Abstract:
A spine screw assembly is adapted to be loaded from a bottom of a receiver member. The spine screw assembly includes a bone fastener with a head having a groove that removably accepts a clip. The clip acts to retain the head of the bone fastener in the receiver member.

Description:
REFERENCE TO PRIORITY DOCUMENT 
       [0001]    This application claims priority of co-pending U.S. Provisional Patent Application Ser. No. 60/920,211 filed Mar. 26, 2007. Priority of the aforementioned filing date is hereby claimed and the disclosures of the Provisional Patent Application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This disclosure is directed to skeletal bone fixation systems, and more particularly to a fixation assembly for vertebrae of a spinal column. 
         [0003]    Spinal fixation systems are used to secure sections of the spinal column, such as vertebral bodies, into a fixed position to correct spinal injuries and defects. Internal fixation is used most frequently in the spine in conjunction with vertebral fusion, and also for the manipulation of the spine to correct spinal deformities. A typical spinal fixation assembly includes a fixation device, such as a screw or hook, that can be attached to a portion of a first vertebral body. The screw can be coupled to a stabilization member, such as an elongate rod, that can be linked to one or more additional vertebral bodies using additional screws. 
         [0004]    Pursuant to a general process, two or more bone screws and/or hooks are secured to a vertebral body that is to be stabilized. After the screws are secured to the vertebral bodies, the screws are coupled to a spinal stabilization rod that restricts movement of the stabilized vertebra. It is important that the screws have a secure coupling with the spinal stabilization rod in order to prevent movement of the rod relative to the screw after placement. 
         [0005]    In several available pedicle screw systems, a tulip-like coupling element with opposing upright arms or walls is used to secure the pedicle screw to the rod. The coupling element and pedicle screw are configured to be coupled to an elongate stabilizer, such as a rod, that is positioned above the head of the pedicle screw. A compression member, such as a compression nut, is configured to mate with the coupling element and provides a compressive force to the rod. The rod is then forced against the head of the pedicle screw, and that force is translated to the coupling element. Accordingly, the forces generated by the compression nut clamp the rod and pedicle screw head together within the coupling element. 
         [0006]    One type of pedicle screw system is a bottom-loaded system wherein the screw is loaded into the coupling element through the bottom of the coupling element. Bottom loading can allow for greater flexibility and adjustment of the coupling element relative to the screw. There is a need for improved bottom-loaded pedicle screw systems. 
       SUMMARY 
       [0007]    Disclosed is a spine screw assembly such as a pedicle screw system. In an embodiment, the spine screw assembly comprises: a fastener having an upper end and a lower end, a head at the upper end, and an anchoring element extending between the upper and lower ends, wherein a groove is positioned in the head of the fastener; a clip sized to be positioned in the groove, wherein the annular clip transitions between a first state of increased size and a second state of decreased size, and wherein the annular clip can be locked within the groove when in the second state of decreased size and can be removed from the groove by transitioning the first state of increased size; a coupling element having an upper opening at an upper end and a lower opening at a lower end, the coupling element including a rod receiving channel adapted to receive a stabilizing rod, a bore extending through the lower end of said coupling element for receiving said fastener, and a seat adapted to engage the head when the fastener is positioned in the bore; and a compression nut engageable with the coupling element, the compression nut adapted to rotatingly move distally into the coupling element to translate a force to the head of the fastener such that the head is forced against the seat of the coupling element to prevent relative movement between the fixation element and the coupling element. 
         [0008]    Other features and advantages will be apparent from the following description of various embodiments, which illustrate, by way of example, the principles of the disclosed devices and methods. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1   a  is an illustration of a human vertebral column. 
           [0010]      FIG. 1   b  is a superior view of a typical human vertebra. 
           [0011]      FIG. 1   c  is a lateral view of the vertebra depicted in  FIG. 1   b.    
           [0012]      FIG. 2  is an illustration of a set of pedicle screws implanted into a human vertebral column. 
           [0013]      FIG. 3  shows an exploded view of a bone fixation assembly according to one embodiment. 
           [0014]      FIG. 4A  is a lateral view of a bone fixation element according to one embodiment. 
           [0015]      FIG. 4B  is a lateral close-up view of a bone fixation element taken at circle  4 B of  FIG. 4A . 
           [0016]      FIG. 5A  is a perspective view of a clip according to one embodiment. 
           [0017]      FIG. 5B  is a cross-sectional view of the clip taken along lines B-B of  FIG. 5A . 
