Abstract:
A posterior fixation system includes a saddle member and an anchoring member. The anchoring member anchors the saddle member to bone. The saddle member includes a pair of upright portions that define a channel. The saddle member further has a hole therethrough bounded by an inner wall, and the hole forms a lower opening in the saddle member. The lower opening in the saddle member may contain angular cutouts placed symmetrically about the axis of the saddle to increase the allowable angulation of the bone screw in relationship to the axis of the saddle. The channel is adapted to receive an orthopedic rod, and the hole in the saddle member is adapted to receive the anchoring member. The saddle member and the anchoring member can be coupled so as to allow multi-axial movement of the members.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to orthopedic implants used for correction of spinal injuries or deformities, and more specifically, but not exclusively, concerns apparatuses for fixing a portion of the spine, such as the cervical spine, to allow correction or healing thereof.  
         [0002]     In the field of spinal surgery, it is known to place implants into vertebrae for a number of reasons, including (a) correcting an abnormal curvature of the spine, including a scoliotic curvature, (b) to maintain appropriate spacing and provide support to broken or otherwise injured vertebrae, and (c) perform other therapies on the spinal column.  
         [0003]     Typical implant systems utilize a rod as the support and stabilizing member. In such an implant, a series of two or more screws are inserted into two or more vertebrae to be instrumented. A rod is then placed within or coupled to the heads of the screws, or is placed within a connecting device that links the rod and a screw head, and the connections are secured. In this way, a supporting structure is fixed to the vertebrae.  
         [0004]     Many varieties of bone fixation screws are mono-axial in construction. That is, such devices are connected to the rod or plate such that a longitudinal axis through the rod or plate and a longitudinal axis through the fixation device are capable of only a single position with respect to each other. While useful in certain circumstances, in many therapeutic situations the degree of precision required to use such an inflexible device is impractical.  
         [0005]     More recently, bone fixation devices having multi-axial capability have been introduced. Examples of such constructs are shown in U.S. Pat. Nos. 5,797,911, 5,954,725, 5,810,818 and 6,485,491 which is hereby incorporated by reference. These devices help to reduce the required precision of placement of the fixation device, since the saddle portion of the fixation device is multi-axially positionable on the anchor member portion. The saddle portion can thus be positioned so as to easily receive the rod, limiting or removing much of the positioning difficulty inherent in prior devices.  
         [0006]     Most such devices are designed for spinal fixation at the thoracic and lumbar levels and allow only a limited angulation of the anchor member in relation to the saddle member. There is a need in the art for a multi-axial bone attachment assembly, and particularly one that is useful in the cervical region of the spine with a greater degree of such angulation.  
       SUMMARY OF THE INVENTION  
       [0007]     One embodiment of the present invention is a unique multi-axial bone attachment assembly that includes a saddle member and a bone anchoring member. The saddle member has a plurality of upright portions that define a channel through the saddle member. The saddle member further has a hole therethrough bounded by an inner wall, and the hole forms a lower opening in the saddle member. The lower opening in the saddle member may contain angular cutouts placed symmetrically about the axis of the saddle to increase the allowable angulation of the bone screw in relationship to the axis of the saddle. The position, angle and number of cutouts may vary as required by the application of the multi-axial bone attachment assembly. The bone-anchoring member extends through the opening. The bone-anchoring member includes a head portion and an anchoring portion. A further embodiment of the present invention includes a washer (crown member). The washer may have a recessed portion for accommodating an orthopedic rod and may include a radially extending projection. The washer is fitted within the hole of the saddle member and atop the bone-anchoring member.  
         [0008]     Further features and practical advantages of different embodiments of the invention will emerge from the description of the exemplified embodiments with reference to the figures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  shows a partial cross-sectional view of a bone anchor assembly according to one embodiment of the present invention.  
         [0010]      FIG. 2  shows a partial cross-sectional view of a bone anchor assembly according to another embodiment of the present invention.  
         [0011]      FIG. 3  shows a side view of a saddle member according to one embodiment of the present invention.  
         [0012]      FIG. 4  shows a side view of a saddle member according to one embodiment of the present invention.  
         [0013]      FIG. 5  shows a cross-sectional view of the saddle member according to one embodiment of the present invention.  
         [0014]      FIG. 6  shows a cross-sectional view of the saddle member according to another embodiment of the present invention.  
