Abstract:
An apparatus and method for fastening a percutaneous medical device to bone is presented, comprising an anchor which comprises an outer threaded segment and an inner cavity comprising an inner threaded segment.

Description:
RELATED APPLICATION 
   This application claims benefits under 35 U.S.C. §119(e) of co-pending U.S. Provisional Patent Application Ser. No. 60/369,422 filed on Apr. 2, 2002, entitled “Bone Anchor for Use With Implantable Fiducial Markers”, the details and disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a bone anchor. More particularly, the present invention relates to a low profile bone anchor for fastening percutaneous medical devices to bone. 
   2. Description of the Prior Art 
   In certain surgical procedures and medical examination techniques, it is necessary for points or sites to be determined and identified within the body. For instance, stereotactic surgery requires a probe or electrode to be advanced into a patient&#39;s brain via a small aperture to a deep-seated brain path or nerve nucleus, while preserving adjacent structures. Likewise, certain imaging medical examination medical procedures, such as Positron Emission Tomography (PET), Roentgen Ray Analysis (X-ray), Computed Tomography (CT), and Nuclear Spin Resonance Tomography (MMR) require identification and location of inner body structures with a high degree of precision. 
   Conventionally, inner-body locations are pinpointed by percutaneously implanting external fixation pins into the patient&#39;s bone, such as his/her skull, to allow attachment of markers or various adaptors or other devices. Each marker contains a substance which provides a contrasting image and the plurality of markers, generally three, can effectively be triangulated to pinpoint inner-body locations. Moreover, the markers can be maintained in position which allows the markers to be positioned in advance of the surgical or examination procedure as well as providing repeatability for follow-on or additional treatments. 
   Typically, markers, often referred to as fiducial markers, are attached to the body through anchors drilled into bone, such as the skull, in a plurality of locations. The marker can be integral with the anchor, for instance the anchor can be formed of a marker (high contrast) material or, contrariwise, anchors can be installed in the body and then a marker attached to each anchor. 
   When the anchor system is used, it is possible to attach more than simply a marker to the anchor. For instance, certain devices needed for performing stereotactic surgery can be positioned at the skull using a bone anchor, such as sterotactic frames used to replace older “halo” type devices. Thus, the surgical device is repeatedly and precisely positioned for the procedure. 
   Conventional bone anchor devices have several drawbacks. First of all, the anchor often protrudes from the skull or other body part to such a degree as to run the risk of snagging or being caught on objects, thus creating the risk of breaking, infection, and causing the anchor to move out of position, in addition to the simple embarrassment and physical discomfort of a protruding anchor. Moreover, attachment methods conventionally used are not as precise as would be desired. Indeed, when a plurality, such as three, anchors is employed, even slight imprecision in each anchor is magnified and exacerbated when the imprecision occurs at each of three anchors. 
   In one instance of a device suggested for use as a fiducial marker, Leibinger, Leiginger, Felber, and Plangger, in U.S. Pat. No. 5,394,457, disclose a device for marking body sites for medical examinations. The described device consists of an anchor portion slip-fit into bone and a marker then slip-fit into the anchor portion. Clearly, the slip-fitting of the anchor into the bone and the marker into the anchor provide two areas where the fit may not be as precise as may be desired, creating imprecision in location. Although, Leibinger et al., disclose an embodiment where a screw can be inserted through the bottom of the anchor to provide a different anchoring mechanism, there is still a great deal of imprecision inherent in what would then be a three-part system (screw, anchor, marker). 
   In U.S. Pat. No. 5,397,329, Allen discloses a fiducial implant for a human body. The implant consists of an anchoring portion, which can be threaded, and a marker portion which extends above the bone to which the anchor portion is anchored. 
   As can be seen, neither of the discussed patents provides the flexibility of an anchor system to which a marker or other device can be removably attached, yet with the degree of precision and low profile sought. 
   What is desired therefore, is a bone anchor system which provides a high degree of precision in locating points in the center of a body, such as the brain, yet which allows either attachment of a marker or other surgical devices, while still maintaining a low profile with respect to the patient&#39;s body. 
   SUMMARY OF THE INVENTION 
   The present invention provides a bone anchor system which can be applied to a patient&#39;s body to assist in surgical intervention or high precision examination techniques. The inventive bone anchor consists of an outer-threaded segment which can be anchored in a patient&#39;s bone, as well as an inner-threaded segment for attachment thereto of a marker or other percutaneous medical device. 
