Abstract:
A method of securing a spinal rod to a spine includes providing a head portion having a channel extending therethrough for receiving a spinal rod, the channel and the head portion being bounded by a first side wall and a second side wall. The method includes providing a bone fastener depending from the head portion, arranging the spinal rod in the channel of the head portion, and providing a locking cap having first and second portions that are rotatable relative to one another, the second portion of the locking cap having an underside with a recess. The method includes assembling the locking cap with the head portion so that the first portion is between the first and second side walls and so that the recess of the second portion of the locking cap is in contact with the spinal rod. While maintaining the recess of the second portion of the locking cap in contact with the spinal rod, the first portion of the locking cap is rotated, such rotation translating into a locking force applied by the second portion of the locking cap onto the spinal rod.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject application is a continuation of U.S. application Ser. No. 10/365,182, filed Feb. 12, 2003, now U.S. Pat. No. 7,608,095, which is a continuation of U.S. application Ser. No. 09/487,942, filed Jan. 19, 2000, now U.S. Pat. No. 6,565,565, which is a continuation-in-part of both U.S. application Ser. No. 09/167,439, filed Oct. 6, 1998, now abandoned, and U.S. application Ser. No. 09/098,927, filed Jun. 17, 1998, now U.S. Pat. No. 6,090,111, the disclosures of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject disclosure relates to implantable spinal stabilization systems for surgical treatment of spinal disorders, and more particularly, to a device for connecting cylindrical spinal rods of a spinal stabilization system to the spine. 
     2. Background of the Related Art 
     The spinal column is a complex system of bones and connective tissue that protects critical elements of the nervous system. Despite these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. Trauma or developmental irregularities can result in spinal pathologies that limit this range of motion. 
     For many years, orthopedic surgeons have attempted to correct spinal irregularities and restore stability to traumatized areas of the spine through immobilization. Over the past ten years, spinal implant systems have been developed to achieve immobilization. Examples of such systems are disclosed in U.S. Pat. Nos. 5,102,412 and 5,181,917 to Rogozinski. Such systems often include spinal instrumentation having connective structures such as elongated rods, which are placed on opposite sides of the portion of the spinal column intended to be immobilized. Screws and hooks are commonly utilized to facilitate segmental attachment of such connective structures to the posterior surfaces of the spinal laminae, through the pedicles, and into the vertebral bodies. These components provide the necessary stability both in tension and compression to achieve immobilization. 
     Various fastening mechanisms have been provided in the prior art to facilitate securement of screws and hooks to the connective structures of a spinal stabilization system. For example, U.S. Pat. No. 5,257,993 to Asher discloses an apparatus for use in retaining a spinal hook on an elongated spinal rod. The apparatus includes a body extending upwardly from a hook portion and having an open-ended recess for receiving a spinal rod and an end cap engageable with the body to close the recess. A set screw is disposed in the center of the end cap to clamp the rod in the recess of the body. The end cap and body are interconnectable by different types of connectors including a bayonet connector, a linear cam connector or a threaded connector. Other examples of fastening mechanism for facilitating attachment of screws and hooks to the connective structures of a spinal stabilization system are disclosed in U.S. Pat. No. 5,437,669 to Yuan et al. and U.S. Pat. No. 5,437,670 to Sherman et al. 
     In each of these prior art examples, threaded fasteners are used to facilitate securement of the connector to the spinal rod. Yet it is well known that threaded fasteners can become loosened under the influence of cyclically applied loads commonly encountered by the spinal column. Furthermore, during assembly, excessive torque applied to a threaded fastener can cause damage to the fastener as well as to the connective device with which it is associated. 
     It would be beneficial to provide a more reliable and effective mechanism for facilitating the attachment of screws, hooks and clamps to the connective structures of a spinal stabilization system. 
     SUMMARY OF THE DISCLOSURE 
     The subject disclosure is directed to a device for securing a spinal rod to a fixation device such as a pedicle screw or a lamina hook. The device disclosed herein includes a head portion configured to receive a spinal rod, a locking cap configured to engage the head portion and the spinal rod upon rotation of the locking cap relative to the head portion to secure the position of the head portion relative to the spinal rod, and a fastener portion extending from the head portion and configured to engage the spine. The fastener portion of the device can be in the form of a screw, hook or clamp, or any other configuration known in the art. 
