Patent Publication Number: US-2019167310-A1

Title: Radiolucent screw with radiopaque marker

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/063,605, filed Mar. 11, 2011, which is a US nationalization of PCT/US2009/056508, filed Sep. 10, 2009, which is a CIP of U.S. application Ser. No. 12/208,968, filed Sep. 11, 2008, now U.S. Pat. No. 7,922,788, issued Apr. 12, 2011, which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to polyaxial and fixed bone screws and components thereof that can be used for stabilizing adjacent vertebrae of the spine or otherwise fixing to bone. 
     2. The Relevant Technology 
     Polyaxial and fixed bone screws (often referred to as pedicle screws) are commonly used in spinal operations for adjusting or stabilizing adjacent vertebrae. For example, in one conventional procedure a first bone screw is screwed into a first vertebra while a second bone screw is screwed into an adjacent second vertebra. A stabilizing rod is then secured between the bone screws so as to fix the adjacent vertebrae relative to each other. Bone screws can be positioned on each side of each vertebra and can be positioned in any number of consecutive vertebrae with one or more stabilizing rods extending between the different bone screws. 
     A conventional bone screw comprises a threaded screw portion having a collar either fixedly or pivotably mounted on the end thereof. The screw portion is threaded into the bone and the stabilizing rod is received within the collar and secured therein. Other conventional bone screws are used for purposes such as securing a bone plate over a facture, fixing a cranial plate, attaching ligaments, mounting an implant and the like. To be strong enough to handle the stresses placed upon them, the bone screws are typically made of titanium or some other biocompatible metal. Being made of metal allows the doctor to view the bone screws using X-ray photographs during and after implantation. 
     However, because the bone screws are made of metal, the bone screws block X-rays passing through the body, in effect obscuring adjacent bone and other X-ray viewable internal structures surrounding the area and thereby preventing the surgeon from viewing those structures on an X-ray photograph. The metal bone screws can also disrupt MRI and other types of images. This can limit a surgeon&#39;s ability to ensure proper placement of the bone screws and diagnose and treat problems that arise near the location of the bone screws after the bone screws have been implanted. 
     Accordingly, what is needed are polyaxial and fixed bone screws that overcome some or all of the above disadvantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. 
         FIG. 1  is a perspective view of a spinal stabilizing system incorporating a polyaxial bone screw according to one embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of the polyaxial bone screw shown in  FIG. 1 ; 
         FIG. 3  is a perspective view of the assembled screw portion of the bone screw shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of the shaft portion of the screw portion shown in  FIG. 3 ; 
         FIG. 5  is a top perspective view of the head of the screw portion shown in  FIG. 3 ; 
         FIG. 6  is a bottom perspective view of the head of the screw portion shown in  FIG. 3 ; 
         FIG. 7  is a bottom plan view of the assembled screw portion shown in  FIG. 3 ; 
         FIGS. 8A-8C  are perspective views of alternative embodiments of cores; 
         FIGS. 9A-9D  are cross-sectional bottom views of alternative embodiments of screw portions of bone screws; 
         FIG. 10  is a cross sectional side view of an assembled screw portion according to one embodiment having a positioning ring disposed within the shaft; 
         FIG. 11  is a perspective view of an assembled screw portion according to one embodiment having a ring layer painted thereon; 
         FIG. 12  is a perspective view of the collar shown in  FIG. 2 ; 
         FIG. 13  is a cross sectional side view of a portion of the assembled polyaxial bone screw shown in  FIG. 1 ; 
         FIG. 14  is a perspective view of impregnated fibers being wound on the core shown in  FIG. 2 ; 
         FIG. 15  is a perspective view of sheets of fibers being wound on the core shown in  FIG. 2 ; 
         FIG. 16  is a perspective view of a blank that is formed during manufacture of the screw portion shown in  FIG. 3  according to one embodiment; 
         FIG. 17  is a perspective view of the screw portion shown in  FIG. 3  in a partially assembled state; 
         FIG. 18  is an exploded perspective view of an alternative embodiment of the screw portion shown in  FIG. 17  wherein the head and the shaft of the screw portion are integrally formed as a unitary member; 
         FIG. 19  is a perspective view of an alternative embodiment of an assembled screw portion of a bone screw according to the present invention; 
         FIG. 20  is a perspective view of the screw portion shown in  FIG. 19  in a partially assembled state; 
         FIG. 21  is a bottom perspective view of the head of the screw portion shown in  FIG. 20 ; 
         FIG. 22  is a bottom perspective view of an alternative embodiment of the head of the screw portion shown in  FIG. 20 ; 
         FIG. 23  is a partial top perspective view of an alternative embodiment of the shaft of the screw portion shown in  FIG. 20 ; 
         FIG. 24  is a top perspective view of a portion of another alternative embodiment of the shaft of the screw portion shown in  FIG. 20 ; 
         FIG. 25  is a perspective view of an alternative embodiment of an assembled screw portion of a bone screw according to the present invention; 
         FIG. 26  is a perspective view of a portion of the screw portion shown in  FIG. 25 ; 
         FIG. 27  is a bottom perspective view of the head of the screw portion shown in  FIG. 25 ; 
         FIG. 28  is one embodiment of a fixed bone screw wherein a collar is rigidly secured to the end of the shaft; 
         FIG. 29  is an exploded view of the bone screw shown in  FIG. 28 ; 
         FIG. 30  is a perspective bottom view of the collar shown in  FIG. 29 ; 
         FIG. 31  is an exploded perspective view of an alternative embodiment of the fixed bone screw shown in  FIG. 28  wherein the collar and the shaft of the bone screw are integrally formed as a unitary member; 
         FIG. 32  is a exploded perspective view of an alternative embodiment of a spinal stabilizing system; 
         FIG. 33  is an exploded perspective view of the screw portion of the spinal stabilizing system shown in  FIG. 32 ; 
         FIG. 34  is a cross sectional side view of the core and integral head of the screw portion shown in  FIG. 33 ; 
         FIG. 35A  is a top perspective view of the saddle shown in  FIG. 32 ; 
         FIG. 35B  is a bottom perspective view of the saddle shown in  FIG. 35A ; 
         FIG. 36  is an exploded perspective view of the fastener shown in  FIG. 32 ; 
         FIG. 37  is a cross sectional side view of the assembled spinal stabilizing system shown in  FIG. 32 ; 
         FIG. 38  is a perspective view of an alternative embodiment of the saddle shown in  FIG. 35A ; 
         FIG. 39  is an exploded perspective view of an alternative embodiment of a screw portion having a modified core; 
         FIG. 40  is an exploded perspective view of another alternative embodiment of a screw portion having a modified core; 
         FIG. 41  is an exploded perspective view of a bone screw having a modified head; and 
         FIG. 42  is an exploded perspective view of another bone screw having a modified head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Depicted in  FIG. 1  is a spinal stabilizing system  101  incorporating features of the present invention. Spinal stabilizing system  101  can be used for stabilizing adjacent vertebrae of a spine as part of a procedure for fusing together the adjacent vertebrae. Spinal stabilizing system  101  can also be used for stabilizing a series of consecutive vertebrae for manipulation of the spine to correct spinal deformities such as scoliosis. It is appreciated that spinal stabilizing system  101  and/or discrete elements thereof can also be used in other procedures for anchoring, manipulating, and/or stabilizing various bones. 
     As depicted in  FIG. 1 , stabilizing system  101  includes a polyaxial bone screw  100  comprising an elongated screw portion  102  and a collar  104  pivotally mounted thereon. Stabilizing system  101  also includes a fastener  106  that is selectively engageable with collar  104  to secure polyaxial bone screw  100  to a stabilizing rod  107 . The above identified components of polyaxial screw  100  and their relative interaction will now be discussed in greater detail. 
     As shown in  FIGS. 2 and 3 , screw portion  102  of bone screw  100  comprises an elongated shaft  108  having a head  110  disposed thereon with a core  112  extending longitudinally through shaft  108  and head  110 . 
     Turning to  FIG. 4 , shaft  108  is elongated and has a proximal end  114  and a spaced apart distal end  116  with a central longitudinal axis  118  extending therebetween. Shaft  108  comprises an elongated shaft body  113  and an attachment member  126  formed at the proximal end thereof. Shaft body  113  has an exterior surface  122  that extends between proximal end  114  and distal end  116 . One or more threads  120  helically encircle and radially outwardly project from exterior surface  122  of shaft body  113  along the length thereof. The one or more threads  120  can have a variety of different pitches and configurations, and, if desired, can be self-tapping. Proximal end  114  of shaft body  113  terminates at an end face  132  while distal end  116  of shaft body  113  terminates at a tapered tip  124 . End face  132  is typically planar and disposed orthogonal to central longitudinal axis  118 , although this is not required. Tapered tip  124  has a substantially conical configuration for ease in penetration into a bone or predrilled hole. A cutting edge  125  can also be disposed on the tapered portion of tip  124  to aid in cutting the bone in bone screw embodiments that are self-tapping. 
     Attachment member  126  centrally projects from end face  132  of shaft body  113 . As discussed below in greater detail, attachment member  126  is used to engage and secure head  110  ( FIG. 2 ) to shaft  108 . As such, attachment member  126  is sized and shaped so as to fit within a complementary attachment recess  128  disposed on head  110  (see  FIG. 6 ). In the embodiment depicted, attachment member  126  has an encircling side wall  130  that proximally extends from end face  132  of shaft body  113  to a terminal end face  134 . End faces  132  and  134  are depicted as being substantially parallel with each other and orthogonal to longitudinal axis  118 , although this is not required. Side wall  130  is depicted as being substantially parallel to longitudinal axis  118 , but this is also not required. 
     In the depicted embodiment, side wall  130  of attachment member  126  comprises a substantially cylindrical portion  135  and a flat  136 . Flat  136  in effect removes a portion of the rounded side of the cylinder portion  135 . In an alternative embodiment side wall  130  is formed without a flat. Other cross sectional attachment shapes can alternatively be used. For example, side wall  130  of attachment member  126  can be oval, polygonal, star shaped, irregular, or the like. Other shapes are also possible. 
