Patent Publication Number: US-2010131017-A1

Title: Multi-Axial Bone Anchor Assembly

Description:
FIELD/BACKGROUND 
     The present disclosure generally relates to orthopedic implants used for correction of spinal injuries and/or deformities, and more specifically, but not exclusively, concerns apparatuses for fixing a portion of the spine to allow correction and/or healing thereof. In some embodiments, the present disclosure is directed to improved apparatus, systems, and assemblies for securing orthopedic implants to bone and, in some embodiments, to vertebrae. 
     Elongated connecting elements, such as rods, plates, tethers, wires, cables, and other devices have been implemented along the spinal column and connected between two or more anchors engaged between one or more spinal motion segments. Such connecting elements may be rigid and resist movement of the spinal motion segment in response to spinal loading or motion of the spinal motion segment. Other connecting elements are flexible to permit at least some limited spinal motion while providing resistance to loading and motion of the spinal motion segment. Typically, the connecting elements are secured to the spinal column by bone anchors, such as bone screws, that are attached to the vertebrae. While prior bone anchors and screws have been satisfactory for their intended purposes, they have not been satisfactory in all respects. 
     Therefore, there remains a need for improved apparatus, systems, and assemblies for securing orthopedic implants to bone. 
     SUMMARY 
     The present disclosure provides improved apparatus, systems, and assemblies for securing orthopedic implants to bone. 
     In one aspect, the present disclosure provides a bone anchor assembly. The bone anchor assembly comprises a bone anchor having a head portion and a bone engaging portion; a crown member shaped and sized to mate with the head portion of the bone anchor; a saddle having an upper portion and a lower portion; and a sleeve configured for positioning around a portion of the saddle. The upper portion of the saddle includes a channel for receiving an elongated member and a threaded portion for receiving a compression member. The lower portion of the saddle is configured to receive the crown member and the head portion of the bone anchor therein. The lower portion includes a plurality of slots rendering the lower portion outwardly flexible to receive the head portion of the bone anchor and inwardly flexible to secure the head portion of the bone anchor therein. The sleeve is configured for positioning around the lower portion of the saddle and sized such that the lower portion of the saddle is prevented from flexing outwardly upon advancement of the sleeve upwardly around the lower portion of the saddle, thereby securing the head portion of the bone anchor therein. The bone anchor is moveable with respect to the saddle when the sleeve is positioned around the lower portion. The bone anchor is secured in a fixed position relative to the saddle by compression of the head portion of the bone anchor between the crown member and the lower portion of the saddle. In some embodiments, the lower portion further comprises a seat portion for interfacing with the head portion of the bone anchor. In some embodiments the seat portion is configured to transmit forces from the head portion of the bone anchor radially outward to the sleeve. In some embodiments, the bone anchor is secured in the fixed position relative to the saddle by compression of the head portion of the bone anchor between the crown member and the lower portion of the saddle created by threadingly advancing a compression member against the elongated member. In some embodiments, the lower portion of the saddle and/or the sleeve includes a plurality of cutout portions configured to allow an increased range of motion between the saddle and the bone anchor. 
     In another aspect, the present disclosure provides a system for orthopedic implantation. The system comprises an elongated member, a compression member, and a bone anchor assembly. The bone anchor assembly comprises a bone anchor having a head portion and a bone engaging portion; a crown member shaped and sized to mate with the head portion of the bone anchor; a saddle having an upper portion and a lower portion; and a sleeve configured for positioning around the saddle. The upper portion of the saddle includes a channel for receiving the elongated member and a threaded portion for receiving the compression member. The lower portion of the saddle is configured to receive the crown member and the head portion of the bone anchor therein. The lower portion includes a plurality of slots rendering the lower portion flexible to receive and retain the head portion of the bone anchor. The lower portion also includes a seat portion for interfacing with the head portion of the bone anchor. The seat portion is an edge at least partially defined by a conically tapered surface. The sleeve is configured for positioning around the lower portion of the saddle and sized such that the lower portion of the saddle is prevented from flexing outwardly upon advancement of the sleeve upwardly around the lower portion of the saddle, thereby securing the head portion of the bone anchor therein. The bone anchor is moveable with respect to the saddle when the sleeve is positioned around the lower portion. The bone anchor is secured in a fixed position relative to the saddle by compression of the head portion of the bone anchor between the crown member and the lower portion of the saddle created by threadingly advancing the compression member against the elongated member. Generally, the lower portion of the saddle is flexible enough to allow entry of the head of the bone anchor and/or the crown, while the sleeve has sufficient strength to prevent outward movement of the lower portion of the saddle to retain the head portion within the saddle in both pivotable and locked positions. 
     In another aspect, the present disclosure provides a method of assembling a bone anchor assembly. The method comprises providing a bone anchor having a head portion and a bone engaging portion; providing a crown member shaped and sized to mate with the head portion of the bone anchor; providing a saddle having an upper portion and a lower portion; and providing a sleeve configured for positioning around the lower portion of the saddle. The upper portion of the saddle includes a channel for receiving an elongated member and a threaded portion for receiving a compression member. The lower portion of the saddle includes an opening in the bottom thereof configured to receive the crown member and the head portion of the bone anchor. The lower portion also includes a plurality of slots rendering the lower portion flexible and a seat portion for interfacing with the head portion of the bone anchor. The seat portion comprises an edge defined at least partially by a sloped surface. The method further comprises inserting the crown member into the opening of the lower portion of the saddle; inserting the head portion of the bone anchor into the lower portion of the saddle; and advancing the sleeve upwardly around the lower portion of the saddle to moveably secure the head portion of the bone anchor within the saddle, the sleeve preventing the lower portion of the saddle from flexing outwardly. 
     Further aspects, forms, embodiments, objects, features, benefits, and advantages of the present disclosure shall become apparent from the detailed drawings and descriptions provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic perspective view of a system according to one aspect of the present disclosure. 
         FIG. 2  is a diagrammatic perspective cross-section view of the system of  FIG. 1 . 
         FIG. 3  is a diagrammatic perspective exploded view of the system of  FIGS. 1 and 2 . 
         FIG. 4  is a diagrammatic side view of a saddle according to one aspect of the present disclosure. 
         FIG. 5  is a diagrammatic top view of the saddle of  FIG. 4 . 
         FIG. 6  is a diagrammatic bottom view of the saddle of  FIGS. 4 and 5 . 
         FIG. 7  is a diagrammatic cross-sectional view of the saddle of  FIGS. 4 ,  5 , and  6  taken along section line  7 - 7 . 
         FIG. 8  is a diagrammatic top view of a crown washer according to one aspect of the present disclosure. 
         FIG. 9  is a diagrammatic cross-sectional view of the crown washer of  FIG. 8  taken along section line  9 - 9 . 
         FIG. 10  is a diagrammatic side view of a bone screw according to one aspect of the present disclosure. 
         FIG. 11  is a diagrammatic bottom view of a sleeve according to one aspect of the present disclosure. 
         FIG. 12  is a diagrammatic cross-sectional view of the sleeve of  FIG. 11  taken along section line  12 - 12 . 
         FIG. 13  is a diagrammatic perspective view of a bone anchor assembly according to one aspect of the present disclosure. 
         FIG. 14  is a diagrammatic front cross-sectional view of the bone anchor assembly of  FIG. 13 . 
         FIG. 15  is a diagrammatic side view of the bone anchor assembly of  FIGS. 13 and 14  engaged with a rod and set screw according to one aspect of the present disclosure. 
         FIG. 16  is a diagrammatic front cross-sectional view of the bone anchor assembly, rod, and set screw of  FIG. 15 . 
         FIG. 17  is a diagrammatic side view of the bone anchor assembly, rod, and set screw of  FIGS. 15 and 16  in a locked position according to one aspect of the present disclosure. 
         FIG. 18  is a diagrammatic front cross-sectional view of the bone anchor assembly, rod, and set screw of  FIG. 17  in the locked position. 
         FIG. 19  is a diagrammatic bottom view of the bone anchor assembly, rod, and set screw of  FIGS. 17 and 18  in the locked position. 
         FIG. 20  is a diagrammatic perspective view of a system according to another embodiment of the present disclosure. 
         FIG. 21  is a diagrammatic bottom view of the system of  FIG. 20   
         FIG. 22  is a diagrammatic perspective cross-section view of the system of  FIGS. 20 and 21 . 
         FIG. 23  is a diagrammatic side exploded view of the system of  FIGS. 20 ,  21 , and  22 . 
         FIG. 24  is a diagrammatic side view of a saddle according to another embodiment of the present disclosure. 
