Patent Description:
Certain surgical procedures require a surgeon to fuse portions of a patient's spine to each other. Implanting a cervical plate reduces a patient's range of motion, and helps relieve pain experienced by a patient. Although cervical plate implantations are used to treat radiculopathy or myelopathy, but one of ordinary skill in the art would recognize that fusion can be used for other types of surgery.

Anterior cervical plate assemblies typically include a base plate defining through openings for bone screws to anchor the base plate to a patient's spine. Some cervical plate assemblies include a blocking mechanism to prevent the bone screws from inadvertently backing out of the base plate. One type of known blocking mechanism includes an automated blocking feature, such that once a bone screw passes a predetermined threshold then the blocking mechanism is automatically activated to block the bone screws from inadvertently backing out of the plate. These automated blocking mechanisms can be complicated for surgeons to operate, and can make removal of the base plate difficult.

Other known blocking mechanisms for anterior cervical plate assemblies rely on a patient's bones to have a certain strength characteristic to withstand the blocking mechanism features. Some known types of blocking mechanisms require force for removing a screw from the blocking mechanism. However, this type of blocking mechanism can cause stripping of the screw holes for bone screws implanted in a patient's bones.

Another known type of blocking mechanism for anterior cervical plate assemblies requires additional steps to install and lock the blocking mechanism screw in the plate. In some devices, this additional step requires a specialty tool or instrument. These steps are time consuming and require the surgeon to perform additional steps during surgery, which is undesirable.

It would be desirable to provide an improved cervical plate assembly that is relatively simple to use and provides a reliable blocking function.

<CIT>, Us6,<NUM>,<NUM>, <CIT> and <CIT> describes plate systems known in the art.

Briefly stated, an improved cervical plate assembly is disclosed.

According to the invention it is provided an anterior cervical plate assembly having the features of the preamble of claim <NUM>. Further advantageous aspects of the invention are set forth in the dependent claims.

The cervical plate assembly includes a base plate including four bone screw seats. Each bone screw seat includes a borehole dimensioned to receive a bone screw. Each borehole defines a central axis that is (a) angled relative to a central lateral axis of the base plate at a first angle and (b) angled relative to a central longitudinal axis at a second angle. In one embodiment, the first angle is at least <NUM> degrees, and the second angle is at least <NUM> degrees. The base plate defines two retention slots that are each positioned between a pair of the four bone screw seats. The assembly includes two blocking mechanisms. Each blocking mechanism includes a biasing element arranged between a first blocking element and a second blocking element. The first blocking element is configured to obstruct a first bone screw seat of the four bone screw seats, and the second blocking element is configured to obstruct a second bone screw seat of the four bone screw seats. The first blocking element and the second blocking element are independently positionable from each other. Each blocking mechanism is retained within a respective one of the two retention slots. The blocking mechanisms are selectively positionable between a closed position in which the blocking mechanism obstructs at least one bone screw seat to retain a bone screw with the base plate, and an open position in which the bone screw seats are unobstructed.

A variety of arrangements and embodiments are described in more detail below and in the claims.

The foregoing summary as well as the following detailed description will be best understood when read in conjunction with the appended drawings. The invention is shown in particular in <FIG> the remaining drawings are reported for a better understanding of the invention. In the drawings:.

It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the description herein or illustrated in the drawings. The teachings of the present disclosure may be used and practiced in other embodiments and practiced or carried out in various ways.

The following discussion is presented to enable a person skilled in the art to make and use embodiments of the present disclosure. Various modifications to the illustrated embodiments will be readily apparent to those skilled in the art, and the principles herein can be applied to other embodiments and applications without departing from embodiments of the present disclosure. Thus, the embodiments are not intended to be limited to embodiments shown, but are to be given the widest scope consistent with the principles and features disclosed herein. The following detailed description is to be read with reference to the figures, in which like elements in different figures have like reference numerals. The figures, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the embodiments. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of the embodiments. References to "embodiments" throughout the description which are not under the scope of the appended claims merely represent possible exemplary executions and are therefore not part of the present invention.

