Abstract:
An apparatus for securing an implantable lead within tissue of a patient includes a base adapted to be secured to a patient&#39;s skull adjacent a craniotomy. The base has an upper surface and a lower surface with a central passage therebetween. The central passage is adapted to receive the implantable lead therethrough. The apparatus also has a cover that is releasably coupled to the base so as to substantially cover the central passage and capture the implantable lead therebetween. A first rotating member is also coupled with the base and the first member is rotationally movable so as to meet and engage the implantable lead at a plurality of positions within the central passage.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application No. 60/908,367, filed Mar. 27, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to medical apparatus and methods, more specifically to instrument immobilizers and even more specifically, but not by way of limitation to an apparatus and methods for anchoring an intracranial probe or lead to the cranium. 
     Implanting medical devices within the cranium is an increasingly important approach for treatment of disorders such as Parkinson&#39;s Disease, essential tremor and dystonia. This approach may also be used to treat a wide array of neuropsychiatric problems, such as depression, epilepsy, obsessive compulsive disorder, obesity and chronic pain. Most of these devices interact with the brain by delivering current through an implanted probe to modulate brain activity. In addition, infusion of drugs through a permanently implanted probe has been proposed as a primary treatment, or as an adjunctive treatment to electrical stimulation, for Alzheimer&#39;s and Parkinson&#39;s Diseases, among others. 
     As part of the implant procedure, the probe must be stabilized in the brain. Ideally, any prosthetic device is attached directly to the tissue on which it operates, in this case, the brain. Direct attachment of electrical and chemical probes to brain tissue is impractical. A more easily implementable solution is a system of flexible probes that bend and float with the brain as the brain moves within the cranial cavity. Such probes are secured to the cranium. In this manner, mechanical forces from outside the cranium are prevented from acting on the brain-to-probe interface. 
     There are a number of current techniques for securing a probe to the cranium. For example, in one approach, a permanently implanted probe is fixed by a sliding door which closes to form a slot just wide enough to slightly compress and grip the body of the probe. A common feature of such devices is that they grip the probe somewhere within the craniotomy opening, and that the slot has a fixed orientation relative to the cranium. 
     In another approach, the probe passes through a narrow aperture at the center of a craniotomy opening. The probe is held in place by a surgeon as it is bent over into a slot leading to the exit from the device. Hinged arms swing into place to narrow the slot and anchor the probe within the slot. 
     Current anchoring devices are typically positioned over the craniotomy opening, and they are attached to the cranium with several peripheral screws. An implantable lead is placed through the cranial opening and the lead is gripped by two opposing thin bars. In some cases, it is possible to damage the lead by crushing it between the thin bars. It would therefore be desirable to grip the lead with wider bars to more evenly distribute the gripping force over a greater axial length of the implantable lead. It would also be desirable to provide a more stable mounting for the skull-mounted portion of the anchoring device. Additionally, current devices often have a small opening for receiving the lead and thus it would be desirable to provide an anchoring device having a wider opening for the lead, to permit adjustment of lead position for optimal placement, especially when using a large multi-channel probe array, a feature shared by only a few currently available anchoring systems. 
     2. Description of the Background Art 
     Prior patents and publications describing anchors for cranial probes include: U.S. Pat. Nos. 4,328,813; 5,464,446; 6,044,304; 2004/0267284; 2005/0192594; and WO 2004/026161. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention generally provides an anchor for securing an implantable lead within tissues in a patient. The terms “lead” and “probe” will be used interchangeably with one another in this disclosure, as will the terms “anchor base” and “cylinder.” Often the lead may comprise an electrode or a catheter, and the lead is often implanted into brain tissue through a craniotomy in the patient&#39;s skull. A current and/or therapeutic agent may be delivered through the lead to the tissue and the anchor is usually composed of materials that are compatible with magnetic resonance imaging. The anchor may be fabricated from metals that do not interfere with MRI and/or polymers such as polyphenylene sulfide, polyetheretherketone (PEEK), polyetherimide, polyimide, polysulfone and the like. 
     In a first aspect of the present invention an apparatus for securing an implantable lead within tissue of a patient comprises a base that is adapted to be secured to a patient&#39;s skull adjacent a craniotomy. The base has an upper surface, a lower surface and a central passage therebetween which is adapted to receive the implantable lead. The apparatus also includes a cover that can be releasably coupled to the base so as to substantially cover the central passage and also to capture the lead therebetween. A first rotating member or door, is coupled with the base and is rotationally movable so as to meet and engage the lead at a plurality of positions within the central passage. Rotating the door also adjusts the position of an opening within the central passage in which the lead may pass through and also closes or reduces the size of the central passage while still allowing the lead to pass therethrough. 
     Often, the first rotating member comprises a removable insert that is adapted to releasably grip the lead and that may be received in a recessed region of the rotating member. The removable insert is usually adapted to be removably coupled to the first rotating member with a rotationally actuated tool that may be coupled to the first rotating member. The first rotating member may have a surface defining a wedge shaped or indented region that is adapted to receive and align the tool. 
