Abstract:
A revisable orthopedic anchor and method of use for removably securing an anchor in bone, such as compromised or degenerated bone, is described herein. In one embodiment, the anchor makes use of dual probes and vector divergence of the distal tips of the probes to achieve superior bone purchase and pull-out resistance. In such an embodiment, the probes can be inserted one at a time into a hole formed in, for example, the pedicle bone. After the probes are inserted and joined at the proximal end, they have a greater pull-out resistance than a threaded anchor. Removing the anchor involves separating the proximal heads and reversing the implantation process. As a result of the unique bone anchor design disclosed herein, the devices and methods of the present invention allow for less complicated implantation and removal of orthopedic anchors, all while providing enhanced bone purchase when implanted in a patient.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. application Ser. No. 13/222,389 filed on Aug. 31, 2011 and entitled “REVISABLE ORTHOPEDIC ANCHOR AND METHODS OF USE,” the entire content of which is hereby incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This invention is related to bone anchors for use in orthopedic surgery and, in particular, to devices and methods for implanting an anchor in bone such that the anchor provides enhanced bone purchase during use as well as easy removal. 
       BACKGROUND 
       [0003]    Bone in human and other mammal bodies is generally classified into two types, cortical bone, also known as compact bone, and trabecular bone, also known as cancellous or spongy bone. Cortical bone is much denser than trabecular bone with a porosity ranging between 5% and 10%. Cortical bone is found primarily in the shaft of long bones and forms the outer shell around trabecular bone at the end of joints and the vertebrae. 
         [0004]    In the vertebrae, each bone is generally heart shaped, with spinous, inferior and superior processes joined to the vertebral body via opposing pedicles. To stabilize or fix deformities in the spine, implantable medical devices, known as spinal fixation devices, can be employed between adjacent vertebrae. These devices can be attached to the vertebrae using screws inserted through the pedicles (i.e., using “pedicle screws”) and other osseous structures such as the lamina and facet joints. 
         [0005]    The outer shell of the pedicles is formed of dense cortical bone, which surrounds spongier trabecular bone. As mentioned above, trabecular bone is normally less dense than cortical bone. Degenerative conditions, which can result from diseases such as osteoporosis or injury, can cause the trabecular bone to weaken or degrade even further. 
         [0006]    As a result of the lower density of the trabecular or compromised bone, screws of all sizes can loosen or shift position after implantation. Prior art attempts to secure bone screws in the pedicles include features to prevent loosening of the screws. These features can include deflectable wings that push outwardly against the bone, or toggle-bolt-like fittings that rotate once inside the cortical bone shell to prevent removal therefrom. Other prior art systems make use of cement or other binding agent to secure the bone screw inside of the pedicle. 
         [0007]    These prior art solutions, however, are not without potential drawbacks. For example, prior art implementations are designed to provide permanent fixation of the bone screw within the pedicle or other bone. Accordingly, removal of these components requires a complicated and oftentimes invasive procedure. In addition, some prior art solutions utilize traditional threaded screws that require pre-implantation operations to correctly drill and tap a hole through the bone. 
         [0008]    Finally, in certain situations it can be desirable to utilize radiolucent materials to avoid interference with medical imaging technologies. Prior art implementations, however, often utilize metal bone screws that are substantially radiopaque. This can be because radiolucent materials (e.g., any of various polymer-based materials) are often not suitable for creating fine features like threading on a screw. 
         [0009]    Hence, there is a need in the art for a device and method for securing an anchor in degenerated bone such that the anchor exhibits enhanced bone purchase while remaining easy to remove after use. Further, there is a need in the art to design these anchors such that they can be formed from radiolucent materials to prevent interference with medical imaging technologies. 
       SUMMARY 
       [0010]    To overcome the above and other drawbacks of conventional systems, the present invention provides a revisable orthopedic anchor and method of use for removably securing an anchor in degenerated bone. In one embodiment, the anchor makes use of dual probes and vector divergence of the distal tips of the probes to achieve superior bone purchase. In said embodiment, the probes can be inserted one at a time into a hole formed in, for example, the pedicle bone. After the probes are inserted and joined at the proximal end, they have a greater pull-out resistance than a threaded anchor. Removing the anchor involves separating the proximal heads and reversing the implantation process. As a result of the unique designs disclosed herein, the devices and methods of the present invention allow for less complicated implantation and removal of orthopedic anchors, all while providing enhanced bone purchase once implanted in a patient. 
         [0011]    In one aspect of the invention, a bone anchor is provided that includes a first probe component in the form of an elongate member having a proximal head and a distal tip. The first probe component further includes a bone engaging edge having a plurality of barbs, an opposed edge, an external surface, and an opposed internal surface having a guide shoulder formed thereon. The bone anchor also includes a second probe component in the form of an elongate member having a proximal head and a distal tip. The second probe component similarly includes a bone engaging edge having a plurality of barbs, a mating edge having a profile complimentary to the guide shoulder and being configured to be seated along the guide shoulder, an external surface, and an opposed internal surface. Optionally, the mating edge of the second probe member is curved. The first and second probe components are configured to be assembled to form a bone anchor such that the barbed bone engaging edges of the first and second probe components are disposed opposite to one another, the mating edge of the second probe component is seated along the guide shoulder of the first probe component, and the distal tip of each probe component diverges away from a central longitudinal axis of the anchor. The first and second probe members can optionally be curved elongate members. 
