Abstract:
Bone (e.g., spinal) fixation assemblies are provided that resist back out of associated bone anchors (e.g., screws). The bone fixation plate may house a bone anchor retaining member that permits partial passage of the bone anchor and subsequently resists back out of the bone anchor from the member and the bone fixation plate. In some embodiments, the bone fixation plate may include an attachable cover member that prevents the bone anchor retaining member from ejecting from the plate. In other embodiments, ejection of the anchor retaining member from the bone fixation plate may be prevented by walls integral to the plate.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to bone fixation plates, and more particularly, to bone (e.g., spinal) fixation plates that resist back out of associated bone anchors (e.g., screws). 
       BACKGROUND OF THE INVENTION 
       [0002]    The spine is a flexible, multi-segmented column that supports upright posture in a human while providing mobility to the axial skeleton. The spine encases and protects vital neural elements while providing structural support for the body by transmitting the weight of the body through the pelvis to the lower extremities. The cervical spine exhibits a wide range of motion due to the orientation of its facets and the lack of supporting structures. The thoracic and lumbar regions of the spine also have a significant range of motion. 
         [0003]    The spine is made up primarily of bone and intervertebral discs, which are surrounded by supporting ligaments, muscle, fascia, blood vessels, nerves, and skin. These elements are subject to a variety of pathological disturbances: inflammation, trauma, neoplasm, congenital anomalies, disease, etc. Trauma to the spine can play a large role in the etiology of neck and low back pain. For example, trauma frequently results in damage at the upper end of the lumbar spine, where the mobile lumbar segments join the less mobile dorsal spine. Excessive forces on the spine not only produce life-threatening traumatic injuries, but may contribute to an increased rate of degenerative change. 
         [0004]    The cervical region of the spine comprises the seven most superior vertebrae of the spine, which begin at the base of the skull and end at the upper torso. Because the neck has a wide range of motion and is the main support for the head, the neck is extremely vulnerable to injury and degeneration. 
         [0005]    Spinal fixation is a common method of treating spinal disorders, fractures, and degeneration. One common device used for spinal fixation is the bone fixation plate, which is typically used in conjunction with a graft device placed between the vertebral bodies. Generally, there are two types of spinal plates: (i) constrained plates and (ii) semiconstrained plates. Generally, a constrained plate completely immobilizes the vertebrae and does not allow for graft settling. In this instance, the plate itself carries a significant portion of the loading. Constrained plates are useful, for example, in patients with highly unstable anatomy, such as with a vertebrectomy, or in patients with little chance of bone growth, such as cancer patients. In contrast, a semiconstrained plate is dynamic and allows for a limited degree of graft settling through micro-adjustments made between the plate and bone screws attaching the plate to the spine. The operation of the semiconstrained plate stimulates bone growth because the loading is transferred through the graft. Each type of plate has its own advantages depending upon the anatomy and age of the patient, and the results desired by the surgeon. 
         [0006]    A typical bone fixation plate includes a relatively flat, rectangular plate having a plurality of apertures formed therein. A corresponding plurality of bone screws may be provided to secure the bone fixation plate to the vertebrae of the spine. A common problem associated with such a bone fixation plate is the tendency for bone screws to become dislodged from the bone and “back out” from the plate, thereby causing the plate to loosen and the screws to protrude from the plate. For example, in a typical anterior cervical fusion surgery, the carotid sheath and sternocleidomastoid muscles are moved laterally and the trachea and esophagus are moved laterally in order to expose the cervical spine. The cervical plate is designed to lie on the anterior face of the spine, dorsal to the esophagus. Due to its relative location to the esophagus and other connective tissue, if the bone screw securing the plate to the cervical spine backs out, the bone screw could pierce the esophagus, causing not only pain and infection, but also posing a serious risk of death to the patient. Bone fixation plates with large anterior-posterior profiles (e.g., thickness) can also make it difficult for the patient to swallow post-surgery. 