         [0015]      FIG. 7  shows a top view of the saddle member of  FIG. 2 .  
         [0016]      FIG. 8  shows a side view of an anchor member according to one embodiment.  
         [0017]      FIG. 9  shows a perspective view of a washer according to one embodiment of the present invention.  
         [0018]      FIG. 10  shows a top view of the washer of  FIG. 10 .  
         [0019]      FIG. 11  shows a cross-sectional view of the washer of  FIG. 9   
         [0020]      FIG. 12  shows a top view of another embodiment of a washer according to the present invention.  
         [0021]      FIG. 13  shows a side view of a set screw according to one embodiment of the present invention.  
         [0022]      FIG. 14  shows a top view of the set screw of  FIG. 13 .  
         [0023]      FIG. 15  shows a cross-sectional view of an external set screw according to another embodiment of the present invention.  
         [0024]      FIG. 16  shows a top view of the external set screw of  FIG. 15 .  
         [0025]      FIG. 17  shows a perspective view of a snap ring for use in the present invention.  
         [0026]      FIG. 17   a  shows a side view of an alternative embodiment of a snap ring for use in the present invention.  
         [0027]      FIG. 18  shows a top view of the snap ring of  FIG. 17 .  
         [0028]      FIG. 19  shows a partial cross-sectional view of a bone anchor assembly according to another embodiment of the present invention.  
         [0029]      FIG. 20  shows a cross-sectional view of an embodiment of a saddle member shown in  FIG. 19 .  
         [0030]      FIG. 21  shows a top view of the saddle member of  FIG. 20 .  
         [0031]      FIG. 22  shows a cross-sectional view of a washer shown in  FIG. 19 .  
         [0032]      FIG. 23  shows a bottom view of the saddle member according to one embodiment of the present invention.  
         [0033]      FIG. 24  shows an isometric view of a saddle member of  FIG. 23 .  
         [0034]      FIG. 25  shows a bottom view of the saddle member according to one embodiment of the present invention.  
         [0035]      FIG. 26  shows an isometric view of a saddle member of  FIG. 25 .  
         [0036]      FIG. 27  shows a cross-sectional view of the saddle member according to one embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     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 invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of embodiments of the invention as illustrated therein, being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0038]     In  FIG. 1 , there is shown an embodiment of a multi-axial bone anchor assembly  20 . Bone anchor assembly  20  includes a saddle member  22 , a bone anchoring member  24 , and set screw member  30 . Saddle member  22  generally has a U-shape, with two upright portions  32  defining a channel  34  extending through saddle member  22 . Channel  34  is then configured to accommodate an elongated member  36 , such as a spinal rod. For posterior cervical fixation, rod  36  may have one of a number of desired lengths and diameters. As seen in  FIG. 1 , the width of channel  34  is slightly larger than the diameter of rod  36 , which allows easier insertion of rod  36  into channel  34 , also allows for compensation for contouring of the rod, and allows use of a range of rod sizes with the same saddle member  22 . The curved bottoms  255 , shown in  FIG. 3 , of channel  34  are arranged such that the top of the head portion  58  of the of bone anchor member  24 , when fully nested into the lower portion of hole  38 , extends above the edge of the curved bottoms  255  of channel  34  such that rod  36  positioned in channel  34  will pressingly engage the head portion  58  of bone anchor member  24 . Saddle member  22  further includes a hole  38  therethrough, the axis of hole  38  being substantially perpendicular to the axis of channel  34 .  
         [0039]     In the particular embodiments of saddle member  22 , illustrated in  FIGS. 1, 2  and  19 , upright portions  32  each have an outer surface  40  and an inner surface  42 . Inner surfaces  42  are substantially parallel to the axis of hole  38 , along a longitudinal axis of saddle member  22 . In the embodiment shown in  FIG. 3 , outer surfaces  40  are angled with respect to inner surfaces  42  and the longitudinal axis of saddle member  22  with an inward taper, which taper allows for easier handling of the saddle member  22  and reduced bulk of saddle member  22 . Near the bottom of saddle member  22 , hole  38  is narrowed by a wall portion  44 . In one embodiment the wall portion  44  contains a section  52  that is circular and the plane of which is substantially perpendicular to the longitudinal axis of hole  38 . However wall portion  44  and section  52  may be of any shape as long as the diameter of hole  38  at section  52  of wall portion  44  is greater than that of shank  72  and less than that of head portion  58  of the of bone anchor member  24 .  