     The apparatus may have a pin or rivet engaged with the first rotating member that secures the first rotating member to the base while allowing rotation of the first rotating member relative to the base. The first rotating member may also have a surface that defines a receptacle that is adapted to receive a tool for turning the first rotating member into a desired position so as to engage the lead and fix the lead into a position. The first rotating member may further comprise a resilient end that is adapted to releasably grip the lead. The resilient end may lie in the same plane as the first rotating member and may be composed of an elastomer. The resilient end often is constructed with a substantially solid core while sometimes it may be porous. Often the resilient end comprises surface features that are adapted to capture the lead. The surface features may include a plurality of convex or concave regions adjacent to one another or the surface features may be scallops. Sometimes the surface features may comprise a plurality of resilient fingers that extend outward from the resilient end. The surface features may also comprise combinations thereof. 
     The apparatus may further comprise a ratchet mechanism that is adapted to restrict the first rotating member to motion in one direction. Often the apparatus also comprises a fixing element such as a set screw that is adapted to immobilize the first rotating member. The apparatus also often comprises a second rotating member that is coupled with the base and a spacer may be used to separate the first and second rotating members from one another. The second rotating member is rotationally movable so as to meet and engage the lead at a plurality of positions within the central passage. Rotating the second door also adjusts the position of an opening within the central passage in which the lead may pass through and also closes or reduces the size of the central passage while still allowing the lead to pass therethrough. Usually, the first and second rotating members are movable independently of one another and they may be retained in the base with a retaining member such as a ring. Also, the first and second rotating members may lie in the base adjacent to one another. Sometimes the second rotating member comprises a removable insert that is adapted to releasably grip the lead. The insert on the second rotating member may take the same form as the insert on the first rotating member. Often the resilient end on the first rotating member lies in a plane between the first and second rotating members. 
     The apparatus may further comprise a locking mechanism coupled with the first and second rotating members. The locking mechanism locks the first and second members together thereby preventing relative motion therebetween. The locking mechanism may be a detent and comprise a protuberance on either the first or second rotating member and a receptacle for receiving the protuberance on the other rotating member. These features allow the rotating members to snap into position with one another thereby ensuring the lead is gripped therebetween. 
     Often, the apparatus further comprises one or more tabs that extend radially outward from the base. The tabs are adapted to be secured to the skull adjacent the craniotomy. The tabs often define apertures that can receive a fastener such as a screw, thereby securing the base adjacent the craniotomy. 
     Sometimes the base is cylindrical and may be sized to fit at least partially within the craniotomy, and at least a portion of the base may be securely press fit into the craniotomy. The base may comprise a discrete upper and a discrete lower portion that are fastened together, or the base may be of unitary construction. The base may be recessed at least partially into the craniotomy, or the lower surface of the base may sit substantially flush with the top of the skull. The base may also have one or more receptacles that are adapted to releasably receive at least a portion of the cover. Often, the upper surface of the base defines one or more channels that are sized and shaped to accept the lead after the lead has been disposed therein. The base may also be adapted to receive and retain other surgical instruments such as instrument positioning guides or other reference devices often used during neurosurgery. These other surgical instruments may releasably lock with a flange in the base, a retaining member in the base or any other portion of the base or components therein. 
     Often the cover is adapted to be removably coupled to the base. Sometimes the cover comprises one or more legs that are adapted to releasably snap fit into engagement with the base. Alternatively, the legs may be disposed on the base or on a retaining member that fits in the base. The cover may have a surface that defines one or more channels that are sized and shaped to accept the lead after it has been disposed therein. One or more plugs may be placed into the channels or a gasket may be disposed between the cover and the base in order to seal any gaps therebetween. 
     In another aspect of the present invention, a system for securing an implantable lead within tissue of a patient comprises an apparatus for securing the implantable lead within tissue. The apparatus comprises a base adapted to be secured to a patient&#39;s skull adjacent a craniotomy, the base having an upper and lower surface and a central passage therebetween. The implantable lead is often disposed in the central passage. The apparatus also comprises a first rotating member coupled with the base and having a removable insert adapted to engage the lead. A retaining pin may couple the insert with the first rotating member. The first rotating member is rotationally movable so as to meet and engage the lead at a plurality of positions within the central passage. Rotating the door also adjusts the position of an opening within the central passage in which the lead may pass through and also closes or reduces the size of the central passage while still allowing the lead to pass therethrough. The system also includes a tool having a proximal end, a distal end and a handle, the tool being adapted to introduce and remove the removable insert to or from the first rotating member. 
     Often the tool also comprises a pin disposed near the distal end that is adapted to retain the insert when the insert is decoupled from the first rotating member. The tool is usually adapted to be rotated so as to simultaneously engage the insert and withdraw the retaining pin from the insert. The tool may have an angled surface that facilitates seating of the tool against the first rotating member. 
     The system may also include a cover that can be coupled to the base so as to substantially cover the central passage and also to capture the lead therebetween. The system may also comprise a potting material that is used to fill gaps between the base and the craniotomy in order to reduce or eliminate leakage of body fluids, such as cerebral spinal fluid (CSF), from around the base. 
     In another aspect of the present invention, a method of securing an implantable lead into tissue of a patient comprises positioning a base having an upper surface, a lower surface and a central passage therethrough, adjacent a craniotomy in a skull of a patient. The base may be secured adjacent the craniotomy and to the skull and an implantable lead is inserted through the central passage into the tissue. Rotating a first rotating member that is coupled to the base moves the rotating member so that it meets and engages the implantable lead at a plurality of positions within the central passage. 
