Abstract:
A cervical plate and one or more locking assemblies that help prevent screw backout without impinging on therapeutically valuable settling of the screws. In some cases, the locking assembly is configured to be permanently attached to the plate, to be securely but efficiently locked, to be readily unlocked for revision surgery, and/or to reduce the possibility of operator error in installation by providing simplified visible and tactile indicia of the locked and unlocked positions.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/077,508 for “Polyaxial Bone Plate and Locking Assembly” filed Nov. 10, 2014, which is incorporated in this application in its entirety by this reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This application relates to implantable internal fixator assemblies for use in stabilizing and supporting the spine. 
       BACKGROUND OF THE INVENTION 
       [0003]    Broken bones heal naturally, albeit slowly compared to most soft tissue, provided they are adequately supported and relieved of stress. In a simple break in an extremity, adequate support and relief may be provided from outside the body with a device as simple as a splint or a cast, which immobilizes the body part containing the broken bone. Such procedures may suffice when the bone can be set and will retain its position without significant intervention, for instance when the break is simple and contained in a body part that can be readily immobilized in a natural posture. Immobilization is also therapeutic to treat damage to connective tissue, by preventing repetitive stress and further injury to, for instance, damaged ligaments, tendons, or cartilage. 
         [0004]    When a break or fracture is in the spine, or when the connective tissue between one or more vertebrae is damaged, external immobilization is significantly less effective for several reasons. Because the spine is the central support column of the human body, externally imposed immobilization is impractical, as it involves immobilizing most of the body. Furthermore, the spine is a load-bearing structure that is subject to repetitive compressive and rotational stresses constantly during the normal waking life of a person; therefore, external immobilization of the spine significantly impacts the mobility and activity of a patient. For practical purposes, externally imposed spinal immobilization often requires that the patient is subjected to bed rest, is wheelchair-bound, is fitted with a significant amount of uncomfortable stabilizing equipment, or a combination of the above. 
         [0005]    Since the advent of sterile surgery, it has been possible for doctors to internally stabilize broken bones and connective tissue with implants. Internal stabilization can be complex, but tends to allow much greater precision in aligning broken bones, and significantly reduces misalignment in healing. Internal stabilization also improves healing time and allows a patient to live a much more normal life while still healing. One such type of implant is a bone plate, which is a shaped rigid or semirigid part usually having several through-holes by which a surgeon will attach the plate to parts of a broken bone, or to parts of two or more proximate bones that require alignment, by means of screws. All such parts are formed of biocompatible materials and may either be left in the body during and after healing, or may be removed after healing. Ideally, bone plates would be painstakingly formed and attached in several directions, so that the plate would conform perfectly to the patient&#39;s body, and would be secured to the bone or bones with an optimal balance of minimal tissue damage and maximal rigidity. In practice, the fact that such devices must be attached in surgery restricts the amount of time and the amount of access to the bone, such that physicians require such devices to attach efficiently and primarily from one direction. 
         [0006]    Another significant challenge to the use of bone plates is the stress placed on the bone by the tightening of the bone screws. Ordinary screws in other fields may be held fast to a surface by the friction between the screw head and the outer surface of the attached part, by friction between the screw threads and the material, or a combination. However, the force generated by tightening screws to achieve such friction in bone may cause excessive damage, and the healing of the bone over time in combination with the motion of the body may act to gradually force the bone screw from its position. Therefore, bone plate implants may require an assortment of apparently contradictory features including but not limited to additional locks to prevent the extrusion of the bone screw from the bone and plate, attachment that is both secure and that provides some wiggle-room, attachment that is very quick but also very secure or conforming, and/or other features. 
         [0007]    Anti-backout mechanisms on bone plates tend to suffer a variety of drawbacks. Parts of conventional anti-backout mechanisms, for instance screws and washers, tend to be small and delicate, and can be broken during installation or lost by the surgeon within the surgical wound. In conventional bone plates that possess internal anti-backout mechanisms, securing the mechanism may require specialized tools, or it may be difficult to ascertain whether the anti-backout mechanism has been fully engaged. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The terms “invention,” “the invention,” “this invention” and “the present invention” used in this patent are intended to refer broadly to all of the subject matter of this patent and the patent claims below. Statements containing these terms should be understood not to limit the subject matter described herein or to limit the meaning or scope of the patent claims below. Embodiments of the invention covered by this patent are defined by the claims below, not this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification of this patent, any or all drawings and each claim. 