         [0007]    In view of the foregoing, it would be desirable to provide bone fixation assemblies that resist back out of associated bone anchors. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention relate to bone plating systems that resist back out of associated bone anchors. 
         [0009]    In an aspect, a bone fixation apparatus is provided that includes a bone fixation plate, a bone anchor (e.g., bone screw), and a bone anchor retaining member. The bone fixation plate includes a top surface, a bottom surface, and at least one aperture between the top surface and the bottom surface for permitting partial passage of a bone anchor through the plate. The bone fixation plate additionally includes a cavity formed between the top surface and the bottom surface, where the cavity at least partially intersects (e.g., is coaxial with) the aperture. The bone anchor retaining member is housed at least partially within the cavity, and is configured to transition from a first position to a second position in response to interaction with the bone anchor. In the first (e.g., open) position, the bone anchor retaining member permits partial passage of the bone anchor through the member. In the second (e.g., closed) position, the member resists back out of the bone anchor. 
         [0010]    In some embodiments, the bone anchor retaining member may be formed generally in the shape of a ring. In one embodiment, in the first position the bone anchor retaining member may have a generally convex shape (e.g., conical). In the second position, the bone anchor retaining member may have a generally concave shape (e.g., inverse conical). In another embodiment, the bone anchor retaining member may have a generally convex shape in both the first and second positions. 
         [0011]    In some embodiments, the bone anchor retaining member may include multiple surfaces (e.g., tabs) extending towards a center of the aperture formed within the bone fixation plate. In the second position, a distance between the plurality of surfaces may be less than a distance between the plurality of surfaces in the first position. 
         [0012]    In still other embodiments, the member may transition from the first position to the second position via a third position, wherein in the third position a distance between the plurality of surfaces is less than both the distance between the plurality of surfaces in the first position and the distance between the plurality of surfaces in the second position. 
         [0013]    In some embodiments, the bone anchor retaining member may be flat (e.g., a flat ring-shaped member) both before and after partial passage of the bone anchor through the member. In response to interaction of the retaining member with the bone anchor, the retaining member may deform in the direction of advancement of the anchor, thus increasing the size of an internal diameter of the member such that it allows partial passage of the anchor. The retaining member may then return to its original, flat configuration in order to resist back out of the anchor. 
         [0014]    In some embodiments, the bone fixation plate includes a generally part-spherical surface adjacent to the at least one aperture and the top surface, for example, for multi-angular articulation with a complimentary part-spherical surface of the bone anchor. 
         [0015]    In some embodiments, the bone fixation plate comprises a generally part-cylindrical surface adjacent to the at least one aperture and the bottom surface. 
         [0016]    In some embodiments, when the bone anchor is advanced fully into the plate, the width of the aperture in the bone fixation plate may be substantially equal to a width of an adjacent portion of the bone anchor. This may constrain movement of the fixation plate subsequent to the procedure (e.g., rigid fixation). In other embodiments, the width of the aperture may be greater than the width of the adjacent portion of the bone anchor, which may allow for dynamic movement of the fixation plate at an implantation site. 
         [0017]    In some embodiments, the width of the cavity in the bone fixation plate may be substantially equal to a width of the bone anchor retaining member, which may constrain movement of the fixation plate. In other embodiments, the width of the cavity may be greater than the width of the bone anchor retaining member, which may allow for dynamic movement of the fixation plate at an implantation site. 
         [0018]    In still other embodiments, the bone fixation plate may include multiple aperture and/or cavity sizes, which may allow the same plate to be used for both rigid and dynamic fixation, at the option of the surgeon. For example, the same bone fixation plate may include a set of apertures and/or cavities configured for rigid fixation, and an independent set of apertures and/or cavities configured for dynamic fixation. 
         [0019]    In some embodiments, the bone anchor may be a bone screw that includes a head, a shoulder in communication with the head, a groove in communication with the shoulder, and a threaded shank in communication with the groove. The head may be configured for articulation with the bone fixation plate. The shoulder may be configured to contact the bone anchor retaining member when the member is in the first position. The groove may be configured to receive the bone anchor retaining member when the member is in the second position. A width of the shoulder may be greater than a width of the threaded shank. 