     The head portion of the device has a channel extending therethrough for receiving a spinal rod and the channel is preferably bounded by opposed side walls each having an arcuate engagement slot defined therein. The locking cap preferably has opposed arcuate engagement flanges configured for reception in the opposed arcuate engagement slots of the head portion upon rotation of the locking cap relative to the head portion. Preferably, the opposed engagement slots are each defined in part by inclined slot surfaces, with the angle of the inclined surface of one engagement slot being opposite that of the opposed engagement slot. Similarly, the opposed engagement flanges are preferably each defined in part by inclined flange surfaces, with the angle of the inclined surface of one engagement flange being opposite that of the opposed engagement flange. The head portion also preferably includes structure for interacting with the locking cap to prevent the opposed side walls of the head portion from expanding radially outwardly when the arcuate flanges are engaged in the arcuate slots. 
     Preferably, the locking cap of the device is configured for rotation between an initial position in which the arcuate engagement flanges are 90° out of phase with the arcuate engagement slots, an intermediate position in which the arcuate engagement flanges are 45° out of phase with the arcuate engagement slots, and a locked position in which the arcuate engagement flanges are in phase and intimately engaged with the arcuate engagement slots. 
     In this regard, the bottom surface of the locking cap preferably includes a first recess oriented to accommodate a spinal rod when the locking cap is in an initial unlocked position, a second recesses which intersects the first recess at a first angle to accommodate a spinal rod when the locking cap is in an intermediate position, and a third recess which intersects the elongate recess at a second angle to accommodate a spinal rod when the locking cap is in a final locked position. In accordance with a preferred embodiment of the subject disclosure, the first recess is an elongate recess, the second recess is a transverse recess, which intersects the elongate recess at a 45° angle, and the third recess is an orthogonal recess, which intersects the elongate recess at a 90° angle. 
     The subject disclosure is also directed to a device for securing a spinal rod to the spine which comprises a head portion having a channel extending therethrough configured to receive a spinal rod, a locking cap including a first portion configured to engage an interior surface of the head portion and a second portion configured to engage an exterior surface of a spinal rod received by the channel to secure the position of the head portion relative to the spinal rod, and a fastener portion depending from the head portion and configured to engage the spine. 
     Preferably, the locking cap is a two-piece structure which includes an upper portion configured to engage an interior surface of the head portion and a lower portion configured to engage an exterior surface of the spinal rod to secure the position of the head portion relative to the spinal rod upon rotation of the upper portion relative to the lower portion and the head portion. The upper portion of the locking cap includes a bottom surface having an axial reception bore defined therein and the lower portion of the locking cap includes an upper surface having an axial post extending therefrom configured to engage the axial reception bore in the bottom surface of the upper portion of the locking cap and facilitate the relative rotation of the two parts. The upper portion further includes opposed arcuate engagement flanges configured for cammed engagement in correspondingly configured opposed arcuate engagement slots formed in the opposed side walls of the head portion upon rotation of the upper portion relative to the lower portion. The lower portion further includes a bottom surface having an elongated hemi-cylindrical recess that is oriented to accommodate a spinal rod extending through the channel in the head portion. 
     In accordance with one aspect of the subject disclosure, the fastener portion is formed monolithic with the head portion. In accordance with another aspect of the subject disclosure, the fastener portion is mounted for movement relative to the head portion. In this regard, the head portion defines a central axis oriented perpendicular to the spinal rod channel and the fastener portion is mounted for angular movement relative to the central axis of the head portion. More particularly, the fastener portion includes a generally spherical head and a threaded body, which depends from the spherical head, and the head portion defines a seat to accommodate the spherical head and an aperture to accommodate the threaded body. In use, upon rotation of the upper portion of the locking cap relative to the lower portion of the locking cap into a locked position, the position of the head portion relative to the spinal rod and the position of the fastener relative to the head portion become fixed. 
