Patent Publication Number: US-10327820-B2

Title: Spinal plate with compression locking

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/265,785, filed on Apr. 30, 2014, which is a divisional of U.S. patent application Ser. No. 12/908,814, filed on Oct. 20, 2010, which claims priority to and benefit of U.S. Provisional Patent Application Ser. No. 61/279,351, filed on Oct. 21, 2009, and U.S. Provisional Patent Application Ser. No. 61/280,950, filed on Nov. 12, 2009, which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Field of Technology 
     This application relates generally to spinal fixation. More specifically, this application is directed to a spinal plate with compression locking and a method of fixating vertebrae using the spinal plate with compression locking. 
     Brief Description of Related Art 
     Spinal surgery frequently requires fixation of the spinal column, e.g., spinal fixation of a plurality of spinal vertebrae. Spinal vertebrae are bony cylindrical structures that are located in front of the spinal cord and nerves; they contribute to the structural support of the axial skeleton. Anterior or lateral spinal fixation can be used to fixate vertebrae along the cervical, thoracic and lumbar regions of the spinal column. 
     Generally, a spinal plate and a plurality of screws are used for spinal fixation. The spinal plate is placed over multiple vertebrae to be fixated with respect to one another. Generally, the openings through the spinal plate have matching geometry to the screws, allowing screw angulations over a range of trajectories through the spinal plate. The screws anchor the spinal plate to the vertebrae. The screw angulations provide for various screw placements given different anatomy of patients and locations along the spinal column. 
     Screw back-out that results from the loosening of the screws with respect to the spinal plate is a significant concern. When screws loosen, their screw heads pivot about the openings of the spinal plate with spinal plate motion in respect to the vertebrae. Unrestricted movement can result in screw dislodgement with respect to the spinal plate, jeopardizing the patient&#39;s health. 
     Multiple back-out mechanisms have been proposed. However, the back-out mechanisms do not adequately lock (rigidly fixate) the screws (via their screw heads) in their trajectories with respect to the spinal plate, but rather attempt to prevent dislodgement of the screws from the spinal plate. For rigid fixation versus back-out prevention, the potential for pivoting of screw heads about the openings in the spinal plate should be restrained. 
     Furthermore, the foregoing back-out mechanisms usually include multiple components that require complex engagement with respect to the spinal plate and screws, blocking visualization of the underlying vertebra and increasing undesirably the size of the spinal plate. 
     SUMMARY 
     In accordance with an embodiment, a polyaxial spinal screw is provided. The polyaxial spinal screw includes a head and a threaded body. 
     The head is defined by a portion of a first sphere that has a first center and a portion of a second sphere that has a second center, wherein the second center is approximately concentric with the first center. 
     The threaded body extends from the portion of first sphere, wherein the portion of the first sphere is compressible approximately through the first center via compression of the portion of the second sphere. 
     The portion of the first sphere can be defined by the first sphere that is truncated by a first diameter and a second diameter, wherein the first diameter is smaller than the second diameter. Moreover, the first diameter and the second diameter can be smaller than a diameter through the first center of the first sphere. 
     The threaded body can extend from the first sphere truncated by the first diameter. The head can further be defined a cylinder that extends from the first sphere truncated by the second diameter, wherein the cylinder is within or circumscribed by the first sphere. 
     The head can further be defined by a truncated cone that extends approximately centrally with respect to the first sphere truncated by the second diameter between the cylinder and the portion of the second sphere. The truncated cone and the portion of the second sphere can intersect to form an engagement surface that extends approximately centrally from the cylinder. 
     The head can further be defined by one or more geometrical shapes that extend approximately centrally with respect to the first sphere truncated by the second diameter between the cylinder and the portion of the second sphere. The one or more geometrical shapes can include a cylindrical, a conical, or a spheroidal shape, or a combination of two or more shapes thereof. The one or more geometrical shapes and the portion of the second sphere can intersect to form an engagement surface that extends approximately centrally from the cylinder. 
     The head can further be defined by a plurality of recesses that extend through the cylinder into the portion of the first sphere, wherein the recesses are configured to engage reciprocal extensions of a tool that rotates the spinal screw. The recesses can extend through a periphery of the cylinder and a periphery of the portion of the first sphere. The recesses can be disposed equidistantly about the cylinder. 
     The portion of the second sphere can be deformable. The portion of the second sphere can include a portion that is deformable. 
     The threaded body can include a shaft, a thread, and a tip. The shaft can have a first end and a second end. The thread can extend about a portion of the shaft between the first end and the second end. The tip can extend from the second end of the shaft. The tip can be self-cutting. 
     For a more thorough understanding of the present invention, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which: 
         FIG. 1  illustrates a perspective view of an example spinal plate; 
         FIG. 2  illustrates a perspective view of an example threaded recess provided in the plate body of the spinal plate that is eccentric (off-center) with reference to an opening of the plate body; 
         FIG. 3  illustrates a cross-sectional view of the plate body illustrated in  FIG. 2 ; 
         FIG. 4  illustrates a perspective view of a vertebral screw (screw); 
         FIG. 5  illustrates a perspective top view of a threaded cap configured to be disposed in one or more alignment positions with respect to the threaded recess of  FIG. 2 ; 
         FIG. 6  illustrates a perspective bottom view of the threaded cap illustrated in  FIG. 5 ; 
         FIG. 7  illustrates a cross-sectional view of the threaded cap illustrated in  FIG. 5 ; 
         FIG. 8  illustrates a perspective view of the screw of  FIG. 4  disposed in the opening of the plate body in the spinal plate of  FIG. 1 ; 
         FIG. 9  illustrates a perspective view of the threaded cap of  FIG. 5  engaging threading of the threaded recess in a screw-loading alignment and in a screw-compression alignment; 
         FIG. 10  illustrates a cross-sectional view of the threaded cap of  FIG. 5 , engaging threading of the threaded recess in the screw-loading alignment and in the screw-compression alignment; 
         FIG. 11  illustrates a perspective view of an example spinal plate; 
         FIG. 12  illustrates a cross-sectional view of an opening and a recess of the spinal plate illustrated in  FIG. 11 ; 
         FIG. 13  illustrates a perspective view of the example crescent-shaped cap; 
         FIG. 14  illustrates a compression ramp of the crescent-shaped cap illustrated in  FIG. 13 ; 
         FIG. 15  illustrates a top view of the crescent-shaped cap engaging the recess of  FIG. 12  in a screw-loading alignment; 
         FIG. 16  illustrates a perspective view of the crescent-shaped cap engaging the recess of  FIG. 12  in the screw-loading alignment; 
         FIG. 17  illustrates a perspective view of the crescent-shaped cap engaging the recess of  FIG. 12  in a screw-compression alignment; 
         FIG. 18  illustrates a cross-sectional view of the example crescent-shaped cap between the screw-loading alignment and the screw-compression alignment with respect to the opening in the spinal plate of  FIG. 11  and the screw of  FIG. 4 ; 
         FIG. 19  illustrates a perspective view of a vertebral screw (screw); 
         FIG. 20  illustrates a perspective view of the screw of  FIG. 19  engaging a compression ramp of the crescent-shaped cap of  FIG. 13 ; 
         FIG. 21  illustrates a cross-sectional view of the example crescent-shaped cap of  FIG. 13  between the screw-loading alignment and the screw-compression alignment with respect to the opening of  FIG. 12  and the screw of  FIG. 19 ; 
         FIG. 22  illustrates a cross-sectional view of the example crescent-shaped cap of  FIG. 13  in the screw-compression alignment with respect to the screw of  FIG. 19  in the opening of  FIG. 12 ; 
         FIG. 23  illustrates a perspective view of an example crescent-shaped cap engaging the recess of  FIG. 12  in the screw-loading alignment with respect to an example screw having a deformable portion; 
         FIG. 24  illustrates a top view of the crescent-shaped cap of  FIG. 23  engaging the recess of  FIG. 12  in a screw-loading alignment with respect to the screw of  FIG. 23 ; 
         FIG. 25  illustrates a perspective view of the crescent-shaped cap of  FIG. 23  engaging the recess of  FIG. 12  in a screw-compression alignment with respect to the screw of  FIG. 23 ; 
         FIG. 26  illustrates a perspective view of an example recess in the plate body of  FIG. 1  that is eccentric (off-center) with reference to multiple openings of  FIG. 1 ; 
         FIG. 27  illustrates a perspective top view of a threaded cap configured to be disposed in one or more alignment positions with respect to the recess illustrated in  FIG. 26 ; 
         FIG. 28  illustrates a perspective bottom view of the threaded cap illustrated in  FIG. 27 ; 
         FIG. 29  illustrates a cross-sectional view of the threaded cap illustrated in  FIG. 27 ; 
         FIG. 30  illustrates a perspective view of the threaded cap of  FIG. 27  engaging threading of the recess of  FIG. 26  in a screw-loading alignment; 
         FIG. 31  illustrates a perspective view of a threaded cap of  FIG. 27  engaging threading of the recess of  FIG. 26  in a screw-compression alignment; and 
         FIG. 32  illustrates a cross-sectional view of a threaded cap of  FIG. 27  engaging threading of the recess of  FIG. 26  in the screw-compression alignment. 
