Abstract:
An implantable cervical plate assembly includes a cervical plate, one or more bone fasteners. The cervical plate comprises an elongated asymmetric body having one or more through-openings extending from the front surface to the back surface of the elongated asymmetric body. The one or more bone fasteners are configured to be inserted through the one or more through-openings, respectively. The bone fasteners comprise a threaded main body and a head that includes one or more breakable structures configured to be broken when inserted into a groove and then unflex and remain captured within the groove.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61/947,605 filed Mar. 4, 2014 and entitled “BONE FASTENER FOR A SPINAL FIXATION ASSEMBLY”, the contents of which are expressly incorporated herein by reference. 
         [0002]    This application is also a continuation in part and claims the benefit of U.S. application Ser. No. 13/742,898 filed Jan. 16, 2013 and entitled “SYSTEM AND METHOD FOR A SPINAL STABILIZATION IMPLANT ASSEMBLY”, the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to a bone fastener used for attaching a spinal plate to vertebral elements, and in particular to a bone fastener with breakable head components. 
       BACKGROUND OF THE INVENTION 
       [0004]    Spine fixation assemblies are used to stabilize diseased or surgically removed vertebral elements. Several prior art spine fixation assemblies utilize rods and/or plates as connecting and stabilization elements between the vertebral elements. The rods and/or plates are usually secured to vertebral bones with screws. In situations and/or spinal locations where the vertebral elements are allowed to move after the rod or plate is attached, stresses associated with this motion or stresses due the motion of adjacent vertebral elements often cause the screws to disengage from the rod or plate and finally from the vertebral elements. Accordingly, there is a need for a locking mechanism that would prevent such a disengagement of the screws from the rod or plate and the vertebral elements. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention relates to a system and method for a bone fastener used for securing a spinal plate assembly to vertebral elements. The bone fastener includes a self-contained locking mechanism that prevents accidental disengagement of the bone fastener due to stresses after they have been attached to the vertebral elements. The bone fastener comprises a threaded main body and a head. The threaded main body comprises threads for engaging a spinal vertebra and the head comprises one or more breakable structures configured to break when inserted into an opening of the spinal plate. 
         [0006]    In general, in one aspect, the invention features an implantable cervical plate assembly for stabilization of two adjacent spinal vertebras, including a cervical plate and two or more bone fasteners. The cervical plate includes an elongated body having two or more through-openings extending from a front surface to a back surface of the elongated body. The two or more bone fasteners are configured to be inserted through the two or more through-openings, respectively, and to be attached to two or more locations in the two adjacent spinal vertebras, respectively, thereby attaching the cervical plate to the spinal vertebras. The through-openings comprise a first diameter at the front surface of the elongated body, a second diameter at the back surface of the elongated body and a third diameter in the area between the front and back surfaces of the elongated body and the first diameter is smaller than the third diameter, thereby forming a lip at the top of the through-openings and the third diameter is larger than the second diameter and the first diameter is larger than the second diameter, thereby forming a groove within the perimeter of the inner wall of the through-openings. The bone fasteners comprise a threaded main body and a head and the threaded main body comprises threads for engaging the spinal vertebras and the head comprises a cylindrical main body and one or more breakable structures configured to be flexed and inserted into the groove and then break and unflex upward and remain captured within the groove. Each of the one or more breakable structures comprises a curved body that extends tangentially from a first location of an outer side of the cylindrical main body and curves around a portion of the cylindrical main body and terminates and attaches to a second location of the outer side of the cylindrical main body. 
