Patent Publication Number: US-7904149-B2

Title: Implantable medical elongated member including fixation elements along an interior surface

Description:
This application is a continuation of application Ser. No. 11/591,279 (U.S. Publication No. US 2008/0103569), entitled, “IMPLANTABLE MEDICAL ELONGATED MEMBER INCLUDING FIXATION ELEMENTS ALONG AN INTERIOR SURFACE,” and filed on Oct. 31, 2006, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to medical device systems and, more particularly, to elongated members in medical device systems. 
     BACKGROUND 
     Electrical stimulation systems may be used to deliver electrical stimulation therapy to patients to treat a variety of symptoms or conditions such as chronic pain, tremor, Parkinson&#39;s disease, multiple sclerosis, spinal cord injury, cerebral palsy, amyotrophic lateral sclerosis, dystonia, torticollis, epilepsy, pelvic floor disorders, gastroparesis, muscle stimulation (e.g., functional electrical stimulation (FES) of muscles) or obesity. An electrical stimulation system typically includes one or more implantable medical leads coupled to an electrical stimulator. 
     The implantable medical lead may be percutaneously or surgically implanted in a patient on a temporary or permanent basis such that at least one stimulation electrode is positioned proximate to a target stimulation site. The target stimulation site may be, for example, a nerve or other tissue site, such as a spinal cord, pelvic nerve, pudendal nerve, stomach, bladder, or within a brain or other organ of a patient, or within a muscle or muscle group of a patient. The one or more electrodes located proximate to the target stimulation site may deliver electrical stimulation therapy to the target stimulation site in the form electrical signal s. 
     Electrical stimulation of a sacral nerve may eliminate or reduce some pelvic floor disorders by influencing the behavior of the relevant structures, such as the bladder, sphincter and pelvic floor muscles. Pelvic floor disorders include urinary incontinence, urinary urge/frequency, urinary retention, pelvic pain, bowel dysfunction, and male and female sexual dysfunction. The organs involved in bladder, bowel, and sexual function receive much of their control via the second, third, and fourth sacral nerves, commonly referred to as S2, S3 and S4 respectively. Thus, in order to deliver electrical stimulation to at least one of the S2, S3, or S4 sacral nerves, an implantable medical lead is implanted proximate to the sacral nerve(s). 
     Electrical stimulation of a peripheral nerve, such as stimulation of an occipital nerve, may be used to mask a patient&#39;s feeling of pain with a tingling sensation, referred to as paresthesia. Occipital nerves, such as a lesser occipital nerve, greater occipital nerve or third occipital nerve, exit the spinal cord at the cervical region, extend upward and toward the sides of the head, and pass through muscle and fascia to the scalp. Pain caused by an occipital nerve, e.g. occipital neuralgia, may be treated by implanting a lead proximate to the occipital nerve to deliver stimulation therapy. 
     In many electrical stimulation applications, it is desirable for a stimulation lead to resist migration following implantation. For example, it may be desirable for the electrodes disposed at a distal end of the implantable medical lead to remain proximate to a target stimulation site in order to provide adequate and reliable stimulation of the target stimulation site. In some applications, it may also be desirable for the electrodes to remain substantially fixed in order to maintain a minimum distance between the electrode and a nerve in order to help prevent inflammation to the nerve and in some cases, unintended nerve damage. Securing the implantable medical lead at the target stimulation site may minimize lead migration. 
     SUMMARY 
     In general, the invention is directed toward an implantable medical elongated member that includes one or more fixation elements along an interior surface of the elongated member, as well as a method for implanting the elongated member in a patient. The elongated member is configured to be coupled to a medical device to deliver a therapy from the medical device to target therapy delivery site in a patient. The therapy may be electrical stimulation, drug delivery, or both. 
     An “interior” surface of the elongated member is a portion of an outer surface of the elongated member that generally faces away from an epidermis layer of a patient (or a scalp of the patient, depending on the particular application of the elongated member) when implanted in subcutaneous tissue of the patient. Accordingly, an “exterior” side of the elongated member generally faces toward the epidermis of the patient when the elongated member is implanted in subcutaneous tissue of a patient. The elongated member may be implanted so that the fixation elements face inward away from an integumentary layer of the patient (e.g., the epidermis, dermis, or scalp), rather than outward so as to avoid damage to the integumentary layer or irritation to the patient from engagement of a fixation member with the integumentary layer. In one embodiment, the elongated member is fixed at one or more points that are distributed about less than a full outer perimeter of the elongated member in order to minimize or eliminate points of stress between the one or more fixation elements and the epidermis or scalp of a patient. 
     In accordance with one embodiment of the invention, at least a section of the exterior surface of the elongated member near a distal end of the elongated member is devoid of any fixation elements in order to help minimize or prevent stress points between the elongated member and an integumentary layer of the patient. In other embodiments, the exterior surface includes fixation elements that are sized to minimize any interference with the epidermis or scalp of the patient. 
     In one embodiment, the elongated member is an implantable medical lead that is coupled to an implantable or external electrical stimulator, which is configured to deliver electrical stimulation therapy to a target stimulation site in a patient via the lead, and more specifically, via at least one electrode disposed adjacent to a distal end of a lead body of the lead. The lead may be, for example, a cylindrical lead or a paddle lead. In another embodiment, the elongated member is a catheter configured to deliver a fluid, such as pharmaceutical agents, insulin, pain relieving agents, gene therapy agents, or the like from an implantable or external fluid reservoir and/or pump to a target tissue site in a patient. 
     The fixation element may be any suitable fixation element that helps substantially fix a position of the elongated member to (e.g., at or near) the target therapy delivery site, thereby reducing migration of the elongated member when the elongated member is implanted in a patient. 
     In one embodiment, the invention is directed to an apparatus comprising an implantable medical elongated member configured to couple to a medical device to deliver a therapy from the medical device to a target therapy delivery site in a patient. The elongated member extends between a proximal end and a distal end and defines an outer surface comprising a first outer surface portion, and a second outer surface portion extending around at least ten percent of an outer perimeter of the elongated member. The apparatus further comprises a fixation element extending a distance from the first outer surface portion of the implantable medical elongated member. A longitudinally-extending section of the second outer surface portion proximate to the distal end of the elongated member and extending around at least ten percent of the outer perimeter of the elongated member is substantially devoid of any fixation elements that extend the distance from second outer surface portion. 
     In another embodiment, the invention is directed to a system including a medical device and an elongated member. The implantable medical elongated member is configured to couple to the medical device to deliver a therapy from the medical device to a target therapy delivery site in a patient. The elongated member extends between a proximal end and a distal end and defines an outer surface comprising a first outer surface portion, and a second outer surface portion extending around at least ten percent of an outer perimeter of the elongated member. The apparatus further comprises a fixation element extending a distance from the first outer surface portion of the implantable medical elongated member. A longitudinally-extending section of the second outer surface portion proximate to the distal end of the elongated member and extending around at least ten percent of the outer perimeter of the elongated member is devoid of any fixation elements that extend the distance from second outer surface portion. 
     In yet another embodiment, the invention is directed toward an implantable medical lead comprising a lead body, one or more electrodes carried by the lead body, and one or more fixation elements extending from an outer surface of the lead body. At least a circumferential sub-section of the outer surface extending over at least ten degrees is substantially devoid of the fixation elements. 
     In yet another embodiment, the invention is directed to method for implanting an elongated member in a patient. The method comprises introducing the elongated member a patient. The elongated member extends between a proximal end and a distal end and defines an outer surface comprising a first outer surface portion and a second outer surface portion extending around at least ten percent of an outer perimeter of the elongated member. The elongated member further comprises a fixation element extending a distance from the first outer surface portion of the implantable medical elongated member. A longitudinally-extending section of the second outer surface portion proximate to the distal end of the elongated member and extending around at least ten percent of the outer perimeter of the elongated member is substantially devoid of any fixation elements that extend the distance from second outer surface portion. The method further comprises orienting the elongated member so that the second outer surface portion faces a superficial direction and advancing the elongated member through the introducer to a target therapy delivery site to deploy the fixation member into tissue of the patient, wherein the fixation element engages with surrounding tissue to substantially fix a position of the elongated member proximate to the target therapy delivery site. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates the implantation of a therapy system, which includes an electrical stimulator coupled to an implantable medical lead, at a location proximate to an occipital nerve. 
         FIG. 2  is a block diagram illustrating various components of the electrical stimulator and implantable lead of the therapy system of  FIG. 1 . 
         FIG. 3A  is a perspective view of the implantable medical lead of  FIG. 1  implanted in subcutaneous tissue. 
         FIG. 3B  is a schematic cross-sectional view of the implantable medical lead of  FIGS. 1-3A . 
         FIG. 3C  is a schematic cross-sectional view of an introducer taken along line  3 C- 3 C in  FIG. 1 . 
         FIGS. 4A-4B  are schematic cross-sectional views of alternate embodiments of implantable medical leads including fixation elements along a first outer surface portion of the lead body. 