     The method may also comprise the step of rotating a second rotating member that is also coupled to the base so as to meet and engage the lead at a plurality of positions within the central passage thereby securing the lead in the tissue. Rotating the second door also adjusts the position of an opening within the central passage in which the lead may pass through and also closes or reduces the size of the central passage while still allowing the lead to pass therethrough. The method may also include rotationally adjusting the first and second rotating members in order to capture the lead therebetween or to release the lead therefrom. Often the method includes attaching and/or removing an insert that is adapted to engage the lead and that is coupled to the first or second rotating members. 
     The method may also comprise inserting one or both of the two rotating members into a secure base ring intraoperatively. In early stages of the lead implantation procedure, a wide lumen is available. After the lead is placed, an opening in such rotating members allows them to pass around the lead and rest in the base, and grip the lead. The method may further comprise retaining the two rotating members within the base by interlocking a retaining member placed over the rotating members and within the base, thereby restricting axial movement of the rotating members relative to the base. 
     Sometimes securing the base comprises press fitting at least a portion of the base into the craniotomy and often securing the base comprises coupling the base to the skull adjacent the craniotomy with a fastener such as a screw. Sometimes securing the base comprises recessing at least a portion of the base in the craniotomy, or the base may be coupled adjacent the craniotomy such that a bottom surface of the base is substantially flush with the craniotomy. 
     Usually, a cover is engaged with the base so as to substantially cover the central passage and capture the implantable lead therebetween. The cover and/or base may have channels which can accept the lead after being positioned therein. Often the first and second rotating members are locked and this may be accomplished by threadably engaging the rotating members with a set screw or by using detents in order to prevent relative motion therebetween. Sometimes, the lead may be bent into a channel that is defined by a top surface of the base and a potting material may be applied in order to fill gaps between the base and the craniotomy, thereby reducing or eliminating leakage of body fluids such as CSF from around the base. 
     These and other embodiments are described in further details in the following description related to the appended drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1D  illustrate cross-sections of several anchor assembly embodiments having fixation tabs that allow the anchor to be placed within a craniotomy at varying depths. 
         FIG. 1E  shows an anchor attached to a patient&#39;s cranium. 
         FIGS. 2A-2G  show top and cross section views of rotating doors. 
         FIGS. 3A-3F  show alternative embodiments of the grip bars. 
         FIGS. 4A-4F  show alternative embodiments of the rotating doors. 
         FIG. 5A  shows a top view of the cylinder without the rotating doors. 
         FIG. 5B  shows a side view of an exemplary embodiment of a cover. 
         FIG. 5C  shows a bottom view of an exemplary embodiment of a cover. 
         FIG. 6A  shows a cross section view of an assembled anchor with rotating doors. 
         FIGS. 6B-6K  show the components of the anchor depicted in  FIG. 6A . 
         FIG. 7A  shows an alternative embodiment of a mechanism for retaining the moving members within the cylinder. 
         FIG. 7B  shows a bottom view of the anchor assembly in  FIG. 7A . 
         FIGS. 8A-8C  show an anchor base of unitary construction. 
         FIGS. 8D-8M  show various components in various stages of assembly with the anchor base of  FIGS. 8A-8C . 
         FIGS. 9A-9C  illustrate the use of set screws to lock the rotating doors in position. 
         FIGS. 10A-10J  show the use of a tool for placement and removal of inserts into the rotating doors. 
         FIGS. 11A-11D  show side views of a tool as it us used to insert, place, attach and detach inserts into the anchor. 
         FIGS. 12A-12C  show an alternative embodiment of rotating doors that are adapted to pass around a placed lead intraoperatively and snap together. 
         FIGS. 13A-13C  show an alternative embodiment of the anchor base which may be used with the doors of  FIGS. 12A-12C . 
         FIGS. 14A-14E  illustrate exemplary embodiments of retaining members which hold the rotating doors in the anchor base. 
         FIGS. 15A-15D  illustrate an exemplary embodiment of a retaining member which retains the rotating doors and the cap. 
         FIG. 16  illustrates an exemplary embodiment of an anchor base with rotating doors that are held in place with a retaining member. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings like numerals describe substantially similar components. Now turning to  FIG. 1A  is a cross section exploded view of the anchor assembly, showing the probe  5 , cap  200 , and cylinder body  10 , also referred to as an anchor base in this application, assembled with parts that grip the probe. In this embodiment, the radial tabs  20  are elevated relative to the bottom surface of base  10  so that the cylinder body  10  may be recessed into a craniotomy. The cylinder  10  is fixed to the cranium by screws which pass through openings  25  in tabs  20  and secure the tabs to the cranium. In an alternative embodiment, the cylinder may have ridges, protrusions or other surface features (not shown) which generate a friction fit with the wall of the craniotomy, in conjunction with or in lieu of the radial tabs. Rotating doors  110  and  120  are shown rotated to a position such that the grip bars  70  are positioned to grip the probe  5 . In the section shown in  FIG. 1A , the grip bars  70  are positioned by removable inserts  140  and  150 , which in turn are captured in the doors  110  and  120  by rotating rivets  130 . An upper rivet plate  134  and a lower rivet plate  136  coupled to rivet  130  help lock the inserts into position. A ring-like spacer  16  separates doors  110  and  120 . The cap  200  has legs or pins  220  with catches  225  which snap into receiving sockets  40  within the cylinder  10 . The receiving sockets  40  not only provide fixation for a cap, but also provide a site and mechanism for attaching other instruments to the device. Examples of other devices that could be attached thereto include positioning guides or other reference instruments commonly used during neurosurgery. 