           [0018]      FIG. 6  is a lateral view of the clip of  FIG. 5A  installed in the bone fixation element of  FIG. 4A . 
           [0019]      FIGS. 7A and 7B  are partially exploded, cross-sectional views of a coupling element and a bone fixation element with clip installed. 
           [0020]      FIG. 8  is a cross-sectional view of a bone fixation assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Before discussing the embodiments in detail, it may be helpful to first briefly review the basic devices and concepts used in orthopedic surgery, and particularly spine surgery. Bone stabilization assemblies are commonly used throughout the skeletal system to stabilize broken, fractured, diseased or deformed bones. In particular, pedicle screw systems are particularly well adapted for the fixation and manipulation of the bones of the vertebral column ( FIG. 1   a ). 
         [0022]    A vertebral pedicle is a dense stem-like structure that projects from the posterior of a vertebra. There are two pedicles per vertebra that connect to other structures (e.g. lamina, vertebral arch). The location of a pedicle P is illustrated in  FIGS. 1   b  and  1   c , which illustrate a typical vertebral column, a superior view of a typical vertebra, and a lateral view of a typical vertebra, respectively. 
         [0023]    Bone screws have been used in spinal instrumentation since the 1960s. A pedicle screw is a particular type of bone screw designed for implantation into a vertebral pedicle. Monoaxial pedicle screws are still used quite often, but the current standard for implantation is a polyaxial pedicle screw made of titanium or titanium alloy. Titanium alloy is useful because it is highly resistant to corrosion and fatigue, and is MRI compatible. The screw is threaded and the head is moveable, allowing it to swivel so as to defray vertebral stress. Polyaxial pedicle screw lengths range from about 30 mm to about 60 mm with diameters ranging from about 5.0 mm to about 8.5 mm. 
         [0024]    Pedicle screws are used to correct deformity, and or to treat trauma. They can be used in instrumentation procedures to affix rods and plates to the spine. They can also be used to immobilize part of the spine to assist fusion by holding bony structures together. Although pedicle screws are most often used in the lumbar (lumbosacral) spine, they can be implanted in the thoracic and sacral vertebra. The surgeon uses fluoroscopy, conventional x-ray, and sometimes computer-assisted visualization to determine the depth and angle for screw placement. A receiving channel is drilled and the screw is inserted. The screws themselves do not fixate the spinal segment, but act as firm anchor points that can then be connected with a rod. As shown in  FIG. 2 , the screws are placed down the small bony tube created by the pedicle on each side of the vertebra, between the nerve roots. This allows the screws to grab into the bone of the vertebral body, giving them a solid hold on the vertebra. Once the screws are placed, one in each of the two pedicles of each vertebra, they are attached to metal rods that connect the screws together. The screws are placed at two or more consecutive spine segments (e.g., lumbar segment  5  and  6 ) and connected by the rods. 
         [0025]    Generally, a poly-axial pedicle screw assembly includes a tulip-like coupling element that can be coupled to a fixation element, such as, for example, a pedicle screw, with a shaft and a head. Poly-axial pedicle screw assemblies can be top-loading and/or bottom-loading assemblies. In a top-loading assembly, the shaft of the fixation element is fed through the top of the coupling element. In bottom-loading assemblies, the head of the fixation element is inserted through the bottom of the coupling element. 
         [0026]      FIG. 3  shows an exploded view of one embodiment of a bottom-loading polyaxial pedicle screw assembly. The assembly  100  generally includes a fixation element  105  that is removably coupled to a coupling element  110  and a clip  115  that is removably coupled to the fixation element  105 . As described in detail below, the fixation element  105  can be coupled to a skeletal structure, such as a spinal vertebra. The coupling element  110  is used to couple the fixation element  105  to a stabilizer, such as an elongate rod  120 , which can be coupled to multiple fixation elements using additional couplers. 
         [0027]    The Fixation Element 
         [0028]      FIG. 4A  shows a detailed view of a fixation element. The fixation element  105  can comprise, for example, an elongate screw having a threaded shank  210  with external threads  215  that can be screwed into a bone structure, e.g., pedicle of a vertebra. A head  220  is positioned at the proximal end of the shank  210 . 
