Abstract:
In general, the invention is directed to apparatus and techniques that aid in the removal or explantation of an implantable medical device (IMD) under the scalp of a patient. The various embodiments of the invention address risks associated with the explantation, such as the risk of damage to leads, the risk of damage to the IMD, the risk that the incision may hinder the explantation, and the risk that the IMD may be difficult to remove. In some embodiments, the invention is directed to apparatus that help the surgeon identify the location of the implanted elements, and that protect the implanted elements from inadvertent damage. In other embodiments, the invention is directed to techniques that facilitate the removal of the IMD.

Description:
[0001]     This application is a divisional of U.S. patent application Ser. No. 10/835,232, filed Apr. 29, 2004, which claims the benefit of U.S. Provisional Application Ser. No. 60/471,262, filed on May 16, 2003, and U.S. Provisional Application Ser. No. 60/503,945, filed on Sep. 20, 2003. The entire content of each of these applications is incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates to implantation and removal of medical devices, and more particularly, to implantable medical devices that deliver therapy to and/or monitor a patient.  
       BACKGROUND  
       [0003]     Implantable medical devices (IMDs) include devices implantable in a mammalian body that sense medical parameters, monitor medical conditions, administer therapy, or any combination thereof. Typical IMDs include a variety of electrical and/or mechanical components, often including a housing that houses the components. Because the components may be fragile, the housing is usually sufficiently robust to protect the components from forces to which they would otherwise be exposed when implanted within the body. Housings may be constructed from titanium, for example. In order to avoid potentially harmful interactions between the components and bodily fluids, such as corrosion, IMD housings are typically hermetically sealed.  
         [0004]     Large components common to most IMDs typically include a battery, a coil, and a hybrid circuit that includes digital circuits, e.g., integrated circuit chips and/or a microprocessor, and analog circuit components. IMDs may include other components as well. The components and the housing each add bulk to the IMD.  
         [0005]     Some medical devices may be implanted in the head of a patient. For example, an IMD may be implanted under the scalp and on top of the cranium, with one or more leads deployed on the head or implanted in the brain. In many cases, the implantation is not permanent, and it may be advantageous to remove the device for reasons such as repair, maintenance, replacement, or because the patient no longer benefits from the device.  
       SUMMARY  
       [0006]     In general, the invention is directed to techniques for explantation of an IMD under the scalp of a patient, i.e., removal of an IMD implanted under the scalp of a patient. Explantation of a cranially implanted IMD includes making an incision in the scalp of a head of a patient to obtain access to the IMD, and removing the IMD. The invention addresses risks that are a part of the surgical procedure.  
         [0007]     One of the risks associated with explantation is that the leads may be damaged. Typical leads can be readily damaged by a scalpel used to incise the scalp. Damage to the leads is often undesirable because removal of one IMD may be followed by implantation of another IMD, and it can be more beneficial to use leads already deployed than to deploy new leads. Accordingly, many of the embodiments of the invention are directed to protecting the leads against inadvertent damage. Some of the embodiments are directed to locating the leads so that the surgeon can plan the incision to avoid the leads, and other embodiments are directed to protecting the leads in the event the incision is made proximate to the leads.  
         [0008]     Another risk associated with explantation is the incision may cut across the IMD itself. As a result, the IMD may be damaged, or the explantation may be hindered or complicated by a poorly placed incision. Many of the embodiments of the invention are directed to protecting the leads against inadvertent damage. Some of the embodiments are directed to locating the IMD so that the surgeon can plan an incision that will achieve the goals of the surgical procedure.  
         [0009]     A further risk associated with explantation is that removal of the IMD may be difficult because of factors such as tissue growth proximate to the implantation site. Some of the embodiments are directed to structural features of the IMD that permit the surgeon to apply force to the IMD to dislodge it or remove it.  
         [0010]     There are additional risks associated with explantation. Incision over the top of an IMD or leads may not only damage the implanted elements, but may also adversely affect the health of the patient by, for example, damaging blood vessels, damaging nerves and increasing the risk of infection. In general, the various embodiments of the invention reduce these and other risks associated with explantation.  
