direction, as described above, are ensured to face in a direction perpendicular to the plane of the assembly  110 , thereby maximizing transmission of the stimulation energy into the target neural tissue when the assembly  110  is fully implanted within the epidural space of the patient&#39;s spine. 
         [0063]    Although a rail and slot arrangement has been disclosed as the preferred means of slidably engaging the primary and stimulation leads  102 / 104 , other means of slidably engaging the leads can be provided. For example, instead of slots, the primary stimulation lead can have loop structures (not shown) that extend along the opposing sides the respective sheath body. The secondary stimulation leads  104  can then be introduced through the respective sets of loop structures in order to couple the leads together. 
         [0064]    In the illustrated embodiment, the slots  150  have distal rail stops (not shown), i.e., the distal ends of the slots  150  terminate prior to the distal tip of the sheath body  112  to prevent the distal ends  136  of the secondary stimulation leads  104  from sliding distal to the distal end  116  of the primary stimulation lead  102 . Alternatively, the distal ends of the slots  150  may have chamfered openings  151 , as illustrated in  FIG. 13 . In this manner, the distal ends of the secondary stimulation leads  104  will diverge from the distal end of the primary stimulation lead  102  when the leads  102 / 104  are slidably engaged with each other. That is, when the rail  152  of a secondary stimulation lead  104  is slid along the respective slot  150  of the primary stimulation lead  102 , the distal end of the rail  152  will be diverted out of the chamfered opening  151  at the distal end of the slot  150 , thereby expanding the footprint of the resulting assembly  110 , as illustrated in  FIG. 14   a . The secondary stimulation lead  104  may have a proximal rail stop (not shown) to prevent further sliding of the respective secondary stimulation lead  104  when fully deployed. 
         [0065]    The distal ends of the secondary stimulation leads  104  can be pre-curved inward towards the primary stimulation lead  102 , as illustrated in  FIG. 14   a , so that the distal ends of the secondary stimulation leads  104 , when deployed from the primary stimulation lead  102 , extend in a parallel direction with the distal end of the primary stimulation lead  102 . The distal ends of the secondary stimulation leads  104  can be pre-curved in any one of a variety of manners. For example, as illustrated in  FIG. 15   a , a pre-curved resilient member  153  composed of a suitable material, such as nitinol, can be formed within the sheath body  132 . Preferably, the cross-section of the resilient member  153  resembles of flat plate, so that the sheath body  132  consistently bends in a pre-defined plane, i.e., within the plane of the assembly  110 . 
         [0066]    Alternatively, as illustrated in  FIG. 14   b , the distal ends of the secondary stimulation leads  104  are not pre-curved, but rather normally exhibit a straight geometry after exiting slot  150  of the primary stimulation lead  102  (shown in phantom in  FIG. 14   b ), such that the distal ends of the secondary stimulation leads  104  diverge from the primary stimulation lead  102 . Alternatively, the distal ends of the secondary stimulation leads  104  may not be resilient. In either case, the secondary stimulation lead  104  comprises a lumen  155  through which a curved stylet  157  is introduced, as illustrated in  FIG. 15   b . The distal end of the stylet  157  is curved, such that, when introduced through the lumen  155 , the distal end of the respective stimulation lead  104  assumes a geometry that curves inward towards the primary stimulation lead  102 , as illustrated in  FIG. 14   b . Differently curved stylets  157  can be used in order to provide the distal end of the secondary stimulation lead  104  with the desired curved geometry. Alternatively, rather than providing a curved stylet  157  and a normally straight secondary stimulation lead  104 , the distal end of the secondary stimulation lead  104  can be pre-curved much like the stimulation lead  104  illustrated in  FIG. 14   a . In this case, the distal end of the stylet  157  can be straight, so that its introduction through the lumen  157  straightens the pre-curved distal end of the secondary stimulation lead  104 , as shown in phantom in  FIG. 14   b.    
