Abstract:
A brain lead anchoring system is described which allows for installation of a brain stimulation lead or a drug delivery catheter without the lead or catheter ever being unsupported during installation so that the lead or catheter does not move during the anchoring procedure. An anchor assembly contains an anchoring mechanism that is biased in the closed or anchoring position and is only open to allow installation of a lead when it is mated to an introducer instrument, which has prongs that open the anchoring mechanism. Once the lead is appropriately positioned within the brain and while the lead is still supported by the installation guiding device, the introducer instrument is withdrawn from the anchoring mechanism, which thus allows the anchoring mechanism to return to its closed position. A locking cap covers the aperture through the anchor assembly into the skull and mechanically ensures that the anchoring mechanism remains in the closed position. The anchor assembly also has channels in its top surface within which the lead may be inserted so that the lead may be lain flat on the surface of the patient&#39;s skull.

Description:
This application Claims the benefit of Provisional Application No. 60/099,119 filed Sep. 3, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an anchoring system for use in neurostimulation techniques. More specifically, the present invention relates to a system for anchoring a brain stimulation lead within a cranial burr hole. 
     BACKGROUND OF THE INVENTION 
     The surgical implantation in the human brain of electrode leads to deliver electrical impulses and catheters to deliver drugs in order to provide various types of therapy is known. Electrical stimulation of the brain, for example, can be considered for use to treat chronic pain or movement disorders. Typically, such stimulation is accomplished by the insertion of a multi-electrode lead into the brain, with the electrodes positioned at the location within the brain indicated by the particular condition requiring treatment. Usually, the electrodes are located on the distal end of the lead and a connector is located on the proximal end of the lead, where the lead is connected to a pulse generator, which may be internally or externally powered. 
     In order to insert the lead into the patient&#39;s brain, a surgeon first drills a hole in the patient&#39;s cranium using a surgical burr. Typically, the hole is 12 to 14 mm in diameter. The surgeon installs a burr hole ring in the burr hole, inserts the lead into the ring and advances the lead through the burr hole ring into the brain. As the surgeon advances the electrode, a test stimulation pulse is delivered to the brain through the electrode and the patient&#39;s response is monitored. When the surgeon observes an appropriate response, the lead is appropriately placed. Placement of the electrode within the brain can be critical, as small changes in position can have an effect on the efficacy of the therapy. Therefore, some type of method for anchoring the lead in place, once the surgeon has determined the optimal location for the electrodes, is required. 
     Prior methods for anchoring the lead include the application of bio-compatible epoxy or the use of a mechanical anchoring device that is part of or connected to a burr hole ring. For example, U.S. Pat. No. 4,328,813 to Ray (“Ray”) and U.S. Pat. No. 5,464,446 to Dressen et al. (“Dressen”) and PCT patent application number WO 98/08554 by Knuth et al. (“Knuth”), all of which are incorporated into this document by this reference, describe anchoring systems that involve mechanical anchoring of the lead to a burr hole ring. An article by Jean Siegfried, M. D., Pierre Comte, Ph.D., and Remy Meier appearing in the August, 1983 issue of the Journal of Neurosurgery entitled “Intracerebral electrode implantation system” (“Siegfried”) also describes an anchoring system that involves mechanical anchoring of the lead to a burr hole ring. 
     Ray describes an anchoring system including an externally threaded burr hole ring that defines a socket into which an anchoring plug is inserted once the lead is correctly positioned within the brain. The anchoring plug is described as being made of sufficiently resilient material that it can be inserted into the socket and deform to accommodate the thickness of the lead. The friction between the socket, the lead and the plug is said to prevent the lead from moving after the plug is inserted into the socket. The anchoring system described in Ray, however, has disadvantages. Because the lead is secured off center, it is difficult to support during installation by stereotactic surgical instruments, which can be used to guide the lead during implantation. Additionally, the lead is subject to movement after the surgeon determines that the lead is correctly positioned but before the surgeon installs the plug, because the lead is unsupported until the plug is actually installed. Finally, the action of installing the plug into the socket can cause movement of the lead. 
