Patent Publication Number: US-2022211419-A1

Title: Anterior cervical plate assembly

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/407,284 which is a continuation-in-part of U.S. Ser. No. 16/356,760, filed Mar. 18, 2019, which is a continuation-in-part of U.S. Ser. No. 15/814,490, filed Nov. 16, 2017, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF INVENTION 
     The present disclosure is related to a surgical implant, and is more particularly related to an anterior cervical plate. 
     BACKGROUND 
     Certain surgical procedures require a surgeon to fuse portions of a patient&#39;s spine to each other. Implanting a cervical plate reduces a patient&#39;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&#39;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&#39;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&#39;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. 
     SUMMARY 
     Briefly stated, an improved cervical plate assembly is disclosed. 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 25 degrees, and the second angle is at least 6 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary as well as the following detailed description will be best understood when read in conjunction with the appended drawings. In the drawings: 
         FIG. 1  is a perspective view of one embodiment of a cervical plate assembly implanted on a patient&#39;s spine. 
         FIG. 2  is a perspective view of the cervical plate assembly of  FIG. 1  including bone screws. 
         FIG. 3A  is a perspective view of the cervical plate assembly of  FIGS. 1 and 2  with a blocking mechanism in an open position. 
         FIG. 3B  is a perspective view of the cervical plate assembly of  FIGS. 1-3A  with the blocking mechanism in a blocked position. 
         FIG. 4A  is a top perspective view of the cervical plate assembly of  FIGS. 1-3B  including an installed bone screw with the blocking mechanism in the open position. 
         FIG. 4B  is a top perspective view of the cervical plate assembly of  FIGS. 1-4A  including an installed bone screw with the blocking mechanism in the blocked position. 
         FIG. 5A  is a top view of a cervical plate assembly according to one embodiment. 
         FIG. 5B  is a side view of the cervical plate assembly of  FIG. 5A . 
         FIG. 5C  is a front view of the cervical plate assembly of  FIGS. 5A and 5B . 
         FIG. 5D  is a cross-sectional view along line  5 D- 5 D of  FIG. 5C . 
         FIG. 5E  is a cross-sectional view along line  5 E- 5 E of  FIG. 5B . 
         FIG. 6A  is a top view of a cervical plate assembly in an open position according to one embodiment. 
         FIG. 6B  is a top view of the cervical plate assembly of  FIG. 6A  in a blocked position. 
         FIG. 6C  is a side cross sectional view of the cervical plate assembly of  FIGS. 6A and 6B  including a bone screw in a blocked position. 
         FIG. 7A  is a top view of a cervical plate assembly in an open position according to one embodiment. 
         FIG. 7B  is a top view of the cervical plate assembly of  FIG. 7A  in a blocked position. 
         FIG. 7C  is a perspective view of the cervical plate assembly of  FIGS. 7A and 7B  in the blocked position. 
         FIG. 7D  is a cross section view of a blocking mechanism of the cervical plate assembly of  FIGS. 7A-7C . 
         FIG. 8  is a perspective view of a cervical plate assembly in a blocked position according to one embodiment. 
         FIG. 9A  is a top view of a blocking mechanism according to one embodiment. 
         FIG. 9B  is a perspective view of the blocking mechanism of  FIG. 9A . 
         FIGS. 9C-9E  are perspective views of sub-components of the blocking mechanism of  FIGS. 9A and 9B . 
         FIG. 10A  is a top view of a cervical plate assembly including a bone screw in a blocked position according to one embodiment. 
         FIG. 10B  is a perspective view of the cervical plate assembly of  FIG. 10A . 
         FIG. 10C  is a side cross section view of the base plate of  FIGS. 10A and 10B . 
         FIG. 11A  is a top view of an embodiment of a cervical plate assembly. 
         FIG. 11B  is a side view of the cervical plate assembly of  FIG. 11A . 
         FIG. 12  is a top view of an embodiment of a blocking mechanism. 
         FIG. 13  is a side view of an embodiment of a blocking mechanism associated with a bone screw. 
         FIG. 14A  is a side view of a blocking mechanism according to one embodiment in an open position. 
         FIG. 14B  is a side view of the blocking mechanism of  FIG. 14A  in a blocked position. 
         FIG. 14C  is a top view of the blocking mechanism of  FIGS. 14A and 14B . 
         FIG. 14D  is a side view of a portion of the blocking mechanism of  FIGS. 14A-14C . 
         FIG. 15A  is a top view of a blocking mechanism according to one embodiment in a blocked position. 
         FIG. 15B  is a top view of the blocking mechanism of  FIG. 15A  in an open positon. 
         FIG. 16A  is a top view of a blocking mechanism according to one embodiment. 
         FIG. 16B  is a side cross section view of the blocking mechanism of  FIG. 16A . 
         FIG. 17A  is a magnified, side view a bone screw according to one embodiment. 
         FIG. 17B  is a side view of the bone screw of  FIG. 17A . 
         FIG. 17C  is a cross section view of the bone screw of  FIG. 17B  along line  17 C- 17 C. 
         FIG. 18A  is a top view of a cervical plate assembly in a blocked position according to one embodiment. 
         FIG. 18B  is a top view of the cervical plate assembly of  FIG. 18A  in an open position. 