         [0011]     In one embodiment, the invention is directed to an implantable medical device comprising at least one module that includes control electronics within a housing, a member that at least partially encapsulates the housing, and a grippable access structure coupled to the member. The device, which is configured to be implanted between a scalp and a skull of a patient, can also include a radiopaque element. The grippable access structure may be, for example, a handle, a loop or a tab.  
         [0012]     In another embodiment, the invention presents an implantable medical device, configured to be implanted between a scalp and a skull of a patient, comprising a module that includes control electronics within a housing, member that at least partially encapsulates the housing, and a radiopaque element. The radiopaque element may be a part of the housing itself, for example, or may be a radiopaque marker.  
         [0013]     In a further embodiment, the invention is directed to an implantable medical device configured to be implanted between a scalp and a skull of a patient. The device includes at least one module that includes control electronics within a housing and a lead management structure. The lead management structure is configured to receive and protect bodies of leads coupled to the implantable medical device. The lead management structure may comprise a groove around the periphery of the device, for example.  
         [0014]     In an additional embodiment, the invention presents burr hole cap, comprising a lead management structure configured to receive and protect coiled bodies of leads passing through the burr hole cap. The lead management structure may comprise a groove in one of the members of the burr hole cap.  
         [0015]     In another embodiment, the invention is directed to an implantable medical device comprising a pouch made of cut-resistant material. The pouch is sized to receive a coil of a lead implanted in a body, and may include a radiopaque element.  
         [0016]     In an added embodiment, the invention is directed to a method comprising receiving an image of a patient, determining a location of an implantable medical device implanted between a scalp and a skull of the patient based on the image, and making an incision in the scalp based upon the determination. The method can optionally include gripping a grippable access structure of the implantable medical device and applying force to the implantable medical device via the grippable access structure.  
         [0017]     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  
       [0018]      FIG. 1  is a conceptual diagram illustrating deployment of a low-profile IMD under the scalp of a patient.  
         [0019]      FIG. 2  is a plan diagram of the top of a head of a patient, illustrating an exemplary implantation of a low-profile IMD.  
         [0020]      FIG. 3  is a conceptual imaging diagram of the top of a head of a patient, illustrating an exemplary technique for identifying the location of an implanted low-profile IMD.  
         [0021]      FIG. 4  is a plan diagram of one embodiment of a low-profile IMD that includes a grippable access structure in the form of a loop.  
         [0022]      FIG. 5  is a plan diagram of another embodiment of a low-profile IMD that includes a grippable access structure in the form of a tab.  
         [0023]      FIG. 6  is a plan diagram of the top of a head of a patient, illustrating an exemplary implantation of a low-profile IMD with a tethered interconnect.  
         [0024]      FIG. 7  is a perspective view of an embodiment of a low-profile IMD that includes a lead management structure.  
         [0025]      FIG. 8  is a perspective view of an embodiment of a burr hole cap that includes a lead management structure.  
         [0026]      FIG. 9  is a perspective view of an embodiment of a protective lead pouch. 
     
    
     DETAILED DESCRIPTION  
       [0027]      FIG. 1  shows a patient  10  with a low-profile IMD  12  deployed beneath his scalp  14 . In  FIG. 1 , IMD  12  is a neurostimulator that provides deep brain stimulation via leads  16 A,  16 B deployed in the brain of patient  10 . In the example of  FIG. 1 , IMD  12  is deployed in proximity to site of stimulation therapy. IMD  12  may be used to treat any nervous system disorder including, but not limited to, epilepsy, pain, psychological disorders including mood and anxiety disorders, movement disorders (MVD) such as, but not limited to, essential tremor and Parkinson&#39;s disease and neurodegenerative disorders.  
