Patent Publication Number: US-2007123923-A1

Title: Implantable medical device minimizing rotation and dislocation

Description:
BACKGROUND OF THE INVENTION  
      Conventional implantable medical devices typically have regular, curved outer surfaces. In  FIG. 1 , the conventional implantable medical device  101  (i.e. pacemaker) having leads  102  provides an example of the shape of such a device. Some implantable medical devices have smooth biocompatible surfaces in order to prevent metabolization by a host body. Having a smooth, regular outer surface, however, does not ensure a stable position in the body. Devices may (1) rotate, (2) revolve, and/or (3) migrate to a different and unintended position. Such movement can lead to compromised functionality, particularly for subcutaneous defibrillation systems that rely on the device for sensing or providing electrical therapy to the heart.  
      One example of undesired device displacement is referred to as Twiddler&#39;s syndrome, where repeated rotation of a subcutaneous pacemaker causes looping of the pacing wires or catheters, potentially causing poor contact between the wires or catheters and the tissue they are intended to monitor and/or stimulate.  FIG. 2  illustrates a normal placement of a conventional subcutaneous pacemaker  101  in a human body. The device sits under the skin with leads  102  which feed through major blood vessels into the heart  203 , sensing and/or stimulating heart tissues  204 .  
       FIG. 3  illustrates more closely the area of detail from  FIG. 2 , including device  101  and leads  102 , and  FIG. 4  illustrates the same area of detail showing one possible result of Twiddler&#39;s syndrome. Here, pacemaker  101  has rotated in place, wrapping leads  102  around itself. Over time, the tension on the leads may result in a poor connection with the tissues of the heart  204 .  
      Presently, implantable devices are sutured or stapled to surrounding tissues in order to prevent dislocation. In general, such acute fixation means have proven to be adequate for most devices. However, in a significant number of cases, failure of the acute fixation means occurs, leaving the implanted device to “float” within the body, as described above. Alternative, chronic means for maintaining device location are needed in these cases. For proper functioning of implantable medical devices, it is desired that the original device orientation and position be maintained throughout the life of the device, without the potential for failure associated with acute fixation means.  
     BRIEF SUMMARY OF THE INVENTION  
      Device housings and methods are provided which minimize rotation and displacement of medical devices implanted within humans and other animals. Device housings include housing shapes, surface features, and/or attached implements which help bind a device to the surrounding tissues. Some embodiments work with a body&#39;s natural healing process, enabling encapsulating tissues to anchor the device in place. Additional embodiments engage surrounding tissues directly, either through manual activation, or through automatic activation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing brief summary of the invention, as well as the following detailed description, is better understood when read in conjunction with the accompanying drawings, which are included by way of example, and not by way of limitation with regard to the claimed invention. In the accompanying drawings, the same or similar elements are labeled with the same reference numbers.  
       FIG. 1  is a prior art example of a conventional implantable medical device.  
       FIG. 2  illustrates a prior art example of a normal placement of a conventional subcutaneous pacemaker in a human body.  
       FIG. 3  illustrates more closely the area of detail from prior art  FIG. 2 , including a device and attached leads.  
       FIG. 4  illustrates more closely the area of detail from prior art  FIG. 2  showing one possible result of Twiddler&#39;s syndrome.  
       FIG. 5  depicts an example of an implantable medical device situated within a human body in accordance with one or more embodiments.  
       FIG. 6  depicts a perspective view of a device and the degrees of motion that are impeded by the shape of the device in accordance with one or more embodiments.  
       FIGS. 7, 8 , and  9  depict perspective views of three embodiments having elongated members and areas around which tissue may grow in accordance with one or more embodiments.  
       FIGS. 10, 11 ,  12 ,  13 , and  14  depict plan views of five embodiments created using overlapping two-dimensional shapes in accordance with one or more embodiments.  
       FIGS. 15, 16 , and  17  depict perspective views of three embodiments having notched portions in accordance with one or more embodiments.  
       FIGS. 18, 19 , and  20  depict perspective views of three embodiments having members with flared ends in accordance with one or more embodiments.  
       FIG. 21  depicts a plan view of an optional enclosure for an implantable device in accordance with one or more embodiments.  
       FIG. 22  depicts a perspective view of an implantable device having several holes or pores in accordance with one or more embodiments.  
       FIGS. 23 and 24  depict cross-sectional views of a pore on the device of  FIG. 22  in accordance with one or more embodiments.  