         FIG. 18C  is a side cross section view of a blocking mechanism for the cervical plate assembly of  FIGS. 18A and 18B . 
         FIG. 19A  is a top view of a cervical plate assembly having a blocking mechanism in a blocked position according to one embodiment. 
         FIG. 19B  is a top view of the cervical plate assembly of  FIG. 18A  in an open position. 
         FIG. 19C  is a top cross section view of the cervical plate assembly illustrated in  FIG. 19B . 
         FIG. 20  is a top view of the cervical plate assembly of  FIGS. 19A-19C  with a fixation element extending therethrough. 
         FIG. 21A  is a perspective view of a cervical plate assembly according to one embodiment. 
         FIG. 21B  is a top view of the cervical plate assembly of  FIG. 21A  having a blocking mechanism in a blocked position. 
         FIG. 21C  is a top view of the cervical plate assembly of  FIG. 21A  having a blocking mechanism in an open position. 
         FIG. 21D  is a top cross section view of the cervical plate assembly illustrated in  FIG. 21C . 
         FIG. 22  is a perspective view of a blocking element in accordance with embodiments of the present disclosure. 
         FIG. 23  is a top view of a cervical plate assembly according to one embodiment. 
         FIGS. 24A-24B  depict an example of a fixation element for use with a cervical plate assembly in accordance with embodiments of the present disclosure. 
         FIGS. 25A-25B  depict an example of a fixation element for use with a cervical plate assembly in accordance with embodiments of the present disclosure. 
         FIGS. 26A-26B  depict a cervical plate assembly with the screws of  FIGS. 24A-25B  in accordance with embodiments of the present disclosure. 
         FIG. 27  depicts an example of a fixation element for use with a cervical plate assembly in accordance with embodiments of the present disclosure. 
         FIG. 28  depicts an example of a fixation element for use with a cervical plate assembly in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     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. 
       FIG. 1  illustrates a cervical region of a patient&#39;s spine  1  with an anterior cervical plate assembly  10  implanted on the spine  1 . As shown in  FIG. 1 , the plate assembly  10  is implanted at the C4-C5 vertebrae of the spine  1 . One of ordinary skill in the art would recognize that the plate assembly  10  could be installed at different regions of the patient&#39;s spine  1 . 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&#39;s anatomy besides the spine. With respect to  FIG. 1 , the anterior cervical plate assembly  10  is shown including four bone screws  12   a - 12   d  and two blocking assemblies  14   a,    14   b,  however one of ordinary skill in the art would recognize from the present disclosure that alternative arrangements of the anterior cervical plate assembly  10  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 4.2 mm and 4.6 mm, although one of ordinary skill in the art would understand that different sizes for bone screws can be used.  FIG. 2  illustrates the plate assembly  10  with the bone screws  12   a - 12   d  and the blocking mechanisms  14   a,    14   b  in an uninstalled state. As shown in  FIG. 2 , the plate assembly  10  includes two pairs of bone screws  12   a - 12   d , with each pair configured to be implanted into a vertebral body. In this embodiment, the blocking mechanisms  14   a,    14   b  each include a blocking screw  15   a,    15   b.  The blocking screws  15   a,    15   b  each include a head  16   a,    16   b  with a varying circumferential edge  17   a,    17   b,  and an engagement recess  20   a,    20   b  configured to be engaged by a tool for rotationally driving the heads  16   a,    16   b.  One of ordinary skill in the art would understand from the present disclosure that the geometry of the engagement recesses  20   a,    20   b  can be varied. In one embodiment, the engagement recess can be omitted and the blocking mechanisms  14   a,    14   b  can be manually engaged/actuated by a user. 
     As shown in  FIG. 2 , the circumferential edges  17   a,    17   b  of the blocking screws  15   a,    15   b  include diametrically opposed cutouts  18   a,    18   b,  and diametrically opposed lobes  19   a ,  19   b.  The cutouts  18   a,    18   b  are dimensioned to allow passage of the bone screws  12   a - 12   d  when the cutouts  18   a,    18   b  overlap an associated bone screw seat. The lobes  19   a,    19   b  are configured to obstruct an associated bone screw seat to block the bone screws  12   a - 12   d  from backing out of the plate assembly  10 . The engagement recesses  20   a,    20   b  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  16   a,    16   b  are engaged by a user and rotated a quarter turn, i.e. 90 degrees, to move from a blocked position, in which bone screws  12   a - 12   d  are retained with the plate assembly, to an open position, in which the bone screws  12   a - 12   d  can be removed from the plate assembly  10 . 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. 3A  illustrates an open configuration for the blocking mechanisms  14   a,    14   b , and  FIG. 3B  illustrates a blocked configuration for the blocking mechanisms  14   a,    14   b.  As shown in  FIGS. 3A and 3B , the plate assembly  10  includes a base plate  11  that defines bone screw seats  22   a - 22   d  for a respective one of the bone screws  12   a - 12   d.  The plate  11  also defines blocking mechanism seats  24   a,    24   b,  shown in  FIG. 3B , which are configured to retain a respective one of the blocking mechanisms  14   a,    14   b.  As illustrated in this embodiment, the blocking mechanism seats  24   a,    24   b  include a retention lip  25   a,    25   b  that prevents the blocking mechanisms  14   a,    14   b  from being removed from the plate  11 . 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  14   a,    14   b  are retained with the plate  11 . For example, the blocking mechanisms  14   a,    14   b  could be retained with the plate  11  by tabs, prongs, elastic elements, slots, channels, or other retention features. In one embodiment, the blocking mechanisms  14   a,    14   b  can include an enlarged head on an opposite end from the screw heads  16   a,    16   b  and the plate  11  can define a retention recess dimensioned to captively secure the enlarged head of the blocking mechanisms  14   a,    14   b.  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  14   a,    14   b  with the base plate  11  while also allowing the blocking mechanisms  14   a,    14   b  to rotate. 
