Abstract:
A bone screw assembly includes a tulip-shaped seat, a bone fixation device, a ring-shaped washer, a rod and a cap. The tulip-shaped seat comprises a through opening dimensioned to receive the bone fixation device and a horizontal channel dimensioned to receive the rod. The washer is placed into the seat on top of the bone fixation device head and comprises first and second side tabs and a first pair of upward extending projections separated by a first gap and a second pair of upward extending projections separated by a second gap. The side tabs interface with a groove formed in the bottom portion of the seat. The rod is placed within the channel and positioned within a groove formed on the top surface of the washer. The cap includes first and second projections extending downward from its bottom surface and each of the first and second downward projections comprises first and second sidewise extending ridges. The first ridges of the first and second projections are aligned and placed within the first and second gaps of the washer, respectively, and interface with a first groove formed on the side portion of the seat and the second ridges interface with a second groove on the side portion of the seat when the cap is rotate.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 61/097,288 filed Sep. 16, 2008 and entitled “SPINAL SCREW ASSEMBLY”, the contents of which are expressly incorporated herein by reference. 
     This application is also a continuation in part of U.S. application Ser. No. 11/121,414 filed on May 4, 2005 and entitled “MULTISTAGE SPINAL FIXATION LOCKING MECHANISM” the contents of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a spinal screw assembly and to a screw insertion tool, and in particular to a spinal screw assembly used in connection with spinal stabilization rods. 
     BACKGROUND OF THE INVENTION 
     The human spine comprises individual vertebras  30  (segments) that are connected to each other to form a spinal column  29 , shown in  FIG. 1A . Referring to  FIGS. 1B and 1C , each vertebra  30  has a cylindrical bony body (vertebral body)  32 , three winglike projections (two transverse processes  33 ,  35  and one spinous process  34 ), left and right facet joints  46 , lamina  47 , left and right pedicles  48  and a bony arch (neural arch)  36 . The bodies of the vertebrae  32  are stacked one on top of the other and form the strong but flexible spinal column. The neural arches  36  are positioned so that the space they enclose forms a tube, i.e., the spinal canal  37 . The spinal canal  37  houses and protects the spinal cord and other neural elements. A fluid filled protective membrane, the dura  38 , covers the contents of the spinal canal. The spinal column is flexible enough to allow the body to twist and bend, but sturdy enough to support and protect the spinal cord and the other neural elements. The vertebras  30  are separated and cushioned by thin pads of tough, resilient fiber known as inter-vertebral discs  40 . Disorders of the spine occur when one or more of the individual vertebras  30  and/or the inter-vertebral discs  40  become abnormal either as a result of disease or injury. In these pathologic circumstances, fusion of adjacent vertebral segments may be tried to restore the function of the spine to normal, achieve stability, protect the neural structures, or to relief the patient of discomfort. 
     Several spinal fixation systems exist for stabilizing the spine so that bony fusion is achieved. The majority of these fixation systems utilize rods that attach to screws which are threaded into the vertebral bodies or the pedicles. These systems can be extended along the sides of the spine by connecting two adjacent pedicles at a time similar to the concept of a bicycle chain. Spinal screw assemblies that allow support of stabilization rods without adding bulk to the lateral aspect of the spine or limiting access to the pars and transverse processes are desirable. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, the invention features a bone screw assembly including a tulip-shaped seat, a bone fixation device, a ring-shaped washer, a rod and a cap. The tulip-shaped seat comprises a semispherical bottom portion and a cylindrical side portion extending upward from the bottom portion. The bottom portion comprises a through opening dimensioned to receive the bone fixation device and to prevent the fixation device from passing entirely therethrough. The side portion comprises first and second concentric grooves extending along its inside periphery wall and a horizontal channel dimensioned to receive the rod. The bone fixation device comprises a bone fixation portion passing through the bottom portion opening and a head residing within the semispherical bottom portion. The ring-shaped washer is placed into the seat on top of the bone fixation device head and includes a first pair of outward extending projections separated by a first gap and a second pair of outward extending projections separated by a second gap. The projections extend from the top surface of the washer and are shaped and dimensioned to interface with the first concentric groove of the seat side portion. The rod placed within the channel and positioned within a groove formed on the top surface of the washer. The cap includes first and second projections extending downward from its bottom surface and wherein each of the first and second downward projections comprises first and second sidewise extending ridges and wherein the first ridges of the first and second projections are aligned and placed within the first and second gaps of the washer, respectively, and interface with the first groove of the seat side portion and the second ridges interface with the second groove of the seat side portion when the cap is rotate. 