       FIG. 25  depicts a perspective view of an implantable device having several modified pores in accordance with one or more embodiments.  
       FIGS. 26 and 27  depict cross-sectional views of a pore on the device of  FIG. 25  in accordance with one or more embodiments.  
       FIG. 28  depicts a perspective view of an implantable device having anchor structures in accordance with one or more embodiments.  
       FIGS. 29 and 30  depict cross-sectional views of a portion of the surface of the device from  FIG. 28  in accordance with one or more embodiments.  
       FIG. 31  depicts a perspective view of an implantable device having a mesh anchor in accordance with one or more embodiments.  
       FIGS. 32 and 33  depict cross-sectional views of a portion of the surface of the device of  FIG. 31  in accordance with one or more embodiments.  
       FIG. 34  depicts a perspective view of an implantable device having through-holes in accordance with one or more embodiments.  
       FIGS. 35 and 36  depict cross-sectional views of a portion of the device of  FIG. 34  in accordance with one or more embodiments.  
       FIGS. 37 and 38  depict perspective views of an implantable medical device having a rotation implement in accordance with one or more embodiments.  
       FIGS. 39-41  depict cross-sectional views of a spring attachment abutting and grasping tissue in accordance with one or more embodiments.  
       FIGS. 42-44  depict side views of an elastic attachment abutting and grasping tissue in accordance with one or more embodiments.  
       FIGS. 45-47  depict cross-sectional views of an expandable slitted attachment abutting and grasping tissue in accordance with one or more embodiments.  
       FIGS. 48-49  depict side views of an orthogonal slitted attachment in accordance with one or more embodiments.  
       FIGS. 50-52  depict cross-sectional views of an inflatable grasper abutting and grasping tissue in accordance with one or more embodiments.  
       FIGS. 53-56  depict cross-sectional side views of the operation of a capped perforator piercing tissue in accordance with one or more embodiments.  
       FIGS. 57-59  depict cross-sectional views of the operation of a buried sharp stylet piercing tissue in accordance with one or more embodiments.  
       FIGS. 60-62  depict cross-sectional views of the operation of an alternative buried stylet piercing tissue in accordance with one or more embodiments.  
       FIGS. 63-65  depict cross-sectional views of the operation of a “pop rivet” affixing to tissue in accordance with one or more embodiments.  
       FIGS. 66-68  depict cross-sectional views of the operation of an axial clasp grasping tissue in accordance with one or more embodiments.  
       FIGS. 69-70  depict cross-sectional views of the operation of a barbed clip affixing to tissue in accordance with one or more embodiments.  
       FIG. 71  depicts a perspective view of rotating barbs for use in affixing a tube to tissue in accordance with one or more embodiments to tissue.  
       FIGS. 72 and 73  depict cross-sectional views of a tension spring affixing to tissue in accordance with one or more embodiments.  
       FIGS. 74-77  depict cross-sectional views of additional deployed tension spring embodiments in accordance with one or more embodiments.  
       FIGS. 78 and 79  depict cross-sectional views of curved barbs in accordance with one or more embodiments.  
       FIGS. 80 and 81  depict cross-sectional views of a tension spring extending into tissue in accordance with one or more embodiments.  
       FIGS. 82 and 83  depict cross-sectional views of a tension spring embodiment extending into tissue in accordance with one or more embodiments.  
       FIGS. 84 and 85  depict perspective views of an implantable device having retractable helices in accordance with one or more embodiments.  
       FIGS. 86 and 87  depict perspective views of an implantable device having heat-activated blades in accordance with one or more embodiments.  
       FIG. 88  depicts a perspective view of an implantable device having curved needles in accordance with one or more embodiments.  
       FIG. 89  is a flow chart depicting a method for affixing an implantable medical device to surrounding tissue in accordance with one or more embodiments. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
      Upon insertion of a foreign object into the bodies of humans (and other animals), steps in the natural healing response are triggered which may bring about the elimination of the foreign object via tissue encapsulation. This process will occur whether the insertion is regarded as harmful (e.g., bullets, slivers) or as beneficial (e.g., chronic medical devices). For such encapsulation, the outside of the tissue capsule or pocket is typically anchored to the surrounding native tissue structure and supplied by its vasculature. The inside of the tissue pocket is typically lined with reactive cells (e.g., macrophages, foreign body giant cells, fibroblasts) that attempt to metabolize as well as encapsulate a foreign object.  