       FIGS. 4A and 4B  illustrate the plate assembly  10  including a single bone screw  12   b.    FIG. 4A  illustrates the blocking mechanism  14   a  in an open position in which the bone screw  12   b  can be removed from the base plate  11 .  FIG. 4B  illustrates the blocking mechanism  14   b  in a blocking position in which the bone screw  12   b  is blocked from backing out of the plate assembly  10 . 
       FIGS. 5A-5E  illustrate a base plate  211 , and specifically illustrate the relatively high angulation of the bone screw seats  222   a - 222   d.  The base plate  211  includes a central window  209 , which can be used by a surgeon during installation to view the disc space (and interbody implant). This window  209  can be used to assist in making sure the base plate  211  is the correct position. Although the window  209  is only illustrated in the embodiment of  FIGS. 5A-5E , one of ordinary skill in the art would recognize from the present disclosure that the window  209  can be integrated into the design of any of the base plates disclosed herein. The plate  211  includes two blocking mechanisms  214   a,    214   b.  As shown in  FIG. 5A , the plate  211  defines channels  207   a ,  207   b  for each of the blocking mechanisms  214   a,    214   b  in which blocking elements can slide. Blocking elements (not shown in  FIG. 5A ) are retained within the channels  207   a,    207   b  and are driven outward to the bone screw seats  222   a - 222   d  to retain bone screws with the plate  211 . A tapered profile defined by the channels  207   a,    207   b  is shown in  FIG. 5D . The blocking elements of the blocking mechanism  214   a,    214   b  have a complementary profile such that the blocking elements are retained in the channels  207   a,    207   b  and can slide within the channels  207   a,    207   b . The channels  207   a,    207   b  define a laterally outer stop surface for the blocking elements. 
     As shown in  FIG. 5A , the base plate  211  is divided by two primary axes: a central lateral axis X 1  and a central longitudinal axis X 2 . The high angulation of the bone screw seats  222   a - 222   d  provides a strong and rigid construction for attaching bone screws to a patient. The plate  211  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  222   a - 222   d  define a borehole  223   a - 223   d  with a central axis CA 1 -CA 4 . Each central axis CA 1 -CA 4  of boreholes  223   a - 223   d  is angled relative to the central lateral axis X 1  of the base plate  211  at a first angle θ 1 . In one embodiment, the first angle θ 1  is between 50-75 degrees. In one embodiment, the first angle θ 1  is at least 65 degrees. In one embodiment, the first angle θ 1  is at least 70 degrees. Each central axis CA 1 -CA 4  of boreholes  223   a - 223   d  is also angled relative to the central longitudinal axis X 2  at a second angle θ 2 . In one embodiment, the second angle θ 2  is between 4-8 degrees. In one embodiment, the second angle θ 2  is at least 6 degrees. Both the first angle θ 1  and the second angle θ 2  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  223   a  and  223   b  (with respect to the view shown in  FIG. 5A ) are angled or canted towards each other on an underside of the base plate  211  due to the second angle θ 2 . Although the angulation of the central axes CA 1 -CA 4  of the boreholes  223   a - 223   d  for the bone screw seats  222   a - 222   d  are only specifically illustrated in  FIGS. 5A-5E  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. 
       FIGS. 6A-6C  illustrate another embodiment of a plate assembly  310  including a base plate  311 . As shown in  FIGS. 6A-6C  the plate assembly  310  includes two bone screw seats  322 , with a single bone screw  312  arranged in one of the bone screw seats  322 . This plate assembly  310  is only illustrated with two bone screw seats  322 , 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  310  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  11  described above. 