         [0028]     Although IMD  12  is depicted as a neurostimulator, the invention is not limited to applications in which the IMD is a neurostimulator. The invention may be employed with IMDs that perform any monitoring or therapeutic functions. The invention is not limited to IMDs that include leads deployed in the brain, but may also be employed with leads deployed anywhere in the head or neck including, for example, leads deployed on or near the surface of the skull, leads deployed beneath the skull such as near or on the dura mater, leads placed adjacent cranial or other nerves in the neck or head, or leads placed directly on the surface of the brain. Nor is the invention limited to IMDs that are coupled to electrodes. The invention may be employed with low-profile IMDs coupled to any sensing or therapeutic elements, such as temperature sensors or motion sensors. The invention may also be employed with different types of IMDs including, but not limited to, IMDs operating in an open loop mode (also referred to as non-responsive operation), IMDs operating in a closed loop mode (also referred to as responsive), and IMDs for providing monitoring and/or warning.  
         [0029]     In the example of  FIG. 1 , IMD  12  is deployed beneath scalp  14  of patient  10 , but on top of the cranium of patient  10 . The invention may be applied to other types of implantation as well, such as implantation of IMD  12  in a trough cut into the cranium of patient  10 .  
         [0030]     A surgeon may implant IMD  12  using any surgical technique. In a typical implantation, the surgeon makes an incision through the scalp  14  of patient  10 , and pulls back the resulting flap of skin to expose the desired area of the cranium. The incision may be a “C-flap” incision, for example. The surgeon drills holes, called “burr holes,” in the cranium and deploys leads  16  through the burr holes into the brain.  
         [0031]     The surgeon typically places caps, called “burr hole caps,” over the burr holes. Before connecting leads  16  to IMD  12 , the surgeon typically “manages” the leads. Lead management includes arranging the excess length of leads  16  using techniques such as coiling and anchoring with anchoring plates. In a typical implantation, the surgeon arranges the leads to provide some slack to reduce the risk of lead migration. Lead management also reduces the risk that the leads will be accidentally damaged during explantation, as described below.  
         [0032]     The surgeon implants IMD  12  between scalp  14  and the skull. In one surgical procedure, the surgeon uses a tool to form a pocket beneath the scalp proximate to the burr holes, and positions IMD  12  in the pocket. The surgeon may fix IMD  12  to the cranium using an attachment mechanism such as bone screws. The surgeon closes the skin flap over IMD  12 , and then staples or sutures the incision.  
         [0033]     At a later date, it may be necessary to remove IMD  12  from patient  10 . Explantation involves considerations that are distinct from implantation. For example, the surgeon may desire to remove IMD  12  but may desire to keep leads  16  deployed as they are. In addition, the surgeon may desire to recover IMD  12  in an undamaged condition. It may also be possible that the implanting surgeon and the explanting surgeon are different people, and the explanting surgeon may be unaware of what implantation and lead management techniques were used by the implanting surgeon. Because of considerations such as these, the explanting surgeon plans the surgery to avoid accidentally damaging the leads or the implanted device when making an incision.  
         [0034]      FIG. 2  illustrates a procedure for explantation of IMD  12  shown in  FIG. 1 .  FIG. 2  is a diagram showing the top of the head of patient  10 . Patient  10  may be under local anesthetic. The surgeon begins explantation by making an incision such as C-flap incision  18  in scalp  14 . In general, the surgeon has discretion concerning the making of an incision based upon the circumstances of each individual patient. Accordingly, the incision need not be a C-flap incision as shown in  FIG. 2 , but may include a straight incision or an S-shaped incision, for example. The incision chosen by the surgeon may be a function of the location of IMD  12 , the location of the leads, or other factors. As shown in  FIG. 2 , the surgeon draws scalp flap  20  away to expose the portion of the patient&#39;s skull  22  beneath scalp flap  20 , and to expose at least a portion of IMD  12 .  
         [0035]     In the example shown in  FIG. 2 , patient  10  has leads  16 A and  16 B deployed in the brain through burr holes  24 A and  24 B. A portion of the bodies of leads  16 A and  16 B, identified with reference numerals  26 A and  26 B, is deployed outside of the brain on the surface of skull  22 . The burr holes may be sealed with burr hole caps, with leads  26 A and  26 B passing therethrough. Leads  26 A and  26 B are depicted as coiled and are anchored by anchoring plates  28 A and  28 B. Leads  26 A and  26 B are coupled to IMD  12 .  