<FIG> illustrates a cervical region of a patient's spine <NUM> with an anterior cervical plate assembly <NUM> implanted on the spine <NUM>. As shown in <FIG>, the plate assembly <NUM> is implanted at the C4-C5 vertebrae of the spine <NUM>. One of ordinary skill in the art would recognize that the plate assembly <NUM> could be installed at different regions of the patient's spine <NUM>. The term plate assembly is generically used herein to generally refer to the plate assemblies described in some of the embodiments. In one embodiment, the plate assemblies are specifically designed to be used for anterior cervical implantations. One of ordinary skill in the art would understand from the present disclosure that the concepts and features of the plate assemblies disclosed herein could be adapted for surgical assemblies and techniques for other portions of a user's anatomy besides the spine. With respect to <FIG>, the anterior cervical plate assembly <NUM> is shown including four bone screws 12a-12d and two blocking assemblies 14a, 14b, however one of ordinary skill in the art would recognize from the present disclosure that alternative arrangements of the anterior cervical plate assembly <NUM> could be used. For instance, the cervical plate assembly can extend to multiple levels of the spine and include additional screw holes for additional fixation to adjacent vertebral bodies. The bone screws used in any of the embodiments described herein can be self-drilling, self-tapping, variable angle, fixed angle, or any other known type of bone screw design. Additionally, the embodiments of the base plate described herein can be configured to accept bone screws having screw diameters of <NUM> and <NUM>, although one of ordinary skill in the art would understand that different sizes for bone screws can be used. <FIG> illustrates the plate assembly <NUM> with the bone screws 12a-12d and the blocking mechanisms 14a, 14b in an uninstalled state. As shown in <FIG>, the plate assembly <NUM> includes two pairs of bone screws 12a-12d, with each pair configured to be implanted into a vertebral body. In this embodiment, the blocking mechanisms 14a, 14b each include a blocking screw 15a, 15b. The blocking screws 15a, 15b each include a head 16a, 16b with a varying circumferential edge 17a, 17b, and an engagement recess 20a, 20b configured to be engaged by a tool for rotationally driving the heads 16a, 16b. One of ordinary skill in the art would understand from the present disclosure that the geometry of the engagement recesses 20a, 20b can be varied. In one embodiment, the engagement recess can be omitted and the blocking mechanisms 14a, 14b can be manually engaged/actuated by a user.

As shown in <FIG>, the circumferential edges 17a, 17b of the blocking screws 15a, 15b include diametrically opposed cutouts 18a, 18b, and diametrically opposed lobes 19a, 19b. The cutouts 18a, 18b are dimensioned to allow passage of the bone screws 12a-12d when the cutouts 18a, 18b overlap an associated bone screw seat. The lobes 19a, 19b are configured to obstruct an associated bone screw seat to block the bone screws 12a-12d from backing out of the plate assembly <NUM>. The engagement recesses 20a, 20b are illustrated with a hexagonal profile, but one of ordinary skill in the art would recognize from the present disclosure that any non-round profile can be used. The heads 16a, 16b are engaged by a user and rotated a quarter turn, i.e. <NUM> degrees, to move from a blocked position, in which bone screws 12a-12d are retained with the plate assembly, to an open position, in which the bone screws 12a-12d can be removed from the plate assembly <NUM>. As used herein, the blocked position corresponds to a position in which the bone screws seats are obstructed and the open position corresponds to a position in which the bone screw seats are unobstructed.