     The plate assembly  310  includes a blocking mechanism  314  having a different profile than the blocking mechanisms  14   a,    14   b  of  FIGS. 1-3B . The blocking mechanism  314  includes a head  316  with a varying circumferential edge  317 , and an engagement recess  320  configured to engage a tool for rotationally driving the blocking mechanism  314 . The edge  317  includes diametrically opposed cutouts  318 , and diametrically opposed lobes  319 . The cutouts  318  are dimensioned to allow insertion of the bone screw  312 , and the lobes  319  are configured to block the bone screw  312  from backing out of the plate assembly  310 . The lobes  319  are also configured to deflect inward when the bone screw is inserted and then blocks the bone screw from backing out. The circumferential edge  317  of the blocking mechanism  314  has a symmetrical profile such that the blocking mechanism  314  provides an identical profile if the blocking mechanism  314  is rotated 180°. Slots  321  are provided on the blocking mechanism  314  which provide grips or insertion points for a tool for rotating the blocking mechanism  314 .  FIG. 6A  illustrates the blocking mechanism  314  in an open position, and  FIG. 6B  illustrates the blocking mechanism  314  in a blocked position without a bone screw  312 .  FIG. 6C  illustrates a side cross section view of the plate assembly  310 . As shown in  FIG. 6C , the plate  311  defines a blocking mechanism seat  324  which axially retains the blocking mechanism  314  within the plate  311 . The plate  311  defines grooves  313  configured to engage a threading  323  defined on the blocking mechanism  314 . It should be noted that, a bone screw can be inserted into screw holes even when the blocking mechanism  314  is in a blocked position as shown in  FIG. 6B , as the lobes of the blocking mechanism may be deformable. As shown in  FIG. 6C , a clearance (c) is defined between the blocking mechanism  314  and the bone screw  312  when the bone screw  312  is fully seated in the bone screw seat  322 . 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. 
       FIGS. 7A-7D  illustrate another embodiment of a plate assembly  410  and a base plate  411 . The blocking mechanism  414  includes a head  416  having a generally X-shaped profile, and including four arms  428 . Resilient blocking elements  426  extend between a respective pair of the four arms  428  on diametrically opposed sides of the head  416 . Reliefs  427  are provided on sides of the head  416  including the resilient blocking elements  426 . These reliefs  427  allow for the resilient blocking elements  426  to deform radially inwardly towards a central rotational axis of the blocking mechanism  414 .  FIG. 7A  illustrates the blocking mechanism  414  in an open position.  FIG. 7B  illustrates the blocking mechanism  414  is a blocked position, with the resilient blocking elements  426  overlapping the bone screw seats  422 . This particular blocking mechanism  414  can be in either the open or blocked position and still allow insertion of the bone screw  412  into the plate assembly  410 . This embodiment allows for the bone screw  412  to be inserted into the plate assembly  410  when the blocking mechanism  414  is in the blocked position ( FIG. 7B ) due to elastic deformation of the resilient blocking elements  426  during insertion of the bone screw  412 . Once fully inserted and installed, the bone screw  412  is prevented from backing out of the plate assembly  410  due to the resilient blocking elements  426 . In one embodiment, the resilient blocking element  426  is formed from spring steel. In another embodiment, the resilient blocking element  426  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  426  can be used as long as the blocking elements  426  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. 7C  illustrates the plate assembly  410  in a perspective view with the blocking mechanism  414  in a blocked position. As shown more clearly in  FIG. 7D , the blocking mechanism  414  includes cutouts  418  that are dimensioned to allow passage of the bone screws  412 . 
       FIG. 8  illustrates another embodiment of a plate assembly  510  in a blocked position. The plate assembly  510  includes a blocking mechanism  514  including blocking lobes  519 . The blocking lobes  519  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. 
       FIGS. 9A-9E  illustrate another embodiment of a blocking mechanism  614 . The blocking mechanism  614  of  FIGS. 9A-9E  can be integrated into any of the plate assemblies described herein. The blocking mechanism  614  includes a hub  650 , a blocking element  654 , and retention washer  658 . The hub  650  defines grooves for accommodating the blocking element  654 . The blocking element  654  defines lobes on diametrically opposite sides of the blocking element  654 , each configured to block a bone screw within a cervical plate assembly. The blocking element  654  can be formed as a wire forming a continuous loop. The blocking element  654  can be snapped into or placed into the grooves formed on the hub  650 , and a retention washer  658  can then be pressed or snapped onto a bottom end of the hub  650 . The retention washer  658  can be snapped into a groove formed on the hub  650  such that the blocking element  654  is retained between the hub  650  and the retention washer  658 . The blocking mechanism  614  can be rotated while retained within a seat of a base plate, such that the blocking element  654  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  654  is rotated away from the bone screw seat and the bone screw seat is unobstructed. 
       FIGS. 10A-10C  illustrate another embodiment of a plate assembly  710 . The plate assembly  710  includes a blocking mechanism  714  including blocking tabs  719  arranged on opposite sides of a central portion  729 . The blocking tabs  719  are configured to provide obstructions to an underlying bone screw seat, such that a bone screw is retained with the plate assembly  710 . The central portion  729  includes resilient arms  731 , such that the blocking mechanism  714  can be deformed by applying inward pressure to the blocking tabs  719 . The resilient arms  731  form an X-shaped configuration with reliefs  733  formed between the arms  731 . A user can manually pinch the blocking tabs  719  towards each other, and the blocking mechanism  714  can be manipulated from an open position to a blocked position without the use of a tool. The blocking mechanism  714  can be retained with the base plate  711  via a slot  721  formed in the base plate  711 , or other retention configuration. As shown in  FIG. 10C , the slot  721  has a tapered profile, and the blocking mechanism  714  has a complementary tapered profile such that the blocking mechanism  714  is retained within the slot  721 . One of ordinary skill in the art would understand from the present disclosure that this slot  721  can be integrated into any of the base plates described herein to retain a blocking mechanism. 