         [0067]    As another alternative, a steering mechanism can be used to control the shape of the secondary stimulation lead  104 . In particular, as illustrated in  FIG. 14   c , the distal ends of the secondary stimulation leads  104  normally exhibit a straight geometry, in which case, the resilient member  153  is likewise formed into a straight geometry. As illustrated in  FIG. 15   c , the secondary stimulation lead  104  comprises a pullwire lumen  159  and an associated pullwire  161  mounted to the inside surface of the distal end of the resilient member  153 . When the pullwire  161  is relaxed, the distal end of the secondary stimulation lead  104  assumes the straight geometry. In this case, the distal ends of the secondary stimulation leads  104  diverge from the primary stimulation lead  102 , as illustrated in  FIG. 14   c . In contrast, when the pullwire  161  is pulled, the distal end of the secondary stimulation lead  104  assumes a geometry (shown in phantom) that curves inward towards the primary stimulation lead  102 . Notably, the proximal-most portion of the distal ends of the secondary stimulation leads  104  does not contain the resilient member  153 , so that the respective stimulation lead  104  bends at this portion when the pullwire  161  is pulled. Rather than providing a normally straight secondary stimulation lead  104 , the distal end of the secondary stimulation lead  104  can be pre-curved much like the stimulation lead  104  illustrated in  FIG. 14   a . In this case, the pullwire  161  can be mounted to the outside surface of the distal end of the resilient member  153 , such that relaxation of the pullwire  161  causes the distal end of the secondary stimulation lead  104  to assume a curved geometry that converges towards the primary stimulation lead  102 , whereas the application of tension on the pullwire  161  causes the distal end of the secondary stimulation lead  104  to assume a lesser curved or straight geometry that diverges from the primary stimulation lead  102 . 
         [0068]    As still another alternative, a portion of the secondary stimulation lead  104  may not have a rail, so that it bows outward after the primary stimulation lead  102  and secondary stimulation lead  104  are fully engaged, as illustrated in  FIG. 14   d . As best seen in  FIGS. 15   d - f , the rail  152  extends along the distal-most and proximal-most portions of the distal end of the second stimulation lead  104 , but does not extend along a medial-portion of the distal end of the second stimulation lead  104 . As a result, after the distal end of the respective rail  152  (i.e., the rail  152  located on the distal-most portion of the second stimulation lead  104 ) abuts the distal rail stop (not shown) in the corresponding slot  150 , further distal movement of the secondary stimulation lead  104  relative to the primary stimulation lead  102  causes the medial-portion, which is not engaged with the slot  150  of the primary stimulation lead  102 , to bow outward from a straight geometry (shown in phantom). In contrast, proximal movement of the secondary stimulation lead  104  relative to the primary stimulation lead  102  causes the medial-portion to return from the bowed geometry back to its straight geometry. 
         [0069]    It should be noted that although the assemblies  110  illustrated in  FIGS. 2 and 14   a - d  are formed of three stimulation leads, less or more than three stimulation leads can be used. For example, if an assembly formed only of two stimulation leads is desired, only one slot  150  on the primary stimulation lead  102  is required. In this case, the primary stimulation lead  102  may only have one slot  150  formed along one side of the respective body  112 , or alternatively, if the primary stimulation lead  102  comprises two opposing slots  150 , only one will be used to couple the lone secondary stimulation lead  104  thereto. On the other hand, if an assembly formed of more than three stimulation leads is desired, the secondary stimulation leads  104  may have a pair of circumferentially opposed rails  152 . For example, if there are five stimulation leads, two secondary stimulation leads  103  (which are similar to the secondary stimulation leads  104 , but with a pair of circumferentially opposing rails  152 ) can be coupled to the primary stimulation lead  102  by sliding the rails  152  of the respective secondary stimulation leads  104  along the respective slots  150  of the primary stimulation lead  102 , thereby forming a partial assembly similar to that illustrated in  FIG. 3 . Then, two additional secondary stimulation leads  105  (which are similar to secondary stimulation leads  105 , but have a pair of circumferentially opposed slots  150 ) can be coupled to the secondary stimulation leads  105  by sliding the slots  150  of the additional secondary stimulation leads  102  along the respective rails  152 , thereby forming a full assembly  160 , as illustrated in  FIGS. 6 and 7 . 
         [0070]    Referring back to  FIG. 1 , the implantable stimulation source  106  is designed to deliver electrical pulses to the stimulation leads  102 / 104  in accordance with programmed parameters. In the preferred embodiment, the stimulation source  106  is programmed to output electrical pulses having amplitudes varying from 0.1 to 20 volts, pulse widths varying from 0.02 to 1.5 milliseconds, and repetition rates varying from 2 to 2500 Hertz. In the illustrated embodiment, the stimulation source  106  takes the form of a totally self-contained generator, which once implanted, may be activated and controlled by an outside telemetry source, e.g., a small magnet. In this case, the pulse generator has an internal power source that limits the life of the pulse generator to a few years, and after the power source is expended, the pulse generator must be replaced. Generally, these types of stimulation sources  106  may be implanted within the chest or abdominal region beneath the skin of the patient. 