         [0029]      FIG. 4B  shows a close-up view of the head  220  taken along circle  4 B of  FIG. 4A . The head  220  has an upper region  305  and a lower region  310  with a groove  315  therebetween. The upper region  305  and lower region  310  each has a distal end  3050 ,  3150  and a proximal end  3055 ,  3155 . The distal end  3050  of the upper region  305  has a diameter D 1  (which can essentially be equal to the diameter of the proximal end  3155  of the lower region  310 ). The groove  315  has a diameter D 2 , which is less than D 1 . Thus, a lip  320  overhangs the groove  315  at its proximal end and lip  325  juts out from underneath the groove  315  at its distal end. 
         [0030]    In an embodiment, the upper region  305  and the lower region  310  are generally curved or spherical in shape, whereas the groove  315  is generally cylindrical in shape. It should be appreciated that other shapes of the upper and lower regions can be used. Similarly, other shapes of the groove can be used. The groove  315  has a shape that is configured to mate with and receive a correspondingly-shaped clip  115  therein, to be discussed in more detail below. 
         [0031]    A drive coupler, such as a drive cavity  225  (best shown in  FIG. 3 ) is located within or on the head  220  of the fixation element  105 . The drive cavity  225  has a shape that is configured to receive a device that can impart rotational movement to the fixation element  105  in order to screw the fixation element into a bone structure. For example, the drive cavity  225  can have a hexagonal shape that is configured to receive therein an Allen-style wrench. 
         [0032]    It should be appreciated that the drive coupler need not be a cavity that mates with an Allen-style wrench and that other types of drive couplers can be used. Moreover, the fixation element  105  can be in forms other than a shank  210 , including, for example, a hook or clamp. Indeed, it should be appreciated that any structure or component configured for attachment to a bone structure can be used in place of the shank  210  of the fixation element  105 . 
         [0033]    The fixation element  105  can be made of various materials, including metallic and non-metallic material, depending on the application involved and the stresses expected in vivo. In one embodiment, the fixation element  105  is made of implant grade titanium (Ti-6A1-4V) per ASTM F-136. 
         [0034]    The Clip 
         [0035]    As mentioned above, the groove  315  in the head  220  of the fixation element  105  has a shape that is configured to mate with and receive the correspondingly-shaped clip  115  therein. As shown in  FIGS. 5A and 5B , the clip  115  is a generally annular ring having a break  405  in its circumference, wherein the break  405  defines two opposing ring surfaces  410 ,  415 . The clip  115  can also take shapes that are not generally circular. The clip  115  has an outer diameter D 4  and an inner diameter D 3 . Inner diameter D 3  defines an opening  430  through which a fixation element  105  can be inserted. 
         [0036]    The clip  115  has an outer surface  420  and an inner surface  425 . As best shown in  FIG. 5B , the outer surface  420  is generally curved or spherical in cross-section, whereas the inner surface  425  is generally flat in cross-section. This cross-section allows the clip  115  to mate with the generally cylindrical shape of the groove  315  at its inner surface  425  and complete the generally rounded shape of the head  220  at its outer surface  420 . 
         [0037]    The clip  115  is designed to be at least partially inserted into the groove  315  of the head  220 . The height of the groove (Hg) is sufficient to receive the height of the clip (Hc), such that there is little play along axis A or in the up and down direction. Although there is little up and down play between the clip  115  and the groove  315 , the clip  115  can be rotated around axis A inside the groove  315 . Further, the width of the clip  115  is such that at least a portion of the clip  115  resides inside the groove  315  between upper lip  320  and lower lip  325  and a portion sits outside the groove  315 . A portion of the proximal edge  435  of the clip  115  abuts the upper lip  320  of the head  220 . Similarly, a portion of the distal edge  440  of the clip  115  abuts the lower lip  325  of the head  220 . 
         [0038]    The head  220  with the clip  115  residing in the groove  315  is configured to be inserted in the coupling element  110  from the bottom, as described in more detail below. While installed in the groove  315 , the clip  115  can be radially compressed. The generally c-shape of the clip  115  and the slightly larger inner diameter D 3  of the clip  115  compared to the groove  315  render the clip  115  flexible in the radial direction, also to be discussed in more detail below. 
         [0039]    The clip  115  can be compressed until the opposing ring surfaces  410 ,  415  meet and the inner surface  425  of the clip  115  approaches the surface of the groove  315 . During compression, the inner diameter D 3  of the clip  115  decreases until it approaches the diameter D 2  of the groove  315 . Similarly, the outer diameter D 4  of the clip  115  decreases until it approaches D 1 . This compression of the clip  115  allows the head to be inserted through the entry port near the bottom of the coupling element  110  into internal bore  505 . Assembly of the device according to a bottom-loading configuration that involves compression of the clip  115  is to be discussed in more detail below. 