     The grip bars  70  may be made of a soft material, for example an elastomer, such as silicone rubber, polyurethane, or Santoprene™, or they may be made of the same material as the doors. Grip bars  70  may be porous or have holes running through them to make them compressible. Pores could be produced by many methods, including gas bubbles forming during the curing process, dissolving filler materials, or by withdrawing filaments introduced at the time the bars are formed or molded. During implantation, the probe  5  is placed intracranially, and the rotating doors  110  and  120  are rotated to place the grip bars  70  against the probe  5 . The probe  5  is bent to course along a groove  30  on the superior surface of the cylinder  10 , and onto the surface of the cranium. The cap  200  is then lowered so that pins of the cap  220  are inserted into sockets of the cylinder  40 , and the cap presses against the probe  5 . In some embodiments, a groove in the cap  210  wraps around the probe  5 . As the cap is lowered, pins  220  and protrusions from the pins  225  are displaced towards the center of the cylinder by catches  45 , until the protrusions snap outward under the catches, retaining the cap. In other embodiments, the cap may have an elastomeric gasket shaped so as to seal the space between the cap and the base, except for allowing passage for the probe through one set of grooves  30  and  210 . In other embodiments, the elastomeric gasket shall leave all sets of grooves open, and the unused probe passages are filled with separate plugs with radial dimension similar to the probe. 
       FIG. 1B  shows the embodiment of  FIG. 1A  with the probe  5  positioned intracranially, and the cap  200  snapped into the closed position. In this embodiment, the tabs  20  are elevated so that cylinder  10  may be recessed in the craniotomy allowing the top of the cylinder to be substantially level with the cranium. Such an embodiment has the advantage that the top of the cap extends minimally above the cranium. 
       FIG. 1C  shows an alternative embodiment of the assembly shown in  FIGS. 1A-B , in which the cylinder  10  is partially recessed into the cranium.  FIG. 1D  shows an alternative embodiment of the assembly shown in  FIGS. 1A-1C , in which the tabs  20  are positioned so that the lower surface of the cylinder  10  is at the level of the outer surface of the cranium. Such an embodiment has the advantage that the craniotomy opening need only be as large as the inner lumen of the cylinder  10 . The rotating door grip mechanism provides the particular advantage that if the probe  5  is inserted through the center of the device as shown in  FIG. 1B , the doors may be rotated together, thereby rotating the probe while still retaining vertical fixation. 
       FIG. 1E  illustrates the anchor base or cylinder  10  attached to a patient&#39;s cranium C. In  FIG. 1E , anchor  10  is positioned over a craniotomy so that a portion of the anchor fits within the craniotomy opening in order to reduce the portion of anchor  10  protruding out of the patient&#39;s cranium C. Fixtures F such as screws removably couple the anchor  10  to the cranium and a lead  5  is place through the central opening of the anchor  10  into the patient&#39;s brain B. A cover  200  may then be snap fit into engagement with the anchor  10 , thereby capturing the lead  5  in a desired position. 
       FIG. 2A  shows the lower rotating door  120 , apart from the rest of the anchor. The door is a disk with a large cutout  122  within its interior. Along one edge of the cutout is a bar  70  which can grip the probe placed in an intracranial position. Near the bar is a ledge depressed into the door  80  into which a gripping insert can be placed. The insert is retained from movement towards the open portion of the disk by terminating the depression at two stops  85 .  FIG. 2B  shows both rotating doors overlayed, with the both rivets  130  in the open position and both inserts removed. In this configuration a relatively large opening in the center of the anchor is available for the probe or any related test or accessory instrumentation. 
       FIG. 2C  shows the upper rotating door  110  with the rivet  130  in the closed position, and gripping insert  140  in place. The upper door also has a cutout  112 , a gripping bar  70  and a place for seating the insert. The insert  140  rests on the depressed ledge  80 . Motion of the insert towards the open part of the door  140  is prevented by the stops  85 , as in the lower door. Motion of the insert up out of the ledge, or rotation of the insert out of the ledge is prevented by the rivet  130 . The upper plate of the rivet  134  is a partial disk. When it is in the closed position, as shown in  FIG. 2C , the upper plate covers the edge of the insert, so that it is locked into place on the depressed seating ledge  80 . When it is open, the insert may be removed. The upper plate has three sockets  132  which may accept prongs from an insertion and removal tool, in order to rotate the rivet. A lower plate of the rivet  136  is similar to the upper plate  134  and also helps hold the insert. Lower plate  136  may be seen in  FIG. 1A . 