<FIG> illustrates an open configuration for the blocking mechanisms 14a, 14b, and <FIG> illustrates a blocked configuration for the blocking mechanisms 14a, 14b. As shown in <FIG>, the plate assembly <NUM> includes a base plate <NUM> that defines bone screw seats 22a-22d for a respective one of the bone screws 12a-12d. The plate <NUM> also defines blocking mechanism seats 24a, 24b, shown in <FIG>, which are configured to retain a respective one of the blocking mechanisms 14a, 14b. As illustrated in this embodiment, the blocking mechanism seats 24a, 24b include a retention lip 25a, 25b that prevents the blocking mechanisms 14a, 14b from being removed from the plate <NUM>. One of ordinary skill in the art would recognize from the present disclosure that alternative retention arrangements could be provided for ensuring the blocking mechanisms 14a, 14b are retained with the plate <NUM>. For example, the blocking mechanisms 14a, 14b could be retained with the plate <NUM> by tabs, prongs, elastic elements, slots, channels, or other retention features. In one embodiment, the blocking mechanisms 14a, 14b can include an enlarged head on an opposite end from the screw heads 16a, 16b and the plate <NUM> can define a retention recess dimensioned to captively secure the enlarged head of the blocking mechanisms 14a, 14b. One of ordinary skill in the art would recognize from the present disclosure that a variety of retention features could be used, as long as the retention features retain the blocking mechanisms 14a, 14b with the base plate <NUM> while also allowing the blocking mechanisms 14a, 14b to rotate.

<FIG> illustrate the plate assembly <NUM> including a single bone screw 12b. <FIG> illustrates the blocking mechanism 14a in an open position in which the bone screw 12b can be removed from the base plate <NUM>. <FIG> illustrates the blocking mechanism 14b in a blocking position in which the bone screw 12b is blocked from backing out of the plate assembly <NUM>.

<FIG> illustrate a base plate <NUM>, and specifically illustrate the relatively high angulation of the bone screw seats 222a-222d. The base plate <NUM> includes a central window <NUM>, which can be used by a surgeon during installation to view the disc space (and interbody implant). This window <NUM> can be used to assist in making sure the base plate <NUM> is the correct position. Although the window <NUM> is only illustrated in the embodiment of <FIG>, one of ordinary skill in the art would recognize from the present disclosure that the window <NUM> can be integrated into the design of any of the base plates disclosed herein. The plate <NUM> includes two blocking mechanisms 214a, 214b. As shown in <FIG>, the plate <NUM> defines channels 207a, 207b for each of the blocking mechanisms 214a, 214b in which blocking elements can slide. Blocking elements (not shown in <FIG>) are retained within the channels 207a, 207b and are driven outward to the bone screw seats 222a-222d to retain bone screws with the plate <NUM>. A tapered profile defined by the channels 207a, 207b is shown in <FIG>. The blocking elements of the blocking mechanism 214a, 214b have a complementary profile such that the blocking elements are retained in the channels 207a, 207b and can slide within the channels 207a, 207b. The channels 207a, 207b define a laterally outer stop surface for the blocking elements.

As shown in <FIG>, the base plate <NUM> is divided by two primary axes: a central lateral axis X<NUM> and a central longitudinal axis X<NUM>. The high angulation of the bone screw seats 222a-222d provides a strong and rigid construction for attaching bone screws to a patient. The plate <NUM> allows bone screws to be inserted at high cephalad and caudal angles. The high angulation also allows for longer bone screws to be used for implantation, which corresponds to increased mechanical purchase and improved strength of the implantation. Each of the bone screw seats 222a-222d define a borehole 223a-223d with a central axis CA<NUM>-CA<NUM>. Each central axis CA<NUM>-CA<NUM> of boreholes 223a-223d is angled relative to the central lateral axis X<NUM> of the base plate <NUM> at a first angle θ<NUM>. In one embodiment, the first angle θ<NUM> is between <NUM>-<NUM> degrees. In one embodiment, the first angle θ<NUM> is at least <NUM> degrees. In one embodiment, the first angle θ<NUM> is at least <NUM> degrees. Each central axis CA<NUM>-CA<NUM> of boreholes 223a-223d is also angled relative to the central longitudinal axis X<NUM> at a second angle θ<NUM>. In one embodiment, the second angle θ<NUM> is between <NUM>-<NUM> degrees. In one embodiment, the second angle θ<NUM> is at least <NUM> degrees. Both the first angle θ<NUM> and the second angle θ<NUM> can be varied depending on the type of bone screw being used in a particular assembly. During installation, the threaded ends of bone screws inserted into bone screw seats 223a and 223b (with respect to the view shown in <FIG>) are angled or canted towards each other on an underside of the base plate <NUM> due to the second angle θ<NUM>. Although the angulation of the central axes CA<NUM>-CA<NUM> of the boreholes 223a-223d for the bone screw seats 222a-222d are only specifically illustrated in <FIG> and explained with respect to these figures, one of ordinary skill in the art would understand that these angulation values are present for any of the other embodiments of the base plate described herein.

