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 . in fig1 , 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 fig1 , 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 fig3 , 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 . in the particular embodiments of saddle member 22 , illustrated in fig1 , 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 fig3 , 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 59 that is circular and the plane of which is substantially perpendicular to the longitudinal axis of hole 38 . however wall portion 44 and section 59 may be of any shape as long as the diameter of hole 38 at section 59 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 . upright portions 32 further include an internally threaded portion 52 , as shown in fig1 . internally threaded portion 52 is configured to be threadedly coupled with set screw 30 , as described below . in other embodiments , as shown in fig2 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 fig2 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 fig4 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 . 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 . as shown in fig2 - 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 fig2 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 fig2 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 . as shown in fig8 , 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 . multi - axial bone anchor assembly 20 may further include a set screw 30 . in the embodiments , illustrated in fig1 - 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 . as illustrated in fig1 - 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 . in another embodiment , as illustrated in fig1 - 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 fig1 , 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 . in fig2 , there is shown a multi - axial bone anchor assembly 20 a according to another embodiment of the present invention . similar to the embodiment in fig1 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 . the illustrated embodiment of fig2 , 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 . referring now to fig9 - 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 fig1 ) therein . lower portion 82 further includes an upper surface 83 that faces snap ring 28 . referring now to fig1 , 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 . in one embodiment , shown in fig9 - 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 fig1 , projections 90 a each include a rounded end surface 100 . 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 fig1 - 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 128 , 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 fig2 , or a circular or other appropriate shape cross - section , and in one particular embodiment is made of a shape memory alloy such as nitinol . another embodiment of snap ring 28 ′ is illustrated in fig1 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 . in fig1 , 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 fig1 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 . 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 fig2 - 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 . referring now to fig2 , 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 . 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 opposite from upper surface 86 d is adapted to accommodate head portion 58 a of anchor member 24 a . in the particular embodiment illustrated in fig2 , 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 . 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 .
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 .