         [0040]    Following compression, the clip  115  can resume its original shape such that the ring surfaces  410 ,  415  separate and the inner diameter D 3  and outer diameter D 4  return to their original, resting dimensions. The radial deformation is within the elastic range of the clip  115  so that no significant permanent deformation of the clip  115  occurs during flexation. 
         [0041]    The clip  115  can be made from many materials, including metallic and non-metallic. In the one embodiment, the clip  115  is an implant grade titanium (Ti-6A1-4V) per ASTM F-136. In another embodiment, the clip  115  is the same material as the head  220  of the fixation element  105 . 
         [0042]    The Coupling Element 
         [0043]    Again with reference to  FIG. 3 , the coupling element  110  is configured to receive a fixation element  105  and an elongate rod  120 . The coupling element  110  has an internal bore  505  that extends through the coupling element  110  along an axis A and defines entry ports near the bottom and near the top of the coupling element  110 . The internal bore  505  has a diameter D 5  (best shown in  FIG. 7A ) at its entry port nearest the bottom of the coupling element  110 . Diameter D 5  is at least as large as diameter D 1 , the diameter of the distal end  3050  of the upper region  305  of the head  220 . Diameter D 5 , however, is less than diameter D 4 , the diameter of the outer surface  420  of the clip  115  in its resting state. As will be discussed in more detail below, diameter D 4  decreases upon compression of the clip  115  such that diameter D 4  becomes less than diameter D 5  of the internal bore  505  allowing the head  220  of the fixation element  105  to be received therethrough. 
         [0044]    A pair of laterally-opposed, upwardly extending projections  510  are separated by the bore  505 . The projections  510  have internal, threaded surfaces  515 . In addition, a pair of u-shaped channels  520  extend through the coupling element  110  for receiving therein a rod  120 , which can extend along an axis that is transverse to the axis A of the bore  505 . 
         [0045]    The upper ends of the projections  510  define an entry port near the top of the coupling element  110  that is sized to receive therein, for example a compression nut  605 , as described below. The compression nut  605  can have outer threads  610  that are configured to mate with the inner threads on the opposed inner surfaces  515  of the projections  510  of the coupling element  110 . As described below, the entry port is sized and shaped to facilitate an easy entry of a compression nut  605  into or over the projections  510  of the coupling element  110 . 
         [0046]    A bottom saddle  705  and a top saddle  805  are configured to be positioned within the coupling element  110 . The saddles  705 ,  805  each define a contact surface  715 ,  815  that has a contour selected to complement a contour of the outer surface of the rod  120 . In one embodiment, the contact surfaces  715 ,  815  have rounded contours that complement the rounded, outer surface of the rod  120 . However, the contact surfaces  715 ,  815  can have any shape or contour that complement the shape and contour of the rod  120 . The contact surfaces  715 ,  815  can also be roughed, serrated, ribbed, or otherwise finished to improve the frictional engagement between the saddles  705 ,  805  and the rod  120 . The rod  120  can also be correspondingly roughed, serrated, ribbed, or otherwise finished to further improve the frictional engagement between saddles  705 ,  805  and the rod  120 . 
         [0047]    The complementing shapes and contours between the contact surfaces  715 ,  815  and rod  120  provide a maximum amount of contact area between the saddles  705 ,  805  and rod  120 . For example, the rod  120  is shown having a rounded or convex outer surface. The contact surfaces  715 ,  815  of the saddles  705 ,  805  are correspondingly rounded or concave such that the elongate rod  120  can fit snug between the saddles  705 ,  805  with the contact surfaces  715 ,  815  of the saddles  705 ,  805  providing a wide area of contact with the outer surface of the elongate rod  120 . It should be appreciated that the contour and shape of the contact surfaces  715 ,  815  can be varied to match any contour of the outer surface of the elongate rod  120  or in any manner to maximize the amount of grip between the saddles  705 ,  805  and the elongate rod  120 . 
         [0048]    The bottom saddle  705  has an internal bore  710  that axially aligns with the bore  505  in the coupling element  110  when the bottom saddle  705  is placed in the coupling element  110 . Furthermore, the bottom saddle  705  has a rounded outer surface that includes a pair of pin cavities  720  positioned, for example, on opposed locations on the bottom saddle  705 . Each of the cavities  720  aligns with a corresponding pin aperture  525  that extends through the coupling element  110 . 