       FIGS. 11A and 11B  illustrate another embodiment of a plate assembly  810  including a blocking mechanism  814 . The blocking mechanism  814  includes a central hub  816  defining a pair of C-shaped arms  816   a,    816   b,  with resilient blocking elements  826  extending therebetween. Blocking tabs  819  are engaged against a respective one of the resilient blocking elements  826 . Reliefs  827  are formed on the hub  816 , and the reliefs  827  are dimensioned to each accommodate one of the resilient blocking elements  826  and one of the blocking tabs  819 .  FIG. 11A  illustrates the blocking mechanism  814  in an expanded state in which the blocking tabs  819  are extended and configured to block a bone screw seat. The blocking mechanism  814  is configured to be rotated by 90° such that the blocking tabs  819  and the resilient blocking elements  826  are pushed inwardly into a respective one of the reliefs  827 , and the blocking tabs  819  and the resilient blocking elements  826  are in a compressed state. The blocking tabs  819  can be retained within slots formed in the base plate  811 , such that the blocking tabs  819  slide within the slots. 
       FIG. 12  illustrates an alternative configuration for a blocking mechanism  914  in which two blocking screws  914   a,    914   b  are provided for blocking a respective bone screw seat. As shown in  FIG. 12 , a single blocking screw  914   a  includes an engagement recess  920  configured to be engaged by a tool to rotate the blocking screw  914   a.  Each of the blocking screws  914   a,    914   b  include teeth  937   a,    937   b  which engage each other such that rotation of the first blocking screw  914   a  drives rotation of the second blocking screw  914   b.  The blocking screws  914   a    914   b  each define a blocking lobe  919   a,    919   b  and a cutout  918   a,    918   b  such that in a blocked position the blocking lobes  919   a,    919   b  overlap with a bone screw seat, and in an open position the cutouts  918   a,    918   b  overlap with a bone screw seat to allow for insertion of a bone screw. 
       FIG. 13  illustrates a blocking mechanism  1014  that is directly integrated with a bone screw  1012 . As shown in  FIG. 13 , the bone screw  1012  is inserted into a base plate  1010 , and the blocking mechanism  1014  provides axial retention of the bone screw  1012  relative to the base plate  1010 . The blocking mechanism  1014  is formed as a split ring element, which is retained within a groove  1013  formed in a head of the bone screw  1012 . A tool including a sleeve can be inserted around the head of the bone screw  1012  to push the blocking mechanism  1014  into the groove  1013  of the bone screw  1012 . The blocking mechanism  1014  can define a tapered edge  1014 ′ which is configured to engage against the base plate  1010  during insertion, such that the tapered edge  1014 ′ slides along the base plate  1010  and the blocking mechanism  1014  is pushed into the groove  1013 . 
       FIGS. 14A-14D  illustrate a blocking mechanism  1114  including two blocking elements  1114   a,    1114   b,  a central resilient element  1126 , and two blocking resilient elements  1126   a,    1126   b.  As shown in  FIG. 14A , the blocking resilient elements  1126   a,    1126   b  are engaged against a base plate  1111 , and retain the blocking elements  1114   a,    1114   b  in a first position, which can correspond to an open configuration. The central resilient element  1126  biases the blocking elements  1114   a,    1114   b  outward from each other, but the blocking resilient elements  1126   a,    1126   b  define a stop against the base plate  1111  and prevent this outward movement in  FIG. 14A . As shown in  FIG. 14B , once the blocking resilient elements  1126   a,    1126   b  are downwardly depressed, then the central resilient element  1126  drives the blocking elements  1114   a,    1114   b  outward to a second position, which can correspond to a blocked position.  FIG. 14C  shows a top view of the blocking mechanism  1114  with the blocking elements  1114   a,    1114   b  in the blocked position and overlapping the bone screw seats  1122   a,    1122   b.    FIG. 14D  illustrates the blocking mechanism  1114  separate from the base plate  1111 . In one embodiment, the blocking resilient elements  1126   a,    1126   b  are pressed downward via a tool. One of ordinary skill in the art would recognize that the blocking resilient elements  1126   a,    1126   b  can be pressed downward by a user/surgeon. In one embodiment, the blocking resilient elements  1126   a,    1126   b  are spring elements (e.g., leaf springs, cantilevered springs, etc.), but one of ordinary skill in the art would recognize from the present disclosure that alternative blocking elements can be used. 
       FIGS. 15A and 15B  illustrate another embodiment of a blocking mechanism  1214 . In this embodiment, the blocking mechanism  1214  includes a central blocking element  1219  that moves between a blocked position shown in  FIG. 15A  and an open position in  FIG. 15B  in which the bone screw seats  1222   a,    1222   b  are either obstructed ( FIG. 15A ) or unobstructed ( FIG. 15B ). The central blocking element  1219  is a flexible plate that extends between lateral sides of the base plate  1211 . A blocking cam  1226  is arranged within a slot  1221  formed on the base plate  1211 . The blocking cam  1226  is configured to be positioned in a lower position within the slot  1221 , as shown in  FIG. 15A , in which the central blocking element  1219  is flexed to overlap with the bone screw seats  1222   a,    1222   b.  The terminal ends of the central blocking element  1219  are understood to be fixed to the lateral sides of the base plate  1211  such that the central blocking element  1219  flexes and exhibits inflexion as shown in  FIG. 15A . As shown in  FIG. 15B , as the blocking cam  1226  is moved upward within the recess  1221 , then the central blocking element  1219  returns to a relatively straight profile such that the central blocking element  1219  does not overlap with the bone screw seats  1222   a,    1222   b.    
