Abstract:
Strain relief loops are forced by being formed into medical leads such that a body of the lead imposes a force to regain the loop if the loop has been disturbed. Because the strain relief loop is forced, the surgeon implanting the medical lead is not required to create the strain relief loop as a step in the implantation procedure. Forcing the strain relief loop ensures that the strain relief is achieved. The forced strain relief loop also ensures that the loop is present to reduce heating at the electrodes of the medical caused by exposure to excessive radiofrequency energy. The forced strain relief loop may be created by heating the lead body while held in the loop configuration by a mold to cause the loop configuration to persist once the medical lead is removed from the mold.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments are related to medical leads that carry electrical signals from a medical device. More particularly, embodiments are related to medical leads with forced strain relief loops. 
       BACKGROUND 
       [0002]    Medical leads provide electrical stimulation from a medical device to a target site within a body of a patient. The medical device is typically implanted or otherwise installed on the body in an accessible area at some distance from the target site, and the medical lead is routed to the target site either through a percutaneous procedure or by surgical implantation depending upon the type and size of the medical lead being implanted. 
         [0003]    Because the medical lead extends some distance between the medical device and the target site within the body, the medical lead is subject to forces imposed by movements of the patient. In particular, the medical lead may be subjected to strain. To address the strain, the medical lead may be routed by creating a loop that relieves the strain by the loop making available an additional length of the lead. 
         [0004]    An additional benefit of the strain relief loop occurs in relation to radiofrequency (RF) heating at the electrodes. RF heating can occur when the patient is exposed to relatively high levels of RE energy such as during a magnetic resonance imaging (MRI) scan. The metal conductors within the lead such as filars connected to the electrodes have current induced by the RF energy. This induced current can produce heating within the medical lead and at the electrodes. The presence of the strain relief loop, particularly if the loop is created in relatively close proximity to the distal end of the medical lead where the electrodes are located, reduces such heating which improves the comfort of the patient and lessens the risk of injury during MRI scans. 
         [0005]    An issue is that strain relief loops may be considered optional and/or may be overlooked by some clinicians and therefore are not necessarily created in all instances. Thus, one implantation of a given medical system that includes a strain relief loop may offer better protection for the patient from RF heating than a same medical system that is implanted in a patient without a strain relief loop. 
       SUMMARY 
       [0006]    Embodiments address issues such as these and others by providing medical leads and related systems where the medical lead has a forced strain relief loop. The strain relief loop is forced by having the strain relief loop be formed by a lead body of the medical lead in such a way that the lead body applies force to regain the formed loop whenever the formed loop is disturbed. In this manner, forming the strain relief loop is not a step of the implantation process and therefore is not optional considering the medical lead already has the forced strain relief loop at the time of implantation. 
         [0007]    Embodiments provide a medical lead that includes an insulative lead body having a formed loop such that the insulative lead body imposes a force to regain the formed loop when the formed loop is disturbed. The medical lead further includes at least one electrical conductor surrounded by the insulative lead body and at least one electrical contact on a proximal end of the lead body, the at least one electrical contact being electrically coupled to the at least one electrical conductor. The medical lead further includes at least one electrode on a distal end of the lead body, the at least one electrode being electrically coupled to the at least one electrical conductor. 
         [0008]    Embodiments provide a medical system that includes a medical device with a stimulation output connector and a medical lead. The medical lead includes an insulative lead body having a formed loop such that the insulative lead body imposes a force to regain the formed loop when the formed loop is disturbed. The medical lead further includes at least one electrical conductor surrounded by the insulative lead body and at least one electrical contact on a proximal end of the lead body, the at least one electrical contact being electrically coupled to the at least one electrical conductor and in electrical contact with the stimulation output connector. The medical lead additionally includes at least one electrode on a distal end of the lead body, the at least one electrode being electrically coupled to the at least one electrical conductor. 