         [0049]    The bottom saddle  705  is secured within the coupling element  110  by positioning the saddle  705  between the projections  510  such that each pin cavity  720  in the bottom saddle  705  aligns with a corresponding pin aperture  525  in the coupling element  110 . Pins  905   a ,  905   b  are then inserted through each pin aperture  525  such that one end of each pin  905   a ,  905   b  pokes into a corresponding pin cavity  720 . The pins  905   a ,  905   b  provide an interfering engagement with the pin cavities  720  and the pin apertures  525  to thereby secure the bottom saddle  705  in place relative to the coupling element  110 . 
         [0050]    The diameters of the pins  905   a ,  905   b  can be smaller than the diameters of the pin cavities  720  so that there is some play therebetween. Furthermore, the pins  905   a ,  905   b  can have lengths that extend only partially into the pin cavities  720  to provide some play therebetween. This permits the bottom saddle  705  to “float” in the coupling element  110  such that the position and the orientation of the bottom saddle  705  can be varied slightly. That is, the bottom saddle  705  can be moved slightly upward or downward and from side to side when mounted in the coupling element  110 . The bottom saddle  705  can also rotate slightly when mounted in the coupling element  110 . Thus, the bottom saddle  705  can movingly adjust into a secure engagement with the elongate rod  120  when compressed against the elongate rod  120  during assembly, as described below. 
         [0051]    Still with reference to  FIG. 3 , the top saddle  805  can be rotatingly mounted within a compression nut  605  that has outer threads  610  that are configured to mate with the threads on the internal surface  515  of the opposed projections  810  of the coupling element  110 . In this regard, the top saddle  805  has an upper projection  510  that rotatingly mates with the compression nut  605  and permits the top saddle  805  to rotate and/or tilt relative to the compression nut  605  when attached thereto. When attached, the top saddle  805  is positioned immediately below the compression nut  605 . In another embodiment, the top saddle  805  is fixedly attached to the compression nut  605  such that it does not rotate relative to the compression nut  605 . In another embodiment, there is no top saddle and the compression nut  605  directly contacts the stabilizer rod  120 . 
         [0052]    When the compression nut  605  is attached to the top saddle  805 , the compression nut  605  is rotatingly coupled to the coupling element  110  by mating the outer threads  610  of the compression nut  605  with the inner threads  515  of the coupling element  110 . The compression nut  605  is repeatedly rotated over a 360 degree rotational angle to lower the compression nut  605  into the coupling element  110 . The compression nut  605  is described herein as having outer threads  610  that mate with inner threads  515  on the opposed projections  510 . As described below, this advantageously permits a thread configuration that prevents projections  510  from spreading apart from one another as the compression nut  605  is screwed into the coupling element  110 . However, it should be appreciated that the compression nut  605  can be modified to have an annular shape with internal threads that mate with corresponding outer threads on the opposed projections  510 . 
         [0053]    In one embodiment, the various components of the assembly  100  are manufactured of an inert material, such as, for example, stainless steel or titanium. 
         [0054]    Relationship Between the Clip, Head and Coupling Element 
         [0055]    The configuration of the clip  115  on the head  220  of the fixation element  105  allows for the operator to load the fixation element  105  from the bottom of the coupling element  100 . The dimensions of the head  220  without the clip  115  installed in the groove  315  are such that the head  220  can be freely inserted through the entry port of the bore  505  nearest the bottom of the coupling element  110 . Although, the head  220  can just as easily slide back out the bottom of the coupling element  110  without the clip  115  positioned in the head  220 . Thus, the clip  115  retains the head  220  of the fixation element  105  inside the coupling element  110  and prevents it from backing out the bottom of the coupling element  110 . Once the clip  115  is installed in the groove  315  of the head  220 , the dimensions of the head  220  are such that the head  220  can no longer be freely inserted through the entry port of the bore  505  near the bottom of the coupling element  110 . Loading the fixation element  105  into the coupling element  110  from the bottom involves reduction of the outer diameter of the clip  115  by radial compression, to be discussed in more detail below. 