         [0009]    Improved bone plates and locking assemblies for immobilizing vertebral bodies in the spine are disclosed herein. The plate is installed in one or more vertebrae with bone screws through a plurality of through-holes in the plate, which are then secured by a locking element or locking assembly. Specifically, the disclosed bone plates and locking assemblies are both secure and capable of rapid installation, being designed for ease of installation and use, with minimal moving parts that could potentially become broken or lost within the surgical wound during installation. 
         [0010]    Some examples of the assembly include a bone plate that has one or more screw holes for bone screws and, additionally, one or more holes for connecting a locking assembly to the bone plate. The bone plate can include one or more counter-bores, with each counter-bore being associated with a through-hole. One or more surface features may be included within each counter-bore and configured to interact with features of the locking assembly. The locking assembly may be one or more parts, and may include a lock that may be rotated between two distinct positions, namely: unlocked and locked. The lock, in the locked position, is configured to mechanically obstruct bone screws from backing out as the body part moves and as the underlying bone heals. 
         [0011]    Another example includes a bone plate having multiple through-holes for attaching multiple locking assemblies to the bone plate, and multiple screw holes or groups of screw holes. Each individual or group of screw holes is associated with a locking assembly, such that each respective lock of each locking assembly can obstruct regions above each respective screw hole or groups of screw holes. In some cases, the number of screw holes in each group can be one, two, or more than two screw holes. In some cases, the number of locking assemblies and associated screw-holes and/or screw hole groups can be one, two, three, or more. In some cases, one number of screw holes can be associated with a particular locking assembly in a plate, and a different number of screw holes can be associated with a different locking assembly in the same plate, according to a surgical need. In some cases, a bone plate can include two locking assemblies, each arranged at an end of a bone plate, with each locking assembly having two associated screw holes. In some cases, a bone plate can include three locking assemblies arranged linearly with respect to one another, each locking assembly being associated with two screw holes. 
         [0012]    Another example includes a bone plate having at least one through-hole for attaching a locking assembly, at least one screw-hole associated with that locking assembly, and a surface feature associated with each locking assembly. Each locking assembly includes a lock having a lock head and a locking feature. The lock head is configured to obstruct a screw hole when rotated into a locked position, and is configured not to obstruct the screw hole when rotated into an unlocked position. The locking feature is configured to interact with the surface feature to create two stable positions of the lock, where one of the stable positions is the unlocked position and the other stable position is the locked position. In some cases, the locking assembly can include an additional locking ring that interacts with the surface feature and with the locking feature to create the locked and unlocked positions, but in some other cases, the locking feature interacts directly with the plate in the absence of a locking ring. 
         [0013]    In some cases, the locking assembly incorporates interacting elements of both the bone plate and the part or parts making up the lock, such that the entire assembly is relatively simple, and so that the plate and lock may be installed as a single piece, without risk of parts dislodging or becoming lost in the surgical wound. The locking mechanism is simple to use, fast, and does not necessarily require any specialized equipment to operate. Moreover, the lock is secure against coming undone accidentally, overtightening, and/or accidental disassembly within the surgical wound. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a top view of a bone plate according to an example; 
           [0015]      FIG. 2  is a perspective view of a locking ring configured to be received by the bone plate of  FIG. 1 ; 
           [0016]      FIG. 3  is perspective view of a lock configured to be received by the bone plate and locking ring of  FIGS. 1 and 2 , respectively; 
           [0017]      FIG. 4  is a bottom view of the lock of  FIG. 3 , shown assembled with the locking ring of  FIG. 2 ; 
           [0018]      FIG. 5A  is a top view of an assembled apparatus including a bone plate, a locking ring, a lock, and bone screws, shown in the unlocked position; 
           [0019]      FIG. 5B  is a top view of the assembled apparatus of  FIG. 5A , shown in the locked position; 
           [0020]      FIG. 6  is a perspective view of a bone plate according to a second example; 
           [0021]      FIG. 7  is a perspective view of a lock configured to be received by the bone plate shown in  FIG. 6 ; 
           [0022]      FIG. 8A  is a top view of an assembled apparatus including the bone plate of  FIG. 6  and the lock of  FIG. 7 , with bone screws, shown in the unlocked position; 
           [0023]      FIG. 8B  is a top view of the assembled apparatus of  FIG. 8A , shown in the locked position; 
           [0024]      FIG. 9  is a top view of a bone plate according to a third example; 
           [0025]      FIG. 10  is a perspective view of the bone plate of  FIG. 9 ; 
           [0026]      FIG. 11  is a perspective view of a lock configured to be received by the bone plate of  FIGS. 9-10 ; 
           [0027]      FIG. 12  is a perspective view of an assembled apparatus including the bone plate of  FIGS. 9-10  and the lock of  FIG. 11 , shown in the locked position; 
           [0028]      FIG. 13A  is a top view of the assembled apparatus of  FIG. 12 , with bone screws, shown in the unlocked position; 
           [0029]      FIG. 13B  is a top view of the assembled apparatus of  FIG. 13A , shown in the locked position; and 
           [0030]      FIG. 14  is a top view of a partially assembled bone plate for receiving three exemplary locking assemblies, showing locks in both the unlocked and locked positions. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The subject matter of embodiments of the present invention is described here with specificity to meet statutory requirements, but this description is not necessarily intended to limit the scope of the claims. The claimed subject matter may be embodied in other ways, may include different elements or steps, and may be used in conjunction with other existing or future technologies. This description should not be interpreted as implying any particular order or arrangement among or between various steps or elements except when the order of individual steps or arrangement of elements is explicitly described. 