         [0020]    In some embodiments, the bone anchor retaining member may be formed from an elastic material. 
         [0021]    In still other embodiments, the bone anchor retaining member may form a plurality of peaks and valleys in top and bottom surfaces of the member. The retaining member may be configured to transition from a first, convex position to a second, convex position in response to interaction of the member with the bone anchor. In the first position, the bone anchor retaining member may permit partial passage of the bone anchor through the member. In the second position, the retaining member may resist back out of the bone anchor. 
         [0022]    In some embodiments, the bone fixation plate may include an attachable cover member (e.g., ring-shaped cover member) that is substantially co-axial with the cavity and the ring-shaped member and that is configured to prevent the ring-shaped member from ejecting from the cavity. In some embodiments, the cover member forms the bottom surface of the bone fixation plate. 
         [0023]    In another aspect, a method for bone fixation is provided. The method includes advancing a bone anchor (e.g., screwing a bone screw) through a bone fixation assembly and into bone, and encountering resistance to the advancing before the bone anchor is advanced fully into the bone, where the resistance is attributable to the bone fixation assembly. The method additionally includes advancing the bone anchor into the bone to overcome the resistance, and subsequent to the further advancing, resisting back out of the bone anchor from the bone fixation assembly. In some embodiments, encountering resistance to the advancing includes contacting a portion of the bone fixation assembly with a shoulder of the bone anchor. 
         [0024]    In still another aspect, a bone fixation apparatus is provided that includes means housed within a bone fixation plate for resisting advancement of a bone anchor before the bone anchor is advanced fully into the bone. Upon further advancement of the bone anchor through the bone fixation plate, the means housed within the bone fixation plate further comprises means for resisting back out of the bone anchor from the bone fixation plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    For a better understanding of the present invention, including the various objects and advantages thereof, reference is made to the following detailed description, taken in conjunction with the accompanying illustrative drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0026]      FIG. 1A  is a perspective view of a bone fixation assembly including a bone fixation plate, at least one bone anchor (e.g., screw), and at least one member for retaining the bone anchor within the plate, according to some embodiments of the present invention; 
           [0027]      FIG. 1B  is a perspective view of another embodiment of a bone anchor in accordance with the present invention; 
           [0028]      FIG. 1C  is a top view of the bone anchor of  FIG. 1B ; 
           [0029]      FIG. 2  is an enlarged, cross-sectional view of the bone fixation assembly of  FIG. 1A , in which the bone anchor retaining member is in an open position for permitting passage of the anchor through the fixation plate and into bone; 
           [0030]      FIG. 3  is an enlarged, cross-sectional view of the bone fixation assembly of  FIG. 1A , in which the bone anchor retaining member is in a closed position for preventing back out of the anchor from the plate; 
           [0031]      FIG. 4  is an enlarged, perspective view of the anchor retaining member of  FIG. 1A  in the open position; 
           [0032]      FIG. 5  is an enlarged, perspective view of the anchor retaining member of  FIG. 1A  in the closed position; 
           [0033]      FIG. 6  is a diagram that illustrates changes to the width of an opening in the anchor retaining member of  FIG. 1A  due to a transition of the member from the open position to the closed position, according to some embodiments of the present invention; 
           [0034]      FIG. 7  shows placement of an anchor retaining member within a bone fixation plate according to an embodiment of the present invention; 
           [0035]      FIG. 8  is an enlarged, perspective view of another embodiment of an anchor retaining member in the open position according to the present invention; 
           [0036]      FIG. 9  is an enlarged, perspective view of the anchor retaining member of  FIG. 8  in the closed position; 
           [0037]      FIG. 10A  is a perspective view of another embodiment of a bone fixation assembly in accordance with the present invention; 
           [0038]      FIG. 10B  is an exploded view of the bone fixation assembly of  FIG. 10A ; 
           [0039]      FIG. 11  is an enlarged, perspective view of the anchor retaining member of  FIGS. 10A and 10B ; and 
           [0040]      FIGS. 12A-C  are cross-sectional views showing deformation and reformation of the anchor retaining member of  FIG. 11  in response to interaction with a bone anchor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 1A  is a perspective view of a bone fixation assembly  100  according to some embodiments of the present invention. Assembly  100  includes bone fixation plate  102 , bone anchors (e.g., screws)  104 , and one or more (e.g., six) installations of anchor retaining member  106 . For example,  FIG. 1A  shows a two-level bone fixation plate  102  that is configured to span across and fixate three vertebrae of the cervical spine. In this embodiment, plate  102  includes six anchor retaining members  106 . In other embodiments, single-level plates and other multi-level plates may be provided with different numbers of anchor retaining members  106 . 