       FIGS. 16A and 16B  illustrate an alternative embodiment of a bone screw  1312  including a screw head  1315 . The screw head  1315  includes a slot  1316  and a relief  1317 . The relief  1317  is centered relative to a shaft of the bone screw  1312 . The relief  1317  acts as a drive feature to help thread the screw  1312  and allows the screw head  1315  to compress to reduce the outer diameter of the screw head  1315 . By having a nominal screw head diameter larger than a bone screw hole diameter defined on the base plate, the screw head  1315  provides an interference fit with the base plate when the driver tool is not engaged. When the bone screw  1312  is fully seated within an associated base plate, and the driver tool is removed from the screw head  1315 , then the screw head  1315  expands back open to its nominal diameter, which is oversized relative to the associated bone screw seat and prevents the bone screw  1312  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  1312  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. 
       FIGS. 17A-17C  illustrate an alternative embodiment of a bone screw  1412 . As shown in  FIG. 17A , the bone screw  1412  has a screw head  1415  with a relief  1415 ′. The relief  1415 ′ is configured to accommodate a portion of a blocking mechanism  1414  (shown in dashed lines in  FIG. 17A ). This relief  1415 ′ allows for the blocking mechanism to partially overlap an axial end of the bone screw  1412 , and as a result allows for a thinner base plate. The relief  1415 ′ also ensures that the blocking mechanism sufficiently overlaps with the bone screw  1412  in the axial direction. 
       FIGS. 18A-18C  illustrate another embodiment of a blocking mechanism  1514 . The blocking mechanism  1514  includes a central biasing element  1526  and two blocking elements  1526   a,    1526   b  on opposite ends of the central biasing element  1526 . The blocking mechanism  1514  is retained to a base plate  1511 , and the base plate  1511  includes bone screw seats  1522   a ,  1522   b  (a single bone screw  1512  is shown within the base plate  1511 ). As shown in  FIG. 18A , the blocking elements  1526   a,    1526   b  are in the extended position due to the biasing force from the biasing element  1526 . In this position, the blocking elements  1526   a,    1526   b  overlap with the bone screw seats  1522   a,    1522   b  and are in a blocked position to retain the bone screws with the base plate  1511 . In  FIG. 18B , the blocking elements  1526   a,    1526   b  are in a compressed configuration with the biasing element  1526  being compressed and housed within cavities of the blocking elements  1526   a,    1526   b.  In this configuration, the blocking elements  1526   a,    1526   b  are positioned away from the bone screw seats  1522   a    1522   b  such that the bone screw  1512  can be removed from the base plate  1511 . The blocking elements  1526   a,    1526   b  can be independently moved with respect to each other such that a bone screw  1512  can be removed from one of the bone screw seats  1522   a,    1522   b  while a bone screw  1512  is blocked in the other one of the bone screw seats  1522   a,    1522   b.  The central biasing element  1526  can be a coil spring, leaf spring, or any other type of elastic component. As shown in  FIG. 18C , the blocking elements  1526   a,    1526   b  are retained in the base plate  1511  via a mating slot feature. As shown in  FIG. 15C , the base plate  1511  defines a slot  1517  with a protrusion  1517 ′ and the blocking element  1526   a  includes a groove  1526   a ′. 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  1526   a ,  1526   b  with the base plate  1511 . For example, the retention can be achieved via a t-slot, dovetail, or other mating feature. The blocking elements  1526   a,    1526   b  are slidingly retained within the slot  1517 . In one embodiment, a cover can be integrated into the base plate  1511  that covers the central biasing element  1526  to protect the central biasing element  1526 . As shown in  FIG. 18C , the blocking elements  1526   a,    1526   b  define a housing cavity  1527  which is dimensioned to house a portion of the central biasing element  1526 . As shown in  FIG. 18C , the blocking mechanism  1514  is completely retained within the slot  1517  such that the blocking mechanism  1514  does not extend above an upper surface  1511   a  defined by the base plate  1511 . This arrangement provides a lower profile for the plate assembly since the blocking mechanism  1514  does not add any additional height to the plate assembly. 
       FIGS. 19A-19C  and  FIG. 20  illustrate another embodiment of a base plate  1911  having a blocking mechanism in accordance with embodiments of the present disclosure. The base plate  1911  is substantially similar to the base plate described above. As such, a description of similar features of the base plate  1911  will be omitted here. The blocking mechanism includes a blocking element  1926  (two blocking elements  1926   a,    1926   b  shown) disposed in a corresponding groove  2125  (two grooves  2125   a,    2125   b ) such that a portion of the blocking element protrudes into the bone screw seat  1922  (two bone screw seats  1922   a,    1922   b  shown). Although only two bone screw seats  1922   a,    1922   b  are shown here, it should be noted that the base plate  1911  may include four screw seats as discussed above with respect to bone plate  11  or up to twelve screw seats to accommodate more bone screws. The bone screw seat  1922  is substantially similar to the bone screw seats described above. As such, a description of the bone screw seat  1922  is omitted here. 
