Abstract:
A polyaxial bone anchor includes an anchor head, an anchor member, a bushing, a sleeve and a fastener. The anchor head has a longitudinal bore and an inner spherical surface on a lower portion. The bushing is received within the longitudinal bore adjacent the lower portion and includes an exterior spherical surface for interacting with the inner spherical surface in the anchor head, an internal space defining an inner spherical surface for mating with a spherical head portion of the anchor member and a slot. The inner spherical surface of the bushing has a spherical centerpoint and the exterior spherical surface of the bushing has a spherical centerpoint. The spherical centerpoint of the inner spherical surface is vertically displaced toward the bottom end relative to the spherical centerpoint of the exterior spherical surface. The sleeve is received within the longitudinal bore. The fastener is mountable to the anchor head.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This claims the benefit of U.S. Provisional Application No. 60/739,100, filed Nov. 21, 2005. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to bone fixation devices and related methods of fixation. More particularly, this invention relates to polyaxial bone anchors, such as pedicle screws and hooks, having increased angulation for use in, for example, the posterior fixation of the spine. 
     BACKGROUND OF THE INVENTION 
     Polyaxial bone anchors and methods of use in treating spinal disorders are known. Typical methods involve anchoring at least two screws or hooks into the vertebrae, and fixing the screws or hooks along a spinal rod to position or immobilize the vertebrae with respect to one another. The screws or hooks commonly have anchor heads with U-shaped channels in which the spinal rod is inserted and subsequently clamped by a fastener, such as, for example, a threaded nut, set screw, or locking cap. These methods commonly involve multiple screws or hooks and multiple spinal rods. The spinal rod(s) may be shaped to maintain the vertebrae in a desired orientation so as to correct the spinal disorder at hand (e.g., to straighten a spine having abnormal curvature). Additionally or alternatively, the screws or hooks may be spaced along the rods(s) to compress or distract adjacent vertebrae. 
     Surgeons may encounter difficulty with spinal fixation and stabilization methods because of difficulty aligning the spinal rod(s) with the U-shaped channels in the anchor heads of the screws or hooks. For example, the anchor heads are often out of alignment with one another because of the curvature of the spine or the size and shape of each vertebrae. To facilitate easier insertion of the spinal rods into the U-shaped channels, and to provide additional flexibility in the positioning of the spinal rods and the screws and hooks, bone anchors have been developed where the anchor member (e.g., screw or hook) and anchor head can initially pivot or rotate with respect to each other. These bone anchors are sometimes referred to as polyaxial bone anchors and the pivot or rotation of the anchor member is referred to as angulation. 
     A disadvantage of many polyaxial bone anchors is the degree to which the anchor head and member can angulate. Typical polyaxial bone anchors have anchor members that can rotate up to about 30° from a central axis extending down through the anchor head. It may be advantageous to provide polyaxial bone anchors with increased angulation. 
     SUMMARY OF THE INVENTION 
     The invention is directed to polyaxial bone anchors and methods of use for attaching a rod, such as a support or spinal rod, to a bone, such as a vertebra. The bone anchor may include a hollow generally cylindrical housing or head (referred to hereinafter as an anchor head), an optional hollow generally cylindrical internal sleeve, an internal locking element, a pedicle screw (or other type of anchor member, such as, for example, a hook or other similar structure), and preferably a locking cap with set screw (alternatively, other types of fasteners and fastening arrangements, such as, for example, a threaded nut or locking sleeve mounted on or over the top portion of the head, are also within the scope of the invention). The anchor head and internal sleeve may have a U-shaped channel for receiving a support/spinal rod (referred to hereinafter as a spinal rod or rod). The locking element preferably is sized and shaped to snap on to the head of the pedicle screw. And the locking cap and set screw may close the top opening of the U-shaped channel after a rod has been placed therein and, in combination with the locking element, lock or clamp the respective positions of the pedicle screw and rod. 
     The anchor head, the internal sleeve, and primarily the locking element have features that allow the locking element to rotate or pivot within the anchor head. This in turn allows the pedicle screw to rotate or pivot around and away from the central axis of the anchor head at large angles. The pedicle screw or hook may be locked with respect to the anchor head at these large angles. The angulation is preferably as much as about 50° in every direction from the central axis. This advantageously provides greater flexibility to the surgeon when aligning spinal rods with the anchor heads of implanted screws and hooks during surgery. 