         [0071]    Alternatively, the implantable stimulation source  106  may take the form of a passive receiver that receives radio frequency (RF) signals from an external transmitter worn by the patient. In this scenario, the life of the stimulation source  106  is virtually unlimited, since the stimulation signals originate from the external transmitter. Like the self-contained generators, the receivers of these types of stimulation sources  106  can be implanted within the chest or abdominal region beneath the skin of the patient. The receivers may also be suitable for implantation behind the ear of the patient, in which case, the external transmitter may be worn on the ear of the patient in a manner similar to that of a hearing aid. Stimulation sources, such as those just described, are commercially available from Advanced Neuromodulation Systems, Inc., located in Plano, Tex., and Medtronic, Inc., located in Minneapolis, Minn. 
         [0072]    The optional extension lead  108  comprises an elongated sheath body  144  having a proximal end  146  and a distal end  148 , much like the sheath bodies  112 / 132  of the stimulation leads  102 / 104 , a distal adapter  154  coupled to the distal end  148  of the sheath body  144 , a connector  156  coupled to the proximal end  146  of the sheath body  144 , and a plurality of electrical conductors (not shown) extending through the sheath body  144 . The length of the extension lead  108  is sufficient to extend from the spine of the patient, where the proximal ends of the implanted stimulation leads  102 / 104  protrude from to the implantation site of the stimulation source  106 —typically somewhere in the chest or abdominal region. The distal adapter  154  is configured to receive the proximal ends of the stimulation leads  102 / 104 , and the proximal connector  156  is configured to couple to the stimulation source  106 . 
         [0073]    Having described the stimulation lead kit  100 , its installation and use in treating chronic pain will now be described with reference to  FIGS. 8A-8D . After the patient has been prepared (which may involve testing the efficacy of spinal cord stimulation on the patient, and, once determining that the patient can be effectively treated with spinal cord stimulation, identifying and marking the appropriate vertebral intervals on the patient&#39;s skin and applying a local anesthetic to this region), a needle  10 , such as, e.g., a Touhy needle, is inserted through the patient&#39;s skin  12  between the desired vertebrae  14 , and into the epidural space  16  within the spine at a position inferior to target stimulation site  18  ( FIG. 8A ). In the illustrated method, the Touhy needle  10  will serve as the primary delivery mechanism for the primary stimulation lead  102 . Alternatively, if an optional introducer (not shown) is used, a guide wire (not shown) is introduced through the needle  10  and advanced to or near the target stimulation site  18 . The needle  10  is removed, the introducer is then introduced over the guide wire and advanced to the target stimulation site  18 , and the guide wire is then withdrawn. In this case, the introducer will serve as the primary delivery mechanism for the primary stimulation lead  102 . 
         [0074]    After the deliver mechanism is in place, the primary stimulation lead  102  is then inserted through the needle or the introducer (whichever is in place), and positioned in the epidural space at the target stimulation site  18 , with the electrodes  120  facing the dural layer  20  surrounding the spinal cord  22  ( FIG. 8B ). If the primary stimulation lead  102  has a stylet lumen, a stylet can be used to provide additional axial stiffness and to facilitate control. Next, the needle  10  or introducer is removed, and one of the secondary stimulation leads  104  is delivered through the percutaneous opening  24  left by the removal of the needle  10 , and into the epidural space  16  by slidably engaging the secondary stimulation lead  104  along the primary stimulation lead  102  ( FIG. 8C ). In particular, the rail  152  of the secondary stimulation lead  104  is inserted into the corresponding slot  150  of the primary stimulation lead  102 , and the secondary stimulation lead  104  is pushed until the distal end of the rail  152  abuts the distal end of the slot  150 , thereby signifying that the secondary stimulation lead  104  is fully engaged with the primary stimulation lead  102  (with the electrodes  120 / 140  of the stimulation leads  102 / 104  adjacent, but offset from, each other) and is in its proper location within the epidural space  16  of the patient. The other secondary stimulation lead  104  is then delivered into the epidural space by slidably engaging it along the primary stimulation lead  102  in the same manner, thereby completing the stimulation lead assembly  110  ( FIG. 8D ). If the secondary stimulation leads  104  have stylet lumens, a stylet can be used to provide additional axial stiffness and to facilitate control. Once the assembly  110  is completed, the electrodes  120 / 140  will span the midline of the spinal cord  22 , much like the electrodes of a standard surgical lead do. 