     Dressen describes an anchoring system including a socket with an axial aperture, a plug with a concentric axial aperture and an external circumferential groove, and a cap with means for anchoring the lead in a bent position. The Dressen system has at least the disadvantage of not allowing the lead to be securely anchored by bending it to lay it flat on the surface of the patient&#39;s skull. Dressen&#39;s system also requires anchoring by tightening a suture within the external circumferential groove in the plug. This suture may be inconsistently tightened and may loosen over time. 
     Knuth and Siegfried describe essentially similar anchoring systems, both of which include, among other elements: (1) a baseplate with a centrally located hole that is adapted to be connected to a burr hole; (2) a compression seal, also with a centrally located hole (e.g., a silicone rubber ring), that is located with its hole aligned with the hole through the baseplate; and (3) a compression screw with a centrally located hole that is used to compress the seal longitudinally so that the seal expands radially inward to engage and therefore anchor the lead. At least one disadvantage associated with the systems described in Knuth and Siegfried is that the twisting of the compression screw may twist the compression seal and thus dislocate the end of the lead before the lead is sufficiently anchored. 
     Thus, a need continues to exist for an anchoring system for a brain stimulation lead in which the lead is always secure and anchored without applying torsion or axial forces to the lead or otherwise moving the distal end of the lead after it is appropriately positioned. 
     SUMMARY OF THE INVENTION 
     The design and implementation of a brain lead anchoring system is generally described. The anchor assembly includes an anchoring mechanism within an anchor housing, which has threads on its outer surface so that the housing can be screwed into a burr hole made in the patient&#39;s cranium. The anchoring mechanism preferably includes three locking tabs, each of which moves radially in a channel in the anchor housing relative to the generally centrally located lead path. The locking tabs are spring-loaded in a closed or anchoring position. 
     The introducer instrument is a generally conical body with a conical aperture that extends axially through the instrument through which the lead is introduced into the anchor assembly and thus the patient&#39;s brain. The introducer instrument also has a distal end having retraction protrusions that fit into complementary slots through the anchor housing. The opening in the distal end of the introducer is large enough that the lead moves easily through the opening. As the introducer instrument is mated to the anchor assembly, the retraction protrusions slide through the slots in the anchor housing and into apertures in the locking tabs, which pushes the locking tabs radially away from the lead path into an open or installation position. When the introducer and anchor assembly are so mated, the conical aperture in the introducer instrument and the centrally located aperture in the anchor assembly define the path on which the lead is introduced into the patient&#39;s brain. The surgeon advances the lead along this lead path using a standard stereotactic frame or a skull mounted guiding device. The lead is advanced into the patient&#39;s brain until the distal end of the lead is correctly positioned. While the lead is still secured in this position within the guiding device, the surgeon activates a mechanism on the introducer instrument or axially withdraws the introducer instrument, causing the retraction protrusions to be withdrawn from the apertures in the locking tabs, which in turn causes the spring-loaded locking tabs to return to the anchoring position, thus “pinching” the lead and anchoring it into position. Thus, the lead is anchored while being supported and without applying torsion or axial forces to the lead. 
     After the lead is anchored by the locking tabs within the anchor assembly, the surgeon removes the introducer instrument, which exposes the top of the anchor assembly. The surgeon then bends the lead so that it lies in one of the radially extending channels in the top surface of the anchor housing. To complete the installation of the lead, the surgeon inserts the locking cap, which preferably has three locking protrusions extending from its distal surface, into the same slots through which the retraction protrusions of the introducer instrument were inserted. Unlike the retraction protrusions, the locking protrusions hold the locking tabs in the locking position to ensure that the lead remains anchored even if the biasing member that spring-loads the locking tabs closed loses its resilience. The installed locking cap also covers the aperture in the top of the anchor housing. 