         [0012]    The above-described bone anchor can include a variety of further features or modifications. For example, in some embodiments, the external surfaces of the first probe component and the second probe component can be convex. When the probes are assembled to form a bone anchor, the resulting cross-sectional shape can better adapt to the non-circular anthropometrics of the pedicle or other bone. 
         [0013]    In other embodiments, the internal surfaces of the first probe component and the second probe component can be substantially linear. Linear internal surfaces provide for easy and secure mating between the first and second probe components when assembled to form a bone anchor. In still other embodiments, alternative profiles can be used, including, for example, convex/concave shapes, interlocking ridges (i.e., tongue and groove surfaces), complementary diagonals, etc. 
         [0014]    The bone anchor can further include a crimp head configured to retain the proximal heads of the first probe component and the second probe component in a fixed relationship with each other. Such a crimp head can prevent the two probe components from separating after implantation. As explained below, maintaining the proximal heads in a fixed relationship with each other can be important to provide the bone anchor with its superior resistance to removal. 
         [0015]    In certain embodiments, the proximal head of the first probe component and the second probe component can include a recess formed therein configured to seat a spinal fixation element. The seat formed by the probe components, in combination with a crimp head that forces the spinal fixation element into the seat, can securely attach a vertebral body to a spinal fixation element. 
         [0016]    In other embodiments, the bone anchor can include a polyaxial receiving head configured to retain the proximal heads of the first probe component and the second probe component in a fixed relationship with each other. In these embodiments, the polyaxial receiving head replaces the above-mentioned crimp head to both hold the probe components together at their proximal ends and to secure a spinal fixation element to the bone anchor. 
         [0017]    In still other embodiments, each of the first probe component and the second probe component can include a rod section joined to the proximal head thereof. Each rod section can be joined to form a spinal fixation element that can, in turn, be attached to adjacent bone anchors. The bone anchor can also include a crimp head configured to retain the proximal heads and rod sections of the first probe component and the second probe component in a fixed relationship with each other. 
         [0018]    In certain embodiments, the bone anchor can include an implant configured to fuse two vertebral bodies together. The implant can include at least one lumen formed therein and configured to receive the first probe component and the second probe component. 
         [0019]    In some embodiments, the implant can include a set screw configured to engage a threaded lumen formed in the implant to secure the first probe component and the second probe component in relation to each other and the implant. 
         [0020]    The first and second probe components, and associated rod sections, if any, can be formed from a variety of materials. For example, the probe components and rod sections can be formed from any of titanium, a titanium alloy, polyether ether ketone (PEEK), and reinforced PEEK. The crimp head, polyaxial receiving head, and implant can each be formed from similar biocompatible materials. 
         [0021]    In some embodiments, the first probe component and the second probe component are formed from a radiolucent material. Forming the probe components from radiolucent materials prevents the implanted anchor from interfering with medical imaging technologies (e.g., X-rays, etc.). 
         [0022]    In another aspect of the invention, an implantable bone anchor is provided including an elongate member having a proximal head with a fixation element receiving seat and dual divergent distal tips. The bone anchor further includes opposed bone engaging edges having a plurality of barbs formed thereon. The bone anchor can be formed of separate matable probe components and each probe component has one of the divergent distal tips. 
         [0023]    In some embodiments, each probe component is curved along its length from the proximal head to the divergent distal tip. This curve, combined with the dual divergent tips, results in an anchor that is wider at its distal end than at its proximal end. This configuration allows the anchor to resist being pulled out of the bone. 
         [0024]    In some other embodiments, each probe component is mated to the other along at least a portion of an internal surface that is opposed to one of the bone engaging edges. As a result, the bone engaging edges of each probe component oppose each other and are configured to engage bone upon implantation. The bone engaging edges can include barbs or other protrusions configured to interface with a bone wall. In some embodiments, these barbs can be directionally oriented to allow travel in one direction (e.g., insertion), but oppose travel in an opposite direction (e.g., removal). 
         [0025]    In a third aspect of the invention, a method of anchoring an implant to bone is provided comprising the steps of inserting a first probe member into a cavity formed in bone, where the first probe member is a curved, elongate member having a distal tip and a proximal head. The method further includes the step of inserting a second probe member into the cavity adjacent to the first probe member where the second probe member is a curved, elongate member having a distal tip and a proximal head, and where the first and second probe members mate to one another such that the heads of the first and second probe members are aligned and the distal tips of the first and second probe members diverge. 
         [0026]    In some embodiments, the method step of inserting the second probe member includes sliding the second probe member along a guide shoulder formed in the first probe member to properly align the first probe member and the second probe member in the cavity. 
         [0027]    In other embodiments, the method can further include the step of applying a crimp head to the proximal heads of the first probe member and the second probe member to retain the proximal heads in relation to each other. 