         [0075]    Next, the proximal ends of the stimulation leads  102 / 104  are connected to a tester (not shown), which is then operated in a standard manner to confirm proper location of the stimulation lead assembly  110  and to adjust the stimulation parameters for optimal pain relief. Once this optimization process has been completed, the tester is disconnected from the stimulation leads  102 / 104 , which are then anchored in place using standard lead anchors (not shown). Next, the stimulation lead assembly  110  is coupled to the stimulation source  106  and implantation is completed (not shown). In particular, a subcutaneous pocket is created in the patient&#39;s abdominal area for implantation of the stimulation source  106 , and a tunnel is subcutaneously formed between the spine region and the subcutaneous pocket. The optional lead extension  108  is passed through the tunnel, after which the adapter  154  of the extension  108  is connected to the proximal ends of the stimulation leads  102 / 104  and the connector  156  of the lead extension  108  is connected to the stimulation source  106 . The stimulation source  106  is programmed and tested, and then placed within the subcutaneous pocket, after which all incisions are closed to effect implantation of the stimulation lead assembly  110  and stimulation source  106 . The stimulation source  106  can then be operated to convey stimulation energy from the stimulation source  106  to the electrodes  120 / 140  of the stimulation lead assembly  110 , where it is, in turn, conveyed into the neural tissue for pain relief. If necessary or desired, e.g., if the electrodes  120 / 140  malfunction or stimulation otherwise ceases to provide therapeutic benefit, the stimulation lead assembly  110  can be subsequently retrieved from the patient&#39;s spine by removing the assembly  110  at the same time or by removing the assembly one stimulation lead  102 / 104  at a time by slidably disengaging the stimulation leads  102 / 104 . In the case of the assembly  110  illustrated in  FIG. 14 , the rail and slot arrangement will pull the deployed distal end of the secondary stimulation leads  104  along side of the primary stimulation lead  102  when retrieved. 
         [0076]    It can be appreciated that the relatively large footprint made by the stimulation lead assembly  110 , much like a prior art surgical lead, provides a more stable platform for the electrodes  120 / 140 . Also, like a prior art surgical lead, the electrodes  120 / 140  face in a single direction, thereby focusing the stimulation energy into the affected neural tissue where it is needed. Unlike a surgical lead, however, the stimulation lead assembly  110  can be percutaneously delivered into the patient&#39;s spine in a minimally invasive and relatively pain-free manner, without requiring extensive patient recovery. 
         [0077]    Referring now to  FIG. 9 , a modular stimulation lead kit  200  arranged in accordance with another preferred embodiment of the present invention is shown. The kit  200  is similar to the previously described kit  100 , with the exception that the kit  200  comprises secondary stimulation leads  204  that minimize the profile of the resulting assembly (shown in  FIG. 10 ), as it exits the spine of the patient. In particular, each secondary stimulation lead  204  comprises a shortened sheath body  232 , electrodes  240  mounted to the sheath body  232 , electrical conductors  242  extending from the sheath body  232 , and a connector  244  that receives the proximal ends of the electrical conductors  242 . The connector  244  comprises a plurality of terminals  238  that are similar to the previously described lead terminals  138 . The sheath body  232  is composed of the same material and has the same general shape as the sheath body  132  of the previously described secondary stimulation lead  104 . The sheath body  232 , as illustrated in  FIG. 9 , however, is much shorter, so that it can be entirely received within the epidural space of the patient, i.e., the sheath body  232  will not extend out of the patient&#39;s back when fully deployed within the epidural space. The electrical conductors  242 , because they are exposed, are preferably composed of an electrically insulative material. 