         [0042]    Bone fixation plate  102  may form a plurality of apertures  108  (e.g., six circular or part-circular apertures in the embodiment of  FIG. 1A ) for permitting passage of a corresponding plurality of bone anchors  104  through plate  102  and into bone. Bone fixation plate  102  may additionally form one or more cavities  110  (e.g., cylindrical or part-cylindrical cavities) for housing a corresponding one or more anchor retaining members  106  (e.g., rings). For example, in  FIG. 1A , bone fixation plate  102  forms a cavity  110  adjacent to each aperture  108 . In other embodiments, only a portion of apertures  108  may have cavities  110  and anchor retaining members  106  adjacent thereto. As described more fully in connection with  FIGS. 2-6 , the relative positions of aperture  108  and cavity  110  and the configuration of anchor retaining member  106  and bone anchor  104  may cause member  106  to move from an open position to a closed position responsive to passage of bone anchor  104  through aperture  108 . 
         [0043]    Bone anchors  104  may be configured at their distal ends  112  for self-tapping or self-drilling. Proximal ends  114  (heads) of bone anchors  104  may include a recess (e.g., having a non-circular cross-sectional shape) and/or other features for receiving a complimentary tip of a surgical tool. For example, in the embodiment of  FIG. 1A , bone anchor  104  includes a hex-shaped feature formed within its proximal end  114 .  FIGS. 1B and 1C  show another embodiment of a bone anchor that includes multiple (e.g., three) prongs  115  along the perimeter of its proximal end and a central, non-circular recess  117  for receiving a surgical tool. In some embodiments, the bone anchor shown in  FIGS. 1B and 1C  may be similar to or the same as the bone anchor of  FIG. 1A  in all other respects. 
         [0044]    Bone fixation plate  102  may also form apertures  116  and slots  118  in communication with cavities  110 , and indentations  120 . Apertures  116  and indentations  120  may be configured for attachment to a delivery tool that positions plate  102  at an appropriate implantation site. In some embodiments, apertures  116  and/or slots  118  may permit access to anchor retaining members  106  to permit passage of a tool that re-opens/re-inverts members  106  from the closed position to the open position. In some embodiments, to re-open (unlock) member  106  after bone anchor  104  is screwed into place, bone anchor  104  may be unscrewed or otherwise backed out (e.g., about 1 thread turn), followed by introduction of a tool through a slot  118  to invert/open member  106 . In other embodiments, member  106  may be re-opened by a tool that is secured to plate  102  and bone anchor  104 . The tool may incorporate a member (e.g., trigger actuated member) that pushes against the top side of plate  102  while the tool is secured to and exerting an upward force on anchor  104 , thus forcing anchor  104  upwardly and causing retaining member  106  to invert and open. 
         [0045]    Bone fixation assembly  100  and its various components may be made from any suitable material or combination of materials. For example, in some embodiments, all of components  102 ,  104 , and  106  may be made from titanium, stainless steel, and/or other biocompatible metal(s). In other embodiments, one or more (e.g., all) of components  102 ,  104 , and  106  may be made from a polymer or one or more biocompatible ceramic(s), such as the doped silicon nitride ceramic described in commonly-owned U.S. Pat. No. 6,881,229, which is hereby incorporated by reference herein in its entirety. The one or more materials used for anchor retaining member  106  preferably have an elastic property. 