         [0028]    In still other embodiments, the first probe member and the second probe member can each further include a rod section connected to the proximal head. The method can also include the step of aligning the rod sections of the first probe member and the second probe member to assemble a spinal fixation element. Still further, the method can include the step of applying a crimp head to retain the proximal heads and rod sections of the first probe member and the second probe member in relation to each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0030]      FIG. 1  is a perspective view of one embodiment of a bone anchor of the present invention comprising two probe components; 
           [0031]      FIG. 1A  is a perspective view of the first probe component of the bone anchor in  FIG. 1 ; 
           [0032]      FIG. 2  is a perspective view of another embodiment of a bone anchor of the present invention having a flat top surface on its proximal end; 
           [0033]      FIG. 2A  is an alternate perspective view of the bone anchor in  FIG. 2 ; 
           [0034]      FIG. 3  is a perspective view of a crimp head configured to retain the proximal heads of the first and second probe components in a fixed relationship with each other and receive a spinal fixation element; 
           [0035]      FIG. 4  is a perspective view of the bone anchor of  FIG. 1 , including the crimp head in  FIG. 3 ; 
           [0036]      FIG. 5  is a perspective view of the bone anchor in  FIG. 2 , including a polyaxial receiving head configured to retain the proximal heads of the first and second probe components in a fixed relationship with each other and receive a spinal fixation element; 
           [0037]      FIG. 6  is a perspective view of the bone anchor in  FIG. 5  with the polyaxial receiving head hidden to reveal a spinal fixation element seat adapter and set screw; 
           [0038]      FIG. 7  is a cross-sectional view of the bone anchor in  FIG. 5  implanted in a pedicle bone of a patient; 
           [0039]      FIG. 8  is a cross-sectional view of the bone anchor in  FIG. 7 , taken along line A-A in  FIG. 7 , showing the complementary non-circular shape of the pedicle bone and probe components; 
           [0040]      FIG. 9  is a perspective view of an embodiment of an implant of the present invention configured to fuse two vertebral bodies together including at least one bone anchor and a set screw; 
           [0041]      FIG. 10  is an alternate perspective view of the implant in  FIG. 9  showing lumens configured to receive first and second probe components, as well as a threaded lumen configured to receive a set screw; 
           [0042]      FIG. 11  is a perspective view of the implant in  FIG. 9  with the implant body hidden to reveal the orientation and interaction between the bone anchors and the set screw; 
           [0043]      FIG. 12  is a perspective view of the implant in  FIG. 9  in position between two vertebral bodies with implanted bone anchors shown in phantom; 
           [0044]      FIG. 13  is a perspective view of an embodiment of a bone anchor of the present invention wherein each probe component includes a rod section connected thereto that forms a spinal fixation element when aligned together; 
           [0045]      FIG. 14  is an alternate perspective view of the bone anchor in  FIG. 13  showing the non-circular cross-sectional shape of the rod sections; 
           [0046]      FIG. 15  is a perspective view of the internal surface of the first probe component of the bone anchor in  FIG. 13 ; 
           [0047]      FIG. 16  is a perspective view of the internal surface of the second probe component of the bone anchor in  FIG. 13 ; 
           [0048]      FIG. 17  is a perspective view of the bone anchor in  FIG. 13  implanted in a vertebral body and including a crimp head to retain the proximal heads and rod sections of each probe component in relation to each other; 
           [0049]      FIG. 18  is a perspective view of the bone anchor in  FIG. 17  showing the crimp head retaining the proximal heads and rod sections of each probe component in relation to each other; 
           [0050]      FIG. 19  is an alternate perspective view of the crimp head in  FIG. 18 ; 
           [0051]      FIG. 20  is a perspective view of an embodiment of a spinal fixation assembly of the present invention including various embodiments of the bone anchors disclosed herein; 
           [0052]      FIG. 21  is a cross-sectional view of an embodiment of a bone anchor of the present invention configured to function as a suture anchor; 
           [0053]      FIG. 22  is a front view of an embodiment of a bone anchor of the present invention including a rod section connected to each probe component and a hinge area that allows each probe component to be folded and introduced through, for example, a laparoscopic port; and 
           [0054]      FIG. 23  is a flow diagram illustrating an embodiment of a method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention. 
         [0056]    In one aspect of the invention, a bone anchor is provided that includes two probe components configured to be assembled to form a complete bone anchor. Each probe component can be in the form of an elongate member having a proximal head and a distal tip. A first probe component can have a bone engaging edge, an opposed edge, an external surface, and an opposed internal surface having a guide shoulder formed thereon. A second probe component can have a bone engaging edge, a mating edge, an external surface, and an opposed internal surface. The mating edge of the second probe component can have a profile complementary to the guide shoulder of the first probe component and be configured to be seated along the guide shoulder. While the first and second probe members are illustrated herein as curved, one skilled in the art will appreciate that they may be linear and non-curved. Similarly, the mating edge of the second probe component is illustrated herein as having a curved profile, but it may alternatively be linear. 