     In some embodiments, the blocking elements  1926   a,    1926   b  are bias elements that move between an initial state (shown in  FIG. 19A ) and a compressed state (shown in  FIGS. 19B  and  19 C). In some embodiments, the blocking elements  1926   a,    1926   b  are spring elements. As shown in  FIG. 19A , the blocking elements  1926   a,    1926   b  are in the initial state due to their intrinsic biasing force. In this position, the blocking elements  1926   a,    1926   b  overlap with the bone screw seats  1922   a,    1922   b  and are in a blocked position to retain the bone screws  2000  within the base plate  1911  (as shown in  FIG. 20 ). In  FIG. 19B , the blocking elements  1926   a,    1926   b  are in a compressed configuration with the blocking elements  1926   a,    1926   b  being compressed and entirely disposed within corresponding grooves  1925   a,    1925   b,  which are formed in the base plate  1911  (shown more clearly in the cross-section of  FIG. 19C ) adjacent to corresponding screw seats  1922   a,    1922   b.  In this configuration, the blocking elements  1926   a,    1926   b  are positioned away from the bone screw seats  1922   a    1922   b  such that the bone screw  2000  can be inserted or removed from the base plate  1911 . 
     During insertion of the bone screw  2000  into one of the screw seats  1922   a,    1922   b , the head of the bone screw contacts a ramped surface  1924  of the corresponding blocking element  1926  to deflect the blocking element  1926  away from the screw seat  1922  as the screw  2000  continues to be advanced. After the screw head passes beneath the blocking element  1926 , the blocking element  1926  returns to its initial state, thus blocking removal of the screw  2000  from the screw seat  1922  (as shown in  FIG. 20 ). In this locked position, the blocking element  1926  resists screw back out by partially protruding into the screw seat  1922  and contacting a portion of the screw head. The blocking element  1926  advantageously provides tactile feedback to the surgeon so that there is confirmation that the blocking element  1926  has returned to its initial position to block the screw  2000 . In addition to this tactile confirmation, the blocking element  1926  also advantageously provides visual confirmation to the surgeon that the blocking element  1926  is being deflected away from the screw seat  1922  during insertion and that the blocking element  1926  has returned to its initial state after the screw  2000  has passes beneath the blocking element  1926 . To remove the screw  2000  after it has passed beneath the blocking element  1926 , the blocking element  1926  must first be compressed (i.e., moved away from the screw seat  1922 ). This may be performed using a driver (not shown) having a taper that compresses the blocking element  1926  as the drive tip is inserted into the screw head. Alternatively, there may be a separate instrument that can compress the blocking element  1926  so that a driver can be inserted into the screw head to remove it. 
     The blocking elements  1926   a,    1926   b  can be independently moved with respect to each other such that a bone screw  2000  can be inserted or removed from one of the bone screw seats  1922   a,    1922   b  while a bone screw is blocked in the other one of the bone screw seats  1922   a ,  1922   b.  One of ordinary skill in the art would recognize from the present disclosure that alternative types of blocking elements that elastically deform to retain a screw within the base plate  1911  are within the scope of the present disclosure. For example, the retention can be achieved via a t-slot, dovetail, or other mating feature. 
     As depicted in  FIG. 20 , in some embodiments, the base plate  1911  may include a through hole  1940  disposed between two screw seats  1922  and at a midline of the base plate  1911 . The through hole  1940  facilitates the insertion of temporary fixation elements (not shown) through the hole  1940  and into a bone (e.g., a vertebra) onto which the plate  1911  is to be coupled to preliminarily capture the plate  1911  onto the bone. In some embodiments, the base plate  1911  may additionally include one or more cuts  1942  formed an outer surface  1944  of the base plate  1911  to allow a tool (not shown) to grasp the base plate  1911  in the predetermined orientation and insert the plate in the predetermined trajectory. 
       FIGS. 21A-21D  illustrate another embodiment of a base plate  2111  having blocking elements  2126  disposed corresponding screw seats  2122  in accordance with embodiments of the present disclosure.  FIG. 22  illustrates a blocking element  2126 . The base plate  2111  is substantially similar to the base plate  1911  described above. As such, a description of similar features of the base plate  2111  will be omitted here. Although the base plate  2111  is illustrated as having six screw seats  2122 , each having a corresponding blocking element  2126 , it should be noted that the base plate  2126  may alternatively have fewer (two or four as with the base plates discussed above) or more (up to twelve) screw seats  2122 . 
     In this embodiment, the blocking element  2126  may alternatively be planar instead of having a ramped surface. As a bone screw (e.g., screw  2000 ) is inserted into the screw seat  2122 , the bottom ramped surface of the screw head contacts an innermost tip  2132  of the blocking element  2126  and moves it towards the compressed state (shown in  FIGS. 21C, 21D ) as the screw is advanced further through the screw seat  2122 . Similar to the blocking element  1926 , the blocking element  2126  returns to its initial state (illustrated in  FIG. 21A, 21B ) after the screw head passes beneath the blocking element  2126  to obstruct the screw seat  2122  and retain the screw within the screw seat  2122 . 