         FIGS. 5A-8  illustrate alternate arrangements of fixation elements on a lead body. 
         FIG. 9  is a perspective view of a lead including fixation elements extending radially outward from a lead body at approximately 90° with respect to a first outer surface portion of the lead body. 
         FIGS. 10A and 10B  are a perspective view and schematic cross-sectional view, respectively, of a lead including a fixation element extending around a first outer surface portion of a lead body. 
         FIG. 11  is a schematic cross-sectional view of a lead including a fixation element extending around less than a full first outer surface portion of a lead body. 
         FIG. 12A  is a side view of a paddle lead including fixation elements along a first outer surface portion. 
         FIG. 12B  is a schematic cross-sectional view of a lead body of a paddle lead taken along line  12 B- 12 B in  FIG. 12A . 
         FIGS. 13A and 13B  are side views of alternate embodiments of a paddle lead including fixation elements along both the first outer surface portion and the second outer surface portion. 
         FIG. 14  is a flow diagram illustrating one example method for implanting a lead including fixation members along a first outer surface portion in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an implantable medical elongated member including one or more fixation elements disposed about an outer surface of the elongated member in an arrangement that minimizes or eliminates points of stress between the fixation elements and an integumentary layer (e.g., an epidermis layer, dermis or scalp) of a patient. For example, the elongated member may include fixation elements that radially extend from less than a full outer circumference of a lead body or fixation elements disposed along one outer surface portion of the elongated member. Due to the arrangement of the fixation elements about the outer surface of the elongated member, the elongated member may be implanted so that the fixation elements do not interfere with the epidermis, dermis, or scalp of the patient. For example, the elongated member may be implanted such that the fixation elements may face away from the epidermis, dermis, or scalp of the patient. 
     The elongated member is configured to be coupled to a medical device to deliver a therapy from the medical device to target tissue in a patient. Various embodiments of the elongated member may be applicable to different therapeutic applications. For example, the elongated member may be a stimulation lead or a lead extension that is used to deliver electrical stimulation to a target stimulation site and/or sense parameters (e.g., blood pressure, temperature or electrical activity) of a patient. In another embodiment, the elongated member may be a catheter that is placed to deliver a fluid, such as pharmaceutical agents, insulin, pain relieving agents, gene therapy agents or the like from a fluid delivery device (e.g., a fluid reservoir and/or pump) to a target tissue site in a patient. The invention is applicable to any configuration or type of implantable elongated member that is used to deliver therapy to a site in a patient. For purposes of illustration, however, the disclosure will refer to a neurostimulation lead. 
       FIG. 1  a schematic perspective view of a therapy system  10 , which includes an electrical stimulator  12  coupled to implantable medical lead  14 . Lead  14  is aligned to be introduced into introducer needle  16 , which is positioned proximate to target stimulation site  18  of patient  20 . In particular, lead  14  is aligned to be implanted and anchored or fixated with fixation elements proximate to target stimulation site  18  within patient  20  for stimulation of one or more occipital nerves. In the example shown in  FIG. 1 , target stimulation site  18  is proximate to at least one of lesser occipital nerve  22 , greater occipital nerve  24  or third occipital nerve  26 . In alternate embodiments, lead  14  may be positioned proximate to one or more other peripheral nerves proximate to occipital nerves  22 ,  24 , and  26  of patient  20 , such as nerves branching from occipital nerves  22 ,  24  or  26 . In addition, therapy system  10  may be used to provide stimulation therapy to any other suitable nerves within patient  20 , such as, but not limited to, trigeminal nerves, branches of trigeminal nerves or nerves within a brain, stomach or spinal cord of patient  20 . 
     In the embodiment shown in  FIG. 1 , electrical stimulator  12  is a neurostimulator that is either implantable or external. For example, neurostimulator  12  may be subcutaneously implanted in the body of a patient (e.g., in a chest cavity, lower back, lower abdomen, or buttocks of patient  20 ). Neurostimulator  12  provides a programmable stimulation signal (e.g., in the form of electrical pulses or substantially continuous-time signals) that is delivered to target stimulation site  18  by implantable medical lead  14 , and more particularly, via one or more stimulation electrodes  28  carried by lead  14 . In some embodiments, neurostimulator  12  may be coupled to two or more leads, e.g., for bilateral or multi-lateral stimulation. Stimulation of occipital nerves  22 ,  24 , and  26  may help alleviate pain associated with, for example, chronic migraines, cervicogenic headaches, occipital neuralgia or trigeminal neuralgia. Neurostimulator  12  may also be referred to as a pulse generator. In some embodiments, lead  14  may also carry one or more sense electrodes to permit neurostimulator  12  to sense electrical signals from target stimulation site  18 . 
     Proximal end  14 A of lead  14  may be both electrically and mechanically coupled to connector  13  of neurostimulator  12  either directly or indirectly (e.g., via a lead extension). In particular, conductors disposed within a lead body of lead  14  electrically connect stimulation electrodes  28  (and sense electrodes, if present) located adjacent to distal end  14 B of lead  14  to neurostimulator  12 . 
     As described in further detail below, lead  14  further includes one or more one fixation elements (not shown in  FIG. 1 ) extending from an interior portion of an outer surface of lead  14  to help substantially fix lead  14  proximate to target stimulation site  18 . In the embodiment shown in  FIG. 1 , an interior portion of the outer surface of lead  14  faces away from scalp  30  of patient  20 . By including one or more fixation elements that face away from scalp  30 , and including substantially no fixation elements on a side facing scalp  30 , the possibility of a fixation element (which may protrude from lead  14 ) extending into or through scalp  30  is minimized or eliminated. At the same time, however, the inward-facing fixation elements are effective in resisting displacement of lead  14 , and particularly electrodes  28 , from the stimulation site. In one embodiment, an exterior portion of the outer surface of lead (i.e., a portion that faces scalp  30  when lead  14  is implanted in patient  20 ) is devoid of any fixation elements, which may contribute to the comfort of patient  20  and avoidance of tissue erosion or damage when lead  14  is implanted in patient  20 . 
     In the application of therapy system  10  shown in  FIG. 1 , implantation of lead  14  may involve the subcutaneous placement of lead  14  transversely across one or more occipital nerves  22 ,  24 , and/or  26  that are causing patient  20  to experience pain. Where treating occipital neuralgia, patient  30  may be placed in a lateral position or in a prone position during implantation of lead  14 . 
     In order to locate the specific occipital nerve causing pain, a clinician may palpate the area of pain. In addition, some embodiments, a screening lead may be used prior to implanting lead  14  to develop optimal stimulation parameters (e.g., various electrode combinations, amplitude, pulse width or rate). 
     In one example method of implanting lead  14  proximate to one or more occipital nerves  22 ,  24 , and/or  26 , a vertical skin incision  33  approximately two centimeters in length is made in the neck of patient  20  lateral to the midline of the spine at the level of the C1 vertebra. Fluoroscopy may be used to identify the location of the C1 vertebra. Typically, local anesthetic is used during the implantation procedure. The length of vertical skin incision  33  may vary depending on the particular patient. At this location, the patient&#39;s skin and muscle are separated by a band of connective tissue referred to as fascia. Introducer needle  16 , which may be a Tuohy needle, is introduced into the subcutaneous tissue, superficial to the fascia and muscle layer but below the skin. In some embodiments, introducer needle  16  may be manually curved by the clinician to conform to the contour of the body of patient  20  proximate to the peripheral nerve, and in the embodiment shown in  FIG. 1 , the clinician may conform introducer needle  16  to the contour of the neck of patient  20 . 
     Occipital nerves  22 ,  24 , and  26  are located within the cervical musculature and overlying fascia, and as a result, introducer needle  16 , and eventually lead  14 , are inserted superior to occipital nerves  22 ,  24 , and  26 . That is, in one embodiment, introducer needle  16  is introduced into the fascia layer of patient  20  such that introducer needle  16  is between the skin of patient  20  and the occipital nerve  22 ,  24 , and/or  26  to be stimulated. 
     Introducer needle  16  may be guided transversely from incision  33  across the midline of the spine of patient  16 . Fluoroscopic observation may aid the clinician in identifying the trunk of the occipital nerve. 
     Once introducer needle  16  is fully inserted, a needle stylet may be removed from the introducer needle, if introducer needle  16  includes a stylet. Lead  14  may then be advanced through introducer needle  16  and positioned to allow stimulation of the lesser occipital nerve  22 , greater occipital nerve  24 , third occipital nerve  26 , and/or other peripheral nerves proximate to an occipital nerve. The position of lead  14  may be verified via fluoroscopy or another suitable technique. In addition, the clinician may confirm that the electrodes proximate to distal end  14 A of lead  14  are properly placed with respect to the particular occipital nerve. For example, the clinician may provide electrical signals to the electrodes and patient  30  may provide feedback relating to the paresthesia coverage. Upon placement of lead  14 , introducer needle  16  may be removed (either before or after confirming the placement of the electrodes). As described below, in one embodiment, upon removal of introducer needle  16 , the one or more fixation elements of lead  14  engage with surrounding tissue to substantially fix a position of lead  14  proximate to target stimulation site  18 . In another embodiment, the one or more fixation elements of lead  14  adhere to surrounding tissue to substantially fix a position of lead  14 . 