     Additional objectives and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded schematic perspective view of an anchor assembly of a brain lead anchoring system consistent with the present invention; 
     FIG. 2A is a perspective view of an anchor screwdriver for use with an anchor assembly of FIG. 1; 
     FIG. 2B is an enlarged view of the encircled portion of the anchor screwdriver of FIG. 2A; 
     FIG. 2C is a side view of the anchor screwdriver of FIG. 2A; 
     FIG. 3A is a side view of an embodiment of an introducer instrument consistent with the present invention; 
     FIG. 3B is a bottom view of the introducer instrument of FIG. 3A; 
     FIG. 3C is a top view of the introducer instrument of FIG. 3A; 
     FIGS. 4A and 4B are exploded views of the introducer instrument of FIG. 3A from different perspectives; 
     FIG. 5A is an exploded cross-sectional view of the embodiment of the introducer instrument of FIG. 4A taken along line  5   a — 5   a;    
     FIG. 5B is an enlarged view of the encircled portion of the embodiment of an introducer instrument shown in FIG. 5A; 
     FIG. 5C is a cross-sectional view of the embodiment of an introducer instrument shown in FIG. 1 taken along line  5   b — 5   b  of FIG. 3A; 
     FIG. 6 is a top view of the embodiment of the anchor housing shown in FIG. 1 in which all locking tabs are shown in the anchoring position; 
     FIG. 7 is a cross-sectional view of the embodiment of the anchor housing and anchor base shown in FIG. 1, but assembled together taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is a bottom view of the embodiment of the anchor housing shown in FIG. 1 without locking tabs installed in the anchor housing; 
     FIG. 9 is a bottom view of the embodiment of the anchor housing shown in FIG. 1 with locking tabs installed and two locking tabs in the anchoring position and one locking tab in the installation position; 
     FIG. 10 is a side view of an embodiment of a locking cap that is compatible with the anchoring assembly shown in FIG. 1; 
     FIG. 11 is a bottom view of the locking cap shown in FIG. 10; 
     FIG. 12 is an enlarged, schematic perspective view of the embodiment of the locking tabs shown in FIG. 1; and 
     FIG. 13 is a schematic perspective view of the embodiment of the anchor base shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to an embodiment of the invention, which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 1 is an exploded schematic perspective view generally from the proximal direction of the anchor assembly portion of the embodiment of a brain lead anchoring system constructed according to the present invention. A locking cap  70  (see FIG.  10 ), as discussed below, is used in connection with the anchor assembly  20 . The anchor assembly includes an anchor housing  30 , locking tabs  40  and anchor base  60 , which all may be made of implant-grade titanium or stainless steel. 
     Anchor housing  30  (which is further illustrated in FIGS. 6-9) is a generally cylindrical structure that has a cavity  33  defined therein. The anchor housing  30  has a large opening at one end and a smaller aperture  32  at the other end. Preferably, three locking tabs  40  are distributed circumferentially within cavity  33  and are biased radially inward toward an anchoring position by biasing member  50 . Three locking tabs  40  are preferable because as locking tabs  40  move from the installation to the anchoring position, they automatically center lead  5  or a drug delivery catheter within aperture  32 . After locking tabs  40  are positioned within anchor housing  30 , anchor base  60  (FIG. 13) is connected, for example, via screws  66 , to anchor housing  30  in order to enclose the open end of anchor housing  30 . As perhaps best illustrated in FIG. 13, anchor base  60  includes three channels  62  evenly distributed on the inner surface  64  of anchor base  60  at approximately 120° angles. Anchor housing  30  includes three complementary channels  38  (see FIG.  8 ). Together, channels  62  in anchor base  60  and channels  38  in anchor housing  30  define three channels in anchor assembly  20  within which locking tabs  40  are free to move radially toward and away from the lead path, but not circumferentially, relative to longitudinal axis  37  (FIG. 7) of anchor housing  30 . 
     Each of locking tabs  40  includes a tab stop  46  (FIG. 12) that interferes with a respective pair of anchor housing stops  34  (FIG. 7) to limit the radial movement toward longitudinal axis  37  of each locking tab  40  caused by biasing member  50 . Biasing member  50  may be an elastomeric ring (e.g., made of bio-compatible natural or synthetic rubber), a radial spring made of an implant-grade titanium alloy or MPN35, or other suitable biasing structure known to those skilled in the art. 
     The assembled anchor assembly  20  is anchored or screwed into a burr hole in a patient&#39;s skull using an anchor screwdriver. Referring to FIG. 2A, a bulb-shaped anchor screwdriver  202  is shown. The large or bulb end of the anchor screwdriver  202  is preferably sized to fit comfortably in a person&#39;s hand. The head or narrow end of the anchor screwdriver  202  has three installation tabs  16  on the head end of the anchor screwdriver  202 . FIG. 2C shows a side view of the instrument screwdriver  202 . 