         [0032]    This patent discloses polyaxial bone plates and locking mechanisms that are configured for immobilization of vertebral bodies via fixation to surfaces thereof, with features for preventing screw backout while minimizing certain risks and the time required for surgical installation. 
         [0033]    As shown in the Figures, a bone plate includes a plurality of through-holes for receiving bone screws. In a polyaxial bone plate, the through-holes are configured to seat bone screws in a variety of directions. The locking mechanism disclosed herein may be applied to monoaxial or polyaxial bone plate designs. The locking mechanism is configured to partially obstruct a region above at least a portion of the head of a bone screw and prevent the screw from inadvertently backing out of the through-hole. In some cases, the locking mechanism includes a plate and a lock, which may be one or multiple parts including a shaft, one or more locking features that restrain movement of the locking mechanism, and a noncircular head element that cooperates with the screw heads to form the partial obstruction described above. When assembled with the bone plate, the lock is seated in a through-hole in the bone plate adjacent to one or more of the screw holes. As illustrated, the lock sits adjacent to and between two screw holes; however, a lock may be configured to secure one, two, or more than two screws without deviating from the design principles herein disclosed. Any part herein disclosed may be composed of any material or combination of materials that is biocompatible and sufficiently rigid to perform the part&#39;s function. 
         [0034]      FIG. 1  shows a bone plate  100  from a top view, with the superior surface  102  visible. The opposing inferior face  104  is configured to attach to two or more vertebrae by a plurality of bone screws that are each inserted through a through-hole  116  of the bone plate  100 . Bone plate  100  includes four through-holes  116  for bone screws, disposed in the lobes  118  of the bone plate  100  at the corners. The through-holes  116  for bone screws may be oriented in a polyaxial configuration to better contact and secure the bone plate  100  to vertebral bodies, and may be configured to permit a limited degree of freedom of motion of the bone screws once installed. One form of a polyaxial configuration can include tilting the through-holes  116  with respect to the long axis  108  of the bone plate  100 , such that bone screws inserted therethrough would point inward toward one another, or alternatively outward away from one another. Another form of a polyaxial configuration can include tilting the through-holes  116  with respect to the short axis  110  of the bone plate  100 . In some cases, through-holes  116  can be oriented in a symmetrical arrangement or in an asymmetrical arrangement relative to one another, and each through-hole  116  may be tilted along one or both of the long axis  108  and short axis  110 . An additional form of a polyaxial configuration can include providing sufficient clearance between the through-holes  116  and the associated bone screws that the screws can be seated in the through-holes at various angles of entry according to a medical need, as determined by a physician performing the installation. As described in more detail below, a lock  300  ( FIG. 3 ) is designed so that it does not radially impinge the bone screws and so that it allows a degree of freedom of movement as the screw threads seat in the bone and as the bone heals around them. 