         [0036]     In  FIG. 2 , IMD  12  is a low-profile device, allowing it to be implanted between scalp  14  and skull  22 , with little discomfort or adverse cosmetic consequences to patient  10 . In addition, low-profile IMD  12  can have the advantages of reducing skin erosion and infection. IMD  12  comprises one or more modules that carry out the various functions of IMD  12 . As shown in  FIG. 2 , IMD  12  includes at least three modules: a control module  30 , a power supply module  32  and a recharge module  34 . One or more of modules  30 ,  32 ,  34  includes a housing that can carry out a variety of functions, including encasing the components of the modules, sealing the modules against contamination, electrically isolating electrical components, and the like. In some embodiments of the invention, at least one of the modules comprises a radiopaque material. The modules are coupled to member  36 , which may be made of a soft, biocompatible material. Member  36  at least partially encapsulates one or more housings of modules  30 ,  32 ,  34 , and generally serves as a smooth interface between the modules and the body tissue. Leads  26 A and  26 B are coupled to IMD  12  at lead connectors  38 A and  38 B. IMD  12  may be anchored with an anchoring mechanism such as a metallic tab  40  that includes an opening for receiving a bone screw.  
         [0037]     In general, member  36  integrates modules  30 ,  32  and  34  into a desired form factor, but, where flexible, allows relative intermodule motion. In some embodiments, member  36  incorporates mechanical features to restrict intermodule motion to certain directions or within certain ranges. Member  36  may be made from silicone, and is some embodiments may be made from two or more materials of differing flexibility, such as silicone and a polyurethane. An exemplary polyurethane for this purpose is Tecothane®, which is commercially available from Hermedics Polymer Products, Wilmington, Mass. Member  36  may also be referred to as an “overmold,” but use of the term “overmold” herein is not intended to limit the invention to embodiments in which member  36  is a molded structure. Member  36  may be a molded structure, or may be a structure formed by any process.  
         [0038]     The invention is not limited to the particular IMD depicted in  FIG. 2 , but includes a number of embodiments, some of which are described in more detail below.  
         [0039]     In  FIG. 2 , it is assumed that the surgeon has successfully made incision  18 , avoiding leads  26  and IMD  12 . The surgeon may also have successfully removed bone screws that anchored IMD  12  to skull  22 . The surgeon can decouple leads  26 A and  26 B from lead connectors  38 A and  38 B by hand or with a tool. In many cases, however, IMD  12  does not easily separate itself from the site of implantation, and the surgeon applies force to remove IMD  12 . Fibrous tissue growth proximate to the implantation site, for example, may resist the efforts of the surgeon to remove IMD  12 .  
         [0040]     IMD  12  includes a grippable access structure  42  that aids in explantation. In  FIG. 2 , grippable access structure  42  is a small handle or handle-like formation in or otherwise coupled to member  36  that can be gripped with a hand or an instrument, so that the surgeon may apply force to remove IMD  12 . A surgeon presented with IMD  12  as shown in  FIG. 2 , for example, can grip IMD  12  at handle  42  with a forceps, and apply force to pull or twist IMD  12 .  
         [0041]     The invention is not limited to the grippable access structure shown in  FIG. 2 . Other exemplary embodiments of grippable access structures will be described below. Some embodiments of grippable access structures have the advantage that they give more implantation and explantation options to the surgeon. In particular, the surgeon can plan an explantation procedure in which the incision is close to the grippable access structure, but safely away from the IMD and the leads.  
         [0042]      FIG. 3  is a conceptual imaging diagram of the top of a head of a patient, illustrating an exemplary technique for identifying the location of an implanted low-profile IMD. Before explanting the implanted device, the surgeon should know where the device is. Accordingly, the surgeon may direct that patient  10  be imaged using one or more medical imaging techniques such as X-ray, magnetic resonance imaging (MRI), CT-scan or fluoroscopy.  
