Abstract:
An improved electrical neurological stimulation paddle lead is described. The paddle lead comprises two flexible concave paddle bodies that are joined together at their opposing convex surfaces. The first paddle body contains a series of electrodes that are embedded on the concave surface that expand to fit the contours of the dura mater. The second paddle body consists of a concave surface that is pressed against the bone of the spinal column to act as a fixation mechanism to keep the paddle assembly in place.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. Provisional Application Ser. No. 61/121,966 filed Dec. 12, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related generally to implantable medical electrical leads. More specifically, the present invention is related to implantable neurological leads. 
     2. Prior Art 
     Neurostimulation is the application of electrical energy to neurological tissue to block the sensation of pain. A medical device, specifically an implanted neurostimulator generates an electrical pulse which is emitted from a connected lead that is implanted in the body. 
     Despite the many advances in neurostimulation, many problems still exist with the technology that have yet to be optimized. One of which is lead migration, the second being energy efficiency. 
     An ideal neurostimulation lead is designed to remain in position and emit electrical energy to a specific targeted nerve or array of nerves in an energy efficient manner. However the geometric constraints of the human anatomy sometimes make it difficult to stimulate the targeted neurological tissue in an energy efficient manner. The confined spaces of the spinal column add an increased element of complexity to neurological tissue stimulation. In addition, the delicate nature of the neurological tissue make lead fixation challenging. 
     One such neurostimulator lead is the percutaneous lead. This lead has a long lumen with a small cylindrical diameter. Discrete metal electrode bands are wrapped circumferentially around the proximal and distal regions of the cylindrical lumen of the lead. 
     The small cylindrical diameter and long slender length of the lead make it advantageous for implantation into a patient with minimal tissue trauma. Percutaneous leads are typically inserted through a small opening in the patient and advanced into position from outside a patient&#39;s body. However, despite their advantages of implantation, percutaneous leads are often ineffective in targeting specific neurological tissue in an energy efficient manner. 
     Using a neurostimulator implantable medical device, electrical signals are programmed to be emitted from selected electrode bands around the lead. Once activated, the percutaneous lead radially broadcast electrical energy all around the circumference of the electrode band. The lead indiscriminately emits electrical energy 360 degrees completely around the lead body in the hope of hitting the desired location of the neurological tissue. 
     This approach does not efficiently utilize the electrical energy of the medical device. A significant amount of electrical energy is transmitted in unintended directions away from the targeted neurological tissue. As a result of the indiscriminately broadcasted energy, a power burden is placed on the implanted medical device. This causes the device&#39;s power supply to be drained at a faster rate, thus requiring the device&#39;s power supply to be frequently replenished either through recharging or replacement. Percutanous leads also lack a fixation mechanism which makes them prone to movement within the body. 
     An alternative neurostimulation lead that has been designed to improve energy efficiency is referred to by those skilled in the art as a paddle lead. As its name implies, the paddle lead has a flat rectangular distal end resembling a paddle. The traditional paddle body is rectangular in shape with flat planar top and bottom sides. Electrical energy is emitted from an array of electrode pads which are typically embedded in one side of the paddle body. 
     Since paddle leads only have electrodes on one side, unlike that of percutaneous leads, the paddle lead can only emit electrical energy in a 180 degree semi-spherical arc from the paddle surface. Therefore, paddle leads are more energy efficient than percutaneous leads. However, a disadvantage to the traditional paddle lead is that they are typically designed with a top and bottom planar surface that do not conform to curved surfaces. This is not an ideal shape for focusing electrical energy to a specific location located around a cylindrical spinal column and spinal cord. Traditional paddle leads are an improvement in energy efficiency from percutaneous leads, however, more is desired in focusing the electrical energy to a specific area or point of neurological tissue. 
     Having a lead with a curved surface facing the spinal column improves the ability to focus electrical energy to specific neurological tissue and would allow for more uniform spacing between the paddle body at the distal end of the lead and the spinal column. Unfortunately, this shape alone without a fixation mechanism, would allow for the paddle body to rotate in the epidural space of the spinal column with little resistance. Paddle leads lack a fixation mechanism and therefore are susceptible to lead migration. 
     A curved wing paddle design is disclosed in U.S. Pat. Nos. 6,999,820 and 7,613,524, both to Jordan. As stated in both the &#39;820 and &#39;524 patents, “the wings on the outer edge of the lead serve to stabilize and immobilize the lead with respect to the targeted tissue and assist in focusing the electrical energy”. 