     Accurate lead placement may affect the success of occipital nerve stimulation, as well as any other nerve stimulation application of therapy system  10 . If lead  14  is located too deep, i.e. anterior, in the subcutaneous tissue, patient  20  may experience muscle contractions, grabbing sensations, or burning. Such problems may additionally occur if lead  14  migrates after implantation. Furthermore, due to the location of implanted lead  14  on the back of the neck of patient  20 , lead  14  may be subjected to pulling and stretching that may increase the chances of lead migration. For these reasons, fixating lead  14  may be advantageous. 
     Although occipital nerve stimulation is shown in  FIG. 1 , therapy system  10  is useful in other neurostimulation applications. In alternate applications of lead  14 , target stimulation site  18  may be a location proximate to other nerves, organs, muscles, muscle groups or other tissue sites in patient  20 , which may be selected based on, for example, a therapy program selected for a particular patient  20 . For example, therapy system  10  may be used to deliver neurostimulation therapy to a sacral nerve, a pudendal nerve, a perineal nerve or other areas of the nervous system, in which cases, lead  14  would be implanted and substantially fixed proximate to the respective nerve. As further examples, lead  14  may be positioned for temporary or chronic spinal cord stimulation for the treatment of pain, for peripheral neuropathy or post-operative pain mitigation, ilioinguinal nerve stimulation, intercostal nerve stimulation, gastric stimulation for the treatment of gastric mobility disorders and obesity, muscle stimulation (e.g., functional electrical stimulation (FES) of muscles), for mitigation of other peripheral and localized pain (e.g., leg pain or back pain), or for deep brain stimulation to treat movement disorders and other neurological disorders. Accordingly, although patient  20  and target stimulation site  18  of  FIG. 1  are referenced throughout the remainder of the disclosure for purposes of illustration, a neurostimulation lead  14  in accordance with the invention may be adapted for use in a variety of electrical stimulation applications. 
     Therapy system  10  may also include clinician programmer  32  and patient programmer  34 . Clinician programmer  32  may be a handheld computing device that permits a clinician to program neurostimulation therapy for patient  20 , e.g., using input keys and a display. For example, using clinician programmer  32 , the clinician may specify neurostimulation parameters for use in delivery of neurostimulation therapy. Clinician programmer  32  supports telemetry (e.g., radio frequency (RF) telemetry) with neurostimulator  12  to download neurostimulation parameters and, optionally, upload operational or physiological data stored by neurostimulator  12 . In this manner, the clinician may periodically interrogate neurostimulator  12  to evaluate efficacy and, if necessary, modify the stimulation parameters. 
     Like clinician programmer  32 , patient programmer  34  may be a handheld computing device. Patient programmer  34  may also include a display and input keys to allow patient  20  to interact with patient programmer  34  and neurostimulator  12 . In this manner, patient programmer  34  provides patient  20  with an interface for control of neurostimulation therapy by neurostimulator  12 . For example, patient  20  may use patient programmer  34  to start, stop or adjust neurostimulation therapy. In particular, patient programmer  34  may permit patient  20  to adjust stimulation parameters such as duration, amplitude, pulse width and pulse rate, within an adjustment range specified by the clinician via clinician programmer  34 , or select from a library of stored stimulation therapy programs. 
     Neurostimulator  12 , clinician programmer  32 , and patient programmer  34  may communicate via cables or a wireless communication, as shown in  FIG. 1 . Clinician programmer  32  and patient programmer  34  may, for example, communicate via wireless communication with neurostimulator  12  using RF telemetry techniques known in the art. Clinician programmer  32  and patient programmer  34  also may communicate with each other using any of a variety of local wireless communication techniques, such as RF communication according to the 802.11 or Bluetooth specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. 
       FIG. 2  is a block diagram illustrating various components of neurostimulator  12  and an implantable medical lead  14 . Neurostimulator  12  includes therapy delivery module  40 , processor  42 , memory  44 , telemetry module  46 , and power source  47 . In some embodiments, neurostimulator  12  may also include a sensing circuit (not shown in  FIG. 2 ). Implantable medical lead  14  includes lead body  48  extending between proximal end  48 A and distal end  48 B. In the embodiment of  FIG. 2 , lead body  48  is cylindrical and defines an outer surface including a first portion  49 A, which is facing away from the plane of the image of  FIG. 2 . The outer surface of lead body  48  further defines a second portion  49 B (not shown in  FIG. 2 ) that faces into the plane of the image of  FIG. 2 . In other embodiments, lead body  48  may be paddle-shaped (i.e., a “paddle” lead), in which case lead body  48  would define two opposing surfaces, as shown in  FIG. 11  with respect to lead  142 . 
     Electrodes  28 A,  28 B,  28 C, and  28 D (collectively “electrodes  28 ”) are disposed on lead body  48  adjacent to distal end  48 B of lead body  48 . In some embodiments, electrodes  28  may be ring electrodes. In other embodiments, electrodes  28  may be segmented or partial ring electrodes, each of which extends along an arc less than 360 degrees (e.g., 90-120 degrees) around the circumference of lead body  48 . The configuration, type, and number of electrodes  28  illustrated in  FIG. 2  are merely exemplary. 
     In embodiments in which lead  14  is a paddle lead, electrodes  28  may extend along one side of lead body  48 . Electrodes  28  extending around a portion of the circumference of lead body  48  or along one side of a paddle lead may be useful for providing an electrical stimulation field in a particular direction/targeting a particular therapy deliver site. For example, in the electrical stimulation application shown in  FIG. 1 , electrodes  28  may be disposed along lead body  48  such that the electrodes face toward occipital nerves  22 ,  24 , and/or  26 , or otherwise away from scalp  30 . This may be an efficient use of stimulation because electrical stimulation of scalp  30  may not provide any or very minimal useful therapy to patient  20 . In addition, the use of segmented or partial ring electrodes  28  may also reduce the overall power delivered to electrodes  28  by neurostimulator  12  because of the efficient delivery of stimulation to occipital nerves  22 ,  24 , and/or  26  (or other target stimulation site) by eliminating or minimizing the delivery of stimulation to unwanted or unnecessary regions within patient  20 . 
     In embodiments in which electrodes  28  extend around a portion of the circumference of lead body  48  or along one side of a paddle lead, lead  14  may include one or more orientation markers  45  proximate to proximal end  14 A that indicate the relative location of electrodes  28 . Orientation marker  45  may be a printed marking on lead body  48 , an indentation in lead body  48 , a radiographic marker, or another type of marker that is visible or otherwise detectable (e.g., detectable by a radiographic device) by a clinician. Orientation marker  45  may help a clinician properly orient lead  14  such that electrodes  28  face the desired direction (e.g., toward occipital nerves  22 ,  24 , and/or  26 ) within patient  20 . For example, orientation marker  45  may also extend around the same portion of the circumference of lead body  48  or along the side of the paddle lead as electrodes  28 . In this way, orientation marker  45  faces the same direction as electrodes, thus indicating the orientation of electrodes  28  to the clinician. When the clinician implants lead  14  in patient  20 , orientation marker  45  may remain visible to the clinician. 
     Neurostimulator  12  delivers stimulation therapy via electrodes  28  of lead  14 . In particular, electrodes  28  are electrically coupled to a therapy delivery module  40  of neurostimulator  12  via conductors within lead body  48 . In one embodiment, an implantable signal generator or other stimulation circuitry within therapy delivery module  40  delivers electrical signals (e.g., pulses or substantially continuous-time signals, such as sinusoidal signals) to targets stimulation site  18  ( FIG. 1A ) via at least some of electrodes  28  under the control of a processor  42 . The implantable signal generator may be coupled to power source  47 . Power source  47  may take the form of a small, rechargeable or non-rechargeable battery, or an inductive power interface that transcutaneously receives inductively coupled energy. In the case of a rechargeable battery, power source  47  similarly may include an inductive power interface for transcutaneous transfer of recharge power. 
     The stimulation energy generated by therapy delivery module  40  may be formulated as neurostimulation energy, e.g., for treatment of any of a variety of neurological disorders, or disorders influenced by patient neurological response. The signal s may be delivered from therapy delivery module  40  to electrodes  28  via a switch matrix and conductors carried by lead  14  and electrically coupled to respective electrodes  28 . 
     Processor  42  may include a microprocessor, a controller, a DSP, an ASIC, an FPGA, discrete logic circuitry, or the like. Processor  42  controls the implantable signal generator within therapy delivery module  40  to deliver neurostimulation therapy according to selected stimulation parameters. Specifically, processor  42  controls therapy delivery module  40  to deliver electrical signal s with selected amplitudes, pulse widths (if applicable), and rates specified by the programs. In addition, processor  42  may also control therapy delivery module  40  to deliver the neurostimulation signals via selected subsets of electrodes  28  with selected polarities. For example, electrodes  28  may be combined in various bipolar or multi-polar combinations to deliver stimulation energy to selected sites, such as nerve sites adjacent the spinal column, pelvic floor nerve sites, or cranial nerve sites. 