     In one embodiment of the invention, the locking element, which can be described as a collet or collet-style bushing, has an upper portion with a plurality of resilient tabs to initially receive and hold the head of a pedicle screw. The internal sleeve has a bottom surface with a preferably corresponding inward taper to mate with the tapered shape of the exterior surface of the tabs on the collet to allow rotation and facilitate locking of the collet. The collet has at least one cutout of preferably about 50° on its lower side and the anchor head has a lower portion with a tapered inner surface that together make possible the large angulation of the pedicle screw mounted in the collet. The anchor head preferably also has an internal ledge for receiving a corresponding lip or projection on the collet to seat it within the head and allow it to rotate about the longitudinal axis of the bore of the anchor head so the cutout can be aligned in a desired direction for full angulation of the pedicle screw. The collet may have one or more cutouts and preferably has multiple cutouts. When the bone anchor is ready to be locked, the bottom interior surface of the internal sleeve presses down on the outside of the tabs of the collet so that the collet compresses around the screw head to lock the position of the screw. 
     In another embodiment of the invention, the locking element, which may be described as a spherical bushing, can rotate or swivel within the anchor head prior to locking. The anchor head has a lower portion with a spherically-cut inner surface that facilitates rotation of the spherical bushing about a point within the anchor head. The spherical bushing has a spherical exterior shape, a spherical interior shape, and preferably at least one slot that permits the bushing to compress the head of a pedicle screw or hook inserted into the interior of the spherical bushing. Preferably, the pedicle screw or hook has an arcuate or spherical upper portion (head) whose shape corresponds to the interior shape of the spherical bushing. The internal sleeve has a bottom interior surface with a spherical shape to mate with the exterior spherical shape of the top portion of the spherical bushing. The interior surface of the spherical bushing has a centerpoint that is preferably offset from the centerpoint of the exterior surface of the spherical bushing and hence the pedicle screw mounted within it. This offset provides additional angulation as follows: The pedicle screw angulates a certain amount before its shank engages an edge of the spherical bushing. The spherical bushing can then rotate with the pedicle screw to provide the additional amount of angulation, the sum of which provides the increased angulation. When the bone anchor is ready to be locked, the internal sleeve is pressed down on the spherical bushing&#39;s top surface, so that the bushing compresses around the screw head to lock the position of the screw. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description will be better understood in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIGS. 1-3  are perspective, side cross-sectional, and front cross-sectional views, respectively, of a first embodiment of a polyaxial bone anchor; 
         FIG. 4  is an exploded view of the polyaxial bone anchor of  FIGS. 1-3 ; 
         FIGS. 5A  and B are elevational views of the locking element of the polyaxial bone anchor of  FIGS. 1-4 ; 
         FIG. 6  is a perspective view of the locking element of  FIGS. 1-4  with a pedicle screw mounted therein; 
         FIG. 7  is a side cross-sectional view of the anchor head of the polyaxial bone anchor of  FIGS. 1-4 ; 
         FIG. 8  is a perspective view showing the bottom of the internal sleeve of the polyaxial bone anchor of  FIGS. 1-4 ; 
         FIGS. 9-11  are perspective, side cross-sectional, and front cross-sectional views, respectively, of a second embodiment of a polyaxial bone anchor; 
         FIG. 12  is an exploded view of the polyaxial bone anchor of  FIGS. 9-11 ; 
         FIG. 13  is a cross-sectional view of the locking element of the polyaxial bone anchor of  FIGS. 9-12 ; 
         FIGS. 14 and 15  are perspective views of two embodiments, respectively, of the locking element of the polyaxial bone anchor of  FIGS. 9-12 ; 
         FIG. 16  is a side cross-sectional view of the anchor head of the polyaxial bone anchor of  FIGS. 9-12 ; and 
         FIG. 17  is a perspective view showing the bottom of the internal sleeve of the polyaxial bone anchor of  FIGS. 9-12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention can be used to treat various spinal disorders including, for example, degenerative instabilities and instabilities due to decompression, tumors, infections, and fractures. 