     However the winged electrode body design by Jordan is one rigid piece that lacks a fixation mechanism. Because of its rigid wing design, the paddle is free to rotate around the spinal cord with little resistance. This kind of movement could cause the paddle electrode to move further away from the target nerve and possibly result in an increase in the amount of cerebrospinal fluid (CSF) between the electrode and target nerve. CSF is a biological fluid that flows between the spinal cord and dura mater. An increase in CSF between the electrodes and targeted neurological tissue is not desired because it decreases the electrical efficiency of the system. 
     What is desired is a more energy efficient lead that conforms to the spinal column and incorporates a fixation mechanism for holding the lead in place to minimize rotation, movement and migration of the lead when implanted in the body. 
     SUMMARY OF THE INVENTION 
     The present invention is a paddle lead that is designed to address the shortcomings of the prior art. The disclosed paddle lead is one that is more energy efficient, provides a fixation mechanism and improves the directional control of the electrical signal. 
     Specifically, the present invention is a neurostimulator lead with a double curved flexible paddle assembly design. The paddle assembly consists of two curved paddles (a right side paddle and a left side paddle) with opposing flexible concave front sides that are connected at the apex region of the curvature of the convex backsides. Both right and left side paddles are composed of a biocompatible polymer that adds flexibility and resiliency to the paddles. The right side of the assembly is designed to be placed in contact with the bone of the spinal column as the fixation mechanism whereas the opposing left side with an array of embedded electrodes is designed to compress against the dura mater, a neurological tissue membrane which surrounds the spinal cord. 
     Once implanted into the epidural space of the spinal column, the concave end portions of the right side compress against the bone of the spinal column, acting like a spring in pushing the opposing left side paddle forward into the dura mater, fixating the paddle assembly in place. 
     The concave surface of the left side expands and conforms to the contours of the curved neurological tissue, such as the dura mater of the spinal cord. The embedded electrodes in the surface of the left paddle are also compressed into the dura mater of the spinal cord, thereby providing improved directional control of the stimulation signal. 
     Compression of the left paddle into the neurological tissue restricts the flow of cerebrospinal fluid (CSF) which flows between the dura mater and the spinal cord on the side of the implanted paddle assembly. As will be discussed in more detail, restriction of CSF is desirable in increasing the energy efficiency of the medical device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a prior art neurological stimulator paddle lead  10 . 
         FIG. 1B  is a side view of the prior art paddle lead  10  shown in  FIG. 1A . 
         FIG. 2  is a perspective drawing of the present invention of a neurological stimulator paddle lead  30 . 
         FIG. 3  is a cross-sectional view of the paddle assembly  34  implantation site taken along a horizontal axis of a patient before implantation. 
         FIG. 4  is a cross-sectional view of the paddle assembly  34  implantation site taken along a horizontal axis of a patient after implantation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1A and 1B  show an example of a traditional prior art paddle lead  10 . The distal end of the lead  12  has an extension that resembles a paddle. The distal extension is typically a rectangular body with a planar top surface  14  and a planar bottom surface  16 . Electrodes  18  are embedded within the top surface  14 . As previously mentioned, this lead design is not optimized to fit properly in the curved confines of the epidural space between the spinal column and the dura mater and, as such, does not provide an efficient means to direct electrical energy to stimulate the spinal cord. In addition, this lead design lacks an anchoring mechanism to prevent the distal end from migrating in the epidural space. 
       FIG. 2  illustrates an electrode  30  according to the present invention. Electrode  30  is comprised of a lead body  32  and a paddle body assembly  34 . As  FIG. 2  shows, the paddle body assembly  34  is attached to the distal region  32 B of the lead body along a longitudinal axis A-A. Lead body  32  has an elongate body that extends from a proximal region  32 A to the distal region  32 B. Electrodes  36 A,  36 B,  36 C and  36 D are embedded in the surface of the first curved paddle body  38 . The electrodes  36 A- 36 D are shown aligned along the longitudinal axis A-A. Alternatively, the electrodes  36  could be embedded anywhere within the surface of the first curved paddle body  38 . Furthermore, the embedded electrodes  36  could also have a multitude of shapes, not limited to, rectangular, square, circular, triangular, or combinations thereof. One could also design an array of electrode bands whereby the bands comprise a series of lines that are arranged in a parallel, perpendicular, circular or random pattern on the surface of the first paddle  38 . 
     In a preferred but not limiting embodiment, the array of electrodes  36 A- 36 D lies within the space between parallel lines B-B and C-C. The space between parallel lines B-B and C-C define an area where paddles  38  and  40  are connected to each other. This allows for the lead body  32  to connect with the series of electrodes in the center of the paddle assembly  34 . 