      The housings for implantable medical devices, leads, and catheters may be fashioned using shapes, surface features and/or attached implements that resist movement within a host body using more reliable chronic fixation means. Such housing designs may work with a body&#39;s encapsulation process such that encapsulating tissues naturally anchor or affix themselves to the device. Alternatively, these housing designs may be affixed to surrounding tissues using attached implements. Over time, the positional stability created by acute fixation means (e.g., staples or sutures) may be naturally replaced by or enhanced by the chronic fixation means described below.  
       FIG. 5  depicts an example of an implantable medical device  501  situated within a human body. As previously described, conventional devices may have a tendency to rotate or shift within the body. However, device  501  has been shaped to resist dislocation and rotation within the host body.  FIG. 6  depicts a perspective view of device  501  along with the degrees of motion that are impeded by the shape of the device. Because device  501  includes elongated members  602  and  603  at angles to each other, device  501  will resist rotation as well as vertical and horizontal displacement, unlike conventional rounded device  101 .  
      Generally, a device having elongated members extending at angles approaching ninety degrees from each other may resist motion due to greater surface area in a given direction of motion. The longer the members, the greater the moment required to rotate the device, and the greater the force required to shift the device, depending on the direction of motion.  
      In addition, a device having elongated members  602  and  603  may take advantage of tissue encapsulation, which is a part of the body&#39;s natural healing process. As tissues grow around the device and affix themselves to surrounding tissue, the encapsulating tissues serve to anchor the device. For conventional devices, tissue growth around the device does not necessarily prevent motion, since tissues may slide around and/or off a device. For device  501 , tissue that grows around member  603  may impede device motion in the vertical direction, and likewise, tissue that grows around member  602  may impede device motion in the horizontal direction.  
       FIGS. 7, 8 , and  9  depict perspective views of three embodiments  701 ,  801 , and  901  which include elongated members and areas around which tissue may grow without easily sliding off. Each includes either a notched or narrow portion (e.g., notch  702 ), once again, where encapsulating tissue may completely surround the device, anchor it to the surrounding tissues, and not easily be loosened from the device.  
       FIGS. 10, 11 ,  12 ,  13 , and  14  depict plan views of five embodiments  1001 ,  1101 ,  1201 ,  1301 , and  1401  created using overlapping two-dimensional shapes. As before, notched or narrow portions provide constrained areas around which encapsulating tissue may grow and anchor the devices. Even the minimal notches of device  1401 , designed using overlapping ovals, provide better stabilization than conventionally shaped devices.  
      It should be noted that the selection of any particular shape may be made based on any number of considerations, including the size and shape of components housed within the device, the area of the body into which the device is to be placed, the size of the incision in through which the device is to be implanted, cost of materials, type of materials used, and so forth.  
       FIGS. 15, 16 , and  17  depict perspective views of three embodiments  1501 ,  1601 , and  1701  which include notched portions. Device  1701  in particular uses both elongated members and notched portions. As with the embodiments above, the notched portions and elongated members provide locations around which encapsulating tissue may grow and anchor the devices.  
       FIGS. 18, 19 , and  20  depict perspective views of three embodiments  1801 ,  1901 , and  2001  which include elongated members with flared ends. Similar to a notched portion, an elongated member with flared ends (e.g., flared end  1802 ) allows tissue to grow around the device. The flared ends help prevent the device from sliding beyond the grip of the encapsulating tissues.  
       FIG. 21  depicts a plan view of an optional enclosure  2102  for an implantable device  1601 . Whereas providing notches, elongated members, and flared ends may prevent an implantable device from shifting or rotating in place, those same elements may make placement of a device more difficult. For example, the notches of implantable device  1601  may get caught on skin, fascia, or other tissues when inserted into the body. Providing separable enclosure  2102  may simplify this procedure by covering the notches and easing insertion. Once inserted, enclosure  2102  may be removed.  
      Enclosure  2102  may additionally be comprised of an absorbable substance to dissolve over a period of time. Examples of such a substance include mannitol, and polyethylene glycol (“PEG”). Other nontoxic, biocompatible, water-soluble materials may be utilized. The enclosure substance may further include a pharmacological agent to enhance healing and/or reduce discomfort associated with the insertion procedure.  