         [0035]    Bone plate  100  also includes one or more locking assembly through-holes  112  for receiving the lock  300  ( FIG. 3 ) that are located between the through-holes  116  for the bone screws. Each of the locking assembly through-holes  112  has a counter-bore  114  on the superior surface and one or more surface features  120 ,  122  disposed within the counter-bore  114  that are configured to interact with the locking ring  200  ( FIG. 2 ) and/or the lock  300  ( FIG. 3 ), as described in more detail below. In some cases, a second counter-bore (not shown) is included on the inferior face  104  around the locking assembly through-hole  112  for accommodating the attachment of the lock  300  ( FIG. 3 ). 
         [0036]    As shown in  FIG. 1 , the counter-bore  114  in the superior surface  102  of the bone plate  100  surrounds the locking assembly through-hole  112  and has two surface features, a large surface feature  120  and small surface feature  122  extending upward for interacting with a locking ring  200  ( FIG. 2 ) and a lock  300  ( FIG. 3 ). The large surface feature  120  extends toward the superior surface  102  from the counter-bore  114 . This large surface feature  120  may also be herein referred to as a rotational stop. Opposite from the rotational stop  120 , the counter-bore  114  has a smaller surface feature  122  that extends into the counter-bore  114  for helping secure the locking ring  200  ( FIG. 2 ). In some cases, the surface features  120 ,  122  are oriented opposite one another and along the long axis  108 , but in other cases, the surface features  120 ,  122  can have other orientations, such as parallel to the short axis  110  of the bone plate  100 . 
         [0037]      FIG. 2  shows a locking ring  200  configured for use with a bone plate such as the bone plate  100  shown in  FIG. 1 . The locking ring  200  is a generally circular element with at least one retention feature, which may include a break  208  and a notch  210 , such that the locking ring  200  can elastically deform when subjected to a radial load. The locking ring  200  is sized and shaped to fit within the counter-bore  114  on the superior surface  102  of the bone plate  100 , and is oriented such that the break  208  rests about the larger surface feature  120 , and the notch  210  fits about the smaller surface feature  122 . The notch  210  alters the elastic stiffness of the locking ring, and the depth and width of the notch  210  can be varied to tune the stiffness. In an unflexed position, the locking ring  200  may sit in contact with one or more of the surface features  120 ,  122  and in contact with the surface of the counter-bore  114 , but cannot freely rotate from its original orientation. The interior radial surface  212  of the locking ring  200  may be non-cylindrical, having concave depressions  216 ,  216 ′ situated at regular intervals with small peaks  214  between them, which may be disposed at various increments about the interior surface, for example, at approximately 45-degree increments. The locking ring  200  may also have depressions or surface features in the outer radial surface that may be configured to alter the stiffness of the locking ring. The interior concave depressions  216 ,  216 ′ in the locking ring  200  may have different depths in an alternating fashion, although they need not. Alternatively, the interior radial surface  212  of the locking ring  200  may have concave depressions situated at approximately 90-degree increments about a cylindrical radial surface, or it may have an irregular internal radius with local maxima disposed at intervals, for example approximately 90-degree or 45-degree intervals. 
         [0038]      FIG. 3  shows a lock  300  that is configured to mate with a bone plate such as the bone plate  100  shown in  FIG. 1  and with a locking ring such as the locking ring  200  shown in  FIG. 2 . Lock  300  includes a generally oval head section  302  with an overhang  310 , a circular connecting shaft  308 , and a shaft section  304  disposed between the head section  302  and the shaft  308 . When the lock  300  is assembled with the bone plate  100 , the shaft section  304  abuts the surface of the counter-bore  114 , and the circular shaft  308  is configured to fit within the locking assembly through-hole  112  at the center of the counter-bore  114 , passing through to the inferior surface of the bone plate  100 . The circular shaft  308  is connected to the bone plate  100  by rotatable attachment with the locking assembly through-hole  112 . In some cases, the circular shaft  308  is hollow along a part of its length, such that the shaft end distal from the head may be widened and the shaft may act as a rivet. In some cases, the connection between the lock  300  and the bone plate  100  is permanent and is achieved before the bone plate  100  is implanted. Effective connection may be achieved by riveting or by any comparable means, which may or may not be permanent. Alternative methods of permanent or semi-permanent attachment between the lock  300  and the bone plate  100  are possible within the scope of the invention; for example, an optional end cap  312  may be installed abutting the lower surface of the lock shaft using threads, welding, an interference fit, or other suitable ways of attachment. The shaft section  304  of the lock  300  also includes a locking feature, for example, radial feature  306 , which may be an oval shape, semicircular protrusions, or any other suitable positive radial feature, and which is configured to abut one or more concave depressions  216 ,  216 ′ of the interior radial surface  212  of the locking ring  200  ( FIG. 2 ). 