     Implementations of this aspect of the invention may include one or more of the following features. The bone fixation device may be a polyaxial screw. The bone screw assembly may further include a locking screw threaded through a central opening formed in the cap for locking the rod down onto the bone fixation head. 
     In general in another aspect the invention features a bone screw assembly including a tulip-shaped seat, a ring-shaped washer, a rod and a cap. The tulip-shaped seat comprises a semispherical bottom portion, a cylindrical side portion extending upward from the bottom portion and a bone fixation device extending downward from the bottom portion. The side portion comprises first and second concentric grooves extending along its inside periphery wall and a horizontal channel dimensioned to receive the rod. The ring-shaped washer is placed into the seat on top of the head of the bone fixation device and comprises a first pair of outward extending projections separated by a first gap and a second pair of outward extending projections separated by a second gap. The projections extend from the top surface of the washer and are shaped and dimensioned to interface with the first concentric groove of the seat side portion. The rod is placed within the channel and positioned within a groove formed on the top surface of the washer. The cap comprises first and second projections extending downward from its bottom surface. Each of the first and second downward projections comprises first and second sidewise extending ridges. The first ridges of the first and second projections are aligned and placed within the first and second gaps of the washer, respectively, and interface with the first groove of the seat side portion and the second ridges interface with the second groove of the seat side portion when the cap is rotate. 
     In general, in another aspect, the invention features a bone screw assembly including a tulip-shaped seat, a bone fixation device, a cylindrically shaped washer, a rod and a cap. The tulip-shaped seat comprises a semispherical bottom portion and a cylindrical side portion extending upward from the bottom portion. The bottom portion comprises a through opening dimensioned to receive the bone fixation device and to prevent the fixation device from passing entirely therethrough. The side portion comprises first and second side through openings arranged opposite to each other and on opposite sides of the side portion and a horizontal channel dimensioned to receive the rod. The bone fixation device comprises a bone fixation portion passing through the bottom portion opening and a head residing within the semispherical bottom portion. The cylindrically shaped washer is placed into the seat on top of the bone fixation device head and comprises a first and second outward extending side projections. The side projections extend from the opposite sides of the washer side surface and are shaped and dimensioned to interface with the first and second side through openings of the seat, respectively and to protrude through the outer surface of the seat side portion. The rod is placed within the channel and positioned within a groove formed on the top surface of the washer. The cap comprises first and second sides extending downward from its bottom surface and the first and second sides comprise first and second grooves, respectively. The first and second grooves of the first and second sides are aligned and placed around the outer surface of the seat side portion, and interface with the first and second side projection protruding through the first and second side through openings when the cap is rotate. 
     In general, in another aspect, the invention features a bone screw assembly including a tulip-shaped seat, a bone fixation device, a cylindrically shaped washer, a rod and a cap. The tulip-shaped seat comprises a semispherical bottom portion and a cylindrical side portion extending upward from the bottom portion. The bottom portion comprises a groove extending along its inside periphery wall and a through opening dimensioned to receive the bone fixation device and to prevent the fixation device from passing entirely therethrough. The side portion comprises first and second concentric grooves extending along its inside periphery wall and a horizontal channel dimensioned to receive the rod. The bone fixation device comprises a bone fixation portion passing through the bottom portion opening and a head residing within the semispherical bottom portion. The washer is placed into the seat on top of the bone fixation device head and comprises first and second side tabs and a first pair of upward extending projections separated by a first gap and a second pair of upward extending projections separated by a second gap. The projections extend from the top surface of the washer and the first and second side tabs extend from opposite external sidewall of the washer and interface with the bottom portion groove. The rod is placed within the channel and positioned within a groove formed on the top surface of the washer. The cap comprises first and second projections extending downward from its bottom surface and each of the first and second downward projections comprises first and second sidewise extending ridges. The first ridges of the first and second projections are aligned and placed within the first and second gaps of the washer, respectively, and interface with the first groove of the side portion of the seat and the second ridges interface with the second groove of the side portion of the seat when the cap is rotate. 