         [0040]     Upright portions  32  further include an internally threaded portion  52 , as shown in  FIG. 1 . Internally threaded portion  52  is configured to be threadedly coupled with set screw  30 , as described below. In other embodiments, as shown in  FIGS. 2 and 19 , the internally threaded portions  52   a  and  52   d  respectively are configured so that they end above rod  36  when set screw  30  is secured in saddle member  22 . In one embodiment, as shown in  FIGS. 2 and 19 , saddle member  22  includes a relief groove  16  that extends around hole  38 . Relief groove  16  eliminates the helical thread run out typically found on internal threads. In other embodiments, saddle member  22  does not contain a relief groove. In further embodiments, upright portions  32  may include an externally threaded portions  164 , as shown in  FIG. 4  instead of an internally threaded portions. Externally threaded portions  164  are configured to be threadedly coupled with external set screw  30   a , as described hereafter. In this embodiment outer surfaces  40  are parallel to one another.  
         [0041]     Below wall portion  44 , hole  38  opens outward by virtue of a conical wall portion  46 . Conical wall portion  46  allows bone anchor member  24  to be positioned in any of an infinite number of limited angular positions relative to the longitudinal axis of the saddle member  22  by reducing interference of the lower portion of saddle member  22  with a shank portion  72  of bone anchor member  24 .  
         [0042]     As shown in  FIGS. 23-27 , wall portion  44  conical wall portion  46  contain angular cutouts  62  placed symmetrically about the longitudinal axis of hole  38  to increase the allowable angulation of the bone screw in relation to the longitudinal axis of hole  38 . Any number of cutouts, the shape of the cutouts, the position of the cutouts in relation to the axis of channel  34 , the angle of the cutouts in relation to a plane that is perpendicular to the axis of hole  38 , the size of the cutouts and the angular spacing between each cutout may vary for specific applications. As shown in the embodiment in  FIGS. 23 and 24 , there are three angular cutouts that are generally cylindrical in shape. Two of angular cutouts  62  being offset 30 degrees from the axis of channel  34  and all three angular cutouts  62  are spaced 120 degrees apart from one another. As shown in the embodiment in  FIGS. 25 and 26 , there are three angular cutouts that are generally cylindrical in shape. One of angular cutouts  62  being on the axis of channel  34  and all three angular cutouts  62  are spaced 120 degrees apart from one another.  
         [0043]     As shown in  FIG. 8 , one embodiment of the bone anchor member  24  of the present invention has a threaded portion  56  containing threads  60 , a shank  72  and a head portion  58 . Head portion  58  of bone anchor member  24  in one embodiment is substantially spherical. However it should be understood that any external contour which is equidistant from the center point of the head portion  58  could be utilized. In the illustrated embodiment, a tool-engaging recess  76  is formed in the upper portion of head portion  58 . The specific shape of tool-engaging recess  76  may be chosen to cooperate with any suitable screw-driving tool. In relation to each other, the diameter of the threaded portion  56  should be less than the diameter of the head portion  58 , and the shank  72  should be narrower than the widest portion of threaded portion  56 . As is apparent, any head design, shaft design, thread pitch or tip taper suitable for insertion into a vertebral body can be utilized. Threaded portion  56  can even be larger than head portion  58  if the thread pitch allows threading through wall portion  44 .  
         [0044]     Multi-axial bone anchor assembly  20  may further include a set screw  30 . In the embodiments, illustrated in  FIGS. 13-14 , set screw  30  is generally cylindrical and has external threads  102 . External threads  102 , in one embodiment, are buttress threads. In another embodiment, threads  102  could be reverse angle threads so as to minimize splaying between the two upright members  32 . An example of such reverse angle threading is disclosed in U.S. Pat. No. 6,296,642, which is hereby incorporated by reference.  
         [0045]     As illustrated in  FIGS. 13-14 , this embodiment of set screw  30  includes a substantially flat end surface  110  in order to minimize the profile of assembly  20 . Set screw  30  further includes a tool-engaging bore  112 . Tool-engaging bore  112  is used in conjunction with a tool for introducing set screw  30  into saddle member  22 .  