         [0007]    Implementations of this aspect of the invention may include one or more of the following features. The diameter of the bone fastener head including the breakable structures in an unflexed position is larger than the first diameter of the through openings and the breakable structures are configured to flex inward toward the outer side surface of the cylindrical main body when they come in contact with the lip while the bone fastener is rotated clock-wise to be driven into the vertebras and then the breakable structures are configured to break and detach from at the second location of the outer side of the main cylindrical body and unflex upward once they are below the lip. The bone fastener head comprises an opening extending into the threaded main body and the opening comprises an inner surface having six inward protruding lobes and a bottom having six grooves. The assembly further includes a driver tool, and the driver tool comprises an elongated shaft, a handle attached to the proximal end of the elongated shaft and a bone fastener-engaging component attached to the distal end of the elongated shaft and the bone fastener-engaging component comprises one or more structures that complement and engage at least one of the grooves and lobes of the bone fastener head opening, respectively. The structures of the fastener-engaging component comprise four lobes that complement and engage four of the six lobes of the bone fastener head opening and two opposite tubular protrusions configured to be positioned and engage two opposite located grooves of the bone fastener head opening. The fastener-engaging component comprises a driver and a locking sleeve and the driver comprises an elongated cylindrical body having the structures at its distal end and a slot extending along the driver tool axis and the cylindrical body flexes and snaps into the bone fastener opening and the locking sleeve is configured to move down and lock the driver into the bone fastener head opening. The locking sleeve comprises a tubular cylindrical body and a central blade and the tubular cylindrical body is dimensioned to fit and slide over the driver cylindrical elongated body and wherein the central blade is configured to be placed within the driver slot. The breakable structures comprise curved, angled or beveled outer surfaces and the breakable structures outer surfaces cooperate with matching outer surfaces of the lip. The bone fastener head comprises an opening extending into the threaded main body and the opening comprises pentagonal, hexagonal or octagonal geometric shape. The bone fastener head comprises an opening extending into the threaded main body and the opening comprises inner threads. The elongated body comprises a first straight side surface, a second contoured side surface opposite to the first side surface, the front and back surfaces and top and bottom surfaces and the elongated body further comprises one or more elongated openings configured to support bone graft material. The through-openings comprise an oval-shaped perimeter at the back surface and the oval-shaped perimeter comprises two parallel straight sides and two opposite curved sides and the distance between the two parallel straight sides is smaller than the major diameter of the threads of the bone fasteners and wherein the distance between the curved sides is equal to or larger than the major diameter of the threads of the bone fasteners. The bone fasteners further comprise a tapered portion extending between the threaded main body and the head and the parallel straight sides of the through-openings cut into the diameter of the tapered portion for a tighter secure lock and fit. The through-openings further comprise laser-etched ridges extending perpendicular to the groove. The back surface of the cervical plate comprises a roughened texture. 
         [0008]    In general in another aspect the invention features a bone fastener including a threaded main body and a head. The threaded main body includes threads and the head includes a cylindrical main body and one or more breakable structures configured to be flexed and inserted into a groove and then break and unflex upward and remain captured within the groove. Each of the one or more breakable structures comprises a curved body that extends tangentially from a first location of an outer side of the cylindrical main body and curves around a portion of the cylindrical main body and terminates and attaches to a second location of the outer side of the cylindrical main body. 
         [0009]    In general in another aspect the invention features a method for stabilizing two adjacent spinal vertebras, including the following. First, providing a cervical plate comprising an elongated body having two or more through-openings extending from the front surface to the back surface of the elongated asymmetric body. Next, inserting two or more bone fasteners through the two or more through-openings, respectively, and attaching them to two or more locations in the two adjacent spinal vertebras, respectively, thereby attaching the cervical plate to the spinal vertebras. The through-openings comprise a first diameter at the front surface of the elongated body, a second diameter at the back surface of the elongated body and a third diameter in the area between the front and back surfaces of the elongated body and the first diameter is smaller than the third diameter, thereby forming a lip at the top of the through-openings and the third diameter is larger than the second diameter and the first diameter is larger than the second diameter, thereby forming a groove within the perimeter of the inner wall of the through-openings. The bone fasteners comprise a threaded main body and a head and the threaded main body comprises threads for engaging the spinal vertebras and the head comprises a cylindrical main body and one or more breakable structures configured to be flexed and inserted into the groove and then break and unflex upward and remain captured within the groove. Each of the one or more breakable structures comprises a curved body that extends tangentially from a first location of an outer side of the cylindrical main body and curves around a portion of the cylindrical main body and terminates and attaches to a second location of the outer side of the cylindrical main body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Referring to the figures, wherein like numerals represent like parts throughout the several views: 