     In general, in another aspect, the invention features an inserter tool for inserting a bone screw assembly. The bone screw assembly includes a bone screw and a tulip-shaped seat. The inserter tool includes a driver shaft, a retention sleeve, a spring and a pawl. The driver shaft comprises an elongated cylindrical body having a screw engaging distal end. The cylindrical body comprises upper and lower portions and the upper portion has a diameter larger than the diameter of the lower portion. The retention sleeve surrounds the driver shaft body and comprises a hollow cylindrical body having an upper portion, a lower portion and an intermediate step portion. The upper portion has a diameter larger than the diameter of the lower portion and is dimensioned to house the driver shaft upper portion and the lower portion is dimensioned to house the driver shaft lower portion. The retention sleeve lower portion comprises first and second flexible parallel segments. The spring surrounds the driver shaft upper portion and the driver shaft is configured to be pushed down or pulled up and slide within the retention sleeve and to compress the spring against the step portion. The pawl is configured to lock the position of the driver shaft relative to the retention sleeve. The first and second flexible segments comprise first and second distal ends, respectively, configured to engage the tulip-shaped seat. 
     Implementations of this aspect of the invention may include one or more of the following features. Each of the distal ends comprises a half annular ridge and first and second elongated protrusions extending downward from the bottom surface of the distal end. The annular ridge interfaces with an annular groove formed in the tulip-shaped seat and the first and second protrusions interface with the walls of a U-shaped opening formed in the tulip-shaped seat. The driver shaft lower portion comprises a diameter larger than the diameter of the lower portion of the retention sleeve and pushing the driver shaft down flexes the first and second flexible segments outward and locks the side annular ridges into the annular groove, thereby locking the tulip-shaped seat onto the first and second distal ends. The inserter tool may further comprise an outer sleeve surrounding the retention sleeve. The inserter tool may further comprise a handle. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the figures, wherein like numerals represent like parts throughout the several views: 
         FIG. 1A  is a side view of the human spinal column; 
         FIG. 1B  is an enlarged view of area A of  FIG. 1A ; 
         FIG. 1C  is an axial cross-sectional view of a lumbar vertebra; 
         FIG. 2  is a front perspective view of a first embodiment of the spinal screw assembly; 
         FIG. 3  is a side perspective view of the spinal screw assembly of  FIG. 2 ; 
         FIG. 4  is an exploded view of the spinal screw assembly of  FIG. 2 ; 
         FIG. 5  is a side view of the anchoring portion and head of the spinal screw assembly of  FIG. 2 ; 
         FIG. 6  is a top view of the head of  FIG. 5 ; 
         FIG. 7  is a side exploded view of the anchoring portion, head and washer of the spinal screw assembly of  FIG. 2 ; 
         FIG. 8  is a top exploded view of the anchoring portion, head and washer of the spinal screw assembly of  FIG. 2 ; 
         FIG. 9  is a top perspective view of the head with the installed washer; 
         FIG. 10  is a side transparent view of the head with the installed washer; 
         FIG. 11  is a side view of the head with the installed washer and stabilization rod; 
         FIG. 12  is a top view of the head with the installed washer and stabilization rod; 
         FIG. 13  is a side view of the head with the installed washer and stabilization rod and a side view of the cap before installation; 
         FIG. 14  is a top view of the head with the installed washer and stabilization rod and a top view of the cap before installation; 
         FIG. 15  is a top view of the head with the installed washer and stabilization rod and the cap inserted in the head; 
         FIG. 16  is a top view of the head with the installed washer and stabilization rod and the cap inserted in the head and rotated by a quarter turn; 
         FIG. 17  is a top view of the head with the installed washer, stabilization rod, cap and locking element; 
         FIG. 18  is a side transparent view of the assembly of  FIG. 15 ; 
         FIG. 19  is a side transparent view of the assembly of  FIG. 16 ; 
         FIG. 20  is another side transparent view of the assembly of  FIG. 15 ; 
         FIG. 21  is another side transparent view of the assembly of  FIG. 16 ; 
         FIG. 22  is a side transparent view of the head with the installed washer, stabilization rod, cap and locking element; 
         FIG. 23  is another side view of the head with the installed washer, stabilization rod, cap and locking element; 
         FIG. 24  is an exploded perspective view of another embodiment of the spinal screw assembly; 