     These and other unique features of the device disclosed herein and the method of installing the same will become more readily apparent from the following description of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those having ordinary skill in the art to which the disclosed apparatus appertains will more readily understand how to construct and use the same, reference may be had to the drawings wherein: 
         FIG. 1  is a perspective view of an elongated spinal rod of a spinal stabilization system having attached thereto a bone screw and a bone hook constructed in accordance with a first embodiment of the subject disclosure; 
         FIG. 2  is a perspective view of a locking cap which forms part of the bone screw and bone hook illustrated in  FIG. 1 , oriented in an inverted position for ease of illustration; 
         FIG. 3  is a perspective view of the bone screw and locking cap of  FIG. 1  separated from one another for ease of illustration; 
         FIG. 4  is a cross-sectional view of the bone screw of the subject disclosure taken along line  4 - 4  of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of the locking cap taken along line  5 - 5  of  FIG. 3 ; 
         FIGS. 6A through 6D  illustrate operative steps associated with attaching the bone fastener of the subject disclosure to a spinal rod wherein: 
         FIG. 6A  illustrates the step of positioning the spinal rod and locking cap in the reception channel of the head portion of a fastening device of the subject disclosure; 
         FIG. 6B  illustrates the initial orientation of the locking cap relative to the head portion of a fastening device of the subject disclosure wherein the locking cap is in an unlocked position; 
         FIG. 6C  illustrates the rotation of the locking cap relative to the head portion of a fastening device of the subject disclosure to a partially locked position; and 
         FIG. 6D  illustrates the rotation of the locking cap relative to the head portion of a fastening device of the subject disclosure to a locked position; 
         FIG. 7  is a perspective view of a fastening device constructed in accordance with a second embodiment of the subject disclosure; 
         FIG. 8  is a perspective view of the fastening device of  FIG. 7  with the locking cap separated for ease of illustration; 
         FIG. 9  is a perspective view of the locking cap of the fastener device of  FIG. 7 , oriented in an inverted position for ease of illustration; 
         FIG. 10  is a cross-sectional view of the fastening device of  FIG. 7  taken along line  10 - 10  of  FIG. 7 ; 
         FIG. 11  is a perspective view of an elongated spinal rod of a spinal stabilization system having attached thereto another version of a bone screw and another version of a bone hook constructed in accordance with another embodiment of the subject disclosure; 
         FIG. 12A  is an exploded perspective view of the bone screw of  FIG. 11  with parts separated for ease of illustration including the two-piece locking cap and multi-axial fastener portion associated therewith; 
         FIG. 12B  is a perspective view, looking upward from below, of the two-piece locking cap of the subject disclosure illustrating the bottom surface features of the component parts thereof: 
         FIG. 13  is a cross-sectional view of the bone screw of  FIG. 11  taken along line  13 - 13  of  FIG. 11  with the two-piece locking cap in a locked position; 
         FIGS. 14A through 14C  illustrate, in counter-clockwise progression, the operative steps associated with attaching the bone screw of  FIG. 11  to a spinal rod by employing the two-piece locking cap of the subject disclosure, wherein: 
         FIG. 14A  illustrates the step of positioning the locking cap within the head portion of the bone screw; 
         FIG. 14B  illustrates the initial unlocked orientation of the upper portion of the locking cap within the head portion of the bone screw; and 
         FIG. 14C  illustrates the step of rotating the upper portion of the locking cap relative to the lower portion of the locking cap and the head portion of the bone screw into a locked position to secure the position of the bone screw with respect to the spinal rod; 
         FIG. 15  is an exploded perspective view of the bone hook of  FIG. 11  with parts separated for ease of illustration including the two-piece locking cap associated therewith; and 
         FIG. 16  is a cross-sectional view of the bone hook of  FIG. 11  taken along line  16 - 16  of  FIG. 11  with the two-piece locking cap in a locked positions. 
     
    
    
     These and other features of the apparatus disclosed herein will become more readily apparent to those having ordinary skill in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein like reference numerals identify similar structural elements of the subject apparatus, there is illustrated in  FIG. 1  a section of a spinal stabilization system constructed in accordance with a preferred embodiment of the subject disclosure and designated generally by reference numeral  10 . 
     Referring to  FIG. 1 , spinal stabilization system  10  includes an elongated spinal rod  12  having a circular cross-section and a substantially smooth outer surface finish. As illustrated, fastening devices in the form of a bone screw  14  and right-angle hook  16  are provided for securing spinal rod  12  to the spine during a spinal stabilization procedure. Both fastening devices employ a novel top-loaded locking cap, designated generally by reference numeral  20 , which will be described in greater detail hereinbelow with reference to  FIG. 2 . The novel locking cap achieves significant clinical advantages over the prior art through its reliability and the ease in which it is installed during a spinal stabilization procedure. 
     It should be recognized that the subject disclosure is not limited in any way to the illustrated bone screw and right-angle hook. Rather, these particular fasteners are merely examples of the type of devices that can employ the novel locking cap disclosed herein. Other fasteners commonly utilized in spinal stabilization systems, such as, for example, hooks having alternative angular geometries as well as clamps are also envisioned. Indeed, it is envisioned that any component designed for attachment to an elongated spinal rod or transverse coupling rod, may incorporate the novel locking cap of the subject disclosure. Also, any number of fastening devices can be applied along the length of the spinal rod. 