         [0046]     In another embodiment, as illustrated in  FIGS. 15-16 , set screw  30   a  comprises internal threading  202  which is intended to mate with external threading  164  on the upwardly extending members  32  of saddle member  22 . The set screw also comprises an inner plug portion  300  having a bottom surface which is intended to seat against the top surface of rod  36  seated in saddle  22 , providing a means for driving the rod  36  downward against the head portion of bone anchor member  24  in one embodiment and against the washer  26  in another embodiment. In another embodiment, the bottom surface  301  of inner plug  300  comprises a plurality of raised metal projections to engage and press into rod  36 . Set screw  30   a  has at one end a tool-engaging bore  112 . As illustrated in  FIG. 15 , set screw  30   a  may also include a rounded end surface  206  to reduce internal trauma to a patient or a substantially flat end surface in order to minimize the profile of assembly  20 . Tool-engaging bore  112  is used in conjunction with a tool for introducing set screw  30   a  onto saddle member  22 .  
         [0047]     In  FIG. 2 , there is shown a multi-axial bone anchor assembly  20   a  according to another embodiment of the present invention. Similar to the embodiment in  FIG. 1  and described above, bone anchor assembly  20   a  includes a bone anchoring member  24  and a set screw member  30 . However, this embodiment also comprises a washer (crown member)  26 . Also in this embodiment, the internally threaded portions  52   a  are configured so that they end above rod  36  when set screw  30  is secured in saddle member  22 . Saddle member  22  further includes a relief groove  16  that extends around hole  38 . In some embodiments, assembly  20   a  will further include a C-shaped snap ring  28 , which are fitted with saddle member  22  as will be described hereafter.  
         [0048]     The illustrated embodiment of  FIGS. 2, 5  and  7  also comprises a saddle member  22  that further includes an inner groove  48  that extends around hole  38 . Groove  48  is configured to accommodate snap ring  28  in a compressed condition, i.e., the outer diameter of groove  48  is at least slightly smaller than the normal uncompressed outer diameter of snap ring  28 . The illustrated embodiment of saddle assembly  22  further includes one or more troughs  50  extending longitudinally within each of upright portions  32 . The one or more troughs  50  accommodates placement of washer  26 , as further described below, and may have a rounded (e.g. cylindrical), squared, or other appropriate shape to accommodate washer  26 . In this embodiment, the curved bottoms  255  of channel  34  are arranged such that when rod  36  is inserted therein, rod  36  will pressingly engage the washer  26  which will itself pressingly engage the head portion  58  of bone anchor member  24 .  
         [0049]     Referring now to  FIGS. 9-12 , there is shown an embodiment of washer  26  of the present invention. Washer  26  includes an upper portion  80 , a lower portion  82 , and a hole  84  therethrough. Upper portion  80  and lower portion  82  may be constructed integrally or may be separately constructed and attached together in any known manner. An upper surface  86  of upper portion  80  may include recessed portions  88  in the illustrated embodiment, which recessed portions  88  form a part of a cylinder sized and configured to accommodate placement of an elongated member (such as rod  36  of  FIG. 1 ) therein. Lower portion  82  further includes an upper surface  83  that faces snap ring  28 .  
         [0050]     Referring now to  FIG. 11 , washer  26  has a hole  84  provided through both upper portion  80  and lower portion  82 . Hole  84  includes a lower concave surface  96  and a cylindrical surface  98 . Concave surface  96  in one specific embodiment has a spherical shape so as to substantially coincide with a portion of head portion  58  of anchoring member  24 . Lower portion  82  is generally in the shape of a circular disc, and may include one or more projections  90  extending radially therefrom. Projections  90  in conjunction with troughs  50  align recessed portions  88  of washer  26  with channel  34   a  and prevent rotation of washer  26  so as to minimize misalignment between rod  36  and recessed portions  88 .  
         [0051]     In one embodiment, shown in  FIGS. 9-10 , projections  90  each include two substantially planar side surfaces  92 , and an end surface  94  that is rounded and may form a portion of a cylinder. Projections  90  are sized and shaped so as to fit and slide easily within the troughs  50  upright portions  32   a  of saddle member  22 . In another embodiment illustrated in  FIG. 12 , projections  90   a  each include a rounded end surface  100 .  