           [0011]      FIG. 1  is a perspective view of a cervical plate assembly, according to this invention; 
           [0012]      FIG. 2A  is a perspective view of the cervical plate of  FIG. 1 ; 
           [0013]      FIG. 2B  is a side view of the cervical plate of  FIG. 2A ; 
           [0014]      FIG. 3  is a top view of the cervical plate of  FIG. 2A ; 
           [0015]      FIG. 4A  is a side view of end  111   b  of the cervical plate of  FIG. 2B ; 
           [0016]      FIG. 4B  is a cross-sectional view of the cervical plate along line  113 ; 
           [0017]      FIG. 5A  is a perspective view of the screw of  FIG. 1  before engaging an opening of the cervical plate; 
           [0018]      FIG. 5B  is a perspective view of the screw of  FIG. 1  after engaging an opening of the cervical plate; 
           [0019]      FIG. 6A  is a top view of the screw of  FIG. 5A ; 
           [0020]      FIG. 6B  is a side view of the screw of  FIG. 5A ; 
           [0021]      FIG. 7A  is a top perspective view of the screw of  FIG. 5A  before engaging an opening of the cervical plate; 
           [0022]      FIG. 7B  is a top perspective view of the screw of  FIG. 7A  as it starts to engage an opening of the cervical plate; 
           [0023]      FIG. 7C  is a top perspective view of the screw of  FIG. 7A  after it has fully engaged an opening of the cervical plate; 
           [0024]      FIG. 7D  is a top view of the screw of  FIG. 7C ; 
           [0025]      FIG. 8  is a cross-sectional view of the cervical plate assembly; 
           [0026]      FIG. 9A  is a detailed side view of area A in  FIG. 8 ; 
           [0027]      FIG. 9B  is a detailed top view of area A in  FIG. 8 ; 
           [0028]      FIG. 10  is a cross-sectional view depicting an angular placement of a the screw within an opening of the cervical place; 
           [0029]      FIG. 11  depicts a two-component driver tool; 
           [0030]      FIG. 11A  depicts a driver tool end in the unlocked position; 
           [0031]      FIG. 11B  depicts the driver tool end of  FIG. 11A  in the locked position; 
           [0032]      FIG. 11C  is a cross-sectional view of  FIG. 11A ; 
           [0033]      FIG. 11D  is a cross-sectional view of  FIG. 11B ; 
           [0034]      FIG. 11E  is a detailed view of the lower end of the driver tool in the locked position; 
           [0035]      FIG. 11F  is an exploded view of  FIG. 11C ; 
           [0036]      FIG. 11G  is a detailed bottom view of the driver  210 ; 
           [0037]      FIG. 11H  is a detailed bottom view of the driver  210  with the lowered blade  226 ; 
           [0038]      FIG. 14A  depicts a driver tool end for removing a bone screw; 
           [0039]      FIG. 14B  is an exploded view of the driver tool end of  FIG. 14A ; 
           [0040]      FIG. 15A  and  FIG. 15B  are perspective views of an intervertebral component assembly, according to this invention; 
           [0041]      FIG. 16  is a front view of the intervertebral component  150  and screw  120  of  FIG. 15A  in the non-engaged position; and 
           [0042]      FIG. 17  is a partially exploded perspective view of the intervertebral component assembly of  FIG. 15B . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]    The present invention relates to a system and method for a cervical plate assembly that includes an asymmetric bone plate and screws attaching the plate to vertebral elements. The screws include a self-contained locking mechanism that prevents accidental disengagement of the screws due to stresses after they have been attached to the vertebral elements. 
         [0044]    Referring to  FIG. 1 , cervical plate assembly  100  includes a cervical plate  110  and screws  120 . Cervical plate  110  is a two-level bone plate configured to stabilize three adjacent vertebras (not shown). Referring to  FIG. 2A ,  FIG. 2B ,  FIG. 3 ,  FIG. 4A  and  FIG. 4B , plate  110  includes an elongated asymmetric body  118  that has six through-openings  114   a - 114   f  extending from the top surface  112   a  to the bottom surface  112   b  of body  118 . Body  118  has one side  109   b  that is straight and an opposite side  109   a  that is contoured around the openings  114   a - 114   c . The width  118   a  of plate  110  in the area inbetween openings  114   a ,  114   b  and inbetween  114   b ,  114   c  is smaller than the width  118   b  in the areas across openings  114   a ,  114   f  at the end  111   a  of the plate, across openings  114   c ,  114   d  at the end  111   b  of the plate and across openings  114   b ,  114   e  at the center  116  of the plate. In one example, body  118  has a length  118   c  of 43 millimeters, a width  118   a  of 13 millimeters and a width  118   b  of 17 millimeters. There are also two additional through-openings  119   a ,  119   b  arranged along the straight side  109   b  of