Abstract:
A cover for receiving an implantable medical device includes self-anchoring protrusions that engage tissue of a pocket where the device is implanted to resist movement including rotation and flipping. The implantable medical device is placed into the cover prior to being placed into the pocket so that once in the pocket, the device may reduce rotating, flipping, or otherwise moving. The self-anchoring protrusions may include barbs of various shapes to frictionally engage the tissue of the pocket. The cover may include features such as a strap and elastic construction to assist in holding the implantable medical device within the cover. Apertures may be included to enable the device. The cover may include additional features like suture tabs to allow additional fixation via suturing the cover to the surrounding tissue.

Description:
RELATED CASES 
     This application is a U.S. National Stage filing under 35 U.S.C. 371 of copending PCT Application Serial No. PCT/US2010/035429, filed May 19, 2010, which claims priority to U.S. Provisional Patent Application No. 61/180,833, filed May 22, 2009, and both applications are hereby incorporated by reference as if re-written in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments are related to covers for implantable medical devices. More particularly, embodiments are related to covers that include self-anchoring protrusions that resist movement of the implantable medical device. 
     BACKGROUND 
     Implantable medical devices (IMD) and associated implantable medical leads provide functions such as stimulation of muscle or neurological tissue and/or sensing of physiological occurrences within the body of a patient. Typically, the IMD is installed in a subcutaneous location that is accommodating and relatively accessible for implantation. For instance, to provide stimulation near the spine or pelvis, the IMD may be installed in a pocket located within the abdomen or upper buttocks region of the patient. For stimulation of the brain or heart, the IMD may be installed in a pocket located in the pectoral region. Stimulation leads are routed to the appropriate stimulation site and terminate at the pocket where they may be connected to the IMD. 
     After installation, movement of the IMD within the pocket, and rotational movement in particular, is a concern. An external housing of the IMD is typically a smooth metal such as titanium that does not resist movement relative to the tissue in the pocket. Such movement may result from the normal daily activities of the patient. Movement may also occur due to the patient manipulating the IMD position by grasping the IMD through the skin. The movement may cause various problems. 
     The implantable medical leads have electrical conductors within them. Typically, the lead is installed so that slack in the lead is removed. For instance, excess length of the lead may be wrapped about the IMD within the pocket to reduce the amount of slack. Excess movement of the IMD, and particularly rotation or flipping of the IMD, may stress the electrical conductors within the leads and cause breakage of the conductors. A surgical procedure to replace the broken lead may be necessary. 
     Another issue for IMDs that utilize unipolar stimulation is that the back side of a metal external housing of the IMD is coated with an insulator so that the front side of the metal can acts as one of the electrodes. Maintaining this housing electrode closest to the surface of the patient&#39;s body reduces the likelihood of uncomfortable sensations occurring within the tissue beneath the IMD. Should the IMD be flipped such that the uninsulated side of the metal can faces the tissue beneath the IMD, then the uncomfortable sensations become more likely. 
     Another issue occurs for IMDs that utilize a rechargeable battery and associated recharging circuitry. For these IMDs, the side of the IMD where a recharging coil is located is positioned closest to the surface of the patient&#39;s body so that energy transfer through an inductive coupling with an external coil can occur. Should the IMD be flipped such that the opposite side of the IMD becomes the closest to the surface of the patient&#39;s body, the inductive coupling is adversely affected and the IMD may not be properly recharged. If the IMD cannot be externally flipped back to the proper orientation, then a surgical procedure may be necessary to flip the IMD back to the original orientation so that recharging can commence. 
     SUMMARY 
     Embodiments address issues such as these and others by providing a cover for an implantable medical device. The cover includes self-anchoring protrusions that engage the tissue of the pocket to resist movement including rotation and flipping. The implantable medical device is placed into the cover prior to being placed into the pocket so that once in the pocket, the implantable medical device is less likely to rotate, flip, or otherwise move in a harmful manner. 
     Embodiments include a method of resisting movement of an implantable medical device having an external housing. The method involves providing a cover that includes self-anchoring protrusions and placing the implantable medical device within the cover. The method further involves placing the implantable medical device within the cover into a subcutaneous pocket of a patient. 
     Embodiments include a cover for an implantable medical device having an external housing. The cover includes a body comprising an inner region adapted to receive the implantable medical device. The cover also includes self-anchoring protrusions extending from the body. 
     Embodiments also include a kit. The kit includes an implantable medical device having an external housing and a cover having self-anchoring protrusions. The cover is sized to allow insertion of the implantable medical device into an inner region of the cover. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative cover embodiment for an implantable medical device. 
         FIG. 2  is a perspective view of the illustrative cover with the implantable medical device positioned within an inner region of the cover. 
         FIG. 3  is a front view of the illustrative cover. 
         FIG. 4  is a rear view of the illustrative cover. 
         FIG. 5  is a side view of the illustrative cover. 
         FIG. 6  is a side cross-sectional detail view of the self-anchoring protrusion of the illustrative cover. 