         FIG. 25  is a diagrammatic top view of the saddle of  FIG. 24 . 
         FIG. 26  is a diagrammatic bottom view of the saddle of  FIGS. 24 and 25 . 
         FIG. 27  is a diagrammatic cross-sectional view of the saddle of  FIGS. 24 ,  25 , and  26  taken along section line  27 - 27 . 
         FIG. 28  is a diagrammatic bottom view of a sleeve according to another embodiment of the present disclosure. 
         FIG. 29  is a diagrammatic cross-sectional view of the sleeve of  FIG. 28  taken along section line  29 - 29 . 
         FIG. 30  is a diagrammatic perspective view of a bone anchor assembly according to another embodiment of the present disclosure. 
         FIG. 31  is a diagrammatic perspective view of a sleeve member according to another embodiment of the present disclosure. 
         FIG. 32  is a diagrammatic perspective view of a bone anchor assembly according to another embodiment of the present disclosure 
         FIG. 33  is a diagrammatic side view of the bone anchor assembly of  FIG. 32 . 
         FIG. 34  is a diagrammatic exploded perspective view of a bone anchor assembly according to another embodiment of the present disclosure. 
         FIG. 35  is a diagrammatic side view of a saddle member according to another embodiment of the present disclosure. 
         FIG. 36  is a diagrammatic exploded perspective view of a bone anchor assembly according to another embodiment of the present disclosure. 
         FIG. 37  is a diagrammatic front cross-sectional view of the bone anchor assembly of  FIG. 36 . 
         FIG. 38  is a diagrammatic front cross-sectional view of a saddle member and a crown member according to one embodiment of the present disclosure. 
         FIG. 39  is a diagrammatic exploded perspective view of a bone anchor assembly according to another embodiment of the present disclosure. 
         FIG. 40  is a diagrammatic front cross-sectional view of the bone anchor assembly of  FIG. 39 . 
         FIG. 41  is a diagrammatic perspective cross-sectional view of the bone anchor assembly of  FIG. 39 . 
         FIG. 42  is a diagrammatic perspective view of a saddle member and a crown member according to one embodiment of the present disclosure. 
         FIG. 43  is a diagrammatic front cross-sectional view of the saddle member and the crown member of  FIG. 42 . 
         FIG. 44  is a diagrammatic perspective view of a saddle member and a crown member according to one embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is intended. Any alterations and further modifications in the described devices, instruments, methods, and any further application of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. In particular, it is fully contemplated that the features, components, and/or steps described with respect to one embodiment may be combined with the features, components, and/or steps described with respect to other embodiments of the present disclosure. 
     Referring to  FIGS. 1-3 , a system  10  for orthopedic implantation is shown according to one embodiment of the present disclosure. The system  10  includes a bone anchor assembly  12 . The bone anchor assembly  12  includes a receiver or saddle  14 , a crown washer  16  ( FIGS. 2 and 3 ), a bone anchor  18 , and a sleeve  20 . A set screw  22  secures a rod  24  to the bone anchor assembly  12 . The system  10  may be part of a larger orthopedic system comprising a plurality of longitudinal members (e.g., rods, plates, etc.), a plurality of bone anchor assemblies, and/or a plurality of connectors. In some embodiments, the system  10  is particularly suited for use in the spinal column. It will be understood that various types of fasteners or connectors (e.g. clamps) can be used in combination with the bone anchor assembly  12  and rod  24 . Further additional or alternative longitudinal members can also be used, such as the plates and/or rods disclosed in U.S. Pat. No. 6,485,491, commonly assigned and hereby incorporated by reference in its entirety. 
     The components of system  10  can be implanted via an open, minimally-invasive, or other surgical approach. Generally, one or more bone anchor assemblies  12  are inserted into one or more bones, then the longitudinal members  24  are contoured, if necessary, and surgically inserted and connected to the bone anchor assemblies. The relative angles of bone anchor assemblies  12  with respect to the longitudinal members  24  can be adjusted as necessary for ease of connection of the longitudinal member to the fasteners. Additional connectors are fitted to the longitudinal members and/or bone anchors as necessary or desired, and all elements are locked against unwanted movement with respect to other parts. 
     Referring now to  FIGS. 4-12 , the components of the bone anchor assembly  12  are shown individually in greater detail. Referring more specifically to  FIGS. 4-7 , the receiver member or saddle  14  is illustrated therein.  FIG. 4  is a diagrammatic side view of the saddle  14 ;  FIG. 5  is a diagrammatic top view of the saddle  14 ;  FIG. 6  is a diagrammatic bottom view of the saddle  14 ;  FIG. 7  is a diagrammatic cross-sectional view of the saddle  14 . 
     Saddle member  14  generally has a U-shape with an upper portion  26  and a lower portion  28 . The upper portion  26  comprises two upright portions  30  that define a channel  32  extending through saddle member  14 . Channel  32  is configured to accommodate an elongated member, such as rod  24 . It is understood that the elongated member may have a number of desired lengths and diameters. In that regard, the width  34  of the channel  32  in the current embodiment is substantially equal to the diameter of elongated member. In some embodiments, the width of the channel is slightly larger than the diameter of the elongated member, which allows easier insertion of elongated member into the channel  32 , allows for contouring of the elongated member, and also allows a variety of elongated member of differing sizes to be used with saddle member  14 . Generally, the elongated member  24  is positioned above the bottom portion  36  of the channel  32  when in a locked position. However, in some embodiments the elongated member  24  may be seated within the bottom portion  36  when in a locked position. Thus, the bottom portion  36  may be shaped or otherwise include features to ensure secure placement of the elongated member. In the current embodiment, the bottom portion  36  has a substantially cylindrical shape, as viewed in  FIG. 4 , to allow clearance of the outer surface of the rod  24 . 
     The upright portions  30  of the saddle member  14  include an inner surface  38  and an outer surface  40 . A bore or hole  42  extends through the upright portions  30  between the outer surface  40  and the inner surface  38 . The holes  42  are substantially aligned with one another and are substantially perpendicular to the channel  32 . In some instances, the holes  42  are utilized for grasping by a surgical tool to facilitate positioning of the rod  24  into the bone anchor assembly  12  within the patient. In the current embodiment and as shown in  FIGS. 4 and 7 , the outer surfaces  40  taper with respect to inner surfaces  38  as they extend upwardly. This taper reduces the bulk and size of the saddle member  14  allowing for easier handling. In that regard, a surgical instrument may engage the holes  42  without substantially increasing the overall width needed to insert to the bone anchor assembly. 
     Generally, the inner surfaces  38  extend substantially coaxially with the axis of a bore  44  extending longitudinally through saddle member  14 . The inner surfaces  38  of the upright portions  30  define an internally threaded portion  46 , as shown in  FIG. 7 . Internally threaded portion  46  is configured to be threadingly coupled with set screw  22 , as described below. The internally threaded portion  46  is configured such that the threads end above the rod  24  when the rod is secured within the saddle member  14 . In some embodiments, as shown in  FIG. 7 , the inner surfaces  38  include an annular relief or cutout  48  that extends radially around bore  44  below the threaded portion  46 . The annular relief  48  eliminates the helical thread run out often found on internal threads. In other embodiments, the inner surfaces do not include an annular relief  48 . Further, in some embodiments the threaded portion  46  may not end above the rod  24  when the rod is secured within the saddle member  14 . In further embodiments, the outer surfaces of the upright portions  30  define an externally threaded portion, instead of or in addition to the internally threaded portion  46 . An externally threaded portion is configured to be threadingly coupled with a locking nut. In some embodiments with an externally threaded portion, at least the externally threaded portion of the outer surfaces  40  extend substantially parallel with the longitudinal bore  44  of the saddle member  14 . Alternatively, saddle member  14  could be externally and/or internally configured for compression members using snapping, twisting or other types of closures. 
     The aperture or bore  44 , which may be generally cylindrical, extends through the saddle member  14  from the upper portion  26  to the lower portion  28 . The bore extends along the longitudinal axis of the saddle member  14  and substantially transversely to and in communication with channel  32 . In the current embodiment, the bore  44  extends entirely through the saddle member  14 . In other embodiments, the bore  44  extends only partially through the saddle member  14 . As described below, the bore  44  can also allow access for a driving tool to engage the bone anchor  18 . In this manner, the bone anchor  18  can be driven into a bone, such as a vertebra, with the bone anchor movably retained within the saddle member  14 . 
     The inner surfaces  38  include a stop portion or shoulder  50  below the annular relief  48  and in communication with bore  44 . The shoulder  50  is provided to act as a stop for the bottom-loaded crown member  16 . Generally, shoulder  50  serves as an upper boundary for the crown member  16  within the bore  44 , preventing movement of the crown beyond the shoulder. Though not shown in the current embodiment, in other embodiments the shoulder  50  and/or crown member  16  include features to facilitate engagement therebetween, such as mating projections and recesses. 