         [0039]      FIG. 4  shows a bottom view of the lock  300  ( FIG. 3 ) assembled with the locking ring  200  ( FIG. 2 ), showing the relative positioning of the overhang  310  of the lock  300 , circular shaft  308 , shaft section  304 , and locking ring  200 . The radial feature  306  of the noncircular shaft section  304  is shown resting in a first concave depression  216  of the locking ring  200 . The radial feature  306  is shaped such that, when the lock  300  is turned within the locking ring  200 , the radial feature  306  contacts the interior radial surface  212  of the locking ring  200 , causing it to flex outwardly as the lock turns, until the radial feature  306  comes to rest in a second concave depression (e.g., second concave depression  216 ′) in the locking ring  200  and the locking ring  200  returns to its unflexed shape. In this non-limiting example, the noncircular shaft section  304  has two radial features  306 , each disposed in a concave depression  216  in the locking ring  200 . 
         [0040]    The lock  300  is shown in its unlocked position relative to a bone plate such as bone plate  100  shown in  FIG. 1 . From this position, the lock  300  may be rotated (counterclockwise from below as in the view of  FIG. 4 , although clockwise when viewed from above)  90  degrees until the lock  300  is secured in the locked position. During rotation, the locking ring  200  will deform elastically as the radial feature  306  of the noncircular shaft section  304  of the lock  300  presses outwards on the interior of the locking ring  200 , and then the locking ring  200  will return to its unflexed position when the lock  300  has passed fully to its locked position. In some cases, the lock passes through  90  degrees of rotation from its unlocked to its locked position, but other configurations are possible by adding additional convex and concave features to the noncircular shaft section  304  and locking ring interior radial surface  212 . 
         [0041]    The curved interior radial surface  212  of the locking ring  200  is shaped to exert at least some rotational force on the lock  300  when the lock  300  is oriented between the locked and unlocked positions, such that the lock  300  will provide tactile feedback to a user, such as a surgeon turning the lock  300  while installing the apparatus in a patient. The combination of the curvature and elastic deformation of the locking ring  200  will exert a circumferential force on the lock  300  resisting an initial turning force when a surgeon begins to turn the lock from its unlocked position. When the lock  300  has been turned to a position sufficiently close to the locked position, which in this non-limiting example is approximately ninety degrees, the curvature of the locking ring  200  in combination with the elastic deformation of the locking ring  200  will exert a circumferential force serving to “snap” the lock  300  into the locked position. 
         [0042]    The locking ring  200  is configured to have a spring stiffness such that the lock  300  may be operated by a physician during surgery without requiring substantial mechanical advantage or putting excessive strain on the underlying bone to which the bone plate may be attached. In some cases, the target stiffness is such that the lock  300  can be turned by hand using an inline screwdriver, providing enough rotational force that it provides tactile feedback along the screwdriver to a surgeon performing the installation. Furthermore, the lock  300  can be configured to accept a driver bit, which may be a standard hex bit, star bit, torx® bit, or other common variety of driver bit. The lock  300  may also be configured to accept the same driver bit as bone screws, further improving the simplicity of installation of the apparatus. 
         [0043]    In some non-limiting examples, the lock  300  is configured to interact with the larger of the two positive surface features (or the “stop”)  120  of the counter-bore  114  ( FIG. 1 ). In some cases, the lock  300  is restricted to a partial arc of rotation by the radial feature  306  of the noncircular shaft section  304  encountering the positive surface feature or stop  120 . For example, when the lock  300  is in the unlocked position, the radial feature  306  abuts the stop  120 , such that the lock  300  can only be turned toward the locked position—that is to say, only in one direction. Likewise, when the lock  300  is in the locked position, it can only be turned toward the unlocked position, which will be in the opposite direction. This binary configuration of the lock aids in preventing operator error in locking or unlocking the apparatus. In some non-limiting examples, there are two radial features  306  in the form of protrusions from the noncircular shaft section  304  arranged symmetrically and opposite, such that a different protrusion abuts the stop  120  in the locked position than in the unlocked position. In the illustrated examples, the stop  120  and radial features  306  are sized such that the lock  300  may rotate approximately 90 degree, but a configuration could be readily achieved that would restrict the rotation of the lock to a different arc, such as 60 degrees, 45 degrees, 30 degrees, or other arcs. In configurations having a symmetrical noncircular section, the effect of said symmetry is that net radial loading when the lock  300  passes between the unlocked and locked positions is minimized, reducing wear on the lock and minimizing the possibility of breakage. 