         [0057]    In use, the distal tips of the first and second probe components can be inserted sequentially into a small hole formed in, for example, a pedicle bone. Once inserted into the pedicle bone, the proximal heads of the first and second probe components can be joined together such that bone engaging edges of the first and second probe components are disposed opposite to one another, the mating edge of the second probe component can be seated along the guide shoulder of the first probe component, and the distal tips of the first and second probe components can diverge away from a central longitudinal axis of the bone anchor. The divergent distal tips of the bone anchor can allow the curved bone engaging surfaces to interface with, for example, the more dense cortical bone that forms the outer shell of the pedicle, thereby providing greater pull-out resistance than a traditional bone screw. 
         [0058]      FIGS. 1 ,  1 A,  2 , and  2 A illustrate a bone anchor  100  of the present invention comprising a first probe component  102  and a second probe component  104 . The probe components  102 ,  104  are separate and designed to be mated together, for example, in the orientation shown in  FIG. 1 . In such an orientation, the proximal head  106  of the first probe component  102  and the proximal head  108  of the second probe component  104  are aligned while the distal tip  110  of the first probe component  102  and the distal tip  112  of the second probe component  104  diverge away from a central longitudinal axis  114  of the bone anchor. 
         [0059]    The first probe component, shown in isolation in  FIG. 1A , can include a bone engaging edge  116  having a plurality of barbs  118  formed thereon. Barbs  118  can be formed in a variety of shapes but, in an exemplary embodiment, the barbs can be formed with a one-way directional slant. The directional slant of the barbs allows advancement of the probe component into bone, but resists its removal. The first probe component can also include an opposed edge  120 , which can have a smooth curved profile. One skilled in the art will appreciate that the barbs may additionally include, or they may be formed from, teeth, porous bone in-growth surfaces, and micro- or nano-features. 
         [0060]    The first probe component can also include an internal surface  122  configured to interface with a portion of the second probe component  104 . The internal surface  122  can be substantially linear to provide a planar surface to interface with the second probe component  104 . The internal surface  122  can also include a guide shoulder  124  formed thereon that is also configured to interface with a portion of the second probe component  104 . The guide shoulder  124  can be formed in a variety of shapes depending on the desired geometry of the bone anchor. In an exemplary embodiment, the guide shoulder  124  forms a diagonally extending curve across the internal surface  122  of the first probe component  102 . In such a configuration, the guide shoulder  124  can gradually urge the distal tip  112  of the second probe component  104  to diverge from the distal tip  110  of the first probe component  102  as the second probe component is advanced down the length of the first probe component along the internal surface  122 . Although not illustrated, the internal surface  122  may alternatively include one or more features formed thereon that are configured to mate with complementary features of the internal surface  206  of the second probe component  104 . 
         [0061]    The first probe component  102  can further include an external surface  202 , as shown in  FIG. 2A . The external surface  202  can have a variety of profiles according to the geometry of the intended implantation site. In an exemplary embodiment, external surface  202  has a convex profile to adapt to the non-circular geometry of the pedicle bone, as discussed below. 
         [0062]    Referring back to  FIG. 1A , the first probe component  102  can include a proximal head  106  at its proximal end. Proximal head  106  can be formed in a variety of shapes and sizes according to the requirements of any receiving heads or other accessories to be attached to the bone anchor. In one embodiment, proximal head  106  can include a recess  126  formed therein and configured to form a fixation element receiving seat  128  (shown in  FIG. 1 ) when combined with a complementary recess formed in the proximal head  108  of second probe component  104 . 
         [0063]    In other embodiments, proximal heads  106 ,  108  of the first and second probe components  102 ,  104  can have alternate geometries. For example, and as illustrated in  FIGS. 2 and 2A , proximal heads  106 ,  108  can each include a flat surface  204 ,  208 , or any other type of surface configured to interface with a receiving head assembly. Proximal heads  106 ,  108  of first and second probe components can also have a bulb-shape on their outer surfaces to allow polyaxial movement of attached receiving head assemblies having a socket-shaped cavity to receive the proximal heads  106 ,  108 . 
         [0064]      FIGS. 1 and 1A  also illustrate a receiving head attachment portion  130  that can be included in the first probe component  102 . Receiving head attachment portion  130  can be located distally from the proximal head  106  and can be configured to receive a mating feature from a receiving head or other accessory. In an exemplary embodiment, receiving head attachment portion  130  can be a narrowed section of the first probe component  102 . 
         [0065]      FIGS. 1 ,  2 , and  2 A also illustrate an exemplary embodiment of the second probe component  104 . The second probe component  104  can have several features in common with the first probe component  102 . These can include a bone engaging surface  132  having a plurality of barbs  118  formed thereon and an external surface  134  that can have a similar profile to the external surface  202  of first probe component  102  (e.g., a convex surface profile). As shown in  FIG. 2A , the second probe component  104  can include an internal surface  206  opposed from external surface  134 . The internal surface  206  can have a substantially linear surface profile in order to interface with the substantially linear internal surface  122  of the first probe component  102 . Alternatively, however, and as noted above, the internal surface  206  can have features (not shown) that are configured to mate with complementary features (also not shown) of the internal surface  122 . 