     The lower portion  28  of the saddle member  14  has a reduced diameter compared to the upper portion  26 . Accordingly, a shoulder  52  is defined between the upper portion  26  and the lower portion  28 . The lower portion  28  includes an inner surface  54  and an outer surface  56 . The inner surface  54  at least partially defines bore  44 . In that regard, the saddle member  14  is configured for bottom-loading. That is, the crown member  16  and the bone anchor  18  are inserted into the saddle member  14  through an opening in the bottom of the lower portion  28 . In that regard the lower portion  28  must have an opening large enough to receive the crown member  16  and bone anchor  18 . However, the opening cannot be too large as the crown member  16  and bone anchor must be retained within the saddle member  14 . Accordingly, in the present embodiment the lower portion  28  includes slotted reliefs  58 . The slotted reliefs  58  extend through the lower portion  28  from the inner surface  54  to the outer surface  56  and extend upwardly from the bottom of the lower portion. The slotted reliefs  58  include an elongated slot portion  60  and a relief  62  having an increased width compared to the slot portion. The slotted reliefs  58  render the lower portion  28  at least partially flexible so that the lower portion is movable between at least two positions; the first position being an enlarged insertion configuration for allowing the head of the bone anchor to pass therethrough; the second position being a reduced dimension retaining configuration for retaining the head of the bone anchor therein. The lower portion  28  may flex outwardly (expand) slightly to allow insertion of the bone anchor  18  and/or crown  16  into bore  44 . The lower portion  28  may also flex inwardly (contract) to secure the bone anchor  18  and crown  16  within the bore  44 . In that regard, as described below engagement with the sleeve  20  may cause the lower portion  28  to contract and thereby retain the bone anchor  18  and crown  16  therein. There are three slotted reliefs  58  in the current embodiment. The three slotted reliefs are equally spaced 120 degrees apart from one another around the circumference of the lower portion  28 . Two of the slotted reliefs  58  are offset approximately 30 degrees from the axis of channel  32 , while the other slotted relief  58  is offset approximately 90 degrees from the axis of the channel  32 . In other embodiments, the number, shape, size, and placement of the slotted reliefs may be varied. 
     As described, the inner surface  54  is configured to receive the head of the bone anchor  18 . In that regard, the inner surface  54  includes a tapered portion  64  extending inwardly from the inner surface to a cylindrical bore  66 , thereby defining an edge  67 . In some embodiments, the tapered portion  64  has a substantially conical shape. The bore  66  defines the narrowest opening of the bore  44  in the lower portion  28 . The intersection of the tapered portion  64  and the cylindrical bore  66  generally define the edge  67  which comprises the seat of the saddle  14  configured for mating with the bone anchor  18 . In that regard, the tapered portion  64  extends from the inner surface  54  at an angle  68 . As illustrated in  FIG. 7 , the angle  68  may have a value between 90 and 170 degrees, which may also be considered 10-90 degrees relative to the longitudinal axis of the saddle  14 . In one particular embodiment, the angle  68  is approximately 20 degrees. In another particular embodiment, the angle  68  is approximately 55 degrees. When assembled, the head of the bone anchor  18  engages the edge  67  such that at least some of the loading forces from the bone anchor are directed radially outward to the sleeve  20 . Distributing the pullout force radially may increase the carrying load capacity of the bone anchor assembly  12  and reduces the risk of disassembly during rod reduction and/or post-operatively. 
     The inner surface  54  also includes a tapered portion  70  extending outwardly from the cylindrical bore  66 . The tapered portion  70  is configured to allow the bone anchor  18  to translate through a plurality of positions corresponding to multi-axial movement with respect to the saddle  14  by reducing the interference of the lower portion  28  with the shaft of the bone anchor. In some embodiments, the tapered portion  70  has a substantially conical shape. 
     Generally, the outer surface  56  is substantially cylindrical and configured to mate with the sleeve  20 . In the current embodiment, the outer surface  56  is substantially flat and coaxial with the longitudinal axis of the saddle  14  when in a neutral position (i.e., not expanded or contracted). In other embodiments, however, the outer surface may be flared outward (see e.g.,  FIG. 34 ) or inward when in a neutral position. In some embodiments the outer surface  56  and/or a portion of the sleeve  20  may be chamfered to facilitate engagement therebetween. In some embodiments, the neither the outer surface  56  nor the sleeve  20  is chamfered. Rather, the engaging edges of the outer surface  56  and the sleeve  20  comprise break edges. In some embodiments, the break edges are less than 0.5 mm. In one particular embodiment, the break edge of the outer surface  56  is less than 0.1 mm and the break edge of the sleeve  20  is less than 0.4 mm. Further, the outer surface  56  and/or the inner surface of the sleeve  20  may be treated (physically, chemically, and/or combinations thereof) to encourage engagement therebetween. The shoulder  52  serves as an upper stop for sleeve  20 . In yet other embodiments, the outer surface  56  and/or the inner surface of the sleeve  20  may include mating features (e.g., projections and recesses) to secure engagement and/or alignment therebetween. 
     Engagement between the sleeve  20  and the outer surface  56  of the lower portion  28  results in the bone anchor  18  and crown  16  being movably held within the saddle member  14 . That is, the bone anchor  18  and crown  16  are securely retained with the saddle member  14 , but are capable of movement with respect to the saddle member. In particular, the bone anchor  18  is capable of multi-axial movement with respect to the saddle member  14 . The bone anchor  18  locked with respect to the saddle  14  upon compression of the rod  24  by the set screw  22 , which in turn compresses the crown member  16  onto the head of the bone anchor  18 , thereby securing the bone anchor between the crown member and the seat of the saddle limiting movement of the bone anchor relative to the saddle. 
     As described above, the inner surface  54  includes the tapered portion  70  configured to allow the bone anchor  18  to translate through a plurality of positions corresponding to multi-axial movement with respect to the saddle  14  The inner surface  54  also includes angular cutouts  72  placed symmetrically about the circumference of the bore  44  to increase the allowable angulation of the bone anchor  18  in relation to the saddle  14 . As shown in  FIG. 6 , there are three angular cutouts that are generally partially cylindrical in shape and at approximately a 45 degree angle with respect to the longitudinal axis of the saddle  14 . The three angular cutouts  72  are equally spaced 120 degrees apart from one another around the circumference of the lower portion  28 . Two of the angular cutouts  72  are offset approximately 30 degrees from the axis of channel  32 , while the other cutout  72  is offset approximately 90 degrees from the axis of the channel  32 . In the current embodiment, the slotted reliefs  58  are substantially aligned with and positioned within the cutouts  72 . In other embodiments, the slotted reliefs  58  and cutouts  72  may be separated. Though a particular arrangement has been described above, it is understood that the number of cutouts, the shape of the cutouts, the position of the cutouts in relation to the axis of the channel  32 , the angle of the cutouts in relation to the longitudinal axis of the saddle  14 , the size of the cutouts, and the angular spacing between each cutout may vary for specific applications. Generally, the cutouts allow a greater range of motion between the saddle  14  and the bone anchor  18 . 
     The illustrated embodiment of saddle member  14  is an “open” variety. That is, channel  32  is open through the top of saddle member  14 , thereby making the saddle member  14  generally U-shaped. It will be understood that the principles of this disclosure apply to equally to “closed” fasteners, i.e., those in which a longitudinal member receiving channel is not open through the top, but is essentially a bore through the saddle member  14 . 
     Referring more specifically to  FIGS. 8 and 9 , the crown member or washer  16  is illustrated therein.  FIG. 8  is a diagrammatic top view of the crown washer  16 ;  FIG. 9  is a diagrammatic cross-sectional view of the crown washer  16 . 
     The crown member  16  is substantially cylindrical with an internal opening  74  and an undersurface  76 . Crown member  16  is sized to fit within bore  44  of the saddle, so that crown member  16  has some freedom of axial movement within the bore. In particular, crown member  16  is sized to move axially between shoulder  50  and the head of the bone anchor  18 . Internal opening  74  in the crown member  16  allows surgical instrument access to the bone anchor  18  when the crown member is positioned above or on top of the bone anchor. Undersurface  76  is configured to accommodate at least a portion of the head of the bone anchor  18 . The undersurface  76  may be shaped (e.g. spherical, rounded, conical, or otherwise) to allow relative multi-axial movement between the crown and the head of the bone anchor  18 . In the current embodiment, the undersurface  76  is partially rounded or spherical to mate with the spherical head of the bone anchor. In that regard, the rounded portion of the undersurface  76  has substantially the same radius of curvature as the head of the bone anchor  18 . The undersurface  76  is shaped such that sufficient compression of the head of the bone anchor  18  between the crown member  16  and the seat of the saddle  14  can fixedly secure the bone anchor relative to the saddle. 