         [0056]      FIG. 7A  shows the clip  115  in a resting state installed in the groove  315 . D 1  is the diameter of the distal end  3050  of the upper region  305  of the head  220 . In an embodiment, D 1  is essentially equal to the diameter of the proximal end  3155  of the lower region  310  of the head  220 . D 1  constitutes the widest portion of the head  220  of the fixation element  105  when the clip  115  is not installed. D 5  is the diameter of the entry port of the internal bore  505  at the bottom of the coupling element  110 . D 5  is at least as large as D 1  allowing for the head of the screw to be freely inserted when the clip  115  is not installed. D 2 , the diameter of the groove  315 , is less than D 1 . 
         [0057]    When installed in the groove  315 , the outer surface  420  of the clip  115  completes a generally rounded shape to the head  220  of the fixation element  105 . D 4  is the outer diameter of the clip  115  when in the resting state. Upon installation of the clip  115  in the groove  315 , D 4  becomes the widest portion of the head  220 . D 4  is greater than D 1  and also D 5 . Because D 4  is greater than D 1  and D 5 , the head  220  of the fixation element  105  with the clip  115  installed cannot freely pass through the entry port of the internal bore  505  at the bottom of the coupling element  110 . Thus, the resting state the head  220  of the fixation element  105  with the clip  115  installed has a diameter that exceeds the diameter of the bore  505 . 
         [0058]    To insert the head  220  through the bore  505  the clip  115  is radially compressed such that D 4  becomes less than D 5  (see  FIG. 7B ). As the clip  115  is compressed, the width of the break  405  decreases until the opposing ring surfaces  410 ,  415  meet and/or the inner surface  425  of the clip  115  abuts the surface of the groove  315 . D 4  decreases until the head  220  and clip  115  together can be inserted through the entry port at the bottom of the coupling element  110 . 
         [0059]    Upon insertion of the head  220  of the fixation element  105  into the coupling element  110 , the clip  115  expands to its dimensions in the resting state. That is, the clip  115  expands such that diameter D 4  is greater than D 5  and the head  220  of the fixation element  105  cannot slide back out the entry port of the coupling element  110 . 
         [0060]    The fixation element  105  rests inside the coupling element  110 , the head/clip assembly of which makes contact with a seat  530  near the bottom of the coupling element  110 . In one embodiment, the seat  530  is formed of one or more inclined or slanted surfaces. The seat  530  can be formed of an annular surface such that the seat  530  is generally conical or the seat  530  can be formed of three or more flat surfaces, such that the seat is pyramidal. The seat  530  can have any of a variety of shapes adapted to support the head  220  and clip  115  therein. For example, the seat  530  can be spherical, partially-spherical, conical, frustoconical or other shapes. In a preferred embodiment, the seat  530  is conical or at least partially conical. 
         [0061]    The seat  530  supports the rounded head/clip assembly of the fixation element  105 . The rounded head/clip assembly abuts against the seat  530  near the bottom of the coupling element  110 , as shown in the cross-sectional view of  FIG. 8 . The rounded head/clip assembly contacts the seat  530  along a contact region that can vary in size and shape. The contact region can be in the form of a contact circle for example if the seat  530  is conically shaped. In the instance the seat  530  is conically shaped, the rounded head/clip assembly can be rotated within the seat  530  to move the axis of the shank portion  210  to a desired orientation relative to the coupling element  110  and thereby provide a poly-axial configuration. 
         [0062]    Assembly of the Device 
         [0063]    The device  100  can be assembled prior to or after driving the fixation element  105  into the bone structure. Similarly, the clip  115  can be assembled in the groove  315  of the fixation element  105  prior to or after driving the fixation element  105  into the bone structure. 
         [0064]    In an embodiment, to install the clip  115  into the groove  315 , the clip  115  can be inserted over the top of the head  220  of the fixation element  105 , for example, when the fixation element  105  is already driven into the bone. Because the inner diameter D 3  of the clip  115  is less than the diameter D 1  of the upper region  305  of the head  220 , the clip expands radially in order to pass over the upper region  305  of the head  220 . Once the clip  115  surpasses the upper region  305  of the head  220 , the clip slides past the upper lip  320  into the groove  315 . The groove has a diameter D 2  that is less than the inner diameter D 3  of the clip  115 . Thus, the clip  115  elastically springs in place such that is returns to its original shape inside the groove  315  between lips  320 ,  325 . 