<FIG> illustrate another embodiment of a plate assembly <NUM> including a base plate <NUM>. As shown in <FIG> the plate assembly <NUM> includes two bone screw seats <NUM>, with a single bone screw <NUM> arranged in one of the bone screw seats <NUM>. This plate assembly <NUM> is only illustrated with two bone screw seats <NUM>, but one of ordinary skill in the art would understand that the features of this embodiment can be adapted for a plate assembly including any number of bone screw seats. This plate assembly <NUM> is also illustrated as having a generic rectangular profile, however one of ordinary skill in the art would understand that the profile of the plate itself can vary, and can resemble the profile of the base plate <NUM> described above.

The plate assembly <NUM> includes a blocking mechanism <NUM> having a different profile than the blocking mechanisms 14a, 14b of <FIG>. The blocking mechanism <NUM> includes a head <NUM> with a varying circumferential edge <NUM>, and an engagement recess <NUM> configured to engage a tool for rotationally driving the blocking mechanism <NUM>. The edge <NUM> includes diametrically opposed cutouts <NUM>, and diametrically opposed lobes <NUM>. The cutouts <NUM> are dimensioned to allow insertion of the bone screw <NUM>, and the lobes <NUM> are configured to block the bone screw <NUM> from backing out of the plate assembly <NUM>. The lobes <NUM> are also configured to deflect inward when the bone screw is inserted and then blocks the bone screw from backing out. The circumferential edge <NUM> of the blocking mechanism <NUM> has a symmetrical profile such that the blocking mechanism <NUM> provides an identical profile if the blocking mechanism <NUM> is rotated <NUM>°. Slots <NUM> are provided on the blocking mechanism <NUM> which provide grips or insertion points for a tool for rotating the blocking mechanism <NUM>. <FIG> illustrates the blocking mechanism <NUM> in an open position, and <FIG> illustrates the blocking mechanism <NUM> in a blocked position without a bone screw <NUM>. <FIG> illustrates a side cross section view of the plate assembly <NUM>. As shown in <FIG>, the plate <NUM> defines a blocking mechanism seat <NUM> which axially retains the blocking mechanism <NUM> within the plate <NUM>. The plate <NUM> defines grooves <NUM> configured to engage a threading <NUM> defined on the blocking mechanism <NUM>. It should be noted that, a bone screw can be inserted into screw holes even when the blocking mechanism <NUM> is in a blocked position as shown in <FIG>, as the lobes of the blocking mechanism may be deformable. As shown in <FIG>, a clearance (c) is defined between the blocking mechanism <NUM> and the bone screw <NUM> when the bone screw <NUM> is fully seated in the bone screw seat <NUM>. Although the clearance (c) is only illustrated with respect to this embodiment, one of ordinary skill in the art would recognize that the clearance can be provided in any of the other embodiments described herein.