     An alternate embodiment of the invention comprises an array or multitude of electrodes which lie outside parallel planes B-B and C-C within paddle  38 . In a further alternate embodiment, one could design the invention with a plurality of electrodes that lie parallel to the longitudinal axis A-A such as in a column or multiple columns. Likewise, one could design the invention with a plurality of electrodes that lie perpendicular to the longitudinal axis A-A such as in a row or multiple rows. One could also design the invention with a combination of electrodes that are both parallel and perpendicular ( 39 A- 39 D) to the longitudinal axis A-A. 
     Conductors connect the respective electrodes  36 A- 36 D to the proximal region of the lead body  32 A. Each conductor separately connects to a metal band (not shown) within the proximal region  32 A of the lead and an individual electrode  36  within the distal region of the lead. The conductors reside within and traverse the length of the lead body, from the proximal region  32 A to the distal region  32 B thereof. The conductors are preferably wires that are composed of a silver cored material. Alternate materials such as stainless steel, platinum, platinum alloy, MP35N, titanium, silver, gold, palladium or nickel alloy in an insulated or uninsulated form can also be used. The conductor wire should be of about the length of the electrical stimulator lead  30  and of a diameter that fits freely with multiple conductor wires inside the hollow lead body  32 . A preferred conductor wire diameter is about 0.1 mm and can range from about 0.025 mm to about 0.25 mm. The conductors are preferably round; however, they can also be flat or in the form of a cable. 
     The proximal end of the lead body  32 A is connected to the header of a medical device (not shown). It is preferred that a neurostimulator is connected to the lead body  32 . It is contemplated that although the present invention is intended for use with a neurostimulator to stimulate neurological tissue, one could also use the invention to stimulate cardiac tissue as well. Therefore, the present invention could be connected and used in conjunction with other implantable medical devices such as pacemakers and defibrillators. 
     Paddle assembly  34  is a fusion of two paddles  38  and  40 . Each paddle has a concave front side  42 ,  46  and a convex backside  44 ,  48 . As  FIG. 2  shows, the two paddles are positioned back to back with the concave front sides  42 ,  46  opposing each other. The convex backsides of the two paddles  44 ,  48  are joined together along the longitudinal axis A-A. The connection region  50  is defined between longitudinal lines B-B and C-C which are parallel to longitudinal axis A-A. It is preferred that the length between longitudinal lines B-B and C-C is about 0.25 mm to about 2.50 mm and that the resultant concave curvature of paddles  38 ,  40  is between 5 to 30 degrees. The overall width of paddle assembly  34  as measured from the right side paddle end portions  38 A,  40 A to the left side paddle end portions  385 ,  40 B is about 1.0 mm to 15 mm. The overall length of paddle assembly  34  as measured from the most proximal point to the most distal point of the paddle assembly  34  along longitudinal axis A-A is about 1.0 mm to about 15.0 mm. 
     To control the curvature and flexibility of the paddles  38 ,  40 , a person skilled in the art could adjust the distance between parallel lines B-B and C-C that defines the connection region  50 . For example, increasing the distance between parallel lines B-B and C-C decreases the degree of curvature and flexibility of the convex paddles  38 ,  40 . In contrast, decreasing the distance between parallel lines B-B and C-C increases the degree of curvature and flexibility of the paddles  38 ,  40  and the resultant paddle assembly  34 . 
     In a preferred embodiment the concave front side surfaces  42 ,  46  of respective paddles  38  and  40  have a continuous curvature. The trough of the concave surface  42 ,  46  is parallel to axis A-A and extends from one end of the paddle to the other. However, both paddles  38 ,  40  could be designed with a planar region at the trough of the concave surfaces  42 ,  46 . Such a planar portion, particularly with regards to paddle  38 , would provide a planar surface to embed electrodes  36 . 
     In the context of the present invention, the term “concave” is meant to describe a curved surface on which neighboring lines normal to the curved surface converge and on which lies the chord joining two neighboring points of the curved surface. The depth of curvature of the concave surface  46  is from about 1 percent to about 25 percent of the distance between a line tangent to where the concave surface  46  meets the end walls  41 A and  41 B. 
     Electrode  36  is shown embedded in the concave front surface of paddle  38 . A portion of the electrode  36  is shown protruding from the concave front surface of the paddle  38 . This protrusion allows for improved contact with the neurological tissue. Although not preferred, one skilled in the art could design the electrode  36  to not protrude from the surface of the paddle  38  and, therefore, be flush with the concave surface. 