         [0044]      FIG. 5A  shows an assembled bone plate apparatus  500   a  from a top view, including bone plate  100 , bone screws  502  inserted in through-holes  116 , and locks  300  seated on locking rings  200  (shown in broken lines where obstructed by the locks  300 ) of  FIGS. 1-4 , with each of the locks  300  in the unlocked position. The bone screws  502  are arranged in a polyaxial configuration, and no part impinges on the screw heads. In particular, the head sections  302  of the two locks  300  are oriented away from the through-holes  116  so as not to overhang the bone screws  502 . The bone plate  500   a  and locking ring  200  have points of minimum and maximum clearance  504  and  506  between each locking ring  200  and a boundary of the counter-bore  114 . In some cases, the locking ring  200  may be in contact with the counter-bore  114  at a point of minimum clearance  504 , and may have a clearance of approximately 0.2 mm radially at a point of maximum clearance  506 . In some cases, the maximum clearance can vary to approximately 0.3 mm, or up to approximately 0.75 mm, or any other suitable distance. 
         [0045]      FIG. 5B  shows the assembled bone plate apparatus of  FIG. 5A , with the locks  300  in the locked position  500   b.  Here, the oval head sections  302  of the two locks  300  lie above and partially obstructing the removal path of the bone screws  502 , preventing backout. In some cases, the locks  300  are designed to clear the bone screws  502  such that the locks do not impinge on the bone screws  502 . By providing a slight clearance between the locks  300  and the bone screws  502 , the screws are permitted to toggle and settle, which in some configurations may be preferred over having fully rigid attachment. The configuration shown provides that, when the bone screws  502  are fully inserted and the locks  300  are in the locked position, the bone screws  502  do not exert an axial load on the locks  300 , although gradual settling and toggling of the screws may initiate contact between screw heads and locks. 
         [0046]      FIG. 6  shows a polyaxial bone plate  600 , from a perspective view, having an alternative structure within the counter-bore  614 . The alternative structure is an extension  624  of the counter-bore wall, extending into the cylindrical space of the counter-bore  614  and creating a radial undercut  626  along a side of the counter-bore that is designed to mate with a positive radial feature  704  of an alternative lock  700  ( FIG. 7 ). In this example, each counter-bore  614  possesses only a single extension  624  and radial undercut  626 , although each counter-bore may include multiple radial undercut features for matching with positive radial locking elements of alternative locks. The radial undercut  626  narrows in a wedge fashion such that a lock  700  ( FIG. 7 ) may be inserted in an unlocked position and secured in a rotatable fashion to the bone plate  600 , such that it can be rotated into a locked position. As with bone plate  100 , the bone plate  600  has at least one through-hole  616  for bone screws adjacent to each counter-bore  614 , and a lock through-hole  612  is arranged in the counter-bore  614 . 
         [0047]      FIG. 7  shows a lock  700  configured to mate with the bone plate  600  ( FIG. 6 ). As described above, the lock  700  includes a locking feature, for example, positive radial feature  704  that is configured to mate with the radial undercut  626  of the counter-bore  614  ( FIG. 6 ). In this example, no additional locking ring part is needed to create the locking action. Assembly of the apparatus as shown may be achieved by assembling the lock  700  into the lock through-hole  612  of the bone plate  600  and attaching the shaft  706  to the lock through-hole  612 . The lock  700  and bone plate  600  can be rotatingly attached together as described above with reference to the bone plate  100  and lock  300  ( FIGS. 1, 3, and 5 ). The unlocked and locked positions may be achieved by rotating the lock  700  until the positive radial feature  704  interacts with the radial undercut  626  ( FIG. 6 ) in the form of a taper lock. Thus, both the receiving space formed by the radial undercut  626  and the positive radial feature  704  of the lock may have a slight taper in a circumferential direction, such that friction between the inner surface of the radial undercut  626  and the outer surface of the positive radial feature  704  of the lock  700  will act to retain the lock  700  in the locked position. The inner surface of the radial undercut  626  ( FIG. 6 ) also performs the function of a stop, such that the lock can no longer rotate in the locking direction once locked. The lock may be prevented from rotating unnecessarily in the unlocking direction by interaction between the positive radial feature  704  and the counter-bore  614 . Additional structures may be included in the counter-bore that form a stop, such as an optional protrusion, or alternatively, no stop may be provided. Alternatively, the radial undercut  626  or the positive radial feature  704  may include one or more additional surface features configured to increase resistance to turning, for instance, rough or jagged surfaces, a positive feature and groove, two interacting ratcheting surfaces, or other similar features. 