      Additional embodiments may unevenly distribute weight throughout an implantable device. Such a configuration may assist in positioning a device by using gravity to automatically orient and stabilize the orientation. For example, a circular implantable device may include a heavy weight placed along one edge of the circle. When inserting the device into a body, the weighted portion of the circle may gravitationally bias the orientation of the device into a preferred position. Similar alteration of weight and density in an implantable device may achieve similar results in other device shapes.  
      In addition to (or instead of) elongated members, notches, flared ends, and other movement-avoiding body shapes, implantable devices may be provided with surface features that help prevent rotation and dislocation of the device. These surface features may work with a body&#39;s natural tissue encapsulation process where the tissue grows around the surface features and subsequently becomes anchored to the implantable device.  
       FIG. 22  depicts a perspective view of a generic implantable device  2201  having several holes or pores  2202 . These pores may serve as anchoring locations into which fibrous tissue may grow and become affixed. Although depicted as small circles or ovals here, other pore shapes and sizes may work just as well. The pores may be distributed evenly over the entire device, or may be placed in strategic locations on only one or more sides, in order to maximize anchoring without unduly complicating device removal. Pore location may be strategically selected based on knowledge of likely directions in which the device may shift or rotate. If too many tissue anchors are attached to a device, removing the device may require more time and or pain to the patient.  
       FIGS. 23 and 24  depict cross-sectional views of pore  2202  on device  2201 . Such pores (also referred to as blind holes) may be designed with an overhanging lip  2303 , such that tissue  2404  may grow into the pore and become wedged in place. When device  2201  attempts to shift or rotate, tissue  2404 , which may be anchored both to device  2201  and to other body tissues, prevents movement due to both the wedge shape of the anchor  2405  and the suction created by the anchor inside the pore. Depending on the angle of the lip  2303 , removing tissue from around the device (such as when the device needs to be replaced), may be more or less difficult. Suction alone may be enough to hold anchor  2405  in place, obviating the need for a lip, and possibly simplifying manufacture of the device surface.  
      The inner walls of pore  2202  may further be textured rather than smooth. The textured character of the walls may be needed so as to provide numerous locations for the fibrotic ingrowth to form anchor attachments to device  2201 .  
       FIG. 25  depicts a perspective view of a generic implantable device  2501  having several modified pores  2502 . The modifications made to the pores are made clear in  FIGS. 26 and 27 , which depict a cross-sectional views of pore  2502 . Here, the pore has been modified to include column  2603 , which provides additional gripping surface around which fibrous tissue  2704  may grow. Following device implantation, the initial, immature tissue encapsulation will first be produced about the entire device, including within the pores. With the further passage of time, the encapsulation material (i.e. collagen) may undergo natural remodeling and condensation thereby bringing about contraction. Contraction of the tissue around column  2603  further increases the grasping of the device by the tissue.  
      It should be noted that column  2603  need not be any particular height. Although shown in  FIGS. 26 and 27  as being the same height as the surface of a device, such surface features may be shorter (below surface level) or taller (above surface level). Moreover, multiple columns  2603  may vary from location to location, shorter in some and taller in others.  
      As with device  2201 , the pores  2502  may take on different shapes and sizes. Furthermore, a porous structure is not required to take advantage of the enhanced grasping caused by tissue contraction. The exterior of a device may be provided with several grooves or contours generally superficial and tangential to the surface of the device. These grooves should, as with column  2603 , form closed loop-like shapes, around which tissue may grow and then contract.  
       FIG. 28  depicts a perspective view of a generic implantable device  2801  having anchor structures  2802 . Anchor structures  2802  are convex to the surface of the device, as opposed to the concave pores of devices  2201  and  2501 . These structures may be created as a part of the original housing, or added after initial manufacture, perhaps using an attachment method such as screws, or through sintering of a porous material.  FIGS. 29 and 30  depict cross-sectional views of the surface of device  2801 , including two anchor structures  2802 . Upon insertion, tissue  3003  grows around anchors  2802 , creating a bond between the two. As with the columns  2603  of device  2501 , tissue  3003  may contract around anchors  2802  over time, further strengthening the bond.  
      Other surface structures may additionally provide surfaces around which encapsulating tissue may attach itself.  FIG. 31  depicts a perspective view of a generic implantable device  3101  having mesh anchor  3102 . The mesh  3102  may be metallic, polymeric, fabric, etc., in nature.  FIGS. 32 and 33  depict cross-sectional views of device  3101  having mesh  3102 . Tissue  3303  may encapsulate device  3101  and once again surround the mesh anchor  3102 , creating wedges  3304  similar to those of device  2201 . The tissue may further contract around the mesh over time, strengthening its grasp of the device. Other similar porous or mesh structures may similarly induce the desired anchoring effect from encapsulating tissue.  