     
    
    
     DETAILED DESCRIPTION 
     A spinal plate with compression locking and a method of fixating vertebrae using the spinal plate with compression locking are disclosed. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of example embodiments. It will be evident, however, to one skilled in the art, that an example embodiment can be practiced without all of the disclosed specific details. 
       FIG. 1  illustrates a perspective view of an example spinal plate  100 . The spinal plate  100  includes a plate body  102  and a plurality of screw-receiving openings (openings)  104 - 114  through the plate body  102 . In some embodiments, the plate body  102  can also include at least one opening  116 . 
     The plate body  102  can be made of a metal (e.g., titanium, stainless steel, or other metal), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed materials that are resilient yet durable to withstand movement of the vertebrae. 
     The plate body  102  can be generally rectangular with planar top and bottom surfaces  120 ,  122  and rounded corners  118 . The plate body  102  has a length, width and height. The dimensions of the plate body  102  depend generally on the region of the spine (e.g., cervical, thoracic, lumbar) as well as the number of vertebrae being fixated. As an example for a cervical application, the plate body  102  can be about 2 mm-to about 4 mm tall; about 15 mm wide; and between about 18 mm-about 70 mm long (depending on the number of vertebrae being fixated). To fixate two (2) vertebra, the length of the plate body  102  can be about 18 mm-about 20 mm. 
     The plate body  102  can have a non-rectangular (e.g., irregular) shape. The top and bottom surfaces  120 ,  122  can also be non-planar (e.g., arcuate), as may be desirable for different spinal regions, anatomies and/or certain spinal procedures. For example, the plate body  102  can have a bone/barbell shape (e.g., wider at ends and narrower in center) and a variety of other shapes. 
     The plate body  102  is configured to be disposed along and to fixate a spinal segment (including a plurality of vertebrae) of the cervical, thoracic or lumbar region of the spinal column (not shown). The plate body  102  can be disposed about the anterior or lateral aspect of the spinal segment. Accordingly, the plate body  102  can have a variety of shapes, dimensions and surface curvatures to accommodate different spinal segments and aspects along the spinal column. 
     The openings  104 - 114  are generally configured to receive vertebral screws (screws), such as screw  400  illustrated in  FIG. 4 , (or other screws  1900 ,  2306  described herein) in one or more trajectories through the plate body  102 , securing the plate  100  to the spinal segment in order to fixate the vertebrae of the spinal segment. The locations of the openings  104 - 114  are shown as examples and openings  104 - 114  can be disposed at one or more locations about the plate body  102  required for particular spinal segments and/or spinal procedures. The openings  104 - 114  can also be biased through the plate body  102  toward a central axis of the plate body  102  to match different curvatures of spinal segment to be fixated. 
     Furthermore, the number, dimension and orientation of the openings  104 - 114  are given as examples. More or fewer openings of the same or different dimensions and orientations can be provided as required for fixation of certain spinal segments in the spinal column. In some embodiments, two openings can be provided, while in other embodiments, more than two openings can be provided, such as openings  104 - 114  shown in  FIG. 1 . 
     The at least one opening  116  is configured to enable viewing of at least one of the vertebrae in the spinal segment to be fixated. The dimension and location of the opening  116  through the plate body  102  are shown as an example. One or more similar or different openings  116  can be provided at one or more locations of the plate body  102 . In some embodiments, the at least one opening  116  is made as open as possible but does not affect the structural stability or strength of the plate body  102 . In some embodiments, the at least one opening  116  is not provided. 
       FIG. 2  illustrates a perspective view of an example threaded recess  202  provided in the plate body  102  that is eccentric (off-center) with reference to the opening  104 . The recess  202  has a center  208 , seat  204  and threading  206 . 
     Whereas the opening  104  extends through the plate body  102 , the recess  202  extends partially into the plate body  102 . The center  208  of the recess  202  is off-center (eccentric) with reference to the center  210  of the opening  104 . Furthermore, the opening  104  is encompassed entirely inside the recess  202 . 
     In some embodiments, no point in the circumference of the opening  104  touches any other point in the circumference of the recess  202 . In other embodiments, at least one point in the circumference of the opening  104  touches at least one point in the circumference of the recess  202 . One or more other recesses can similarly be provided in the plate body  102  with reference to the other openings  106 - 114  of  FIG. 1 , such as a recess  212  illustrated in  FIG. 2  with respect to opening  114 . 
     The seat  204  is generally a planar surface below the top surface  120  and above the bottom surface  122  of the plate body  102 . The seat  204  can be, but does not have to be, parallel to the top and bottom surfaces  120 ,  122  of the plate body  102 . For example, the top and bottom surfaces  120 ,  122  can be arcuate while the seat  204  can be planar. The seat  204  is configured to provide a final stop to a threaded cap  500 , which is described in greater detail below with reference to  FIGS. 5 and 6 . 
     The threading  206  is formed along the inner circumference of the recess  202  and is configured to threadably engage a threaded cap  500  of  FIGS. 5 and 6 , such that the cap  500  can be inserted (screwed) into and removed (unscrewed) from the recess  202 . In some embodiments, the threading  206  can be configured for in-plane rotation, such that the cap  500  can rotate in-plane about the recess  202 . The seat  204  provides the inner-most extent to which the threaded cap  500  can be inserted in the recess  202 . 
     The threading  206  (lead, form and other thread factors) is configured such that any possible motion (wobble) of the screw  400  in the opening  104  is less likely to be converted to rotary motion, further mitigating the possibility of the threaded cap  400  from being unscrewed from the thread  206  of the recess  202 . 
       FIG. 3  illustrates a cross-sectional view of the plate body  102  along plane A-A as illustrated in  FIG. 2 . Opening  104  is configured to secure the screw  400  of  FIG. 4  in one or more trajectories through the plate body  102 . 
     The opening  104  is generally defined by a sphere  302  that is truncated by a first diameter  304  along the bottom surface  122  of the plate body  102  and a second diameter  306  along the seat  204  of the recess  202 . The first diameter  304  is smaller than the second diameter  306 . 
     The opening  104  is generally configured to have a matching configuration to a screw  400  that will be described in greater detailed below with reference to  FIG. 4  (or any other screw described herein, such as screw  1900  or  2306 ). Accordingly, the opening  104  is configured to receive a similarly configured head  402  of the screw  400  in one or more trajectories through the plate body  102 . 
       FIG. 4  illustrates a perspective view of a vertebral screw (screw)  400 . 
     The screw  400  is a poly-axial screw that is configured to secure the spinal plate  100  to a vertebra of a spinal segment through the opening  104  of the plate body  102  in one or more trajectories. One or more additional screws  400  can be used to secure the spinal plate to the same or different vertebrae through openings  106 - 114  in the plate body  102  of the spinal plate  100 . 
     The screw  400  can be made of a metal (e.g., titanium, stainless steel, other metal or metal alloys), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed suitable materials that are resilient yet durable to withstand movement of the vertebrae. 
     The screw  400  includes a head  402  and body  424 . The head  402  is defined by a sphere  404  that is truncated by a first diameter  406  and second diameter  408 . The first diameter  406  is smaller than the second diameter  408 , and the second diameter  408  is smaller than the diameter of the sphere  404  (not shown) that defines the head  402 . The head  402  is further defined by a cylinder  410  that extends from the truncated sphere  404  to a top surface  414 . The height of the cylinder  410  is within or circumscribed by the shape of the sphere  404 , such that the head can pivot in the opening  104 . 
     The head  402  of the screw  400  includes an engagement surface  416  and a plurality of recesses  412 . The head  402  is configured to be disposed in the opening  104  in one or more trajectories with respect to the seat  204 . More specifically, because of the matching configuration of the head  402  to the opening  104 , the head  402  can be pivoted and rotated in the opening  104 . 
     Accordingly, the geometry of the head  402  is configured similarly to the geometry of the opening  104 , enabling the head  402  of the screw  400  to pivot and rotate in the opening  104  and to be secured in the opening  104  (as well as other openings  106 - 114 ) of the plate body  102  in one or more trajectories. The top surface  414  is configured to be approximately planar with seat  204  when the head  402  is disposed in the opening  104  approximately transversely to the seat  204 . 
     In some embodiments, the truncated sphere  404  can be defined by the diameters  406 ,  408  such that the top surface  414  of the head  402  is below the seat  204  when the head  402  is disposed in the opening  104  approximately transversely to the seat  204 . Disposing the top surface  414  of the head  402  below the seat  204  can provide a greater number of trajectories through the openings  104 . 
     The engagement surface  416  is arcuate and extends approximately centrally above the top surface  414 . The engagement surface  416  is configured to approximate a center of the sphere  404  that defines the head  402 . The engagement surface  416  is further configured to engage the threaded cap  600  of  FIGS. 5 and 6  such that the screw head  402  is pressed via approximately its center into at least a portion of the opening  104  to secure the screw  400  with respect to the spinal plate  100  in a selected trajectory. 