         [0066]    Referring back to  FIGS. 1 and 2 , the second probe component  104  can include a mating edge  136  opposed from the bone engaging edge  132 . The mating edge  136  can have a profile complementary to the profile of the guide shoulder  124 . In an exemplary embodiment, the mating edge  136  has a curved profile matching the curve of guide shoulder  124 . The complementary profiles allow the mating edge  136  to slide along the guide shoulder  124  as the second probe component  104  is implanted adjacent to the first probe component  102 . 
         [0067]    The second probe component  104  can also include similar components at its proximal end as the first probe component  102 . For example, the second probe component can include a proximal head  108  configured to interface with a variety of spinal fixation components. Proximal head  108  can include, for example, a recess  138  formed therein that, in combination with recess  126  of the first probe component  102 , can form a fixation element receiving seat  128 . Alternatively, proximal head  108  can include a flat surface  208 , as shown in  FIG. 2 . The second probe component  104  can also include a receiving head attachment portion  140  similar to the receiving head attachment portion  130  of the first probe component  102 . 
         [0068]    The first and second probe components can be formed from a variety of biocompatible materials suitable for implantation in a patient. These materials include, for example, metals such as titanium and titanium alloys, as well as polymers such as polyether ether ketone (PEEK) and reinforced PEEK. The design of the bone anchor of the present invention can be particularly well suited to the use of polymer-based materials. This is in contrast to traditional bone screw designs that have fine thread forms that cannot be reliably created with polymers. Another advantage of utilizing polymers like PEEK is the radiolucency of these materials. Unlike prior art metal bone screws, X-Ray and other medical imaging technologies can see through bone anchors formed from these radiolucent materials, providing medical professionals with a better image of the surrounding bone structure. 
         [0069]    The first and second probe components can also be formed in a variety of sizes suited to the particular implantation site. In exemplary embodiments, the bone anchors are implanted in the pedicle bones of human vertebrae and are therefore sized accordingly. As discussed below, however, bone anchors of the present invention can be utilized in many different operations where tissue or implants need to be secured to bone. Variations on the size of the bone anchors to accommodate different implantation site geometries are considered within the scope of the invention. 
         [0070]    As the foregoing description of the first and second probe components illustrates, an inventive aspect of the bone anchor disclosed herein is the ability to insert the first probe component  102  into a cavity formed in a bone, then insert the second probe component  104  and utilize the first probe component to aid in positioning the second probe component. This can be accomplished, for example, through the interaction of the guide shoulder  124  of the first probe component  102  and the mating edge  136  of the second probe component. After both probe components are implanted in the bone cavity, the proximal heads of the first and second probe components can be secured together to lock the probe components in the orientation shown in  FIGS. 1 and 2 . 
         [0071]      FIG. 3  illustrates an exemplary embodiment of a crimp head  300  that can be used to secure the proximal heads  106 ,  108  of the first and second probe components  102 ,  104  together. Crimp head  300  includes an arch portion  302  that extends between two sets of clamping arms  304 . Crimp head  300  can further include recesses  306  configured to receive the bulb-shaped proximal heads  106 ,  108  of the first and second probe components  102 ,  104 . 
         [0072]      FIG. 4  illustrates crimp head  300  in position over the first and second probe components shown in  FIG. 1 . In  FIG. 4 , crimp head  300  is shown in an un-compressed state in which there is a small amount of clearance between the clamping arms  304  and the receiving head attachment portion  130 . There is also a small amount of clearance between the recesses  306  and the proximal heads  106 ,  108  of the first and second probe components  102 ,  104 . This is done so that crimp head  300  can be placed over the top of the proximal head  106 ,  108  of first and second probe components  102 ,  104  without encountering resistance. To secure the crimp head  300  (and thereby secure the proximal heads  106 ,  108  in a fixed relationship with each other) a crimping tool (not shown) or other device suitable to provide compressive force to the clamping arms  304  can be used to position the clamping arms securely on the receiving head attachment portion  130 ,  140  of the first and second probe components. 
         [0073]      FIG. 4  also illustrates that the arch portion  302  of crimp head  300 , in combination with fixation element receiving seat  128 , forms a closed lumen that can be used to secure a spinal fixation element to the bone. As a result, the bone anchor and crimp head of the present invention can be used to anchor spinal fixation elements to the vertebrae in place of traditional bone screws. An exemplary prior art system of spinal fixation is disclosed in U.S. Pat. No. 7,527,638 to Anderson et al., which is hereby incorporated by reference in its entirety. 
         [0074]    Crimp head  300  can be formed from any of the same biocompatible materials mentioned above with respect to the first and second probe components. However, consideration should be given to the ability of the material to hold its shape under stress once crimped in position. In an exemplary embodiment, the crimp head  300  can be formed from a malleable material, such as titanium or a titanium alloy, in order to both allow deformation into the desired shape and to provide the necessary rigidity after implantation. 