       FIG. 34  depicts a perspective view of a generic implantable device  3401  having through-holes  3402 . Through-holes  3402  are formed merely by creating holes in the housing of implantable device  3401 .  FIGS. 35 and 36  depict cross-sectional views of device  3401  having through-hole  3402 . Once inserted into a body, the holes may become filled with cellular agents that form numerous fibrous tissue bridges  3604  through the device that connect to the tissue pocket  3603  at both openings. Tissue bridges  3604  through device  3401 , especially if situated in multiple through-holes  3402 , prevent movement and rotation within the body. Although through-holes  3402  are depicted as circles or ovals, other shapes and sizes may work just as well. However, a tnrough-hole which is too narrow may result in a tissue bridge which is either too brittle, or non-existent.  
      As discussed above with regard to pores  2202 , each of the surface features may benefit from the addition of textured surfaces that provide additional grasping sites for tissue ingrowth. For example, through-holes  3402  may include a rough or otherwise textured surface to provide additional grasping locations for tissue bridges  3604 .  
      The above surface features may be combined with each other, creating hybrid forms and strengths of tissue anchoring. For example, the sintered structures may be combined with the pores to maximize anchoring. Alternatively, a device may include a radial elongated member or arm extending outwards (not shown), the radial arm having a through-hole or other anchoring surface feature. This arm may provide both resistance to shifting and rotating, as well as provide a through-hole through which a tissue bridge may anchor the device. When the device needs to be replaced, the radial arm can merely be broken or detached, and the device removed.  
      The above mentioned mesh or porous structures, through-holes, blind holes, or grooves may be filled with a biocompatible, inert, water-soluble agent that temporarily fills the structure to generate a smooth surface which may facilitate implantation. Shortly after implantation, the water-soluble agent will dissolve to expose the anchoring means provided. The water-soluble agent may be specially selected to promote the production of tissue encapsulation.  
      The implantable devices described thus far have included devices designed to passively anchor themselves to surrounding tissues and to resist movement. Additional embodiments may provide active means for affixing a device to surrounding tissues. Such embodiments may require upon implantation and placement the manual activation of one or more implements attached to the device. Alternatively, automatic activation of such anchoring implements may also be utilized. Thus far, examples of implantable medical devices have included defibrillators, but catheters, leads, and other implantable devices may also take advantage of the embodiments and concepts described.  
       FIGS. 37 and 38  depict perspective views of implantable medical device  3701  having rotation implement  3702 . Such a device may be a part of a larger device, and may vary in both size and shape. In  FIG. 37 , device  3701  is depicted in an inactivated state. This configuration allows easier insertion since no notches or elongated members are exposed to catch on tissues. Once inserted, however, rotation implement  3702  can be rotated around axis  3703  so as to create both notches and elongated members, as shown in  FIG. 38 . Similar to the passive embodiments described above, these notches and/or elongated members provide anchoring points around which encapsulating tissue may grow, assisting in the stabilization of device  3701 .  
       FIG. 39  depicts a cross-sectional view of an inactive spring attachment  3901  abutting tissue  3902 . Spring attachment  3901  may be attached to the outer housing of an implantable medical device (e.g., a pacemaker or associated lead) and can be activated once the device has been implanted.  FIG. 40  depicts a cross-sectional view of the same spring attachment  3901  now in an activated state. Here, spring  3901  has been stretched, either by exerting force in the directions of force arrows  4004 , or by twisting the spring by applying a moment at one or both ends. Once activated, portions of tissue  3902  may become lodged between widening adjacent coils of spring  3901 , such as tissue section  4003 . Once the force or moment has been removed, as shown in  FIG. 41 , spring  3901  relaxes, catching tissue section  4003  in the gap between narrowing coils, effectively grasping the tissue. It is possible that multiple sections of tissue  3902  could be grasped between multiple coils of spring  3901 , creating an even stronger bond between the medical device and surrounding tissue.  