     In some embodiments illustrated in  FIG. 4 , the engagement surface  416  can be defined by the intersection of a truncated cone  418  and sphere  420 , such that a continuous arcuate surface  416  is formed centrally on the top surface  414 . The cone  418  is disposed between the surface  414  and the sphere  420 . The engagement surface  416  (via center of sphere  420 ) approximates the center of the sphere  404 . 
     The dimensions of the cone  418  and sphere  420  can be selected such that the centers of the spheres  404 ,  420  are approximately concentric, e.g., the center of sphere  420  approximates the center of the sphere  404 . Accordingly, the screw head  402  (sphere  404 ) can be pressed through approximately its center via the center of sphere  420  into at least a portion of the opening  104  to secure the screw  400  with respect to the spinal plate  100  in a selected trajectory. The sphere  420  can be any dimension such that its center is approximately concentrically disposed in relation to the center of the sphere  404 . 
     In other embodiments, the engagement surface  416  can be defined by one or more shapes, such as cylindrical, conical, spherical and/or other shapes. For example, the cone  418  and sphere  420  can be substituted with a centrally disposed sphere (e.g., hemisphere or another truncated portion of a sphere) the center of which is concentric with the center of sphere  404 , such that the sphere  404  can be pressed approximately via its center. 
     The recesses  412  are configured to engage reciprocal extensions of a driving tool (not shown) that can be used to drive (rotate) the screw  400  into a vertebra of the spinal segment to be engaged. The recesses  412  can be disposed at various locations about the periphery of the top surface  414 . In some embodiments as illustrated in  FIG. 4 , two recesses  412  are disposed on opposite sides of the engagement surface  416 , e.g., approximately 180 degrees with respect to one another. In other embodiments, more than two recesses—which are equidistantly or variously spaced about the periphery of the top surface  414 —can be provided. 
     The body  424  of the screw  400  includes a shaft  426 , thread  428  and tip  430 . The thread  428  is provided along a portion of the shaft  426  below the head  402 , such that the screw  400  can be disposed in the opening  104  of the plate body  102  in a plurality of trajectories and such that the screw  400  can engage the vertebra. The tip  430  is configured to enable the screw  400  to penetrate the vertebra. The thread  428  and tip  430  can be self-cutting and/or the vertebra can be pre-drilled. 
       FIG. 5  illustrates a perspective top view of a threaded cap  500  that is configured to be disposed in one or more alignment positions with respect to the recess  202  of  FIG. 2 . 
     The threaded cap  500  can be made of a metal (e.g., titanium, stainless steel, other metal or metal alloys), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed suitable materials that are resilient yet durable to withstand movement of the vertebrae. The threaded cap  500  includes openings  502 - 506 , alignment opening  508  and threading  510 . 
     The openings  502 - 506  are configured to engage respective extensions of a tool (e.g., prong screw driver) to threadably engage the threading  510  of the threaded cap  500  with the threading  206  of the recess  202 , and further to rotate the threaded cap  500  in the recess  202  between a screw-loading alignment and a screw-compression alignment. Instead of the openings  502 - 506 , a different engagement mechanism can be used to engage threading  510  of threaded cap  500  with threading  206  of the opening  202 . 
     In the screw-loading alignment, the alignment opening  508  approximates the opening  104 , enabling the receipt of the screw  400  through the threaded cap  500  into the opening  104 , pivoting of the head  402  of the screw  400  in the opening  104  into a selected trajectory, and rotation (threading) of the screw  400  into a vertebra through the threaded cap  500  in the selected trajectory. Because of the matching configuration of the head  402  and the opening  104  and the alignment of the opening  104  and the opening  508 , the screw  400  can be threaded into the vertebra through the plate body  102  of the spinal plate  100  in one of many trajectories, as may be advantageous in order to achieve better engagement with the vertebra. 
       FIG. 6  illustrates a perspective bottom view of the threaded cap  500  illustrated in  FIG. 5 . 
     The threaded cap  500  further includes thread extensions (starts)  602 - 608 , planar bottom surface  610 , compression ramp  612 , and recess (detent)  614 . 
     The thread extensions  602 - 608  are a part (starts) of the threading  510  and are disposed about the circumference of the threaded cap  500 . The thread extensions  602 - 608  are sized and dimensioned to be received in respective alignment slots  802 - 808 , which will be described in greater detail below with reference to  FIG. 8 . Further, the extensions  602 - 608  are configured to enable engagement of threading  510  of threaded cap  500  with threading  206  of the recess  202 , such that the threaded cap  500  can screw into the recess  202  via the threading  206 . In some embodiments, more or fewer extensions (starts) in the threaded cap  500  and respective alignment slots in the recess  202  can be provided. For example, three extensions and alignment slots can be provided. 
     In some embodiments, the bottom surface  610  can mate in a planar configuration with the seat  204  of the recess  202 . 
     The compression ramp  612  is configured to approximate and progressively compress or engage the engagement surface  416  of the screw  400  into the opening  104 , as the threaded cap  500  rotates along engagement path  616  between the screw-loading alignment and the screw-compression alignment in the recess  202 . 
     The recess (detent)  614  approximates the engagement surface  416  and is configured to receive at least a portion of the engagement surface  416  in a compression engagement with respect to the opening  104 . In some embodiments, the recess  614  is configured to accommodate at least a portion of the sphere  420  of the engagement surface  416 . The recess  614  can provide a clicking that indicates successful compression engagement. Further, the recess  614  mitigates any possible motion (wobble) of the head  402  of screw  400  in the opening  104  and about engagement surface  612 , such that it is less likely that such motion is converted to rotary motion that can unscrew the threaded cap  500  from the recess  202 . 
       FIG. 7  illustrates a cross-sectional view of the threaded cap  500  along plane B-B illustrated in  FIG. 5 . 
     The compression ramp  612  is a chamfered surface with respect to the bottom surface  610  along the engagement path  616  that the engagement surface  416  of the screw  400  follows as the threaded cap  500  is rotated in the recess  202 . As the threaded cap  500  is rotated into the screw-compression alignment, the recess (detent)  614  engages the engagement surface  416  of the screw  400  or a portion thereof. 
       FIG. 8  illustrates a perspective view of the screw  400  of  FIG. 4  disposed in the opening  104  of the plate body  102  in the spinal plate  100  of  FIG. 1 . 
     It is noted that the screw  400  is shown in the opening  104  to illustrate the interface of these elements. However, it is intended that the screw  400  will be inserted and threaded into a vertebra through the threaded cap  500  of  FIGS. 5 and 6 , as will be described in greater detail below with reference to  FIGS. 9 and 10 . 
     The threading  206  of the recess  202  is configured to include thread alignment (engagement) slots  802 - 808 . The thread alignment slots  802 - 808  of the recess  202  are configured to receive reciprocal thread extensions  602 - 608  of the threaded cap  500 , such that the threaded cap  500  can be received in alignment with respect to the recess  202 . Mores specifically, the foregoing slots/extensions enable precise alignment of the threaded cap  500  and the recess  202  into a screw-loading alignment and screw-compression alignment. The alignment slots  802 - 808  represent respective starts to the threading  206  of the recess  202 . The number and configuration of the alignment slots  802 - 808  can be adjusted based on the respective number and configuration of the thread extensions  602 - 608 . 
     As particularly illustrated in  FIG. 8 , the screw head  402  of the screw  400  is seated into the opening  104  of the plate body  102  in a generally vertical (transverse) trajectory, such that the planar surface  414  approximates the planar seat  204 . Other trajectories of the screw  400  through the opening  104  of the plate body  102  are of course possible. Because of matching spherical configuration of the head  402  of the screw  400  and the opening  104  of the spinal plate  100 , the screw  400  can pivot in the various trajectories through the spinal plate  100  via the opening  104 . Accordingly, in various trajectories, the engagement surface  416  extends into the recess  202  above the seat  204 . 
       FIG. 9  illustrates a perspective view of a threaded cap  500  engaging threading  206  of the recess  202  in a screw-loading alignment and in a screw-compression alignment. 
     The extensions  602 - 608  of the threaded cap  500  have been received via respective alignment slots  802 - 808  into the recess  202 . Furthermore, the threaded cap  500  has been rotated at least partially (e.g., clockwise) via the openings  502 - 506  to engage the threading  510  with the threading  206  and to align the alignment opening  508  of the threaded cap  500  with the opening  104  of the plate body  102  in the screw-loading alignment. 
     It is to be noted that the spinal plate  100  with the threaded cap  500  engaging the recess  202  in the screw-loading alignment is inserted into position with respect to the vertebrae of spinal segment to be fixated via the spinal plate  100 . Additional caps  500  in relation to one or more of the openings  106 - 114  are also engaged in screw-loading alignment before insertion into position with respect to the spinal segment to be fixated. In alternate embodiments, the spinal plate  100  and threaded cap  500  can be inserted separately. 