the plate between two adjacent main openings  114   f ,  114   e  and  114   e ,  114   d , respectively. The reduced width  118   a  of the plate due to the contoured side  109   a  and the presence of openings  119   a ,  119   b  along the straight side  109   b  help improve the line of sight. Openings  119   a ,  119   b  are also used for inserting bone graft material. Cervical plate  110  is also curved along its width and is thicker along the center  108  relative to the sides  109   a ,  109   b . In one example, the plate thickness at the center is 2.55 millimeters, the width at the sides is 2.3 millimeters and the curvature R along its width 27 millimeters. The increased thickness along the center  108  provides stability and additional strength. The overall plate thickness is kept at a minimum level in order to maintain a low profile and the overall contour of the plate is configured to provide improved anatomical interface. Cervical plate ends  111   a ,  111   b  are chamfered to minimize damage of the adjacent soft tissue. Through-openings  114   a - 114   f  receive the screws  120 , which are used to attach the plate  110  to the vertebras. Openings  114   a - 114   f  have an essentially circular perimeter at the top surface  112   a  of the plate. The diameter  131   a  of each opening  114   a - 114   f  near the top surface  112   a  is larger than the diameter  131   b  near the bottom surface  112   b , as shown in  FIG. 4B . Both top and bottom diameters  131   a ,  131   b  are smaller than the diameter  131   c  at the center of the opening. In one example, diameter  131   a  is 6 millimeters, diameter  131   b  is 4.20 millimeters and diameter  131   c  is 6.4 millimeters. A lip  132  is formed around each opening  114   a - 114   f  near the top surface  112   a . Lip  132  is designed to interface with breakable components  121   a - 121   c  of the screw head  122  and thereby to lock the screw  120  onto the plate  110 , as will be explained below. Openings  114   a - 114   f  have a chamfered bottom portion  117 , as shown in  FIG. 4B . Chamfered bottom portion  117  allows the screws  120  to assume variable trajectory and angled orientation when engaged in the vertebral bone, as shown in  FIG. 10 . In some embodiments, polyaxial screws  120  are used and the chamfered bottom  117  allows them to be positioned at a desired angular orientation  146  prior to being locked. The bottom portion  117  of the openings  114   a - 114   f  is oval-shaped and has two parallel straight sides  117   a ,  117   b  and two opposite curved sides  117   c ,  117   d . The distance between the two parallel straight sides  117   a ,  117   b  (width of the opening)  131   b  is smaller than the major diameter  91  of the threaded portion  124  of the screw  120  and equal or larger than the minor diameter  92  of the threaded portion  124 , shown in  FIG. 5A . The distance  131   d  between the curved sides  117   c  and  117   d  of the opening (diameter) is larger or equal to the major diameter  91  of the threaded portion  124  of the screw. The oval-shaped structure of the bottom portion  117  of openings  114   a - 114   f  cooperates with the screw threads  124   a  to allow the screw  120  to move downward or upwards through the opening when the screw  120  is rotated and prevents backing out or moving forward of the screw  120  when the screw is pushed up or down, respectively. Since the width  131   b  of the opening at the bottom portion  117  is smaller than the major diameter  91  of the threaded portion  124  of the screw  120  and the diameter  131   d  is larger or about the same size as the major diameter  91  of the threaded portion  124 , the screw threads  124   a  move through the opening as they are rotated clock-wise only when they are in line with the diameter  131   d . Once the screw threads  124   a  pass below the bottom portion  117  of the opening, they cannot be accidentally pushed straight up because they will hit the straight parallel sides  117   a ,  117   b  of the oval-shaped opening, whose spacing  131   d  is smaller than the major diameter  91  of the screw. This “threading” of the screw  120  through the oval-shaped opening (i.e. “captive geometry”) of the bottom portion  117  of the plate  110  locks the screw  120  to the plate  110  and prevents accidental backing out of the screw  120 . Furthermore, screw  120  includes a tapered portion (angled sides  125   a ,  125   b ) and at this tapered portion the straight parallel sides  117   a ,  117   b  cut into the diameter of the tapered portion for a tighter secure lock and fit. Plate  110  is also described in U.S. application Ser. No. 13/785,279 filed Mar. 5, 2013 and entitled “CERVICAL PLATE ASSEMBLY”, which is commonly owned and the contents of which are expressly incorporated herein by reference. 
         [0045]    Referring to  FIG. 5A  to  FIG. 10 , bone screw  120  has a threaded main body  124  and a head  122 . Main body  124  includes threads  124   a  for engaging the vertebral bone. Head  122  has a flat top  123 , a cylindrical center  126 , three breakable components  121   a ,  121   b ,  121   c , and a tapered portion  125  with angled bottom sides  125   a ,  125   b , as shown in  FIG. 8 . Top  123  includes an opening  128  extending into the main body  124 . Opening  128  has six lobes  127   a - 127   f , and at the bottom between two adjacent lobes six grooves  99   a - 99   f  are formed, as shown in  FIG. 11F . As will be explained later, the geometry of opening  128  interfaces with the geometry of a screw engaging component  284  to lock a driver tool  200  into the opening  128 , as shown in  FIG. 11B . 
