Polyaxial bone plate and locking assembly

[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.

[0002] This application relates to implantable internal fixator assemblies for use in stabilizing and supporting the spine.

[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'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.

[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.

[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'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. 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] FIG. 5A shows an assembled bone plate apparatus 500a 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 500a 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.

[0044] FIG. 5B shows the assembled bone plate apparatus of FIG. 5A, with the locks 300 in the locked position 500b. 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.

[0045] 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.

[0046] 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.

[0047] FIG. 8A shows a bone plate assembly in an unlocked position 800a, 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.

[0048] FIG. 8B shows a locked bone plate assembly 800b 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'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.

[0049] 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.

[0050] 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).

[0051] 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.

[0052] 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.

[0053] FIG. 13A shows an unlocked bone plate assembly 1300a 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.

[0054] FIG. 13B shows a locked bone plate assembly 1300b based on locking the assembly 1300a 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.

[0055] 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 1406a-c and 1406a′-c′ (collectively, 1406), and three through-holes 1408a-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 1406a, 1406a′ 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 1406b, 1406b′ are oriented downward, and the third pair of screw holes 1406c, 1406c′ 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.

[0056] In the bone plate assembly 1400 shown, a first lock 300 is shown engaged in the first through-hole 1408a and is oriented in an unlocked position, where the overhang 310 of the lock is not obstructing the adjacent screw holes 1406a and 1406a′. The second through-hole 1408b 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 1408b. 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 1408c 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 1406c, 1406c′. Notably, the screw holes 1406 can be directly opposite one another across their associated through-hole 1408, as in 1408b and 1406b, 1406b′; or can be substantially opposite one another, with an offset, as in 1408a and 1406a, 1406a′.

[0057] 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.

[0058] 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.