         [0046]    In some embodiments, bone fixation plate  102  may have a lordotic curvature that corresponds to a lordotic curvature of the human cervical spine. For example, an anterior face of plate  102  may be contoured and rounded so as to reduce or eliminate irritation of the esophagus and the surrounding tissues. 
         [0047]    In some embodiments, bone fixation plate  102  may be configured to promote bone ingrowth to the plate. For example, in some embodiments, at least a portion of bone fixation plate  102  may be made from a porous material, such as the porous silicon nitride ceramic described in commonly-owned U.S. Pub. Appln. No. 20050049706, which is hereby incorporated by reference herein in its entirety. Alternatively or additionally, one or more bone contacting surfaces of bone fixation plate  102  may be roughened, for example, by mechanical blasting and/or plasma spraying with metal particles of one or more sizes. 
         [0048]    In some embodiments, bone fixation plate  102  may be coated with a bio-active material having an osteoconductive property, such as hydroxyapatite or a calcium phosphate material. Alternatively or additionally, bone fixation plate  102  may carry one or more therapeutic agents, for example, for enhancing bone fusion and ingrowth. Examples of such therapeutic agents include natural or synthetic therapeutic agents such as bone morphogenic proteins (BMPs), growth factors, bone marrow aspirate, stem cells, progenitor cells, antibiotics, and other osteoconductive, osteoinductive, osteogenic, bio-active, or any other fusion enhancing material or beneficial therapeutic agent. In some embodiments, bone anchor  104  and/or anchor retaining member  106  may be porous, roughened, and/or coated with one or more bio-active and/or therapeutic materials. 
         [0049]      FIG. 2  is an enlarged, cross-sectional view of bone fixation plate  102 , bone anchor  104 , and anchor retaining member  106 , in which member  106  is in an open position within cavity  110 . Cavity  110  is located between (e.g., about midway between) top surface  202  and bottom surface  204  of bone fixation plate  102 . Cavity  110  may at least partially intersect aperture  108  formed by surfaces  206  and  208  of plate  102 . For example, cavity  110  may be coaxial with aperture  108 , although cavity  110  may be wider than aperture  108  (e.g., the portion of aperture  108  formed by the innermost portion of surface  206 ). Surface  206  of bone fixation plate  102  may form a part-spherical seat configured for multi-angular articulation with a complimentary part-spherical surface  210  of bone anchor  104 . Surface  208  may be generally cylindrical and may have a diameter that is wider than the threads of shank  212  of bone anchor  104 . Bone anchor  104  may include shoulder  214 , and groove  216  for receiving anchor retaining member  106  when member  106  is in a closed position. 
         [0050]    In the open position, anchor retaining member  106  may form an opening having a width greater than width  218  of threaded shank  212 , where width  218  is equal to the major diameter of the threads. In other embodiments, the width of the opening within anchor retaining member  106  in the open position may be greater than (e.g., only slightly greater than) the width of the minor diameter of the threads, such that the screw is threaded through the anchor retaining member  106 . The width of aperture  108  formed by surfaces  206  and  208  may also be greater than width  218 . This may allow threaded shank  212  to pass through bone fixation plate  102  and anchor retaining member  106  and into bone. Shoulder  214  of bone anchor  104  may be wider than the opening in member  106 , which may cause shoulder  214  to contact member  106  as anchor  104  is screwed into the bone. For example, shoulder  214  may contact anchor retaining member  106  just prior (e.g., less than about 1 screw turn prior) to bone anchor  104  being fully screwed into the bone. Additional screwing of bone anchor  104  into the bone after the occurrence of such contact may cause anchor retaining member  106  to transition from the open position to the closed position. In some embodiments, providing anchor retaining member  106  within plate  102  causes member  106  to interact with shoulder  214  and not the top of screw head  114  ( FIG. 1A ). Advantageously, this may prevent member  106  from, for example, interfering with a tool that screws and/or unscrews bone anchor  104  into and/or out of plate  102 . 