     Note that while the polyaxial bone anchor is described and illustrated herein with reference to certain preferred or exemplary embodiments, the invention should not be limited to those preferred or exemplary embodiments. Furthermore, the features described and illustrated herein can be used singularly or in combination with other features and embodiments. 
       FIGS. 1-3  show a first embodiment of a polyaxial bone anchor. Polyaxial bone anchor  100  includes a fastener  102 , an anchor head  104 , and an anchor member  106 . Fastener  102  is a locking cap that includes a locking ring  112  and a set screw  122  and may be similar or identical to that described in International Patent Application PCT/US2006/015692, internationally filed Apr. 25, 2006, which is incorporated herein by reference in its entirety. Alternatively, fastener  102  may be any known fastener, and anchor head  104  may have any corresponding features required to permit attachment and operation of fastener  102  (e.g., threaded upper arms). Anchor head  104  is preferably cylindrically hollow having a generally longitudinal bore  1014  along longitudinal axis  109 . Anchor head  104  also has a generally U-shaped opening  103  transverse to longitudinal bore  1014  for receiving a spinal rod  108  or other similar part. Longitudinal bore  1014  has a top opening  194  and a bottom opening  184 . Anchor member  106 , which may be a bone or pedicle screw, hook, or other similar structure (and will be referred to hereinafter as pedicle screw  106 ), extends out of bottom opening  184 . Anchor member  106  may be coupled to anchor head  104  such that the head and screw can polyaxially rotate with respect to each other when in an unlocked position, but the angle of the longitudinal axis of anchor member  106  may be fixed with respect to the longitudinal axis of anchor head  104  in a locked position. 
     One or more polyaxial bone anchors  100  may be attached, for example, to the vertebrae via respective anchor members  106 , and a spinal rod  108  or other similar part can be inserted into the U-shaped openings  103 . The spinal rod may thereafter be locked with respect to anchor head  104 . A system of bone anchors and rods could be used to correctly align the spine or treat other spinal disorders. 
     Representative dimensions of bone anchor  100  include an anchor head height  114  of about 11.5 mm, a width  124  of about 9.5 mm, and a length  134  of about 8.2 mm. Pedicle screw  106  has a shank diameter  116  of about 4 mm, a neck diameter  126  of about 2.75 mm, and head diameter  136  of about 5.4 mm. Alternatively, bone anchor  100  may be of other dimensions. 
     Advantageously, pedicle screw  106  can angularly rotate (before being locked or clamped in place) about central axis  109  by an angle θ of preferably about 50° in any direction (i.e., the angular rotation of the head of anchor member  106  in the anchor head forms a cone of preferably about 100°). 
       FIG. 4  shows an exploded view of the assembly of bone anchor  100 , which includes locking ring  112  and set screw  122  of locking cap  102 , a hollow generally cylindrically shaped internal sleeve  405 , a spinal rod  108 , an internal locking element  407 , a pedicle screw  106  (shown mounted in locking element  407 ), and an anchor head  104 . Bone anchor  100  is first assembled by snap-fitting locking element  407  over the head of pedicle screw  106 . 
     As shown in  FIGS. 5A  and B, locking element  407  may be described as a collet or collet-styled bushing (referred to hereinafter as collet  407 ). Collet  407  is made of a resilient material that can be compressed around the head of pedicle screw  106  to retain pedicle screw  106  securely in place. Preferably the material of the collet is softer than the material of internal sleeve  405  and pedicle screw  106 . 
     Preferably, the upper portion  487  of collet  407  provides the collet with most, if not all, of its screw head retention capabilities. Upper portion  487  has a plurality of resilient tabs  427 . The exterior surface of tabs  427  preferably are tapered inward at an angle Φ of preferably about 30°, although other angles Φ are alternatively possible and contemplated. Tabs  427  can deflect outward to allow the head  186  of pedicle screw  106  to be inserted within the internal space of the collet, as shown in  FIG. 6 . The internal space  477  of collet  407  is shaped to substantially match the shape of the pedicle screw head such that the collet has to be pressed over the screw head in a friction fit. Preferably, the head  186  of the pedicle screw and the internal space of the collet have at least a portion which is spherically shaped. Tabs  427  are separated by slots  437 , which may also have a radius or circular shaped portion  447  as a stress relief and/or to provide better resiliency to tabs  427 . The arrangement, shapes, and dimensions of the tabs/slots optionally may be different than shown. 