     Processor  42  may also control therapy delivery module  40  to deliver each stimulation signal according to a different program, thereby interleaving programs to simultaneously treat different symptoms or provide a combined therapeutic effect. For example, in addition to treatment of one symptom such as sexual dysfunction, neurostimulator  12  may be configured to deliver neurostimulation therapy to treat other symptoms such as pain or incontinence. 
     Memory  44  of neurostimulator  12  may include any volatile or non-volatile media, such as a RAM, ROM, NVRAM, EEPROM, flash memory, and the like. In some embodiments, memory  44  of neurostimulator  12  may store multiple sets of stimulation parameters that are available to be selected by patient  20  via patient programmer  34  ( FIG. 1 ) or a clinician via clinician programmer  32  ( FIG. 1 ) for delivery of neurostimulation therapy. For example, memory  44  may store stimulation parameters transmitted by clinician programmer  32  ( FIG. 1 ). Memory  44  also stores program instructions that, when executed by processor  42 , cause neurostimulator  12  to deliver neurostimulation therapy. Accordingly, computer-readable media storing instructions may be provided to cause processor  42  to provide functionality as described herein. 
     In particular, processor  42  controls telemetry module  46  to exchange information with an external programmer, such as clinician programmer  32  and/or patient programmer  34  ( FIG. 1 ), by wireless telemetry. In addition, in some embodiments, telemetry module  46  supports wireless communication with one or more wireless sensors that sense physiological signals and transmit the signals to neurostimulator  12 . 
     As previously discussed, migration of lead  14  following implantation may be undesirable, and may have detrimental effects on the quality of therapy delivered to a patient  20 . For example, with respect to the occipital nerve stimulation application shown in  FIG. 1 , migration of lead  14  may cause displacement of electrodes  28  carried adjacent to distal end  14 B of lead  14  with respect to target stimulation site  18 . In such a situation, the electrodes may not be properly positioned to deliver therapy to target stimulation site  18 , resulting in reduced electrical coupling, and possibly undermining therapeutic efficacy of the neurostimulation therapy from system  10 . 
     Substantially fixing lead  14  to surrounding tissue may help prevent lead  14  from migrating from target stimulation site  18  following implantation, which may ultimately help avoid harmful effects that may result from a migrating lead  14 . However, while it may be desirable to fix lead  14  such that electrodes  28  remain proximate to target stimulation site  18  ( FIG. 1 ), in some situations, it may also be desirable to minimize discomfort to patient  20  from lead  14 . For example, when lead  14  is implanted in a dermis or subcutaneous tissue of patient  20  ( FIG. 1 ), such as in the occipital nerve stimulation application shown in  FIG. 1 , patient  20  may be more aware of lead  14  due to the location of lead  14  near an epidermis, scalp  30  or another integumentary layer of patient  20 . 
     To that end, lead  14  includes fixation elements  50 ,  52 , and  54  (not shown in  FIG. 3A ) along a first portion  49 A of outer surface  49  of lead body  48  to minimize migration of lead  14  and substantially fix a position of electrodes  28  proximate to target stimulation site  18 . When lead  14  is implanted proximate to target stimulation site  18 , lead body  48  may be oriented such that first portion  49 A (“first outer surface portion”) of outer surface  49  generally faces away from the scalp  30  of patient and in an inward, deep direction and a majority of second portion  49 B (“second outer surface portion”) (shown in  FIG. 3B ) faces outward in a superficial direction. In this way, first portion  49 A of outer surface  49  may also be referred to as an “interior surface” of lead body  48  and second portion  49 B may also be referred to as an “exterior surface” of lead body  48 . During implantation, the caregiver will take note of the interior and exterior surfaces  49 A,  49 B of lead body  48  and appropriately position lead  14  so that the interior and exterior surfaces face inward and outward, respectively. 
     Fixation elements  50 ,  52 , and  54  engage with surrounding tissue at target stimulation site  18  to fix a position of lead  14 . When lead  14  is implanted in patient  20  such that first outer surface portion  49 A of lead body  48  faces away from scalp  30  (or another integumentary layer) of patient  20 , fixation elements  50  and  54  extend substantially parallel to scalp  30  and fixation element  52  extends away from scalp  30 . In the embodiment of lead  14  shown in  FIG. 2 , second portion  49 B of outer surface  49  of lead body  48  is devoid of any fixation elements. As a result, lead  14  does not include any fixation elements that may extend into scalp  30 , and possibly protrude through scalp  30 , thereby resulting in an implantable medical lead  14  that is more comfortable to patient  20  and reduces the possibility of erosion or damage to subcutaneous tissue. 
     Although lead  14  includes three fixation elements  50 ,  52 , and  54  in the embodiment shown in  FIG. 2 , lead  14  may include any suitable number of fixation elements  50 ,  52 , and  54 . Furthermore, fixation elements  50 ,  52 , and  54  need not be protrusions that extend from lead  14 . Fixation elements  50 ,  52 , and  54  may be any suitable actively or passively deployed fixation element that helps prevent migration of lead  14  when lead  14  is implanted in patient  20 , such as, but not limited to, one or more tines, barbs, hooks, wire-like elements, adhesives (e.g., surgical adhesives), balloon-like fixation elements, pinning fixation elements, collapsible or expandable fixation structures, and so forth. Fixation elements  50 ,  52 , and  54  may be composed of any suitable biocompatible material, including, but not limited to, polymers, titanium, stainless steel, Nitinol, other shape memory materials, hydrogel or combinations thereof. 
     Fixation elements  50 ,  52 , and  54  may be any suitable size, which may depend on the particular application of lead  14 . In particular, it may be desirable to select the size of or otherwise configure fixation elements  50 ,  52 , and  54  to fix lead  14  to a particular region of the patient proximate to the target stimulation site (e.g., a peripheral nerve stimulation site), which may involve selecting the size of fixation elements  50 ,  52 , and  54  to accommodate the specific anatomical configuration of a region of the patient proximate to the peripheral nerve. 
     Furthermore, fixation elements  50 ,  54 , and  54  may not be attached directly to lead  14 , but may be carried by another apparatus that is attached to the elongated member, such as a sleeve or mounting band. An example of a mounting band is described in commonly-assigned U.S. Pat. No. 6,999,819, entitled “IMPLANTABLE MEDICAL ELECTRICAL STIMULATION LEAD FIXATION METHOD AND APPARATUS” and issued on Feb. 14, 2006, which is hereby incorporated by reference in its entirety. 
     Examples of suitable hydrogel fixation elements are described in commonly assigned U.S. Patent Application Publication No. 2006/0095077, entitled “EXPANDABLE FIXATION STRUCTURES and filed on Oct. 29, 2004, U.S. Patent Application Publication No. 2006/0095078, entitled “EXPANDABLE FIXATION MECHANISM” and filed on Oct. 29, 2004, and U.S. patent application Ser. No. 11/591,174 by Martin T. Gerber, entitled “IMPLANTABLE MEDICAL ELONGATED MEMBER INCLUDING EXPANDABLE FIXATION MEMBER” and filed on the same date as the present disclosure. 
     Other suitable fixation elements may include wire-like fixation elements as described in commonly assigned U.S. Patent Application Publication No. 2005/0096718, entitled “IMPLANTABLE STIMULATION LEAD WITH FIXATION MECHANISM” and filed on Oct. 31, 2003 and commonly-assigned U.S. patent application Ser. No. 11/591,282 by Martin T. Gerber, entitled “IMPLANTABLE STIMULATION LEAD INCLUDING WIRE-LIKE FIXATION ELEMENTS” and filed on the same date as the present disclosure. An example of tine fixation elements is described in U.S. Pat. No. 6,999,819, entitled “IMPLANTABLE MEDICAL ELECTRICAL STIMULATION LEAD FIXATION METHOD AND APPARATUS” and filed on Nov. 9, 2001. 
     An example of a suitable lead including a tissue-receiving cavity is described in commonly-assigned U.S. patent application Ser. No. 11/591,294 by Martin T. Gerber, entitled “IMPLANTABLE MEDICAL ELONGATED MEMBER INCLUDING A TISSUE RECEIVING CAVITY” and filed on the same date as the present disclosure. An example of a suitable in situ formed fixation element is described in commonly-assigned U.S. patent application Ser. No. 11/591,433 by Martin T. Gerber, entitled “IMPLANTABLE MEDICAL ELONGATED MEMBER WITH IN SITU FORMED FIXATION ELEMENT” and filed on the same date as the present disclosure. An example of suitable balloon-like fixation elements are described in commonly-assigned U.S. patent application Ser. No. 11/591,447 by Martin T. Gerber, entitled, “IMPLANTABLE STIMULATION LEAD INCLUDING BALLOON FIXATION ELEMENT” and filed on the same date as the present disclosure. 