<FIG> illustrate another embodiment of a plate assembly <NUM> and a base plate <NUM>. The blocking mechanism <NUM> includes a head <NUM> having a generally X-shaped profile, and including four arms <NUM>. Resilient blocking elements <NUM> extend between a respective pair of the four arms <NUM> on diametrically opposed sides of the head <NUM>. Reliefs <NUM> are provided on sides of the head <NUM> including the resilient blocking elements <NUM>. These reliefs <NUM> allow for the resilient blocking elements <NUM> to deform radially inwardly towards a central rotational axis of the blocking mechanism <NUM>. <FIG> illustrates the blocking mechanism <NUM> in an open position. <FIG> illustrates the blocking mechanism <NUM> is a blocked position, with the resilient blocking elements <NUM> overlapping the bone screw seats <NUM>. This particular blocking mechanism <NUM> can be in either the open or blocked position and still allow insertion of the bone screw <NUM> into the plate assembly <NUM>. This embodiment allows for the bone screw <NUM> to be inserted into the plate assembly <NUM> when the blocking mechanism <NUM> is in the blocked position (<FIG>) due to elastic deformation of the resilient blocking elements <NUM> during insertion of the bone screw <NUM>. Once fully inserted and installed, the bone screw <NUM> is prevented from backing out of the plate assembly <NUM> due to the resilient blocking elements <NUM>. In one embodiment, the resilient blocking element <NUM> is formed from spring steel. In another embodiment, the resilient blocking element <NUM> is formed from an elastomeric material. One of ordinary skill in the art would recognize from the present disclosure that alternative types of blocking elements <NUM> can be used as long as the blocking elements <NUM> are configured to elastically deform during insertion of the bone screws and return to an initial position after insertion that blocks or overlaps with the inserted bone screw. <FIG> illustrates the plate assembly <NUM> in a perspective view with the blocking mechanism <NUM> in a blocked position. As shown more clearly in <FIG>, the blocking mechanism <NUM> includes cutouts <NUM> that are dimensioned to allow passage of the bone screws <NUM>.

<FIG> illustrates another embodiment of a plate assembly <NUM> in a blocked position. The plate assembly <NUM> includes a blocking mechanism <NUM> including blocking lobes <NUM>. The blocking lobes <NUM> may have a rectangular profile. One of ordinary skill in the art would recognize from the present disclosure that the profile of the lobes can be modified.

<FIG> illustrate another embodiment of a blocking mechanism <NUM>. The blocking mechanism <NUM> of <FIG> can be integrated into any of the plate assemblies described herein. The blocking mechanism <NUM> includes a hub <NUM>, a blocking element <NUM>, and retention washer <NUM>. The hub <NUM> defines grooves for accommodating the blocking element <NUM>. The blocking element <NUM> defines lobes on diametrically opposite sides of the blocking element <NUM>, each configured to block a bone screw within a cervical plate assembly. The blocking element <NUM> can be formed as a wire forming a continuous loop. The blocking element <NUM> can be snapped into or placed into the grooves formed on the hub <NUM>, and a retention washer <NUM> can then be pressed or snapped onto a bottom end of the hub <NUM>. The retention washer <NUM> can be snapped into a groove formed on the hub <NUM> such that the blocking element <NUM> is retained between the hub <NUM> and the retention washer <NUM>. The blocking mechanism <NUM> can be rotated while retained within a seat of a base plate, such that the blocking element <NUM> moves from a position overlapping with a bone screw seat to block the bone screw seat, to an open position in which the blocking element <NUM> is rotated away from the bone screw seat and the bone screw seat is unobstructed.

<FIG> illustrate another embodiment of a plate assembly <NUM>. The plate assembly <NUM> includes a blocking mechanism <NUM> including blocking tabs <NUM> arranged on opposite sides of a central portion <NUM>. The blocking tabs <NUM> are configured to provide obstructions to an underlying bone screw seat, such that a bone screw is retained with the plate assembly <NUM>. The central portion <NUM> includes resilient arms <NUM>, such that the blocking mechanism <NUM> can be deformed by applying inward pressure to the blocking tabs <NUM>. The resilient arms <NUM> form an X-shaped configuration with reliefs <NUM> formed between the arms <NUM>. A user can manually pinch the blocking tabs <NUM> towards each other, and the blocking mechanism <NUM> can be manipulated from an open position to a blocked position without the use of a tool. The blocking mechanism <NUM> can be retained with the base plate <NUM> via a slot <NUM> formed in the base plate <NUM>, or other retention configuration. As shown in <FIG>, the slot <NUM> has a tapered profile, and the blocking mechanism <NUM> has a complementary tapered profile such that the blocking mechanism <NUM> is retained within the slot <NUM>. One of ordinary skill in the art would understand from the present disclosure that this slot <NUM> can be integrated into any of the base plates described herein to retain a blocking mechanism.