     The installation tabs  16  fit into detents  36  in anchor housing  30  to enable the anchor screwdriver to turn the anchor assembly  20  when the anchor assembly  20  is positioned in a burr hole of a patient&#39;s skull. When the anchor screwdriver  202  is fully engaged with the anchor assembly  20 , installation tabs  16  mate with detents  36  in the proximal end surface  35  of anchor housing  30 . The installation tabs  16  enable the surgeon to use the anchor screwdriver  202  to turn anchor assembly  20  to implant or withdraw the anchor assembly  20  that is in the cranial burr hole in the patient&#39;s skull. (See FIG. 7 for a cross-sectional profile of threads  31 .) Installation tabs  16  bear the twisting load applied when the surgeon uses the anchor screwdriver  202  to install or remove the anchor assembly  20 . 
     Referring to FIG. 3A, after the anchor assembly  20  has been inserted or installed into a patient&#39;s cranium  100 , an introducer instrument  300  is used to guide the lead  5  into and through anchor assembly  20  into the patient&#39;s skull. The introducer instrument  300  has three retraction protrusions  14  on the distal end  13  of the introducer instrument  300  to push out locking tabs  40  and retract the locking tabs  40  into the installation position. The introducer instrument  300  may be made of surgical stainless steel or aluminum. 
     Each of the distal ends of retraction protrusions  14  includes a retraction surface  15 . As retraction protrusions  14  are advanced into slots  39  (FIGS.  5 B and  8 ), each of retraction surfaces  15  comes into contact with a corresponding retraction surface  42  of the corresponding locking tab  40  and causes locking tabs  40  to be pushed radially outward from the anchoring position to the installation position. FIG. 9 depicts, for illustrative purposes only, two locking tabs  40   b  in the anchoring position and locking tab  40   a  in the installation position within anchor housing  30 . Preferably, in operation, all locking tabs  40  move substantially simultaneously. The retraction protrusions  14  extends from a retractable introducer  402  (FIGS. 4A and 4B) of the introducer instrument  300  and are operative to slide axially relative to the introducer body. A distal end portion of the retraction protrusion extends from the distal end of introducer instrument  300 . 
     The introducer instrument  300  includes an introducer guide channel  304 , an introducer retaining sleeve  308 , an introducer retraction knob  312 , a retractable introducer  402  (FIG. 4A) and a spring  410  (FIG.  4 A). The introducer guide channel  304  includes wing members  320  for use in securing the introducer instrument  300  to the skull  100 . The wings  320  have screw holes  324  defined therein for receiving screws  326  that secure the instrument introducer  300  to the patient&#39;s skull. The screws  326  extend through the wings  320  into the patient&#39;s skull  100 . 
     The introducer instrument  300  allows the surgeon to withdraw the retraction protrusions  14  while introducer instrument  300  and anchor assembly  20  are mated, which secures lead  5  within anchor assembly  20 , without physically separating the distal end  13  of introducer instrument  300  from the top surface  35  of anchor housing  30 . Thus, locking tabs  40  move from the installation position to the anchoring position without any other part of the system coming in contact with lead  5  and dislodging it. 
     As the introducer instrument  300  is mated to the anchor assembly  20 , the retraction protrusions  14  slide through slots in the anchor housing and into apertures in the locking tabs  40 , which pushes the locking tabs  40  radially away from the path of the lead  5  or a catheter into an open or installation position. When the introducer instrument  300  and anchor assembly  20  are mated, a conical aperture  530  (FIG. 5A) in the introducer retaining sleeve  308 , a cylindrical channel  540  (FIG. 5A) and centrally located aperture in the anchor assembly define the path in which the lead  5  is introduced into the patient&#39;s brain. The surgeon advances the lead along this path using a standard stereotactic frame or a skull mounted guiding device. The lead  5  is advanced into the patient&#39;s brain until the distal end of the lead is correctly positioned. While the lead  5  is still secured in this position within the introducer instrument, the surgeon activates a mechanism on the introducer instrument  300  that causes the retraction protrusions  14  to be withdrawn from the apertures in the locking tabs. The withdrawal of the retraction protrusions  14 , in turn, causes the spring-loaded locking tabs to return to the anchoring position, thus “pinching” the lead  5  and anchoring it into position. Thus, the lead is anchored while being supported by the introducer instrument  300 . 
