the following description is merely exemplary in nature and is in no way intended to limit the invention , its application , or uses . for example , although the present teachings are illustrated for internal fixation of the cervical spine , the present teachings can also be used for other orthopedic procedures in which it is necessary to secure two bone portions relative to one another . referring to fig1 , an exemplary bone fixation system 100 according to the present teachings is illustrated as implanted in the anterior cervical spine for securing two cervical vertebrae 90 , 90 ′ relative to one another , without interfering with any bone graft 80 , which may also be implanted therebetween . referring to fig2 - 5 , the bone fixation system 100 can include a fixation plate 102 , and a plurality of fixation fasteners , including angulatable ( semi - constrained ) fixation fasteners 104 and non - angulatable ( constrained ) fixation fasteners 106 . as used herein , the term “ constrained ” is intended to mean fixed in angular orientation . the term “ semi - constrained ” is intended to mean having a variable angle of angulation for orientation within a cone of angulation . the fixation plate 102 can include a large viewing window 112 , which allows graft visualization during surgery , and also in postoperative x - ray and mri diagnostic procedures . the fixation plate 102 can also include a plurality of fixation holes 108 having threads 110 for interchangeably receiving the semi - constrained and / or constrained fixation fasteners 104 , 106 . the fixation fasteners 104 , 106 can include self - tapping and / or self - drilling bone engaging portions . although the fixation plate 102 is illustrated with four fixation holes 108 pairwise arranged in two fixation levels for fixation of two bone portions , it will be appreciated that the fixation plate 102 can include additional fixation holes and corresponding fixation levels for fixation of three or more bone portions . each fixation hole 108 defines a center axis a which is substantially perpendicular to the fixation plate 102 . referring to fig4 , the constrained fixation fastener 106 can include a bone engaging portion 116 which can be threaded , and a head 113 . the head 113 can include an interior hex recess or other driver - receiving formation 118 to facilitate inserting the constrained fixation fastener 106 into the bone through one of the fixation holes 108 of the fixation plate 102 . the hex recess 118 can also be engaged by a driver to remove the fixation fastener 106 from the fixation plate 102 . the head 113 can also include external threads 114 for engaging the threads 110 of the fixation hole 108 to prevent backing out of the constrained fixation fastener 106 after implantation . the constrained fixation fastener 106 , once threaded into the fixation plate 102 , remains in a fixed orientation . in the embodiment illustrated in fig4 , the longitudinal axis of the constrained fixation fastener 106 coincides with the center axis a of the fixation hole 108 and is substantially perpendicularly to the fixation plate 102 . referring to fig5 - 10 , the semi - constrained fixation fastener 104 includes a shank 120 and a coupler 124 . the shank 120 defines a longitudinal axis b and has a bone - engaging portion 122 with threads 123 , and a coupler - engaging portion , such as a ball hex 128 . the ball hex 128 includes six spherical or curved portions 140 interconnected at ridges 142 . the ball hex 128 can also include a driver - receiving recess , such as a hex recess 130 . the coupler 124 can include external threads 132 engaging the threads 110 of the fixation hole 108 of the fixation plate 102 for preventing backing out of the semi - constrained fixation fastener 104 after implantation . when the coupler 124 is engaged with the fixation hole 108 , the coupler &# 39 ; s center axis coincides with the center axis a of the hole . the coupler 124 also includes a prismatic hex opening 126 which receives and retains the ball hex 128 , and allows the shank 120 to angulate relative to the coupler 124 , as illustrated in fig7 . the shank 120 can be non - removably retained in the coupler 124 by crimping a retaining portion 127 of the coupler 124 around the shank 120 , as shown in fig5 . it will be appreciated that other known retaining devices can be used to the same effect . referring to fig5 and 6 , the coupler 124 and the shank 120 can rotate as an integral unit about the center axis a , without relative rotation between the shank 120 and the ball hex 128 about the center axis a , by using a driver that engages the recess 130 of the ball hex 128 . the shank 120 can angulate about the ball hex 128 by rotating relative to the ball hex 128 about axes that are orthogonal to the center axis a , as illustrated in fig7 . in the angulated position , the center axis a and the longitudinal axis b of the shank define an angle α . in this manner , the shank 120 can rotate relative to the fixation plate 102 within a cone of angulation that is defined by the maximum angle α that can be accommodated by the geometry of the semi - constrained fixation fastener 104 . for example , the angle α can take values ranging from 0 ° to 30 °, although other angle ranges are possible . the fixation plate 102 and the fixation fasteners 104 , 106 can be made of biocompatible materials , including , but not limited to , titanium , titanium alloys , such a ti - 6al - 4v , for example , or other materials . referring to fig1 , 2 and 3 , in a cervical fixation procedure , the fixation plate 102 can be provisionally secured to the vertebrae 80 using bone pins through small holes 107 of the fixation plate 102 . one or more constrained fixation fasteners 106 can be threaded into corresponding fixation holes 108 of the fixation plate 102 and implanted into a first bone portion or vertebra 90 . one or more semi - constrained fixation fasteners 104 can be threaded into corresponding fixation holes 108 of the fixation plate 102 and implanted into a second bone portion or vertebra 90 ′. the semi - constrained fixation fasteners 104 can be angulated within a cone of angulation relative to the fixation plate 102 . it will be appreciated that the fixation plate 102 can be used with any combination of constrained and semi - constrained fixation fasteners 106 , 104 , including all constrained fixation fasteners 106 or all semi - constrained fixation fasteners 104 . the foregoing discussion discloses and describes merely exemplary arrangements of the present invention . one skilled in the art will readily recognize from such discussion , and from the accompanying drawings and claims , that various changes , modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims .
a bone fixation system . the bone fixation system includes a fixation plate including at least one fixation hole , and at least one fastener , the fastener including a coupler and a shank for engaging a bone . the coupler is threadably engaged with the fixation hole . the shank is angulatably retained by the coupler and orientable relative to the fixation plate at a selectable angle .