         [0009]    Embodiments provide a method of making a medical lead that involves surrounding a conductor with an insulative lead body and mounting a contact on a proximal end of the insulative lead body and in electrical connection to the conductor. The method further involves mounting an electrode on a distal end of the insulative lead body and in electrical connection to the conductor. Additionally, the method involves forming a loop in the insulative lead body such that the insulative lead body imposes a force to regain the formed loop when the formed loop is disturbed. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows an example of a medical system environment where a forced strain relief loop is present on a medical lead. 
           [0011]      FIG. 2  shows a process for creating an example of a medical lead having a forced strain relief loop. 
           [0012]      FIG. 3  shows two mold halves that form the strain relief loop onto the medical lead, 
           [0013]      FIG. 4A  shows a first view of an example of the forced strain relief loop according to various embodiments of a medical lead. 
           [0014]      FIG. 4B  shows a second view of an example of the forced strain relief loop according to various embodiments of a medical lead. 
           [0015]      FIG. 5  shows another example of the forced strain relief loop according to various embodiments of a medical lead where a protective band is present about two adjacent leads. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Embodiments provide forced strain relief loops on medical leads. The strain relief loop is formed on the lead body so that the loop is regained whenever disturbed by force imposed by the lead body. Thus, the strain relief loop is not optional but is present on the medical lead when being implanted and is re-established automatically by the action of the lead body if the loop is manipulated. 
         [0017]      FIG. 1  shows an example of a medical system  100  that includes a medical device  102  and a medical lead  104 . The medical device  100  produces stimulation signals and the medical lead  104  which is attached to a stimulator output of the medical device  100  delivers the stimulation signals to a set of electrodes  106  on a distal end of the medical lead  104 . The set of electrodes  106 , such as a paddle having a grid of electrodes, are positioned at a target stimulation site so that the stimulation signals provide stimulation therapy to the body  112  of the patient. 
         [0018]    The medical lead  104  includes a forced strain relief loop  108 . This strain relief loop  108  is already formed on the lead  104  prior to implantation. Thus, a surgeon implanting the medical lead  104  is not required to form the strain relief loop  108  but instead positions the medical lead  104  including the forced loop  108  within the body of the patient  112 . The strain relief loop  108  retains the loop shape during implantation and thereafter. Thus, benefits of the strain relief loop  108  are ensured, such as the strain relief itself as well as the reduction in RF heating should the patient  112  be exposed to significant RF energy such as during an MRI scan. 
         [0019]    It has been found that at typical MRI frequencies, producing the medical lead  104  with the forced strain relief loop  108  at between 5 and 15 centimeters from the distal end  106  provides the best level of heating reduction at the distal end  106  where the electrodes are present. The distance of the forced strain relief loop  108  to the distal end may be measured from an intersection point  110  where the medical lead  104  has looped back onto itself which is also referred to as the crossover. 
         [0020]    This intersection point  110  tends to develop more heating from RF energy than at other points along the medical lead  104 . Therefore, the medical lead  104  may include features to alleviate any issues that might arise from the additional heating at the intersection point  110 . These features may be of various forms. Some examples include an increased thickness of the lead body at the intersection point  110  and/or a protective sleeve that covers the intersection point  110 . Such examples are discussed in more detail below with reference to  FIGS. 4 and 5 . 
         [0021]    While  FIG. 1  shows a single medical lead  104 , some embodiments may provide for multiple adjacent medical leads, with each having a loop  108  and the loops may be made adjacent. For example, a distal end  106  with a paddle may have many electrodes such that a single paddle is connected to two adjacent lead bodies. As another example where there are multiple medical leads  104 , each lead  104  may have a separate distal end  106  and the forced loops  108  of each lead are may be formed adjacently to one another. 
         [0022]      FIG. 2  shows a process for creating one example of a medical lead  104  with a forced strain relief loop  108 . In this example, the process begins with one or more filar conductors  202  which may be coiled filars as shown or may be cabled filars. Generally, there is a filar conductor present for each proximal contact and each distal electrode that will be present on the completed medical lead  104 . The one or more conductors  202  may be constructed of various conductive biocompatible materials such as MP35N, MP35N with a silver core, titanium molybdenum, and the like. Each conductor typically has an insulative coating. 