         [0051]      FIG. 3  is an enlarged, cross-sectional view of the bone fixation assembly of  FIG. 1A , in which bone anchor retaining member  106  is in the closed (locked) position. In the closed position, the width of the opening in anchor retaining member  106  may be reduced to less than a width of threaded shank  212  (e.g., less than major diameter  218  or the minor diameter of the threads), which may prevent threaded shank  212  from backing out of member  106  and thus member  102 . For example, the width of the opening may be slightly greater than the width of groove  216  formed in bone anchor  104 . In some embodiments, the width of the inner most part of surface  206  in bone fixation plate  102  may be approximately equal to the width of shoulder  214  of bone anchor  104 . This may prevent lateral movement of bone anchor  104  within plate  102  and cause rigid fixation between surface  206  of plate  102  and surface  210  of anchor  104 . In other embodiments, the width of the inner most part of surface  206  may be greater than the width of shoulder  214 . This may allow for movement of bone anchor  104  within plate  102  and dynamic articulation of surfaces  206  and  210 . 
         [0052]      FIG. 4  is an enlarged, perspective view of anchor retaining member  106  in the open position. In some embodiments, member  106  may be generally ring-shaped. Member  106  may include one or more (e.g., three) tabs  402  that form the opening (e.g., part-circular opening) allowing for passage of threaded shank  212  ( FIG. 2 ). Tabs  402  may slope upwardly (e.g., conically) from outer surface  404  of member  106  towards the center of member  106 . Thus, in the open position, member  106  may be convex. In the embodiment of  FIG. 4 , anchor retaining member  106  is a single piece of solid, although elastic, construction. In other embodiments, member  106  may be formed from multiple pieces. In the open position, the opening formed by tabs  402  may have width  406  (shown generally as the distance between surfaces  408  and  410  of tabs  402 ), which may be greater than major diameter  218  of threaded shank  212  or greater than the minor diameter of the threads. Tabs  402  may be configured to contact shoulder  214  of bone anchor  104  and to resist further advancement of anchor  104  into the bone, prior to surface  210  of anchor  104  being seated within surface  206  of bone fixation plate  102 . Such resistance may be overcome by further advancement (e.g., screwing) of bone anchor  104  into the bone. 
         [0053]      FIG. 5  is an enlarged, perspective view of anchor retaining member  106  in the closed position. Tabs  402  may slope downwardly (e.g., inverse conically) from outer surface  404  of member  106  towards the center of member  106 . Thus, in the closed position, member  106  may be concave. The opening formed by tabs  402  may have reduced width  502 , which may be less than both width  406  formed by tabs  402  in the open position and a width of threaded shank  212  (e.g., less than major diameter  218  or the minor diameter of the threads). Width  502  is shown as the reduced distance between surfaces  408  and  410  of tabs  402 . This reduced inner dimension of member  106  may prevent threaded shank  212  from backing out of member  106  and thus plate  102 . 
         [0054]      FIG. 6  is a diagram that illustrates changes to the width of the opening in anchor retaining member  106  during a transition of the member from the open position to the closed position. More specifically,  FIG. 6  is an enlarged, side view of the path of surfaces  408  and  410  of tabs  402  during the transition, where the distance between surfaces  408  and  410  represents the width of the opening. In some embodiments, the paths traversed by surfaces  408  and  410  may form an hourglass-shape. In the open position, tabs  402  may form a generally convex shape and surfaces  408  and  410  may be separated by distance  406 . Distance  406  may be greater than a width of threaded shank  212  (e.g., greater than major diameter  218  or the minor diameter of the threads). At an intermediate position along the paths, tabs  402  may be generally parallel and surfaces  408  and  410  may be separated by distance  602 . Distance  602  may be less than the width of threaded shank  212 , and substantially equal to or greater than the width of groove  216  in bone anchor  104 . In the closed position, tabs  402  may form a generally concave shape and surfaces  408  and  410  may be separated by distance  502 . Distance  502  may be less than a width of threaded shank  212  (e.g., less than major diameter  218  or the minor diameter of the threads) but greater than distance  602 . An elastic property of retaining member  106  may be sufficiently low enough to allow member  106  to invert (e.g., from the open to the closed position) responsive to the mechanical force exerted by the interaction of the anchor and the plate, while being sufficiently high enough to resist inverting responsive to the physiological forces imposed upon the assembly in vivo. 