     An upper surface  78  of the crown member  16  is configured to engage with the elongated member of spinal rod  24 . In particular, the crown member  16  is configured to be compressed downwardly by the spinal rod  24  to secure the bone anchor in place. In some embodiments, the upper surface  78  includes features to facilitate engagement with the elongated member. For example, in at least one embodiment the upper surface  78  includes a recess shaped to match the outer contours of the elongated member. In other embodiments, the crown member  16  includes additional features to facilitate proper positioning of the crown member within the saddle  14 , such as external grooves or projections configured to mate with corresponding projections or grooves of the saddle. 
     Referring more specifically to  FIG. 10 , the bone anchor  18  is illustrated therein.  FIG. 10  is a diagrammatic side view of the bone anchor  18 . As shown, the bone anchor  18  is a bone screw having a head portion  80  and a shaft portion  82 . The shaft portion  82  includes a bone engaging portion  84  that, in the current embodiment, includes a series of threads  86 . The threads  86  are particularly suited for engaging bone. The shaft portion  82  also includes a non-threaded shank portion  88 . In the current embodiment, the head portion  80  of the bone anchor  18  includes a substantially spherical outer contour. However it should be understood that any external contour that allows for multi-axial movement could be utilized. In the current embodiment, the spherical surface  18  of the head portion  80  provides for bearing contact with the seat of the saddle  14  so that the bone screw can be arranged at a variety of angular orientations relative to the saddle corresponding to the multi-axial movement. In the illustrated embodiment, a tool-engaging recess  90  is formed in the upper portion of head portion  80 . The specific shape of tool-engaging recess  90  may be chosen to cooperate with any suitable driving tool. In the current embodiment, the recess  90  is a hex-shaped recess configured to receive a hex-driver. In relation to each other, the maximum diameter of the bone-engaging portion  84  may be greater, smaller, or equal to the maximum diameter of the head portion  80 , and at least a portion of the non-threaded portion  88  has a maximum diameter less than the maximum diameter of the head portion  80 . The reduced diameter of the non-threaded portion  88  may increase the available range of motion of the bone anchor  18  when assembled within the saddle  14 . 
     The size of the bone anchor  18  is selected based on the intended use. Generally, the bone anchor  18  may have a length between 10-52 mm. However, in some circumstances the bone anchor  18  may be larger or smaller than this range. It will be understood that other bone anchors may be utilized. For example, in some embodiments a bone anchor including a hook element is utilized. Such a hook includes a head portion identical or substantially similar to head portion  80  of bone anchoring member  18 . However, the shank portion of such a bone anchor would include or extend into a curved portion for engaging with and/or connecting to a bone. It should be understood that while specific bone anchors have been described herein, bone anchors of various head design, shaft design, thread pitch, and/or tip taper suitable for orthopedic use may be utilized. 
     Referring more specifically to  FIGS. 11 and 12 , the sleeve  20  is illustrated therein.  FIG. 11  is a diagrammatic bottom view of the sleeve  20 ;  FIG. 12  is a diagrammatic cross-sectional view of the sleeve  20 . 
     The sleeve  20  is substantially cylindrical with an opening  92  extending therethrough. The sleeve  20  is sized to friction fit around the lower portion  28  of the saddle  14 . In particular, the sleeve  20  is sized to retain the displaceable lower portion  28  around the head portion  80  of the bone anchor  18  to secure the bone anchor therein. In that regard, the sleeve  20  prevents the lower portion  28  of the saddle  14  from flexing outwardly to ensure that the head of the bone anchor is retained therein. It is recognized that in some embodiments the sleeve  20  allows some outward flexing of the lower portion  28 , but still retains the bone anchor securely therein. In some embodiments, the sleeve  20  compresses the lower portion  28  of the saddle  14  inwardly to retain the bone anchor. In the current embodiment the diameter of the opening  92  is slightly less than the diameter of the outer surface  56  of the lower portion  28  when in a neutral position. Thus, when the sleeve  20  is frictionally engaged with outer surface  56  the lower portion  28  is prevented from expanding outwardly. To facilitate the initial advancement of the sleeve  20  over the outer surface  56 , the sleeve includes a chamfer  94  between an upper surface  96  and an inner surface  98 . The chamfer  94  serves to guide the sleeve  20  around the outer surface  56 . In other embodiments, the lower portion  28  of the saddle  14  may include a chamfer and the chamfer  94  of the sleeve may be omitted. To ensure that the sleeve  20  has sufficient strength to prevent outward flexing of the lower portion  28  of the saddle and also prevent the lower portion of the saddle from stretching the sleeve  20 , the sleeve  20  may be formed of a material having an increased hardness with respect to the saddle  14  or at least the lower portion of the saddle. In addition to or in lieu of using a harder material for the sleeve, in some embodiments the thickness of the sleeve  20  may be increased and the thickness of the lower portion  28  decreased to increase the strength of the sleeve relative to the lower portion (e.g., see  FIGS. 20-24 ). 
     The upper surface  96  of the sleeve  20  is configured to engage with the shoulder  52  of the saddle  14 . In the current embodiment the upper surface  96  is substantially smooth and mates with the substantially smooth surface of the shoulder  52 . However, in other embodiments the upper surface  96  and/or the shoulder  52  may include mating features to secure the position of the sleeve  20  relative to the saddle  14 . Similarly, in other embodiments the inner surface  98  and/or the outer surface  56  may include mating features to secure the position of the sleeve  20  relative to the saddle  14 . 
     Referring again to  FIGS. 1-3 , the set screw  22  is illustrated as an externally threaded element. The set screw  22  may be a standard set screw or a break-offset screw such as those disclosed in U.S. Pat. No. 6,478,795, the entirety of which is incorporated herein by reference. The set screw may also include reverse angle threads as disclosed in U.S. Pat. No. 6,296,642, the entirety of which is incorporated herein by reference. In the current embodiment, the set screw  22  is configured to thread into threaded portion  46  of the saddle  14 . The set screw  22  advances along threaded portion  46  until it engages the rod  24  and urges the rod towards the bottom portion  36  of the channel  32 , thereby compressing crown member  16  downward and locking the head portion  80  of the bone anchor  18 . The set screw  22  includes an upper surface  96  having a tool-engaging recess  98 . The set screw  22  is sized such that the upper surface  96  will be substantially aligned with or below the top of the saddle  14  after locking engagement to minimize the overall profile of the system  10 . In other embodiments, the set screw  22  may extend beyond the top of the saddle  14  after locking engagement. The upper surface  96  may be rounded to reduce internal trauma to a patient or a substantially flat to minimize the profile. The tool-engaging recess  98  is configured to mate with a tool used for introducing the set screw  22  into saddle member  14 . In the current embodiment, the recess  98  is a hex-shaped recess configured to receive a hex-driver. In some embodiments, the recess  98  is substantially similar to the recess  90  of the bone anchor  18  such that the same surgical driving instrument may be utilized with both the set screw  22  and the bone anchor. The specific shape of tool-engaging recess  98  may be chosen to cooperate with any suitable driving tool. 
     Alternatively or additionally, set screw  22  can comprise an external element such as a nut or cap. The external element which may or may not include threads or other features for holding the external element to receiver member  30 . If an external element is used, the saddle  14  may be provided with compatible threads or other features for mating with the external element. Generally, set screws, locking screws, locking nuts, nuts, and combinations thereof used in this manner may be referred to herein as compression members. 
     Referring now to  FIGS. 13 and 14 , shown therein is the bone anchor assembly  12 .  FIG. 13  is a diagrammatic perspective view of the bone anchor assembly  12 ;  FIG. 14  is a diagrammatic front cross-sectional view of the bone anchor assembly  12 . In some embodiments, the bone anchor assembly  12  is assembled as follows. The crown member  16  is introduced into the bore  44  of the saddle  14  through the lower portion  28 . In some embodiments, the crown member  16  is sized such that it has a diameter less than the diameter of the opening of the lower portion  28  of the saddle  14  such that the crown member  16  may be inserted into the bore  44  without expansion or flexing of the lower portion. In other embodiments, the crown member  16  is sized such that it has a diameter larger than the diameter of the opening of the lower portion  28  of the saddle  14  such that the lower portion is expanded about the slotted reliefs  58  as the crown member is introduced. Upward movement of the crown member  16  within the bore  44  is limited by shoulder  50 . 