         [0023]    In this particular example, an inner insulative lead body layer  204  is fitted to or otherwise applied to surround the one or more conductors  202 . The inner insulative lead body layer  204  may surround the conductor either by forming a lumen that the conductor  202  is present within or by encapsulating the conductor  202 . The inner insulative lead body layer  204  may be constructed of various non-conductive biocompatible materials such as polyurethane, silicone, silicone-polyether-urethane, and the like. The inner insulative lead body layer  204  may be fitted to the conductors  202  by building the layer  204  separately and then stringing the conductors  202  through the lumen of the layer  204 . Alternative manners of applying the layer  204  are also applicable, such as for example extruding or injection molding the layer  204  onto the conductors  202 . 
         [0024]    In some embodiments, the medical lead  104  may now be ready to have proximal contacts  210  and distal electrodes  216  on a distal paddle  218  installed and also have the forced strain relief loop  108  formed in the lead body layer  204 . However, in some cases it may be desirable to provide additional structure such as shielding to further protect the medical lead  104  from unwanted RE heating on the filar conductors  202  and electrodes  216 . 
         [0025]    In one embodiment for providing such additional structure, a next action taken is to provide a conductive shield  206  about the inner insulative lead body  204 . In the particular example shown, the conductive shield  206  is a braided shield that has been placed about the inner insulative lead body  204 . The braided shield  206  may be applied in various ways, such as by being braided directly onto the insulative inner lead body  204 . The braided shield may be constructed from various biocompatible conductive materials such as tantalum, titanium, and other similarly conductive materials. 
         [0026]    The conductive shield  206  may take other forms as well. One example is a foil tube that wraps about the inner insulative lead body  204 . Another example is a metallic layer that has been sputtered onto the outer surface of the insulative inner lead body  204 . Utilizing a carbon nanotube structure as a dopant or a coating to the body  204  is yet another example. 
         [0027]    The shield  206  may then be protected by applying an insulative outer lead body  208  that surrounds the shield  206  and hence the insulative inner lead body  204  and conductors  202 . The insulative outer lead body  208  may be constructed of various non-conductive biocompatible materials such as the same materials listed above for the inner body layer  204 . The insulative outer lead body layer  208  may be applied in various ways, such as by fitting the layer  208  onto the shield  206  and layer  204  and utilizing a reflow process, extrusion, or injection molding. 
         [0028]    In addition to applying the insulative outer lead body  208 , the proximal contacts  210  and distal paddle  218  and/or distal electrodes  216  are installed and electrically connected to the corresponding electrical conductors  202  in the conventional manner. At this point, the medical lead  104  is complete. However, the strain relief loop  108  may then be formed in the insulative lead bodies  204  and/or  208  in such a manner to force the strain relief loop  108  to persist on the medical lead  104 . 
         [0029]    One manner of forming the stain relief loop  108  is to utilize a mold  220  that includes a looped passageway  222  that the medical lead  104  is positioned within at the appropriate point along the medical lead  104 . Heat is then applied to the mold  220  and medical lead  104  within the mold  220  to force the strain relief curve to persist in the insulative lead body of the medical lead  104 . The medical lead  104  is then removed from the mold  220  and is ready to be packaged for shipment with the forced strain relief loop  108  present. For embodiments where multiple lead bodies are looped adjacently, each lead body may be baked in separate molds and then made adjacent once removed or alternatively, the mold  220  may be sized to accommodate multiple adjacent lead bodies and form the loop  108  in the multiple leads  104  simultaneously. 
         [0030]    Some specific examples for heating the medical lead  104  to create the strain relief loop are as follows. In one example, a polyurethane lead body with a hardness of 80 Shore A, the lead  104  is heated at approximately 220 to 270 degrees Fahrenheit for approximately 20-30 minutes. In another example, a polyurethane lead body with a hardness of 55 Shore D, the lead  104  is heated at approximately 230 to 280 degrees Fahrenheit for approximately 20-30 minutes. 