         [0048]      FIG. 8A  shows a bone plate assembly in an unlocked position  800   a,  from the top view, including a bone plate  600 , locks  700 , and bone screws  802 , as shown in  FIGS. 6 and 7 . As shown, the locks  700  are oriented in the unlocked position. The bone screws  802  are arranged in a polyaxial configuration, and no part impinges on the screw heads. In particular, the oval head sections  702  of the two locks  700  shown are oriented in line with the long axis  808  of the bone plate  600  such that no parts of the head sections  702  overlap with a region above any bone screw  802 . 
         [0049]      FIG. 8B  shows a locked bone plate assembly  800   b  with the locks  700  oriented in the locked position. In the view shown, both locks  700  have been rotated approximately ninety degrees (albeit in opposite directions) relative to the configuration of  FIG. 8A , such that the positive radial feature  704  of each lock  700  has become trapped as in a taper lock by the cavity of the radial undercut  626  ( FIG. 6 ) formed by the extensions  624  in each counter-bore wall. When locked, friction between the radial undercut  626  and the positive radial feature  704  resists rotation, such that ordinary motion within a patient&#39;s body will not dislodge the lock  700 . The resistance may be tuned by adjusting the taper of the undercut  626  and positive radial feature  704 , or by the provision of additional locking features on one, the other, or both surfaces as described above. In the locked position, the head sections  702  of the locks  700  obstruct regions above the bone screws  802  in order to prevent screw backout. The locking directions that each of the two locks  700  must be turned may be the same or different; and the plate  600  may be rotationally symmetrical instead of mirror-symmetrical (as shown), in which case the locks could rotate identically in order to lock. 
         [0050]      FIG. 9  shows a bone plate  900  from a top view that is configured to mate with a lock  1100  ( FIG. 11 ). As with the bone plates described above, the bone plate  900  also includes counter-bores  914 , concentric with lock through-holes  912  for receiving the lock  1100  ( FIG. 11 ). The lock  1100  has positive, downwardly-directed surface features  1104  ( FIG. 11 ) that are sized and spaced to mate with grooves  918  in the counter-bore  914  radiating from the lock through-hole  912 . The counter-bores  914  are adjacent to at least one bone screw hole  916 . 
         [0051]      FIG. 10  shows the bone plate  900  of  FIG. 9  in a perspective view, showing in more detail the surface features of the counter-bore  914 . The counter-bore  914  possesses an interior surface that has a series of grooves  918  radiating from the lock through-hole  912 , at set circumferential spacing. In this example, four grooves  918  are disposed at approximately  90  degree increments. The grooves  918  are configured to mate with downwardly-directed positive protrusions  1104  on the underside of an oval head section  1102  of a lock  1100  ( FIG. 11 ). 
         [0052]      FIG. 11  shows a lock  1100  configured for assembly with the bone plate  900  of  FIGS. 9 and 10 . Lock  1100  includes an oval head section  1102 , and a locking feature including two positive protrusions  1104  extending downward from the oval head section  1102 , and a shaft  1106 . The two positive protrusions  1104  extend downward to mate with the grooves  918  of the counter-bore  914  ( FIGS. 9 and 10 ), such that the lock  1100  can be stably seated within the grooves  918  with minimal tension in the shaft  1106 . When rotated between the grooves  918 , the positive protrusions  1104  press against the surface of the counter-bore  914  and produce axial tension in the shaft  1106  and/or bending stress in the oval head section  1102 . Slight deformation may occur in the lock  1100  when the lock  1100  is rotated through intermediate positions where the positive protrusions are in-between the grooves  918 . The positive protrusions  1104  possess curved surfaces where the lock  1100  interacts with the grooves  918  of the counter-bore, such that the lock  1100  resists rotation when the positive protrusions  1104  are seated in a groove  918 , and such that the lock may “snap” into place when the lock is rotated such that the positive protrusions  1104  enter a groove  918 . 