         [0055]      FIG. 7  shows placement of an anchor retaining member  106  within cavity  110  of bone fixation plate  102  during manufacturing according to an embodiment of the present invention. As shown, retaining member  106  may be assembled into plate  102  through the bottom side of the plate. Bore  208  may include a lead-in angle or radius  702  to guide member  106  into cavity  110 . Bore  208  may be slightly smaller than the outside diameter of retaining member  106 , and may have a defined edge  704  to ensure that the retaining member  106  cannot easily slip out. Retaining member  106  may utilize adjacent surface  706  to provide opposing force when shoulder  214  ( FIG. 2 ) engages member  106 . Tool  708  may use a center bore of retaining member  106  to guide member  106  into cavity  110 . To allow passage of member  106  through bore  208 , the surfaces of plate  102  that form bore  208  may apply axial force to member  106 , thus causing member  106  to collapse slightly. When retaining member  106  reaches cavity  110 , an elastic property of member  106  may cause it to expand into and become captured within cavity  110  of plate  102 . 
         [0056]      FIG. 8  is an enlarged, perspective view of another embodiment of an anchor retaining member  802  in an open position according to the present invention. Anchor retaining member  802  may include a plurality of tabs  802  (e.g., 8 tabs) that form the opening allowing for passage of threaded shank  212  ( FIG. 2 ). Each of tabs  804  may be formed by a pair of arms  806  that slope upwardly from an outer surface toward the center of the opening. This upwardly sloping characteristic of arms  806  may form a generally convex, crinkle-shaped surface with a plurality of peaks and valleys in the top and bottom surfaces of anchor retaining member  802 . In the embodiment of  FIG. 8 , anchor retaining member  802  is a single piece of solid, although elastic, construction. In other embodiments, member  802  may be formed from multiple pieces. In the open position, the opening formed by opposed tabs  804  may have width  808 , which may be greater than width  218  of threaded shank  212 . Tabs  804  may be configured to contact shoulder  214  of bone anchor  104  and to resist further advancement of anchor  104  into the bone, prior to surface  210  of anchor  104  being seated within surface  206  of bone fixation plate  102 . Such resistance may be overcome by further advancement (e.g., screwing) of bone anchor  104  into the bone. 
         [0057]      FIG. 9  is an enlarged, perspective view of the anchor retaining member of  FIG. 8  in the closed position. Generally, in the closed position, anchor retaining member  802  may have a more flattened, albeit still generally convex, topology. Arms  806  that support tabs  804  may still slope upwardly from the outer surface of member  802  towards the center of member  106 , although the magnitude of the slope may be reduced. The opening formed by tabs  804  may have reduced width  902 , which may be less than both width  808  formed by tabs  804  in the open position and width  218  of threaded shank  212 . Width  902  is shown as the reduced distance between two opposed tabs  804 . This reduced inner dimension of member  802  may prevent threaded shank  212  from backing out of member  802  and thus plate  102 . In some embodiments, the procedure for inserting member  802  into bone plate  102  during manufacturing may be the same as the procedure described above in connection with  FIG. 7 . 