     With continuing reference to  FIG. 1 . bone screw  14  includes a head portion  22  defining a horizontal axis and a vertical axis. A shank portion  24  depends from the head portion and a threaded portion  26  having a helical thread extending about the outer periphery depends from the shank portion. The helical thread is particularly adapted to securely engage the vertebral bodies of the spine. A channel  28  extends through the head portion  22  along the horizontal axis thereof for receiving elongated spinal rod  12 . As best seen in  FIG. 3 , channel  28  is defined by the interior surfaces of side walls  30  and  32  and the curved lower surface  29 , which extends therebetween. Locking cap  20  is dimensioned and configured for reception and engagement in locking channel  28  to secure the position of bone screw  14  with respect to spinal rod  12  during a spinal stabilization procedure. 
     Referring again to  FIG. 1 , right-angle hook  16  includes a head portion  42  defining a horizontal axis and a vertical axis. A hook portion  46  depends from the head portion  42  for securement to a vertebral body of the spine. A channel  48  extends through the head portion  42  along the horizontal axis thereof for receiving elongated spinal rod  12 . Channel  48  is defined by the interior surfaces of opposed side walls  50  and  52  and a curved lower surface extending therebetween. Locking cap  20  is dimensioned and configured for reception and engagement in channel  48  to secure the position of hook  16  with respect to spinal rod  12  during a spinal stabilization procedure. 
     Referring now to  FIG. 2 , there is illustrated locking cap  20  in an inverted position to best illustrate structural aspects thereof Locking cap  20  includes a cylindrical head  62  and a flanged portion  64 . The bottom surface  66  of flanged portion  64  includes an elongate recess  68  having a curvature complementary to spinal rod  12  for accommodating the spinal rod when locking cap  20  is in an unlocked position, shown for example in  FIG. 6B . In such a position, the fastening device may be moved freely along or rotated about the longitudinal axis of the spinal rod. Bottom surface  66  also includes a bifurcated orthogonal recess  70  which intersects the elongate recess at a 90° angle and has a curvature complementary to spinal rod  12  to accommodate the spinal rod when locking cap  20  is in a locked position, shown for example in  FIG. 6D  and  FIG. 4 . In addition, bottom surface  66  includes bifurcated first and second transverse recesses  72  and  74  which intersect the elongate recess  68  at opposite angles of intersection and have curvatures which are complementary to spinal rod  12  to accommodate the spinal rod when the locking cap  20  is in either of two intermediate positions, one of which is shown for example in  FIG. 6C . In such a position, the fastening device retains the spinal rod but is not fully secured, and if desired by the surgeon, locking cap  20  can be rotated from the intermediate position and the fastener moved to an alternative location on the spinal rod. Preferably, the transverse recesses intersect the elongate recess at opposed 45° angles. However, those skilled in the art will readily appreciate that the transverse recess can be oriented at alternative intersecting angles. It is also contemplated that the bottom surface can be flat without any recesses. 
     Referring to  FIGS. 3 and 5 , the cylindrical head  62  of locking cap  20  includes a hexagonal axial bore  80  extending partially therethrough for receiving a working implement such as a wrench to facilitate rotation of the locking cap  20  relative to the head portion  22  of the fastening device about the vertical axis defined thereby. It envisioned that alternative tooling configurations known in the art can also be utilized to facilitate axial rotation of locking cap  20  during a surgical procedure. Curved notches  76  and  78  are formed in the inner surfaces of opposed walls  30  and  32  for accommodating the cylindrical head  62  of locking cap  20  when the locking cap is received and rotated within channel  28 . 
     The flanged portion  64  of locking cap  20  is defined in part by two diametrically opposed arcuate engagement flanges  82  and  84  which are dimensioned and configured for operative engagement with two complementary diametrically opposed arcuate engagement slots  86  and  88  defined in the interior surfaces of the opposed side walls  30  and  32  of head portion  22 . (See  FIG. 4 ). 
     With continuing reference to  FIGS. 3 through 5 , engagement flanges  82  and  84  define ramped camming surfaces  92  and  94 , respectively. Camming surfaces  92  and  94  are of opposite angular inclination with respect to one another. More particularly, each engagement flange has a low side (e.g.,  82   a  of flange  82 ) and a high side (e.g.,  82   b  of flange  82 ), whereby the low sides of the two flanges are diametrically opposed from one another as are the high sides. Actually, the camming surfaces of the flanges are mirror images of one another. Thus, the locking cap can be initially oriented with either flange aligned to engage either slot. This versatility adds to the ease in which the locking cap is installed during a surgical procedure. 
     As best seen in  FIG. 4 , the arcuate engagement slots  86  and  88  in head portion  22  of fastener  14  have inclined surfaces that mate with the ramped camming surfaces  92  and  94  of flanges  82  and  84 . As best seen in  FIG. 5 , the ramped camming surfaces  92  and  94  are tapered radially inwardly to enhance the interlock with the mating surfaces of arcuate engagement slots  86  and  88 , which are also tapered to complement the radially inward taper of camming surfaces  92  and  94 . This interlocking relationship serves to prevent the opposed side walls  30  and  32  of head portion  22  from spreading radially outward as the arcuate flanges are engaged with the arcuate slots when the locking cap  20  is rotated to a locked position. 
       FIGS. 6A through 6D  illustrate the steps in securing the fastening device to the spinal rod during a surgical procedure. Although attachment of a bone screw  14  is shown, it should be understood, as noted above, that other fastening devices, e.g., bone hooks, can be secured to the spinal rod  12  using the locking cap and head portion structure of the present disclosure. Initially, as illustrated in  FIG. 6A , spinal rod  12  is moved into approximation with the horizontal channel  28  of head portion  22  such that the periphery of the spinal rod  12  is in registration with the curved surface  29  of the channel  28 . Locking cap  20  is then top loaded into the channel along the vertical axis of the fastener in the direction of arrow a. At such a time, spinal rod  12  is accommodated within the elongate recess  68  defined in the bottom surface  66  of locking cap  20  and the bone screw  14  may be moved freely relative to the spinal rod. The opposed flanged sections  82  and  84  of locking cap  20  are 900 out of phase from the opposed arcuate engagement slots  86  and  88  defined in head portion  22 , as shown for example in  FIG. 6B . 
     Thereafter, as shown in  FIG. 6C , locking cap  20  is rotated 45° relative to head portion  22  about the vertical axis thereof. At such a time, spinal rod  12  is accommodated within one of the two transverse recesses  72  or  74 , depending upon the initial orientation of the locking cap  20  with respect to the head portion. Thereupon, the opposed arcuate engagement flanges  82  and  84  of locking cap  20  are only partially engaged with the opposed arcuate engagement slots  86  and  88  defined in head portion  22 , as they are 45° out of phase with the slots. Consequently, the locking cap holds the fastener  22  and spinal rod  12  together, but does not lock the fastener. In this position, the locking cap  20  can be readily rotated in the opposite direction to disengage from the spinal rod  12  to adjust the position of the bone screw  14  with respect to the spinal rod  12 . 
     Once the desired position and orientation of the bone screw  14  has been attained, locking cap  20  is rotated another 90° to the locked position illustrated in  FIG. 6D . At such a time, spinal, rod  12  is accommodated within the orthogonal recess  70  defined in the bottom surface of locking cap  20 . Thereupon, the opposed engagement flanges  82  and  84  of flanged portion  64  are fully engaged with the opposed engagement slots  86  and  88  of head portion  22  and the longitudinal and angular orientations of the bone screw  14  are fixed with respect to spinal rod  12 , as illustrated in  FIG. 4 . It should be readily apparent that the manner and method by which bone screw  14  hook is attached to spinal rod  12  is identical to the manner and method by which hook  16  or other fasteners are attached to spinal rod  12 . 
     Since the rotational range of locking cap  20  is limited, i.e., the locking cap can only be rotated 90°, it will be readily appreciated that the cap cannot be over-torqued. Thus, the damage often caused by over-tightening a conventional threaded locking mechanism, such as a set screw, is avoided. Furthermore, since the locking cap of the subject disclosure has a predetermined locked position, it is unlikely that it will be under-torqued or left in a loose condition after installation as is common with threaded set screws found in the prior art. That is, by having a predetermined locked position, uniform locking forces are provided for all of the fastening devices used to secure the spinal rod  12  along its length and cross threading is reduced. 