     Both paddles  38  and  40  are composed of a biocompatible polymeric material, preferably silicone rubber. This material gives paddles  38 ,  40  a solid yet flexible structural form. Paddles  38 ,  40  are designed to be flexible and bend under compression without tearing or creating damage to the paddle assembly  34 . Specifically end portions  38 A,  38 B,  40 A and  40 B of the respective paddles  38  and  40  are design to bend and flex independent of each other. Other biocompatible polymeric materials such as polytetrafluoroethylene (PTFE), polyurethane, and polyimide could also be used. 
     The flexing action of paddle end portions  40 A,  40 B which are curved in a concave form, create a spring like action that pushes against the bone of the spinal column and fixates the lead in place once implanted. The fusion of the two curved paddles  38 ,  40  create interstitial spaces  52  and  54  between the end portions of the paddles  38 ,  40 . As the paddle end portions  38 A,  383 ,  40 A and  40 B are compressed, the interstitial spaces  52  and  54  decrease. Once the compression is relieved, the interstitial spaces  52 ,  54  increase. 
     The spring like action of the flexible end portions of the paddle  40 , compress paddle  38  into the dura mater  60 . By compressing paddle  38  into the dura mater, the flexible curved end portions of paddle  38  conform to the curved shape of the dura mater  60 , thereby stabilizing and fixating the paddle assembly  34  into place. 
     Compression of the first paddle  38  into the neurological tissue also improves the stimulation efficiency by focusing the electrical energy directly to the area of intended stimulation. The conforming front surface  42  of the first paddle  38  directs the electrical energy to a focused point or area of stimulation. The electrical energy is no longer being emitted indiscriminately in an array of directions. 
     As shown in  FIGS. 3 and 4 , compression of paddle  38  into the dura mater  60  restricts the flow of cerebral spinal fluid (CSF)  62  along the side of the implanted paddle assembly  34 . CSF is a biological fluid that flows between the spinal cord  64  and the dura mater  60 . The restriction of CSF  62  improves the efficiency of the electrical energy in reaching the targeted neurological tissue of the spinal cord.  64  from the electrodes  36 A- 36 D. First, reduction of the distance between the electrodes  36  of the paddle assembly  34  and the spinal cord  64  reduces the amount of CSF through which electrical energy must pass. Secondly, CSF  62  has been known to diffuse electrical signals. Reducing the amount of CSF  62  reduces undesirable signal diffusion and improves electrical signal efficiency. 
       FIG. 2  depicts the preferred embodiment in which there are four individual electrodes  36 A- 36 D. One of the conductors is connected to a specific electrode pad. Although it is preferred to have four electrodes  36 A- 36 D, one skilled in the art could design such a lead with fewer or more than four electrodes as desired. 
     The paddle assembly  34  is implanted in the epidural space between the spinal column  58  and spinal cord  64 , specifically the space between the spinal column  58  and the dura mater  60  of the spinal cord  64 . Paddle  38 , with embedded electrodes  36 A- 36 D, is positioned towards the dura mater  60  so the electrodes are in contact with the dura mater  60 . 
     The lead is implanted by first accessing the targeted area along the spinal column. The curved paddles  38 ,  40  of the paddle assembly  34  are compressed together for insertion into the epidural space. Once inserted, paddles  38 ,  40  are released, expanding the area of the interstitial space  52 ,  54  and compressing the paddle assembly  34  into place. 
       FIG. 4  illustrates the implanted paddle assembly  34  after implantation between the spinal column  58  and dura mater  60 . As the figure illustrates, the concaved paddle  40  is compressed against the bone of the spinal column  58  and the concaved paddle  38  is compressed against the dura mater  60 . Paddle end portions  40 A and  40 B are pushing against the spinal column  58 . The paddle lead is now confined into place and cannot move. As the figure illustrates, the space between the dura mater  60  and spinal cord  64  is reduced due to the compression of the paddle assembly  34  towards the spinal cord  64 . Reduction in the space between the electrode side of the paddle assembly and the dura  60  not only confines the paddle assembly  34  into place, but also restricts the flow of CSF and reduces the gap between the dura mater  60  and the targeted neurological tissue  64 . Therefore, the electrical efficiency of the medical device system is improved. A shorter transmission distance is now required and there is less impedance created by CSF  62  which diffuses the electrical signal being emitted from the electrode  36 . 
     It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.