     In the screw-loading alignment, the alignment opening  508  allows receipt of the screw  400  through the threaded cap  500  into the opening  104 , pivoting of the head  402  of the screw  400  in the opening  104  into a selected trajectory, and rotation of the body  424  of the screw  400  into the vertebra in the selected trajectory through the threaded cap  500  via recesses  412 . Because of the matching configuration of the screw head  402  and the opening  104 , the screw  400  can be threaded into a vertebra of the spinal segment through the plate body  102  in one of many trajectories, as may be advantageous in order to achieve better engagement with the vertebra. 
     Once the body  424  of the screw  400  has engaged the vertebra securely through opening  104 , the head  402  of the screw engages at least a portion of the opening  104  of the plate body  102 . Thereafter, the threaded cap  500  is threaded in the recess  202  from the screw-loading alignment into a screw-compression alignment. The rotation of the threaded cap  500  progressively compresses the head  402  of the screw  400  via engagement surface  416  into the opening  104  of the plate body  102  in the selected trajectory. 
     More specifically, the compression ramp  612  progressively compresses the head  402  along the engagement path  616  into the opening  104  until the recess (detent)  614  engages the engagement surface  416  (or a portion thereof) of the screw head  402  in the screw-compression alignment. The recess  614  also compresses the engagement surface  416  of the screw head  402  in the screw-compression alignment. Generally, the screw-compression alignment can be approximately up to 180 degrees or less with respect to the screw-loading alignment. Other alignments between loading and compression can be used. 
     In the screw-compression alignment, the threaded cap  500  engages the engagement surface  416  (or a portion thereof) of the head  402  of the screw  400  via recess  614 , compressing the screw head  402  (sphere  404 ) of the screw  400  into the engaged portion of the opening  104  in the selected trajectory via the sphere  420  of the engagement surface  416 . This mitigates the wobbling of poly-axial screws in the openings of prior art spinal plates. The screw  400  does not wobble in the opening  104  if and when the body  424  of the screw  400  loosens with respect to the vertebra. Similarly, the body  424  of the screw  400  is also less likely to loosen with respect to the vertebra because the screw  400  does not wobble in the opening  104 . Accordingly, rigid fixation can be provided across the vertebrae. 
       FIG. 10  illustrates a cross-sectional view of a threaded cap  500  along plane C-C in  FIG. 9 , engaging threading  206  of recess  202  in the screw-loading alignment and in the screw-compression alignment. 
     In the screw-loading alignment, the threaded cap  500  has been advanced (threaded) partially into the recess  202  to align opening  508  with the opening  104 , such that the screw  400  can be received through the opening  508  into the opening  104 . The bottom surface  610  of the threaded cap  500  is disposed at a first distance to the seat  204 . 
     The screw  400  is received through the aligned opening  508  of the threaded cap  500  and the opening  104 . A trajectory for the screw  400  in relation to a vertebra is selected and the screw  400  is threaded into the vertebra to secure the plate  100  to the vertebra via the opening  104 . The screw head  402  of the screw  400  engages at least a portion of the opening  104  in the selected trajectory. 
     Thereafter, the threaded cap  500  is advanced further in the recess  202  into the screw-compression alignment in which the threaded cap  500  engages the engagement surface  416  (sphere  420 ) of the head  402  via recess  614 , compressing the screw head  402  (sphere  404 ) of the screw  400  into the engaged portion of the opening  104  in the selected trajectory via sphere  420  of the engagement surface  416 . This compression reduces or eliminates the wobbling of the screw  400  in the opening  104  if and when the screw  400  loosens with respect to the vertebra and also reduces the likelihood that the screw will loosen with respect to the vertebra. 
     In the screw-compression alignment, the bottom surface  610  of the threaded cap  600  can engage the seat  204 , providing friction across the engaging surfaces  204 ,  610  to further counteract any unscrewing forces that can compel the threaded cap  500  from the recess  202 . In other embodiments, the bottom surface  610  of the threaded cap  500  is at second distance to the seat  204 . The second distance of the screw-compression alignment is smaller than the first distance of the screw-loading alignment. 
       FIG. 11  illustrates perspective view of an example spinal plate  1100 . 
     The spinal plate  1100  includes a plate body  1102 , plurality of openings  1104 ,  1116  through plate body  1102  and associated plurality of threaded recesses  1106 ,  1120 . In some embodiments, the spinal plate  1100  can also include at least one opening through plate body  102  (not shown in  FIG. 11 ) configured to enable viewing of at least one of the vertebrae in the spinal segment to be fixated such as opening  116  described with reference to  FIG. 1 . Although only several openings  1104 ,  1116  and associated recesses  1106 ,  1120  are illustrated in  FIG. 11 , it should be noted that multiple openings and associated recesses can be provided to fixate multiple vertebrae of the spinal segment. 
     The plate body  1102  can be made of a metal (e.g., titanium, stainless steel, or other metal), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed suitable materials that are resilient yet durable to withstand movement of the vertebrae. 
     The plate body  1102  is generally rectangular with planar top and bottom surfaces  1202 ,  1204  (illustrated in  FIG. 12 ) and rounded corners  1122 . The plate body  1102  has a length, width and height, which can be similar to or different than described with reference to the plate body  102  of  FIG. 1 . The plate body  1102  can have a non-rectangular (e.g., irregular) shape. The top and bottom surfaces  1202 ,  1204  can also be non-planar (e.g., arcuate), as may be desirable for certain regions of the spine or spinal procedures. 
     The plate body  1102  is configured to be disposed along and to fixate a spinal segment (including a plurality of vertebrae) of the cervical, thoracic or lumbar region of the spinal column (not shown). The plate body  1102  can be disposed about the anterior or lateral aspect of the spinal segment. Accordingly, the plate body  1102  can have a variety of shapes, dimensions and surface curvatures to accommodate different spinal segments and aspects along the spinal column. The shapes, dimensions and configurations of the plate body  1102  can be similar to or different than described with reference to the plate body  102  of  FIG. 1 . 
     The openings  1104 ,  1116  (and/or other similar openings) are generally configured to receive vertebral screws (screws)  400  of  FIG. 4  (or other screws such as, screws  1900  or  2306 ), in one or more trajectories through the plate body  1102 , securing the plate  1100  to the spinal segment in order to fixate the vertebrae of the spinal segment. The locations of the openings  1104 ,  1116  are shown as examples and these and/or other openings can be disposed at one or more locations about the plate body  1102  required for particular spinal segments and/or spinal procedures. 
     The threaded recesses  1106 ,  1120  of plate  1100  are eccentric (off-center) with reference to their associated openings  1104 ,  1116 , respectively, as described below. For simplicity and to facilitate understanding of the subject matter disclosed herein, only the representative opening  1104  and associated threaded recess  1106  will be described in greater detail below with reference to  FIGS. 11-18 . It is understood that the other openings (e.g., opening  1116  and/or other openings) and associated recesses (e.g., recess  1120  and/or other recesses) are configured similarly to the representative opening  1104  and associated recess  1116  described below. 
     The opening  1104  has a center  1112  and the recess  1106  has a center  1114 . The centers  1112 ,  1114  are off-center (eccentric). Further, the recess  1106  intersects the opening  1104  at points  1108 ,  1110 , such that only a portion of the opening  1104  is encompassed inside the recess  1106 . Furthermore, the intersection is such that the center  1112  of the opening  1104  is encompassed in the recess  1106 . 
       FIG. 12  illustrates a cross-sectional view of the opening  1104  and recess  1106  of spinal plate  1100  along plane D-D illustrated in  FIG. 11 . 
     The opening  1104  is generally defined by cylindrical openings  1206 ,  1208  and spherical opening  1210  that is disposed between the openings  1206 ,  1208 . 
     Specifically, the first cylindrical opening  1206  has a first diameter  1212  and a first height. The first opening  1206  extends through and from the bottom surface  1204  partially into the plate body  1102 . The second cylindrical opening  1208  has a second diameter  1214  and a second height. The second opening  1206  extends through and from a seat  1216  partially into the plate body  1102 . The first and second height can be the same. 
     The spherical opening  1210 —disposed between openings  1206 ,  1208 —is defined by a sphere  1210  that is truncated by the first diameter  1212  and the second diameter  1214 . The first diameter  1212  is smaller than the second diameter  1214 . The spherical opening  1210 —defined as diameters  1212 ,  1214 —approximates the sphere  404  defined by diameters  406 ,  408  of the head  402  in the screw  400  illustrated in  FIG. 4 . 
     The opening  1104  (via spherical opening  1210 ) has an approximate matching configuration to the head  402  of the screw  400 . Furthermore, the heights of the openings  1206 ,  1208  can approximate the height of a cylinder  410  of the screw head  402 . Accordingly, the head  402  can pivot in the opening  1104  such that at least a portion of the engagement surface  416  of the screw head  402  extends into the recess  1106 . 
     The recess  1106  includes a seat  1214 , extension  1218  and channel  1220 . The seat  1214  is generally a planar surface below the top surface  1202  and above the bottom surface  1204  of the plate body  1102  of the spinal plate  1100 . The seat  1214  can be, but does not have to be, parallel to the top and bottom surfaces  1202 ,  1204  of the plate body  1102 . As will be described in greater detail with reference to  FIGS. 15-18 , the seat  1412  is configured to receive a crescent-shaped cap  1300  of  FIG. 13  in a generally planar configuration. 