     Referring now to  FIGS. 7 and 8 , there is illustrated another fastening device constructed in accordance with a preferred embodiment of the subject disclosure and designated generally by reference numeral  110 . Fastening device  110  is similar to fastening devices  12  and  14  in that it is particularly designed to facilitate securement of a spinal rod to the spine in a convenient manner. Fastening device  110  includes a head portion  122  having opposed side walls  130  and  132 , which define a horizontal channel  128  in conjunction with the curved lower surface  129  extending therebetween. Arcuate tabs  176  and  178  project upwardly from side walls  130  and  132 , respectively, for interacting with locking cap,  120 . 
     Referring to  FIG. 9 , locking cap  120 , which is shown in an inverted position for ease of illustration, includes a hexagonal head  162  a cylindrical body  163  and a flanged portion  164 . The hexagonal head  162  is adapted and configured for interaction with a wrench or similar work implement. An annular channel  165  extends into the bottom surface of hexagonal head  162  for receiving arcuate tabs  176  and  168  This positive interaction serves to prevent the opposed side walls  130  and  132  of head portion  122  from spreading radially outwardly when arcuate flanges  182  and  184  of locking cap  120  are engaged in arcuate slots  186  and  188  of head portion  122  upon rotation of locking cap  20  into a locked position. Thus, in this embodiment, the ramped camming surfaces  192  and  194  of the arcuate engagement flanges  182  and  184  need not be provided with radially inwardly directed tapers as provided on flanges  82  and  84  of the locking cap  20  of the embodiment of  FIGS. 1-6 . 
     With continuing reference to  FIG. 9 , the bottom surface  166  of the flanged portion  164  of locking cap  120  is configured in substantially the same manner as the bottom surface  66  of locking cap  20  in that it is provided with an elongate recess  168  for accommodating a spinal rod when the locking cap  120  is in an unlocked position, first and second bifurcated transverse recesses  172  and  174  which intersect the elongate recess  168  at opposite 45° angles to accommodate the spinal rod when the locking cap  120  is in either of two intermediate positions, and a bifurcated orthogonal recess  170  which intersects the elongate recess at a 90° angle to accommodate the spinal rod when the locking cap  120  is in a final locked position, as shown in  FIG. 10 . It will be readily appreciated that locking cap  120  is engaged with fastening device  110  in a manner that is substantially similar to the manner in which locking cap  20  is engaged with bone fastener  14  and hook  16 , and that the configuration of the bottom surface of flanged portion  164  provides the same benefits afforded by the flanged portion  64  of locking cap  20 . 
     Referring now to  FIG. 11 , there is illustrated two additional fastening devices constructed in accordance with the subject disclosure in the form of a multi-axial bone screw  214  and a right-angle hook  216  which are provided for securing spinal rod  212  to the spine during a spinal stabilization procedure. Both fastening devices employ a novel top loaded two-piece locking cap, designated generally by reference numeral  220 , which will be described in greater detail hereinbelow with reference to  FIGS. 12 and 13 . The novel two-piece locking cap achieves significant clinical advantages over the prior art through its reliability and the ease in which it is installed during a spinal stabilization procedure. As with respect to the previously described embodiments of  FIG. 1 , the novel two-piece locking cap may be used in conjunction with other types of fasteners commonly employed in spinal stabilization procedures. Moreover, while the two-piece locking cap illustrated in  FIG. 11  is employed with a multi-axial bone screw, it is readily apparent that the same two-piece locking cap could be employed with a fixed axis bone screw such as that which is illustrated in  FIG. 1 . 
     Referring to  FIGS. 12A and 13 , the multi-axial bone screw  214  includes a head portion  222  defining a horizontal axis “x” and a vertical axis “y”. A channel  228  extends through the head portion  222  along the horizontal axis “x” for receiving an elongated spinal rod  212 . Channel  228  is defined by the interior surfaces of the side walls  230  and  232  of head portion  222 . Bone screw  214  further includes a fastener portion  224 , which includes a generally spherical head  225  and a threaded body  226 . Threaded body  226  depends from and is monolithically formed with the spherical head  225 . The threaded body includes a helical thread formation that is particularly adapted to securely engage the vertebral bodies of the spine. 
     The head portion  222  of multi-axial bone screw  214  further defines a generally cylindrical vertical channel  227 , which extends through and is aligned with the vertical axis “y” of the head portion  222 . Vertical channel  227  is configured to receive and accommodate the fastener portion  224  of bone screw  214 . More particularly, as best seen in  FIG. 13 , a lower interior surface portion of vertical channel  227  defines an annular seating surface  229  configured to cooperate with the lower hemi-spherical region of spherical head  225 . The cooperative engagement between the two structures permits the relative movement of the fastener portion  224  with respect to the head potion  222  about the vertical axis y. The multi-axial motion afforded thereto, enhances the operational range of bone screw  214 , providing greater flexibility to the surgeon during a spinal stabilization procedure. 