       FIG. 2D  shows a side view of the two rotating doors  110 ,  120 , with the inserts  140 ,  150  in place, and the rivets in the closed position. When the rotating doors  110 ,  120  are rotated, the inserts are pushed toward each other by their corresponding doors.  FIG. 2E  shows the two rotating doors  110 ,  120  overlayed, with the inserts locked in place by the rivets  130 .  FIG. 2F  also shows the two rotating doors overlayed, with the inserts removed and the doors opened to their maximum aperture.  FIG. 2G  shows both top and side views of the two inserts  140  and  150 . The head or top portion  164  of insert  150  along with the head or top portion  162  of insert  140  is seen in the side view of  FIG. 2G . The divots in the inserts  165 ,  166  accommodate the rivets. When the rivets are rotated into the closed position, the tails of the inserts  160  fit between the head of the rivet  134  and the seating depression in the rotating door  80 . When the inserts are in place, their grip bars  70  are continuous with the grip bars of the rotating doors. The tails of the inserts  160  sit in recessed ledges  80  in the rotating doors. 
       FIGS. 3A-3F  show alternative embodiments of the grip bars  70 , with greater contact between the grip bars and the probe compared to the embodiment shown in previous Figures. Only views from above are shown. In  FIG. 3A , the grip bars are scalloped to conform to the shape of the probe, and the spacing between scallops is less than the diameter of the probe, allowing many prospective positions where the probe could be placed. In  FIG. 3B , the grip bars are also scalloped, but with a shape complementary to the shape in  FIG. 3A . This shape generates as many prospective positions as the shape in  FIG. 3A , but instead of apposing the probe with conforming surfaces, this shape contacts the probe at 4 points, compared to two points in the embodiment shown in the other Figures. In  FIG. 3C , the grip bars completely surround the probe, generating fewer prospective fixation positions compared to the embodiments of  FIGS. 3A-3B . In  FIG. 3D , thin flanges or resilient fingers protrude from the grip bars, such that the flanges from one grip bar are out of phase or alternate with the flanges from the other grip bar.  FIGS. 3E-3F  are similar to the embodiment of  FIG. 3D , except that the flanges on opposite grip bars are in phase with one another so that they oppose each other, rather than the out of phase or alternating pattern seen in  FIG. 3D .  FIG. 3E  has longer flanges, while  FIG. 3F  has shorter flanges. These different embodiments illustrate examples of how the contact area of the grip bar with the probe may be increased compared to the embodiments shown in the other Figures. 
       FIGS. 4A-4F  show alternative embodiments of the grip bars  70 , with one or both grip bars attached directly to the rotating door  110 ,  120 , without an insert or the possibility of removing a portion of the grip bar  70 . In  FIGS. 4A-4D , the grip bars are centered on a plane between the rotating doors, as in  FIG. 2D , while in  FIGS. 4E and 4F , the grip bars  70  are centered in the planes of their respective rotating doors. When the grip bars are centered on a plane between the rotating doors, they do not transmit a bending moment to a probe inserted parallel to the axis of the cylindrical anchor body, while the embodiment in  FIGS. 4E-4F  the grip bars could potentially transmit a bending moment to such a probe. 
       FIGS. 4A-4B  show an embodiment with an upper rotating door  110  similar to the embodiments shown in  FIGS. 2A-2G , while the lower rotating door has no insert, and its grip bar is one continuous member.  FIG. 4A  shows the rotating doors in position to grip the probe, while  FIG. 4B  shows the rotating doors opened to their maximum aperture. The maximum aperture of this embodiment is nearly the same as the maximum aperture illustrated in  FIG. 2F , except near the center of the cylinder. 
       FIGS. 4C-4D , show an embodiment in which neither rotating door has an insert, and both grip bars are single, continuous members.  FIG. 4C  shows the rotating doors in position to grip the probe, while  FIG. 4D  shows the rotating doors opened to their maximum aperture. In this embodiment, the maximum aperture is smaller than in the embodiments of  FIGS. 2A-2G  and  FIGS. 4A-4B . 
       FIGS. 4E-4F  show an embodiment in which neither rotating door has an insert, and both grip bars  70  are single, continuous members, as in  FIGS. 4C-4D .  FIG. 4E  is a cross section view, which shows that in this embodiment the grip bars  70  are centered in the plane of their respective rotating doors.  FIG. 4F  shows that the maximum aperture of this embodiment is wider than any of the other illustrated embodiments, except near the center. 
     In other embodiments, the grip bars could be attached directly to the rotating doors for their full length, without any inserts or rivets. It will be obvious to those skilled in the art that many other specific forms are possible. 
       FIG. 5A  shows a top view of the cylinder or anchor base  10 , without the rotating doors. The anchor is fixed to the cranium by screws through screw-holes  25  in radial tabs  20 . A relatively short set screw  50  inserts into a threaded hole  56  to impinge upon the upper rotating door  110 , (not shown) and lock it into place. A relatively long set screw  52  having a flat point  51  inserts into a threaded hole  56  to impinge upon the lower rotating door  120 , (not shown) and lock it into place. Another relatively long set screw  54  having a cone point  55  inserts into a threaded hole  56  to impinge upon both rotating doors  110  and  120  (not shown) and lock them into place. In the illustrated embodiment, screws  50  and  52  have a flat tip, and impinge upon the outer upper corner of the rotating doors, while the screw  54  has an angled tip, and impinges upon the flat edge of both rotating doors. Receiving sockets  40  having catches  45  are adapted to receive the cap thereby snap fitting the two components together. Additionally, grooves or channels  30  radially extend outward from anchor  10  and provide a channel for holding the lead when the lead is captured between the anchor  10  and the cap. 