     Once the crown member  16  is positioned within the bore  44 , the head portion  80  of the bone anchor  18  is introduced into the bore  44  of the saddle  14  through the lower portion  28 . Introduction of the bone anchor  18  may require expansion of the lower portion  28  about the slotted reliefs  58 . Once positioned within the bore  44 , the upper section of the head portion  80  moveably engages the undersurface  76  of the crown member  16  and the lower section of the head portion moveably engages the seat of the saddle  14 . After the crown member  16  and bone anchor  18  have been positioned within the bore  44 , the sleeve  20  is positioned around the lower portion  28  until the upper surface  96  of the sleeve engages the shoulder  52  of the saddle  14 . The sleeve  20  thereby holds the lower portion  28  around the head portion  80  of the bone anchor  18  securing the head portion therein. The sleeve  20  prevents outward movement or flexing of the lower portion  28  that results in the bone anchor  18  and crown  16  being movably held within the saddle member  14 . That is, the bone anchor  18  and crown  16  are securely retained with the saddle member  14 , but are capable of movement with respect to the saddle member. In particular, the bone anchor  18  is capable of multi-axial movement with respect to the saddle member  14  after the sleeve  20  has been introduced (e.g., see the ghost views of the bone anchor  18  in  FIGS. 13 and 14 ). 
     In some embodiments, the sleeve  20  and the saddle member  14  engage one another via a friction fit. In that regard, the diameter of the inner opening of the sleeve  20  may be substantially similar to the outer diameter of the lower portion of the saddle member  14 . In some embodiments, the diameter of the inner opening of the sleeve  20  is less than the outer diameter of the lower portion of the saddle member. To encourage the friction fit, the outer surface  56  of the saddle  14  and/or the inner surface of the sleeve  98  may be roughened, textured, knurled, grit-blasted, or otherwise treated. In that regard, it is contemplated that the surfaces may be physically or chemically etched. Further, it is also contemplated that a biocompatible adhesive be utilized to further secure the sleeve  20  and the saddle member  14 . In other embodiments, the sleeve  20  and/or the saddle  14  may include mating features (e.g., projections and recesses) to secure engagement and/or alignment therebetween. Once the sleeve  20  is securely attached to the saddle member  14 , via friction fit or otherwise, the bone anchor assembly  12  is assembled. After assembly, the bone anchor assembly  12  may be provided to a surgeon for use in a surgical procedure. In many instances a plurality of bone anchor assemblies  12  will be provided as part of a surgical kit. 
     In use, the bone anchor assembly  12  may be implanted with the rod  24  and the set screw  22  as part of a orthopedic system as follows. One or more surgical exposures are made proximate to an area of the spine or other bones to be instrumented. The surgical exposures may be open, minimally-invasive, or of other types that are known in surgical practice. The vertebrae or other surgical site is prepared, for example by retracting tissue, removing tissue, drilling pilot holes, adjusting bony or other tissue, and/or other steps to prepare and fixate a bone or bones. 
     Prior to insertion of the bone anchor  18 , the saddle  14 , crown member  16 , bone anchor  18 , and sleeve  20  are assembled as described and shown above. In this pre-insertion state, the saddle  14  is multi-axially positionable and rotatable with respect to bone anchor  18 , so that the channel  32  can be oriented in a plurality of positions with respect to the bone anchor. The surgeon is able to change the relative orientation of saddle  14  with respect to bone anchor  18  immediately prior to and during surgery as desired by rotating the saddle about the head portion  80  of the bone anchor or vice-versa. Crown assembly  16  is held within the saddle  14  between the head portion  80  of the bone anchor  18  and shoulder  50 . It will be appreciated that assembly of these parts can take place at any time prior to insertion, by the surgeon, manufacture, or otherwise, and that kits including one or more of each type of part described above, in one or more sizes can be provided for the surgeon&#39;s convenience. 
     Once the surgical site is prepared, the assembled bone anchor assembly  12  is implanted into the site. In the embodiment in which bone anchor  18  is a bone screw, the bone engaging portion  84  of the shaft  82  is inserted into a bone. In some embodiments, the bone is a vertebra or part thereof, such as a pedicle. In that regard, the vertebra or other bone may be prepared to receive the bone anchor  18 . In some embodiments, the pilot hole or bore is predrilled or tapped into the vertebra for receiving the bone screw prior to the bone screw being inserted. In some embodiments, the bone screw is a self-drilling or self-tapping screw, and predrilling an opening may be omitted. An appropriate surgical tool or driver is engaged with the tool-engaging recess  90  of the bone anchor  18 . The surgical driver is inserted through the upper portion  26  of the saddle  14  along bore  44 , through the opening  74  in the crown member  16  to engage the bone anchor  18 . The surgical driver is then utilized to rotationally insert the bone anchor, and thereby the bone anchor assembly  12 , into the bone or vertebra. Preferably, the bone anchor  18  is driven into the vertebra to a recommended depth for adequate fixation, but preferably not so deep that the bottom of the saddle  14  will contact or press against the vertebral bone limiting the available movement thereof. In order for the multi-axial capability of the bone anchor assembly  12  to be utilized, the saddle  14  must be allowed to pivot in three dimensions about the head portion  80  of the bone anchor  18 . 
     After the bone anchor  18  has been inserted into the bone to the desired depth, the surgical tool is removed, and the surgeon makes any desired adjustments to the orientation of saddle  14  with respect to bone anchor  18 . For example, the surgeon may rotate or angle the saddle  14  relative to the bone anchor  18  to achieve a desired orientation. In particular, the surgeon may adjust the position of the saddle  14  to accommodate the reception of a longitudinal member, such as rod  24 . A longitudinal member, such as rod  24 , can be bent or otherwise contoured and then inserted into the surgical site. In particular, the rod  24  is inserted into channel  32  of the saddle. The rod  24  is inserted into towards the bottom portion  36  of the channel  32  at least to a point so that set screw  22  can be threadingly engage the threaded portion  46  of the saddle  14  to hold the rod  24  within the channel, as shown in  FIGS. 15 and 16 . 
     As shown in  FIGS. 15 and 16 , the rod  24  engages the upper surface  78  of the crown member  16 . As shown, the crown member  16  extends at least partially above the bottom of the channel  32 . In this position, the bone anchor  18  is still capable of multi-axial movement as illustrated by the space between the head portion  80  of the bone anchor  18  and the undersurface  76  of the crown member  16 . It is recognized that there may not actually be a space between the undersurface  76  and the head portion  80 . However, the space is shown to clearly illustrate that the head portion  80  is not locked in place in this orientation. Further, the lower section of the head portion  80  of the bone anchor  18  engages the seat portion of the saddle  14 . This engagement between the head portion  18  and the seat portion directs the at least some of the loading forces from the bone anchor  18  outwardly toward the sleeve  20 . Accordingly, at least some of the pullout forces may be distributed to the sleeve  20 . Distributing the pullout forces radially increases the load to the sleeve  20  and reduces the risk of disassembly during rod reduction and/or post-operatively. Other types of longitudinal members may be used instead of rod  24 . In that regard, the other longitudinal members may similarly be positioned within the channel  32  of the saddle  14 . As the rod  24  and saddle  14  are still moveable with respect to the bone anchor  18 , the surgeon can manipulate the spine and the implanted devices so that the spine is corrected or placed in a therapeutically improved position. 
     When the spine and implants are positioned as the surgeon desires, the rod  24  is locked within the channel  32  of the saddle  14  by advancing the set screw  22  against the rod. As the set screw  22  is advanced it urges the rod  24  towards the bottom portion  36  of the channel  32 . The rod  24 , in turn, presses against crown member  16  urging the crown member towards the head portion  80  of the bone anchor  18 . The rod  24  is advanced until the head portion  80  is locked in place between the crown member  16  and the seat of the saddle  14 . Generally, the rod  24  is positioned at least slightly above the bottom portion  36  of the channel  32  when in a locked position. The result is that the rod  24 , saddle  14 , and bone anchor  18  are locked in position with respect to one another, as shown in  FIG. 17 . As shown, in the locked position the top surface  96  of the set screw  22  is positioned substantially equal to or below the uppermost portion of the saddle  14 , the rod  24  is seated within the bottom portion of the channel  32 , and the undersurface  76  of the crown member  16  is engaged about the head portion  80  of the bone anchor. It should be noted that in some embodiments, the saddle  14 , the head portion  80 , and/or the rod  24  may be configured such that crown member  16  may be omitted. For example, in some embodiments the rod  24  may directly contact the head portion  80  to lock the relative positions of the bone anchor, saddle, and longitudinal member. Other bone anchor assemblies  12  and/or other implant devices are similarly tightened to hold the rod  24  and/or other members in the desired position. 