       FIG. 42  depicts a side view of an inactive elastic attachment  4201  abutting tissue  4204 . Elastic attachment  4201  includes an elastic sleeve  4202  having a multitude of gripping bands  4203 , and may be attached or incorporated into the exterior of an implantable medical device. Elastic sleeve  4202  may be composed of silicone or some other polymeric substance. In  FIG. 43 , elastic attachment  4201  has been activated through the application of force at the ends of sleeve  4202  in the direction of force lines  4306 . When activated, gripping bands  4203  are separated from each other along the axis of the sleeve. Abutting tissue, such as tissue section  4305 , may become trapped in the gaps created. Once elastic attachment  4201  is again relaxed, as shown in  FIG. 44 , tissue is trapped between the gripping bands, holding an associated device in place.  
       FIG. 45  depicts a cross-sectional view of an expandable slitted attachment  4501  abutting tissue  4505 . Such an attachment may be affixed to the exterior of a larger implantable device, such as a pacemaker, or may be integrated into the outer portion of a catheter or lead. Slitted attachment  4501  is composed of inflatable bladder  4502 , and elastic ring  4503  having slits  4504 . It should be noted that the shape used here is merely representative, and other shapes may work just as well.  FIG. 46  depicts the same slitted attachment  4501 , although with inflatable bladder  4502  fully inflated, such that elastic ring  4503  has distended, opening slits  4504 . As the slits open, portions  4606  of abutting tissue  4505  may enter the openings. Once inflatable bladder  4502  has been deflated, as in  FIG. 47 , the tissue portions  4606  are grasped by the open slits  4504  of elastic ring  4503 .  
       FIG. 48  depicts a side view of an orthogonal slitted attachment  4801 . As with expandable slitted attachment  4501 , orthogonal slitted attachment  4801  may be affixed to the exterior of a device, or integrated into the outer surface of a catheter or lead. Orthogonal slitted attachment  4801  includes a multitude of slitted regions  4802 . Here, the slits are made in a simple “X” pattern, but other patterns are available. In  FIG. 49 , as with slitted attachment  4501 , the internal pressure of attachment  4801  is increased using an inflatable bladder (not shown) or some other inflation method. The outer surface of the attachment is distended, and slits  4802  expand as shown. Once the internal pressure is returned to normal, slits  4802  return to normal size, grasping abutting tissue (not shown) with the corners created by the pattern.  
       FIG. 50  depicts a cross-sectional view of an inactive inflatable grasper  5001  abutting tissue  5003 . Inflatable grasper  5001  includes grasping barbs  5002 , which are crossed when the grasper is an inactive state. Grasper  5001  is inflated, increasing its circumference, and separating the grasping barbs  5002  as shown in  FIG. 51 . Once the expansion of grasper  5001  stops, grasping barbs  5002  pierce tissue  5003 . In  FIG. 52 , pressure within grasper  5001  has returned to normal, and grasping barbs  5002  have returned to their crossed position, pulling tissue  5003  down and locking the tissue in place. As with other attachment implements, alternative shapes and sizes of graspers may be used. Furthermore, increasing the circumference of grasper  5001  may be accomplished using other means, such as inserting a member into the center of the grasper which widens the diameter.  
       FIGS. 53-56  depict cross-sectional side views of the operation of a capped perforator  5301  piercing tissue  5305 .  FIG. 53  depicts capped perforator  5301 , which is composed of spring-loaded elements  5302 , held in a retracted position by a restraining rod  5303 , capped by a pointed cap  5304 . Such a perforator may be affixed to the housing of a medical device, or to the end of a catheter or lead and used to semi-permanently affix the device, catheter, or lead to tissue  5305 . In  FIG. 54 , capped perforator  5301  has pierced tissue  5305 . In  FIG. 55 , restraining rod  5303  is thrust forward, releasing spring-loaded elements  5302  from under pointed cap  5304 . In  FIG. 56 , restraining rod  5303  is retracted, leaving capped perforator  5301  in place. The wedge created by spring-loaded elements  5302  helps prevent capped perforator  5301  from dislodging from tissue  5305 .  
       FIGS. 57-59  depict cross-sectional views of the operation of a buried sharp stylet  5701  piercing tissue  5705 .  FIG. 57  depicts a retracted position for sharp stylet  5701 . Outer surface  5702  is formed in specialized “pucker” formation, around which tissue  5705  is pressed. Beneath outer surface  5702 , sending tunnel  5703  and receiving tunnel  5704  are formed with metal or otherwise protected interior surface. In  FIG. 58 , sharp stylet  5701  is advanced through sending tunnel  5703  and into tissue  5705 . In.  FIG. 59 , sharp stylet  5701  continues on through receiving tunnel  5704 . Additional buried stylets, or additional pucker formations using the same stylet, may be used to enhance the grasping effect. Alternative formations may be used which place tissue in proximity to a retractable stylus.  