     The collet/screw assembly is then inserted screw-shank first through the top opening  194  in anchor head  104  until circumferential lip  417  of collet  407  is seated against circumferential internal ledge  144  of anchor head  104  (see  FIGS. 2 ,  3 , and  7 ). Screw shank  146  of pedicle screw  106  now protrudes through the bottom opening  184  of anchor head  104  as shown in  FIGS. 1-3 . The diameter of collet  407  at lip  417  preferably is such that it can pass through the top opening  194  of anchor head  104 , but cannot pass through the bottom opening  184  and more particularly internal ledge  144  of anchor head  104 . 
     The lower portion  497  of collet  407  has one or more cutouts  457  of angle α, which is measured from the bottom of collet  407  at central axis  509  (which coincides with central axis  109  when collet  407  is seated in anchor head  104 ) to the top of the cutout, as shown in  FIG. 5A . Preferably angle α is about 50° (other angles α are alternatively possible). The embodiment of collet  407  shown in  FIG. 5A  has three 50° cutouts  457 . Embodiments with two, four, or more cutouts are possible. Cutouts  457  make possible the larger angulation between the anchor head and pedicle screw where cutouts are located. Collet  407  may rotate or swivel about axis  109  within anchor head  104 , prior to the locking of the pedicle screw, to position a cutout in a preferred direction in which to provide full (i.e., maximum) angulation. For example, if a cutout is not aligned as desired, pedicle screw  106  will press against a prong  467  as a surgeon angulates anchor head  104  in a desired direction. This pressing preferably causes collet  407  to rotate within anchor head  104  until cutout  457  is aligned in the desired direction. 
     The number of cutouts represents a tradeoff between versatility and screw retention capability. That is, a collet with more cutouts has more positions at which to provide full angulation and is thus more easily aligned (i.e., such a collet does not need to be rotated as much to be aligned as a collet with fewer cutouts). However, retention capability (e.g., friction and gripping strength) is in part a function of the amount of surface area in contact with the pedicle screw head. If more area is in contact with the screw head (e.g., because the collet has fewer cutouts), more friction to provisionally hold the pedicle screw in place before locking and more gripping strength to lock the screw in place is available. If more cutouts are provided, less surface area may contact the pedicle screw head, and less friction and gripping strength may be provided. 
     Also facilitating the angulation of pedicle screw  106  is a preferably tapered lower portion inner surface  1004  of anchor head  104 , as shown in  FIG. 7 . Pedicle screw  106  will angulate until the neck  156  of the screw butts against inner surface  1004 , as best shown in  FIG. 2 . 
     Internal sleeve  405 , which may be optional in some embodiments, is next inserted downward into anchor head  104 . Internal sleeve  405  preferably provides a U-shaped channel  455  transverse to a longitudinal bore in sleeve  405 . Internal sleeve  405  preferably has a pair of retention tabs  415   a,b  on its outer surface that snap into respective slots  154   a,b  on opposite walls of anchor head  104  (best seen in  FIG. 2 ). This aligns the U-shaped channels of anchor head  104  and sleeve  405 . Slots  154   a,b  of anchor head  104  allow sleeve  405  to move up and down from an unlocked screw position to a locked screw position, respectively, on top of collet  407 , while retaining the sleeve within the anchor head. Tabs  415   a,b  may also keep the U-shaped channel in sleeve  405  aligned with the U-shaped opening in the anchor head. Alternatively, other means of keeping U-shaped channel  455  in sleeve  405  aligned with U-shaped opening  103  in anchor head  104  may used, such as, for example, protruding tabs along the boundary of U-shaped channel  455  that project or snap into space provided by the U-shaped opening in anchor head  104 . 
     With fastener  102  removed from the assembly of the anchor head, internal sleeve, collet, and pedicle screw, the pedicle screw may be attached to a bone. The head of pedicle screw  106  preferably has a recess  166  (as shown in  FIG. 3 ) or slot  166  (as shown in  FIG. 4 ) keyed to receive a hex wrench, torque wrench, or other known driver (through the aforementioned assembly) to implant the pedicle screw by rotating into, for example, a vertebra. 