         FIG. 25  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer; 
         FIG. 26  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer and rod; 
         FIG. 27  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer and rod and the cap before insertion; 
         FIG. 28  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer, rod and the cap; 
         FIG. 29  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer, rod and the cap rotated by a quarter turn; 
         FIG. 30  is a top perspective view of the spinal screw assembly of  FIG. 24  depicting the head with the installed washer, rod, cap and locking element; 
         FIG. 31  is another embodiment of the spinal screw assembly for a polyaxial screw; 
         FIG. 32  is an exploded perspective view of the spinal screw assembly of  FIG. 31 ; 
         FIG. 33A  is side cross-sectional view of the tulip-shaped head  120  of  FIG. 32 ; 
         FIG. 33B  is a top perspective view of the tulip-shaped head  120  of  FIG. 32 ; 
         FIG. 34  is a side perspective view of the washer  150  of  FIG. 32 ; 
         FIG. 35  is a side cross-sectional view of the spinal screw assembly of  FIG. 32 ; 
         FIG. 36A  is a perspective view of the inserter tool for the spinal screw assembly of  FIG. 32 ; 
         FIG. 36B  is an exploded view of the inserter tool of  FIG. 36A ; 
         FIG. 37A  depicts capturing the tulip-shaped screw head of  FIG. 32  with the inserter tool of  FIG. 36A ; 
         FIG. 37B  depicts locking the tulip-shaped screw head of  FIG. 32  onto the inserter tool of  FIG. 36A ; 
         FIG. 38A  is a detailed view of the inserter tool end portion prior to engaging the tulip-shaped screw head; 
         FIG. 38B  is a detailed view of the inserter tool end portion with the engaged tulip-shaped screw head; 
         FIG. 38C  is a detailed view of the inserter tool end portion with the screwdriver tip pushed down; 
         FIG. 39A  is a detailed cross-sectional view of the inserter tool end portion prior to engaging the tulip-shaped screw head; 
         FIG. 39B  is a detailed cross-sectional view of the inserter tool end portion inserted into the tulip-shaped screw head; and 
         FIG. 39C  is a detailed cross-sectional view of the inserter tool end portion inserted into the tulip-shaped screw head and with the screwdriver tip pushed down. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 ,  FIG. 3  and  FIG. 4 , a spinal screw assembly  100  includes an anchoring portion  110 , a tulip-shaped head  120 , a washer  150 , a stabilization rod  130 , a cap  140  and a locking element  160 . The anchoring portion  110  is usually a screw having outer threads, as shown in  FIG. 24 . In other embodiments, anchoring portion  110  is a pin (shown in  FIG. 2 ) or a hook (not shown). The head  120  includes a U-shaped slot  122  (shown in  FIG. 5 ) dimensioned to receive the stabilization rod  130 . The U-shaped slot is formed by two legs  123 ,  124  extending from a base  125 . Base  125  includes a groove  126  dimensioned to receive the washer  150 , shown in  FIG. 6 . Referring to  FIG. 7 , and  FIG. 8 , washer  150  is a circular ring-shaped plate having a first set of wings  152   a ,  152   b  and a second set of wings  154   a ,  154   b , extending from opposite edges of the top surface  151   a . Wings  152   a ,  152   b  are dimensioned to engage a groove  127   a  formed in the inner lower portion of leg  123 . Wings  154   a ,  154   b  are dimensioned to engage a grove  128   a  formed in the inner lower portion of leg  124 . Wings  152   a ,  152   b  are separated by a slot  153   a  and wings  154   a ,  154   b  are separated by a slot  153   b . Slots  153   a ,  153   b  are dimensioned to receive lower protrusions  142   a ,  142   b , respectively, extending from the lower surface of cap  140 , shown in  FIG. 4 . The top surface  151   a  of washer  150  also includes a groove  155  dimensioned to accommodate the stabilization rod  130  when the assembly is locked so that the groove axis  156  aligns parallel with the stabilization rod axis  132 , as shown in  FIG. 9 . Cap  140  also includes upper protrusions  144   a ,  144   b  extending from the lower surface of cap  140  and formed above the lower protrusions  142   a ,  142   b , respectively, as shown in  FIG. 13 . Upper protrusions  144   a ,  144   b  are dimensioned to engage grooves  127   b ,  128   b  formed in the inner upper portion of legs  123 ,  124 , respectively. Cap  140  further includes a central bore  145  dimensioned to receive the locking element  160 , which in this case is a threaded screw configured to engage inner threads in the bore  145 , shown in  FIG. 4 . The anchoring portion  110  is shown in the embodiment of  FIG. 2  to be integral with the head  120 . In other embodiments the anchoring portion  110  is a separate component from the head  120  and the two are joined together via a threading mechanism or a snap-in mechanism, as shown in the embodiments of  FIG. 24  and  FIG. 31 . 