         [0046]    Each of the breakable components  121   a - 121   c  includes a curved body that extends tangentially from a first location of the outer side of the cylindrical center  126 , curves around the center  126  and terminates at a second location of the outer side of the cylindrical center  126 . Breakable components  121   a - 121   b  are initially attached to the cylindrical center  126  at both the first and second locations, and gaps  66   a - 66   c  are formed between the outer surface of the cylindrical center  126  and the breakable component  121   a - 126   c . In one example, breakable component  121   a  extends from location  126   a  of the outer surface of the cylindrical center  126  and terminates at location  126   b  of the outer side of the cylindrical center  126 . Breakable component  121   a  is initially attached to both locations  126   a  and  126   b  and a gap  66   a  is formed between the outer surface of the cylindrical center  126  and the breakable component  121   a . The connection between the breakable component  121   a  and the outer surface of the cylindrical center  126  at point  121   b  is weak and breaks when the screw head  122  engages the opening  114   a  of the cervical plate  110 , as shown in  FIG. 5B .  FIG. 7A  depicts the head  122  of screw  120  just before it engages opening  114   a  of the cervical plate  110 .  FIG. 7B  depicts the head  122  of screw  120  immediately after it engaged opening  114   a  of the cervical plate  110 . As shown, one end of each of the breakable components  121   a - 121   c  breaks and detaches from the cylindrical center  126 . As the screw  120  continues to engage the bone, the detached ends of the breakable components  121   a - 121   c  flex upward, as shown in  FIG. 7C . 
         [0047]    The effective diameter  136  of the screw head  122  including the breakable components  121   a - 121   c  in the initial position of  FIG. 7A  is larger than the top diameter  131   a  of openings  114   a - 114   f . Breakable components  121   a - 121   c  flex inward toward the central axis  140  when they come in contact with lip  132  of the openings  114   a - 114   f  while the screw  120  is being rotated clock-wise to be driven into the vertebral body. This inward flexing causes the breakable components  121   a - 121   c  to break away from the cylindrical body  126  at their corresponding second locations. The effective diameter  136   a  of the screw head  122  including the breakable components  121   a - 121   c  in the inward flexed position is smaller than the top diameter  131   a  of openings  114   a - 114   f , and this allows the screw head  122  including the breakable components  121   a - 121   c  to move below the lip  132 , as shown in  FIG. 7B . Once the breakable components  121   a - 121   c  are below the lip  132  they expand back up to their unflexed position within the space  133  formed in the opening  114   a  between the lip  132  and the chamfered sides at the bottom portion  117  of the opening, as shown in  FIG. 7C . Once the entire screw head  122  is in place within space  133 , the lip  132  prevents the screw head from accidentally moving up (i.e., backing out) from space  133  due to stresses applied during spinal motion. In cases where the mounted screw is rotated counter-clockwise, breakable components  121   a - 121   c  hit the lip  132  and sidewall  133   a  and flex outward away from the central axis  140 , thereby increasing the effective diameter of the screw head so that it is even larger than the top diameter  131   a . This outward flexing of the breakable components  121   a - 121   c  prevents the screw head  122  from accidentally moving up and out of space  133 . The surgeon may pull out the screw with a driver tool, as will be described below. 
         [0048]    In operation, plate  110  is attached to the vertebras with the screws  120 . During the driving in of the screws into the selected vertebral locations, the screw threads  124   a  cooperate with the “captive geometry” at the bottom portion of the plate  117 . The breakable components  121   a - 121   c  are flexed inward and break away from the cylindrical center  126  at their corresponding second locations and then move in space  133  where they expand back up to their unflexed state. The combination of these two mechanisms, i.e., “threading” the screw  120  though the bottom portion  117  of the plate  110  and the breaking, positioning and locking of the breakable components  121   a - 121   c  in space  133 , lock the screw  120  onto the plate  110  and prevent accidental disengagement due to stresses generated during motion. 