     Anchor head  104  may now be aligned to receive a rod  108 . Rod  108  is preferably snapped into internal sleeve  405 . The distance between upright arms  425   a,b  of sleeve  405  across the narrowest widths  435  of the U-shaped channel is preferably slightly less than the diameter of rod  108 . For example, if rod  108  has a diameter of about 3.5 mm, the aforementioned distance would preferably be about 3.26 mm. In this manner, the sleeve may provisionally retain the spinal rod but still permit the rod to slide in the U-shaped channel or be removed. Alternatively or additionally, sleeve  405 , with or without the spinal rod, can be pushed down in the anchor head (e.g., by pushing down on the spinal rod in the U-shaped channel) so that the under surface of sleeve  405  interacts with tabs  427  on collet  407  to provisionally lock the pedicle screw with respect to the anchor head. In this manner, the spinal rod is still permitted to slide within and/or be removed from the sleeve. 
     With the spinal rod in the U-shaped channel (with or without the head of the screw or hook being locked in the anchor head), the locking cap  102  may be placed on anchor head  104 , closing the U-shaped channel. In this embodiment, locking cap  102  is first positioned on top of anchor head  104  and pressed downward until it snaps into position. The locking cap is then rotated until oppositely-positioned projections  132   a,b  on locking ring  112  contact corresponding structures  164   a,b , respectively, on anchor head  104 . As this occurs, a pair of oppositely-positioned, preferably dovetailed, lateral flanges  142   a,b  on locking ring  112  slide within corresponding, preferably dovetailed, grooves  174   a,b , respectively, on anchor head  104 . Preferably, locking ring  112  and the upper surfaces of anchor head  104  do not engage each other with screw threads, although screw threads may be used, as well as different locking caps. 
     At this stage, rod  108  can still be positioned (e.g., moved) relative to anchor head  104  and pedicle screw  106 . Upon satisfactory positioning of the rod and pedicle screw, set screw  122  is driven downward to lock the rod and anchor head in place. Set screw  122  has external threads  152  that mate with internal threads  162  of locking ring  112 . Preferably, the set screw is screwed into the locking ring before the locking cap is inserted into the anchor head, and preferably the set screw cannot be screwed out of the locking ring because of a flared portion  172  at the bottom of the set screw. Set screw  122  preferably also has a star socket  182 . Alternatively, set screw  122  can have other types of sockets or recesses keyed to other known drivers or tools. A single instrument/tool may be used with locking cap  102  to drive in a single action both locking ring  112  and set screw  122  simultaneously to lock locking cap  102  in place on the anchor head and then to continue driving set screw  122  alone until rod  108  and pedicle screw  106  are clamped in place. 
     As set screw  122  contacts rod  108 , rod  108  pushes down on internal sleeve  405 . As the downward rotation of set screw  122  continues, if internal sleeve  405  is in the upper position in anchor head  104 , internal sleeve  405  moves downward within anchor head  104  compressing and ultimately crush-locking collet  407  around the head of pedicle screw  106 , locking pedicle screw  106  with respect to anchor head  104 . As shown in  FIG. 8 , internal sleeve has a bottom interior surface  455  preferably tapered inward by preferably about 30° so as to mate with the tapered tabs  427  of collet  407 . Set screw  122  may be driven downward until (1) retention tabs  415   a,b  of sleeve  405  contact the bottom of slots  154   a,b  on anchor head  104 , (2) the bottom edge  445  of sleeve  405  and lip  417  of collet  407  are clamped against internal ledge  144  of anchor head  104 , and/or (3) tabs  427  are compressed against the head of the pedicle screw such that the sleeve can no longer travel down the bore of the anchor head. The set screw will push the spinal rod into the bottom of the U-shaped channel in sleeve  405  in order to move the sleeve down the bore of the anchor head. Once the sleeve can no longer move the sleeve down the bore of the anchor head, the set screw will apply pressure to the spinal rod so that it becomes locked in a final position in the sleeve (and in anchor head  104 ) so that the rod cannot slide and/or be removed from the anchor head. 
     Alternatively, other fasteners or caps may be used. 
     Collet  407  may be advantageously used with other types of anchor heads, internal sleeves, fasteners, and pedicle screws than those shown herein. For example, collet  407  may be used with similar corresponding bone anchor elements disclosed in the previously cited U.S. Provisional Patent Application No. 60/674,877, filed Apr. 25, 2005, incorporated herein by reference in its entirety. 