<FIG> illustrate another embodiment of a plate assembly <NUM> including a blocking mechanism <NUM>. The blocking mechanism <NUM> includes a central hub <NUM> defining a pair of C-shaped arms 816a, 816b, with resilient blocking elements <NUM> extending therebetween. Blocking tabs <NUM> are engaged against a respective one of the resilient blocking elements <NUM>. Reliefs <NUM> are formed on the hub <NUM>, and the reliefs <NUM> are dimensioned to each accommodate one of the resilient blocking elements <NUM> and one of the blocking tabs <NUM>. <FIG> illustrates the blocking mechanism <NUM> in an expanded state in which the blocking tabs <NUM> are extended and configured to block a bone screw seat. The blocking mechanism <NUM> is configured to be rotated by <NUM>° such that the blocking tabs <NUM> and the resilient blocking elements <NUM> are pushed inwardly into a respective one of the reliefs <NUM>, and the blocking tabs <NUM> and the resilient blocking elements <NUM> are in a compressed state. The blocking tabs <NUM> can be retained within slots formed in the base plate <NUM>, such that the blocking tabs <NUM> slide within the slots.

<FIG> illustrates an alternative configuration for a blocking mechanism <NUM> in which two blocking screws 914a, 914b are provided for blocking a respective bone screw seat. As shown in <FIG>, a single blocking screw 914a includes an engagement recess <NUM> configured to be engaged by a tool to rotate the blocking screw 914a. Each of the blocking screws 914a, 914b include teeth 937a, 937b which engage each other such that rotation of the first blocking screw 914a drives rotation of the second blocking screw 914b. The blocking screws 914a 914b each define a blocking lobe 919a, 919b and a cutout 918a, 918b such that in a blocked position the blocking lobes 919a, 919b overlap with a bone screw seat, and in an open position the cutouts 918a, 918b overlap with a bone screw seat to allow for insertion of a bone screw.

<FIG> illustrates a blocking mechanism <NUM> that is directly integrated with a bone screw <NUM>. As shown in <FIG>, the bone screw <NUM> is inserted into a base plate <NUM>, and the blocking mechanism <NUM> provides axial retention of the bone screw <NUM> relative to the base plate <NUM>. The blocking mechanism <NUM> is formed as a split ring element, which is retained within a groove <NUM> formed in a head of the bone screw <NUM>. A tool including a sleeve can be inserted around the head of the bone screw <NUM> to push the blocking mechanism <NUM> into the groove <NUM> of the bone screw <NUM>. The blocking mechanism <NUM> can define a tapered edge <NUM>' which is configured to engage against the base plate <NUM> during insertion, such that the tapered edge <NUM>' slides along the base plate <NUM> and the blocking mechanism <NUM> is pushed into the groove <NUM>.

<FIG> illustrate a blocking mechanism <NUM> including two blocking elements 1114a, 1114b, a central resilient element <NUM>, and two blocking resilient elements 1126a, 1126b. As shown in <FIG>, the blocking resilient elements 1126a, 1126b are engaged against a base plate <NUM>, and retain the blocking elements 1114a, 1114b in a first position, which can correspond to an open configuration. The central resilient element <NUM> biases the blocking elements 1114a, 1114b outward from each other, but the blocking resilient elements 1126a, 1126b define a stop against the base plate <NUM> and prevent this outward movement in <FIG>. As shown in <FIG>, once the blocking resilient elements 1126a, 1126b are downwardly depressed, then the central resilient element <NUM> drives the blocking elements 1114a, 1114b outward to a second position, which can correspond to a blocked position. <FIG> shows a top view of the blocking mechanism <NUM> with the blocking elements 1114a, 1114b in the blocked position and overlapping the bone screw seats 1122a, 1122b. <FIG> illustrates the blocking mechanism <NUM> separate from the base plate <NUM>. In one embodiment, the blocking resilient elements 1126a, 1126b are pressed downward via a tool. One of ordinary skill in the art would recognize that the blocking resilient elements 1126a, 1126b can be pressed downward by a user/surgeon. In one embodiment, the blocking resilient elements 1126a, 1126b are leaf springs, but one of ordinary skill in the art would recognize from the present disclosure that alternative blocking elements can be used.

<FIG> illustrate another embodiment of a blocking mechanism <NUM>. In this embodiment, the blocking mechanism <NUM> includes a central blocking element <NUM> that moves between a blocked position shown in <FIG> and an open position in <FIG> in which the bone screw seats 1222a, 1222b are either obstructed (<FIG>) or unobstructed (<FIG>). The central blocking element <NUM> is a flexible plate that extends between lateral sides of the base plate <NUM>. A blocking cam <NUM> is arranged within a slot <NUM> formed on the base plate <NUM>. The blocking cam <NUM> is configured to be positioned in a lower position within the slot <NUM>, as shown in <FIG>, in which the central blocking element <NUM> is flexed to overlap with the bone screw seats 1222a, 1222b. The terminal ends of the central blocking element <NUM> are understood to be fixed to the lateral sides of the base plate <NUM> such that the central blocking element <NUM> flexes and exhibits inflexion as shown in <FIG>. As shown in <FIG>, as the blocking cam <NUM> is moved upward within the recess <NUM>, then the central blocking element <NUM> returns to a relatively straight profile such that the central blocking element <NUM> does not overlap with the bone screw seats 1222a, 1222b.

<FIG> illustrate an alternative embodiment of a bone screw <NUM> including a screw head <NUM>. The screw head <NUM> includes a slot <NUM> and a relief <NUM>. The relief <NUM> is centered relative to a shaft of the bone screw <NUM>. The relief <NUM> acts as a drive feature to help thread the screw <NUM> and allows the screw head <NUM> to compress to reduce the outer diameter of the screw head <NUM>. By having a nominal screw head diameter larger than a bone screw hole diameter defined on the base plate, the screw head <NUM> provides an interference fit with the base plate when the driver tool is not engaged. When the bone screw <NUM> is fully seated within an associated base plate, and the driver tool is removed from the screw head <NUM>, then the screw head <NUM> expands back open to its nominal diameter, which is oversized relative to the associated bone screw seat and prevents the bone screw <NUM> from backing out of the plate. Although an interference type fit is described with respect to this embodiment, one of ordinary skill in the art would recognize from the present disclosure that alternative arrangements could be provided to ensure that the bone screw <NUM> has a secure connection to the base plate after insertion. Additionally, although a slot/relief arrangement is described with respect to this embodiment, one of ordinary skill in the art would recognize from this disclosure that alternative geometries can be used to achieve the same result of fixing the bone screw relative to a bone screw seat defined by a base plate.

<FIG> illustrate an alternative embodiment of a bone screw <NUM>. As shown in <FIG>, the bone screw <NUM> has a screw head <NUM> with a relief <NUM>'. The relief <NUM>' is configured to accommodate a portion of a blocking mechanism <NUM> (shown in dashed lines in <FIG>). This relief <NUM>' allows for the blocking mechanism to partially overlap an axial end of the bone screw <NUM>, and as a result allows for a thinner base plate. The relief <NUM>' also ensures that the blocking mechanism sufficiently overlaps with the bone screw <NUM> in the axial direction.