         [0043]     Some of the imaging techniques employ electromagnetic radiation.  FIG. 3  represents an image  50  obtained with radiation, such as an X-ray. The image may include images of features or landmarks  52 ,  54  of the skull, which assist in locating the implanted device. In addition,  FIG. 3  shows images of modules  56  of the implanted device. Images of modules  56  appear in contrast to the most of the balance of image  50 . Modules  56  appear because the housings include a radiopaque material that causes the modules to stand out in image  50 . In the exemplary illustration of  FIG. 3 , the member, being made of a non-radiopaque material such as silicone, does not appear in image  50 .  
         [0044]     In some embodiments of the invention, however, the member includes one or more radiopaque markers, so that the location of the member can be identified as well. The invention supports any of several techniques for including one or more radiopaque markers in the member, such as outlining the member with radiopaque wire and loading the member with radiopaque powders or fibers.  
         [0045]     In  FIG. 3 , the image of leads  58  is visible as well, as the leads may include radiopaque markers. In addition, image  50  includes a radiopaque incision mark  60 , which may have been created by the surgeon who implanted the device. The surgeon can use a radiopaque marker to make radiopaque incision mark  60  on the skull of the patient during the implantation procedure. In some cases, radiopaque incision mark  60  can assist the surgeon in locating the IMD and leads by providing a reference on the skull itself. In addition to imaging as shown in  FIG. 3 , the surgeon could palpate for the IMD and could use the implantation incision scar as a reference. Radiopaque incision mark  60  may show the surgeon whether the implantation incision scar is proximate to its original site, or whether the implantation incision scar has migrated anteriorly or posteriorly. The surgeon can correct for scar migration, thereby reducing the risk of making an incision that cuts across the IMD. In addition, the surgeon can reduce the risk of making an incision that inadvertently cuts across the leads, which may be difficult to locate by palpation.  
         [0046]     In general, the explanting surgeon takes one or more images of the patient, and uses the images to determine the location of the implanted device and the leads. In particular, the surgeon uses the image to learn about the size and configuration of the implanted device, and the lead management techniques that have been employed. The surgeon may also take into consideration the site of an incision used during the implantation procedure.  
         [0047]     Using this information, the surgeon plans an incision strategy. The incision strategy takes into account the safety and effectiveness of a given incision, based upon the information obtained from the images. The surgeon implements the incision strategy in the operating room and removes the implanted device.  
         [0048]      FIG. 4  shows an alternate exemplary embodiment of the invention. IMD  70  is a low-profile IMD that includes one or more modules  72  with housings that are at least partially encapsulated by a member  74 . In addition, radiopaque markers  76 ,  78  are coupled to member  74 . Markers  76 ,  78 , which appear more plainly on an image than member  74 , can assist the surgeon in locating the position of the member. Markers  76 ,  78  may include additional information about member  74 , such as a model number, that would assist the surgeon in identifying the shape and dimensions of member  74 . Markers  76 ,  78  may be affixed to exterior of member  74  or may be embedded in member  74 .  
         [0049]     In addition, IMD  70  includes a grippable access structure  80  coupled to member  74 , in the form of a loop. Loop  80 , like handle  42  in  FIG. 2 , can be formed integral with the member or may be mechanically coupled to the member. Loop  80  can be dimensioned such that a surgeon may grip loop  80  with an instrument such as a forceps, or the surgeon has the option to grip loop  80  with her fingers. Loop  80  may include a wire or other radiopaque element (not shown) that would make loop  80  visible during imaging.  
         [0050]      FIG. 5  illustrates another exemplary embodiment of the invention. IMD  90  is a low-profile IMD that includes one or more modules  92  with housings that are at least partially encapsulated by a member  94 . In addition, radiopaque markers  96 ,  98 ,  100  are coupled to member  94 , and can assist the surgeon in locating the position of member  94  in an image. In particular, radiopaque markers  96 ,  98 ,  100  assist the surgeon in identifying the edges of IMD  90 . Radiopaque markers  96 ,  98 ,  100  may be affixed to exterior of member  94  or may be embedded in member  94 , such as by loading radiopaque powders or fibers in member  94 .  