       FIGS. 9-11  show a second embodiment of a polyaxial bone anchor. Polyaxial bone anchor  900  includes fastener  102 , an anchor head  904 , and an anchor member  106 . Anchor head  904  is substantially similar to anchor head  104  and is cylindrically hollow having a longitudinal bore  9014 , top opening  9194 , bottom opening  9184 , and a generally U-shaped opening  903  transverse to the longitudinal bore for receiving spinal rod  108  or other similar part. Unlike anchor head  104 , however, side lower portion  994  of anchor head  904  may have an inward taper. In one embodiment, side lower portion  994  may taper inward by about 0.65 mm on each side such that, for example, an upper width  924  of about 9.5 mm results in a lower width  9004  of about 8.2 mm. Bone anchor  100  may have the taper illustrated in this second embodiment and alternatively, bone anchor  900  may not have a taper as illustrated. Other representative dimensions of bone anchor  900  may be identical to those of bone anchor  100 , and bone anchor  900  alternatively may be of other dimensions. 
     As with bone anchor  100 , anchor member  106  (which will again be referred to hereinafter as pedicle screw  106 ) may be associated with or coupled to anchor head  904  such that the head and screw can polyaxially rotate with respect to each other. In particular, pedicle screw  106  can advantageously polyaxially rotate (before being locking or clamped in place) about central axis  909  of anchor head  904  by an angle θ of preferably about 50° in any direction (i.e., the angular rotation of the head of pedicle screw  106  in anchor head  904  forms a cone of preferably about 100°). 
       FIG. 12  shows an exploded view of the assembly of bone anchor  900 , which includes locking ring  112  and set screw  122  of locking cap  102 , a hollow internal sleeve  905 , spinal rod  108 , an internal locking element  907 , pedicle screw  106  (shown mounted in locking element  907 ), and anchor head  904 . The assembly of bone anchor  900  is substantially, if not completely, identical to bone anchor  100 . Locking element  907  is first snap-fitted onto the head  186  of pedicle screw  106 . The locking element/screw assembly is then inserted screw-shank first through the top opening  9194  of anchor head  904  until the lower exterior surface  977  of locking element  907  rests against corresponding spherical inner surface  984  on the lower portion of anchor head  904 . This causes screw shank  146  of pedicle screw  106  to protrude through the bottom opening  9184  of anchor head  904 . Internal sleeve  905  may be inserted through top opening  9194  so that the sleeve is retained in anchor head  904 . 
     As shown in  FIGS. 12-15 , locking element  907  may be described as a spherical bushing (referred to hereinafter as bushing  907 ). Bushing  907  is made of a resilient material that can be compressed around the head of pedicle screw  106  to retain pedicle screw  106  securely in place. Preferably the material of the bushing is softer than the material of internal sleeve  905  and pedicle screw  106 . Internal space  917  of bushing  907  is shaped to substantially match the shape of the pedicle screw head such that the bushing has to be pressed over the screw head. Preferably, internal space  917  of the bushing has an arcuate or spherical shape to correspond to the preferably spherical or arcuate shape of the head of the pedicle screw. The exterior surface  987  of bushing  907  preferably has an arcuate or spherical shape. The inner surface  984  of lower portion  994  of anchor head  904  preferably has a corresponding arcuate or spherical shape so that bushing  907  can rotate or swivel in anchor head  904  about a point inside the anchor head and/or within the bore of the spherical bushing. 
     Bushing  907  has a slot  927  to provide resiliency. Slot  927  may extend completely through from the exterior the side of bushing  907  to the interior side of the bushing and from the top end of the bushing to the bottom end of the bushing as shown in  FIG. 14 . Another embodiment of a spherical bushing according to the invention is shown in  FIG. 15 . Bushing  1507  has additional slots  937  that do not extend completely from the top end of the bushing to the bottom end of bushing  1507 . Slots  937  may also have a radius or circular shaped portion  947  as a stress relief and/or to provide better resiliency. The arrangement, shapes, and dimensions of the slots of bushings  907 / 1507  alternatively may be different than shown. For example, although slots  937  are shown as extending from the lower or bottom end of bushing  1507 , some or all of slots  937  alternatively can extend from the top end of bushing  1507 . 