     The extension  1218  and channel  1220  extend about the periphery of the recess  1106 , except for a portion of the periphery eliminated as a result of the intersection of the opening  1104  with the recess  1106 . 
     The extension  1218  is generally planar with the top surface  1202  of the plate body  1102 . The extension  1218  overhangs the recess  1106 , forming the channel  1220  that extends between the seat  1214  and the extension  1218 . The channel  1220  is configured to receive a lip  1302  of the crescent-shaped cap  1300  of  FIG. 13 , while the extension  1218  is configured to movably engage the lip  1302  such that the crescent-shaped cap  1300  is rotationally engaged and retained in the recess  1106 . 
       FIG. 13  illustrates perspective view of the example crescent-shaped cap  1300 . The crescent-shaped cap  1300  includes a lip  1302 , wall  1304 , top surface  1306 , chamfer  1308 , alignment opening  1310 , and compression ramp  1316 . 
     The shape of the crescent-shaped cap  1300  is defined by an alignment opening  1310  that approximates the opening  1104 . The lip  1302  is configured to engage extension  1218  of the recess  1106 . More specifically, the lip  1302  is generally planar and is configured to be disposed in a generally planar configuration in the seat  1216  such that the lip  1302  can rotate in the channel  1220  of the recess  1106 . The wall  1304  extends generally upwardly from the lip  1302  to the top surface  1306 . The chamfer  1308  smoothes the intersection or transition between the wall  1304  and top surface  1306 . 
     In some embodiments, the crescent-shaped cap  1300  can be rotated in the recess  1106  via a tool (not shown) that engages the opening  1302  and the wall  1304 . In other embodiments, openings  1312 ,  1314  can be provided. The openings  1312 ,  1314  are configured to engage respective extensions of a tool (e.g., prong screw driver) to rotate the crescent-shaped cap  1300  in the recess  1106 . Alternate engagement mechanism(s) can also be provided to rotate the crescent-shaped cap  1300  in the recess  1106 . 
     The crescent-shaped cap  1300  can be rotated about the recess  1106  between a screw-loading alignment and a screw-compression alignment. In the screw-loading alignment, the alignment opening  1310  approximates the opening  1104 . The configuration of the compression ramp  1316  approximates the engagement surface  416  of the screw  400  (or other screw, such as screw  1900  or  2306 ). The compression ramp  1316  is configured to progressively compress or engage the engagement surface  416  of the screw  400  as the crescent-shaped cap  1300  rotates between the screw-loading alignment and the screw-compression alignment. The compression ramp  1316  will be described in greater detail below with reference to  FIG. 14 . 
       FIG. 14  illustrates the compression ramp  1316  of the crescent-shaped cap  1300  of  FIG. 13 . 
     The bottom surface  1402  of the crescent-shaped cap  1300  is generally planar and is configured to mate in a planar configuration with the seat  1216  of the recess  1106 , such that the crescent-shaped cap  1300  can be rotated about the recess  1106 . 
     The compression ramp  1316  is configured to approximate and progressively compresses the engagement surface  416  of the screw  400  into the opening  1104 , as the crescent-shaped cap  1300  rotates between the screw-loading alignment and the screw-compression alignment in the recess  1106 . 
     The compression ramp  1316  includes a ramped surface  1404 , ridge  1406  and a locking recess (detent)  1408 . The ramped surface  1404  extends from the inner opening  1310  to the locking recess  1408 , along a travel path of the engagement surface  416  about the crescent-shaped cap  1300 , as the crescent-shaped cap  1300  rotates in the recess  1106  between the screw-loading alignment and the screw-compression alignment. 
     The ramped surface  1404  progressively provides more compression along the travel path, with the ridge  1406  providing the most compression of the compression ramp  1316 . The locking recess  1408  is slightly less compressive than the ridge  1406 , such that engagement surface  416  can be locked by the ridge  1406  in the locking recess  1408 . 
     The locking recess  1408  approximates the engagement surface  416  and is configured to receive at least a portion of the engagement surface  416  in a compression engagement with respect to the opening  1104 . In some embodiments, the locking recess  1408  is configured to accommodate at least a portion of the sphere  420  of the engagement surface  416 . The recess  1408  can provide a clicking that indicates successful compression engagement. Further, the locking recess  1408  mitigates any possible motion (wobble) of the head  402  of screw  400  in the opening  1104 , such that it is less likely that such motion is converted to rotary motion that can rotate the crescent-shaped cap  1300  from the screw-compression alignment to the screw-loading alignment. 
       FIG. 15  illustrates a top view of the crescent-shaped cap  1300  engaging the recess  1106  in a screw-loading alignment. 
     In the screw-loading alignment, the alignment opening  1310  in crescent-shaped cap  1300  approximates the opening  1104  in the plate body  1102 , allowing receipt of the screw  400  into the opening  1104 , pivoting of the head  402  of the screw  400  in the opening  1104  into a selected trajectory, and rotation of the body  424  of the screw  400  into the vertebra in the selected trajectory through the moon shaped cap  1300 . 
     Because of the matching configuration of the screw head  402  and the opening  1104 , the screw  400  can be received in the opening  1104  in one of many trajectories so that the vertebra can be engaged (threaded) in the trajectory that is advantageous for better engagement. Recesses  412  in the head  402  are used to thread the screw  400  into the vertebra. 
       FIG. 16  illustrates perspective view of the example crescent-shaped cap  1300  engaging the recess  1106  of the spinal plate  1100  in the screw-loading alignment. 
     The bottom surface  1402  of the crescent-shaped cap  1300  is disposed in a planar configuration with the seat  1216  of the recess  1106 . The crescent-shaped cap  1300  is secured in the recess  1106  by the lip  1302  that is disposed in the channel  1220  and engaged by the extension  1218 , such that the crescent-shaped cap  1300  can be rotated about the recess  1106 . 
     In the screw-loading alignment, the opening  1310  of the crescent-shaped cap  1300  approximates the opening  1104  in the plate body  1102  of the spinal plate  1100 . The screw  400  has been inserted through the opening  1310  into the opening  1104 . Furthermore, the screw  400  has been threaded via recesses  412  into the vertebra of the spinal segment in the selected trajectory. The head  402  of the screw  400  engages at least a portion of the opening  1104  of the plate body  1102 . 
     The engagement surface  416  of the screw  400  extends at least partially into the recess  1106 . The compression ramp  1316  at the inner opening  1310  generally approximates the extension of the engagement surface  416  into the recess  1106 , such that compression ramp  1316  can easily slide over or engage the engagement surface  416 . 
       FIG. 17  illustrates a perspective view of the crescent-shaped cap  1300  engaging the recess  1106  of the spinal plate  1100  in a screw-compression alignment. 
     Once the body  424  of the screw  400  has engaged the vertebra securely through opening  1104  in the selected trajectory, the head  402  of the screw  400  engages at least a portion of the opening  1104  in the plate body  1102  of the spinal plate  1100 . The crescent-shaped cap  1300  is thereafter rotated in the recess  1106  from the screw-loading alignment into the screw-compression alignment. In the screw-compression alignment, the crescent-shaped cap  1300  covers a substantial portion of screw head  402 . 
     The rotation of the crescent-shaped cap  1300  in the recess  1006  from the screw-loading alignment into the screw-compression alignment progressively compresses the head  402  of the screw  400  via engagement surface  416  into the engage portion of the opening  1104  of the plate body  102  in the selected trajectory. 
     More specifically, the ramped surface  1404  of the compression ramp  1316  progressively compresses the head  402  into the opening  1104  until the recess (detent)  1408  of the compression ramp engages at least a portion of the engagement surface  416 . Generally, the screw-compression alignment can be up to 180 degrees or less with respect to the screw-loading alignment. In various embodiments, the alignment can be different between loading and compression. 
     In the screw-compression alignment, the locking recess  1408  of compression ramp  1316  engages the engagement surface  416  (or a portion thereof) of the head  402  of the screw  400 , compressing of the head  402  (sphere  404 ) of the screw  400  into the engaged portion of the opening  1104  in the selected trajectory via the sphere  420  of the engagement surface  416 . This mitigates the wobbling of poly-axial screws in the openings of prior art spinal plates. The screw  400  does not wobble in the opening  1104  if and when the body  424  of the screw  400  loosens with respect to the vertebra. Similarly, the body  424  of the screw  400  is also less likely to loosen with respect to the vertebra because the screw  400  does not wobble in the opening  104 . Accordingly, rigid fixation can be provided across the vertebrae. 
       FIG. 18  illustrates a cross-sectional view of the example crescent-shaped cap  1300  between the screw-loading alignment and the screw-compression alignment with respect to the opening  1104  in the plate body  1102  and screw  400 . 
     The compression ramp  1316  of the crescent-shaped cap  1300  rides over the engagement surface  416  as the crescent-shaped cap  1300  is rotated in the recess  1106 , with the ramped surface  1404  progressively increasing compression along its path and the ridge  1406  providing the most compression on the engagement surface  416  of the screw  400 , until the locking recess  1408  (shown in  FIG. 14 ) engages the engagement surface  416  past the ridge  1406 . 