         [0051]     In addition, IMD  90  includes a grippable access structure  102  coupled to member  94 , in the form of a tab. Like loop  80  in  FIG. 4 , tab  102  can be formed integral with the member or may be mechanically coupled to the member, and can be dimensioned to give the surgeon flexibility to grip the structure by hand or with an instrument. In the embodiment shown in  FIG. 5 , tab  102  includes a radiopaque marker  104  that would make tab  102  visible during imaging.  
         [0052]      FIG. 6  shows a further exemplary embodiment of the invention in an explantation procedure. In particular,  FIG. 6  demonstrates a technique for lead management that may be advantageous during explantation.  
         [0053]      FIG. 6  shows the top of the head of the patient, with the scalp being invisible for clarity. As in  FIG. 2 , leads  26 A and  26 B are coiled proximate to burr holes  24 A and  24 B, and IMD  12  is implanted nearby. In  FIG. 6 , leads  26 A and  26 B are coupled to IMD  12  via tethered interconnect module  110 . In the embodiment shown in  FIG. 6 , tethered interconnect module  110  couples to the lead connectors  38 A and  38 B of IMD  12  and leads  26 A and  26 B, and is interposed between the lead connectors and the leads. With tethered interconnect module  110 , the surgeon has more options for coupling leads  26 A and  26 B to IMD  12 . The surgeon may elect, for example, to deploy leads  26 A and  26 B so as to create a substantial space between the leads and IMD  12 .  
         [0054]     During explantation, an incision  112  can cause damage to the interconnecting leads  114  of tethered interconnect module  110 . Even so, the integrity of leads  26 A and  26 B is preserved. In other words, tethered interconnect module  110  can be sacrificed during explantation to avoid damage to IMD  12  and leads  26 A and  26 B by the incision. Once tethered interconnect module  110  is decoupled from IMD  12  and from leads  26 A and  26 B, the surgeon can remove IMD  12  without disturbing from leads  26 A and  26 B.  
         [0055]     Tethered interconnect module  110  may include a radiopaque material that enhances its visibility during imaging. In addition, tethered interconnect module  110  may include one or more anchoring structures (not shown) that hold tethered interconnect module  110  in position. The configuration of tethered interconnect module  110  shown in  FIG. 6  is exemplary, and the invention is not limited to the particular configuration shown.  
         [0056]      FIG. 7  is a perspective view of an embodiment of a low-profile IMD  120  that includes a lead management structure. IMD  120  includes one or more modules  122  within housings and a member that at least partially encapsulates the housings. IMD  120  is configured to be implanted between a scalp and a skull of a patient.  
         [0057]     Leads  126 A and  126 B are coupled to lead connectors  128 A and  128 B. Leads  126 A and  126 B are deployed around IMD  120  in a lead management structure. A lead management structure is a structure in IMD  120  that is configured to receive and protect the bodies of leads that are coupled to the IMD. In particular, a lead management structure is a structure that is configured to receive and protect the bodies of the leads as opposed to the terminals of the leads. Lead management structures include, but are not limited to, structures that route, fixate or anchor the lead bodies. Examples of a lead management structure include a groove or a cavity that receives a lead body.  
         [0058]     One of the practical problems associated with the leads is that the leads can be difficult to manage. The leads can twist, bend, slide and otherwise move. The propensity of leads to move can be inconvenience during implantation, and can also be a problem during explantation. If the leads move after implantation, there is an increased risk of damage to leads during explantation.  
         [0059]     In  FIG. 7 , the lead management structure is a groove  130  formed in member  124 , and leads  126 A and  126 B are wrapped around IMD  120  in groove  130 . The dimensions of the groove may a function of the length of the leads and the dimensions of IMD. The placement of groove  130  around the periphery of IMD  120  is for illustrative purposes, and the invention is not limited to the particular lead management structure shown in  FIG. 7 .  