<FIG> illustrate an embodiment of a blocking mechanism <NUM>, according to the invention. The blocking mechanism <NUM> includes a central biasing element <NUM> and two blocking elements 1526a, 1526b on opposite ends of the central biasing element <NUM>. The blocking mechanism <NUM> is retained to a base plate <NUM>, and the base plate <NUM> includes bone screw seats 1522a, 1522b (a single bone screw <NUM> is shown within the base plate <NUM>). As shown in <FIG>, the blocking elements 1526a, 1526b are in the extended position due to the biasing force from the biasing element <NUM>. In this position, the blocking elements 1526a, 1526b overlap with the bone screw seats 1522a, 1522b and are in a blocked position to retain the bone screws with the base plate <NUM>. In <FIG>, the blocking elements 1526a, 1526b are in a compressed configuration with the biasing element <NUM> being compressed and housed within cavities of the blocking elements 1526a, 1526b. In this configuration, the blocking elements 1526a, 1526b are positioned away from the bone screw seats 1522a 1522b such that the bone screw <NUM> can be removed from the base plate <NUM>. The blocking elements 1526a, 1526b can be independently moved with respect to each other such that a bone screw <NUM> can be removed from one of the bone screw seats 1522a, 1522b while a bone screw <NUM> is blocked in the other one of the bone screw seats 1522a, 1522b. The central biasing element <NUM> can be a coil spring, leaf spring, or any other type of elastic component. As shown in <FIG>, the blocking elements 1526a, 1526b are retained in the base plate <NUM> via a mating slot feature. As shown in Figure 15C, the base plate <NUM> defines a slot <NUM> with a protrusion <NUM>' and the blocking element 1526a includes a groove 1526a'. One of ordinary skill in the art would recognize from the present disclosure that alternative types of retention/mating features can be used to slidingly retain the blocking elements 1526a, 1526b with the base plate <NUM>. For example, the retention can be achieved via a t-slot, dovetail, or other mating feature. The blocking elements 1526a, 1526b are slidingly retained within the slot <NUM>. In one embodiment, a cover can be integrated into the base plate <NUM> that covers the central biasing element <NUM> to protect the central biasing element <NUM>. As shown in <FIG>, the blocking elements 1526a, 1526b define a housing cavity <NUM> which is dimensioned to house a portion of the central biasing element <NUM>. As shown in <FIG>, the blocking mechanism <NUM> is completely retained within the slot <NUM> such that the blocking mechanism <NUM> does not extend above an upper surface 1511a defined by the base plate <NUM>. This arrangement provides a lower profile for the plate assembly since the blocking mechanism <NUM> does not add any additional height to the plate assembly.

Claim 1:
An anterior cervical plate assembly (<NUM>) comprising:
a base plate (<NUM>; <NUM>) including:
at least two bone screw seats (222a-222d), each bone screw seat including a borehole (223a-223d) dimensioned to receive a bone screw (12a-12d), and
a first blocking seat (24a, 24b) positioned between the at least two bone screw seats(222a-222d); and
a blocking mechanism (14a, 14b; 214a, 214b; <NUM>) retained within the first blocking seat (24a, 24b) of the base plate (<NUM>; <NUM>), the blocking mechanism (14a, 14b; 214a, 214b; <NUM>) being selectively positionable between a closed position in which the blocking mechanism (14a, 14b; 214a, 214b; <NUM>) obstructs at least one bone screw seat of the base plate (222a-222d) to retain a bone screw with the base plate (<NUM>; <NUM>), and an open position in which the bone screw seats (222a-222d)are unobstructed;
wherein the blocking mechanism (14a, 14b; 214a, 214b; <NUM>) includes a central biasing element (<NUM>) and a first blocking element (1114a, 1114b; 1526a, 1526b) and a second blocking element (1114a, 1114b; 1526a, 1526b), wherein the first blocking element (1114a, 1114b; 1526a, 1526b) is configured to obstruct a first bone screw seat (222a-222d) of the at least two bone screw seats (222a-222d), and the second blocking element (1114a, 1114b; 1526a, 1526b) is configured to obstruct a second bone screw seat (14a, 14b; 214a, 214b) of the at least two bone screw seats (14a, 14b; 214a, 214b),
and wherein the first blocking element (1114a, 1114b; 1526a, 1526b) and the second blocking element (1114a, 1114b; 1526a, 1526b) are independently positionable from each other and wherein the first blocking element (1114a, 1114b; 1526a, 1526b) and the second blocking element (1114a, 1114b; 1526a, 1526b) define a housing cavity which is dimensioned to house a portion of the central biasing element (<NUM>; <NUM>).