     Continuing with  FIG. 4 , shaft  108  includes an internal surface  138  that bounds a first passageway  140  extending longitudinally through shaft  108  between proximal end  114  and distal end  116 . First passageway  140  extends along central longitudinal axis  118 , through terminal end face  134  of attachment member  126  and through tapered tip  124 . In the embodiment depicted, first passageway  140  has a substantially circular cross-sectional shape. Other cross-sectional shapes can alternatively be used for first passageway  140 . For example, first passageway  140  can be oval shaped, star shaped, polygonal shaped, irregular or the like. First passageway  140  can also be symmetrically or non-symmetrically shaped. In alternative embodiments, first passageway  140  need not extend the full length of shaft  108 . For example, first passageway  140  need not extend through tip  124 . 
     Shaft  108  can be comprised of a radiolucent material that will allow viewing of adjacent bone or other internal structures on an X-ray photograph that are in the viewing path of shaft  108 . Using radiolucent material for the shaft  108  will also minimize scattering caused by commonly used metallic or other radiopaque shafts in X-Rays, CAT scans, MRI&#39;s, and other types of imaging systems. 
     One example of a radiolucent material that can be used in shaft  108  is a radiolucent biocompatible fiber and adhesive matrix. In this embodiment, an adhesive is applied to one or more elongated biocompatible fibers that are then wound about core  112 , a rod, or other object to form shaft  108 . This is typically done by winding two or more layers of fibers about core  112  or other object. The fibers can be wound one fiber at a time or multiple fibers at a time in a fiber bundle or tow. The fibers are typically of indefinite length and are wound from a spool or other carrier and then cut when the winding is completed. Alternatively, the fibers can comprise one or more shorter fibers that are wound or otherwise disposed within shaft  108 . In still other embodiments, the fibers can be included in a sheet or other structure and then wound about core  112  or other object in one or two or more layers. Various winding patterns and fiber orientation can also be used. Methods of manufacturing the shaft  108  are discussed in more detail below 
     Many different types of biocompatible fibers and adhesives can be used to form radiolucent shaft  108 . For example, the fibers can be comprised of carbon, fiberglass, poly paraphenylene terephthalamide (PPTA, more commonly known as Kevlar®), other aramids, and ceramics. Other radiolucent, biocompatible fibers having desired properties can also be used. 
     Although fibers having multiple different properties can be used, typical fibers have a diameter in a range between about 5 microns to about 18 microns with about 5 microns to about 7 microns being more common and a tensile strength in a range between about 300 ksi to about 1000 ksi with about 600 ksi to about 1000 ksi being more common. Other diameters and tensile strengths can be used. The fibers can be sized or unsized. By “unsized,” it is meant that the fibers have not been coated with a material to improve adhesion of the resin or adhesive. If the fibers are sized, biocompatibility of the sizing needs to be considered. When bundles of fibers are used, the tow of the fibers (i.e., the number of fibers per bundle) can range from about 1 k to about 72 k with about 3 k to about 24 k being more common. Other tow ranges can also be used. In one specific embodiment, the fibers comprise a continuous high strength, PAN based carbon fiber, 34-700, 12 k (tow), “unsized”. In another specific embodiment, the fibers comprise a continuous high strength, PAN based carbon fiber, 34-700, 3 k (tow), sized. 
     Examples of biocompatible adhesives that can be used with the fibers include thermoplastic materials, thermoset materials and ceramics. Examples of thermoplastic materials that can be used include polyester, vinylester, polycarbonate, polyetheretherketone (PEEK), polyaryletherketone (PAEK), polyethylene, polyurethane, and polyamide. Examples of thermoset materials that can be used include epoxies and polyimides. Exemplary biocompatible epoxies include the Master Bond Inc. epoxies EP42HT-2 and EP45HT MED and the Epotek epoxies 301-2 and 375. Examples of ceramics that can be used include alumina and zirconia. Other epoxies, ceramics, plastics and resins that are implantable, biocompatible, sterilizable, and have the desired strength properties can also be used. In an alternative embodiment, the radiolucent material used in shaft  108  can simply comprise the adhesive materials as discussed above without the fibers. If desired, other additives and fillers can be combined with the adhesive materials. 
     Returning to  FIGS. 2 and 3 , head  110  is disposed on proximal end  114  of shaft  108  so as to engage with attachment member  126 . As shown in  FIG. 3 , head  110  comprises a rounded substantially semi-spherical bottom portion  150  that can bias and rotate against collar  104 . Bottom portion  150  has a first end  142  on which a face  144  is formed and a second end  146 . A top portion  152  centrally projects from face  144  and is shaped to allow a tool to engage and rotate screw portion  102 . An annular neck  154  extends from the second end  146  of bottom portion  150  of head  110  to a bottom surface  156  (see  FIG. 6 ). Neck  154  has an encircling exterior surface  155  having a substantially concave transverse cross section. In the depicted embodiment, top portion  152  has an encircling sidewall  158  that extends from face  144  to a top surface  160 . Sidewall  158  typically has a polygonal shape so that it can mate with a driver or other tool for tightening and loosening bone screws. Other shapes, such as oval or irregular, can also be used. Alternatively, a socket can be formed within top surface  160  or on face  144  of bottom portion  150  for engaging a tool. 
     Turning to  FIG. 5 , head  110  includes a central longitudinal axis  162  extending through head  110  between top surface  160  of top portion  152  and bottom surface  156  of neck  154 . When screw portion  102  is assembled, axis  118  of shaft  108  (see  FIG. 4 ) and axis  162  of head  110  can be aligned with each other. 
     An engagement slot  164  is formed on head  110 . Engagement slot  164  comprises a pair of opposing side walls  166  and  168  that are generally disposed in parallel planes and extend to a rounded floor  170  and a back wall  172 . Back wall  172  typically intersects with floor  170  at a right angle while back wall  172  is disposed generally parallel to central longitudinal axis  162  at a distance spaced apart therefrom. In alternative embodiments, floor  170  need not be rounded but can be flat, V-shaped, or have other configurations. It is appreciated that engagement slot  164  can have a variety of different configurations and merely needs to be sized, shaped, and oriented to permit the desired pivoting of collar  104  and rotation of screw portion  102  as discussed below in greater detail. 
     Turning to  FIG. 6 , attachment recess  128  is formed in bottom surface  156  of head  110  to mate with attachment member  126  of shaft  108  ( FIG. 4 ). As such, attachment recess  128  is sized and shaped so as to receive attachment member  126 . For example, in the depicted embodiment, attachment recess  128  is bounded by an encircling side wall  174  that extends from bottom surface  156  to a floor  176 . Attachment recess  128  has a straight section  178  of side wall  174  corresponding to flat  136  of side wall  130  of attachment member  126 . ( FIG. 4 ) In an alternative embodiment, attachment member  126  is disposed on head  110  and attachment recess  128  is formed on shaft  108 . It is appreciated that attachment member  126  and attachment recess  128  can have a variety of different configurations and merely need to be sized, shaped, and oriented to permit attachment member  126  and attachment recess  128  to selectively mate with each other when head  110  and shaft  108  are secured together, as discussed below in greater detail. 
     Returning to  FIG. 5  in conjunction with  FIG. 6 , similar to shaft  108 , head  110  includes an internal surface  180  bounding a second passageway  182  that extends through head  110 . Second passageway  182  extends along central longitudinal axis  162 , between top surface  160  and attachment recess  128  (or attachment member  126 , if attachment member  126  is disposed on head  110 ). Second passageway  182  can be of the same cross-sectional shape as first passageway  140  or can be of a different shape. For example, in the depicted embodiment, second passageway  182  has a substantially circular cross sectional shape except for a straight portion  184  on one of the sides. Other shapes can also be used. 
     Head  110  can comprise a radiolucent material, such as any of those listed above for shaft  108 . In one embodiment, head  110  comprises the same or different radiolucent material as shaft  108 . Alternatively, head  110  can comprise a radiopaque material in place of or in addition to a radiolucent material. Examples of radiopaque metals that can be used in head  110  are titanium, stainless steel, tungsten, cobalt based alloys, cobalt chrome alloys, nickel titanium alloys such as Nitinol, platinum/iridium, gold, barium and alloys thereof. Other radiopaque materials that can be used include cortical bone and synthetic bone. The radiopaque material may also comprise the radiolucent materials discussed above having a radiopaque filler disposed therein. Other biocompatible metals and other radiopaque materials having desired properties can also be used. 
     Applicant notes that due to the electric potential between carbon and titanium, corrosion may occur between the two surfaces in the presence of an electrolyte. However, because the electron potential is small, the corrosion would be very small, if it occurs at all. Furthermore, the adhesive used in the matrix acts as an insulator. To combat any corrosion that may occur, anodization or passivation of the metals can be performed before assembly. 
     Returning to  FIG. 2 , core  112  comprises a slender rod having an encircling outer surface  198  that extends between a proximal end  200  and an opposing distal end  202 . Core  112  is designed to be disposed within first and second passageways  140  and  182  of assembled shaft  108  and head  110 , respectively. As discussed below, this can be accomplished by forming the shaft  108  about core  112  or by inserting core  112  into passageway  140  after the passageway  140  has been formed. It is appreciated that core  112  need not extend all the way through shaft  108  but can be disposed only along a portion thereof. Thus, both core  112  and first passageway  140  can extend only along a portion of shaft  108 . 
     Core  112  comprises a head portion  204  at proximal end  200  and a shaft portion  206  at distal end  202 . Head portion  204  of core  112  is shaped to be disposed within second passageway  182  of head  110  and shaft portion  206  is shaped to be disposed within first passageway  140  of shaft  108 . For example, in the embodiment depicted, shaft portion  206  has a substantially circular cross section (see  FIG. 7 ) to match the circularly shaped first passageway  140 , and head portion  204  has a substantially circular cross section with a segment removed to form a straight section  208  so as to match the shape of second passageway  182 . In some embodiments, the cross-sectional shapes of head portion  204  and shaft portion  206  comprise the same shape. 