     As a part of the process of adjusting the position of the spinal column, one or more intervertebral implants may be inserted between adjacent vertebrae. Examples of such devices are disclosed in U.S. Pat. Nos. 5,984,967 and 6,113,637, which are incorporated herein by reference in their entirety. “Cage”-type intervertebral implants may also be packed or otherwise provided with one or more substances for inducing or promoting bone growth, as disclosed in U.S. Pat. No. 5,984,967, herein incorporated by reference in its entirety. Also, it is understood that the bone anchor assembly  12  and the associated devices may be implanted anteriorly, posteriorly, laterally, obliquely, combinations thereof, and/or in any other appropriate or necessary approach. Also, the bone anchor assembly  12  and associated devices may be used as a supplement to a Smith-Robinson technique. 
     In some aspects, the bone anchor assembly  12  as described above may be utilized to simplify a surgical procedure. For example, the bone anchor assembly  12  is threaded onto the vertebra from the top and the spinal rod  24  is loaded from the top, allowing a common surgical implantation approach. Also, the rod  24  does not need to be preloaded into the bone anchor assembly  12 . Rather, the rod  24  can be loaded after implantation of the bone anchor assembly  12 . Moreover, the multi-axial capability of the bone anchor assembly  12  allows the rod  24  to be connected with minimal contouring of the rod. 
     Referring to  FIGS. 20-23 , a system  110  for orthopedic implantation is shown according to another embodiment of the present disclosure. The system  110  includes a bone anchor assembly  112 . The bone anchor assembly  112  includes a receiver or saddle  114 , a crown washer  116  ( FIGS. 22 and 23 ), a bone anchor  118 , and a sleeve  120 . A set screw  22  secures a rod  24  to the bone anchor assembly  112 . In some aspects the system  110  is similar to the system  10  described above and may be assembled and utilized in substantially similar manners and therefore these aspects will not be described in detail. However, the sleeve  120  of system  110  has a larger thickness than the sleeve  20  of the bone anchor assembly  12  and the lower portion of the saddle  114  has smaller thickness than the lower portion  28  of the saddle  14 . In some embodiments, the increased thickness of the sleeve  120  relative to the lower portion of the saddle  114  allows more of the pullout force from the bone anchor  118  to be transferred to the sleeve. Additional features of the bone anchor assembly  112  will now be described. 
     Referring now to  FIGS. 24-29 , components of the bone anchor assembly  112  and, in particular, the saddle  114  and the sleeve  120  are shown individually in greater detail. Referring more specifically to  FIGS. 24-27 , the receiver member or saddle  114  is illustrated therein.  FIG. 24  is a diagrammatic side view of the saddle  114 ;  FIG. 25  is a diagrammatic top view of the saddle  114 ;  FIG. 26  is a diagrammatic bottom view of the saddle  114 ;  FIG. 27  is a diagrammatic cross-sectional view of the saddle  114 . 
     Saddle member  114  generally has a U-shape with an upper portion  126  and a lower portion  128 . The upper portion  126  comprises two upright portions  130  that define a channel  132  extending through saddle member  114 . Channel  132  is configured to accommodate an elongated member, such as rod  24 . Generally, the elongated member  24  is positioned above a bottom portion  136  of the channel  132  when in a locked position. The upright portions  130  of the saddle member  114  include an inner surface  138  and an outer surface  140 . A bore or hole  142  extends through the upright portions  130  between the outer surface  140  and the inner surface  138 . The holes  142  are substantially aligned with one another and are substantially perpendicular to the channel  132 . In the current embodiment and as shown in  FIGS. 24 and 27 , the outer surfaces  140  taper with respect to inner surfaces  138  as they extend upwardly. 
     The inner surfaces  138  generally extend substantially coaxially with the axis of a bore  144  extending longitudinally through saddle member  114 . The inner surfaces  138  of the upright portions  130  define an internally threaded portion  146 , as shown in  FIG. 27 . Internally threaded portion  146  is configured to be threadingly coupled with set screw  22 . The internally threaded portion  146  is configured such that the threads end above the rod  24  when the rod is secured within the saddle member  114 . In the current embodiment and as shown in  FIG. 27 , the inner surfaces  138  include an annular relief or cutout  148  that extends radially around bore  144  below the threaded portion  146 . 
     The aperture or bore  144 , which may be generally cylindrical, extends through the saddle member  114  from the upper portion  126  to the lower portion  128 . The bore  144  extends along a longitudinal axis of the saddle member  114  and substantially transversely to and in communication with channel  132 . In the current embodiment, the bore  144  extends entirely through the saddle member  114 . The bore  144  is configured to allow a driving tool access to the bone anchor  118 . By engaging the bone anchor  118  through the bore  144  and driving the bone anchor into a bone, such as a vertebra, the bone anchor assembly  112  can be secured to the bone with the bone anchor movably retained within the saddle member  114 . The inner surfaces  138  also include a stop portion or shoulder  150  below the annular relief  148  and in communication with bore  144 . The shoulder  150  acts as a stop for the crown member  116 . 
     The lower portion  128  of the saddle member  114  has a reduced diameter compared to the upper portion  126 . A shoulder  152  is defined between the upper portion  126  and the lower portion  128  by the change in diameter. The lower portion  128  includes an inner surface  154  and an outer surface  156 . The inner surface  154  at least partially defines bore  144 . The lower portion  128  of the saddle member  114  is configured for bottom-loading. In that regard, the crown member  116  and the bone anchor  118  may be inserted into the saddle member  114  through an opening in the bottom of the lower portion  128 . The bottom of the lower portion  128  includes opening large enough to receive the crown member  116  and bone anchor  118 . However, the opening is not so large that the crown member  116  and bone anchor  118  cannot be retained within the saddle member  114 . 
     In the present embodiment the lower portion  128  includes slotted reliefs  158 . The slotted reliefs  158  extend through the lower portion  128  from the inner surface  154  to the outer surface  156  and extend upwardly from the bottom of the lower portion. The slotted reliefs  158  include an elongated slot portion  160  and a relief  162  having an increased width compared to the slot portion. In the current embodiment, the relief  162  is positioned at the uppermost end of the elongated slot portion. The slotted reliefs  158  render the lower portion  128  at least partially resiliently displacable or flexible. Accordingly, the lower portion  128  may flex outwardly (expand) slightly to allow insertion of the bone anchor  118  and/or crown  116  into bore  144 . It is contemplated, however, that the opening in the bottom of the lower portion  128  be sized such that the head of the bone anchor  118  may be inserted without expansion of the lower portion. The lower portion  128  may also flex inwardly (contract) to secure the bone anchor  118  and crown  116  within the bore  144 . In that regard, engagement with the sleeve  120  may cause the lower portion  128  to contract and thereby retain the bone anchor  118  and crown  116  therein. In some embodiments, the lower portion  128  may retain the bone anchor  118  and crown  116  therein in a substantially neutral position, that is, neither expanded nor contracted. There are three slotted reliefs  158  in the current embodiment. The three slotted reliefs  158  are equally spaced 120 degrees apart from one another around the circumference of the lower portion  128 . 
     As mentioned above, the inner surface  154  is configured to receive the head of the bone anchor  118 . In that regard, the inner surface  154  includes a tapered portion  164  extending inwardly from the inner surface to a cylindrical bore  166 , thereby defining an edge  167 . In some embodiments, the tapered portion  164  has a substantially conical shape. The bore  166  defines the narrowest opening of the bore  144  in the lower portion  128 . The intersection of the tapered portion  164  and the cylindrical bore  166  generally define an edge  167 , which comprises a seat  168  of the saddle  114  configured for mating with the bone anchor  118 . In that regard, the tapered portion  164  may extend from the inner surface  154  at an angle between 90 and 170 degrees, which may also be considered 10-90 degrees relative to a longitudinal axis of the saddle  114 . In the current embodiment, the angle is approximately 20 degrees. In another particular embodiment, the angle is approximately 55 degrees. When assembled the head of the bone anchor  118  engages the edge  167  such that at least some of the loading forces from the bone anchor are directed radially outward to the sleeve  120 . Distributing the pullout force radially increases the load on the sleeve  120  and reduces the risk of disassembly. 