         FIG. 7  is a top detail view of a self-anchoring protrusion of the illustrative cover. 
         FIG. 8  is a cross-sectional side view of the illustrative cover. 
         FIG. 9  is a procedural flow for utilizing the illustrative cover. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments include covers for implantable medical devices (IMD) that provide self-anchoring protrusions. These self-anchoring protrusions engage with tissue of the pocket once the IMD has been placed into the cover and then implanted into the pocket. The self-anchoring protrusions thereby resist rotation, flipping, and other movements of the IMD. 
       FIGS. 1-5  and  8  show an illustrative embodiment of a cover  100 . The cover  100  of this example forms a pouch having a front side  102 , back side  122 , left side  117 , right side  116 , and bottom side  119 . The cover  100  of this example also includes a strap  108  that extends from a front top edge  112  to a back top edge  110 . However, embodiments of the cover  100  may have various forms other than the pouch shown, such as a pouch with no strap  108 , a sleeve with no bottom side  119  and no strap  108  or with the strap  108  and a second strap lapping over the bottom in place of the bottom side  119 . 
     The cover  100  may be formed of various elastic biocompatible materials. For instance, liquid silicone rubber may be molded into the desired shape to form the cover  100 . Examples of other such elastic biocompatible materials include enhanced tear resistance silicone (ETR). The elasticity allows the cover  100  to be stretched to accommodate insertion of an IMD  200  and to ultimately grip the IMD  200  to hold the IMD  200  in place within the cover  100 . The elasticity may also allow self-anchoring protrusions  106  to be more compliant than a rigid material. This compliance allows the self-anchoring protrusions  106  to engage the tissue of the pocket to prevent troublesome rotation and other movement of the IMD  200  relative to the pocket yet not damage the tissue due to slight movement of the IMD  200  and cover  100  within the pocket. 
     As shown in  FIG. 2 , the IMD  200  may be placed within the inner region of the cover  100 . For this particular example, the IMD  200  may be placed into the cover  100  by manually stretching the cover  100 , and particularly the strap  108 , so that the open top of the cover  100  is stretched to a sufficiently large size to allow the IMD  200  to be positioned into the cover  100 . In this example, the metal external housing  204  of the IMD  200  extends down to the bottom side  119  of the cover  100  while a header  202  resides above the top edges  110 ,  112 . The strap  108  laps over the header  202  in a slightly stretched state such that the strap  108  may apply force against the IMD  200  to aid in holding the IMD  200  within the cover  100 . The lead openings  206  of the header  202  are exposed so that leads can be easily connected to the IMD  200  once the IMD  200  is placed within the cover  100 . 
     In this particular example, the cover  100  includes apertures  104  on the front side  102  while the back side  122  remains solid. Additionally, in this example, the front top edge  112  is lower than the back top edge  110  and a side top edge  114  forms a taper. The apertures  104  and/or the lower front top edge  112  may be useful when the IMD  200  is employing unipolar stimulation which utilizes the metal outer surface of a can  204  of the IMD  200  as an electrode for the stimulation signal as it returns from an electrode of the lead back through the tissue to the IMD  200  to complete the electrical circuit. The metal outer surface of the can  204  being exposed through the apertures  104  and/or above the lower front top edge  112  provides the electrode contact for the unipolar stimulation. 
     The solid back side  122  provides adequate insulation of the metal outer surface of the can  204  on the back side of the IMD  200  such that the metal on the back side does not act as an electrode. This provides the added benefit that a non-conductive coating such as a parylene coating on the back side of the IMD  200  may be omitted thereby potentially reducing the costs associated with manufacturing the IMD  200 . Furthermore, as the metal outer surface of the can  204  frictionally resides against the cover  100 , the solid back side  122  provides a large surface area for gripping the IMD  200 . 
     The apertures  104  and/or lower front top edge  112  may be included for additional purposes. For instance, where the IMD  200  uses a recharge coil, such as an external recharge coil  212 , the apertures  104  and/or lower front top edge  112  may allow for ventilation of the recharge coil  212  where the recharge coil  212  is located in proximity to the apertures  104  once the IMD  200  is inserted into the cover  100 . The apertures  104  and/or lower front top edge  112  may also allow for improved inductive coupling to an external coil during the recharging process as compared to a cover  100  not having apertures  104  or the lower front top edge  112 . 
     The self-anchoring protrusions  106  may be of a variety of shapes. As shown in  FIGS. 1-5  and  8 , and in additional detail in  FIGS. 6 and 7 , an example of the self-anchoring protrusions  106  may be barbs having multiple prongs. Various other styles of self-anchoring protrusions may also be applicable, such as barbs having a different configuration of prongs, for instance longer or shorter prongs and/or prongs at different angles. 