     The locking recess  1408  compresses the engagement surface  416  of the screw  400  in which compression of sphere  420  compresses the head  402  (sphere  404 ) of the screw  400 —as the centers of spheres  420 ,  404  are approximately concentric—into the engaged portion of the opening  1104  in the selected trajectory. 
       FIG. 19  illustrates a perspective view of a vertebral screw (screw)  1900 . 
     The screw  1900  is a poly-axial screw that is configured to secure the spinal plate  1100  to a vertebra of a spinal segment through the opening  1104  of the plate body  1102  in one or more trajectories. One or more additional screws  1900  can be used to secure the spinal plate  1100  to same or different vertebrae through respective openings in the plate body  1102  of the spinal plate  1100 . 
     The screw  1900  can be made of a metal (e.g., titanium, stainless steel, or other metal), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed suitable materials that are resilient yet durable to withstand movement of the vertebrae. 
     The screw  1900  includes a head  1902  and body  1924 . The head  1902  is defined by a sphere  1904  that is truncated by a first diameter  1906  and second diameter  1908 . The first diameter  1906  is smaller than the second diameter  1908 , and the second diameter  1908  is smaller than the diameter of the sphere  1904  (not shown) that defines the head  1902 . The head  1902  is further defined by a cylinder  1910  that extends from the truncated sphere  1904  to a top surface  1914 . The height of the cylinder  1910  is within or circumscribed by the shape of the sphere  1904 . 
     The head  1902  of the screw  1900  includes an engagement surface  1916  and a plurality of recesses  1912 . The head  1902  is configured to be disposed in the opening  1104  in one or more trajectories with respect to the seat  1216 . More specifically, because of the matching configuration of the head  1902  to the opening  1104 , the head  1902  can be pivoted and rotated in the opening  1104 . 
     Accordingly, the geometry of the head  1902  is configured similarly to the geometry of the opening  1104 , enabling the head  1902  of the screw  1900  to pivot and rotate in the opening  1104  and to be secured in the opening  1104  (as well as other openings) of the plate body  1102  in one or more trajectories. 
     The top surface  1914  is configured to be approximately planar or recessed with respect to a seat  1216  of the recess  1106  when the head  1902  is disposed in opening  1104 . In various embodiments, the truncated sphere  1904  can be defined by the diameters  1906 ,  1908  such that the top surface  1914  of the head  1902  is approximately planar with or recessed below the seat  1216 . Recessing the top surface  1914  of the head  1902  can provide a greater number of trajectories through the openings  1104  as well as keeping a low profile of the spinal plate  1100  and cap. 
     The engagement surface  1916  is arcuate and extends approximately centrally above the top surface  1914 . The engagement surface  1916  is configured to approximate a center of the sphere  1904  that defines the head  1902 . The engagement surface  1916  is further configured to engage a cap (e.g., crescent-shaped cap  1300 ,  2300 ), such that the screw head  1902  is pressed via approximately its center into at least a portion of the opening  1104  to secure the screw  1900  with respect to the spinal plate  1100  in a selected trajectory. 
     The engagement surface  1916  is defined by approximately hemisphere (or a different portion of a sphere)  1920  that extends approximately centrally above the top surface  1914 . The engagement surface  1916  approximates the center of the sphere  1904 . More specifically, the centers of the spheres  1904 ,  1920  are approximately concentric, e.g., the center of sphere  1920  approximates the center of the sphere  1904 . Accordingly, the screw head  1902  (sphere  1904 ) can be pressed through approximately its center via the center of sphere  1920  into at least a portion of the opening  1104  to secure the screw  1900  with respect to the spinal plate  1100  in a selected trajectory. The sphere  1920  can be any dimension such that its center is approximately concentrically disposed in relation to the center of the sphere  1904 . 
     The recesses  1912  are configured to engage reciprocal extensions of a driving tool (not shown) that can be used to drive (rotate) the screw  1900  into a vertebra of the spinal segment to be engaged. The recesses  1912  can be disposed at various locations about the periphery of the top surface  1914 . In some embodiments as illustrated in  FIG. 19 , three recesses are equidistantly disposed about the engagement surface  1916 . In other embodiments, more or less than three recesses—which are equidistantly or variously spaced about the periphery of the top surface  1914 —can be provided. 
     The body  1924  of the screw  1900  includes a shaft  1926 , thread  1928  and tip  1930 . The thread  1928  is provided along a portion of the shaft  1926  below the head  1902 , such that the screw  1900  can be disposed in the opening  1104  of the plate body  1102  in a plurality of trajectories and such that the screw  1900  can engage the vertebra. The thread  1928  is configured for self-cutting into the vertebra. The tip  1930  is self-cutting to facilitate the screw  1900  in penetrating the vertebra. The vertebra can also be pre-drilled to enable easier penetration into the vertebra. The body  1924  can also be similar to body  424  of screw  400  illustrated in  FIG. 4 . 
       FIG. 20  illustrates a perspective view of the screw  1900  engaging compression ramp  1316  of the crescent-shaped cap  1300 . 
     The compression ramp  1316  approximates the engagement surface  1916  (e.g., sphere  1920 ) of the screw  1900 . The compression ramp  1316  can be adjusted as required based on the dimensions of the engagement surface  1916 . 
     It is assumed that the screw  1900 —in the screw-loading alignment—has been received through the aligned opening  1310  of the crescent-shaped cap  1300  and opening  1104  in the plate body  1102 , has further been pivoted into a selected trajectory through the plate body  1102 , and then rotated (threaded) via body  1924  into the vertebra in the selected trajectory. 
     Once the body  1924  of the screw  1900  has engaged the vertebra securely through the opening  1104  in the selected trajectory, the head  1902  of the screw  1900  engages at least a portion of the opening  1104  in the plate body  1102  of the spinal plate  1100 . The crescent-shaped cap  1300  is thereafter rotated in the recess  1106  of the plate body  1102  from the screw-loading alignment into the screw-compression alignment. 
     The compression ramp  1316  is configured to engage the engagement surface  1916  of the screw  1900  and to progressively compress the engagement surface  1916  of the screw  400  as the crescent-shaped cap  1300  rotates between the screw-loading alignment and the screw-compression alignment. 
       FIG. 21  illustrates a cross-sectional view of the example crescent-shaped cap  1300  between the screw-loading alignment and the screw-compression alignment with respect to the opening  1104  in the plate body  1102  and screw  1900 . 
     The compression ramp  1316  of the crescent-shaped cap  1300  rides over the engagement surface  1916  as the crescent-shaped cap  1300  is rotated in the recess  1106 , with the ramped surface  1404  progressively increasing compression along its path and the ridge  1406  (shown in  FIG. 22 ) providing the most compression on the engagement surface  1916  of the screw  1900 , until the locking recess  1408  (shown in  FIG. 22 ) engages the engagement surface  1916  (or a portion thereof) past the ridge  1406 . 
       FIG. 22  illustrates a cross-sectional view of the example crescent-shaped cap  1300  in the screw-compression alignment with respect to the screw  1900  in the opening  1104  of the plate body  1102 . 
     The compression ramp  1316  rides over the engagement surface  1916  (e.g., sphere  1920 ) as the crescent-shaped cap  1300  is rotated in the recess  1106  between the screw-loading alignment and the screw-compression alignment with respect to the opening  1104 . The ramped surface  1404  of the compression ramp  1316  progressively increases compression along its path until ridge  1406 . Thereafter, the engagement surface  1916  (or a portion thereof) engages the locking recess  1408 . 
     The locking recess  1408  compresses the engagement surface  1916  of the screw  1900 —in which compression of sphere  1920  compresses the head  1902  (sphere  1904 ) of the screw  1900  because as the centers of spheres  1904 ,  1920  are approximately concentric—into the engaged portion of the opening  1104  in the selected trajectory. 
       FIG. 23  illustrates a perspective view of an example a crescent-shaped cap  2300  engaging the recess  1106  of the spinal plate  1100  in the screw-loading alignment with respect to an example screw  2306 . 
     The configuration of the crescent-shaped cap  2300  is similar to crescent-shaped cap  1300 , except as follows. The crescent-shaped cap  2300  omits the compression ramp  1316 . Instead, the crescent-shaped cap  2300  includes bottom surface  2302  and chamfer  2304 . The bottom surface  2302  is configured to mate in a planar configuration with seat  1216  of the recess  1106 . The chamfer  2304  is configured to facilitate engagement of the crescent-shaped cap  2300  with the engagement surface  1916  of the screw  2306 . Further the crescent-shaped cap  2300  is approximately planar with the top surface  1202  of the plate  1102 , providing a lower profile configuration. 
     The configuration of the screw  2306  is similar to screw  1900 , except as follows. The engagement surface  1916  of the screw  2306  can deform to accommodate dimensional differences between the screw  2306 , opening  1104  and crescent-shaped cap  2300 . The screw  2306  can be made of similar materials described herein with reference to screws  400 ,  1900  of  FIGS. 4 and 19 , respectively. 