       FIGS. 60-62  depict cross-sectional views of the operation of an alternative buried stylet  6001  piercing tissue  6003 . The operation, which begins in  FIG. 60 , is similar to that for buried stylet  5701 , except that a curved sharp stylet  6001  is advanced through tunnel  6002 . Although depicted as being utilized at the end of a cylindrical housing  6004 , such a fixation means could be used on the outer housing of implantable devices, including pacemakers, catheters and leads. In  FIG. 61 , when curved stylet  6001  departs tunnel  6002 , it arcs around and back towards the device, piercing and grasping tissue  6003  en route. In  FIG. 62 , curved stylet advances back into the device, either to be received into a second tunnel (not shown) or possibly to embed itself in a malleable surface such as silicone.  
       FIGS. 63-65  depict cross-sectional views of the operation of a “pop rivet”  6301  affixing to tissue  6305 . Affixing pop rivet  6301  to tissue  6305  is similar to the process of perforating cap  5301 . In  FIG. 63 , pop rivet  6301 , consisting of piercing head  6302 , stylet  6303 , and shoulder  6304 , is advanced towards tissue  6305 . In  FIG. 64 , the piercing head advances through tissue  6305 , up to shoulder  6304 . Finally, in  FIG. 65 , stylet  6303  is retracted, modifying the shape of piercing head  6302  so that it assumes a “mushroom cap” shape which helps prevent the removal of pop rivet  6301 . Using a pop rivet embodiment, the stylet may later be advanced, reforming piercing head  6302 , and allowing removal and repositioning of pop rivet  6301  and its associated device.  
       FIGS. 66-68  depict cross-sectional views of the operation of a rotary clasp  6601  grasping tissue  6605 . Such a clasp may be incorporated into the outer housing of an implantable device, including a pacemaker, a catheter, or a lead.  FIG. 66  depicts clasp  6601  in its relaxed initial position, abutting tissue  6605 . Clasp  6601  includes clasp members  6602  and  6603 , one of which includes sharp stylet  6604 . In  FIG. 67 , when clasp  6601  is opened while abutting tissue  6605 , sharp stylet  6604  is exposed, and a tissue section  6706  enters the gap. When the clasp is again closed in  FIG. 68 , sharp stylet  6604  pierces tissue section  6706 , and relaxing clasp members  6602  and  6603  squeeze and grip the tissue section. Once closed, the clasp both grasps and pierces the tissue section, creating a strong bond between device and tissue.  
       FIGS. 69-70  depict cross-sectional views of the operation of barbed clip  6901  affixing to tissue  6902 . Barbed clip  6901  functions in much the same way as a pen clip. In FIG.  69 , barbed clip  6901  includes sharp barb  6903 , and is abutted by tissue  6902 . In  FIG. 70 , as the device to which sharp barb  6901  is attached is moved in the direction of arrow  6904 , tissue  6902  is pierced, and barb  6903  holds the tissue in place. Multiple barbs may be incorporated into a device surface to further secure device position.  
       FIG. 71  depicts a perspective view of rotating barbs  7101  for use in affixing tube  7102  to tissue. Tube  7102 , which may be attached to any device (e.g., a catheter), can be placed gently against tissue and turned in the direction of arrow  7103 , securing rotating barbs  7101  into the tissue. Although two barbs are pictured here, additional barbs may be utilized.  
       FIGS. 72 and 73  depict cross-sectional views of tension spring  7201  affixing to tissue  7204 .  FIG. 72  depicts an undeployed tension spring  7201  having curved extensions  7203  placed inside a rigid tube  7202  (made of e.g., glass, metal, or rigid biocompatible polymer). In  FIG. 73 , tension spring  7201  is deployed, piercing tissue  7204 . Deploying the spring may be accomplished by forcing the spring out of tube  7202  using rod  7305 . Rod  7305  may terminate or interact with plunger  7306 . Rod  7305  may otherwise be provided a groove or slot to guide its progression and interaction with tension spring  7201 . Other rod configurations may aid stable deployment of the spring. A tension spring such as the embodiment shown here may be used in conjunction with any type of implantable device, including pacemakers, catheters, leads, and so forth.  