     Other variations can also be incorporated into the head portion  204  and/or the shaft portion  206  of core  112 . For example, one or more channels or projections can be incorporated into core  112  to increase engagement between core  112  and shaft  108  or head  110 , thereby minimizing the potential for separation therebetween. In  FIG. 8A , a core  112   a  includes two channels  400   a  and  400   b  longitudinally spaced apart from each other. Each channel  400  is bounded by an encircling side wall  402  having a substantially circular cross section with a diameter less than the diameter of outer surface  198 . As such, channels  400  are also bounded by a first end face  404  and a second end face  406  that extend between the outer surface  198  and the sidewall  404  at either end of channels  400 . The end faces can be substantially orthogonal to the outer surface  198  (as shown), or form some other angle with the outer surface  198 . Although  FIG. 8A  shows two channels  400 , it is appreciated that one or three or more channels  400  can alternatively be used. 
     Furthermore, instead of or in conjunction with channels  400 , one or more projections can be formed along core  112   a . The projections can comprise a flange  401   a  that encircles or partially encircles core  112   a , one or more ribs  401   b  that extend along core  112   a , knobs, or projections having a variety of other configurations. Instead of having a diameter less than the diameter of outer surface  198 , the projections  401  have a diameter greater than the diameter of outer surface  198 . As such, the projections extend out from outer surface  198 . 
     The sizes and locations of channels  400  or projections  401  can vary widely. In some embodiments, the locations of the channels or projections are chosen so as to provide a length indicator when the core  112   a  is viewed on an X-ray. That is, when viewed on an X-ray, the channel or projection can identify to the doctor a predefined length of the core  112   a.    
       FIG. 8B  shows another surface shape variation incorporated into a core  112   b  to help minimize the potential for separation from shaft  108  or head  110 . In  FIG. 8B , a helical thread  408  is formed on outer surface  198 . If the first and second passageways  140  and  182  are likewise threaded, the shaft  108  and/or the head  110  can be threaded onto the core  112  during manufacturing and assembly, if desired. The size, shape, and pitch of the helical thread can vary. 
       FIG. 8C  shows another variation incorporated into a core  112   c . In  FIG. 8C , core  112   c  has a cannula  410  that longitudinally extends completely through core  112   c  between proximal end  200  and distal end  202 . Cannula  410  can be used during implantation to pass a guidewire or other surgical device through is assist in positioning bone screw  100  and/or can be used for performing other surgery techniques. The cross sectional size and shape of cannula  410  can vary, depending on the cross-sectional size and shape of core  112   c.    
     It is appreciated that any of the core variations described above can be combined, if desired, in the same core  112 . For example, in one embodiment, a cannula and one or more channels or projections could be included in the same core, while in another embodiment a cannula could be included in a threaded core. Other combinations are also possible. 
     Various geometric cross sectional shapes can alternatively be used for the head portion  204  and/or the shaft portion  206  of core  112 . For example,  FIGS. 9A-9D  disclose various embodiments of shaft portion  206  having different cross sectional shapes.  FIG. 9A  shows an embodiment in which shaft portion  206   a  is oval shaped.  FIG. 9B  shows an embodiment in which shaft portion  206   b  is generally star shaped.  FIG. 9C  shows an embodiment in which shaft portion  206   c  is generally polygonal shaped. In some embodiments head portion  204  and/or shaft portion  206  have a symmetrical cross sectional shape, such as shaft portion  206   c  shown in  FIG. 9C ; in other embodiments head portion  204  and/or shaft portion  206  have a non-symmetrical cross sectional shape, such as shaft portion  206   d  shown in  FIG. 9D . Head portion  204  and/or shaft portion  206  can also use a combination of curved and linear segments, such as head portion  204  shown in  FIG. 2 . It is appreciated that the aforementioned core shapes are exemplary only and that other shapes, that are typically non-circular, can alternatively be used. It is appreciated that the passageways in shaft  108  and head  110  in which core  112  is received can have the same complementary configuration as core  112 . One benefit of producing core  112  with a non-circular configuration is that greater engagement can be formed between core  112  and screw portion  102 , thereby minimizing the potential for separation therebetween. 
     Core  112  typically has a maximum outer diameter in a range between about 1 mm to about 3.5 mm, with about 2 mm to about 3 mm being common. In one embodiment, core  112  has a maximum diameter that is less than about 3 millimeters and more commonly less than about 2 millimeters. Other diameters can also be used. 
     Core  112  is typically comprised of a radiopaque material, such as those previously discussed with regard to head  110 . Core  112  can be comprised of the same radiopaque material as head  110  or can be comprised of a different radiopaque material. One advantage of using a radiopaque material in core  112  while using a radiolucent material in shaft  108  is that only the thin core  112  will be seen on an X-ray during and after implantation of screw portion  102 . This aids the surgeon in positioning screw portion  102  when implanting screw portion  102 , yet allows other internal body structures to be viewed by X-ray during and after screw portion  102  implantation. Where core  112  is comprised of a radiopaque material, core  112  comprises a marker for screw portion  102 . 
     In alternative embodiments, core  112  can be comprised of a radiolucent material, such as those previously discussed with regard to shaft  108 . For example, core  112  can comprise an adhesive as discussed with regard to shaft  108  that is free of fibers or that that has elongated or chopped fibers embedded therein. In these embodiments, screw portion  108  can be completely free of any radiopaque markers or, alternatively, one or more radiopaque markers can be added thereto, as discussed below. In some embodiments, core  112  is comprised of the same material as shaft  108 . In still other embodiments, core  112  can be comprised of both radiolucent and radiopaque materials. For example, small pieces of radiopaque material, such as small pieces of metal, i.e., metal particles, fibers, and/or spheres, can be embedded within or spaced between a matrix of a radiolucent material such as an epoxy. 
     In one method of manufacture, the radiolucent fibers and adhesive can be wound around a removable rod. Once shaft portion  108  is formed by the radiolucent material about the rod, the rod is removed leaving passageway  140 . Passageway  140  can then be backfilled with a radiolucent material as discussed above or a combination of radiolucent and radiopaque materials. As a result, if desired, radiopaque material can be positioned at a defined location or at select, spaced apart locations along passageway  140  to form one or more defined markers under X-ray. 
     Based on the foregoing, it is appreciated that inventive screw portion  102  can be comprised of a radiolucent shaft  108  with a radiopaque core  112 ; a radiolucent shaft  108  with a radiolucent core  112 ; and/or a radiolucent shaft  108  with a core  112  having both radiolucent and radiopaque sections. Other material combinations can also be used. In combination with each of the above three alternative designs, it is appreciated that radiopaque markers can be formed on or along the radiolucent shaft  108 . Such markers can further aid the surgeon in the implantation and positioning of screw portion  102 . 
     One example of a radiopaque marker is an encircling marker disposed within or on shaft  108  such that the marker is spaced apart or is disposed directly against core  112 . For example,  FIG. 10  shows an embodiment of a screw portion  102  in which a biocompatible positioning marker  147 A is embedded within shaft  108  between proximal end  114  and distal end  116 . In the depicted embodiment, positioning marker  147 A can comprise a ring that completely encircles passageway  140  or a partial ring that partially encircles passageway  140 . In other embodiments, positioning marker  147 A can be linear or any other desired shape. Each positioning marker  147 A can be positioned so as to be exposed on the exterior surface of shaft  108  (such as positioning marker  147 A), completely embedded within shaft  108  (such as positioning marker  147 B), positioned against core  112  (such as positioning marker  147 C), or can extend between core  112  and the exterior surface of shaft  108 . Furthermore, a positioning marker  147 D, such as in the form of a ring or other structure, can be disposed on the exterior surface  122  of shaft  108 . This can be accomplished by welding, crimping, adhering, or otherwise securing positioning marker  147 D on exterior surface  122 . Other configurations and placement of positioning markers  147  can also be used. For example, a positioning marker can form a helix that spirals in one or more partial or complete revolutions about passageway  140  or can form a linear strand that extends along the length of shaft  108 . 
     Positioning markers  147  are comprised of a radiopaque material so as to be viewable on an X-ray photograph. As such, positioning markers  147  can be comprised of the same types of radiopaque materials discussed above with regard to head  110 . During implantation and positioning of screw portion  102 , the X-ray image of positioning markers  147  can help the physician determine the position and orientation of screw portion  102 . 
     In one embodiment, a positioning marker  147  is positioned about midway between proximal end  114  and distal end  116  of shaft  108 . In other embodiments, a positioning marker  178  is positioned substantially closer to proximal end  114  or distal end  116  or at any desired location. In some embodiments, as shown in  FIG. 10 , it is appreciated that two or more positioning markers  147  can be positioned along shaft  108  at spaced apart locations. 
     Depicted in  FIG. 11  is another embodiment of a positioning marker  147 E. Positioning marker  147 E is again comprised of a radiopaque material but in this embodiment is in the form of paint or ink that is painted or printed onto exterior surface  122  of shaft  108 . Positioning marker  147 E can be used in place of or in combination with one or more additional positioning markers as discussed above. Positioning marker  147 E can form a continuous ring that encircles shaft  108  or can be any other type of configuration such as linear, semi-circular, helical configuration or the like. For example, positioning marker  147 E can be painted on a single helical revolution of threads  120 . Furthermore, a single or two or more spaced apart positioning markers  147 E can be formed along shaft  108 . 
     It is appreciated that radiopaque markers can be any desired shape and be located at any position or orientation that will produce a desired marking. For example, in other embodiments, pieces of radiopaque material can be embedded within the shaft matrix as radiopaque positioning markers. These pieces can comprise small or large particles that are placed within the shaft matrix during manufacture either randomly or in a particular pattern. Many different shapes and patterns can be used for these radiopaque positioning markers. Also, these pieces of radiopaque material can be used with or without any of the other types of positioning markers discussed above. 
     Turning to  FIG. 12 , collar  104  comprises a tubular side wall  220  having an interior surface  222  and an exterior surface  224  that each extend between a first end  226  and an opposing second end  228 . First end  226  terminates at a terminal end face  230 . Interior surface  222  bounds a longitudinal passage  232  that longitudinally extends through collar  104 . Internal threads  233  are formed on interior surface  222  at or toward first end  226 . 
     Side wall  220  is formed having a pair of channels  234  and  236  that are disposed on opposing sides of side wall  220  and that transversely extend through side wall  220 . In the embodiment depicted, channels  234  and  236  each have a substantially U-shaped configuration. Each channel  234  and  236  has an open mouth  238  that extends through end face  230  and an opposing floor  240  that is rounded. Each channel  234  and  236  is configured so that stabilizing rod  107  ( FIG. 1 ) can be received therein. In alternative embodiments, floor  240  need not be rounded but can be flat, V-shaped, or have other configurations. Each of channels  234  and  236  is also bounded by opposing side surfaces  242  and  244 . Although side surfaces  242  and  244  are shown as being in substantially parallel alignment, in alternative embodiments side surfaces  242  and  244  can be designed to diverge or converge as they project away from floor  240 . Other configurations can also be used. Channels  234  and  236  form a portion of a transverse passage that transversely extends through collar  104 , as identified by arrow  246  (see  FIG. 1 ). 
     As shown in  FIG. 12 , collar  104  further comprises a shoulder  248  that downwardly and radially inwardly projects from second end  228  of side wall  220 . Shoulder  248  terminates at an inside edge  247  that bounds an opening  249 . Opening  249  forms part of a longitudinal passage that also extends through collar  104 , as identified by arrow  232 , and that orthogonally intersects with transverse passage  246  ( FIG. 1 ). 
     Shoulder  248  has a tapered interior surface that forms an annular seat  250 . As discussed below in greater detail, bottom portion  150  of head  110  of screw portion  102  ( FIG. 3 ) rests against seat  250  so that collar  104  can pivot relative to screw portion  102 . In this regard, as depicted in  FIG. 13 , bottom portion  150  of head  110  has a maximum diameter larger than opening  249  of collar  104  so that head  110  cannot pass therethrough. It is also noted that when head  110  is received within opening  249 , top surface  160  of head  110  projects slightly above floor  240  of channels  234  and  236  of collar  104 . As a result, as discussed further below, when stabilizing rod  170  ( FIG. 1 ) is received within channels  234  and  236 , stabilizing rod  170  biases against top surface  160  of head  110  so as to wedge head  110  within opening  249  and thereby lock screw portion  102  relative to collar  104 . 
     As also depicted in  FIG. 13 , a pin hole  252  transversely extends through side wall  220  and/or shoulder  248  at second end  228  of side wall  220 . Although not required, pin hole  252  is typically disposed orthogonal to transverse passage  246 . As also discussed below in greater detail, pin hole  252  is adapted to receive a pin  254  which has a first end  256  and an opposing second end  258 . Collar  104  and pin  254  are typically comprised of a radiopaque material such as those previously discussed with regard to core  112 . In alternative embodiments, however, collar  104  and/or pin  254  can be comprised of a radiolucent material, such as those previously discussed with regard to shaft  108 . 
     Returning to  FIG. 1 , fastener  106  comprises a locking screw  270  having an encircling side wall  272  that extends between a top end face  274  and an opposing bottom end face  276 . Optionally, movably attached to bottom end face  276  of locking screw  270  is an alignment cap  278  having a substantially U-shaped channel  280  extending transversally therethrough. Channel  280  is bounded by two side surfaces  286  and  288 . Alignment cap  278  is rotatably attached to locking screw  270  so that as locking screw  270  is rotated, alignment cap  278  can remain rotationally stationary so as to bias against rod  107 . 
     Radially outwardly projecting from side wall  272  of locking screw  270  so as to encircle locking screw  270  are one or more helical threads  282 . Threads  282  of locking screw  270  are configured to threadedly engage with internal threads  233  of collar  104  ( FIG. 12 ). Recessed on top surface  274  of locking screw  270  is a polygonal socket  284  adapted to receive a driver. Accordingly, once stabilizing rod  107  is disposed within transverse passage  246  of collar  104 , locking screw  270  can be screwed into longitudinal passage  232  of collar  104  so that fastener  106  biases stabilizing rod  107  against head  110  of screw portion  102 . If alignment cap  278  is used, surfaces  286  and  288  of the U-shaped channel  280  bias against stabilizing rod  107 ; otherwise bottom end face  276  of locking screw  270  biases against stabilizing rod  107 . In this configuration, stabilizing rod  107  is secured from unwanted movement by being compressed between fastener  106  and head  110  of screw portion  102  and/or between fastener  106  and floor  240  of channels  234  and  236 . Furthermore, as stabilizing rod  107  pushes against head  110 , head  110  is wedged against seat  250  of collar  104 , thereby also locking collar  104  relative to screw portion  102 . 
     Collar  104  and fastener  106  are simply one example of a collar and fastener that can be used with screw portion  102  described herein. Other collars and associated fasteners can alternatively be used, such as the collars and fasteners described in U.S. patent application Ser. No. 11/863,133, filed Sep. 27, 2007, the entirety of which reference is incorporated herein by specific reference. 
     Methods of manufacturing and assembling the screw portion  102  and bone screw  100  will now be discussed. It is appreciated that while reference is made to screw portion  102  and its corresponding components, the methods of manufacturing and assembly given below can also be used with the other embodiments disclosed herein or otherwise encompassed by the invention. To manufacture screw portion  102 , core  112  is formed from a radiopaque material, a radiolucent material, or a combination of such materials. Examples of such materials are discussed above. Core  112  can be formed by any conventional method known in the art. 
     Shaft  108  is then formed about shaft portion  206  of core  112  to produce a blank  292 , as shown in  FIGS. 14-16 . Blank  292  can be formed in a number of ways. For example, blank  292  can be formed by winding a fiber and adhesive mixture about core  112  to produce a fiber and adhesive matrix. For example, in the embodiment depicted in  FIG. 14 , a filament winding process is used as is known in the art. In this process, filaments or fibers  294  are wound under tension over the shaft portion  206  of core  112 . Core  112  rotates while a carriage (not shown) moves back and forth along the longitudinal direction of core  112 , laying down fibers  294  in a desired pattern. Fibers  294  are coated with an adhesive as the fibers  294  are wound about core  112 . Many types of biocompatible fibers and adhesives can be used, as discussed above. If positioning marker  147  (such as marker  147 A-C in  FIG. 10 ) is used, the positioning marker  147  can be positioned in its desired location during the filament winding process so that positioning marker  147  becomes enveloped by the outer layers of fibers  294 . The marker can also be positioned before or after the winding process. The winding process continues until the diameter of the blank  292  is equal to or greater than the desired diameter of the finished shaft  108  of screw portion  102 . Blank  292  is then allowed to cure or harden. If required, blank  292  can be placed in an oven during the curing process. 
     In an alternative embodiment, blank  292  is formed using a roll wrap or table wrap process, as depicted in  FIG. 15 . In this process, one or more sheets  296  of fiber are coated with the adhesive. Many types of biocompatible fibers and adhesives can be used, as discussed above. If required, the coated sheet or sheets  296  are then allowed to partially cure. Once the desired amount of partial curing has been obtained, the sheet or sheets  296  are then wrapped about the shaft portion  206  of core  112  to produce a fiber and adhesive matrix. Again, if a positioning marker  147  ( FIG. 11 ) is used, it can be positioned in its desired location during the wrapping process so that positioning marker  147  becomes enveloped by the outer layers of sheets  296 . That is, multiple different layers can be wrapped on top of each other. The marker can also be positioned before or after the wrapping. The wrapping continues until the diameter of the blank  292  is greater than or equal to the desired diameter of the finished shaft  108  of screw portion  102 . Blank  292  is then allowed to cure in a similar manner to the filament winding process, described previously. 
     It is also appreciated that non-winding methods can also be used for forming blank  292  about core  112 . For example, compression, injection, rotational and other molding processes can be used to mold an adhesive, a fiber/adhesive mixture, or a mixture of an adhesive and other types of fillers about core  112 . In this embodiment, the fibers can be short or chopped fiber pieces that are distributed throughout the adhesive. As another alternative, shaft  108  can be formed about core  112  by a direct or indirect extrusion process, where the fiber/adhesive matrix or other adhesive matrix is extruded about core  112 . Other known methods can alternatively be used to form blank  292 . 
     As the fibers  294  or sheets  296  are only wound around shaft portion  206  of core  112 , the head portion  204  of core  112  remains open and uncovered, as shown in  FIG. 16 . To allow for a better bond between core  112  and the wound fiber and adhesive matrix, the surface of core  112  can be etched or otherwise abraded before the fibers  294  or sheets  296  are wound thereon. This can be accomplished by sand blasting, rubbing with sandpaper, chemical etching, or other known roughening process, if desired. 
     Once the blank  292  has been formed and allowed to cure, a grinder or other finishing process can be used, if desired, to smooth out or cut down any sharp edges remaining on the exterior surface  298  of the blank  292  to form the exterior surface  122  of shaft  108 . Attachment member  126  and helical threads  120  ( FIG. 4 ) are then formed on the exterior surface  298  of the blank  292  to further form shaft  108 . This can be accomplished by removing a portion of the exterior surface  298  of the blank  292  by using a grinder, lathe, or other cutting tool as is known in the art. Other methods of forming attachment member  126  and threads  120  can alternatively be used. If positioning marker  147 D or  147 E is used ( FIGS. 10 and 11 ), it is positioned or painted on the exterior surface  122  of shaft  108  after blank  292  has been processed. 
     Tapered tip  124  ( FIG. 4 ) can also be formed at the distal end of the shaft  108 , if desired. In one embodiment, tapered tip  124  is formed by removing a portion of the exterior surface  298  of the blank  292 . Any other features, such as those needed for self tapping, can also be formed if desired. 
     In an alternative method of manufacturing stabilizing screw portion  102 , shaft  108  can initially be formed by winding a radiolucent fiber/adhesive matrix about a removable core. In contrast to prior embodiments, however, removable core is then slid out of shaft  108 . The remaining passageway  140  can then be backfilled by injecting a radiolucent material, such as an epoxy or other adhesive, or a combined radiolucent and radiopaque material into passageway  140 . Alternatively, a radiolucent core can be slid into the passageway and secured in place by an adhesive or other method of securing. As a result, the entire shaft and core are radiolucent. Again, any number or type of radiopaque positioning marker can be used. 
     Turning to  FIG. 17 , once attachment member  126  and threads  120  have been formed on the shaft  108 , head  110 , which has been previously formed, is then attached to the threaded shaft  108 . To do this, bottom surface  156  of head  110  is positioned adjacent head portion  204  of core  112  so that second passageway  182  of head  110  aligns with core  112 . Head  110  is then advanced toward shaft  108  so that head portion  204  of core  112  is received within second passageway  182 . Head  110  is further advanced along core  112  until attachment member  126  is received within attachment recess  128 . Head  110  is then rigidly secured to core  112  and to shaft  108  by a securing method known in the art, such as by adhesive, laser welding, and/or other known method. For example, in addition to using an adhesive between head  110  and shaft  108  and between head  110  and core  112 , if desired, the exposed end of core  112  can be directly welded to head  110 . Any portion of core  112  that extends out of second passageway  182  and past top surface  160  of head  110  can be cut off, if desired. 
     In an alternative method of manufacturing screw portion  102 , after core  112  has been formed, blank  292  is configured so that both head portion  204  and shaft portion  206  can be formed therefrom. Specifically, depicted in  FIG. 18 , screw portion  102  is shown as being comprised of a body  290  and core  112  that is positioned therein. Body  290  comprises shaft  108  and head  110 . However, in contrast to the prior embodiment where head  110  is attached to shaft  108 , in this embodiment shaft  108  and head  110  are integrally formed as a single, unitary structure. That is, both shaft  108  and head  110  are milled, cut or otherwise formed from a single blank that is formed about core  112 . As such, in this embodiment the entire body  290  is comprised of a radiolucent material, such as those previously discussed with regard to shaft  108 , while core  112  is typically comprised of a radiopaque material but can also be comprised of a radiolucent material or combination. As with other embodiments, the positioning markers  147  ( FIGS. 10 and 11 ) can also be used with body  290 . 
     In one similar method of manufacture, body  290  can initially be formed by winding a radiolucent fiber/adhesive matrix about a removable core as discussed above. The removable core can then be slid out of body  290 . The remaining passageway can then be backfilled by injecting a radiolucent material such as an epoxy or other adhesive within the passageway. Alternatively, a radiolucent or radiopaque core can be slid into the passageway and secured in place by an adhesive, welding or other method of securing. As a result, the entire body and core can be radiolucent. Again, to help facilitate placement, positioning marks  147  ( FIGS. 10 and 11 ) can be used with the radiolucent body. 
     Once screw portion  102  has been manufactured and assembled as described above, the polyaxial bone screw  100  can be assembled with screw portion  102  as one of its components. For example, turning to  FIG. 13 , to assemble polyaxial bone screw  100 , shaft  108  of assembled screw portion  102  is passed down through longitudinal passage  232  and opening  249  of collar  104 . Head  110  of screw portion  102 , however, has a maximum diameter that is greater than the minimum diameter of opening  249  extending through seat  250  of collar  104 . As such, head  110  of screw portion  102  rests on seat  250  of collar  104  and is prevented from passing through opening  249 . As a result of the rounded configuration of bottom portion  150  of head  110  and the tapered sloping of seat  150 , head  110  can freely slide on seat  250  such that screw portion  102  and collar  104  can freely pivot relative to each other. 
     Once screw portion  102  is seated within collar  104 , pin  254  is advanced into pin hole  252 . First end  256  of pin  254  is secured within pin hole  252  such as by welding, adhesive, press fit, or other mechanical engagement, such as threaded engagement. In this position, second end  258  of pin  254  projects into engagement slot  164  of screw portion  102 . It is noted that pin  254  is spaced apart above floor  170  of engagement slot  164 . As a result, screw portion  102  and collar  104  can continue to freely pivot relative to each other. However, because pin  254  extends over floor  170 , head  110  is prevented from passing back up through collar  104 . Pin  254  also functions to couple screw portion  102  and collar  104  together so that rotation of collar  104  or screw portion  102  also facilitates rotation of the other of the collar  104  or screw portion  102 . As such, screw portion  102  can be implanted or removed simply by rotating collar  104 . In alternative embodiments, it is appreciated that pin  62  can come in a variety of different configurations and can be mounted at a variety of different orientations and locations. Pin  62  can also be comprised of a radiolucent or radiopaque material. 
     In an alternative embodiment, head  110  is mounted on the collar  104  using pin  254 , as described above, before head  110  is attached and secured to core  112  and shaft  108 . 
     Depicted in  FIG. 19  is an alternative embodiment of a screw portion  350  incorporating features of the present invention that can be used with polyaxial bone screw  100 . Like elements between screw portion  350  and other screw portions described herein are identified by like reference characters. 
     As depicted in  FIG. 20  and similar to screw portion  102 , screw portion  350  comprises an elongated shaft  352  having a head  354  disposed thereon with a core  356  extending longitudinally through shaft  352  and head  354 . 
     Screw portion  350  is similar to screw portion  102  except for the attachment structure between shaft  352  and head  354 . For example, instead of attachment member  126  of shaft  108  having a flat  136  and projecting from an end face  132  of shaft body  113 , attachment member  358  of shaft  352  is simply an extension of shaft body  113  having the same diameter as shaft body  113 . That is, attachment member  358  projects from shaft body  113  in such a manner that no end face  132  is formed. In other words, attachment member  358  has an encircling exterior surface  360  that is aligned with exterior surface  122  of shaft body  113  at proximal end  114 . Exterior surface  360  extends to terminal end face  134 . 
     Correspondingly, head  354  is similar to head  110  except that head  354  further comprises a shoulder  362  extending from the outer perimeter of bottom surface  156 . As shown in  FIG. 21  in conjunction with  FIG. 20 , shoulder  362  comprises an encircling perimeter wall  364  having an inner surface  366  and an opposing outer surface  368  extending from bottom surface  156  to a terminal end face  370 . Inner surface  366  of perimeter wall  364  bounds an attachment recess  372  that is sized and shaped so as to snugly fit over attachment member  358 . As such, attachment recess  372  is substantially cylindrical in shape in the depicted embodiment, having a mouth  373  defined by terminal end face  370 . Because of attachment recess  364 , no attachment recess is necessary within bottom surface  156 , although attachment recess  364  could also extend into bottom surface  156  if so desired. 
     Because of the size and shape of attachment member  358  and attachment recess  372 , the amount of surface area that can be used for bonding the two together is increased over other embodiments. This can allow for a stronger bond that can withstand more torque. 
     Similar to head  110 , head  354  also includes a second passageway  374 . Second passageway  374  is similar to second passageway  182  except second passageway  374  has a substantially circular cross-sectional shape without a straight portion  184 . 
     Core  356  is similar to core  112 , except that head portion  204  remains substantially circular in cross section to match the shape of second passageway  364 . However, any of the cores described herein or contemplated by the invention can be used with screw portion  350 , and first and second passageways will reflect this. Furthermore, flats or other surface structures can be formed on attachment member  358  and inner surface  366  of head  354 . 
     As with screw portion  102 , shaft  352 , head  354 , and core  356  can respectively be comprised of the same materials as discussed above regarding shaft  108 , head  110 , and core  112 . Also, screw portion  350  can be manufactured and assembled similar to that described above with regard to screw portion  102 . One small difference from assembled screw portion  102  is that, as shown in  FIG. 19 , when screw portion  350  is assembled, shoulder  362  extends slightly further away from the longitudinal axis  118  then exterior surface  122  as extension  362  fits over attachment member  358 . 
     Furthermore, it is appreciated that many of the alternative design features as previously discussed with regard to screw portion  102  are also applicable to screw portion  350 . For example, to aid in the implantation of screw portion  350 , positioning markers  147  ( FIGS. 10 and 11 ), as previously discussed, can again be formed on or within shaft  352 . Likewise, as with screw portion  102 , by forming shaft  352  out of a radiolucent material while core  356  is formed from a radiopaque material, screw portion  350  can be properly positioned while limiting unwanted obstructions. Specifically, the thin core  356  can be easily viewed by X-ray to determine proper positioning of the screw portion  350  but the larger shaft  352  is radiolucent so as to not obstruct surrounding structure. 
     To increase the bonding strength and ability to transfer torque, one or more keyed splines and corresponding grooves can be disposed within attachment recess  372  and attachment member  358 . For example,  FIG. 22  shows a spline  378  projecting into attachment recess  372  from the inner surface  366  of perimeter wall  364 . Spline  378  comprises a sidewall  380  that extends longitudinally from a first end  382  disposed at or near bottom surface  156  to a spaced apart second end  384  disposed at or near the mouth  373  of the attachment recess  372 . 
     Turning to  FIG. 23 , a corresponding groove  386  is formed in exterior surface  360  of attachment member  358 . Groove  386  is bounded by a sidewall  388  that extends longitudinally from terminal end face  134  to a spaced apart end wall  390 . Groove  386  is sized and shaped so as to snugly receive spline  380  when attachment member  358  is received within attachment recess  372 . As shown in the depicted embodiment, spline  380  is substantially parallel to longitudinal axis  162  of head  354  and groove  386  is substantially parallel to longitudinal axis  118  of shaft  352  so as to be aligned when assembled. Other matching shapes can alternatively be used. For example, spline  380  and groove  386  can be helical in nature, if desired. In that case, head  354  would be screwed onto shaft  352  during assembly. Other mating shapes are also possible. For example, attachment member  358  can be formed with one more flats or can be formed into a polygonal, oval, irregular or other non-circular shape. Attachment recess  372  would have a complementary configuration. 
     It is appreciated that more than one spline and groove can be used in the present invention. For example, in  FIG. 24 , four grooves  386   a - d  are formed in exterior surface  360  of attachment member  358 . Although not shown, it is appreciated that a head  354  incorporating four splines  378  that mate with grooves  386   a - d  would correspondingly be used. In the depicted embodiment, the grooves  386   a - d  are similar to each other and equidistant from each other, although this is not necessary. Grooves  386  and splines  378  can alternatively be spaced with respect to each other so as to form a sort of key. In this manner heads  354  will only attach to certain shafts  352  in a particular orientation depending on the keyed fit. Alternatively, one or more of the grooves  386  can be shaped differently than the other grooves so as to also form a key. Of course, head  354  will incorporate splines  378  that match the keyed grooves  386 , so as to attach to shaft  352  in the particular orientation. 
     It is appreciated that more or less splines and grooves can be used with the present invention. For example, screw portion  350  can comprise two or three or more splines and grooves. 
     Depicted in  FIG. 25  is another alternative embodiment of a screw portion  420  incorporating features of the present invention that can be used with polyaxial bone screw  100 . Like elements between screw portion  420  and other screw portions described herein are identified by like reference characters. 
     Screw portion  420  is similar to screw portion  350  ( FIG. 20 ) except that the shoulder  362  of head  354  includes an extension of the threads  120  that are formed on the exterior surface  122  of shaft body  113 . 
     As shown in  FIG. 26 , attachment member  358  of shaft  352  is sized so as to have a smaller diameter than the exterior surface  122  of shaft body  113 . As a result, similar to end face  132 , an end face  422  is formed on shaft body  113  between the exterior surface  360  of attachment member  358  and exterior surface  122  of shaft body  113 . End face  422  is generally planar and orthogonal to longitudinal axis  118  of shaft  352 , but this is not required. Attachment member  358  centrally projects from this end face  422  to terminal end face  134 . Because of the smaller diameter of attachment member  358 , shoulder  362  can correspondingly have a smaller diameter. Again, if desired one or more flats, grooves, splines, threads, or other structures can be formed on attachment member  358  with a complementary structure being formed on head  354 . 
     As shown in  FIG. 27 , the end face  370  of shoulder  362  of head  354  is shaped so as to match the shape of end face  422  and inner surface  366  is dimensioned with a smaller diameter so as to snugly receive the smaller diameter attachment member  358 . Due to the smaller dimensions, when assembled the outer surface  368  of shoulder  362  and the exterior surface  122  of shaft body  113  are aligned, as shown in  FIG. 25 . Also as shown in  FIG. 25 , one or more threads  424  helically encircle and radially outwardly project from outer surface  368  of shoulder  362 . The threads  424  are configured to align with the threads  120  of shaft body  113  so that as the screw portion  420  is threaded into the bone, the threads  424  will also engage the bone. 
     Because the threads extend onto the shoulder  362 , the shaft body  113  can be shorter so that the attachment member  358  and the shoulder  362  can be longer than in screw portion  350 , thereby providing even more surface area for bonding between the head  354  and shaft  352 . This results in a stronger bond. Furthermore, the threads  424  on the shoulder  362  cause a better bone-to-screw connection when threads  424  are threaded into cortical bone. 
     Depicted in  FIG. 28  is one embodiment of a fixed bone screw  300  incorporating features of the present invention. In general, fixed bone screw  300  comprises a collar rigidly secured to or formed on the end of a threaded shaft so that the collar cannot pivot relative to the shaft. Like elements between bone screw  300  and the prior discussed embodiments are identified by like reference characters. 
     As depicted in  FIG. 29 , in one embodiment bone screw  300  comprises shaft  108 , core  112 , and a collar  302 . Core  112  is secured within first passageway  140  of shaft  108 . The previously discussed materials, configurations, methods of manufacture and alternatives thereof of shaft  109  and core  112  are also applicable to bone screw  300 . 
     As depicted in  FIGS. 29 and 30 , collar  302  comprises a base  304  that extends from a first end  306  to a floor  308 . Base  304  has an interior surface  309  that bounds an attachment recess  310  extending from floor  308  to a first end face  311  at first end  306 . Attachment recess  310  thus has the configuration of a blind socket. Interior surface  309  has a substantially circular transverse cross section with a flat  314  formed thereon. Attachment recess  310  has a configuration complementarily to and is configured to receive and secure to attachment member  126  of shaft  108  in the same manner that attachment member  126  is received and secured within attachment recess  128  of head  110  ( FIG. 6 ). 
     Floor  308  also has an interior surface  316  that bounds a second passageway  312  that extends through floor  308  so as to communicate with attachment recess  310 . Interior surface  316  also has a substantially circular transverse cross section with a flat  318  formed thereon. 
     Second passageway  312  is positioned so that when attachment member  126  is secured within attachment recess  310 , first passageway  140  of shaft  108  is aligned with second passageway  312 . It is also appreciated that second passageway  312  is also configured to receive and secure to head portion  204  of core  112  in the same manner that head portion  204  is received and secured within second passageway  182  of head  110  ( FIG. 6 ). 
     A pair of spaced apart arms  320  and  321  project from opposing sides of base  304  in substantially parallel alignment. Each arm  320  and  321  has an interior surface  322 . The opposing interior surfaces bound a substantially U-shaped channel  323  in which stabilizing rod  107  ( FIG. 1 ) can be received. Furthermore, each interior surface  322  has a thread portion  324  formed thereon. Thread portions  324  enable locking screw  270  ( FIG. 1 ) or an alternative embodiment thereof to threadedly engage with arms  320  and  321  so as to secure stabilizing rod  107  within channel  323 . It is appreciated that many of the alternative design features as previously discussed with regard to collar  104  are also applicable to collar  302 . Likewise, collar  302  can be comprised of the same materials as previously discussed with regard to collar  104 . 
     To aid in the implantation of bone screw  300 , positioning markers  147  ( FIGS. 10 and 11 ), as previously discussed, can again be formed on or within shaft  108 . Likewise, as with screw portion  102 , by forming shaft  108  out of a radiolucent material while core  112  and collar  302  are formed from a radiopaque material, bone screw  300  can be properly positioned while limiting unwanted obstructions. Specifically, the thin core  112  can be easily viewed by X-ray to determine proper positioning of the bone screw but the larger shaft  108  is radiolucent so as to not obstruct surrounding structure. 
     Depicted in  FIG. 31  is an alternative embodiment of bone screw  300  incorporating features of the present invention wherein like element are identified by like reference characters. In this embodiment, bone screw  300  is shown as being comprised of a body  330  and core  112  that is positioned therein. Body  330  comprises shaft  108  and collar  302 . However, in contrast to the prior embodiment where collar  302  is secured to shaft  108 , in this embodiment shaft  108  and collar  302  are integrally formed as a single unitary structure. That is, both shaft  108  and collar  302  are milled, cut or otherwise formed from a single blank that is formed about core  112 . As such, in this embodiment the entire body  330  is comprised of a radiolucent material, such as those previously discussed with regard to shaft  108 , while core  112  is typically comprised of a radiopaque material but can also be comprised of a radiolucent material. As with other embodiments, one or more positioning markers  147  ( FIGS. 10 and 11 ) can also be used with body  330 . Furthermore, as discussed in prior embodiments, core  112  can be removed and replaced with an adhesive or an alternative core. 
     Depicted in  FIG. 32  is another alternative embodiment of a spinal stabilizing system  450  wherein like elements are identified by like reference characters. Stabilizing system  450  includes a polyaxial bone screw  452  comprising an elongated screw portion  454 , a collar  456  pivotally mounted thereon and a saddle  458  that is disposed within collar  456 . Stabilizing system  450  also includes a fastener  460  that is selectively engageable with collar  456  to secure polyaxial bone screw  452  to stabilizing rod  107 . 
     As depicted in  FIG. 33 , screw portion  454  of bone screw  452  comprises a shaft  462 , an elongated core  464  that extends within shaft  462 , and a head  466  formed on an end of core  464 . Shaft  462  is substantially identical to shaft body  113  discussed with regard to  FIGS. 3 and 4  and thus like reference characters reference like elements. Shaft  462  can be made from the same radiolucent materials and with the same methods and alternatives as discussed with regard to shaft body  113 . Furthermore, the various markers as discussed herein can be used in association with shaft  462 . 
     In contrast to the embodiment shown in  FIG. 2  wherein head  110  and core  112  are formed as separate discrete elements, in the present embodiment head  466  and core are integrally formed as a single, unitary structure. In other embodiments, head  466  and core  464  can be rigidly fixed together such as by welding, press fit, or other connection techniques. Core  464  has an exterior surface  468  extending between a proximal end  469  and an opposing distal end  470 . Core  464  is secured within passageway  140  of shaft  462  using the methods previously discussed. In this embodiment, however, a helical thread  472  is formed on exterior surface  468  and extends along a length of core  464 . Helical thread  472  has a thread orientation opposite that of thread  120  on shaft  462 . As a result, core  464  further engages shaft  462  when bone screw  452  is being backed out of a bone, thereby helping to prevent separation between core  464  and shaft  462 . In alternative embodiments, core  464  can have the other shapes and/or protrusions as discussed with regard to the other cores herein. 
     Head  466  comprises an annular shoulder  474  that extends between a flat bottom end face  475  and a recessed annular neck  476 . Core  464  extends from bottom end face  475 . Upwardly and outwardly extending from neck  476  is an annular, rounded head portion  477  that terminates a flat top face  478 . If desired, texture, such as micro grooves or other patterns can be formed on the exterior surface of head portion  477  to facilitate locking between head  466  and collar  456  as discussed below in greater detail. As depicted in  FIG. 34 , an engagement socket  480  is recessed on top face  478 . Engagement socket  480  is bounded by an encircling sidewall and typically has a polygonal or other non-circular transverse cross section so that a driver can engage with socket  480  for rotating bone screw  452 . As also shown in  FIG. 34 , core  464  and head  466  each have an interior surface  482  that bounds a cannula  483  extending from engagement socket  480  through distal end  470  of core  464 . Again, cannula  483  can be used to receive a guide wire for implanting bone screw  452  and/or can be used for other surgical techniques. In other embodiments, cannula  483  can be eliminated. Head  466  and core  464  can be made from the same radiopaque materials, such as radiopaque metals, as discussed with regard to the other heads and cores disclosed herein. During manufacture, shaft  462  is formed on or otherwise secured to core  464  so that shaft  462  is disposed against bottom face  475  of head  466 . 
     Returning to  FIG. 32 , collar  456  comprises a tubular side wall  522  having an interior surface  524  and an exterior surface  526  that each extend between a first end  528  and an opposing second end  530 . Interior surface  524  bounds a longitudinal passage  532  that longitudinally extends through collar  456 . Internal threads  534  are formed on interior surface  524  at or toward first end  528 . 
     Side wall  522  is formed having a pair of channels  536  and  538  that are disposed on opposing sides of side wall  522  and that transversely extend through side wall  522 . In the embodiment depicted, channels  536  and  538  each have a substantially U-shaped configuration. Other channel shapes can also be used. Channels  536  and  538  form a portion of a transverse passage that transversely extends through collar  456  so as to intersect with the longitudinal passage  532  that also extends through collar  456 . Each channel  536  and  538  is configured so that stabilizing rod  107  can be received therein as stabilizing rod  107  is placed within the transverse passage. 
     As depicted in  FIG. 37 , collar  456  further comprises a shoulder  541  that radially inwardly projects from second end  530  of side wall  522  so as to encircle longitudinal passage  532 . Shoulder  541  has a tapered interior surface that forms an annular seat  542 . In alternative embodiments, seat  542  need not completely encircle passage  532 . Seat  542  can also comprise two or more spaced apart portions. 
     During assembly of bone screw  452 , shaft  462  is passed down through longitudinal passage  532  of collar  456 . Head  466 , however, has a maximum diameter that is greater than the minimum diameter of longitudinal passage  132  extending through seat  542  of collar  456 . As such, head  466  rests on seat  542  of collar  456  and is prevented from passing through collar  456  as shown in  FIG. 37 . As a result of the spherical configuration of head  466  and the tapered sloping of seat  542 , head  466  can freely slide on seat  542  such that shaft  462  and collar  456  can freely pivot relative to each other. 
     As shown in  FIG. 32 , fastener  460  can be used to secure stabilizing rod  107  to bone screw  452 . Fastener  460  comprises a locking screw  600  having an encircling side wall  602  that extends between a top end face  604  and an opposing bottom end face  606 . Radially outwardly projecting from side wall  602  of locking screw  600  so as to encircle locking screw  600  are one or more helical threads  608 . Threads  608  of locking screw  600  are configured to threadedly engage with internal threads  534  of collar  456 . A socket  610  or other type of engaging member or recess adapted to receive a driver can be disposed on top surface  604  of locking screw  600 . 
     Fastener  460  is threaded into threads  534  formed on interior surface  524  of collar  456  to secure stabilizing rod  107  to bone screw  452  within channels  536  and  538  of collar  456 . That is, once stabilizing rod  107  is disposed within the transverse passage of collar  456 , locking screw  600  is screwed into collar  456  so that bottom end face  606  of locking screw  200  presses against stabilizing rod  107 , which in turn causes stabilizing rod  107  to press against head  466 . As a result, head  466  is pressed within seat  542  of collar  456  which locks screw portion  454  relative to collar  456 . 
     Although not required, saddle  458  can be used to provide a seat for stabilizing rod  107  so as to reduce localized stress points. More specifically, without saddle  458 , stabilizing rod  107  sits directly over engagement socket  480  on head  466  ( FIG. 34 ). The perimeter edge of engagement socket  480  produces localized stress points on stabilizing rod  107  which can damage stabilizing rod  107  and/or distort engagement socket  480 . Saddle  458  separates stabilizing rod  107  from the perimeter edge of engagement socket  480  and more uniformly distributes the clamping forces around stabilizing rod  107 . For example, as depicted in  FIG. 37 , saddle  458  can be positioned between head  466  and stabilizing rod  107  such that when fastener  460  is threaded into collar  456 , stabilizing rod  107  presses against saddle  458 , which in turn presses against head  466 . Again, as a result, head  466  is pressed within seat  542  of collar  456  which locks screw portion  454  relative to collar  456 . 
     To be able to retain saddle  458  within passage  532  in a particular positioning arrangement, collar  456  can also include one or more channels or lips. For example, the embodiment depicted in  FIG. 32  includes collar  456  having a channel  548  formed on interior surface  524 . Channel  548  is generally aligned with longitudinal passage  532  and is designed to receive a key formed on saddle  458 , as discussed in more detail below. Furthermore, collar  456  can also includes an inwardly projecting annular lip  618  formed on interior surface  524  that at least partially encircles longitudinal passage  532 . Lip  618  is sized so as to have a slightly smaller diameter than the general diameter of interior surface  524 . 
     Turning to  FIGS. 35A and 35B , saddle  458  has a top surface  622  and an opposing bottom surface  624  with an encircling outer side wall  626  extending therebetween. An internal side wall  628  also extends between top and bottom surfaces  622  and  624  so as to bound a central opening  630  that extends all the way through saddle  458 . Opening  630  is generally circular and is sized so as to allow a driver tool to access the socket  480  on the head  466  ( FIG. 37 ) when saddle  458  is disposed against head  466 . The opening  630  causes saddle  458  to be generally ring shaped when viewed from a direction generally normal to the top and bottom surfaces  622  and  624 . 
     A substantially U-shaped channel  632  is formed on top surface  622  that extends transversally through saddle  458  so as to intersect the opening  630 . Channel  632  is bounded by a curved side surface  634  sized so as to snugly receive stabilizing rod  107 . As discussed in more detail below, when locking screw  600  is screwed into collar  456  (see  FIG. 37 ), surface  634  of the U-shaped channel  632  presses against stabilizing rod  107 . Although depicted as being substantially smooth, the channel surface  634  can be textured for improved gripping. Examples of the types of texture that can be used on channel surface  634  include: ribs, grooves, a waffle pattern, and an abrasive pattern. Other types of textures can also be used. See, e.g., the waffle-like texture shown in  FIG. 38 . 
     A generally concave cavity  636  is formed on bottom surface  624  of saddle  458  so as to encircle opening  630 . Cavity  636  is bounded by an annular curved side surface  638  sized so as to snugly receive head  466  (see  FIG. 37 ). As such, when locking screw  600  is screwed into collar  456 , side surface  638  presses against head  466 . As noted above, however, opening  630  in saddle  458  still allows access to socket  480  of head  466  when saddle  458  presses against head  466 . 
     Saddle  458  has an outside diameter that is generally the same as the inner diameter of longitudinal passage  532  extending through collar  456 . In some embodiments, a slit is formed in saddle  458  to allow saddle  458  to be able to be flexed for insertion into collar  456 . For example, as shown in the depicted embodiment, a slit  640  is formed in saddle  458  that extends all the way between top and bottom surfaces  622  and  624  and between outer and internal side walls  626  and  628 . 
     Slit  240  is bounded by side surfaces  642  and  644  that face each other across the slit  640 . The slit  640  causes the saddle  620  to be generally “C” shaped, with the slit  640  being the mouth of the “C.” As a result of the slit  640 , the portions of saddle  458  on either side of slit  640  can be flexed toward each other, causing the diameter of saddle  458  to slightly decrease, thereby allowing saddle  458  to be inserted into longitudinal passage  132  of collar  456  and past lip  618  during assembly (see  FIG. 37 ). Once positioned therein, the saddle  458  resiliently springs back to its original diameter and is retained within the passage  532  by the lip  618 , which has a diameter that is slightly less then that of saddle  458 . 
     To help keep saddle  458  oriented in a desired position within collar  456 , a key  646  is also positioned thereon. Key  646  comprises a spline projecting out from outer side wall  626  and extending generally orthogonally between top and bottom surfaces  622  and  624 . In the depicted embodiment, key  646  is positioned on the opposite side of saddle  458  as slit  640 , although this is not required; key  646  can be positioned anywhere along the outer side wall  626 . As noted above, key  646  is designed to fit within corresponding channel  648  formed on interior surface  524  of collar  456  (see  FIG. 32 ). Other types of keys can alternatively be used, or, if desired, saddle  458  can be formed without a key. In some alternative embodiments the key  646  outwardly projects from the interior surface  524  of collar  456  and the corresponding channel  648  is formed on the outer side wall  626  of saddle  458 . Saddle  458  is typically comprised of a radiopaque material such as those previously discussed with regard to head  110 . However, other high strength, biocompatible materials can also be used. 