     More specifically, the sphere  1920  can include a deformable portion  2308  that can be deformed by engagement with the crescent-shaped cap  2300 . The compressible portion  2308  can be a hollow space or can be a material that is resilient yet compressible (e.g., PEEK). 
     In the screw-loading alignment, the opening  1310  of the crescent-shaped cap  2300  approximates the opening  1104  in the plate body  1102  of the spinal plate  1100 . The screw  2306  has been inserted through the opening  1310  into the opening  1104 . Furthermore, the screw  2306  has been threaded via recesses  1912  (shown in  FIG. 24 ) into the vertebra of the spinal segment in the selected trajectory. The head  1902  of the screw  2306  engages at least a portion of the opening  1104  of the plate body  1102 . 
     The engagement surface  1916  of the screw  2306  extends at least partially into the recess  1106 . The chamfer  2304  at the opening  1310  generally approximates the extension of the engagement surface  1916  into the recess  1106 , such that the bottom  2302  of the crescent-shaped cap  2300  can engage and deform the engagement surface  1916  as the crescent-shaped cap  2300  is rotated from the screw-loading alignment into screw-compression alignment. 
       FIG. 24  illustrates a top view of the crescent-shaped cap  2300  engaging the recess  1106  in a screw-loading alignment. 
     In the screw-loading alignment, the alignment opening  1310  in the crescent-shaped cap  2300  approximates the opening  1104  in the plate body  1102 , allowing receipt of the screw  2306  into the opening  1104 , pivoting of the head  1902  of the screw  2306  in the opening  1104  into a selected trajectory, and rotation of the body  424  of the screw  2306  into the vertebra in the selected trajectory through the crescent-shaped cap  2300 . 
     As illustrated in  FIG. 24 , the screw  2306  has been inserted through the cap  2300  into the opening  1104  and threaded into the vertebra of the spinal segment in the selected trajectory with respect to the plate body  1102 . 
     Because of the matching configuration of the screw head  1902  and the opening  1104 , the screw  2306  can be received in the opening  1104  in one of many trajectories so that the vertebra can be engaged (threaded) in the trajectory that is advantageous for better engagement. Recesses  1912  in the head  402  are used to thread the screw  2306  into the vertebra in the selected trajectory. 
     The engagement surface  1916  of the screw  2306  extends at least partially into the recess  1106 . The chamfer  2304  at the alignment opening  1310  generally approximates the extension of the engagement surface  1916  in the recess  1106 , such that crescent-shaped cap  2300  can slide over and engage the engagement surface  1916 . 
       FIG. 25  illustrates a perspective view of the crescent-shaped cap  2300  engaging the recess  1106  of the spinal plate  1100  in a screw-compression alignment. 
     Once the body  1924  of the screw  2306  has engaged the vertebra securely through opening  1104  in the selected trajectory, the head  1902  of the screw  2306  engages at least a portion of the opening  1104  in the plate body  1102  of the spinal plate  1100 . The crescent-shaped cap  2300  is thereafter rotated in the recess  1106  from the screw-loading alignment into the screw-compression alignment. 
     The rotation of the crescent-shaped cap  2300  in the recess  1106  from the screw-loading alignment into the screw-compression alignment deforms engagement surface  1916 , compressing the head  1902  of the screw  2306  via engagement surface  1916  into the engaged portion of the opening  1104  of the plate body  102  in the selected trajectory. 
     Generally, the screw-compression alignment can be at any location along the bottom  2302  of the crescent-shaped cap  2300 . However, in order to cover a substantial portion of screw head  1902  in the screw-compression alignment, the crescent-shaped cap  2300  is rotated up to approximately 180 degrees with respect to the screw-loading alignment. In various embodiments, these alignments can be different. 
     In the screw-compression alignment, the engagement surface  1916  is deformed to form engagement surface  2502 . The engagement surface  2502  not only compresses the head  1902  into the opening  1104  but also engages bottom surface  2302  of the crescent-shaped cap  2300  across a larger contact area in the selected trajectory. This mitigates the wobbling of poly-axial screws in the openings of prior art spinal plates. The screw  2306  does not wobble in the opening  1104  if and when the body  1924  of the screw  2306  loosens with respect to the vertebra. Similarly, the body  1924  is also less likely to loosen with respect to the vertebrae. 
       FIG. 26  illustrates perspective view of an example recess  2602  in the plate body  102  that is eccentric (off-center) with reference to the openings  104 ,  114 . The recess  2602  has a center  2606 , and seat  2604  and threading  2612 . 
     Whereas openings  104 ,  114  extend though the plate body  102 , the recess  2602  extends partially into the plate body  102 . The center  2606  of the recess  2602  is off-center (eccentric) with reference to the centers  2608 ,  2610  of the opening  104 ,  114 , respectively. The openings  104 ,  114  are configured to receive screws  400  and can also be configured similarly to the opening  1104  described in detail hereinabove with reference to  FIG. 12 . 
     The recess  2602  intersects the openings  104 ,  114 , such that only a portion of the openings  104 ,  114  is encompassed inside the recess  2602 , including at least the centers  210 ,  2610  of the respective openings  104 ,  114 . 
     In some embodiments, openings  104 ,  114  are encompassed in the recess  2602 , where no point in the circumferences of the openings  104 ,  114  touches any other point in the circumference of the recess  2602 . In other embodiments, at least one point in the circumferences of the openings  104 ,  114  touches at least another point in the circumference of the recess  2602 . One or more other recesses—similar to recess  2602 —can be provided in the plate body  102  with reference pairs of openings  106 - 112  in  FIG. 1 . 
     The seat  2604  is generally a planar surface below the top surface  120  and above the bottom surface  122  of the plate body  102 . The seat  2604  can be, but does not have to be, parallel to the surfaces  120 ,  122  of the plate body  102  depending on the configuration of the plate body  102 . The seat  2604  is configured to provide a final stop to threaded cap  2700  described in greater detail below with reference to  FIGS. 27-29 . 
     The threading  2612  is formed along the circumference of the recess  2602  and is configured to threadably engage the threaded cap  2700  of  FIG. 27 , such that the threaded cap  2700  can be inserted (screwed) into and removed (unscrewed) from the recess  2602 . The seat  2604  provides the inner-most extent to which the threaded cap  2700  can be inserted in the recess  2602 . In some embodiments, the threaded cap s 700  can also rotate in-plane with respect to the recess  2602 , as described with reference to  FIG. 2 . 
     The threading  2612  (lead, form and other factors) is configured such that any possible motion (wobble) of the screws  400  in the openings  104 ,  114  is less likely to be converted to rotary motion, further mitigating the possibility of the threaded cap  2700  from being unscrewed from the threading  2612  of the recess  2602 . 
     The threading  2612  includes thread alignment (engagement) slots  2614 - 2618 , which are configured to receive reciprocal thread extensions  2802 - 2806  of the threaded cap  2700  (shown in  FIG. 28 ), such that the threaded cap  2700  can be received in alignment with respect to the threading  2612 . The alignment slots  2614 - 2618  represent respective starts to the threading  2612  of the recess  2602 . The number and configuration of the alignment slots  2614 - 2618  can be adjusted based on the respective number and configuration of the  2802 - 2806 . The foregoing alignment enables precise alignment of the threaded cap  2700  and the recess  2602  into a screw-loading alignment and screw-compression alignment with rotation of the threaded cap  2700  in the recess  2602 . 
       FIG. 27  illustrates a perspective top view of a threaded cap  2700  that is configured to be disposed in one or more alignment positions with respect to the recess  2602  in the plate body  102  illustrated in  FIG. 26 . 
     The threaded cap  2700  can be made a metal (e.g., titanium, stainless steel, or other metal), polyethylethylketone (PEEK), ceramic material, bio-absorbable material, other medically-surgically acceptable material, and combinations of these and/or conventional or later-developed suitable materials that are resilient yet durable to withstand movement of the vertebrae. The threaded cap  2700  includes alignment openings  2702 ,  2704 , openings  2706 - 2710 , and threading  2712 . 
     The openings  2706 - 2710  are configured to engage respective extensions of a tool (e.g., prong screw driver) to threadably engage the threading  2712  of the threaded cap  2700  with the threading  2612  of the recess  2602 , and further to rotate the threaded cap  2700  in the recess  2602  between a screw-loading alignment and a screw-compression alignment. Instead of the openings  2706 - 2710 , a different engagement mechanism can be used to engage threading  2712  of threaded cap  2700  with threading  2612  of the opening  2602 . 
     In the screw-loading alignment, the alignment openings  2702 ,  2704  approximate the respective openings  104 ,  114 , enabling the receipt of the screws  400  (or other screws, such as screws  1900  or  2306 ) through the threaded cap  2700  into the openings  104 ,  114 , pivoting of the head  402  of the screws  400  in the openings  104 ,  114  into selected trajectories, and rotation (threading) of the screws  400  into a vertebra or vertebrae through the threaded cap  2700  in the selected trajectories. 
     It is noted that, that because of the matching configuration of the head  402  of the screws and the openings  104 ,  114  and the alignment of the openings  104 ,  114  and the openings  2702 ,  2704 , the screws  400  can be threaded into the vertebra through the plate body  102  of the spinal plate  100  in one of many trajectories, as may be advantageous in order to achieve better engagement with the vertebra or vertebrae. 