         [0078]    The kit  200  further comprises a pusher  214  that can be used to facilitate introduction of the respective secondary stimulation lead  204  along the primary stimulation lead  102  once the entire sheath body  232  of the secondary stimulation lead  204  is within the patient&#39;s back. The pusher  214  comprises a cylindrical rod  216  having a distal tip  218  and a proximal end  220 , and a handle  222  mounted on the proximal end  220  of the rod  216 . The distal tip  218  of the rod  216  is adapted to be received within an opening  224  (shown in  FIG. 11 ) at the proximal end of the sheath body  232 , thereby facilitating stable engagement between the pusher  214  and respective secondary stimulation lead  204 . 
         [0079]    The kit  200  can be installed and used in the same manner as the previously described kit  100  in treating chronic pain. In particular, the patient is prepared and the primary stimulation lead  102  is delivered into the epidural space  16  of the patient&#39;s spine, so that the electrodes  120  are placed adjacent the target stimulation site  18  in the same manner described above with respect to  FIGS. 8A and 8B . One of the secondary stimulation leads  204  is then delivered into the epidural space  16  in the same manner as the secondary stimulation lead  104  described above was delivered, with the exception that the pusher  214  is used to advance the secondary stimulation lead  204  along the primary stimulation lead  102  until fully deployed within the epidural space  16  ( FIG. 12A ). The remaining stimulation lead  204  is delivered into the epidural space  16  in the same manner to complete the stimulation lead assembly  210  ( FIG. 12B ). 
         [0080]    Notably, because the percutaneous opening  24  need only support, at most, two sheath bodies at one time, it can be made smaller, or alternatively, additional stimulation leads with shortened sheath bodies can be introduced within the epidural space  16  without increasing the size of the percutaneous opening. After the stimulation lead assembly  210  has been formed within the epidural space, it is tested and optimized. The extension lead  108  is then connected between the stimulation leads  102 / 204  and the stimulation source  106 , and the incisions are closed to fully implant the system, as previously described above. 
         [0081]    Referring now to  FIG. 16 , an alternative embodiment of a secondary stimulation lead  304  engaged with one side of the primary stimulation  102  is illustrated. Although not shown, another secondary stimulation lead  304  can be engaged with the opposite side of the primary stimulation lead  102 . The secondary stimulation lead  304  is similar to the previously described secondary stimulation lead  104 , with the exception that the secondary stimulation lead  304  comprises a flap  303  on which the electrodes  140  are mounted. The secondary stimulation lead  304  can, alternatively, have a shortened sheath body much like the secondary stimulation lead  204  illustrated in  FIG. 9 . The flap  303  is designed to be constrained by the primary stimulation lead  102  to facilitate percutaneous delivery of the secondary stimulation lead  102 , and released by the primary stimulation lead  102  to deploy the electrodes  140  into contact with the neural tissue. 
         [0082]    In particular, the edge of the flap  303  comprises a coupling mechanism  305  that is designed to fit snugly within the respective slot  150  of the primary stimulation lead  102 , along with the rail  152  of the secondary stimulation lead  102 , when the secondary stimulation lead  304  is slidably engaged with the primary stimulation lead  102 , as illustrated in  FIG. 17 . As the rail  152  of the secondary stimulation lead  102  exits the slot  150  of the primary stimulation lead  102 , however, the coupling mechanism  305  of the flap  303  will release from the slot  150 , thereby allowing the flap  303  to deploy, placing the electrodes  140  into contact with the underlying tissue, as illustrated in  FIG. 18 . It should be noted, that, when the secondary stimulation lead  304  is used in the kit  100  or kit  200 , the slots  150  in the primary stimulation lead  102  will not terminate as hereinbefore described, but will rather open up at the distal tip of the primary stimulation lead  102 , so that the flap  303  can exit the respective slot  150  and be released by the primary stimulation lead  102 . 
         [0083]    Installation and use of the secondary stimulation lead  304  in forming the stimulation lead assembly  110  illustrated in  FIG. 2 , or alternatively the stimulation lead assembly  210  illustrated in  FIG. 10 , is similar that previously described above. 
         [0084]    Although in all of the previous embodiments, the primary stimulation lead  102  was used to provide a means of guiding the secondary stimulation leads  104  into the percutaneous opening within the patient and adjacent the target tissue region, as well as to provide a means of stimulating the tissue region, a guide member similar to the primary stimulation lead  102 , but lacking stimulation capability, can be alternatively used to similarly guide the secondary stimulation leads  104  through the percutaneous opening to the target tissue region. In this case, only the secondary stimulation leads  104  will be used to stimulate tissue. 
         [0085]    Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.