     Each of the aforementioned patents and patent applications relating to suitable fixation elements are herein incorporated by reference in their entirety. 
       FIG. 3A  illustrates a schematic cross-sectional view of skin  61  of patient  20 , which includes epidermis  62 , dermis  64 , and subcutaneous tissue  66 , as well as a portion of a nerve  68 .  FIG. 3A  further shows a perspective view of implantable medical lead  14  of  FIGS. 1  and  2  implanted in a subcutaneous tissue  66  proximate to nerve  68 .  FIG. 3B  illustrates a schematic cross-sectional view of lead  14  taken along line  3 B- 3 B in  FIG. 3A . 
     Distal end  48 B of lead body  48  is shown in  FIG. 3A . Proximal end  48 A of lead body  48 , which contains contacts (not shown in  FIGS. 3A and 3B ) to electrically couple lead  14  (and in particular, electrodes  28 ) to a lead extension or a neurostimulator (e.g., neurostimulator  12  in  FIG. 1 ). Skin  61  and nerve  68  are general representations of a region of patient  20  and are shown to aid in the description of the invention, and thus, are not necessarily specific to a specific nerve of patient  20 , nor drawn to any particular scale. 
     If neurostimulator  12  ( FIGS. 1 and 2 ) is implanted in patient  20 , the entire length of lead  14  is typically implanted in patient  20 . On the other hand, if neurostimulator  12  is external, lead  14  may be partially implanted within patient  20  and lead  14  (or a lead extension to which proximal end  14 A (shown in  FIG. 1 ) is coupled) may extend through epidermis layer  62  via a percutaneous port. 
     As previously discussed, lead body  48  defines outer surface  49 , which includes first (“interior”) outer surface portion  49 A and second (“exterior”) outer surface portion  49 B, which are demarcated by line  69  in  FIG. 3A . In general, first outer surface portion  49 A and second outer surface portion  49 B do not overlap and have center points  70  and  72  that are generally opposite each other. Center points  70  and  72  are shown in  FIG. 3B  and referred to herein as reference points to aid in the description of the invention. Fixation elements  50 ,  52 , and  54  (shown in  FIG. 3B ) are distributed about less than a full outer perimeter of lead body  48  because first outer surface portion  49 A extends around less than a full outer perimeter of lead body  48 . The outer perimeter of lead body  48  is the outer circumference of lead body  48 . In embodiments in which a lead includes a noncircular cross-section, the outer perimeter of the lead body is defined by the outermost edge of a cross-section of the lead body. 
     In the embodiment shown in  FIG. 3A , first outer surface portion  49 A has a larger size (i.e. measured in terms of surface area) than second outer surface portion  49 B. In particular, in the embodiment shown in  FIGS. 3A-B , second outer surface portion  49 B extends around less than or equal to about 50 percent (%) of the outer perimeter of lead body  48 . In one embodiment, second outer surface portion  49 B extends around at least 10% of the outer perimeter of lead body  48 , while second outer surface portion  49 A extends around about 50% to about 90% of the outer perimeter of lead body  48 . In the embodiment shown in  FIGS. 3A-B , first outer surface portion  49 A extends around approximately 75% of the outer perimeter of lead body  48 . Therefore, fixation elements  50 ,  52 , and  54  are distributed about approximately 75% of the outer perimeter of lead body  48 . In some embodiments, first and second portions  49 A and  49 B may be the same size. For example, demarcation line  69  may extend through a center of lead body  48 , such that first and second outer surface portions  49 A and  49 B each define a half of lead body  48 . Alternatively, demarcation line  69  may be moved toward center point  70  of first portion  49 A to define a second portion  49 B that is a greater (i.e., has a greater surface area) than first portion  49 A. 
     In embodiments in which lead body  48  has a circular cross-section, the percentages given above can be translated to a percentage of a circle. For example, in the embodiment shown in  FIGS. 3A-B , first outer surface portion  49 A extends around approximately 75% of the outer perimeter of lead body  48 , or alternatively, extends over about 270°. 
     Regardless of the respective sizes of first and second outer surface portions  49 A and  49 B, lead  14  may be oriented and implanted in patient such that first outer surface portion  49 A (particularly center point  70 ) generally faces a deep direction  67 A (i.e., faces away from epidermis  62 ), while a majority of second outer surface portion  49 B (particularly center point  72 ) faces a superficial direction  67 B (i.e., faces toward epidermis  62 ). Of course, due to the cylindrical shape of lead body  48  in the example of  FIGS. 3A and 3B , at least some of first outer surface portion  49 A neither faces toward nor away from epidermis  62 , but rather faces a direction  63  (shown in  FIG. 3B ) that is generally parallel to epidermis  62 . Only a small percentage of second outer surface portion  49 B faces direction  63 , and thus, it can be said that the “majority” of second outer surface portion  49 B faces a superficial direction. 
     Lead  14  may include a visible marker  65  (shown in  FIG. 2  in phantom) on the proximal end  48 A of lead body  48 . Visible marker  65  may provide a reference point for a clinician during implantation of lead  14  in patient  20 . For example, the clinician may use visible marker  65  to orient lead  14  such that first outer surface portion  49 A faces a deep direction when lead  14  is implanted in patient  20 . Visible marker  65  may be a printed marking on lead body  48 , an indentation in lead body  48 , a radiographic marker, or another type of marker that is visible or otherwise detectable (e.g., detectable by a radiographic device) by a clinician. In  FIG. 2 , visible marker  65  is shown in phantom because visible marker  65  is located on second outer surface portion  49 B of lead body  48 , which is facing into the plane of the image of  FIG. 2 . Alternatively, visible marker  65  may be on first outer surface portion  49 A. In other embodiments, visible marker  65  may be any suitable configuration (e.g., another shape, size, etc.). 
     In addition to or instead of visible marker  65 , introducer  16  ( FIG. 1 ) may have orientation marks to properly orient fixation elements  50 ,  52 , and  54  of lead  14  with respect to epidermis  62  of patient  20 . For example, proximal end  16 A of introducer  16  may include printed markings for aligning with visible marker  65  of lead  14  to orient lead  14  such that first outer surface portion  49 A faces away from epidermis  62  when lead  14  is implanted in patient  20 . In some embodiments, visible marker  65  and orientation marker  45  may overlap and may effectively be a single marker. 
       FIG. 3C  is a schematic cross-sectional view of introducer  16  taken along line  3 C- 3 C in  FIG. 1 . In embodiments in which fixation elements  50 ,  52 , and  54  protrude from lead body  48  during an implantation procedure, introducer  16  may be keyed to receive lead  14  in a certain orientation. In the embodiment shown in  FIG. 3C , introducer  16  defines lumen  79  for receiving lead body  48 , where lumen  79  defines channels  80 A-C that are sized and otherwise configured to receive fixation elements  50 ,  52 , and  54 , respectively, so that lead  14  may be introduced into introducer  16  in a limited number of orientations. In this way, once introducer  16  is properly oriented with respect to epidermis  62  of patient, introducer  16  may be used to force proper orientation of lead  14  with respect to epidermis  62 . Introducer  16  may be properly oriented with respect to epidermis  62  via any suitable means, such as, for example, visible or radiographic marker  81  on introducer  16 . For example, when the clinician is introducing introducer  16  into patient  20 , the clinician may orient introducer  16  such that visible or radiographic marker  81  is facing the clinician or facing a particular direction in order to properly orient channels  80 A-C with respect to epidermis  62 . In addition, channels  80 A-C defined by lumen  79  of introducer  16  may help minimize the overall diameter of introducer  16 , which may help minimize the invasiveness of an implantation procedure. 
     In addition to or instead of visible marker  65  on lead body  48  or orienting features of introducer  16 , a keyed stylet may be used to guide lead  14  into an orientation that results in fixation elements  50 ,  52 , and  54  facing away from epidermis  62 . For example, just as introducer  16  is shown in  FIG. 3C  to be configured to receive lead  14  in one orientation, a stylet may be keyed to receive lead  14  in one orientation. 
     At least a longitudinally-extending section (i.e., extending in the direction between proximal end  48 A and distal end  48 B of lead body  48 ) of second outer surface portion  49 B near electrodes  28  is devoid of any fixation elements or of any fixation elements that may extend into epidermis layer  62  when lead  14  is implanted in subcutaneous tissue  66 . For example, in the embodiment shown in  FIG. 3A , the entire second outer surface portion  49 B is devoid of any fixation elements. However, in other embodiments, second outer surface portion  49 B may include fixation elements sized to engage with subcutaneous tissue  66 , but not epidermis  62  or dermis  64 , or fixation elements to engage with dermis  64 , but not epidermis  62 . In another embodiment, a portion of second outer surface portion  49 B other than the portion proximate to electrodes  28  may include fixation members that are the same size or greater than fixation elements  50 ,  52 , and  54 . 