         [0060]     The lead management structure need not be formed in member  124 . In some embodiments, the lead management structure can be constructed of a separate material, such as a protective material that would resist damage in the event the incision should cut across IMD  120 . Cut-resistant materials include, but are not limited to, metals and materials including embedded wire or polymer meshes. Furthermore, the lead management structure need not be located around the periphery as shown in  FIG. 7 , but in some embodiments can be located underneath member  124  and modules  122 . Lead management structures can not only direct lead bodies around IMD  120 , but can direct the lead bodies over or under IMD  120 .  
         [0061]     The lead management structure offers several possible benefits. First, it can protect the leads from damage in circumstances in which the incision cuts across the IMD. Second, it can in some circumstances offer a more efficient lead management option than coiling as illustrated in  FIGS. 2, 3  and  6 . Third, if the leads include radiopaque materials, an image of the leads can show not only the position of the leads, but also the position of the IMD.  
         [0062]      FIG. 8  is a perspective view of an embodiment of a burr hole cap  140  that includes a lead management structure. Burr hole cap  140  comprises a ring member  142  and a cover member  144  that couples to ring member  142  by a coupling mechanism (not shown). Burr hole cap  140  is configured to close a burr hole in a bony structure such as a skull, while allowing a lead to pass through.  
         [0063]     Ring member  142  includes a lead management structure. The lead management structure is groove  146 , which receives lead  148 . The implanting surgeon can coil lead  148  inside groove  146 , and draw lead through exit  150 , before coupling cover member  144  to ring member  142 . Ring member  142 , cover member  144  or both can be constructed from a protective material that would resist damage in the event the incision should cut across burr hole cover  140 .  
         [0064]     The lead management technique illustrated in  FIG. 8  can protect the lead from damage in circumstances in which the incision is close to the burr holes. Burr hole cap  140  can, in some circumstances, offer a more efficient lead management option than coiling outside of the burr hole cap. The configuration of the burr hole cap and the lead management structure are for illustrative purposes, and the invention is not limited to the burr hole cap or lead management structure shown in  FIG. 8 . For example, the invention includes burr hole caps that include a lead management structure that supports winding of a lead around the exterior of the burr hole cap.  
         [0065]      FIG. 9  is a perspective view of a protective pouch  160  that can be used in lead management. Pouch  160  is sized to slip over coils of lead  162  and protect the coils from accidental damage. Pouch  160  can be constructed of a cut-resistant protective material and may also include a radiopaque material that enhances visibility of pouch  160  during imaging. Pouch  160  may be constructed of any of a number of biocompatible materials, such as silicone, and may further incorporate cut-resistant materials. Cut-resistant materials include, but are not limited to, metals and materials including an embedded metallic wire mesh, embedded threads, or a polymer mesh such as a Dacron mesh.  
         [0066]     The invention is not limited to the particular embodiment of the pouch shown in  FIG. 9 . The invention encompasses, for example, pouches that are configured to hold more than one lead, pouches that have anchoring structures, and pouches that include closing structures that reduce the risk that the pouch will disengage from the coiled lead.  
         [0067]     Although the invention has been described in connection with explantation of a device implanted on the head, the invention is not limited to the area of the head. A low-profile IMD such as the devices described herein may be implanted anywhere in the body. Implantation and explantation techniques may be similar to techniques for explantation and implantation under the scalp. In particular, the surgeon may make an incision in the skin of a patient. The surgeon may retract the incision to expose a bone, muscle or other anatomical structure. The surgeon may wish to avoid damage to the IMD or the leads, and may wish to remove the IMD without disturbing the leads.  
         [0068]     The invention supports implantation of an IMD that performs any of several functions. The invention supports explantation of IMDs that provide monitoring, IMDs that administer therapy, and IMDs that do both. The invention is not limited to any particular number of modules or to any particular functionality.  
         [0069]     Various embodiments of the invention have been described. As mentioned above, the invention is not limited to the particular embodiments described or shown in the figures. These and other embodiments are within the scope of the following claims.