     The inner surface  154  also includes a tapered portion  170  extending outwardly from the bore  166 . The tapered portion  170  is configured to allow the bone anchor  118  to translate through a plurality of positions corresponding to multi-axial movement with respect to the saddle  114  by reducing the interference of the lower portion  128  with the shaft of the bone anchor. In some embodiments, the tapered portion  170  has a substantially conical shape. The inner surface  154  also includes angular cutout portions  172  to increase the allowable angulation of the bone anchor  118  in relation to the saddle  114 . As shown in  FIG. 26 , there are three angular cutout portions  172  that are generally partially cylindrical in shape and extend at approximately a 45 degree angle with respect to the longitudinal axis of the saddle  114 . In the current embodiment, the three angular cutout portions  172  are equally spaced 120 degrees apart from one another around the circumference of the lower portion  128 . In the current embodiment, the slotted reliefs  158  are substantially aligned with and positioned within the cutout portions  172 . 
     The outer surface  156  of the lower portion  128  of the saddle  114  has a substantially cylindrical profile and is configured to mate with the sleeve  120 . In the current embodiment, the outer surface  156  is substantially flat and coaxial with the longitudinal axis of the saddle  114  when in a neutral position (i.e., not expanded or contracted). Engagement between the sleeve  120  and the outer surface  156  of the lower portion  128  results in the bone anchor  118  and crown  116  being movably held within the saddle member  114 . That is, the bone anchor  118  and crown  116  are securely retained within the saddle member  114 , but are capable of movement with respect to the saddle member. In particular, the bone anchor  118  is capable of multi-axial movement with respect to the saddle member  114 . The bone anchor  118  is locked with respect to the saddle  14  upon compression of the rod  24  by the set screw  22 , which in turn compresses the crown member  116  onto the head of the bone anchor  118 , thereby securing the bone anchor between the crown member and the seat  168  of the saddle movement of the bone anchor relative to the saddle. 
     In the current embodiment, the sleeve  120  has a thickness greater than the thickness of the lower portion  128  of the saddle  114 . In that regard, it is contemplated that the ratio of thicknesses between the sleeve  120  and the lower portion  128  of the saddle be in the range of 1.25:1 and 10:1. In some embodiments, the ratio of the thicknesses is approximately 2:1. Referring now to  FIGS. 26 ,  28 , and  29 , the sleeve  120  is shown therein.  FIG. 28  is a diagrammatic bottom view of the sleeve  120 ;  FIG. 29  is a diagrammatic cross-sectional view of the sleeve  120 . 
     In the current embodiment the sleeve  120  includes angular cutout portions  174 . The angular cutout portions  174  are configured to be substantially aligned with the angular cutout portions  172  of the lower portion  128  of the saddle  114  to increase the allowable angulation of the bone anchor  118  in relation to the saddle  114 . Accordingly, there are three angular cutout portions  174  that are generally partially cylindrical in shape and extend at approximately a 45 degree angle with respect to the longitudinal axis of the sleeve  120 . The three angular cutout portions  174  are equally spaced 120 degrees apart from one another around the circumference of the sleeve  120 . The angular cutout portions  174  may have slightly increased width as compared to a continuation of angular cutout portions  172  to allow the cutout portions  172 ,  174  to be sufficiently aligned even when not perfectly aligned. In that regard, the angular cutout portions  174  may be slightly offset with respect to the angular cutout portions  172 , yet the overall function of the cutouts is not adversely affected. In some embodiments, the outer surface  156  and/or the inner surface  198  of the sleeve  120  include mating features (e.g., projections and recesses) to facilitate alignment of the cutout portions  172  and  174  and/or engagement between the sleeve and saddle  114 . 
     For example, referring now to  FIGS. 30 and 31 , the sleeve  120  may include a recess  176  configured to receive a projection  178  of the saddle  114 . As shown, the projection  178  may extend from the shoulder  152  of the saddle  114  and the recess  176  may be positioned adjacent an upper portion  196  of the sleeve  120  such that as the sleeve is urged over the lower portion  128  the projection engages the recess. If the projection  178  and recess  176  do not engage as the sleeve  120  is advance upwardly around the lower portion  128 , then translation of the sleeve will be limited by engagement of the projection with an upper surface of the sleeve. The recess  176  and projection  178  are positioned such that alignment of the recess and projection corresponds with alignment of the angular cutout portions  172  and  174 . It is understood that numerous other combinations of structures may be utilized to align the sleeve  120  with the saddle  114 . Also, it is understood that the position of the structures may be varied. For example, it is contemplated that the outer surface  156  of the saddle  114  may include features for mating with corresponding features of the inner surface of the sleeve  120 . 
     In some embodiments, the sleeve  20 ,  120  is not substantially cylindrical. Rather, in some embodiments at least one side of the sleeve is substantially planar. In some embodiments, two opposing outer portions of the sleeve have substantially planar outer profiles and two other opposing outer portions of the sleeve have substantially cylindrical outer profiles. For example, referring to  FIGS. 32 and 33 , a sleeve  200  having planar surface  202  may be configured for use with a saddle  204  having planar outer surfaces  206  such that the planar surfaces of the sleeve and saddle are substantially aligned upon engagement between the sleeve and saddle. The planar surfaces  202 ,  206  of the sleeve and saddle may serve to limit the overall size and profile of the bone anchor assembly. An example of a saddle member having planar outer surface portions is disclosed in U.S. Pat. No. 5,728,098, herein incorporated by reference in its entirety. 
     Referring now to  FIGS. 34 and 35 , shown therein is a bone anchor assembly  212  according to another embodiment of the present disclosure.  FIG. 34  is an exploded perspective view of the bone anchor assembly;  FIG. 35  is a side view of a saddle  214  of the bone anchor assembly. The bone anchor assembly  212  includes the receiver or saddle  214 , a crown washer  216 , a bone anchor  218 , and a sleeve  220 . Referring more specifically to  FIG. 35 , the saddle  214  may be substantially similar to the saddles  14  and  114  in some aspects. The saddle  214  includes an upper portion  222  and a lower portion  224 . The lower portion  224  of the saddle member  214  has a reduced diameter compared to the upper portion  222  such that a shoulder  226  is defined between the upper and lower portions. The lower portion  224  is configured for bottom-loading. That is, the crown member  216  and the bone anchor  218  may be inserted into the saddle member  214  through an opening in the bottom of the lower portion  224 . In that regard, the lower portion  224  must have an opening large enough to receive the crown member  216  and bone anchor  218 . However, the opening cannot be so large as to prevent the crown member  216  and bone anchor from being retained within the saddle member  214 . Accordingly, in the present embodiment the lower portion  224  includes slotted reliefs  228 . The slotted reliefs  228  extend through the lower portion  224  and extend upwardly from the bottom of the lower portion. The slotted reliefs  228  include an elongated slot portion and a relief having an increased width compared to the slot portion. In the current embodiment, the elongated slot portion includes surfaces  229  that extend at an oblique angle with respect to a longitudinal axis of the saddle  214 . In some embodiments, the angle of the surface  229  substantially matches the angle of an outer surface  230 , as shown in  FIG. 35 . The slotted reliefs  228  render the lower portion  28  at least partially flexible or resiliently deformable. 
     In the current embodiment, the lower portion  224  is movable between a first position for receiving the crown and the head of the bone anchor and a second position for retaining the crown and the head of the bone anchor therein. In the first position, the lower portion  224  is flared outwardly with respect to a longitudinal axis of the saddle  214  such that an outer surface  230  of the lower portion extends at an oblique angle with respect to the longitudinal axis. In the second position, the lower portion  224  is contracted by the sleeve  220  to retain the crown  216  and bone anchor  218  therein. In that regard, a tool  231  may be utilized to initially contract the lower portion  224 , as indicated by the arrows of  FIG. 35 , and then the sleeve  220  may be positioned around the lower portion to retain it in the second, contracted position. In  FIG. 35 , the dashed lines  229 ′ and  230 ′ illustrate the positions of the surfaces  229  and  230 , respectively, when in the second position. In some embodiments, the outer surface  230  of the lower portion  224  extends substantially coaxially with the longitudinal axis of the saddle in the second position. In some embodiments, the outer surface  230  extends at an oblique angle with respect to the longitudinal axis in the second position. 
     Referring now to  FIGS. 36-38 , shown therein is a bone anchor assembly  232  according to another embodiment of the present disclosure.  FIG. 36  is a diagrammatic exploded perspective view of the bone anchor assembly  232 ;  FIG. 37  is a diagrammatic front cross-sectional view of the bone anchor assembly  232 ;  FIG. 38  is a diagrammatic front cross-sectional view of a saddle member  234  and a crown member  236  of the bone anchor assembly  232 . The bone anchor assembly  232  includes the receiver or saddle  234 , the crown washer  236 , a bone anchor  238 , and a sleeve  240 . In some aspects the bone anchor assembly  232  and its components, the saddle  234 , the crown  236 , the bone anchor  238 , and the sleeve  240 , may be substantially similar to the bone anchor assemblies and components described above. Therefore, only some aspects of the bone anchor assembly  232  will be described in detail. 
