Abstract:
A spinal fixation system includes an elongated rod sized to extend between at least two vertebrae and a number of anchor devices for anchoring the rod to the spine. The anchor devices include a bone engaging fastener having a head defining a spherical socket. A ball insert is configured to be placed within the socket and rotated so that a spherical surface of the ball insert is juxtaposed with the spherical surface of the socket. The insert has a first dimension less than the socket opening and a second dimension that upon rotation causes the insert to be captively retained in the socket. The anchor device further includes a yoke defining a yoke channel for receiving the rod and including a stem that is engaged to the ball insert when the insert is captured within the socket. A sleeve is disposed between the yoke channel and the fastener head for supporting the rod thereon. A set screw is threaded into the yoke channel for clamping the rod against the sleeve. The set screw further operates to draw the ball insert into clamped engagement within the socket.

Description:
BACKGROUND 
     The present invention relates to spinal fixation systems and particularly to an anchor device that incorporates multi-axial fixation to the spine. 
     Several techniques and systems have been developed for correcting and stabilizing injuries to or malformation of the spine. In one type of system, an elongated member such as a bendable rod is disposed longitudinally along a length of the spine, spanning two or more vertebral levels. In certain applications, the rod is bent to correspond to the normal curvature of the spine in the particular region being instrumented, such as the normal kyphotic curvature of the thoracic region or the lordotic curvature of the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of anchor devices that utilize a variety of fixation elements configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook that is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a screw that can be threaded into various parts of the vertebrae or other bones. 
     Early rod-type spinal fixation systems incorporated anchor devices that permitted very limited relative orientations of the rod relative to the fixation element. As these system evolved, various degrees of freedom of relative orientation were integrated into the system. For instance, in one system a bone screw may be engaged to the spinal rod at a range of planar angles. This so-called variable angle screw allows pivoting of the bone screw in a single plane parallel to the plane of the spinal rod. One goal achieved by the variable angle screw is that the surgeon can apply vertebral fixation elements to the spine in more appropriate anatomic positions. 
     Another rod-type fixation system utilizes fixation elements having a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to secure the rod within the body of the fixation element. One benefit of this type of fixation element is that the fixation element may be positioned directly beneath the elongated rod, thereby reducing the overall bulkiness of the implant construct and minimizing trauma to the surrounding tissue. 
     On the other hand, these so-called “open back” fixation elements are capable only of pivoting about the spinal rod to achieve variable angular positions relative to the rod. While this limited range of relative angular positioning is acceptable for many spinal pathologies, many other cases require more creative orientation of a bone fastener relative to a spinal rod. Certain aspects of this problem are addressed by some prior multi-axial or poly-axial screws that are capable of various three-dimensional orientations with respect to the spinal rod. One type of poly-axial screw design, shown in U.S. Pat. No. 6,537,276 to Metz-Stavenhagen et al., includes a spherical projection on the top of the bone screw. An internally threaded receiver member pivotally supports the bone screw and a spinal rod on top of the spherical projection. An inner set screw is tightened into the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. A similar multi-axial screw is disclosed in U.S. Pat. No. 5,466,237 to Byrd et al., except an outer nut is provided to secure the rod against the head of the bone screw. 
     In another approach shown in U.S. Pat. No. 4,946,458 to Harms et al., a spherical headed bone screw is supported within separate halves of a receiver member. The bottom of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. One detriment of this system is that the spinal rod must be threaded in order to accept the compression nuts, which has a tendency to weaken the spinal rod in the face of severe spinal loads. Harms et al. also describes in U.S. Pat. No. 5,207,678 another multi-axial pedicle screw wherein a compression member is provided between the rod and the head of the screw to exert a force on the screw head to lock the screw against the inner spherical surface of the receiver member. 
     Yet another approach is illustrated in U.S. Pat. No. 5,797,911 to Sherman et al., in which a U-shaped holder is provided that receives a bone fastener topped with a crown member. The holder accommodates a rod in a channel above the crown member and a compression member above the rod. The compression member presses on the rod and crown member to lock the fastener against the holder in any of a number of angles in three dimensions with respect to the rod. Another system shown in U.S. Pat. No. 5,733,285 to Errico et al., includes a holder having a tapered and colleted portion into which a bone fastener head is inserted. A sleeve is provided that translates down around the colleted portion to crush lock the colleted portion around the head of the bone fastener. This apparatus is bulky and difficult to manipulate given the external sliding locking mechanism. It is further dependent on the fit of the external sleeve and the relative strength of the collet and its bending and crushing portions for secure locking of the bone fastener head. 
     There is thus a need for a multi-axial or poly-axial fastener for use with a spinal fixation system that is simple to construct yet strong enough to withstand harsh spinal loads. This need should also be fulfilled by an anchor device that avoids the bulkiness of prior systems but can still achieve a simple and easy fixation of the spinal rod to the bone fastener mounted within the spine. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a spinal fixation system that incorporates multi-axial fixation characteristics in a low-profile, easy to construct anchor device. The system includes an elongated member, such as a spinal rod, that extends between spinal segments. A series of anchor devices anchor the rod to the spinal segments, with at least some of the anchor devices providing multi-axial fixation. In one embodiment of the invention, the multi-axial anchor device includes a bone engaging fastener that is adapted to engage a portion of the spine. In one specific embodiment, the fastener is a bone screw adapted to be threaded into the pedicle of a vertebra. 