     In operation the anchoring portion  110  is implanted into a spinal location and the washer  150  is threaded into base  125 , so that the bottom edge  157  engages the groove  126  and the wings  152   a ,  152   b  and  154   a ,  154   b  engage the lower grooves  127   a ,  128   a  of the legs  123 ,  124 , respectively, as shown in  FIG. 9  and  FIG. 10 . Next the stabilization rod  130  is placed in the slot  122 , as shown in  FIG. 11 ,  FIG. 12 , and then the cap  140  is inserted in the head  120  so that the lower protrusions  142   a ,  142   b  engage the spaces  153   a ,  153   b  between the washer protrusions  152   a ,  152   b  and  154   a ,  154   b , respectively, while the upper protrusions  144   a ,  144   b  engage the upper grooves  127   b ,  128   b  of the legs  123 ,  124 , respectively, as shown in  FIG. 15 ,  FIG. 18  and  FIG. 20 . Next, the cap  140  with the engaged washer  150  are rotated together in the direction of arrow  172  by a small segment to lock the position of the cap and to align the axis  148  of the cap perpendicular to the rod axis  132 , as shown in  FIG. 15 ,  FIG. 16 ,  FIG. 19  and  FIG. 21 . This cap and washer rotation also aligns the axis  156  of the washer groove  155  parallel with the stabilization rod axis  132 , as was mentioned above. In one example, the cap  140  and the washer  150  are rotated by a quarter turn. Finally, the locking element  160  is threaded into the bore  145  to tighten the position of the stabilization rod  130  down onto the washer  150 , as shown in  FIG. 17 ,  FIG. 22  and  FIG. 23 . 
     Referring to  FIG. 24-FIG .  30 , another embodiment of the spinal screw assembly  200  includes an anchoring portion  210 , a tulip-shaped head  220 , a washer  250 , a stabilization rod  230 , a cap  240  and a locking element  260 . The anchoring portion  210  is a screw having outer threads  212 . The head  220  includes a U-shaped slot  222  dimensioned to receive the stabilization rod  130 . The U-shaped slot is formed by two legs  223 ,  224  extending from a base  225 . Legs  223 ,  224  include slots  226   a ,  226   b , respectively, dimensioned to receive protrusions  252   a ,  252   b , respectively of the washer  250 , shown in  FIG. 25 . In this embodiment, screw  210  is not integral with the base  220  and includes a spherical head  214  dimensioned to sit in the base  225  while the threaded portion of the screw passes through a bore formed in the base  225  of the head  220 . This embodiment provides for multiaxial orientation of the screw  220  relative to the stabilization rod  230 . Cap  240  includes two legs  242 ,  244 , extending from the edges of the lower surface  241   b . The inner surfaces of legs  242 ,  244  include threads  243  dimensioned to engage outer threads  221  formed in the outer surfaces of legs  223 ,  224  of head  220 . 
     In operation, screw  210  is threaded through the bore in the base  225  of head  220  and is implanted into a spinal location. Next, washer  250  is inserted into head  220 , so that the side protrusions  252   a ,  252   b  engage the slots  226   a ,  226   b , respectively, as shown in  FIG. 25 . Next the stabilization rod  230  is placed in the slot  222 , as shown in  FIG. 26 , and then the cap  240  is placed on the head  220  so that the threaded legs  242 ,  244  engage the outer threads  221  in the head legs  223 ,  224 , respectively, as shown in  FIG. 28 . Next, the cap  240  is rotated in the direction of arrow  282  by a small segment to engage side protrusions  252   a ,  252   b  of the washer  250  through the slots  226   a ,  226   b , respectively, and thereby to lock the position of the washer  250  and the underlying spherical screw head  214 . In one example, the cap  240  is rotated by a quarter turn, thereby aligning the axis  248  of the cap perpendicular to the rod axis  232 , as shown in  FIG. 29 . Finally, the locking element  260  is threaded into the bore  245  to lock the position of the stabilization rod  230  down onto the washer  250 , as shown in  FIG. 30 . 