     Bone screw  214  further includes an annular retention ring  232  that is accommodated within a corresponding annular groove  234  formed within the cylindrical wall of vertical channel  227  (see  FIG. 13 ). Retention ring  232  is adapted to positively engage the spherical head  225  and aiding in its stabilization. In addition, as best seen in  FIG. 13 , the lower hemi-spherical region of head  225  is scored with a series of circular ridges adapted to enhance the frictional coefficient of the seating surface defined thereby. 
     Referring to  FIG. 12A , bone screw  214  further includes a two-piece locking cap  220  which is dimensioned and configured for reception and engagement in the horizontal channel  228  of head portion  220  to secure the position of head portion  222  with respect to spinal rod  212  during a spinal stabilization procedure. In addition, as described in detail hereinbelow with respect to  FIG. 13 , the securement of locking cap  220  within channel  228  also achieves positive fixation of the angular position of the fastener portion  224  with respect to the head portion  222  and the vertical axis “y” defined thereby. 
     As illustrated in  FIGS. 12A and 12B , locking cap  220  includes an upper portion  220   a  and a lower portion  220   b . The upper portion  220   a  includes a cylindrical cap body  280  defining an axial reception port  282  for receiving a tool or working implement that applies torque to the cap during installation. Upper portion  220   a  further includes a pair of circumferentially opposed arcuate engagement flanges,  284  and  286  which extend radially outwardly from cap body  280 . Engagement flanges  284  and  286  include inclined radially inwardly slopping camming surfaces for cooperating with complementary opposed arcuate engagement slots  294  and  296  formed in the opposed side walls  230  and  232  of head portion  222  (see  FIG. 13 ). As described in more detail hereinbelow with respect to  FIGS. 14   a - 14   d , the flanges  284 ,  286  become engaged in corresponding slots  294 ,  296  upon rotation of the upper portion  220   a  of locking cap  220  relative to the head portion  222  of bone screw  214 . 
     The lower portion  220   b  of locking cap  220  is configured for cooperative reception within the cylindrical vertical channel  227  of head portion  222  and is adapted to engage the spinal rod  212  extending through the horizontal channel  228  of head portion  222 . More particularly, the body  285  of the lower portion  220   b  has curved exterior surfaces, which complement the curvature of the walls defining vertical channel  227 . Thus, when the locking cap  220  is loaded into vertical channel  227 , a positive mating relationship is achieved between the lower portion  220   b  of locking cap  220  and vertical channel  227 . As a result, the axial position of lower portion  220   b  becomes fixed with respect to head portion  222  and spinal rod  212 . Furthermore, as best seen in  FIG. 12B , a hemi-cylindrical channel  299  is formed in the undersurface of lower portion  220   b  for intimately cooperating with the cylindrical spinal rod  212  upon loading the locking cap  220  in vertical channel  227 . Body portion  285  includes an extension flange  302  which aides in the alignment and positioning of the lower cap portion  220   a  with respect to spinal rod  212 . 
     As best seen in  FIG. 12B , the bottom surface of the upper portion  220   a  of locking cap  220  includes a recessed seating area  287  and an associated axial reception bore  288 . The recessed seating area  287  is dimensioned and configured to accommodate the body of the lower portion  220   b  of locking cap  220 , while the reception bore  288  is dimensioned and configured to receive and engage an axial post  298  which projects from the upper surface  295  of the lower portion  220   b  of locking cap  220 . More particularly, during assembly, when the axial post  298  is received by the reception bore  288 , the top end of the post is swaged (flared out) to join the two components together (see  FIG. 13 ). The interaction of the axial post  298  and axial reception bore  288  facilitates relative rotational movement of the upper portion  220   a  relative to the lower portion  220   b  when the locking cap  220  is loaded into and locked in the head portion  222  of bone screw  214  during a spinal stabilization procedure. 
     As described in detail hereinbelow with reference to  FIGS. 14A-14C , the two-part locking cap enables a surgeon to load the locking cap  220  into vertical channel  227  and properly position the lower portion  220   b  against the spinal rod  212  so as to ensure an intimate engagement between the hemi-cylindrical channel  299  and the cylindrical surface of the spinal rod. Thereafter, the upper portion  220   a  may be rotated into a locked portion relative to the lower portion  220   b.    