         [0065]    In another embodiment, the clip  115  is installed such that the tip  230  of the fixation element  105  is inserted through the opening  430  in the clip  115 , similar to a washer and screw assembly. Because the inner diameter D 3  of the clip  115  is greater than the diameter of the shank  210 , the clip  115  is easily moved along the length of the fixation element  105 . The clip  115  is passed along the shank  210  of the fixation element  105  until it reaches the lower region  310  of the head  220 . Because the inner diameter D 3  of the clip  115  is less than the diameter of the lower region  310 , the clip radially expands in order to pass over the lower region  310  of the head  220 . The radial expansion is allowed due, in part, to the break  405  in the clip  115 . Once the clip  115  surpasses the lower portion  310  of the head  220 , the clip  115  slides past the lower lip  325  into the groove  315 . The groove has a diameter D 2  that is less than the inner diameter D 3  of the clip  115 . Thus, the clip  115  elastically springs in place such that it returns to it original shape and is retained inside the groove  315  between lips  320 ,  325 . 
         [0066]    Further, the clip  115  can be radially expanded such that the ends  410 ,  415  of the clip  115  spread apart and allow for the clip  115  to be installed directly into the groove  315  of the head  220  instead of past lips  320  or  325 . The ends  410 ,  415  spread apart such that the width of the break  405  expands and approaches the diameter D 2  of the groove  315 . The clip  115  is then pushed into the groove  315  between lips  320 ,  325  and returns to its original shape. A portion of the edge of the clip  115  remains inside the groove  315  and overlaps with lips  320 ,  325 . 
         [0067]    To insert the head  220  of the fixation element  105  with the clip  115  installed, through the entry port near the bottom of the coupling element  110 , the clip  115  is compressed until D 4  is less than D 5 . The clip  115  can be compressed, for example, with a tool such as a pliers-like tool that radially compresses the clip  115 . 
         [0068]    In another embodiment, the clip  115  is introduced through the top of the coupling element  110  and the head  220  of the fixation element  105  is introduced through the bottom of the coupling element  110 . After the two components (the clip  115  and the fixation element  105 ) are introduced into the fixation element  105 , the clip  115  is coupled to the head  220  while the head is inside the coupling element  110 . This is accomplished by expending the clip  115  onto the head  220  of the fixation element  105  until it positions into the groove  315  of the head  220 . The clip  15  then collapses into the groove  315 . After the assembly, the head  220  of the fixation element  110  is locked in the coupling element  110  such that the head  220  cannot separate from the coupling element  110 . In this embodiment, the assembled head/clip does not fit through the bottom opening in the coupling element  110 . 
         [0069]    As described above, compression of the clip is possible due, in part, to the break  405  in the clip  115 ; the resting inner diameter D 3  of the clip  115  exceeds the diameter D 2  of the groove  315 ; and the flexible nature of the clip  115  material. The clip  115  is compressed until the ends  410 ,  415  meet and/or D 3  approaches D 2 . Upon insertion of the head  220  into the coupling element  110 , the outer diameter D 4  of the clip  114  returns to a default diameter that interfaces with the seat to  530  to retain the head  220  within the coupling element  110 . 
         [0070]    The rounded head  220  abuts against and sits within a correspondingly-shaped seat  530  in the bottom of the coupling element  110  in a ball/socket manner, as shown in the cross-sectional view of  FIG. 8 . The seat  530  can have a rounded shape that is configured to provide a secure fit between the head/clip assembly and the coupling element  110 . Because the seat  530  is rounded, the head/clip assembly can be rotated within the seat  530  to move the axis of the shank  210  to a desired orientation relative to the coupling element  110  and thereby provide a poly-axial configuration. 
         [0071]    With the fixation element  105  positioned in the coupling element  110 , the bottom saddle  705  is attached to the coupling element using the pins  905   a ,  905   b , which mate with the pin cavities  720  in the side of the bottom saddle  705 . As discussed, there is some play between the pins  905   a ,  905   b  and the pin cavities  720 , such that the bottom saddle  705  essentially floats and can move somewhat relative to the coupling element  110 . That is, the bottom saddle  705  is attached to the coupling element  110  in a manner that permits movement of the bottom saddle  705  relative to the coupling element  110  and/or relative to the elongate rod  120 . Thus, the bottom saddle  705  adjusts in position as the compression nut  605  is tightened downward into the coupling element  110 , as described below. 