         [0058]      FIG. 10A  is a perspective view of another embodiment of a bone fixation assembly  1000  in accordance with the present invention.  FIG. 10B  is an exploded view of the bone fixation assembly of  FIG. 10A . Bone fixation assembly  1000  includes bone fixation plate  1002 , one or more (e.g., four) anchor retaining members  1004 , one or more (e.g., four) cover members  1006  that may be substantially co-axial with members  1004 , and one or more bone anchors (not shown). In this example, assembly  1000  is a single-level assembly that spans two vertebral bodies, although other multi-level configurations may be provided in other embodiments. Each cover member  1006  may prevent a corresponding retaining member  1004  from ejecting from a cavity within bone fixation plate  1002 . Generally, once an anchor retaining member  1004  is loaded into the cavity formed within plate  1002  and cover member  1006  is fixed to the assembly, the cross-section of assembly  1000  may be similar to the cross-sections shown in  FIGS. 2 and 3 , with the exception that member  1004  may be flat when viewed from the side. Each cover member  1006  may be fixed within bone fixation plate  1002  by any suitable approach including, for example, welding, bonding, and/or a threaded connection. In other embodiments, retaining member(s) with different cross-sectional characteristics (e.g., the retaining members shown in  FIGS. 4 ,  5 ,  8 , and  9 ) may be used in connection with bone fixation plate  1002  and cover member(s)  1006 . In the embodiment shown in  FIGS. 10A and 10B , cover member  1006  forms the bottom surface of bone fixation plate  1002  once it is fixed to the plate. In other embodiments, cover member  1006  may form the top surface of the bone fixation plate. 
         [0059]      FIG. 11  is an enlarged, perspective view of anchor retaining member  1004 . Anchor retaining member  1004  may be flat (e.g., a flat ring-shaped member) when viewed from the side both before and after partial passage of the bone anchor through the member. In response to interaction of retaining member  1004  with the bone anchor, retaining member  1004  may deform in the direction of advancement of the anchor, thus increasing the size of an internal diameter  1102  of the member (shown generally as the distance between tabs  1104  and  1106 ) until it allows partial passage of the anchor. Retaining member  1004  may then return to its original, flat configuration in which it resists back out of the anchor. In the embodiment of  FIG. 11 , anchor retaining member  1004  is a single piece of solid, although elastic, construction. In other embodiments, member  1004  may be formed from multiple components. 
         [0060]      FIGS. 12A-C  are cross-sectional views showing deformation and reformation of anchor retaining member  1004  in response to interaction with bone anchor  1202 . In the embodiment shown in  FIGS. 12A-C , anchor retaining member  1004  is held in place by walls that are integral to plate  1204  (e.g., a plate similar to if not the same as plate  102  (FIG.  1 A)), although placement of member  1004  within other types of bone fixation plates is of course possible (e.g., the plate shown  FIG. 10A ). As shown in  FIG. 12A , anchor retaining member  1004  may be flat prior to interaction with shoulder  1206  of bone anchor  1202 . In some embodiments, shoulder  1206  may have a ramped (e.g., conical) configuration with its diameter increasing in the direction opposite to the direction of advancement of the anchor.  FIG. 12B  shows that interaction of shoulder  1206  with the tabs (e.g., including but not necessarily limited to tabs  1104  and  1106 ) of member  1004  causes the tabs to deform in the direction of advancement of the anchor.  FIG. 12C  shows that anchor retaining member  1004  may return to its flat configuration once shoulder  1206  passes through tabs  1104  and  1106 . The tabs may then rest within groove  1208  of bone anchor  1202 , which may be formed in bone anchor  1202  between shoulder  1206  and anchor head  1210 . The tabs of member  1004  may resist back out of bone anchor  1202  when they are positioned within groove  1208 . 
         [0061]    Thus it is seen that bone fixation plates with anchor retaining members are provided. Although particular embodiments have been disclosed herein in detail, this has been done by way of example for purposes of illustration only, and is not intended to be limiting with respect to the scope of the appended claims, which follow. In particular, it is contemplated that various substitutions, alterations, and modifications may be made without departing from the spirit and scope of the invention as defined by the claims. Other aspects, advantages, and modifications are considered to be within the scope of the following claims. The claims presented are representative of the inventions disclosed herein. Other, unclaimed inventions are also contemplated. The applicant reserves the right to pursue such inventions in later claims.