         [0053]      FIG. 12  shows a partially assembled, locked bone plate apparatus  1200  including lock  1100  and bone plate  900  in accordance with  FIGS. 9-11 , with the lock assembled in the bone plate  100  and rotated into the locked position. In this non-limiting example, the lock  1100  is permanently or semi-permanently attached into the lock through-hole  912  ( FIGS. 9-10 ) in the bone plate  900 . In this view, a positive protrusion  1104  extending from the underside of the oval head section  1102  of the lock  1100  is visible resting within a groove  918  of the underlying surface of the counter-bore  914 . 
         [0054]      FIG. 13A  shows an unlocked bone plate assembly  1300   a  including the bone plate  900  and lock  1100  shown in  FIG. 12 , with bone screws  1302  in the at least one bone screw hole  916 . As in previously described examples, the oval head section  1102  in the unlocked position does not interfere with the bone screws  1302 . 
         [0055]      FIG. 13B  shows a locked bone plate assembly  1300   b  based on locking the assembly  1300   a  shown in  FIG. 13A . As in previously described examples, the oval head section  1102  in the locked position partially projects above the region above one or more of the bone screws  1302  without contacting the bone screws. 
         [0056]      FIG. 14  shows a bone plate assembly  1400  in a top plan view, the bone plate assembly employing locks  300  and locking rings  200  in various illustrative states. The bone plate assembly  1400  includes six screw-holes  1406   a - c  and  1406   a ′- c ′ (collectively,  1406 ), and three through-holes  1408   a - c  (collectively  1408 ) for locks. Each through-hole  1408  is disposed adjacent to two respective screw-holes  1406 . The screw-holes  1406  are arranged in a polyaxial configuration to permit the insertion of screws (not shown) therein at different angles, in order to achieve improved attachment to bone. For example, the first pair of screw holes  1406   a,    1406   a ′ are oriented such that screws placed therein would extend toward one another and toward the first end of the bone plate; the second pair of screw holes  1406   b,    1406   b ′ are oriented downward, and the third pair of screw holes  1406   c,    1406   c ′ are oriented such that the screws placed therein extend toward one another and toward the second end of the bone plate. In some cases, the screw holes can be oriented in various other directions, which can be symmetrical or asymmetrical, in order to direct screws in various other directions. 
         [0057]    In the bone plate assembly  1400  shown, a first lock  300  is shown engaged in the first through-hole  1408   a  and is oriented in an unlocked position, where the overhang  310  of the lock is not obstructing the adjacent screw holes  1406   a  and  1406   a ′. The second through-hole  1408   b  is shown without a lock, so as to illustrate one exemplary placement of a locking ring  200  in the counter-bore  1404  around the through-hole  1408   b.  The locking ring  200  rests in the counter-bore  1404  and is rotationally secured relative to the bone plate assembly  1400  by a counter-bore  1404 . The third-through-hole  1408   c  is shown with an additional lock  300  oriented in a locked position, where the overhang  310  of the additional lock is obstructing both of the adjacent screw holes  1406   c,    1406   c ′. Notably, the screw holes  1406  can be directly opposite one another across their associated through-hole  1408 , as in  1408   b  and  1406   b,    1406   b ′; or can be substantially opposite one another, with an offset, as in  1408   a  and  1406   a,    1406   a′.    
         [0058]    Alternative designs may include bone plates having any suitable number of associated locks and screw-holes. Screw holes may be arranged in pairs around a lock having two overhanging portions, in triplicate around a lock having three overhanging portions, in quadruplicate around a lock having four overhanging portions, or in any other suitable, comparable arrangement, provided that the associated locking features thereof allow the lock to rotate between unlocked and locked positions at various locking angles as appropriate. For example, where a lock has two associated screw holes, an appropriate locking angle may be approximately 90 degrees. Where a lock has three associated screw holes, an appropriate locking angle may be approximately 60 degrees. Where a lock has four associated screw holes, an appropriate locking angle may be approximately 45 degrees, and so on. 
         [0059]    Different arrangements of the components depicted in the drawings or described above, as well as components and steps not shown or described are possible. Similarly, some features and subcombinations are useful and may be employed without reference to other features and subcombinations. Embodiments of the invention have been described for illustrative and not restrictive purposes, and alternative embodiments will become apparent to readers of this patent. Accordingly, the present invention is not limited to the embodiments described above or depicted in the drawings, and various embodiments and modifications can be made without departing from the scope of the claims below.