     Alternatively, one of the rotating doors could be held in place by a one-way ratcheting mechanism. In such an embodiment, a no-back pawl is a beam integrated with the anchor cylinder, in the plane of one of the rotating doors. The outer edge of the corresponding rotating door has the gear teeth. The pawl permits the gear teeth to pass freely in the direction which moves the grip bar  70  towards the probe, closing the door, but prevents the rotating door from opening. Such an embodiment makes fixing the doors faster, as only one set screw must be tightened, yet still permits the opening between the doors to be adjusted to any angular position, multiple times if necessary. 
       FIG. 5B  shows a cross section view of the cap  200 . It is dome shaped. Three pins  220  protrude downward, one of which is visible in this view.  FIG. 5C  shows a bottom view of the cap. The shape is a dome, truncated adjacent to pins  220  which protrude downward to snap into sockets  40  in the cylinder  10 . The dome-shaped disk is truncated adjacent to the pins so that a tool may be inserted into the socket  40 , alongside a pin  220  to facilitate removing the cap  200  when necessary. In the preferred embodiment, grooves  210  in the cap  200  increase the area of the cap  200  contacting the probe, compared to grooveless embodiments.  FIG. 5C  shows a bottom view of cap  200  highlighting grooves  210  and pins  220 . 
     Initially the probe is gripped by the rotating doors and fixed into position. The probe is then bent to lay in grooves  30  on the upper surface of the cylinder. The cap is lowered, with pins  220  sliding into sockets  40  and protrusions  225  from the pins snapping into place under catches  45 . When the cap is snapped in place, it presses upon the probe. In the preferred embodiment, grooves in the cap  210  increase the surface area of the cap in contact with the probe, increasing stability and decreasing point pressure on the probe. 
       FIG. 6A  shows an exemplary embodiment of an anchor base assembled with all of its components.  FIGS. 6B-6K  show the various components of the assembly in  FIG. 6A . In  FIG. 6A , the anchor base is composed of upper  12  and lower  14  portions. In the illustrated embodiment, radial tabs  20  are attached to the upper portion  12  of the cylinder  10 , so that the cylinder may be recessed into the craniotomy opening. In other embodiments the tabs may be attached to the lower portion  14  of the cylinder  10 . A shelf  26 , which retains the moving members within the cylinder, is integrated into the lower portion of the cylinder  14 . The upper portion  12  of the cylinder is the more massive, because it must contain the threaded holes  56  for the set screws (seen in  FIG. 6B ). Within the cylinder the upper  110  and lower  120  rotating doors are separated by a spacer ring  16 . The upper  12  and lower  14  portions of the cylinder are attached by an adhesive. In alternative embodiments, the base could be attached by welding or other mechanism of plastic deformation, by screws or other mechanisms which will be obvious to those skilled in the art.  FIG. 6B  shows the upper  12  portion of the anchor assembly while  FIG. 6C  shows a cross-section take along line  6 C- 6 C and  FIG. 6D  shows a cross section taken along line  6 D- 6 D.  FIG. 6E  shows the upper door  110  with insert  140  and rivet  130  that is positioned in the upper  12  portion of the anchor assembly. A spacer ring  6 F is then positioned next in the anchor assembly and a cross section of ring  16  taken along line  6 G- 6 G is shown in  FIG. 6G . Next lower door  120  with rivet  130  and insert  150  is loaded into the anchor assembly. The lower  14  portion of the anchor base is seen in  FIG. 6I . When the lower portion  14  is fastened to the upper  12  portion, the upper and lower doors  110 ,  120  and spacer  16  are captured therebetween.  FIG. 6J  shows a cross section of lower portion  14  taken along line  6 J- 6 J and  FIG. 6K  shows a cross section of lower portion  14  taken along line  6 K- 6 K. 
       FIGS. 7A-7B  show an alternative embodiment of assembling the anchor employing a plurality of pins  17  penetrating the anchor cylinder wall, and extending beneath the lower rotating door  120 . The pins course through narrow channels  18  in the cylinder wall. Together, the pins provide a support that retain the moving members within the cylinder.  FIG. 7A  shows a cross section of the anchor assembled with all of its components and  FIG. 7B  shows a bottom view of the anchor base with channels  18 . It is clear to those skilled in the art that this embodiment may be combined with the embodiments shown in  FIGS. 6A-6I  and  FIGS. 8A-8M . In embodiment of  FIGS. 6A-6I , the pins would provide the additional advantage of helping to retain the base of the cylinder. In the embodiment of  FIG. 5 , the pins provide further support for the moving members around the cutout that facilitates insertion of the rotating doors  28 . 