         [0075]    The crimp head  300  is one embodiment of a receiving head assembly that can be attached to the bone anchor of the present invention.  FIG. 5  illustrates another suitable receiving head assembly embodiment in the form of a polyaxial receiving head. Polyaxial receiving head  500  is similar to polyaxial receiving heads known in the prior art. U.S. Pat. No. 7,682,377 to Konieczynski et al., which is hereby incorporated by reference in its entirety, illustrates an exemplary prior art polyaxial receiving head. The polyaxial receiving head  500  can include a U-shaped transverse passage and a lumen for receiving the proximal heads  106 ,  108  of first and second probe components  102 ,  104 . Polyaxial receiving head  500  can also include a spinal fixation element seat adapter  502  for use in embodiments having a flat surface  204 ,  208  on proximal heads  106 ,  108 , as well as a set screw  504  that can be used to secure a spinal fixation element within the U-shaped transverse passage.  FIG. 6  illustrates an alternate view of the bone anchor in  FIG. 5  with the polyaxial receiving head hidden to better depict the spinal fixation element seat adapter  502  and set screw  504 . 
         [0076]    Having described exemplary components of the present invention,  FIG. 7  depicts an exemplary bone anchor implanted within the pedicle bone of a patient&#39;s vertebrae in order to better illustrate each component&#39;s function. The partial cross section of the vertebrae  700  shows the outer layer of dense cortical bone  702  and the inner core of less dense trabecular bone  704 . Implanted through the anterior portion of the pedicle bone  706  is bone anchor  100  including polyaxial receiving head  500 . 
         [0077]    Bone anchor  100  is able to achieve superior bone purchase in the pedicle  706  as a result of the curved shape of the first and second probe components  102 ,  104  and the associated divergent distal tips  110 ,  112 . As  FIG. 7  illustrates, the curved shape of the first and second probe components  102 ,  104  allows the bone engaging surfaces  116 ,  132  of the first and second probe components to interface with the stronger cortical bone forming the posterior rim  708  of the vertebral body. In addition, the combined probe width  710 , which is measured as the distance between the divergent distal tips  110 ,  112  of the first and second probe components, is greater than the width  712  of the pedicle bone  706 . As a result, the bone anchor  100  cannot be removed through the pedicle  706  so long as the polyaxial receiving head  500  retains the proximal ends  106 ,  108  of the first and second probe components  102 ,  104  in a fixed relationship with each other. 
         [0078]    However, the width between divergent tips is not the only manner in which the bone anchor of the present invention utilizes anatomical geometry to its advantage.  FIG. 8  illustrates a cross sectional view along the line A-A in  FIG. 7 . The figure illustrates that pedicle bone  706  (again, comprising a shell of cortical bone  702  and a core of trabecular bone  704 ) has a non-circular cross-sectional profile. Many known bone screws utilize circular cross sectional shapes, mostly due to limitations of processing the materials used to form the screws. Forming the first and second probe components  102 ,  104  of the present invention from polymer-based materials such as PEEK or reinforced PEEK allows for the creation of non-circular geometries that can better fill—and thus better anchor within—the pedicle bone  706  or other bone having non-circular geometry. 
         [0079]    In another embodiment, shown in  FIG. 9 , the bone anchor of the present invention includes an implant  900  configured to fuse two vertebral bodies together. The implant  900  can be used to replace a degenerated or injured intervertebral disc that exists between adjacent vertebrae. The implant  900  can include opposing textured surfaces  902 ,  904  configured to interface with a vertebral body. The implant  900  can further include an inner void  904  that can be filled with a biologic agent or other bone growth promoting material to stimulate natural bone growth that further anchors the implant  900  and fuses the vertebrae together. 
         [0080]    Implant  900  can be initially anchored to adjacent vertebral bodies using bone anchors  100   a ,  100   b . As shown in  FIG. 10 , implant  900  can include one or more lumens  1002  configured to receive sets of first and second probe components that form bone anchors  100   a  and  100   b , as discussed above. The lumens can be shaped to appropriately interface with the proximal heads  106 ,  108  of the first and second probe components, similar to the crimp head  300  and polyaxial receiving head  500  discussed above. Implant  900  can further include a threaded lumen  1004  configured to receive a set screw  906 , which is shown in  FIG. 9 . 
         [0081]    As illustrated in  FIG. 11 , which shows the configuration of the set screw  906  and bone anchors  100   a ,  100   b  with the implant hidden, the set screw  906  can include an interface portion  1102  that interfaces with, for example, the flat surface  204 ,  208  of the first and second probe components of each bone anchor  100   a ,  100   b . The set screw  906 , in combination with the implant lumen  1002 , retains the proximal heads  106 ,  108  of the first and second probe components  102 ,  104  in a fixed relationship with each other. 
         [0082]      FIG. 12  illustrates an exemplary embodiment of implant  900  in position between two vertebrae  1202 ,  1204 . Bone anchors  100   a ,  100   b  are shown in phantom implanted within the vertebrae. The bone anchors illustrated in  FIG. 12  are retained in position by the lumens formed in implant  900  and the set screw  906 . Also shown in  FIG. 12  is an exemplary applicator tool  1206  as known in the art for inserting and adjusting the set screw  906 . 