     The self-anchoring protrusions  106  of the example shown are barbs having four prongs. A center prong  125  extends perpendicularly from the surface of the cover  100 . Three peripheral prongs  124 ,  126 , and  128  surround the center prong  125 . The peripheral prongs  124 ,  126 , and  128  are evenly spaced about the center prong  125  and extend from the center prong  125  so as to form an angle relative to the center prong  125  as well as relative to the surface of the cover  100 . These prongs  124 ,  125 ,  126 , and  128  engage the tissue so as to create a frictional relationship that resists movement. While a particular number and pattern of the self-anchoring protrusions  106  are shown, it will be appreciated that other numbers and patterns of self-anchoring protrusions  106  are also applicable. 
     The cover  100  may include additional features such as one or more suture tabs  118 . The suture tab  118  includes holes  120  that allow a physician to suture the tab  118  to the tissue to further assist in fixing the position of the IMD  200  within the pocket. As shown, the suture tab  118  is positioned on the bottom side  119  and closer to the back side  122  than the front side  102 . This may aid in the suturing where the physician sutures the tab  118  to the tissue that is immediately adjacent the back side  122 . While a single suture tab  118  is shown on the bottom side  119 , it will be appreciated that the suture tab  118  may be located on other sides of the cover  100  and/or that additional suture tabs may be located on the other sides. 
     The cover  100  may be sized such that it is slightly smaller in dimension when in the relaxed state than the IMD  200 . As a result, where the cover  100  is made of an elastic material, the cover  100  may stretch to accommodate the IMD  200  within the inner region of the cover  100 . The cover  100  grips the IMD  200  as the cover  100  attempts to shrink back to the dimensions of the relaxed state. 
     As one specific example which is provided for purposes of illustration, the illustrative cover  100  may have the following dimensions when in the relaxed state. These dimensions are complementary to an IMD  200  having a side to side width of 1.95 inches (4.95 cm), a top to bottom height of 2.60 inches (6.604 cm), and a front to back depth of 0.60 inches (1.524 cm). 
     The illustrative cover  100  may have a width from side  116  to side  117  of 1.992 inches (5.059 cm). The cover  100  may have a height from the bottom side  119  to the top of the strap  108  of 2.592 inches (7.498 cm). The height of the front top edge  112  from the bottom side  119  may be 1.870 inches (4.75 cm) while the height of the back top edge  110  from the bottom side  119  may be 2.120 inches (5.385 cm). The depth from the front side  102  to the back side  122  may be 0.593 inches (1.506 cm). The strap  108  may have a width of 0.500 inches (1.27 cm). The thickness of the silicone rubber may be 0.020 inches (0.050 cm). 
     The self-anchoring protrusions  106  of this illustrative example include various dimensions. The center prong  125  extends from the surface by 0.150 inches (0.381 cm) and has a diameter of 0.030 inches (0.076 cm). Each peripheral prong  124 ,  126 , and  128  extends from the center prong  125  to form an angle of 60 degrees relative to the surface of the cover  100  and has a diameter of 0.020 inches (0.05 cm) with the tip being 0.130 inches (0.33 cm) away from the surface of the cover  100 . The peripheral prongs are spaced about the center prong  125  by 120 degrees. The self-anchoring protrusions  106  are positioned in a staggered configuration where the center of the lowest protrusion on each side is 0.547 inches (1.389 cm) from the bottom edge with the center of the next lowest protrusion being 0.141 inches (0.358 cm) higher and 0.141 inches (0.358 cm) offset to the side. The center of the row of lowest protrusions  106  on the front and back sides  102 ,  122  is 0.293 inches (0.744 cm) from the bottom  119  with the center of the next lowest row of protrusions  106  being 0.255 inches (0.648 cm) higher and 0.255 inches (0.648 cm) offset to the side. 
     Other dimensions of this illustrative example may include an aperture diameter of 0.250 inches (0.635 cm). A suture tab height is 0.241 inches (0.612 cm) and a width is 0.075 inches (0.191 cm). The suture tab  118  is spaced from the back side  122  by 0.183 inches (0.465 cm). 
       FIG. 9  shows a set of steps that may be performed to implement embodiments of the cover  100 . The cover  100  and the IMD  200  may be provided as a kit to a physician. Initially, for embodiments where the kit includes the cover  100  and the IMD  200  as separate pieces, the cover  100  is stretched to a more open state so that the IMD  200  may be inserted into the inner region within the cover  100  at an insertion step  302 . At this point, the pocket has been created either before or after the IMD  200  has been inserted into the cover  100 . The IMD  200  which is within the cover  100  is then placed into position within the pocket at a placement step  304 . The confinement of the pocket is such that the self-anchoring protrusions  106  engage the tissue of the walls of the pocket to provide frictional stabilization of the IMD  200 . Furthermore, over time after the implantation there may be tissue in-growth around the engagement of the self-anchoring protrusions  106  to provide additional fixation. 
     While the self-anchoring protrusions  106  may provide resistance to movement, the physician may also decide to suture the IMD  200  to surrounding tissue of the pocket in a suturing step  306 . The physician may suture the IMD  200  by suturing through the suture openings  210  and/or  208  of the IMD  200  that are exposed on each side of the strap  108 . The physician may additionally or alternatively choose to suture through one or more of the holes  120  of the suture tab  118  on the cover  100 . 
     While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.