     The mechanism used to withdraw the retraction protrusions  14  is the introducer retraction knob  312 . The introducer retraction knob  312  is preferably initially set to a position that causes the retraction protrusions  14  to be extended to the maximum distance away from the bottom of the introducer guide channel  304 . The retraction knob  312  may be turned to cause the retraction protrusions  14  of the retractable introducer  402  (FIGS. 4A and 4B) to be withdrawn from the anchor assembly  20  as discussed above. FIG. 3B shows a bottom view of the introducer instrument  300  and FIG. 3C shows a top view of the introducer instrument  300 . 
     Referring to FIGS. 4A and 4B, exploded prospective views of the introducer instrument  300  are illustrated. As discussed above, the introducer instrument  300  includes the introducer retaining sleeve  308 , spring  410  for preventing backlash during movement of the retractable introducer  402 , an introducer retraction knob  312 , and an introducer guide channel  304 . The introducer retaining sleeve  308  has a conical aperture  530  (FIG. 5A) that directs the lead  5  into the body of the introducer instrument  300 . The retractable introducer  402  has threaded protrusions  412  extending from the conical surface of the retractable introducer  402  at a proximal end of the retractable introducer  402 . Each set of the threaded members are preferably evenly distributed at 120° intervals on the retractable introducer  402 . At the distal end of the retractable introducer  402 , the three retraction protrusions  14  are preferably evenly distributed at 120° intervals at the distal end of the retractable introducer  402  as illustrated. The threaded protrusions  412  of the retractable introducer  402  extend into corresponding locking channels  420  of the introducer guide channel  304 . 
     The three locking channels  420  extend lengthwise along and through the arc-shaped sidewalls  426  of the introducer guide channel and are located at 120° intervals such that the threaded members  412  of the retractable introducer  402  slide into the locking channels  420  in a mating position. The introducer retraction knob  312  has threads  424  which mate with the threaded protrusions  412  of the retractable introducer  402 . The introducer retraction knob  312  rests upon a shelf  434  of the introducer guide channel  304 . By rotating the introducer retraction knob  312 , the retractable introducer  402  is moved up or down corresponding to the direction of rotation of the introducer retraction knob  312 . The introducer retaining sleeve  308 , when assembled with the other components of the introducer instrument  300 , attaches to threaded portions  440  of the arc-shaped sidewalls  426  that define the threaded guide channels. The introducer retaining sleeve  308  has threads  448  that screw onto the threaded protrusions  440  of the arc-shaped sidewalls  426  of the introducer guide channel. 
     Referring to FIGS. 5A,  5 B, and  5 C, cross-sectional views of the introducer instrument are illustrated. FIG. 5A illustrates a cross-sectional view of the introducer element taken along line  5   a — 5   a  of FIG.  4 B. The spring element  410  fits into the threaded channel  512  (FIG. 5A) of the instrument retaining sleeve  308  when the introducer instrument is assembled. FIG. 5B is an enlarged view of the encircled portion of introducer instrument  300 . Retraction surfaces  15  are canted relative to the longitudinal axis  17  of the retractable introducer  402  in order to act as ramps on which retraction surfaces  42  of locking tabs  40  ride as retraction protrusions  14  are inserted into actuation apertures  48  (FIG. 12) of locking tabs  40 . Once retraction protrusions  14  are fully advanced into actuation apertures  48 , shoulders  19  at the base of retraction protrusions  14  come into contact with lands  49  (FIG. 12) of locking tabs  40  in order to prevent further advancement of retraction protrusions  14  through actuation apertures  48 . FIG. 5C is a cross-sectional view of the assembled introducer instrument  300  taken along line  5   b — 5   b  of FIG.  3 A. 