         [0083]    Still another embodiment of the invention provides a bone anchor as described above that further includes a rod section joined to each proximal head of the first and second probe components. In such an embodiment, when the first and second probe components are joined together, the associated rod sections can also be aligned to form a complete spinal fixation rod. This rod can be attached to additional bone anchors of the type described herein to create a complete spinal fixation assembly using fewer parts than was previously possible. 
         [0084]      FIG. 13  illustrates an exemplary bone anchor  1300  including first and second probe components  1302 ,  1304  that are connected at their proximal ends to first and second rod sections  1306 ,  1308 . The connection can include a hinge portion  1310  in each rod section that allows the rod sections  1306 ,  1308  to rotate relative to the probe components  1302 ,  1304 . This rotation can be used to connect the rod sections  1306 ,  1308  to additional bone anchors, or to collapse the bone anchor  1300 , e.g., for easier introduction to a surgical site. Hinge portion  1310  can include any mechanical hinge known in the art. In some embodiments, hinge portion  1310  can be a “living” hinge integrated into the material of rod sections  1306 ,  1308 . This can be accomplished, for example, by incorporating a different, more pliable material into the rod sections  1306 ,  1308  in the area of hinge portion  1310 . Alternatively, hinge portion  1310  can be formed by simply using a thinner portion of material to allow bending. A notch or other deformation can also be included at hinge portion  1310  to promote bending in a particular location, as shown later in  FIG. 22 . 
         [0085]    Referring to  FIG. 14 , an alternate view of bone anchor  1300  is shown illustrating the cross-sectional shape of the rod sections  1306 ,  1308 . In some embodiments, the rod sections can form extensions of the probe components  1302 ,  1304 . For example, in  FIG. 14 , each rod section  1306 ,  1308  includes an external face  1402 ,  1404  that can have a convex shape, similar to the external faces of probe components  1302 ,  1304 , as discussed above. Rod sections  1306 ,  1308  can each further include an internal face  1406 ,  1408  that can be substantially linear, similar to the internal faces of the first and second probe components. However, in other embodiments, the rod sections, and even the probe components themselves, can have a variety of other cross-sectional shapes that complement each other and are configured to mate together. Any of these other cross-sectional shapes (e.g., half circles, interlocking ridges, diagonals, tongue and groove, etc.) are considered within the scope of the present invention. 
         [0086]      FIGS. 15 and 16  illustrate the internal surfaces of exemplary probe components  1302 ,  1304  with connected rod sections  1306 ,  1308 . With reference to  FIG. 15  in particular, probe component  1302  is very similar to the first probe component  102  discussed above. The probe component  1302  includes an internal surface  1406  and a guide shoulder  1502  configured to interface with probe component  1304 . Probe component  1304 , illustrated in  FIG. 16 , includes an internal surface  1408  and a mating edge  1602  configured to interface with internal surface  1406  and guide shoulder  1502  of probe component  1302 . 
         [0087]      FIG. 17  illustrates an embodiment of bone anchor  1300  implanted in a vertebrae  1702 . The probe components  1302 ,  1304  are implanted inside the vertebrae, and rod sections  1306 ,  1308  extend outside the vertebrae. Also shown is an exemplary embodiment of a crimp head  1704  that is configured to work with the bone anchor  1300 . An alternate perspective view of bone anchor  1300  and crimp head  1704  is shown in  FIG. 18  for clarity. 
         [0088]    Crimp head  1704  is shown in isolation in  FIG. 19 . Similar to the crimp head  300  discussed above, crimp head  1704  can include clamping arms  1902  to securely retain probe components  1302 ,  1304  in a fixed relationship with each other. Crimp head  1704 , however, can also include an additional set of clamping arms  1904  configured to retain the rod sections  1306 ,  1308  in a fixed relationship with each other. The sets of clamping arms  1902 ,  1904  can be connected by a supporting portion  1906 . Supporting portion  1906  can be configured to provide additional clamping force to the rod sections  1306 ,  1308 . In addition, supporting portion  1906  can be configured to provide stress relief and support for the hinge portion  1310  after implantation. 
         [0089]    The various embodiments disclosed herein can be combined as modular components to create a complete spinal fixation assembly requiring fewer components and less complicated procedures to implant within a patient. An exemplary spinal fixation assembly  2000  is illustrated in  FIG. 20 . The assembly  2000  includes two bone anchors  1300   a ,  1300   b  having connected rod sections  1306   a ,  1308   a ,  1306   b ,  1308   b . The anchors are secured by crimp heads  1704   a  and  1704   b . The rod sections are connected to additional bone anchors  100   a ,  100   b  by being seated in the fixation element receiving seats  128   a ,  128   b  and secured by crimp heads  300   a ,  300   b . The spinal fixation assembly  2000  can also serve as a foundation for attaching various other spinal fixation accessories, such as one or more transverse supporting members  2002 . Crimp heads  1704  can also be provided in transversely connected pairs, incorporating the transverse supporting feature. Transverse supporting members  2002  can be formed from any biocompatible material and, in exemplary embodiments, are formed from titanium or a titanium alloy. Many additional modular combinations of the components disclosed herein are possible and all are considered within the scope of the present invention. 