     Referring to  FIG. 31  and  FIG. 32 , another embodiment of the spinal screw assembly  300  includes an anchoring portion  310 , a tulip-shaped head  120 , a washer  150 , a stabilization rod  130 , a cap  140  and a locking element  160 . The anchoring portion  310  is a screw having outer threads. The head  120  includes a base  125  and a U-shaped slot  122  dimensioned to receive the stabilization rod  130 . The U-shaped slot  122  is formed by two legs  123 ,  124  extending from the base  125 . Base  125  includes a groove  126  dimensioned to receive the washer  150 , shown in  FIG. 34 . Referring to  FIG. 34 , washer  150  is cylindrically shaped and has a first set of wings  152   a ,  152   b  and a second set of wings  154   a ,  154   b , extending upward from opposite edges of its top surface  151   a . Wings  152   a ,  152   b  are dimensioned to engage a groove  127   a  formed in the inner lower portion of leg  123 , shown in  FIG. 33A . Wings  154   a ,  154   b  are dimensioned to engage a grove  128   a  formed in the inner lower portion of leg  124 , shown in  FIG. 33A . Wings  152   a ,  152   b  are separated by a slot  153   a  and wings  154   a ,  154   b  are separated by a slot  153   b . Slots  153   a ,  153   b  are dimensioned to receive lower protrusions  142   a ,  142   b , respectively, extending from the lower surface of cap  140 , shown in  FIG. 32 . Washer  150  also includes side tabs  159   a ,  159   b  extending from opposite sides of its perimeter. Side tabs  159   a ,  159   b  interface with the upper lip  301   a  of annular groove  301  formed in the inner surface of base  125  and prevent the washer from being pushed out of the head  120  by the screw  310 , as shown in  FIG. 35 . Annular groove  301  also includes two opposite loading cutouts  302   a ,  302   b . The washer side tabs  159   a ,  159   b  are aligned with the cutouts  302   a ,  302   b , respectively, when the washer is placed in the slot  122  and then the washer  150  is rotated so that the tabs are underneath the upper lip  301  of the annular groove  301 . The top surface  151   a  of washer  150  also includes a groove  155  dimensioned to accommodate the stabilization rod  130  when the assembly is locked so that the groove axis  156  aligns parallel with the stabilization rod axis  132 , as shown in  FIG. 9 . Cap  140  also includes upper protrusions  144   a ,  144   b  extending from the lower surface of cap  140  and formed above the lower protrusions  142   a ,  142   b , respectively, as shown in  FIG. 13  and  FIG. 32 . Upper protrusions  144   a ,  144   b  are dimensioned to engage grooves  127   b ,  128   b  formed in the inner upper portion of legs  123 ,  124 , respectively. Cap  140  further includes a central bore  145  dimensioned to receive the locking element  160 , which in this case is a threaded screw configured to engage inner threads in the bore  145 , shown in  FIG. 32 . In this embodiment, screw  310  is not integral with the base  125  and includes a spherical head  314  dimensioned to sit in the base  125  while the threaded portion of the screw passes through a bore  310  formed in the base  125  of the head  120 . This embodiment provides for multiaxial orientation of the screw  310  relative to the stabilization rod  130 . 
     An inserter tool is used for capturing the screw assembly  300  and placing the screw  310  into a desired vertebral location. Referring to  FIG. 36A  and  FIG. 36B  the inserter tool  400  includes a handle  402  and an elongated cylindrical driver shaft  408  surrounded by a hollow cylindrical retention sleeve  406 . An outer sleeve  404  surrounds the retention sleeve  406 . The elongated driver shaft includes an upper portion  427 , a lower portion  426  and a distal end  413 . The diameter  428  of the upper portion  427  of the diver shaft  408  is larger than the diameter  429  of the lower portion  426  of the driver shaft  408  and a step  439  is formed between the upper  427  and the lower portion  426  of the driver shaft  408 . The distal end  413  of the driver shaft  408  has a tip  410  extending from it. The diameter of the tip  410  is smaller than the diameter of the rest of the driver shaft and is dimensioned and shaped to interface and engage with the opening  161  of the locking element  160  and/or the opening  315  at the top of the screw head  314 . In one example, the tip  410  has a hexagonal shape, as shown in  FIG. 38C . In other examples, tip  410  may be rectangular, conical, or crossed-shaped. The screw-engaging tip  410  engages the screw head  134  or the locking element  160  so that when the driver shaft  408  is rotated clockwise or counter-clockwise, the screw  310  or the locking element  160  is advanced or retracted to or from the