   Accordingly, it is an object of the present invention to provide a fiducial implant capable of assisting a surgeon or other medical personnel, especially in identifying and locating points within a patient&#39;s body. 
   It is another object of the invention to provide a bone anchor which will assist in identifying points within a patient&#39;s body with a desired degree of precision, and repeatability. 
   It is yet another object of the present invention to provide such a bone anchor which is sufficiently low-profile as to reduce the chances of snagging or other undesirable effects. 
   These objects and others which will be apparent to the skilled artisan can be achieved by providing an apparatus for fastening a percutaneous medical device to bone, the apparatus including an anchor having an outer threaded segment and an inner cavity with an inner threaded segment. Preferably, the inner cavity further has a polygonal-shaped segment to permit it to be engaged by a device for implanting the anchor into bone. 
   Advantageously, the anchor also includes a flange capable of limiting the distance the anchor can be implanted in bone. The flange preferably contains a shaped portion capable of being engaged by a device for implanting the anchor into bone. More particularly, the flange can either have a polygonal-shaped inner surface or an outer surface which assumes a polygonal shape. 
   The anchor can also have a spherical depression sized to receive a localizing guide device useful for guiding a percutaneous medical device into the anchor. 
   Also in accordance with the present invention, a method is presented for fastening a percutaneous medical device to bone, the method involving implanting in bone the anchor of the present invention. The method includes drilling a pilot hole into the bone in which the anchor is to be implanted, and then implanting the anchor into the bone by engaging the bone by the threaded outer segment of the anchor. The percutaneous medical device can then be fastened to the anchor by engaging the inner threaded segment of the anchor with a threaded segment of the percutaneous medical device. 
   It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top perspective view of the bone anchor of the present invention. 
       FIG. 2  is a top-plan view of the bone anchor of  FIG. 1 . 
       FIG. 3  is a side-plan view of the bone anchor of  FIG. 1 . 
       FIG. 4  is a cross-sectional view of the bone anchor of  FIG. 3  taken along lines A—A. 
       FIG. 5  is a cross-sectional view of the bone anchor of  FIG. 1  having a fiducial marker threaded therein. 
       FIG. 6  is a side plan view of an alternate embodiment of a bone anchor in accordance with the present invention. 
       FIG. 7  is a top perspective view of the bone anchor of  FIG. 6 . 
       FIG. 8  is a cross-sectional view of the bone anchor of  FIG. 6 . 
       FIG. 9  is a side plan view of the bone anchor in accordance with the present invention implanted in bone, and with a localized guide device being mated therewith. 
       FIG. 10  is a top perspective view of the bone anchor of  FIG. 9 , having a localized guide device mated therewith. 
       FIG. 11  is a side plan view of the bone anchor of  FIG. 9 , having a localized guide device mated therewith. 
       FIG. 12  is a cross-sectional view of another alternate embodiment of a bone anchor in accordance with the present invention, having a spherical depression for mating with a localized guide device. 
       FIG. 13  is a top perspective view of the bone anchor of  FIG. 12 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will now be described in greater detail. Like or similar reference numerals will be used whenever possible, although, for the sake clarity, not all reference numbers are necessarily shown in each drawing. Likewise, the invention will be described with respect to a specific orientation and relationship of elements with respect to each other, but it will be recognized by the skilled artisan that other orientations and relationships will be equally applicable. In addition, although the primary use of the inventive bone anchor is for humans, it will be recognized that it is equally applicable to non-human (i.e., animal) uses. 
   A bone anchor in accordance with the present invention is described and denoted by the reference numeral  10 . Bone anchor  10  comprises an outer segment  20  having threads formed therein, which can be threaded into a hole drilled in a skull or other bone. As illustrated in  FIGS. 1–4 , anchor  10  advantageously further comprises a cap or flange portion  22 , the bottom surface  22 A of which sits against the skull or other bone when anchor  10  is drilled thereinto. Preferably, cap  22  comprises a beveled outer circumferential area  24  to assist in avoiding snags of anchor  10  by the patient. Bone anchor  10  can include a leading point which is shaped to provide self-centering, drilling or tapping features. Alternatively, however, bone anchor  10  can be formed without cap  22 , as shown in  FIGS. 6–8 . 
   Typically, threaded outer segment  20  of anchor  10  need only be less than about 5 millimeters in diameter at its base  20 A to adapt to the conventional size hole drilled in a bone for anchoring. It will be recognized, however, that in practice the diameter of threaded outer segment  20 , and of anchor  10  itself, can any diameter needed to engage the hole drilled into the bone in which anchor  10  is to be anchored. As is apparent to the skilled artisan, that diameter should be as small as practicable, for reasons of safety and speed of healing. 