       FIG. 28  illustrates a perspective bottom view of the threaded cap  2700  illustrated in  FIG. 27 . 
     The threaded cap  2700  further includes thread extensions (starts)  2802 - 2806 , planar bottom surface  2816 , compression ramps  2808 ,  2812 , and recesses (detents)  2810 ,  2814 . 
     The thread extensions  2802 - 2806  are a part (starts) of the threading  2712  and are disposed about the circumference of the threaded cap  2700 . The thread extensions  2802 - 2806  are sized and dimensioned to be received in respective alignment slots  2614 - 2618 . Further, the extensions  2802 - 2806  are configured to enable engagement of threading  2712  of threaded cap  2700  with threading  2612  of the recess  2602 , such that the threaded cap  2700  can screw into the recess  2602  via the threading  2612 . In some embodiments, more or fewer extensions (starts) in the threaded cap  2700  and respective alignment slots in the recess  2602  can be provided. For example, two or four extensions and respective alignment slots can be provided. 
     In some embodiments, the bottom surface  2816  can mate in a planar configuration with the seat  2604  of the recess  2602 . 
     The compression ramps  2808 ,  2812  are configured to approximate and progressively compresses or engage the engagement surfaces  416  of the screws  400 , such that the heads  402  of the screws  400  can be compressed into engaged portions of the openings  104 ,  114 , as the threaded cap  2700  rotates between the screw-loading alignment and the screw-compression alignment in the recess  2602 . 
     The recesses (detents)  2810 ,  2814  of respective compression ramps  2802 ,  2812  approximate the engagement surfaces  416  of the screws  400  and are configured to receive at least portions of the engagement surfaces  416  in a compression engagement with respect to the openings  104 ,  114 . In some embodiments, the recesses  2810 ,  2814  are configured to accommodate at least a portion of the spheres  420  of the engagement surfaces  416 . The recesses  2810 ,  2814  can provide a clicking that indicates successful compression engagement. 
     Furthermore, recesses  2810 ,  2814  mitigate any possible motion (wobble) of the heads  402  of screws  400  in the respective openings  104 ,  114  and about the engagement surface  2816 , such that it is less likely that such motion is converted to rotary motion that can unscrew the threaded cap  2700  from the recess  2602 . 
       FIG. 29  illustrates a cross-sectional view of the threaded cap  2700  illustrated in  FIG. 27 . As illustrated, the bottom surface  2816  is planar, while the top  2902  is developed or sloped (cone-shaped) around a center  2902 . 
     The compression ramps  2808 ,  2812  are disposed along the engagement path that the engagement surfaces  416  of the screws  400  follow as the threaded cap  2700  is rotated in the recess  2602 . As the threaded cap  2700  is rotated into the screw-compression alignment, the compression ramps  2808 ,  2812  engage at least portions of engagement surfaces  416  of the screws  400 , engaging the screw heads  402  into at least portions of the openings  104 ,  114 . In some embodiments, the compression ramps  2808 ,  2812  compressively engage at least portions of the spheres  420  and the recesses  2810 ,  2814  engage at least portions of the spheres  420 . 
       FIG. 30  illustrates a perspective view of a threaded cap  2700  engaging threading  2612  of the recess  2602  in a screw-loading alignment. 
     The extensions  2802 - 2806  of the threaded cap  2700  have been received via respective alignment slots  2614 - 2618  into the recess  2602 . Furthermore, the threaded cap  2700  has been rotated (threaded) at least partially via the openings  2706 - 2710  to engage the threading  2712  with the threading  2612  and to align the alignment openings  2702 ,  2704  of the threaded cap  2700  with the respective openings  104 ,  114  of the plate body  102  in the screw-loading alignment. 
     It is to be noted that the spinal plate  100  with the threaded cap  2700  engaging the recess  2602  in the screw-loading alignment is inserted into position with respect to the vertebrae of spinal segment to be fixated via the spinal plate  100 . Additional caps  2700  in relation to one or more of the other pairs of openings  106 - 112  are also engaged in screw-loading alignment before insertion of the plate  100  into position with respect to the spinal segment to be fixated. In alternate embodiments, the spinal plate  100  and the treaded cap  2700  can be inserted separately. 
     In the screw-loading alignment, the alignment openings  2702 ,  2704  of the threaded cap  2700  allow receipt of the screws  400  through the threaded cap  2700  into the openings  104 ,  114 , pivoting of the heads  402  of the screws  400  in the openings  104 ,  114  into selected trajectories, and rotation (threading) of the bodies  424  of the screws  400  into the vertebra or vertebrae in the selected trajectories through the threaded cap  2700  via recesses  412  of the screws  400 . Because of the matching configuration of the screw heads  402  and the openings  104 ,  114 , the screws  400  can be threaded into the vertebra or vertebrae of the spinal segment through the plate body  102  in various trajectories, as may be advantageous in order to achieve better engagement with the vertebra or vertebrae. 
       FIG. 31  illustrates a perspective view of a threaded cap  2700  engaging threading  2612  of the recess  2602  in a screw-compression alignment. 
     Once the screws  400  have engaged the vertebra or vertebrae securely through openings  104 ,  114 , the heads  402  of the screws  400  engage at least portions of the openings  104 ,  114  of the plate body  102 . Thereafter, the threaded cap  2700  is rotated (e.g., threaded) in the recess  2602  from the screw-loading alignment into a screw-compression alignment. The rotation of the threaded cap  2700  progressively compresses the heads  402  of the screws  400  via engagement surfaces  416  into the engage portions of the openings  104 ,  114  of the plate body  102  in the selected trajectories as described below. 
     More specifically, the compression ramps  2808 ,  2812  progressively compress the heads  402  of the screws into the openings  104 ,  114  until the recesses (detents)  2810 ,  2814  engage the engagement surfaces  416  of the screw heads  402  in the screw-compression alignment. The recesses (detents)  2810 ,  2814  also compress the engagement surfaces  416  of the screw heads  402  in the screw-compression alignment. Generally, the screw-compression alignment can be approximately up to 180 degrees or less with respect to the screw-loading alignment. In various embodiments, these alignments can be different. 
     In the screw-compression alignment, the threaded cap  2700  engages the engagement surfaces  416  of the heads  402  of the screws  400  via recesses  2810 ,  2814 , compressing the screw heads  402  (spheres  404 ) of the screws  400  into the engaged portions of the openings  104 ,  114  in the selected trajectories via the spheres  420  of the engagement surfaces  416 . This mitigates the wobbling of poly-axial screws in the openings of prior art spinal plates. The screws  400  do not wobble in the openings  104 ,  114  if and when the bodies  424  of the screws  400  loosen with respect to the vertebra or vertebrae. Similarly, the bodies  424  are less likely to loosen with respect to the vertebrae. 
       FIG. 32  illustrates a cross-sectional view of a threaded cap  2700  engaging threading  2612  of recess  2602  in the screw-compression alignment. 
     It is noted that the screws  400  have been received through the aligned threaded cap  2700  and the openings  104 ,  114 . Trajectories for the screws  400  in relation to the vertebra(e) have been selected and the screws  400  threaded into the vertebra or vertebrae to secure the plate  100  to the vertebra(e) via the openings  104 ,  114 . The screw heads  402  of the screws  400  engage at least portion of the openings  104 ,  114  in the selected trajectories. 
     Thereafter, the threaded cap  2700  is advanced further in the recess  2602  into the screw-compression alignment in which the threaded cap  2700  engages the engagement surfaces  416  (spheres  420 ) of the heads  402  via recesses  2810 ,  2814 , compressing the screw heads  402  (spheres  404 ) of the screws  400  into the engaged portion of the openings  104 ,  114  in the selected trajectories via spheres  420  of the engagement surfaces  416 . This compression reduces or eliminates the wobbling of the screws  400  in the openings  104 ,  114  if and when the screws  400  loosen with respect to the vertebra(e). 
     In the screw-compression alignment, the bottom surface  2816  of the threaded cap  2700  can be at distance to the seat  2604  or can engage the seat  2604  in order to provide friction, counteracting any unscrewing forces that can compel the threaded cap  2700  from the recess  2602 . 
     Thus, a spinal plate with compression locking and a method of fixating vertebrae using the spinal plate with compression locking have been described. Although specific example embodiments have been described, it will be evident that various modifications and changes can be made to these embodiments without departing from the broader scope of this application. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter can be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments can be utilized and derived therefrom, such that structural substitutions and changes can be made without departing from the scope of this application. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter can be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention, inventive concept or embodiment. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose can be substituted for the specific embodiments shown. This application is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     The Abstract is provided to comply with 37 C.F.R. § 1.72(b) and will allow the reader to quickly ascertain the nature of the technical disclosure of this application. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
     In the foregoing description of the embodiments, various features can be grouped together in a single embodiment for the purpose of streamlining the disclosure of this application. This method of disclosure is not to be interpreted as reflecting that the claimed embodiments have more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment.