Abstract:
A threaded bone anchor includes a plurality of recesses formed lengthwise along an exterior cylindrical surface for engagement with a driver instrument through which a user applies clockwise or counterclockwise torque for the insertion or removal of the bone anchor into and from bone. The bone anchor includes a cross-sectional geometry that minimizes the anchor&#39;s outer diameter, survives high insertion/removal torque without compromising the anchor&#39;s bending/shear strength, and allows a mating clamp to be attached to the entire non-threaded surface, including the recessed portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority to U.S. Provisional Patent Application No. 61/140,716, filed Dec. 24, 2008, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to orthopedics, and in particular relates to a bone anchor incorporating a spline drive mechanism. 
     BACKGROUND 
     A variety of fixation devices for the reduction of bone or bone fragments or for spinal fixation are well known. Spinal fixation devices including intervertebral implants, spinal rods, and the like, are used to replace intervertebral discs, fuse or align adjacent vertebrae, and address other spinal issues. Long bone fixation devices commonly include both external and internal fixators that are attached to underlying bone. Spinal fixation devices and long bone fixation devices typically are affixed to underlying bone via one or more bone anchors. 
     For instance, a typical bone plate includes screw holes that accommodate bone screws which are drilled into underlying bone on opposing sides of a fracture to join bone segments together. A typical cervical spine implant can likewise include screw holes that accommodate screws which are drilled into adjacent vertebral bodies in order to fix the position of the implant. In certain applications it is desired to provide relatively small bone screws. For instance, spinal screws are having thread and head diameters less than 10 mm are commonly used. Bone screws of this size can become compromised when exposed to high torque/force that are applied when drilling, tapping, or otherwise inserting the anchor into underlying bone. 
     What is desirable is bone anchor configured to accept high torque/force without compromising the anchor&#39;s bending/shear strength. 
     SUMMARY 
     A bone anchor is provided, including an externally threaded shaft extending along a longitudinal axis, and a head connected to the shaft. The head defines a spline drive mechanism including plurality of equidistantly spaced longitudinally elongate recesses extending into the head such that each recess defines a pair of spaced side walls each defining opposing ends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of a preferred embodiment of the application, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the bone anchor of the present application, there is shown in the drawings a preferred embodiment. It should be understood, however, that the application is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1A  is a perspective view of a spine fixation assembly including a plurality of bone anchors attached to pedicles of adjacent vertebrae, clamps attached to the bone anchors, and fixation rods connected between the clamps; 
         FIG. 1B  is a perspective view of a bone anchor assembly including driver instrument attached to one of the bone anchors  FIG. 1A , and attaching the bone anchor to a pedicle; 
         FIG. 2A  is a perspective view of the bone anchor illustrated in  FIG. 1B ; 
         FIG. 2B  is a side elevation view of the bone anchor of  FIG. 2A ; 
         FIG. 3  is a perspective view of the driver instrument illustrated in  FIG. 1B ; 
         FIG. 4A  is a partial side elevation view of the bone anchor assembly illustrated in  FIG. 1B , showing the driver instrument attached to the bone anchor; 
         FIG. 4B  is an end elevation view of the bone anchor assembly illustrated in  FIG. 4A ; 
         FIG. 4C  is a sectional side elevation view of the bone anchor assembly illustrated in  FIG. 4B , taken along line  4 C- 4 C; 
         FIG. 4D  is a sectional side elevation view of the bone anchor assembly illustrated in  FIG. 4B , taken along line  4 D- 4 D; 
         FIG. 5A  is a side elevation assembly view of the bone anchor assembly including an auxiliary extension configured to be attached to the bone anchor; 
         FIG. 5B  is a side elevation view of the bone anchor assembly illustrated in  FIG. 5A , wherein the auxiliary extension is attached to the bone anchor; 
         FIG. 5C  is a sectional side elevation view of the bone anchor assembly illustrated in  FIG. 5B , showing the driver instrument attached to the bone anchor; 
         FIG. 6A  is a perspective view of a bone anchor constructed in accordance with an alternative embodiment; and 
         FIG. 6B  is a sectional side elevation view of a portion of the bone anchor illustrated in  FIG. 6A , showing a recess. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” or “distally” and “outwardly” or “proximally” refer to directions toward and away from, respectively, the geometric center of the bone anchor and related parts thereof. The words, “anterior”, “posterior”, “superior,” “inferior” and related words and/or phrases designate preferred positions and orientations in the human body to which reference is made and are not meant to be limiting. The terminology includes the above-listed words, derivatives thereof and words of similar import. 
     Referring to  FIG. 1A , bone fixation assembly  20  includes a plurality of bone anchors  22  attached to underlying bone and joined by a fixation member, illustrated as an internal spinal fixation rod  29 . In accordance with alternative embodiments, the fixation member can alternatively be an internal bone fixation plate, an external spinal fixation rod, or any internal or external fixation member configured to stabilize any bones via threaded anchors. In accordance with the illustrated embodiment, the bone fixation assembly  20  is illustrated as a spine fixation assembly whereby the anchors are attached to the pedicles  24  of a plurality of vertebrae  25  to be fixed. Thus, the bone anchors  22  can be provided as vertebral anchors, though the anchors can alternatively be configured for attachment to any underlying bone or bone segment as desired. In the illustrated bone fixation assembly  20 , a clamp connector  26  is attached to each bone anchor  22  at one end. The clamp connector  26  extends inwardly to another end that is attached to the fixation rod  29 . Accordingly, a first fixation rod  29   a  and a second fixation rod  29   b  extend generally along the caudal-cranial direction to fix the vertebrae as desired. In accordance with one embodiment, the bone fixation can be provided as a USS Fracture Assembly, commercially available from Synthes, Inc, having a place of business in West Chester, Pa. Unless otherwise indicated, the bone fixation system  20  and its components can be manufactured from any suitable biocompatible material known in the art including but not limited to 316L stainless steel, cobalt-chrome, CP titanium, Ti alloys including, but not limited to Ti-7A1-6Nb, TAV, or Ti-Moly, polymers such as PEEK, or allograft bone. The bone anchor  22  can further be coated with a bone-growth stimulating surface such as hydroxyapatite, plasma-sprayed titanium, anodic plasma spray coating, etc. 
     As shown in  FIG. 1B , the bone anchor assembly  20  can include the bone anchor  22  and a driver instrument  28  configured to be attached to the bone anchor  22  so as to impart a driving torque or force onto the bone anchor  22  that causes the bone anchor  22  to attach to the underlying bone, such as a vertebral body or pedicle, a long bone in the arm, (ex. Humerus), leg (ex. femur) or pelvis, and/or a smaller bones (in the hands, feet, head, face, and the like) 
     Referring now to  FIGS. 2A-B  and  4 A-D, the bone anchor  22  includes a shaft  30  that extends longitudinally along a central longitudinal axis L 1 . The shaft  30  defines longitudinally opposing proximal, or upper, and distal, or lower, ends  30   a  and  30   b , respectively, and a head  32  integrally coupled to the proximal end  30   a . The distal end  30   b  of the shaft  30  is configured to be attached to underlying bone. In particular, helical threads  34  extend radially out from the shaft  30  at locations at and between the proximal and distal ends  30   a - b  that are configured to engage underlying bone. Thus, when the head  32  receive a driving torque/force from the driver instrument  28  (for instance when inserting the anchor into an underlying bone, or removing the anchor from underlying bone), the threads  34  advance into the underlying bone. 
     The threads  34  can be continuous as illustrated or discontinuous so as to define a plurality of teeth that define threads having multiple starts (for instance double lead, triple lead, and the like). Thus, a substantial entirety of the shaft  30  can be threaded, or only a potion of the shaft can be threaded as in a lag screw used in long bone fixation. As illustrated in  FIG. 2B , the threads  34  define an outer diameter OD that can increases in a direction from the distal end  30   b  toward the proximal end  30   a , or can remain substantially constant between the proximal and distal ends  30   a - b . It should thus be appreciated that the bone anchor  22  can alternatively be provided as a locking screw within a bone anchor or rod connector assembly, as desired. The bone anchor  22  can additionally or alternatively be self-tapping if desired, and/or can be configured to attach within a predrilled bore formed in the underlying bone. The bone anchor  22  can be provided as a Schanz or post-type bone anchor, or any alternatively constructed bone anchor as desired. 
     The head  32  includes an annular body  36  that defines a radially outer surface  38 . The head  32  can be cannulated if desired, so as to define a radially inner surface  40  opposite the radially outer surface  38 . Of course, the head  32  can assume any other suitable alternative shape as desired. The head  32  defines a proximal, or upper, end  32   a  and a distal, or lower, end  32   b , such that the proximal end  32   a  of the head defines a proximal end of the bone anchor, and the distal end  30   b  of the shaft defines a distal end of the bone anchor  22 . The distal end  32   b  of the head  32  is integrally coupled to the proximal end  30   a  of the shaft  30 , either directly or indirectly via an unthreaded neck  41 , which can provide a stop of larger diameter than the head, but not necessarily larger than the diameter of the bone threads that is coupled between the proximal end  30   a  of the shaft  30  and the distal end  32   b  of the head  32 . The proximal end  32   a  of the annular body  36  defines a longitudinally outer end  33  that defines an annular longitudinally outer lip  35 . 
     The outer surface  38  of the annular body  36  can be cylindrical as illustrated, that extends along the central longitudinal axis L 1  as illustrated. Thus, the shaft  30  and the head  32  can be longitudinally co-extensive, or extend along the same longitudinal axis. The outer surface  38  defines a diameter or other cross-sectional outer dimension that can be the same as, greater than, or less than, the outer diameter OD of the threads  34 . 
     With continuing reference to  FIGS. 2A-B  and with further reference to  FIGS. 5A-B , the head  32  can define a bore  42  extending longitudinally distally through the longitudinally outer end  33  and radially surrounded by the annular lip  35  and the annular body  36 . The bore  42  terminates within the annular body  36 , and thus does not extend entirely through the bone anchor  22 . The bore  42  defines the radially inner surface  40 , which can present helical threads  44  projecting inwardly into the bore. Thus, the head  32  includes an engagement member configured to attach an auxiliary device, such as a locator  46 . The locator  46  can include a longitudinally elongate body  47  that defines a cylindrical proximal end  46   a  having an engagement member in the form of helical threads  48  extending therefrom. The threads  48  and  44  are configured to mate so as to attach the locator  46  to the bone anchor  22 . 
     Certain embodiments appreciate that the attachment of the bone anchor  22  to an underlying pedicle involves displacing a large amount of soft tissue in order to access the pedicle. Once the anchor  22  is attached to the pedicle, the soft tissue can return to its normal position. The elongate body  47  can be in the form of a flexible wire having a marker  49  at its distal end  46   b  positioned to extend beyond the soft tissue so that the anchor  22  can be easily identified, for instance when it is desired to attach clamps or other structure to the anchor  22 . For example, the marker  49  can be colored differently than the surrounding structure of the body  47 . The locator  46  can define any length as desired, such as between approximately 25 mm and approximately 200 mm. Once the bone fixation assembly  20  has been completed, the threads  48  of the locator  46  can be removed from the anchor head  32 , thereby detaching the locator  46  from the bone anchor  22 . 
     The head  32  can include an external spline drive mechanism  50  formed in the outer surface  38  of the head  32  that is configured to attach to a complementary engagement member of the driver instrument  28 . Thus, the spline drive mechanism  50  can be referred to as an external spline drive mechanism. In particular, the head  32  defines a plurality of recesses  52  projecting into the radially outer surface  38  in a radially inward direction. The recesses  52  are longitudinally elongate, and thus parallel with the longitudinal axis L 1 . The recesses  52  are circumferentially spaced about the head  32  and configured to engage the driver instrument  28 . In accordance with one embodiment, eight recesses  52  are circumferentially spaced equidistantly about the outer surface  38  of the head  32 , though it should be appreciated that the head  32  can alternatively include any number of recesses  52  as desired, such as four, six, ten, twelve or more recesses  52 . 
     Each recess  52  defines a pair of opposing radially extending side walls  56  and a base  58  disposed and connected between the radially inner ends of the side walls  56 . It should be appreciated that the side walls  56  can extend radially, meaning that the side walls can extend in a pure radial direction or in a direction that includes a radial directional component. The side walls  56  of each recess  52  can extend along intersecting directions. For instance, the side walls  56  can converge toward each other with respect to a radially outward direction, or can diverge away from each other with respect to a radially outward direction. Alternatively still, the side walls  56  can extend parallel to each other. Additionally, portions of the side walls  56  of each recess  52  can be parallel, while other portions can converge or diverge. In accordance with the illustrated embodiment, the side walls  56  diverge away from each other with respect to a radially outward direction so as to provide an increased torque-receiving surface area, thereby increasing the torsional strength of each recess  52 . 
     The base  58  can extend circumferentially, meaning that the base  58  can extend in a pure circumferential direction, or in a direction that includes a circumferential directional component, including a direction that extends tangential to the circumferential direction. The base  58  of each recess  52  can be perpendicular to one or both of the side walls  56 , or can define an acute or obtuse angle with respect to one or both of the side walls  56 . Within any or all of the recesses  52 , the side walls  56  may be blended into the base  58  with radii, angled/chamfered corners, or filleted corners. The head  32  thus defines a first cross-sectional distance or diameter D 1  defined by the radially outer surface  38  at opposing locations between adjacent recesses  52 , and a second cross-sectional distance or diameter D 2  defined by the outer surface  38  as defined by the base  58  of opposing recesses  52  that is less than the first distance or diameter D 1  (see  FIGS. 4C-D ). The first distance D 1  can be substantially equal to the outer diameter OD of the threads  34 . The second distance or diameter D 2  can increase along the tapered region  59  at the distal end of at least one, and up to all, of the recesses  52 . In this regard, while the head  32  is illustrated as having a cylindrical outer surface  38 , it is appreciated that the head  32  can define any alternative geometric shape as desired. For instance, the head  32  can define any desired polygon, such as an octagon corresponding to the eight recesses  52 . 
     The bases  58  include a tapered region  59  that flares radially outwardly with respect to a longitudinal direction from the proximal end  32   a  of the head  32  toward the distal end  32   b  of the head  32 . Accordingly, the distance or diameter between the radially inner surfaces of opposing bases  58  can increase at the distal end  32   b  of the head  32 , for instance at the distal 2-5 mm of the recess  52 , while the distance or diameter between the radially inner surfaces of opposing bases  58  at proximal end  32   a  of the head  32  is substantially constant. In accordance with one embodiment, the distance or diameter between the radially inner surfaces of opposing bases  58  along the proximal 5-10 mm can be substantially constant at approximately 4.2 mm. 
     Bone anchors of the type illustrated with respect to the bone anchor  22  are relatively small. In accordance with one embodiment, the outer diameter of the threads  34  and the outer diameter of the outer surface  38  are within the range of approximately 0.5 mm and approximately 9 mm, such as between approximately 0.5 mm and approximately 2 mm, between approximately 2 mm and approximately 4 mm, and between approximately 4 mm and approximately 9 mm. In accordance with one embodiment, the diameter of the inner surface  40  is within the range of approximately 1 mm and 8 mm, such as between approximately 1 mm and 6 mm, for instance between approximately 4 mm and approximately 6 mm at locations circumferentially between recesses  52 . The annular body  36  can define a thickness extending normally between the outer surface  38  and the inner surface  40  of between approximately 0.25 mm and approximately 5 mm, such as between approximately 3 mm and approximately 5 mm. The head  32  can extend longitudinally a length within the range of approximately 5 mm and 50 mm, such as between approximately 17 mm and approximately 20 mm in accordance with the illustrated embodiment. Advantageously, the head  32  is constructed so as to receive and transmit levels of torque that allow the threads  34  to be driven into underlying dense bone, in spite of the small size of the bone anchor  22 , and in particular of the head  32  and shaft  30 . In accordance with one embodiment, the bone anchor  22  is configured to receive torque/force above at and above ten Newton-meters (10 Nm) without bending, yielding, fracturing or failing, for instance when the bone anchor  22  is driven into underlying dense bone between approximately 5 mm and approximately 100 mm in depth. 
     With continuing reference to  FIG. 2A , the head  32  can define a plurality of notches  37  extending longitudinally into the annular lip  35 . The notches  37  can define substantially rectangular or alternatively shaped cutouts or pockets  39  that are equidistantly spaced circumferentially about the lip  35 . In accordance with one embodiment, each pocket  39  is longitudinally aligned with a corresponding one of the recesses  52 . As illustrated, the head  32  includes four pockets  39  aligned with a corresponding four of the recesses  52 , such that recesses aligned with pockets  39  are separated by recess that are not aligned with a corresponding pocket. It should be appreciated, of course, that the head  32  can alternatively include any number of pockets  39  as desired. In accordance with another embodiment, the lip  35  is devoid of pockets  39 , and is thus circumferentially continuous at its longitudinally outer end. 
     Referring now to  FIGS. 3-4D , the driver instrument  28  includes a driver body  60  that defines a proximal attachment end  60   a  configured to be attach to any suitable handle, and a distal engagement end  60   b  configured to engage the bone anchor  22 . The distal end  60   b  defines a longitudinally outward-facing end surface  61 . The driver body  60  can be generally annular about a central longitudinal axis L 2 . While the body  60  is generally tapered radially inwardly along a direction from the distal end  60   b  toward the proximal end  60   a , the body  60  can assume any desired size and geometric configuration suitable to drive the bone anchor  22 . The driver body  60  defines a cannulation  62  extending longitudinally into the outer surface  61  at the distal end  60   b . The cannulation  62  can define a shape corresponding to the shape of the outer surface  38  of the head  32 . Accordingly, the cannulation  62  can be cylindrical, polygonal, or can define any alternative suitable shape as desired. 
     The driver body  60  defines a radially inner surface  64  that defines the interior cannulation  62 , and an opposing radially outer surface  66 . The driver body  60  includes a plurality of protrusions illustrated in accordance with one embodiment as teeth  68  that project radially in from the inner surface  64 . Each tooth  68  generally corresponds in shape to the recesses  52 . For instance, each tooth  68  includes a pair of opposing side walls  70 , and a radially inner base  72  extending and connected between the radially inner ends of each side wall  70 . The teeth  68  are longitudinally elongate, and thus parallel with the longitudinal axis L 2 . The teeth  68  are circumferentially spaced about the driver body  60 , and thus define a spline drive engagement mechanism  65  configured to engage the spline drive mechanism  50  of the head  32  of the bone anchor  22 . In accordance with one embodiment, eight teeth  68  are circumferentially spaced equidistantly about the outer surface  66  of the driver body  60 , though it should be appreciated that the driver instrument  28  can alternatively include any number of teeth  68  as desired, such as four, six, ten, twelve or more recesses  68 . 
     The side walls  70  can extend radially, or in a direction that includes a radial directional component. The side walls  70  of each recess  52  can extend along intersecting directions. For instance, the side walls  70  can converge toward each other with respect to a radially inward direction, or can diverge away from each other with respect to a radially inward direction. Alternatively still, the side walls  70  can extend parallel to each other. Additionally, portions of the side walls  70  of each tooth  68  can be parallel, while other portions can converge or diverge. In accordance with the illustrated embodiment, the side walls  70  converge toward each other with respect to a radially inward direction so as to provide an increased torque-receiving surface area, thereby increasing the torsional strength of each tooth  68 . 
     The base  72  can extend circumferentially, or in a direction that includes a circumferential directional component. For instance, the base  72  can extend in a plane that is tangential to the circumferential direction. The base  72  of each tooth  68  can be perpendicular to one or both of the side walls  70 , or can define an acute or obtuse angle with respect to one or both of the side walls  70 . The side walls  70  may be blended into the base  72  with radii, angled/chamfered corners, or filleted corners. The driver instrument  28  thus defines a first cross-sectional distance or diameter D 3  defined by the inner surface  64  the base  72  of opposing teeth  68  that is less than the first distance or diameter D 1  (see  FIGS. 4C-D ), and a second cross-sectional distance or diameter D 4  defined by the inner surface  64  at opposing locations between adjacent teeth  68 . The first distance D 3  is greater than the second distance D 2  at the proximal end of the recesses  52 , such that the base  72  of each tooth  68  is sized and configured to fit over and slide along the corresponding base  58  of a complementary recess  52  at the proximal end of the recess. The second distance D 4  of the driver instrument  28  is greater than the first distance D 1  of the head  32 . 
     With continuing reference to  FIGS. 4A-D , in operation, the proximal end  60   a  is grasped either manually or via an auxiliary handle, such that the cannulation  62  of the driver instrument  28  is generally longitudinally aligned with the outer surface  38  of the head  32 , and the teeth  68  are generally aligned with the recesses  52 . The driver instrument  28  attaches to the bone anchor  22  by translating one or both of the driver instrument  28  and the bone anchor toward each other along the longitudinal direction such that the cannulation  62  receives the proximal end  32   a  of the head  32 . In particular, each of the teeth  68  is aligned with a select one of the recesses  52 . Accordingly, as the driver instrument  28  and the bone anchor  22  are attached, the teeth  28  slide into the recesses  52  toward the longitudinally distal such that the base  72  of each tooth  68  faces the base  58  of the corresponding recess  52 , and the side walls  70  of each tooth  68  face the side walls  56  of each corresponding recess  52 . If pockets  39  are formed in the lip  35  as illustrated in  FIG. 2A , the pockets can assist in removing an inserted threaded anchor that is surrounded by another implant (for instance a bone anchor connector) or bone. 
     Thus, once the distal end of the shaft  30  is placed against an underlying bone, the driver instrument  28  can rotated in a clockwise direction, thereby causing one of the side walls  70  to impart a torque/force against a corresponding one of the side walls  56  that causes the bone anchor  22  to correspondingly rotate in a clockwise direction, thereby advancing the bone anchor  22  into underlying bone. The driver instrument  28  can be subsequently rotated in a counterclockwise direction, thereby causing the other one of the side walls  70  to impart a torque/force against the corresponding side wall  56  that causes the bone anchor  22  to correspondingly rotate in a counterclockwise direction, thereby removing the bone anchor  22  from the underlying bone. 
     In accordance with one embodiment the tapered region  59  disposed at the distal end of at least one, up to all, of the recesses  52  causes the or distance or diameter D 2  to gradually increase toward the proximal end of the recess  52 . In accordance with one embodiment, the tapered region  59  tapers radially outward at an angle between approximately 0.25 degrees and approximately 5 degrees with respect to the longitudinal axis L 1 . The distance or diameter D 2  of the recess  52  at the tapered region  59  increases to a distance greater than the second diameter D 4  of the bases  72  of the teeth  68 , thereby causing the teeth  68  to interfere with, or bite into, the corresponding base  58  of the recess  52  that receives the tooth  68  once the driver instrument  28  as the teeth  68  are translated into the distal ends of the recesses  52 , so that the tooth  68  becomes wedged in the spline drive mechanism  50 . The interference between the base  72  of the teeth  68  and the base  58  of the corresponding recesses  52  generally prevents the bone anchor  22  retained in the driver  28  from loosening or articulating during insertion or removal of the bone anchor  22 . 
     As an additional strength benefit, the gradually increasing core diameter of the distal portion of the recesses  52  gradually improves the bending strength of the bone anchor  22  as the base diameter increases distally. In accordance with one embodiment, the bases  58  of the recesses  52  are blended with a radius into the converging angled side walls to further improve torsional strength and enhance the manufacturability of the recesses  52 . 
     Alternatively or additionally, the distal end of the radially outer surface  38  can flare radially outward at a location between adjacent recesses  52 , such that the inner surface  64  can interfere with the outer surface  38  at a location circumferentially between adjacent teeth  68 . Alternatively or additionally, one or both the side walls  56  of at least one, up to all, of the recesses  52  can flare radially inward into the recess  52  at the distal end of the recess, such that the distal end of the recess defines a circumferential distance between the side walls  70  of the corresponding tooth  68  or teeth  68 . Accordingly, as the teeth  68  are inserted into the distal end of the recesses  62 , the teeth become pinched within the recesses  62 , and interfere or bite into the side walls  56  of the recesses  52 . Alternatively or additionally still, the proximal ends of at least one, up to all, the teeth  68  can flare radially outward thereby causing the proximal ends of the bases  72  to interfere with and bite into the base  58  of the corresponding recess  52  once the driver instrument  28  has engaged the head  32 . 
     Accordingly, at least one of the teeth  68  and at least one of the recess  52  defines a surface that tapers in a direction toward the other of the tooth  68  and the recess  52 , thereby causing at least one of the teeth  68  to frictionally interfere with the head  32  within the recess  52  that resists inadvertent removal of the driver instrument  28  from the head  32 . In one embodiment, the tapered surface is a radially outwardly tapered region of the base  58  of the recess  52 . Once the anchor  22  has been attached to underlying bone, a laterally outward force can overcome the frictional engagement caused by the tapered region so as to facilitate removal of the driver instrument  28  from the anchor  22 . 
     As illustrated in  FIG. 5C , the auxiliary device, such as the locator  46 , can be threadedly connected to the head  32  in the bore  40  of the head  32  prior to attaching the driver instrument  28  to the head  32 . Thus, the locator  46  has a length sized to fit within the cannulation  62  when the driver instrument  28  is fully attached to the head  32 . 
     Referring again to  FIG. 1A , once the bone anchors  22  are implemented as spine anchors and attached to an underlying pedicle, the clamp connector  26  is then applied over the head  32  at any desired position, including over the recesses  52  or a portion thereof, and the spinal rod  29  is connected between the clamp and a separate pedicle screw assembly, lamina hook, or other clamp assembly anchored to a separate vertebra to form a pedicle screw and rod construct for spinal correction. 
     Referring now to  FIGS. 6A-B , a bone anchor  122  can be provided in accordance with an alternative embodiment. The bone anchor  122  is illustrated having reference numerals corresponding to like structure of the bone anchor  22  incremented by 100 for the purposes of form and clarity. Thus, the bone anchor  122  includes a head  132  having a spline drive mechanism  150  formed in the inner surface  140  that is configured to attach to a complementary engagement member of the driver instrument. Thus, the spline drive mechanism  150  can be referred to as an internal spline drive mechanism. In particular, the head  132  includes an internal spline drive mechanism  150  disposed in the bore  142  that defines a plurality of recesses  152  projecting into the radially inner surface  140  in a radially outward direction. The radially outer surface  138  can define a diameter or other cross-sectional dimension that is greater than, substantially equal to, or less than that of the threads  134 . The recesses  152  are longitudinally elongate, and circumferentially spaced about the head  32 . In accordance with one embodiment, eight recesses  152  are circumferentially spaced equidistantly about the inner surface  140  of the head  32 , though it should be appreciated that the head  132  can alternatively include any number of recesses  152  as desired, such as four, six, ten, twelve or more recesses  152 . 
     Each recess  152  defines a pair of opposing radially extending side walls  156  and a base  158  disposed and connected between the radially inner ends of the side walls  156 . It should be appreciated that the side walls  156  can extend radially, or in a direction that includes a radial directional component. The side walls  156  of each recess  152  can extend along intersecting directions. For instance, the side walls  156  can converge toward each other with respect to a radially outward direction, or can diverge away from each other with respect to a radially outward direction. Alternatively still, the side walls  156  can extend parallel to each other. Additionally, portions of the side walls  156  of each recess  152  can be parallel, while other portions can converge or diverge. 
     The base  158  can extend circumferentially, or in a direction that includes a circumferential directional component. For instance, the base  158  can extend in a plane that is tangential to the circumferential direction. The base  158  of each recess  152  can be perpendicular to one or both of the side walls  156 , or can define an acute or obtuse angle with respect to one or both of the side walls  156 . Within any or all of the recesses  152 , the side walls  156  may be blended into the base  158  with radii, angled/chamfered corners, or filleted corners. At least one, up to all, of the recess  152  can include an inwardly tapered region  159  that flares radially inwardly with respect to the longitudinal axis L 1  so as to engage a driver in the manner described above. For instance, the base  158  can be tapered radially inward at the distal end of one or more, up to all, of the recesses  152 , such that the driver can be wedged between tapered regions of one or more opposing recesses  152  when the driver is inserted into the bore  142  along the direction of Arrow I. Alternatively or additionally, the side walls  156  of one or more, up to all, recesses  152  can be tapered toward each other at their distal ends. 
     The operation of the bone anchor  120  is similar to the operation of the bone anchor  20  shown in  FIG. 1 , with the exception that the driver instrument  28  is modified to operatively interlock inside the recesses  152 . Accordingly, the protrusions or teeth  68  would project radially outward so as to fit in one or more, up to all, of the recesses  152  such that rotation of the driver instrument  28  correspondingly imparts torque/force to the bone anchor  122  in the manner described above. It should be appreciated that a distal region or end of the base  158  of at least a select one or more, up to all, of the recesses  152  tapers inwardly towards a corresponding one of the protrusions or teeth  68  that is disposed in the select one or more, up to all, of the recesses  152 , such that the tapered region defines a cross-sectional distance of the head  28  that is less than a cross-sectional distance defined by bases  72  of opposing protrusions of the driver instrument  28 . 
     The bone anchors  22  and  122 , while illustrated as pedicle screws in accordance with one embodiment, can be constructed in accordance with any desired bone fixation application. For instance, the anchors  22  and  122  can provide polyaxial or monoaxial top-loading or side-loading pedicle screw assemblies with or without separate heads for attachment to spinal rods, bone screws used to anchor bone plates, wires, or other connectors for the stabilization of long bone fractures, cranial/maxillofacial fractures, or pelvic/sternal fractures, locking screws to attach “closed head” spinal hooks and screws to give the user an attachment point to manipulate the bone screw/hook amidst the bony anatomy, and before inserting a rod or tightening a locking screw, and any other bone anchor that is rotated to attach the bone anchor into underlying bone unless otherwise indicated. 
     The embodiments described in connection with the illustrated embodiments have been presented by way of illustration, and the present invention is therefore not intended to be limited to the disclosed embodiments. Furthermore, the structure and features of each the embodiments described above can be applied to the other embodiments described herein, unless otherwise indicated. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, for instance as set forth by the appended claims.