     The saddle member  234  generally has a U-shape with an upper portion  242  and a lower portion  244 . The upper portion  242  is configured to receive an elongated member, such as rod  24  described above, and also configured to be threadingly coupled with a set screw, such as set screw  22  describe above. The lower portion  244  of the saddle member  234  has a reduced diameter compared to the upper portion  242 . Accordingly, an inner shoulder  246  and an outer shoulder  248  are defined between the upper portion  242  and the lower portion  244 . The inner shoulder  246  is provided to act as a stop for the crown member  236 . Generally, the inner shoulder  246  serves as an upper boundary for the crown member  236  within the saddle  234 , preventing upward movement of the crown beyond the shoulder. In the current embodiment, the inner shoulder  246  is positioned below the u-shaped channels of the upper portion. As shown, the crown member  236  includes an upper portion  250 , a lower portion  252 , and a shoulder  254 . The upper portion  250  has a reduced diameter compared to the lower portion  252  and is sized such that the upper portion may extend upwards beyond the inner shoulder  246  of the saddle  214  to engage with the rod  24 . In that regard, the shoulder  254  of the crown member  236  engages with the inner shoulder  246  of the saddle  234  to limit the upward translation of the crown member  236  within the saddle. 
     Referring more specifically to  FIG. 38 , the lower portion  244  includes an inner surface  256  configured to receive at least the head of a bone anchor. In that regard, the inner surface  256  includes a tapered portion  258  extending inwardly from the inner surface to a surface  260  defined by a cylindrical bore, the intersection of the tapered portion  258  and the surface  260  defining an edge  262 . In some embodiments, the tapered portion  258  has a substantially conical shape. The surface  260  defines the narrowest opening in the lower portion  244  of a bore extending through the saddle  234  along the longitudinal axis of the saddle. In the current embodiment, the diameter of the bore defining the surface  260  is substantially similar to the diameter of a bore defining a surface  264  adjacent the inner shoulder  246  of the saddle  234 . The diameter of the upper portion  250  of the crown  236  is sized such that the upper portion may pass through the bores defined by the surfaces  260 ,  264 . In other embodiments, especially where the surfaces  260 ,  264  and/or the upper portion  250  of the crown  236  are not cylindrical, the dimensions of each of these components may be sized to work in a similar manner, but with different shapes. Accordingly, in some embodiments the outer dimension of the upper portion of the crown is sized such that it may pass through the openings defined by inner surfaces of the saddle  34 . 
     In the current embodiment, the diameter of the upper portion  250  of the crown  236  is substantially similar to or slightly less than the diameter of the bores defining the surfaces  260 ,  264 . In that regard, in some embodiments the upper portion  250  of the crown  236  may be positioned within the bore defining surface  260  to align the crown with the opening. Once the crown  236  is aligned with the opening in the saddle  234 , the lower portion  252  of the crown is advanced into and through the opening—thereby expanding or flexing the lower portion  244  of the saddle—until the crown is positioned entirely within the saddle. In some embodiments the crown  236  includes a tapered surface extending between the upper portion and the lower portion to facilitate introduction of the crown into the saddle  234 . In such embodiments, the tapered surface may be in addition to the shoulder  254  or in lieu of the shoulder. 
     Referring now to  FIGS. 39-43 , shown therein is a bone anchor assembly  312  according to another embodiment of the present disclosure.  FIG. 39  is a diagrammatic exploded perspective view of the bone anchor assembly  312 ;  FIG. 40  is a diagrammatic front cross-sectional view of the bone anchor assembly  312 ;  FIG. 41  is a diagrammatic perspective cross-sectional view of the bone anchor assembly  312 ;  FIG. 42  is a diagrammatic perspective view of a saddle member  314  and a crown member  316  of the bone anchor assembly  312 ;  FIG. 43  is a diagrammatic front cross-sectional view of the saddle member  314  and the crown member  316 . In some aspects the bone anchor assembly  312  and its components, the saddle  314 , the crown  316 , the bone anchor  318 , and the sleeve  320 , may be substantially similar to the bone anchor assemblies and components described above. Therefore, only some aspects of the bone anchor assembly  312  will be described in detail. 
     Referring more specifically to  FIGS. 40 and 41 , the crown member  316  has a rounded outer surface  320 , a substantially cylindrical inner surface  322 , an undersurface  324 , and an upper surface  326 . Crown member  316  is sized to fit within the saddle  314  such that crown member has some axial and rotational freedom of movement therein. For example, crown member  316  is sized to move axially between a shoulder  328  of the saddle  314  and the head of the bone anchor  318 . Further, in some embodiments crown member  316  is sized such that it may be rotated within saddle  314  to facilitate insertion of the crown member therein. In that regard, the outer surface  320  may be partially spherical in shape. In some embodiments, the outer surface  320  has a radius of curvature substantially similar to the radius of curvature of the outer surface of the head of the bone anchor  318 . 
     Undersurface  324  is configured to accommodate at least a portion of the head of the bone anchor  318 . The undersurface  324  may be shaped (e.g. spherical, rounded, conical, or otherwise) to allow relative multi-axial movement between the crown and the head of the bone anchor  318 . In the current embodiment, the undersurface  324  is partially rounded or spherical to mate with the spherical head of the bone anchor. In that regard, the rounded portion of the undersurface  324  has substantially the same radius of curvature as the head of the bone anchor  318 . The undersurface  324  is shaped such that sufficient compression of the head of the bone anchor  318  between the crown member  316  and the seat of the saddle  314  can fixedly secure the bone anchor relative to the saddle. The upper surface  328  of the crown member  316  is configured to engage with the an elongated member, such as spinal rod  24 . In particular, the upper surface  328  is configured such that the crown member  316  may be compressed downwardly by an elongated member to secure the bone anchor  318  in place. 
     Referring now to  FIGS. 42 and 43 , in some aspects the crown member  316  is bottom-loaded into the saddle  314 . In some embodiments, the crown member  316  is loaded such that the saddle  314  is not expanded. For example, as shown in  FIG. 42  the crown member  316  is oriented such that the cylindrical bore defined by the inner surface  322  is substantially transverse to a longitudinal axis of the saddle  314 . Then, utilizing at least a portion of at least one of the cutouts in the bottom portion of the saddle  314 , the crown member  316  is inserted through the bottom of the saddle without expanding the saddle. As best seen in  FIG. 40 , the outer diameter of the crown member  316  is greater than the narrowest part of the bore in the lower portion of the saddle  314 . Accordingly, the crown member  316  cannot be inserted with the cylindrical bore defined by the inner surface  322  being substantially parallel with the longitudinal axis of the saddle  314  without expanding the saddle. Once positioned within the saddle  314 , the crown member  316  is rotated approximately 90 degrees with respect to the longitudinal axis of the saddle. As shown in  FIG. 43 , the outer diameter of the crown member  316  is such that it may be rotated within the saddle  314 . In that regard and as mentioned above, in some embodiments the outer diameter of the crown member  316  may be substantially similar to that of the head of the bone anchor  318 . After being rotated within the saddle, the crown member  316  is then movably held therein by the shoulder  328  and the lower portion of the saddle. 
     Referring now to  FIG. 44 , in some aspects the crown member  316  is top-loaded into the saddle  314 . As shown, the crown member  316  is oriented such that the cylindrical bore defined by the inner surface  322  is substantially transverse to a longitudinal axis of the saddle  314 . Further, the crown member  316  is aligned such that the crown member may be lowered into the saddle without interference from the portion of the saddle defining the shoulder  328 . In the current embodiment, the crown member  316  is substantially aligned with the channels defined in the upper portion of the saddle. Then, the crown member  316  is inserted through the top of the saddle and into the lower portion of the saddle. Once positioned within lower portion of the saddle  314 , the crown member  316  is rotated approximately 90 degrees with respect to the longitudinal axis of the saddle. Again, as shown in  FIG. 43 , the outer diameter of the crown member  316  is such that it may be rotated within the saddle  314 . After being rotated within the saddle, the crown member  316  is then movably held therein by the shoulder  328  and the lower portion of the saddle. 
     While the several embodiments of the disclosure have been illustrated and described in detail in the drawings and foregoing description, this is to be considered as illustrative and not restrictive in character, it being understood that all changes and modifications that come within the spirit of the disclosure are desired to be protected. For example, it is fully contemplated that the features described with respect to one embodiment may be selectively combined with the features of other embodiments. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.