         [0052]     In certain embodiments, multi-axial bone anchor assembly  20   a  includes snap ring  28  in order to secure washer  26  against anchoring member  24 . One embodiment of such a snap ring  28  is shown in  FIGS. 17-18 . Snap ring  28  has a central opening  114  and a compression slot  116  defined therein. Snap ring  28  further has a first surface  118 , an opposite second surface  120 , an inner lateral surface  122  defining opening  114 , and an outer lateral surface  124 . Compression slot  116  allows snap ring  28  to compress and fit into inner groove  48  of saddle member  22 . The diameter of the entrance of groove  48  is at least slightly smaller than the outer diameter  126  of an uncompressed snap ring  28 . Opening  114  of snap ring  28  has an inner diameter, which allows snap ring  28  to fit around upper portion  80  of washer  26 . One of the surfaces  118  and  120  engage the upper surface  83  of lower portion  82  in order to secure washer  26 . Snap ring  28  can have a square cross-section, as shown in  FIG. 2 , or a circular or other appropriate shape cross-section, and in one particular embodiment is made of a shape memory alloy such as nitinol.  
         [0053]     Another embodiment of snap ring  28 ′ is illustrated in  FIG. 17   a . Snap ring  28 ′ is non-planar, and in one embodiment has a series of undulations forming relative crests  129   a  and relative troughs  129   b  therein. Alternatively, non-planar snap ring  28 ′ could have other curved configurations, or could have extending finger-spring elements along it. When assembly  20   a  is assembled, non-planar snap ring  28 ′ allows less play between saddle member  22 , anchoring member  24  and washer  26  because non-planar snap-ring  28 ′ fills a greater portion of groove  48  of saddle member  22 .  
         [0054]     In  FIG. 19 , there is shown another embodiment of a multi-axial bone anchor assembly  20   d  according to another embodiment of the present invention. Similar to the embodiment in  FIG. 1  and described above, bone anchor assembly  20   d  includes a bone anchoring member  24  and a set screw member  30 . However, this embodiment also comprises a washer (crown member)  26   d . In some embodiments, assembly  20   d  will further include a C-shaped snap ring  28 , which are fitted with saddle member  22   d  as will be described hereafter.  
         [0055]     The particular illustrated embodiment of saddle member  22   d  may include an inner groove  48   d . As illustrated, groove  48   d  extends around hole  38   d , and in this particular embodiment, groove  48   d  is uniform between a top portion of groove  48   d  and the bottom portion thereof. Groove  48   d  is configured to accommodate snap ring  28  in a compressed condition. Groove  48   d  has a thickness  264  that is, in one form, larger than snap ring  28 . Further, the illustrated embodiment of saddle assembly  22   d  in  FIGS. 20-21  does not include a trough  50  that extends longitudinally within each of upright portions  32   d . Upright portions  32   d  further include internally threaded portions  52   d , which are configured to be threadedly coupled with set screw  30 .  
         [0056]     Referring now to  FIG. 22 , there is shown another embodiment of washer  26   d  according to the present invention. Washer  26   d  includes an upper portion  80   d , a lower portion  82   d , a snap ring recess  266 , and a hole  84   d  therethrough. Upper portion  80   d , lower portion  82   d , and snap ring recess  266  may be constructed integrally or may be separately constructed and attached together in any known manner. Snap ring  28  fits within recess  266  in order to secure washer  26   d  within saddle member  22   d . In one embodiment, assembly  20   d  is assembled by inserting anchoring member  24   a  through hole  38   d  in saddle member  22   d . Washer  26   d , with snap ring  28  in at least a portion of recess  266 , is then inserted into hole  38   d . Snap ring  28  contracts into recess  266  as washer  26   d  goes through saddle member  22   d , and expands into groove  48   d  to hold washer  26   d  within saddle member  22   d . An elongated member is then inserted in channel  34   d , and a set screw (such as those described above) is threaded into internally threaded portions  52   d , saddle member  22   d  to lock the elongated member, washer  26   d  and anchoring member  24   a  together.  
         [0057]     Washer  26   d  has a hole  84   d  provided through both upper portion  80   d  and lower portion  82   d . Hole  84   d  includes a lower concave surface  96   d  and a cylindrical surface  98   d . Lower concave surface  96   d  is adapted to accommodate head portion  58   a  of anchor member  24   a . In the particular embodiment illustrated in  FIG. 22 , lower portion  82   d  is generally in the shape of a circular disc. In this particular embodiment, lower portion  82   d  does not have projections  90 .  
         [0058]     While embodiments of the invention have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It should be understood that only the preferred embodiments have been shown and described.