     Fixation elements  50 ,  52 , and  54  are angled toward proximal end  48 A of lead body  48 , which may help distal end  48 B of lead body  48  resist the pulling force from proximal end  48 A. However, in some applications, it may also be desirable for lead  14  to resist pulling forces from distal end  48 B. Accordingly, the invention contemplates configurations of fixation elements  50 ,  52 , and  54  that are angled both toward and away from proximal end  48 A of lead body  48  (e.g., as shown in  FIG. 8 ). 
     Fixation elements  50 ,  52 , and  54  radially extend from lead body  48  at an acute angle with respect to first portion  49 A. However, in other embodiments, fixation elements  50 ,  52 , and  54  radially extend from lead body  48  at a 90° angle (e.g., as shown with respect to fixation elements  126 ,  128 , and  130  in  FIG. 9 ). Extending from lead body  48  at an acute angle enables fixation elements  50 ,  52 , and  54  to engage with surrounding tissue to prevent both axial and radial movement of lead body  48 . 
       FIG. 3B  is a schematic cross-sectional view of lead  14  taken along line  3 B- 3 B in  FIG. 3A . Lead body  48  carries a plurality of conductors  74  (shown in  FIG. 3B  as a single conductive center of lead body  48  for clarity of illustration) for electrically coupling electrodes  28  ( FIG. 3A ) to therapy delivery module  40  of neurostimulator  12  ( FIG. 2 ). Typically, a separate conductor electrically couples each electrode  28 A-D to therapy delivery module  40 . Separate conductors permit independent selection of individual electrodes  28 A-D. Furthermore, each of the conductors electrically coupled to separate electrodes  28 A-D are electrically insulated from each other. Insulating layer  76  surrounds conductors  74  in order to electrically insulate conductors  74  from subcutaneous tissue  66  when lead  14  is implanted in patient  20  and to help protect a clinician who may be handling lead  14  from shock. 
     As previously described, each of fixation elements  50 ,  52  or  54  may be directly coupled to lead body  48 , as shown in  FIG. 3B , or indirectly coupled to lead body  48  (e.g., carried by a fixation sleeve). In the embodiment shown in  FIG. 3B , fixation elements  50 ,  52  or  54  are each attached to lead body  48  with an adhesive. For example, adhesive  78  is disposed between fixation element  54  and lead body  48 . 
     As  FIGS. 3A and 3B  illustrate, when lead  14  is implanted in subcutaneous tissue  66 , fixation elements  50  and  54  extend from lead body  48  substantially parallel to epidermis layer  62 , while fixation element  52  extends away from epidermis layer  62 . Implanting lead  14  within subcutaneous tissue  66  such that one or more fixation elements  50 ,  52  or  54  extended through dermis layer  64  or even epidermis layer  66  may increase discomfort to patient  20 . In addition, one or more fixation elements  50 ,  52  or  54  may be visible (e.g., a protrusion may be seen protruding into epidermis  62 ). Discomfort to patient  20  may be attributable to the one or more fixation elements  50 ,  52  or  54  causing stress points at the interface between the one or more fixation elements  50 ,  52  or  54  and dermis layer  64 , or even epidermis layer  62 . In addition, fixation elements  50 ,  52  or  54  may rub against dermis layer  64  and/or even epidermis layer  62 , which may lead to erosion of and possible damage to dermis layer  64  and/or epidermis layer  62 . As discussed, in the embodiment of lead  14  shown in  FIGS. 3A-3B , second outer surface portion  49 B is devoid of fixation elements. As a result, second outer surface portion  49 B provides a relatively smooth surface for interfacing with epidermis layer  62  or dermis layer  64 . Thus, in some applications, such as when lead  14  is implanted in subcutaneous tissue  66 , it may be desirable for lead  14  to be implanted in a specific orientation. 
     In some embodiments, second outer surface portion  49 B may include structural fixation elements that also engage with surrounding tissue to prevent migration of lead  14 . However, these fixation elements do not extend as far from second outer surface portion  49 B as fixation elements  50 ,  52  or  54  extend from first portion  49 A in order to minimize the extent to which the fixation elements on second portion  49 B engage with epidermis layer  62  or dermis layer  64 .  FIG. 4A  is a schematic cross-sectional view of lead  55  including fixation element  56  extending from second outer surface portion  49 B of lead body  48 . As  FIG. 4A  illustrates, fixation element  56  is relatively small compared to fixation elements  50 ,  52 , and  54 . Fixation element  56  does not extend as far from lead body  48  as fixation elements  50 ,  52 , and  54 . In particular, in the embodiment shown in  FIG. 4A , fixation element  52  (as well as fixation elements  50  and  54 ) extends a distance A from outer surface  49  of lead body  48 , while fixation element  56  extends a distance B from outer surface  49  of lead body  48 . Distance B of fixation element  56  may be selected such that fixation element  56  engages with subcutaneous tissue  66 , rather than epidermis layer  62  or dermis layer  64 , when lead  55  is implanted in subcutaneous tissue  66  of patient  20  and oriented such that first outer surface portion  49 A faces away from epidermis layer  66 . 
     Also shown in  FIG. 4A  are lines  57 A and  57 B, which are shown to demonstrate that lead body  48  defines four quadrants  58 A-D. Rather than describing the arrangement of fixation elements  50 ,  52 ,  54 , and  56  about lead body  48  with respect to first and second outer surface portions  49 A and  49 B ( FIG. 3B ), respectively, the arrangement of fixation elements  50 ,  52 ,  54 , and  56  may also be described with respect to quadrants  58 A-D of lead body  48 . In the embodiment of lead  55  shown in  FIG. 4A , fixation element  50  extends from first quadrant  58 A of lead body  48 , fixation element  52  extends from second quadrant  58 B, third fixation element  54  extends from third quadrant  58 C, and fixation element  56  extends from fourth quadrant  58 D. Fixation element  56  extending from fourth quadrant has a smaller cross-sectional profile than fixation elements  50 ,  52 , and  54 . 
     Lead body  48  also defines quadrants  58 A-D in each of the schematic cross-sectional views shown in  FIGS. 3B ,  4 B,  5 B,  6 B,  10 B, and  11 . However, for clarity of illustration and description, quadrants are not labeled in  FIGS. 3B ,  4 B,  5 B,  6 B,  10 B, and  11 . Each of the leads shown in  FIGS. 3B ,  4 B,  5 B,  6 B,  10 B, and  11  may be described with respect to quadrants  58 A-D. For example, lead  14  shown in  FIG. 3B  includes fixation element  50  extends from first quadrant  58 A of lead body  48 , fixation element  52  extends from second quadrant  58 B, and third fixation element  54  extends from third quadrant  58 C, while fourth quadrant  58 D is devoid of any fixation elements. 
     In yet another embodiment, second outer surface portion  49 B may include a non-structural fixation element, such as a surgical adhesive.  FIG. 4B  is a schematic cross-sectional view of lead  58  including adhesive layer  59  along second outer surface portion  49 B of lead body  48 , and fixation elements  51 ,  52 , and  54  along first outer surface portion  49 A. As  FIG. 4B  illustrates, adhesive layer  59  does not protrude from second portion  49 B of adhesive layer to the extend fixation elements  50 ,  52 , and  54  protrude from first portion  49 A. In another embodiment, an adhesive may be embedded in second portion  49 B of lead body  48 , rather than being a separate adhesive layer  59 . Regardless of the type of fixation element, if any, carried by second outer surface portion  49 B, it is desirable to minimize the extent to which the fixation element along second outer surface portion  49 B engages with and protrudes into epidermis layer  62  or dermis layer  64  in order to increase the comfort to patient  20 . 
     Adhesive layer  59  may be, for example, surgical adhesive elements disposed on or embedded with second outer surface portion  49 B of lead body  48 . Examples of suitable adhesive elements are discussed in commonly assigned U.S. patent application Ser. No. 11/591,443 by Martin T. Gerber, entitled “IMPLANTABLE MEDICAL ELONGATED MEMBER WITH ADHESIVE ELEMENTS” and filed on the same date as the present disclosure, which is hereby incorporated by reference in its entirety. The adhesive properties of adhesive layer  59  may be activated by any suitable means, including exposure to fluids, a certain temperature, or by activating agents. For example, adhesive layer  59  may be separated from surrounding tissue by a sheath until lead  14  reaches a targets stimulation site, at which time a clinician may withdraw the sheath to expose adhesive layer  59  to surrounding tissue, which may activate adhesive layer  59  via moisture, temperature or otherwise. 
     Fixation element  51  of lead  58  is disposed on first outer surface portion  49 A, but extends in a superficial direction toward epidermis  62  when lead  58  is implanted in patient such that first outer surface portion  49 A faces a deep direction. Although fixation element  51  extends toward epidermis  62 , fixation element  51  does not contact epidermis  62  because of its placement on first outer surface portion  49 A. In particular, fixation element  51  does not extend as far demarcation line  69  separating first and second outer surface portions  49 A and  49 B, respectively. 