       FIGS. 8A-8M  show an alternative embodiment of an anchor assembly employing a different assembly method. In this embodiment, the body of the cylinder is monolithic. The bottom of the cylinder has a shelf  26  which retains the moving members. One side of the shelf is cut away  27  so that the rotating doors may be inserted from below during assembly. Such an embodiment is most compatible with a cylinder body which recesses into the craniotomy, because in such embodiments the slot is not impeded by the radial attachment tabs  20 . To assemble this embodiment, the upper rotating door  110  is slid into the central chamber of the cylinder. Next, the spacer  16  is inserted below the upper rotating door. Finally, the lower rotating door  130  is inserted. One side of the bottom of the cylinder is cutout  28  to facilitate sliding the rotating doors and the spacer parts into the center of the cylinder. The rivets  130  may be attached to the rotating doors in sequence after each is inserted into the central chamber, or after both rotating doors have been inserted. The rotating doors may be prevented from exiting the central chamber by tilting the slot slightly, so that the final door is strained as it is inserted and then snaps into place, or by placing one or more pins in the slot opening so as to constrain the motion of the lower door to rotational motion only. Alternatively, in both of these embodiments, an extended shelf may be fixed in the entry slot.  FIG. 8A  shows the anchor base that holds the upper  110  and lower  120  rotating doors.  FIG. 8B  shows a cross section of the anchor base of  FIG. 8A  taken along line  8 B- 8 B and  FIG. 8C  shows a cross section of the anchor base taken along line  8 C- 8 C.  FIG. 8D  shows the bottom of the anchor base and  FIG. 8E  shows the anchor base after upper door  110  has been inserted into the base.  FIG. 8F  shows the anchor base after both upper  110  and lower  120  doors and spacer  16  have been loaded into the anchor base.  FIGS. 8G-8L  illustrate the sequence of loading components into the anchor base during assembly and  FIG. 8M  shows the assembled anchor. 
       FIGS. 9A-9C  show cross section views, illustrating how set screws can be positioned in three different positions, so as to impinge on the upper rotating door  110  alone, lower rotating door  120  alone, or on both rotating doors  110 ,  120  simultaneously. Exemplary embodiments are shown, illustrating how the rotating doors may be fixed with standard set screws. Small diameter screws, such as 0-80, are appropriate for this application, because the cylinder body  10  is thin. A thin body  10  is desired so that it does not protrude much above the surface of the cranium. 
       FIG. 9A  shows a set screw  50  positioned to fix the upper rotating door  110 . In this embodiment, a flat set screw is used. The tip of such a screw typically has a wide flat surface orthogonal to the screw&#39;s axis of symmetry, bounded by a narrow conical ring  51 . When the screw is deployed with its long axis tilted at approximately 30 degrees from horizontal, one edge of the conical ring is nearly parallel to the outer edge of the upper rotating door  110 . As the screw is tightened, the conical ring  51  impinges upon the outer edge of the upper rotating door, but away from the lower rotating door  120 . 
       FIG. 9B  shows a similar set screw  52  positioned to fix the lower rotating door  120 . This screw is similar to the upper door fixation screw  50 , except that it is longer.  FIG. 9C  shows a set screw  54  positioned so as to impinge upon both rotating doors  110  and  120  simultaneously. In this embodiment a cone-point set screw is illustrated. Such a set screw has a wide conical ring  55  terminating at the tip of the screw, with a tip angle of approximately 118 degrees. When the screw  54  is deployed with its long axis tilted approximately 60 degrees from horizontal, it fixes both rotating doors. 
       FIGS. 10A-10J  show an insertion tool  300  with handle  350  for placement and removal of inserts  140  and  150  into the rotating doors  110  and  120 .  FIGS. 10A-10F  show portions of the tool  300  from several views. A side view of the tool is seen in  FIGS. 10A-10C  and the tool is seen from a top view in  FIGS. 10D-10F .  FIGS. 10A and 10D  show only the lowest portion, which interfaces directly with the insert, rotating door, and upper plate of the rivet. An orienting edge  320  at the bottom of the tool is complementary to the shape of the upper plate of the rivet  134 . Tabs  310  at the bottom of the tool fit precisely into matching sockets  132  in the upper portion of the rivets. In an alternative embodiment of the tool and the top of the rotating rivet, the tabs  310  are slightly larger at their lower most position, and/or the sockets  132  are narrower at their upper most position, to facilitate a snap fit of the tool with the rivet rotor. 
       FIGS. 10B and 10E  show a platform  340  at the base of the insertion/removal tool. The platform forms a bridge between the small features and tight tolerances of the components shown in  FIGS. 10A-10B , and the grip or handle  350  through which the surgeon applies torque, is shown in  FIGS. 10C and 10F . In the embodiment illustrated, the grip  350  is a hexagonal post with an angled handle, which my be turned digitally or with a wrench. In other embodiments, the grip may take another form, for example, a cap screw. In another embodiment, it could be a cylindrical post, with one or a plurality of radial holes into which a lever arm can be inserted. 
       FIGS. 10G-10J  show how the tool mates to the upper plate of the rivet  134  and couples with an insert  150  on lower rotating door  120 . The lower portion of the tool has an angled shape  320  complementary to the edge of the upper plate of the rivet  134 , to facilitate alignment of the tool with the rivet, and to apply torque to the rivet as the tool is rotated. For fine positioning and additional torque, the tool has tabs  310  which insert into matching divots in the upper plate of the rivet  132 . A curved pin  335  holds an insert  140  or  150  in position next to the tool  300  while the insert is placed into or removed from a rotating door  110  or  120 . A bulge  330  is provided for mounting the pin  335 . This mounting bulge  330  is positioned so that it does not impinge upon the upper portion of the insert  140  as the tool is rotated. 