         [0031]    An example of the mold  220  is shown further in  FIG. 3  in the opened state. Here, the mold  220  has two halves  302 ,  304 . Each half  302 ,  304  may be constructed of a material such as stainless steel, aluminum, and the like that is capable of holding the lead  104  in the looped configuration while also distributing heat to the medical lead  104 . The half  302  includes a looped channel  306  where one direction of the loop has a deeper channel portion  310  at the intersection point to allow the medical lead  104  to cross over itself within the mold  220 . Similarly, the half  304  includes a looped channel  308  where one direction of the loop has a deeper channel portion  312  at the intersection point to also allow the medical lead  104  to cross over itself within the mold  220 . These two halves  302 ,  304  are brought into engagement with the medical lead  104  present within the channels  306 ,  308 , and then heat is applied such as by placing the mold  220  including the medical lead  104  within an oven. 
         [0032]    Other manners of forcing the strain relief loop  108  are also available. These include bonding the two portions of the outer body  208  together at the intersection point. This bond may be produced in various ways, such as by using an adhesive or by melting the two portions together. The bond may later be broken while preserving the loop upon implanting the lead to maintain the strain relief function of the loop. 
         [0033]      FIGS. 4A and 4B  show an example of a lead  104 ′ that has a modified lead body portion  402  present at the intersection point  110 . In this example, the modified lead body portion  402  has a greater diameter and hence a greater thickness than the remainder of the medical lead  104 ′. This increased thickness at the modified portion  402  maintains a greater separation between internal conductive items of the medical lead  104 ′ such as filar conductors and/or shields of the intersecting passes of the medical lead  104  at the intersection point  110 . Such separation reduces the amount of RE heating that is produced by the intersection point  110 . This separation is best viewed in the cross-sectional view of  FIG. 4B  take at the intersection point, where the shields  206  and conductors  202  of each pass of the lead  104  have increased separation due to the enlarged lead body portion  402 . 
         [0034]    Additionally, the increased thickness of the modified portion  402  creates a greater separation between the conductive items within the medical lead  104 ′ at the intersection point  110  and the tissue of the patient present at the intersection point  110 . Thus, the tissue of the patient is also better insulated from the RF heating by the modified portion  402 . In  FIG. 4B , the modified portion  402  is present on one pass of the lead  104 . If the same amount of separate of the shield  206  and conductors  202  are desired for both passes, then both passes of the lead shown in  FIG. 49  would be provided with the increased thickness as shown for modified portion  402 . 
         [0035]      FIG. 5  shows an example of two adjacent medical leads  104 ,  105  that also includes a sleeve  502  positioned on the medical leads  104 ,  105  so as to cover the medical leads  104 ,  105  at the intersection point  110  of both. This sleeve  502  may be installed on the leads  104 ,  105  prior to forming the strain relief loops  108  and the sleeve  502  can be formed into the loop configuration as well. Alternatively, the sleeve  502  can be added after the loops  108  have been formed. The sleeve  502  may be constructed of materials such as PEEK, polyurethane, silicone, and the like. The sleeve  502  provides the same benefits as the modified portion  402  of  FIG. 4 . The sleeve  502  provides separation of conductive items between the two passes of the lead body  104  at the intersection point  110  to reduce overall RF heating of the intersection point  110 . Additionally, the sleeve  502  provides an additional layer of insulation to further separate the conductive items of the medical lead  104  at the intersection point  110  from the tissue of the patient. For embodiments where multiple leads  104 ,  104  are looped adjacently as shown, an alternative is for a sleeve  502  to be present on each lead  104 ,  105  individually rather than a single sleeve  502  surrounding the multiple lead bodies. Additionally, a sleeve  502  may be providing around the medical lead  104  for scenarios where only the single medical lead  104  is present. 
         [0036]    While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.