     Referring now in detail to  FIGS. 14A-14C , during a spinal stabilization procedure, the fastener portion  226  of bone screw  214  is first seated within the head portion  222 . Then, the head portion  222  is positioned at the surgical site in such a manner so that the elongated spinal rod  212  extends through the horizontal channel  228  as illustrated in  FIG. 14A . Thereafter, if necessary, the fastener portion  226  may be moved into a desired angular orientation by the surgeon and subsequently mounted to the spinous process using suitable surgical instruments. 
     With reference to  FIG. 14B , once the appropriate position of the fastener portion  226  has been established by the surgeon, the locking cap  220  is loaded into the vertical channel  227  of head portion  222  along the vertical axis “y” defined thereby. At such a time, the hemi-cylindrical channel  299  on the undersurface of lower portion  220   b  will become intimately engaged with the cylindrical surface of the spinal rod  212  and it will be maintained in a fixed axial orientation with respect to the spinal rod due to the mating relationship between the body of the lower portion  220   b  and the vertical channel  227 . 
     Locking cap  220  must be loaded in such a manner so that the radially outwardly extending engagement flanges  284  and  286  of upper portion  220   a  are parallel to the axis of the spinal rod  212 , as illustrated in  FIG. 14B . Otherwise, the flanges will interfere with the opposed side walls  230  and  232  of the head portion  222 . Furthermore, care must be taken to ensure that the upper portion  220   a  of locking cap  220  is positioned in such a manner so that the low sides of the flanges (e.g.  284   a ) are aligned with the high sides of the engagement channels (e.g.  294   a ), or the flanges will not cammingly engage the channels upon rotation of the upper portion  220   a  of the locking cap  220  relative to the head portion  222  of bone screw  214 . 
     Once the upper portion  220   a  of locking cap  220  has been properly oriented with respect to head portion  222  with the extension flange  302  in alignment with spinal rod  212 , it is rotated in a clockwise direction about the vertical axis “y” relative to the lower portion  220   b  of locking cap  220  and the head portion  222  of bone screw  214  using an appropriate surgical implement or tool (not shown). Thereupon, the arcuate engagement flanges  284 ,  286  of upper portion  220  cammingly engage the corresponding engagement slots  284 . Once rotated into a locked portion, the lower portion  220   b  of the locking cap  220  will be seated within the recessed seating area  287  defined in the bottom surface  285  of the upper portion  220   a  of locking cap  220  (See  FIG. 13 ). At such a time, the position of the head portion  222  of bone screw  214  is fixed with respect to longitudinal axis of spinal rod  212  and the position of the fastener portion  226  of bone screw  214  is fixed with respect to the vertical axis “y” defined by head portion  222  of bone screw  214 , as illustrated in  FIG. 14C . 
     Referring now to  FIGS. 15 and 16 , the right-angle hook  216  of the subject disclosure includes a head portion  242  defining a horizontal axis x and a vertical axis “y”. A hook portion  246  depends from the head portion  242  to facilitate securement of the device to a vertebral body of the spine. A channel  248  extends through the head portion  242  along the horizontal axis thereof for receiving elongated spinal rod  212 . Channel  248  is defined by the interior surfaces of opposed upstanding side walls  250  and  252  and a contoured lower surface extending therebetween for complementing the shape of the rod. Channel  248  is further configured to receive a two-piece locking cap  220  adapted to secure the position of hook  216  with respect to spinal rod  212  during a spinal stabilization procedure. 
     As discussed hereinabove with respect to multi-axial bone screw  214 , the locking cap  220  includes an upper portion  220   a  and a lower portion  220   b , which are rotatably joined together. The upper portion includes a pair of circumferentially opposed arcuate engagement flanges  284  and  286  for cooperating with complementary opposed arcuate engagement slots  255  and  257  formed in the opposed side walls  250  and  252  of head portion  242 . As described in more hereinabove with respect-to  FIGS. 14A-14C , the flanges  284 ,  286  become engaged in corresponding slots  255 ,  257  upon rotation of the upper portion  220   a  of locking cap  220  relative to the lower portion  220   b  of the locking cap and the head portion  242  of right-angle hook  216 . 
     Although the apparatus disclosed herein has been described with respect to preferred embodiments, it is apparent that modifications and changes can be made thereto without departing from the spirit and scope of the invention as defined by the claims.