     FIG. 6 is a top view of anchor housing  30  in which all locking tabs  40  are shown in the anchoring position. The shape of lead aperture  32  is illustrated, along with the arrangement of slots  39  in lead aperture  32  and detents  36  in top surface  35  of anchor housing  30 . Also illustrated, to a degree, is the arrangement within cavity  32  of locking tabs  40 , a portion of which are visible through lead aperture  32 . 
     FIG. 8 is a bottom view of anchor housing  30  with anchor base  60  removed and without locking tabs  40  installed in anchor housing  30 . This view illustrates the distribution of slots  39  around the circumference of lead aperture  32  and the distribution of channels  38 , and screw holes  67 , all of which are distributed at approximately 120° intervals around anchor housing  30 . Also clearly illustrated is the distribution of three anchor housing stops  34 , which limit radial movement inward of locking tabs  40  within channels  38 . 
     FIG. 10 is a side view of locking cap  70  for use in covering aperture  32  in anchor housing  30  and positively securing locking tabs  40  in the anchoring position in which three anchoring protrusions  72  are use. It should be recognized that two or more anchoring protrusions could be used. FIG. 11 is a bottom view of locking cap  70  in which the location of all three anchoring protrusions  72  is illustrated. After the surgeon positions lead  5  at the correct location within the patient&#39;s brain and withdraws introducer instrument  10  from anchor assembly  20 , which causes locking tabs  40  to anchor lead  5 , the external portion of lead  5  is folded into one of detents  36  in anchor housing  30  so that lead  5  is bent at approximately a 90° angle and the external portion of lead  5  (or a separate extension of lead  5 ) can be run along the surface of the patient&#39;s skull toward an implantable pulse generator. Once lead  5  is positioned in one of detents  36 , anchoring protrusions  72  of locking cap  70  are inserted into anchor assembly  20  through slots  39  in anchor housing  30  and into actuation apertures  48  of locking tabs  40 . The geometric arrangement of anchoring protrusions  72  causes them to force, via contact with locking surfaces  44 , locking tabs  40  into the anchoring position. Thus, a reduction in resilience of biasing member  50  will not result in a reduction of the anchoring force that locking tabs  40  apply to lead  5 . 
     The method of using the anchoring system according to the present invention is as follows. Once the burr hole in the patient&#39;s cranium is ready for installation of the anchor assembly, the surgeon mates introducer instrument  300  with anchor assembly  20  so that retraction protrusions  14  open locking tabs  40  to the installation position and installation tabs  16  fit within detents  36 . The surgeon then uses the screwdriver  202  to screw anchor assembly  20  into the burr hole in the patient&#39;s cranium. The surgeon securely mounts lead  5  into a suitable guiding device and uses the guiding device to advance lead  5  into the patient&#39;s brain through the installation aperture  11 , and lead aperture  32 . The lead  5  is advanced into the patient&#39;s brain until the distal end of the lead is correctly positioned. While the guiding device is still securely holding lead  5  in this position, the surgeon activates a mechanism, such as te knob  312 , on introducer instrument  300  or withdraws introducer instrument  300  from anchor assembly  20 , causing retraction protrusions  14  to withdraw from actuation apertures  48 . Biasing member  50  then causes locking tabs  40  to return to the anchoring position (see e.g., locking tabs  40   b  in FIG.  9 ), thus “pinching” lead  5  and anchoring it into position. This ensures that lead  5  does not move away from a proper position during the post-placement anchoring procedure. 
     After lead  5  is anchored by locking tabs  40  within anchor assembly  20 , the surgeon removes the guiding device and introducer instrument  10 , which exposes the top surface  35  of anchor housing  30 . The surgeon then bends lead  5  so that it lies in one of detents  36  in anchor housing  30 . To complete the installation of lead  5 , the surgeon installs locking cap  70  by inserting anchoring protrusions  72  into slots  39  and through actuation apertures  48  in locking tabs  40 . Unlike retraction protrusions  14  of introducer instrument  10 , locking protrusions  72  force locking tabs  40  into the anchoring position (see, e.g., locking tabs  40   b  in FIG. 9) to anchor lead  5  even if biasing member  50  loses resilience. The installed locking cap  70  also covers lead aperture  32  in anchor housing  30 . After locking cap  70  is installed, lead  5  is ready to be connected to an appropriate pulse generator. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the anchoring system of the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.