   Likewise, cap  22  need only extend above threaded outer segment  20  a sufficient distance to provide the required structural stability, and to act as a stop to prevent anchor  10  from being implanted into the bone too far, or, in a worst-case scenario, screwed through the bone and into the tissue on the other side. More particularly, in the case of bone anchor  10  being implanted in a skull, without cap  22 , there would be the danger that anchor  10  could be screwed right through the skull and into the brain matter, with potentially harmful results for the patient. Typically, cap  22  need only be on the order of about less than 3 millimeters and typically, less than 2 millimeters in height above threaded outer segment  20  and thus above the bone into which anchor  10  is anchored. In this way, anchor  10  adopts an extremely low profile when implanted in a patient&#39;s bone to avoid snagging. 
   In addition, as illustrated in  FIGS. 1–4 , anchor  10  further comprises an internal cavity defined by inner wall  30 . In practice, the internal cavity can be “blind,” that is, it has a closed bottom, denoted  30 A as shown in  FIG. 5 ; alternatively, the internal cavity of anchor  10  can be a through hole, depending on the particular medical procedure to be performed, as shown in  FIG. 8 . A first segment of the inner cavity defined by wall  30  is inner threaded segment  32 , which is threaded to engage a complementary threaded portion of a marker  100  or other surgical device to thereby attach the marker or other surgical device to anchor  10 , as illustrated in  FIG. 5 . 
   The cavity in anchor  10  defined by wall  30  includes a second segment  34  which adopts a shape useful for engaging anchor  10  in order to implant (i.e., screw) it into the bone to which it is being engaged. Typically, segment  34  adopts a polygonal shape which will allow it to be engaged by an Allen® wrench, hex key or the like and thus permit use of such tool to screw anchor  10  into the bone. Alternatively, other shapes or structures can be formed in cap  22  to permit anchor  10  to be implanted in the bone by use of a screwdriver or other device. 
   In another embodiment (not shown), the outer edge of cap  22  can assume a polygonal shape to permit anchor to be screwed into the bone by use of a hex head socket, or the like. 
   Marker  100  or other surgical device can then be attached to anchor  10  by screwing marker  100  into the cavity formed in anchor  10 ; external threads formed on marker  100  engage the inner threaded segment  32  of anchor  10  to thus securely mount marker  100  to anchor  10 . Indeed, the use of threads to anchor the anchor  10  in bone and attach marker  100  to anchor  10  provides a precise mounting method which will permit greater precision in locating an internal site or point in the patient&#39;s body. 
   Anchor  10  (as well as marker  100 ) can be formed of any material having the desired structural characteristics. Most preferably, the material selected is one which is non-corrosive and which will not react with human tissue, and which can be sterilized. Most preferred, include titanium, polymers such as polytetrafluoroethylene (PTFE) and polysulfon, and ceramics. The material used as the marker within marker  100  can be any material having the desired contrasting characteristics. 
   In use, a pilot hole is drilled in a patient&#39;s skull or other bone in a plurality of locations (preferably three) and anchor  10  screwed into each drilled hole, such that the bottom surface  22 A of cap  22  sits against the bone into which anchor  10  is screwed. Marker  100  or other surgical device is then screwed fully into anchor  10 , such that its location is precise and repeatable. When not in use, overlying skin can be closed over anchor  10 , and a cap placed thereon to avoid tissue obstructing the cavity. 
     FIGS. 9–13  illustrates how a percutaneous medical device  900  might be placed into a subcutaneously implanted bone anchor  10  using a localizing guide device  200 . In  FIG. 9 , the previously implanted bone anchor  10  is shown having previously been placed into bone  999 . The localizing guide device with a spherical end feature  202  is used to locate a mating spherical depression  22 B (shown in the embodiments of  FIGS. 12 and 13 ) in anchor  10 . Once located, a threaded end  902  of percutaneous medical device  900  (such as marker  100 ) can be easily guided into the matching threaded inner hole of the bone anchor  10 . The localizing guide tube can then be removed leaving the percutaneous medical device  900  in place in anchor  10 , as illustrated in  FIG. 11 . 
   By use of bone anchor  10  of the present invention, a desirable low profile, precise and repeatable anchoring system, not heretofore seen in the prior art, is provided. 
   All cited patents and publications referred to in this application are incorporated by reference. 
   The invention thus being described, it will be apparent that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention and all such modifications as would be apparent as one skilled in the art are intended to be included within the scope of the following claims.