     Bushing  907  (and bushing  1507 , referred to collectively hereinafter as bushing  907 ) can rotate or swivel about a point in the interior of the anchor head and/or bushing within anchor head  904  prior to locking. Inner surface  984  of anchor head  904  facilitates the rotation of bushing  907 . As shown in  FIG. 13 , the inner surface of bushing  907  has a spherical centerpoint  957  that is preferably offset from the spherical centerpoint  196  of the exterior surface of bushing  907 . This offset  967  is preferably about 0.6 mm (alternatively, offset  967  can be of other dimensions). In use, the pedicle screw may first angulate a certain amount until its shank  146  engages a lower edge  977  of bushing  907 . At that point, bushing  907  can rotate with the pedicle screw to provide an additional amount of angulation, the sum of which provides the total angulation of the screw within the anchor head. The angulation of the screw within the bushing is preferably up to about 20° to about 30° of movement and the angulation of the bushing within the anchor head is preferably up to about 20° to about 30° of movement. 
     Alternatively, the centerpoints of bushing  907  and the pedicle screw head can be the same, which may maximize the retention capability of bushing  907  with respect to the screw head. 
     As with bone anchor  100 , internal sleeve  905 , which may be optional, is next inserted downward into anchor head  904 . Internal sleeve  905  is positioned on top of bushing  907  and its insertion into and movement within anchor head  904  is substantially identical to that of internal sleeve  405  and anchor head  104 . That is, internal sleeve  905  has a pair of retention tabs  915   a,b  on its outer surface that snap into respective slots  954   a,b  on opposite walls of anchor head  904 . This insertion aligns the U-shaped channels of anchor head  904  and sleeve  905 . Slots  954   a,b  of anchor head  904  allow sleeve  905  to move up and down from an unlocked screw position to a locked screw position, respectively. The sleeve may have the provisional locking features as described for bone anchor  100 . Alternatively, other means of keeping U-shaped channel  955  of sleeve  905  aligned with U-shaped opening  903  in anchor head  904  may used, such as, for example, protruding tabs along the boundary of U-shaped channel  955  that project or snap into space provided by the U-shaped opening in anchor head  904 . 
     With fastener  102  removed, the pedicle screw may be implanted in a bone, such as, for example, a vertebra, and anchor head  904  can thereafter be aligned to receive a spinal rod  108 , which is snapped into internal sleeve  905  in a substantially, if not completely, identical manner as the corresponding parts of bone anchor  100 . 
     Locking cap  102  is next placed on anchor head  904  and tightened to lock the rod and screw positions in a manner identical to that described above with respect to bone anchor  100 . In particular, as set screw  122  contacts rod  108 , rod  108  pushes down on internal sleeve  905 . This causes internal sleeve  905  to move downward, compression locking bushing  907  against the head of pedicle screw  106 . As shown in  FIG. 17 , internal sleeve  905  has a bottom surface  975  with a preferably corresponding arcuate or spherical shape that mates with the top portion of bushing  907 . As with bone anchor  100 , set screw  122  may be driven downward until retention tabs  915   a,b  of sleeve  905  contact the bottom of slots  954   a,b  on anchor head  904 , until the bottom edge  945  of sleeve  905  and bushing  907  are clamped against surface  984  of anchor head  904 , or until sleeve  905  contacts bushing  907  so that further movement of sleeve  905  is not possible. Alternatively, other fasteners or caps may be used. 
     As with collet  407 , bushing  907  may be advantageously used with other types of anchor heads, internal sleeves, fasteners, and pedicle screws than those shown herein. Bushing  907  may be used with similar corresponding bone anchor elements disclosed in the previously cited International Patent Application PCT/US2006/015692, internationally filed Apr. 25, 2006, incorporated herein by reference in its entirety. Bushing  907  provides an additional degree of freedom as compared to collet  407 . Bushing  907  not only will rotate about the longitudinal axis extending through the bore of the anchor head, but will rotate about an axis extending transverse to the longitudinal axis. 
     The present invention has been described in connection with the preferred embodiments. These embodiments, however, are merely for example and the invention is not restricted thereto. It will be understood by those skilled in the art that other variations and modifications can easily be made within the scope of the invention as defined by the appended claims, thus it is only intended that the present invention be limited by the following claims.