     As depicted in  FIG. 22 , the blocking element  2126  is a monolithic planar element having a blocking portion  2202 , a base portion  2204 , and an arm  2206  coupling the blocking portion  2202  to the base portion  2204 . In some embodiments, the base portion  2204  has a bulbous shape and is retained in a correspondingly shaped opening  2208  formed in the base plate  2111  (shown more clearly in the cross-section of  FIG. 21D ). When the blocking element  2126  is compressed by the screw head, the blocking portion  2202  is pushed into a groove  2125  formed in the base plate  2111  such that the screw seat  2122  is unobstructed. 
     Although several embodiments have been disclosed in the foregoing specification, it is understood that many modifications and other embodiments will come to mind, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the scope of the embodiments described herein are not limited to the specific embodiments disclosed hereinabove, and that many modifications and other embodiments are intended to be included within the scope of the appended claims. For example, although the base plates (e.g. base plates  11 ,  1911 ,  1511 ,  2111 ) are shown with two or four screw seats, it should be noted that the present disclosure encompasses a base plate  2311  having up to twelve screw seats  2322 , as depicted in  FIG. 23 . 
     The above-disclosed base plates may utilize a variety of different fixation elements to secure the base plate to a vertebra such as, for example, fixed angle screws, variable angle screws, self-drilling tip screws, self-tapping tip screws, primary diameter screws, rescue diameter screws, and double rescue diameter screws.  FIGS. 24A-24B, 25A-25B, 27, and 28  depict examples of fixation elements (e.g., screws) for use with the base plates (e.g. base plates  11 ,  1911 ,  1511 ,  2111 ) in accordance with embodiments of the present disclosure.  FIGS. 26A-26B  depict a cervical plate assembly with the screws of  FIGS. 24A-25B  in accordance with embodiments of the present disclosure. 
     When comparing fixed angle screws with variable angle screws, there is a difference in a diameter of the necks of the screws. Each screw consists of a spherical screw head with a drive feature used to thread the screw into the bone. This spherical screw head sits in a corresponding spherical hole in the base plate, which provides the capability of polyaxial motion of the screw. The fixed angle screw has a larger neck diameter which allows for less angulation in the plate compared to the variable angle screw. Such a scenario may be beneficial when, for example, a surgeon wants to limit post-operative settling. The variable angle screw has a smaller neck diameter which allows for more angulation of the screw in the plate. Additionally, different types of fixation elements may include identifying features to clearly distinguish them from one another. For example, as will be discussed below, screw heads may include a predetermined pattern of cuts and laser marks to distinguish the various types of fixation elements (i.e., screws) from one another. 
       FIGS. 24A-24B  depict a fixed angle screw  2400 . As illustrated in  FIG. 24A , the screw  2400  includes a head  2402 , a threaded shaft  2404  extending from the head  2402 , and a neck portion  2406  disposed between the head  2402  and the threaded shaft  2404 . As illustrated in  FIG. 24B , the screw head  2402  includes a tool engagement feature  2402  so that driver (not shown) can engage the screw  2400  and drive it into a bone. The screw head  2402  further includes identifying features  2410 . In some embodiments, the identifying features  2410  may be cutouts filled with laser marks and arranged in a predetermined configuration to identify the screw  2400  as a fixed angle screw. 
       FIGS. 25A-25B  depict a variable angle screw  2500 . As illustrated in  FIG. 25A , the screw  2500  includes a head  2502 , a threaded shaft  2504  extending from the head  2502 , and a neck portion  2506  disposed between the head  2502  and the threaded shaft  2504 . As illustrated in  FIG. 25B , the screw head  2502  includes a tool engagement feature  2502  so that driver (not shown) can engage the screw  2500  and drive it into a bone. The screw head  2502  further includes identifying features  2510 . In some embodiments, the identifying features  2510  may be cutouts filled with laser marks and arranged in a predetermined configuration to identify the screw  2500  as a variable angle screw. 
     As noted above, the base plates of the present disclosure may include various different fixation elements. For example, as depicted in  FIG. 26A , the base plate may include the fixed angle screw  2400  disposed in one screw seat and the variable angle screw  2500  disposed in a different screw seat.  FIG. 26B  is a top view of the variable angle screw  2500  disposed in its screw seat with the blocking element disposed above the screw head to prevent any possible backing out of the screw. 
       FIGS. 27 and 28  depict additional examples of different fixation elements that may be used with the base plates of the present disclosure.  FIG. 27  depicts a screw  2700  having a self-drilling tip  2702 .  FIG. 28  depicts a screw having a self-tapping tip  2802 . The self-drilling tip  2702  is sharp, which allows a user to insert the screw  2700  without drilling a pilot hole in the bone. The self-tapping tip  2802  is blunt, which requires a user to pre-drill pilot hole prior to inserting the screw  2800 . In some embodiments, the screws may also include a cut  2704 ,  2804  on the top of the screw head to allow for additional clearance blocking element. This advantageously helps block the screw disposed in the screw seat by keeping the screw head contact surface farther away from the blocking element. 
     It is further envisioned that features from one embodiment may be combined or used with the features from a different embodiment described herein. Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described embodiments, nor the claims which follow.