     While lead  14  includes three fixation elements  50 ,  52 , and  54  spaced about 90° with respect to each other about lead body  48 , and particularly, on first outer surface portion  49 A, in other embodiments, a lead may include any suitable number of fixation elements in any suitable arrangement about the lead body. These and other embodiments of leads including alternate numbers and/or arrangements of fixation elements are shown in  FIGS. 5A-13B  and described in reference thereto. For clarity of description, like numbered reference numbers designate substantially similar elements throughout  FIGS. 2-12 . In  FIGS. 1-13B , the components of the leads, as well as any other components that may be illustrated, are not necessarily drawn to scale. For example, each of the hydrogel fixation members  50 ,  52 , and  54  shown in  FIGS. 2-4B  are not necessarily drawn in correct proportion to the length or diameter of lead body  48 . 
       FIGS. 5A and 5B  are a perspective view and schematic cross-sectional view, respectively, of lead  82 , which includes fixation elements  83 ,  84 , and  86  disposed between proximal end  48 A and electrodes  28  for fixing lead  82  proximate to target stimulation site  18 . As previously described, proximal end  48 A of lead body typically includes contacts (not shown in  FIG. 5A ), for electrically connecting electrodes  28  of lead  82  with a neurostimulator (e.g., neurostimulator  12  in  FIG. 1A ), a lead extension or other medical device. Fixation elements  83 ,  84 , and  86  are coupled to first outer surface portion  49 A of outer surface  49  of lead body  48 . In  FIG. 5A , first outer surface portion  49 A is shown, while second outer surface portion  49 B faces into the plane of the image of  FIG. 5A . In  FIG. 5B , line  69  indicates the demarcation between first outer surface portion  49 A and second outer surface portion  49 B. 
     As  FIG. 5B  illustrates, fixation elements  83  and  84  are spaced about angle J with respect to each other about the outer perimeter of lead body  48 , while fixation elements  84  and  86  are spaced about angle K with respect to each other. Angles J and K may, but need not be equal. In contrast to fixation elements  50 ,  52 , and  54  of lead  14  ( FIGS. 2-4B ), fixation elements  83 ,  84 , and  86  are spaced less than about 90° with respect to each other about first portion  49 A of lead body  48 . For example, angles J and K may each be about 45°. As a result of the arrangement of fixation elements  83 ,  84 , and  86  about first outer surface portion  49 A of lead body  48 , when lead  82  is implanted in subcutaneous tissue  66  ( FIG. 3A ), fixation elements  83 ,  84 , and  86  extend away from epidermis layer  62 . Thus, neither fixation element  83 ,  84  or  86  radially extend from first portion  49 A lead body  48  in a direction that results in fixation element  83 ,  84  or  86  that are substantially parallel to epidermis layer  62 , as with fixation elements  50  and  54  of lead  14  ( FIGS. 2-3B ). 
     First outer surface portion  49 A of the lead body  48  of each of leads  14  and  82  both include one set of fixation elements (i.e., a group of fixation elements that substantially share an axial position with respect to lead body  48 ) located proximal to electrodes  28 . In other embodiments, a lead may include more than one set of fixation elements, and the fixation elements may be otherwise arranged, such as between electrodes  28 , distal to electrodes  28 , or combinations thereof. Examples of these embodiments are shown in  FIGS. 6A-8 . 
       FIGS. 6A and 6B  are a perspective view and schematic cross-sectional view of lead  90 , which includes two sets  92  and  94  of fixation elements on first outer surface portion  49 A that are axially displaced from each other with respect to lead body  48 . First set  92  include fixation elements  95 - 97 , and second set  94  includes fixation elements  98 - 102 . As shown in  FIG. 6B , in the cross-sectional view of lead  90 , the fixation elements  95 - 102  do not overlap. Thus, fixation elements  95 - 97  of first set  92  each have different radial locations about first outer surface portion  49 A than fixation elements  98 - 102  of second set  94 . Accordingly, fixation elements  95 - 102  each extend from first outer surface portion  49 A of lead body  48  in different radial directions. 
     First and second sets  92  and  94  of fixation elements are shown proximate to electrode  50 . However, as shown in  FIG. 7 , lead  106  may include first set  108  of fixation elements separated from second set  110  of fixation elements by electrodes  28 . First and second sets  108  and  110  of fixation elements are each disposed along first outer surface portion  49 A of lead body  48 . In the embodiment of lead  106  shown in  FIG. 7 , first and second sets  108  and  110  of fixation elements, which may each include any suitable number of fixation elements, may be located proximate and distally, respectively, with respect to electrodes  28 . While both leads  14  ( FIGS. 3A-3 ) and  106  include fixation elements for fixing leads  14  and  106 , respectively, to target stimulation site  18  ( FIG. 1 ), which may be within subcutaneous tissue  66  ( FIG. 3A ), lead  106  may be useful for locally fixing distal end  48 B of lead body  48 . In some applications of therapy system  10  ( FIGS. 1 and 2 ), such as when therapy system  10  is used to stimulate a pudendal nerve, it may be desirable to locally fix distal end  48 B of lead body  48 . 
       FIG. 8  is a perspective view of yet another embodiment of lead  112  in accordance with the invention. In addition to first and second sets  108  and  110  of fixation elements along first outer surface portion  49 A of lead body  48 , lead  112  includes third set  114  of fixation elements between electrodes  28 B and  28 C. Third set  114  includes fixation elements  116 ,  118 ,  120 , and  122 . In other embodiments, lead  112  may include a set of fixation elements between all of electrodes  28  or between any other combination of electrodes (e.g., between electrodes  28 A and  28 B). 
     While each of leads  14 ,  82 ,  90 , and  106  of  FIGS. 3A ,  5 A,  6 A, and  7  include fixation elements angled toward proximal end  48 A of lead body  48 , in other embodiments, the fixation elements may also be angled toward distal end  48 B of lead body  48 . For example, fixation elements  116  and  122  of third set  114  of fixation elements of lead  112  shown in  FIG. 8  are angled toward proximal end  48 A of lead body  48 , while fixation elements  118  and  120  are angled toward distal end  48 B. 
     Fixation elements  116 ,  118 ,  120 , and  122  each extend from lead body  48  at an acute angle with respect to first portion  49 A of lead body  48 . As previously discussed, fixation elements of a lead in accordance with the invention may also extend radially outward at about 90° with respect to first portion  49 A of lead body  48 . An embodiment of such a lead is shown in  FIG. 9 , which is a perspective view of lead  124  including fixation elements  126 ,  128 , and  130  that extend radially outward at approximately 90° with respect to first outer surface portion  49 A of lead body  48 . 
     While tine-shaped fixation elements are shown in  FIGS. 2-9  above, a fixation element may have any suitable shape.  FIGS. 10A and 10B  are a perspective view and cross-sectional view, respectively, of lead  134 , which includes fixation element  136  that extends around first outer surface portion  49 A of lead body  48 . Although fixation element  136  is shown in  FIGS. 10A and 10B  as expanding radially outward without an angular component (i.e. at 90° with respect to first outer surface portion  49 A), in alternate embodiments, fixation element  136  may extend from lead body  48  at an angle with respect to first portion  49 A. 
     As shown in  FIGS. 10A and 10B , a single fixation element  136  is disposed between electrodes  28  and proximal end  48 A of lead body  48 . In alternate embodiments, lead  134  may include any suitable number of fixation elements that extend around first outer surface portion  49 A of lead body  48  and/or fixation element  134  may be used in combination with tine-like or other fixation members that do not extend around first outer surface portion  49 A of lead body  48 . For example, in another embodiment, fixation member  136  may extend around 25%, 50% or 75% of the outer perimeter of first outer surface portion  49 A of lead body  48 . For example,  FIG. 11  shows a schematic cross-sectional view of lead  138 , which includes fixation element  140  extending around about 75% of first outer surface portion  49 A of lead body  48 . 
     In each of leads  14 ,  82 ,  90 ,  106 ,  112 ,  124 ,  134  of  FIGS. 3A ,  5 A,  6 A,  7 - 9 , respectively, lead body  48  is cylindrical and defines first outer surface portion  49 A carrying fixation elements, while second outer surface portion  49 B is devoid of any fixation elements or includes one or more fixation elements that do not engage with epidermis  62  ( FIG. 3A ) when lead  14  is implanted in subcutaneous tissue  66 . In another embodiment, a lead may include a paddle shape (i.e. a paddle lead) and may include fixation elements along one surface of the paddle lead, to thereby define a paddle lead including interior fixation when the paddle lead is implanted in subcutaneous tissue  66  ( FIG. 2 ) of patient  20  or near occipital region  29  ( FIG. 1 ) of patient  20 . 