     Returning to  FIG. 32 , fastener  460  can also include an alignment cap  650  movably attached to bottom end face  606  of locking screw  600  to further distribute the clamping forces around stabilizing rod  107 . More specifically, as shown in  FIG. 36 , alignment cap  650  has a generally planar, circular top surface  652  with an encircling perimeter sidewall  654  extending downward therefrom. A post  656  extends upward from the center of top surface  652 . Post  656  is designed to fit within a corresponding hole  658  on bottom end face  606  of locking screw  600 . Alternatively, post  656  can be positioned on locking screw  600  and hole  658  can be formed on alignment cap  650   
     Similar to saddle  458 , alignment cap  650  has a substantially U-shaped channel  662  extending transversally therethrough. Channel  662  is bounded by a curved side surface  664  sized so as to snugly receive stabilizing rod  107 . Alignment cap  650  is rotatably attached to locking screw  600  by inserting post  656  into hole  658  so that as locking screw  600  is rotated, alignment cap  650  can remain rotationally stationary so as to press against stabilizing rod  107 . Once inserted through hole  658 , the end of post  656  can be splayed or otherwise spread apart so as to prevent the post  656  from being pulled back through hole  658 , while still allowing locking screw  600  to rotate with respect to alignment cap  650 . When locking screw  600  is screwed into collar  456 , surface  664  of the U-shaped channel  662  presses against stabilizing rod  107 . Similar to channel surface  634  of saddle  458 , the channel surface  664  of alignment cap  650  can be substantially smooth or textured for improved gripping. Examples of some of the types of textures that can be used on channel surface  664  are as listed above regarding saddle  458 . 
     Alignment cap  650  can be comprised of the same type of materials discussed above regarding saddle  458 . Furthermore, alignment cap  650  can be comprised of the same material as saddle  458  or a different material. 
       FIG. 37  shows how the saddle  458  and alignment cap  650  combine to secure stabilizing rod  107  within collar  456 . As discussed above, when locking screw  600  is screwed into collar  456  while stabilizing rod  107  is disposed within channels  536  and  538  ( FIG. 32 ), surface  664  of alignment cap  650  presses against stabilizing rod  107 . This pressure causes stabilizing rod  107  to, in turn, press against surface  634  of saddle  458 , which causes surface  638  of saddle  458  to press against head  466 . As a result, stabilizing rod  107  is rigidly attached to bone screw  452  while the clamping forces are distributed around stabilizing rod  107  by saddle  458  and alignment cap  650 . 
     As can be appreciated, saddle  458  and alignment cap  650  can be used together, as shown in  FIG. 37  or separately. That is, saddle  458  and alignment cap  650  are not reliant on each other and thus can be used with or without the other, as desired. Furthermore, the surfaces  634 ,  654 , and  656  of channels  632  and  648  can be textured the same or have different textures from each other. Alternatively, a texture may be used on only one or more of the surfaces or, of course, all of the surfaces can be free of any texturing. 
       FIG. 38  shows an alternative embodiment of a saddle  670  that can be used in the current invention. Saddle  670  is similar to saddle  458 , except that there is no opening or slit extending through the saddle. Instead, a closed end cavity  636  is formed on bottom surface  624  that is configured to receive head  466  ( FIG. 37 ). Saddle  670  also includes a waffle-like texture  672  on side surface  634 . Of course, as discussed above, other types of textures can also be used. 
     Depicted in  FIG. 39  is another alternative embodiment of a screw portion  680  that can be used as part of a polyaxial bone screw. Screw portion  680  is similar to screw portion  102  shown in  FIG. 2  and thus like elements are identified by like reference characters. Screw portion  680  includes shaft  108  and head  110  as previously discussed. However, in contrast to core  112 , screw portion  680  includes a core  682 . Core  682  comprises an elongated solid, inner core  684  in the form of a pin. Inner core  684  can have substantially the same configuration as core  112  previously discussed but can be made of a radiolucent material, such as those previously discussed with regard to shaft  108  or can be made of a radiopaque material, such as those previously discussed with regard to head  110 . 
     Core  682  also includes an outer core that extends over at least a portion of inner core  684 . In one embodiment an elongated, tubular outer core  686 A is provided. Outer core  686 A is comprised of a metal wire or ribbon that is coiled into the tubular configuration so as to bound a passage  688 A longitudinally extending therethrough. The material for outer core  686 A is selected so that outer core  686 A is resiliently flexible like a coiled spring. For example, in one embodiment the wire or ribbon of outer core  686 A is comprised of Nitinol and is heat treated when in the coiled configuration so that it obtains a coiled memory. Other metals can also be used. The wire or ribbon can be coiled directly around inner core  684  or can be separately coiled and then placed over inner core  684 . Alternatively, an elongated, tubular outer core  686 B can positioned over inner core  684 . Outer core  686 B comprises a solid tubular sleeve that bound a passage  688 B longitudinally extending therethrough. Outer cores  686 A and B can be comprised of a radiolucent material such as the radiolucent metals previously discussed with regard to head  110 . It is appreciated that inner core  684  can be fabricated and then the outer core secured thereto. Alternatively, the outer core can first be formed and then inner core  684  can be formed by back filling, such as by injection, a material into the passage extending through inner core  684 . 
     Outer cores  686 A and B can be secured to inner core  684  by an epoxy, other adhesives or by other fastening techniques. Outer cores  686 A and B can cover all or substantially all of inner core  684  so that the outer core is received within and is secured to head  110 . Alternatively, the outer core can be sized to cover only a portion inner core  684 . For example, the outer core can be sized to cover the portion of the inner core  684  within shaft  108  but not cover the portion of inner core  684  within head  110 . To that end, the outer core can cover not more than 75% and more commonly not more than 85% of the length of inner core  684 . It is appreciated that the physical properties of the bone screw can be adjusted by forming the core from different materials and elements. 
     Depicted in  FIG. 40  is another alternative embodiment of a screw portion  690  that can be used as part of a polyaxial bone screw. Screw portion  690  is similar to screw portion  680  except that inner core  682  has been eliminated. Thus, screw portion  690  comprises a core which is either outer core  686 A or outer core  686 B as discussed above. The cores are secured to shaft  108  and head  110  in the same manner that core  112  is secured to these elements as previously discussed. By having the core formed from a coiled and/or tubular member, flexible properties of the bone screw can be adjusted. 
     The bone screws previously disclosed herein have primarily been designed as polyaxial or fixed bone screws for use with spinal stabilization systems. It is appreciated, however, that the bone screws of the present invention need not be designed as a polyaxial or fixed bone screw for use with spinal stabilization systems but can be configured like any number of conventional bone screws that are used for applications such as securing bone plates over a facture, attaching cranial plates, securing joint or other implants to bone, fixing ligaments and other soft tissue to bone, and the like. 
     By way of example and not by limitation, depicted in  FIG. 41  is an exploded bone screw  700  incorporating features of the present invention wherein like elements are identified by like reference characters. Bone screw  700  comprises shaft  108 , core  112  and a head  702 . Head  702  is configured similar to a conventional screw head. Specifically, head  702  has a side wall  703  that extends between a proximal end  704  and an opposing distal end  706 . Distal end  706  terminates at a bottom end face on which attachment recess  128  ( FIG. 6 ) is formed. Attachment recess  128  permits head  702  to engage with attachment member  126  of shaft  108  in the manner previously discussed. 
     Side wall  703  flares outwardly as it extends from distal end  706  to proximal end  704 . Proximal end  706  terminates at a substantially flat top end face  708 . In one embodiment of the present invention, means are provided for engaging a driver to the inventive bone screws. The drivers can then be used for rotating the bone screws for implanting of the bone screws. By way of example and not by limitation, an engagement socket  710  is formed on top end face  708 . Engagement socket  710  can be of any desired configuration such as polygonal, irregular or other non-circular configuration that permits a driver to engage engagement socket  710  for rotating bone screw  700 . Engagement socket  710  can also be in the form of one or more slots such as are commonly used for engaging a driver such as a screw driver. In other embodiments, the means for engaging a driver can comprise top portion  152  as shown in  FIG. 3  or other forms of projections to which a driver having a complementary socket can engage. Other locking structures commonly used for engaging a driver can also be used. It is appreciated that each of the different bone screws as disclosed herein can include such means for engaging a driver. 
     Second passageway  183  ( FIG. 6 ) can be formed on the floor of attachment recess  128  and extend to or toward engagement socket  710 . Second passageway  183  permits head  702  to engage with core  112  in the manner previously discussed. It is appreciated that the alternative materials, methods of manufacture, use of markers and other alternatives as previously discussed with regard to the shaft, core and head of screw portion  102  are also applicable to the shaft, core and head of bone screw  700 . 
     It is appreciated that head  702  can have a variety of different configurations and that it can be integrally formed with the core. By way of example, depicted in  FIG. 42  is an exploded view of a bone screw  720  incorporating features of the present invention wherein like elements are identified by like reference characters. Similar to screw portion  454  shown in  FIG. 33 , bone screw  700  comprises shaft  462 , core  464  and a head  722 . Core  464  and head  722  are integrally formed as a single, unitary structure or can be rigidly fixed together such as by welding, press fit or other securing techniques. Head  722  comprises a cylindrical stem  724  that terminates at a bottom end face  726 . Bottom end face  726  is designed to position against top end face  132  of shaft  462 . Formed on the opposing end of stem  724  is a substantially semi-spherical head portion  728 . Head portion  728  has a flat bottom surface that radially outwardly projects from stem  724  and has a top crown on which engagement socket  710  is formed. Again, engagement socket  710  can also be in the form of one or more slots for engaging a driver such as a screw driver. It is appreciated that the alternative materials, methods of manufacture, use of markers and other alternatives as previously discussed with regard to the shaft, core and head of screw portion  454  are also applicable to the shaft, core and head of bone screw  720 . 
     In many of the foregoing embodiments, it is discussed that the core can be comprised of a radiopaque material while the shaft is comprised of a radiolucent material. In each of the embodiments, however, it is also appreciated that that both the core and the shaft can be comprised of a radiolucent material. For example, in each of the embodiments, the core can be comprised of a ceramic or rigid thermoplastic that may include fibers or other fillers while the shaft is comprised of an epoxy fiber matrix. Thus, in some embodiments, the core and the shaft can be comprised of different radiolucent materials. In still other embodiments, the core and shaft can be made of the same radiolucent material. In each embodiment, however, the various makers discussed herein can be used with the core and/or shaft. 
     A number of different methods and embodiments are disclosed herein. It is appreciated that the different methods and components from the different embodiments can be mixed and matched to produce a variety of still other different embodiments. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.