desired location, respectively. The retention sleeve  406  includes an upper portion  430  and a lower portion  435 . A central elongated opening extends along direction  412  of the retention sleeve and is dimensioned to house the driver shaft  408 . The diameter  431  of the upper portion  430  of the retention sleeve  406  is larger than the diameter  436  of the lower portion  435  of the retention sleeve  406 . A step  438  is formed in the transition between the upper  430  and lower portion  435  of the retention sleeve  406 . The upper portion  427  of the driver shaft  408  is dimensioned to be accommodated within the upper portion  430  of the sleeve  406  and to be prevented from entering into the lower portion  435  of the sleeve  406 . The driver shaft  408  is configured to move in the direction of arrow  411 , as shown in  FIG. 37B . A spring  409  surrounds the upper portion  427  of the driver shaft  408  and is compressed against the inner surface of step  438  when the driver shaft  408  is pushed down in the direction  411 . The position of the driver shaft  408  within the upper portion  430  of the retention sleeve  406  is locked with a pawl  407  which is secured within the handle  402 . The lower portion  435  of the retention sleeve  406  is slotted and includes two parallel flexible half-cylindrical hollow segments  414   a ,  414   b , as shown in  FIG. 38A . The distal ends  420   a ,  420   b  of segments  414   a ,  414   b  have an inner radius  421  that is smaller than the inner radius  422  of the main portions  423   a ,  423   b  of the segments  414   a ,  414   b , as shown in  FIG. 39A . The transition portions  424   a ,  424   b  between the main portions  423   a ,  423   b  of each segment to the distal ends  420   a ,  420   b  have the shape of a truncated cone, as shown in  FIG. 39A . Segment  414   a  includes a half-annular ridge  417   a  extending around the outer surface of the distal end  420   a  and two elongated protrusions  416   a ,  416   d  extending from the bottom surface of the distal end  420   a . Similarly segment  414   b  includes elongated protrusions  416   b ,  416   c  extending from the bottom surface of its distal end  420   b  and a half-annular ridge  417   b  extending around the outer surface of the distal end  420   b . Distal ends  420   a ,  420   b  are inserted into opening  122  of the tulip-shaped head  120  and half annular ridges  417   a ,  417   b  interface with grooves  127   b ,  128   b  of the tulip-shaped head  120  to engage the driver shaft  408  onto the tulip-shaped head  120 . Protrusions  416   a - 416   d  have curved outer edges  419  configured to interface with the curvature of the inner walls of opening  122 , as shown in  FIG. 39B . The diameter of the driver shaft lower portion is slightly larger than the diameter of the lower portion of the retention sleeve. When the instrument is at rest, pawl  407  is locked and the distal ends  420   a ,  420   b  of segments  414   a ,  414   b  of the retention sleeve  406  are pinched together. In operation the distal ends  420   a ,  420   b  of the inserter  400  are placed into opening  122  of the tulip shaped head  120  of the screw assembly  300 , as shown in  FIG. 37A  and  FIG. 39B . Next, the pawl  407  is released and the driver shaft  408  is pushed down in the direction of arrow  411 . Pushing the driver shaft  408  down causes the distal ends  420   a ,  420   b  of segments  417   a ,  417   b  to expand within the opening  122  of the head  120 , respectively, and inserts the tip  410  of the driver  408  into the opening  315  of the screw head  314 . The expansion of segments  417   a ,  417   b  interlocks annular ridges  417   a - 417   b  within the annular grooves  127   b ,  128   b  of the tulip-shaped head  120  and this locks the head  120  onto the distal ends  420   a ,  420   b  of the inserter tool, as shown in  FIG. 37B  an  FIG. 39C . Once the distal ends  420   a ,  420   b  of the inserter tool is locked onto the tulip-shaped head  120 , the pawl  407  is locked again to prevent accidental disengagement. The reverse procedure is followed in order to remove the inserter tool  400  from the tulip-shaped head  120 . Unlocking the pawl  407  and withdrawing the driver shaft  408  in the opposite direction of  411  brings segments  417   a ,  417   b  together and disengages the distal ends  420   a ,  420   b  from the tulip-shaped head  120 . 
     Other embodiments are within the scope of the following claims. The screw assembly is made of metal, plastic, ceramic, bone, polymers, composites, absorbable material, biodegradable material, or combinations thereof. The insertion toll is made of metal, alloy or composite material. 
     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.