       FIGS. 74-77  depict cross-sectional views of additional deployed tension spring embodiments. Spring  7401  employs curved extensions having a tighter curve. Spring  7501  employs a smaller tube opening  7502 , causing a shallower, more-controlled tissue entry. Springs  7601  and  7701  both employ barbed or crooked extensions that lodge themselves into tissue.  
       FIGS. 78 and 79  depict differing cross-sectional views of curved sharp implements  7801 , similar to buried stylet  6001 .  FIG. 78  depicts a side cross-sectional view curved implements  7801 . Before the implements are extended (not shown), they initially sit buried in tunnel  7802  within device  7803 . Once device  7803  is in place, an implement  7801  is extended by forcing it through tunnel  7802 . The implements are curved such that they will curl through adjacent tissue and arc back into device  7803 .  FIG. 79  depicts a front cross-sectional view of implements  7801 . The sharp implements may be angled away from each other as shown in order to spread the attachment points with the tissue. Sharp implements  7801  may include barbs to hinder removal. Alternatively, smooth sharp endings may facilitate retraction of the implements if needed.  
       FIGS. 80 and 81  depict cross-sectional views of tension spring  8001  extending into tissue. Similar to previously described tension springs, in  FIG. 80 , spring  8001  lies in a tense or wound up state within tunnel  8002  until the associated device is placed. Once placed, spring  8001  is extended into the surrounding tissue as shown in  FIG. 81 , and spring arms  8003  unfold and extend broadly into the tissue.  
       FIGS. 82 and 83  also depict cross-sectional views of a separate tension spring embodiment  8201  extending into tissue. As shown in  FIG. 82 , the opening  8204  through which tension spring  8201  extends is narrowed. This may cause spring arms  8203  to embed themselves in the tissue at a shallower level, as can be seen in  FIG. 83 . It also may allow the movement and extension of spring arms  8203  to be more controlled and deliberate.  
       FIGS. 84 and 85  depict perspective views of a generic implantable device  8401  having retractable helices  8402 .  FIG. 84  shows retractable helices  8402  completely retracted, which is the position they would be in while device  8401  is being placed within a patient. Once placed, a doctor may extend helices  8402  (e.g., by rotating them from the opposite side) as shown in  FIG. 85 . Helices  8402  twist into the underlying tissues (e.g., fascia) and become attached, similar to a corkscrew. If device  8401  needs to be replaced at a later date, helices  8402  may be unscrewed from surrounding tissues, and the device removed. Although two helices are shown, additional helices may be used.  
       FIGS. 86 and 87  depict perspective views of a generic implantable device  8601  having heat-activated blades  8602 .  FIG. 86  shows heat-activated blades  8602  when they are in their initial inactive state. Blades  8602  may be manufactured using a heat-activated substance, one that takes a new shape when heated. For example, the blades may be manufactured using a nickel-titanium alloy (e.g., nitinol) which, when heated, returns to a predetermined shape.  FIG. 87  depicts blades  8602  after placement in a patient, when they have been warmed and reshaped into a predetermined curl. The curls  8703  grasp surrounding tissues, holding device  8601  in place.  
       FIG. 88  depicts a perspective view of a generic implantable device  8801  having curved needles  8802 . The curved needles  8802  displayed here, similar to previously described stylets and barbs, can be threaded through device  8801  after placement within a patient. Alternatively, needles  8802  may be formed with a heat-activated substance as described above, a substance that bows upon entry into a warm body.  
       FIG. 89  is a flow chart depicting a method for affixing an implantable medical device to surrounding tissue. At step  8901 , a medical device is implanted in a host body, whether it is a catheter, a lead, a pacemaker, and so forth. At step  8902 , a physical aspect of the medical device is modified, enabling the device to engage with the surrounding tissue. Different types of modified physical aspects have been previously described.  
      It should be noted that the devices and methods described above are not limited to use with human patients. Other animals may benefit from preventing displacement of implantable medical devices.  
      While devices and methods embodying the present invention are shown by way of example, it will be understood that the invention is not limited to these embodiments. The devices and housings described are merely examples of the invention, the limits of which are set forth in the claims which follow. Those skilled in the art may make modifications, particularly in light of the foregoing teachings. For example, each of the elements of the aforementioned embodiments may be utilized alone or in combination with elements of the other embodiments.