         [0072]    The rod  120  is loaded into the coupling element  110  by inserting the rod  120  downwardly between the projections  510  through the unshaped channels  520 . As the rod  120  is moved downwardly into the coupling element  110 , the outer surface of the rod  120  will eventually abut and sit against the corresponding rounded contact surface  715  of the bottom saddle  705 . The compression nut  605  and attached upper saddle  805  are then threaded downward into the coupling element  110  by mating the external threads  610  on the compression nut  605  with the internal threads  515  on the projections  510  of the coupling element  110 . The compression nut  605  can be threaded downward until the rod  120  is compressed between the top and bottom saddles  705 ,  805  with the compression nut  605  providing the compression force. 
         [0073]    As mentioned, the coupling element  110  has an entry port for the compression nut  605  that facilitates entry or coupling of the compression nut  605  into the coupling element  110 . The entry port is defined by the upper edges of the projections  510 . The entry port can have a structure that guides the compression nut  605  into a proper engagement with the coupling element  110 . For example, one or more large chamfers can be located on the upper, inner edge of the projections  510  of the coupling element  110  to provide ease of entry for the compression nut  605  into the coupling element  110 . The chamfers can be angled with the angle being in the range of thirty degrees to sixty degrees relative to vertical axis A, although the angle can vary. The chamfers can help to guide the compression nut  605  into proper alignment with the coupling element  110  such that the threads  610  on the compression nut  605  properly engage the threads on the opposed projections  510  without any cross-threading. 
         [0074]    The compression nut  605  is then threaded downwardly by repeatedly rotating the compression nut  605  about a 360 degree rotation. As the compression nut  605  lowers into the coupling element  110 , the rounded contact surface of the top saddle  805  abuts the rod  120  and compresses the rod  120  against the rounded contact surface  715  of the bottom saddle  705 . As mentioned the bottom saddle  705  has a floating arrangement with the coupling element  110  and the top saddle  805  is movable and rotatable relative to the compression nut  605 . This permits the saddles  705 ,  805  to gradually reposition themselves into a secure purchase with the rod  120  as the compression nut  605  moves downward. The contact surface of the saddles  705 ,  805  provide a continuous and maximized area of contact between the saddles  705 ,  805  and the rod  120  for a secure and tight fit therebetween. 
         [0075]    Moreover, the top saddle  805  is shaped so that opposed wings or protrusions  820  are located on opposed sides of the top saddle  805 . The opposed protrusions  820  are positioned on either side of the rod  120  so as to automatically guide the saddle  805  into alignment with the rod  120  as the saddle  805  lowers onto the rod  120 . Because the top saddle  805  can freely rotate as the compression nut  605  lowers onto the rod  120 , the protrusions  820  will abut opposed sides of the rod  120  as the top saddle  805  is lowered into the coupling element  110 . The top saddle  805  thus self-aligns into a secure engagement with the rod  120  as the top saddle  805  is lowered onto the rod  120 . 
         [0076]    In one embodiment, the protrusions  820  of the top saddle  805  are formed by a concave contour of the top saddle contact surface. It should be appreciated that the protrusions  820  need not be formed from curved surfaces, but can also be formed from straight surfaces. Moreover, the protrusions  820  need not be formed from a continuous, elongated surface, but can rather comprise one or more discrete protrusions, such as spikes, that extend downwardly from the top saddle  805 . 
         [0077]    As the compression nut  605  is threaded downward, the downward force of the compression nut  605  is transferred to the bottom saddle  705  via the top saddle  805  and the rod  120 . This causes the bottom saddle  705  to also move downward so as to press downward against the head/clip assembly of the fixation element  105 . The head/clip assembly is thereby pressed downward into the seat  530  in a fixed orientation. In this manner, the position of the fixation element  105  relative to the coupling element  110  is fixed. That is, the head/clip assembly of the fixation element  105  is pressed downward into the seat  530  of the coupling element  110  with a force sufficient to lock the position of the head/clip assembly relative to the coupling element  110 . 
         [0078]    The compression nut  605  can be tightened to provide a sufficient downward force that locks the positions of the saddles  705 ,  805  relative to the coupling element  110  and the elongate rod  120 . The compression nut  605  thereby provides a downward force that locks the relative positions of the elongate rod  120 , saddles  705 ,  805 , coupling element  110 , and fixation element  105 . After this is complete, the upper portion of the opposed projections  510  of the coupling element can be snapped off at a predetermined location along the length of the projections  510 . 
         [0079]    Although embodiments of various methods and devices are described herein in detail with reference to certain versions, it should be appreciated that other versions, embodiments, methods of use, and combinations thereof are also possible. Therefore the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.