         [0090]    In addition, the bone anchors of the present invention can be easily configured to provide anchoring in any application requiring securing a component or tissue to bone. For example, an exemplary bone anchor  2100  of the present invention is shown in  FIG. 21  functioning as a suture anchor. Bone anchor  2100  includes first and second probe components  2102 ,  2104  similar to bone anchor  100  discussed above. First and second probe components  2102 ,  2104  each further include a suture receiving portion  2106 ,  2108 , which can be a bore formed through the probe component and configured to receive a threaded suture  2110  therethrough. Suture  2110  can then extend out of the cavity formed in bone  2112  and be utilized to accomplish any of a variety of tasks. In the embodiment illustrated in  FIG. 21 , the proximal heads of probe components  2102 ,  2104  can be retained in a fixed relationship with each other by a ring clip  2114 . Ring clip  2114  provides the same retaining function as the crimp heads and polyaxial receiving heads discussed above, but has a lower profile design that minimizes the protrusion of the bone anchor from the bone surface. 
         [0091]    The present invention also provides methods for using the bone anchors disclosed herein. Bone anchors of the present invention can be utilized in a variety of procedures, including open surgery and minimally invasive procedures. In minimally invasive procedures, each bone anchor component must be delivered to the surgical site through, for example, an appropriately sized and shaped port similar to the port  2202  illustrated in  FIG. 22 . In order to facilitate delivery through restricted spaces such as port  2202 , bone anchor components of the present invention can include features to reduce their size or area footprint. Exemplary features include the hinge portion  1310  discussed above and illustrated in  FIG. 22 . Hinge portion  1310  of bone anchor  1300  in  FIG. 22  utilizes a thinner portion of material and a pre-formed notch  2204  to allow the bone anchor to be collapsed as shown in the figure. Once at the surgical site, the rod sections  1306 ,  1308  can be rotated back into position to connect with other bone anchor components to form a spinal fixation assembly like that shown in  FIG. 20 . In addition, the probe components and attached rod sections can be delivered together, as shown in  FIG. 22 , or separately. 
         [0092]    Turning now to  FIG. 23 , a method of anchoring an implant to bone is provided that includes the step of inserting a first probe component or member into a cavity formed in bone [step  2302 ]. The probe member can be inserted straight into the cavity formed in bone, or along an arced path. 
         [0093]    In the embodiments discussed above, inserting the first probe member  102  first can be important, because the guide shoulder  124  can direct the second probe member as it is inserted. In some other embodiments, however, the first probe member may not include a guide shoulder  124 . In such embodiments, the divergence of the distal tips can be accomplished through the alignment and fixed relationship of the proximal heads of the probe members alone. In these embodiments, it does not matter whether the first probe member or the second probe member is inserted first. 
         [0094]    Following insertion of the first probe member, a second probe component or member can be inserted into the cavity formed in the bone adjacent to the first probe member [step  2304 ]. This can be accomplished, for example, by sliding the second probe member along an internal surface of the first probe member such that a mating edge of the second probe member interfaces with a guide shoulder formed on the internal surface of the first probe member. The guide shoulder can cause the distal tip of the second probe member to diverge from the distal tip of the first probe member as the second probe member is inserted into the bone cavity. 
         [0095]    Following insertion of both probe members, the proximal heads of the first and second probe members can be aligned [step  2306 ]. If the probe members include any connected rod sections, these can be aligned as well and rotated into position to connect with additional bone anchors. In addition, any other spinal fixation elements that need to be mated to the bone anchor can be seated, for example by seating the element in the fixation element receiving seat formed by the proximal heads of the first and second probe members. 
         [0096]    Finally, a crimp head can be applied to the aligned proximal heads of the first and second probe members to retain them in a fixed relationship with each other [step  2308 ]. The crimp head can further retain any spinal fixation elements seated in, for example, the fixation element receiving seat of a bone anchor. 
         [0097]    In embodiments that utilize an implant for fusing two vertebral bodies together, a polyaxial receiving head, or any other receiving head assembly having a closed lumen, steps  2302 - 2308  can be performed after positioning the implant, receiving head, or other receiving head assembly in position over a hole formed in the bone. Each probe member can then be inserted through both the implant, receiving head, or screw assembly, as well as the bone. Following insertion, and in place of step  2308  above, a set screw can be applied to retain the proximal ends of the probe members in a fixed relationship with each other and to retain a spinal fixation element to the bone anchor. 
         [0098]    Bone anchors of the present invention provide an additional benefit over prior art anchors in that they are easily removable following spinal fixation and natural healing. To remove the bone anchors, the steps of the method illustrated in  FIG. 23  are simply reversed. For example, any crimp head, polyaxial bone screw, implant set screw, or other accessory can be removed to free the first and second probe members from one another. The second probe member can then be removed from the cavity formed in the bone by rotating the second probe member such that its distal tip converges with the distal tip of the first probe member. This movement will disengage the bone engaging edge of the second probe member, thereby allowing its removal from the cavity. The first probe member can then be similarly repositioned to disengage its bone engaging edge and subsequently removed from the cavity. 
         [0099]    All papers and publications cited herein are hereby incorporated by reference in their entirety. One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.