       FIGS. 12A and 12B  are a side view and a schematic cross-sectional view of paddle lead  142 , respectively, which includes substantially flat, paddle-like shaped lead body  144  coupled to distal end  146 A of lead body connector  146 . A proximal end (not shown in  FIG. 12A ) of lead body connector  146  may be mechanically coupled to a neurostimulator (e.g., neurostimulator  12  of  FIGS. 1 and 2 ) or another medical device. Lead body  144  defines a “paddle” like shape, including first surface  144 A and second surface  144 B, which is opposite first surface  144 A. As shown in  FIG. 12B , which is a schematic cross-sectional view of lead body  144  taken along line  12 B- 12 B in  FIG. 12A , lead body  144  defines an outer perimeter  145  (shown in phantom lines). Line  147  demarcates first and second surfaces  144 A and  144 B, respectively, which each extend around about fifty percent of outer perimeter  145  of lead body  144 . 
     In the embodiment shown in  FIG. 12A , electrodes  148  are carried by first surface  144 A of lead body  144 . In another embodiment, paddle lead  142  may also include electrodes along second surface  144 B of lead body  144 . Each of electrodes  148  may be electrically coupled to the neurostimulator, lead extension or other medical device via electrical conductors disposed within lead body connector  146 . A proximal end (not shown in FIG.  12 A) of lead body connector  146  may include electrical contacts for electrically connecting the electrical conductors within lead body connector  146  with the neurostimulator. 
     First surface  144 A of lead body  144  includes fixation elements  149 A-E, while second surface  144 B is devoid of any fixation elements. In the embodiment shown in  FIG. 12A , fixation elements  149 A-D are tine-like structures that are angled toward lead body connector  146 . In other embodiments, fixation elements  149 A-D on first surface  144 A of paddle lead body  144  may be any suitable shape or type of fixation element, and may extend from first surface  144 A at any suitable angle (e.g., radially outward or away from lead body connector  146 ). 
     When paddle lead  142  is implanted in patient  20 , lead  142  may be oriented such that first surface  144 A, and accordingly fixation elements  149 A-D, faces away from epidermis  62 , scalp  30  or another integumentary layer of patient  20  in order to help minimize or eliminate any discomfort or irritation to patient attributable to fixation elements  149 A-D. Second surface  144 B of lead body  144  provides a relatively smooth surface for interfacing with epidermis  62  ( FIG. 2 ) or scalp  30  of patient  20  because second surface  144 B does not have any fixation elements that may engage with epidermis  62  or scalp  30  to create points of stress. 
       FIG. 13A  is a side view of another embodiment of paddle lead  150 , which includes substantially flat, paddle-like shaped lead body  144  coupled to distal end  146 A of lead body connector  146  and electrodes on first surface  144 A of lead body  144 . In addition to fixation elements  149 A-D on first surface  144 A of lead body  144 , lead  150  includes fixation element  152  on second surface  144 B of lead body  144 . In particular, fixation element  152  is a layer of surgical adhesive that helps prevent migration of lead  150  following implantation in patient  20 . Alternatively, surgical adhesive layer  152  may be adhesive elements disposed on or embedded with second surface  144 B of lead body, as discussed by commonly assigned U.S. patent application Ser. No. 11/591,443 by Martin T. Gerber, entitled “IMPLANTABLE MEDICAL ELONGATED MEMBER WITH ADHESIVE ELEMENTS” and filed on the same date as the present disclosure, which is hereby incorporated by reference in its entirety. 
       FIG. 13B  is a side view of another embodiment of paddle lead  154 , which includes fixation elements  156 A-D along second surface  144 B of lead body  144 . Fixation elements  149 A-D as well as fixation elements  156 A-D may engage with surrounding tissue to help substantially fix a position of lead  154  proximate to target stimulation site  18 . In the embodiment shown in  FIG. 13B , fixation elements  156 A-D are tine-like structures that are angled toward lead body connector  146 . In other embodiments, fixation elements  156 A-D on second surface  144 B of paddle lead body  144  may be any suitable shape or type of fixation element (e.g., one or more balloon fixation elements) and may extend from second surface  144 B at any suitable angle (e.g., radially outward or away from lead body connector  146 ). 
     Fixation elements  156 A-D do not extend from second surface  144 B of lead body  144  to the extent that fixation elements  149 A-D extend from first surface  144 A of lead body  144 . That is, in  FIG. 13A , dimension E, which is the overall distance each of fixation elements  149 A-D extend from first surface  144 A of lead body  144  is greater than dimension F, which is the overall distance each of fixation elements  156 A-D extend from second surface  144 B. Dimension F is selected such that fixation elements  156 A-D do not engage with and protrude into an integumentary layer (e.g., epidermis layer  62  (and in some cases, dermis layer  64 ) or scalp  30  ( FIG. 1 )) of patient  20  when lead  154  is implanted in patient  20  such that first surface  144 A of lead body  144  faces away from the integumentary layer. The selective sizing of fixation elements  156 A-D may thus help minimize any discomfort to patient  20  that is attributable to fixation elements  156 A-D. 
       FIG. 14  is a flow diagram of process  160  for implanting a lead  14  ( FIGS. 1-3B ) including fixation elements along first surface  49 A of lead body  48  in accordance with the invention. While lead  14  is referenced in the description of  FIG. 14 , it should be understood that process  160  may be used to implant any of leads  82 ,  90 ,  106 ,  112 ,  124 ,  134 , and  138  of  FIGS. 5A ,  6 A,  7 ,  8 ,  9 ,  10 A, and  11 , respectively, or any other lead including fixation elements disposed along a portion (e.g., first portion  49 A) of an outer surface of lead body  48  in accordance with the invention. Furthermore, while implantation of lead  14  into subcutaneous tissue  66  ( FIG. 3A ) of patient  20  is described, in other embodiments, lead  14  may be implanted proximate to any suitable target therapy delivery site in patient  20 . 
     A lead introducer, such as an introducer needle, is introduced into epidermis layer  62  ( FIG. 3A ) of patient  20  ( FIG. 1 ) and a distal end of the introducer is guided into subcutaneous tissue  66  proximate to nerve  68  ( 162 ). The introducer needle may be inserted into the patient percutaneously or via an incision (e.g., incision  33  in  FIG. 1 ) in epidermis layer  62 . Lead  14  is introduced into a lumen of the introducer ( 164 ). In particular, distal end  14 B of lead  14  is introduced into the lumen before proximal end  14 A. 
     Lead  14  is advanced through the lumen until electrodes  28  adjacent to distal end  48 B of lead body  48 B of lead  14  are positioned proximate to nerve  68  ( 166 ). For example, distal end  14 B of lead  14  may be advanced through the lumen of the introducer until at least distal end  14 B protrudes past the lumen and into tissue of patient  20  and fixation elements  50 ,  52 , and  54  are deployed from the introducer (i.e., are advanced past a distal end of the introducer). Alternatively, fixation elements  50 ,  52 , and  54  may be deployed from the introducer by withdrawing the introducer or another sheath separating fixation elements  50 ,  52 , and  54  from tissue of patient  20 , thereby exposing lead  14 . 
     If necessary, lead  14  is oriented (e.g., rotated) such that first outer surface portion  49 A of lead body  48  faces away from epidermis layer  62  ( 168 ). Lead  14  may be oriented prior to or subsequent to positioning electrodes  28  proximate to nerve  68  ( 166 ). Positioning of the introducer and/or orientation and positioning of lead  14  may be aided by imaging techniques, such as by fluoroscopy using markers (e.g. radio-opaque or otherwise visible) on lead body  48 . The markers may help indicate a location of fixation elements  50 ,  52 , and  54  with respect to the introducer needle. Alternatively, lead  14  may be oriented using visible marker  65  ( FIG. 2 ) and/or a stylet or introducer  16  may include features (e.g., channels for receiving fixation elements  50 ,  52 , and  54 ) for orienting lead  14  such that first outer surface portion  49 A of lead body  48  faces away from epidermis  62  when lead  14  is implanted in patient  20 . 
     Upon deployment into body tissue, fixation elements  50 ,  52 , and  54  engage with surrounding subcutaneous tissue  66  to substantially fix electrodes  28  within subcutaneous tissue  66  proximate to nerve  68 . After lead  14  is positioned, the lead introducer is withdrawn from patient  20  ( 170 ). If an adhesive is also used to help prevent migration of lead  14  after implantation in patient  20  (e.g., an adhesive disposed along second outer surface portion  49 B of lead body  48 ), the adhesive may begin reacting with surrounding tissue or otherwise activating upon deployment into body tissue. 
     A lead including a vacuum cavity for receiving tissue may be useful for various electrical stimulation systems. For example, the lead may be used to deliver electrical stimulation therapy to patients to treat a variety of symptoms or conditions such as chronic pain, tremor, Parkinson&#39;s disease, multiple sclerosis, spinal cord injury, cerebral palsy, amyotrophic lateral sclerosis, dystonia, torticollis, epilepsy, pelvic floor disorders, gastroparesis, muscle stimulation (e.g., functional electrical stimulation (FES) of muscles) or obesity. 
     In addition, the fixation element arrangement described herein with respect to leads  14 ,  82 ,  90 ,  106 ,  112 ,  124 ,  134 , and  138  may also be useful for fixing a catheter, such as a drug deliver catheter, proximate to a target drug delivery site. 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.