       FIGS. 11A-11D  show the tool and insert through the cycle of positioning, attachment and detachment. In  FIG. 11A , two insertion tools are above the anchor, and the inserts are seated in the rotating doors, retained by the rivets. In  FIG. 11B , the tools are lowered to a position adjacent to the upper portion of the rivets  134  and the inserts  140  and  150 . The inserts are seated in the rotating doors, retained by the rivets. In  FIG. 11C , the tools have been rotated as indicated by the double headed arrows, so that the holding pins retain the inserts to the bottom of the insertion tools. The rivets no longer retain inserts. In  FIG. 11D , the inserts  140  and  150  are retained against the insertion tools by the holding pins  335  and lifted away from the rotating doors. The lower surface of the insertion tool fits into divots  165  and  166 , (not shown) in the inserts, so that the insert has a definite position relative to the insertion tool. The rotating doors and rivets lie below the tool as the tool is lifted away. 
       FIG. 12A-12C  show an exemplary embodiment of the rotating doors adapted for intraoperative assembly. In  FIG. 12A  the rotating doors  110  and  120  have gaps  71  positioned so that they can be passed around an indwelling medical lead and placed in a receiving anchor base. The gaps  71  may be positioned as in  FIG. 12B , so that the doors may be passed around the lead in a single movement. Intraoperative handling is facilitated by holes  74  in the doors. Once inserted into the receiving base, the doors can be rotated as in  FIG. 12C  in order to grip the medical lead. A snap mechanism can operate whereby a protrusion or detent from one door  73  lodges into a cavity  72  on the other, so as to maintain the doors in apposition against the lead. 
       FIGS. 13A-13C  show exemplary embodiments of the anchor base  10  and cap  200  adapted for intraoperative assembly with doors such as shown in  FIGS. 12A-12C . In the exemplary embodiment of  FIG. 13C , base  10  has two tabs  20  for attachment to the cranium, but the number of tabs may be modified as required. The doors pass around the lead, and they are placed so that the lower door rests upon a shelf  26 , and the upper door rests upon the lower door. A retaining member, such as those illustrated in  FIGS. 14A-14E  may optionally be inserted interfacing with an annular groove  41  in such a way as to partially occlude the lumen of the base  10  and prevent removal of the rotating doors. Two embodiments of the cap  200  are shown in  FIGS. 13A and 13B , with pins  220  placed so that the cap  200  can be attached to the base  10  by protrusions  225  from the pins  220  into the annular groove  41 . In the embodiment of  FIG. 13B , cavities  226  are placed in the cap  200 , so as to extend the effective length of the pins  220  and control the strain of the pin and mating forces, as will be familiar to those skilled in the art. The annular groove  41  can also be a point of attachment for additional instruments used intraoperatively such as a positioning guide or other reference instruments often used during neurosurgery. The retaining member may similarly be modified to permit attachment of other instruments used intraoperatively. The base  10  and cap  200  could optionally have features to force a particular alignment of the cap and base. For example, a pin may extend from the cap and seat in a groove on the base. 
       FIGS. 14A-14E  show several exemplary embodiments of a retaining member which may be placed intraoperatively, so as to hold or retain the doors within the base. All of these embodiments include a hole feature to facilitate manipulation of the member. One embodiment  400  is a conventional retaining ring, as will be well familiar to those skilled in the art and this is seen in  FIG. 14A . In  FIG. 14B , retaining member  410  includes a member  415  to increase the security of placement of the retention member. Additional security may be desirable if mounting features for a cap or intraoperative instruments are added to the retention feature. In  FIG. 14C , the retaining member  420  occupies half, more or less, of the annular groove, so as to generate less interference with a medical lead placed in the lumen of the base. In  FIGS. 14D and 14  E the ends of retaining members  430  and  440  interface with a groove, such as  41  of  FIG. 13C , but the body of these retaining members cross through the lumen of the base. Such disposition of the body of the retaining member keeps the groove free to accept other attachments. Retaining member  430  passes straight across, while retaining member  440  curves away from the center, so that it is clear of the center during placement. The depictions of retaining members  430  and  440  also include material  450  above the plane of the annular groove. Such material may be arranged so as to strengthen or stiffen the retaining member, or to interface with other parts. 
       FIGS. 15A-15D  show an embodiment where retaining member  460  has pins  220  extending in such a way that they could snap into the cap  200  and thereby attach it to the base  10 .  FIG. 15A  is a perspective view of the anchor base  10  with retaining member  460  and cap  200  assembled together.  FIG. 15B  shows cap  200  and  FIG. 15C  shows the retaining member  460 . Anchor base  10  is seen in  FIG. 15D .  FIG. 16  is a perspective view of anchor base  10  with the doors  110  and  120  and retaining member  440  assembled together. The retaining member  440  seats into an annular groove  41 , but its body is within the center of the base, leaving much of the groove  41  clear. 
     While the exemplary embodiments have been described in some detail for clarity of understanding and by way of example, a variety of additional modifications, adaptations and changes may be clear to those of skill in the art. Hence, the scope of the present invention is limited solely by the appended claims.