         [0049]    Referring to  FIG. 11  to  FIG. 13 , a two-component tool  200  is used to drive screw  120  through the openings  114   a - 114   f  of the cervical plate  100  into the bone. Tool  200  includes an elongate shaft  280  having a handle  282  at its proximal end and a screw engaging component  284  at its distal end. Screw engaging component  284  includes a driver  210  and a locking sleeve  220 . Driver  210  has an elongated cylindrical body  212  with a cylindrical top  214  and a driver end  216 . The driver end  216  includes four lobes  217   a - 217   d  that match and interface with four of the six lobes  127   a ,  127   c ,  127   d ,  127   f  of opening  128  in the screw top  123 , respectively. Driver end  216  also includes two tubular protrusions  218   a ,  218   b  positioned between lobes  217   a ,  217   d  and  217   d ,  217   c , respectively. Protrusions  218   a ,  218   b  fit within opposite located grooves  99   a  and  99   d  formed between adjacent lobes in opening  128 . The interfacing of the driver end geometry with the screw head opening  128  geometry engages the driver  210  to the screw head  122 . In this engaged position, the driver is used to rotate screw  120  clockwise or counter-clockwise. An elongated slot  215  extends along the length of the cylindrical body  212  through its center and allows the body  212  to flex and snap into opening  128  of the screw head. Once the driver end  216  is snapped into opening  128 , the locking sleeve  220  is moved down to lock the driver  210  into the opening  128  of the screw head. Locking sleeve  220  has a cylindrical body  222  with a diameter larger than the diameter of the cylindrical body  212  of the driver. Cylindrical body  222  has a central opening  224  extending the entire length of body  222  and a central blade  226  extending from about the middle of body  222  toward and past the lower end  222   a  of body  222 . Driver  210  is inserted into the central opening  224  of the locking sleeve  220  and slot  215  is aligned with and placed over blade  226 , as shown in  FIG. 11A  and  FIG. 11C . After placing the driver end  216  into the screw opening  128 , the locking sleeve  220  is moved down in the direction  219  so that the blade  226  is positioned in the slot area of the driver end  216 , shown in  FIG. 11B  and  FIG. 11D . The two parallel sides  226   a ,  226   b  of blade  226  protrude through the sides of slot  215 , as shown in  FIG. 11E . The protruding blade sides  226   a ,  226   b  interface with two opposite lobes  127   b ,  127   e  in opening  128 , respectively. The placing of the blade  226  within the slot  215  in the screw head opening  128  prevents the lower end of body  212  from flexing and thereby locks the driver  210  within the screw head opening  128 . The locked driver  210  is then used to rotate clockwise or counter-clockwise screw  120  into or out of the desired bone location, respectively, and to drive or pull the screw  120  in or out of place. 
         [0050]    Referring to  FIG. 14   a , and  FIG. 14B , the driver tool  200  includes an inner cylindrical shaft  232  having a screw  236  at its distal end, instead of an inner central blade  226 . Screw  236  is used for removing a bone screw from a vertebral location. In this case, opening  128  in the bone screw head top  123  includes inner threads. Inner cylindrical shaft  232  rotates clockwise independently of the outer sleeve  238  and attaches screw  236  to the threaded hole  128 , thereby locking the driver tool  200  to the screw  120 . Rotating the driver tool  200  counter-clockwise removes the screw  120  from its place. 
         [0051]    Referring to  FIG. 15A-FIG .  16 B, screw  120  is used for securing an intervertebral component  150  to adjacent vertebras. Intervertebral component  150  includes three openings  154   a ,  154   b ,  154   c  that are configured to receive corresponding rings  156   a ,  156   b ,  156   c  and screws  120 , as shown in  FIG. 17  and  FIG. 15B . Each openings  154   a ,  154   b ,  154   c  has an essentially circular perimeter at the top surface of the intervertebral component  150 . The diameter  151   a  of each opening  154   a ,  154   b ,  154   c  near the top surface is larger than the diameter  151   b  near the bottom surface, as shown in  FIG. 16A . Both top and bottom diameters  151   a ,  151   b  are smaller than the diameter  151   c  at the center of the opening. A lip  152  is formed around each opening  154   a ,  154   b ,  154   c  near the top surface. Lip  152  is designed to interface with breakable components  121   a - 121   c  of the screw head  122  and thereby to lock the screw  120  onto the intervertebral component  150 , as was explained above. Intervertebral component  150  is made of polyether ether ketone (PEEK) and rings  156   a - 156   c  and screws  120  are made of metal. Metal rings  156   a - 156   c  are inserted into openings  154   a - 154   c  and prevent the metal screw head  122  from scraping and damaging the intervertebral component  150  within the openings  154   a - 154   c . Metal breakable components  121   a - 121   c  in each screw head  122  interface with the metal rings  156   a - 156   c  in the corresponding opening  154   a - 154   c  and compress inward and break without contacting the PEEK material. 
         [0052]    Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.