     The head of the bone engaging fastener in such specific embodiment is provided with a spherical socket facing the spinal rod. A ball insert element is provided that incorporates a spherical surface for variable angular interface with the socket. The ball insert is further configured so that the insert may be introduced into the socket and then rotated within the socket so that the spherical surface is juxtaposed to the socket for captive retention therein. 
     In one configuration, the socket has an interior diameter and a smaller diametrical opening communicating therewith. The ball insert is configured as a truncated sphere having a spherical diameter slightly less than the interior diameter of the socket. A portion of the ball insert is formed to have an outer curved surface defining a cylinder having a maximum diameter less than the spherical diameter of the ball insert and less than the diameter of the socket opening. The ball insert is introduced into the socket by aligning the cylindrical diameter with the diameter of the opening and then placing the insert into the socket. The insert is thereafter rotated in the socket to juxtapose the spherical surface of the insert with the interior diameter of the socket to captively retain the insert. 
     Connection to the spinal rod is provided by way of a yoke that is engaged to the ball insert. In one embodiment, this engagement is accomplished by a threaded bore in the ball insert and a mating threaded stem of the yoke. The ball insert is free to swivel in the fastener socket and since the yoke is attached to the ball insert it is thereby also free to move in a multi-axial manner. The yoke defines a channel between opposing arms of the yoke, with the channel configured to snugly seat the rod therein. A sleeve is provided that fits about an upper portion of the head of the bone engaging fastener. This upper portion provides a spherical surface to interface with a spherical lower cavity of the sleeve so that the sleeve may adapt a range of spherical angles relative to the bone engaging fastener as necessary to accommodate the position of the spinal rod relative thereto. The sleeve includes in one configuration opposing notches to receive and support the rod. 
     While the yoke does not itself support the spinal rod, it does support a set screw that is used to clamp the spinal rod to the notches in the sleeve. In one embodiment, the set screw is carried by a cap that fits over and around the arms of the yoke. The set screw is configured to engage internal threads defined in the yoke arms so that as the set screw is driven into the yoke a lower face of the screw contacts the spinal rod to drive it into the sleeve. In one embodiment, the set screw is supported within the cap so that the screw may rotate independently of the cap. The cap and the set screw may each define opposing grooves for mutually carrying a retaining ring used to fix the set screw against axial movement relative to the cap. The retaining ring does permit relative rotation so the set screw may be used to clamp the spinal rod. 
     The angular orientation of the yoke is adjusted relative to the bone engaging fastener to accommodate the position of the spinal rod relative to the portion of the spine. In one feature of the invention, this angular orientation is fixed by pressure engagement between the ball insert and the spherical socket of the head of the bone fastener. Thus, in accordance with this feature, the present invention contemplates that the set screw not only operates to firmly clamp the spinal rod within the yoke and against the sleeve, it also generates an array of forces that press the ball insert into the spherical socket. In particular, as the set screw is tightened within the threaded arms of the yoke, the pressure face of the set screw first contacts the spinal rod. As the set screw is advanced further into the yoke, the pressure face clamps the rod against the sleeve. At this point, the rod is generally firmly fixed to the yoke, although the yoke itself is not yet firmly fixed to the bone engaging fastener. 
     As the set screw is tightened further into the internal threads of the yoke arms, a reaction force is generated against the yoke itself, since the set screw cannot be driven any further into the rod of lower sleeve. This reaction force pulls the yoke upward, which in turn pulls the ball insert upward due to the threaded engagement between the ball insert and the threaded stem of the yoke. As the yoke and ball insert are pulled upward, the ball insert is pressed into the upper portion of the spherical socket of the head of the bone screw, thereby clamping the ball insert relative to the bone screw. With the ball insert clamped, further tightening of the set screw pushes against the rod to drive the sleeve into firm locking engagement with the spherical outer surface of the head of the bone fastener. 
     In a particular aspect of the invention, an improved anchor device for anchoring an elongated rod to the spine is of the type comprising a fastener having a bone engaging portion and a yoke having a rod receiving channel that is supported by the fastener for swivel movement relative thereto. In accordance with the improvement, the fastener includes a head defining a socket. An insert is captively retained in the socket and is configured for swivel movement therein. A yoke comprises a portion attached to the insert so that the yoke is capable of movement with the insert. The yoke and insert are attached by a process of initially captively retaining the insert in the socket and then attaching the yoke portion to the insert once captively retained. 
     One benefit of the present invention is that it provides for solid anchoring between a spinal rod and a bone engaging fastener at variable spherical angles. A further benefit is that a common clamping element is provided to clamp the spinal rod and fix the angular position of the anchor device. Yet another benefit resides in one aspect of the anchor device that reduces the overall prominence and profile of the components of the device. Other benefits of the invention can be discerned from the following written description and accompanying figures. 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a transverse view of a portion of a spine with a fixation system utilizing an elongated members engaged between successive vertebrae. 
         FIG. 2  is a side perspective view of an anchor device according to one embodiment of the invention for use in the fixation system shown in  FIG. 1 . 
         FIG. 3  is a top plan view of the anchor device shown in  FIG. 2 . 
         FIG. 4  is a side cross-sectional view of the anchor device shown in  FIG. 2 . 
         FIG. 5  is a longitudinal cross-sectional view of the anchor device illustrated in  FIG. 2  along the longitudinal axis of the elongated member. 
         FIG. 6  is a top plan view of a ball insert element of the anchor device shown in  FIG. 2 . 
         FIG. 7  is a side elevational view of the ball insert shown in  FIG. 6 . 
         FIGS. 8   a - 8   f  are side perspective views of a sequence of assembly of the components of the anchor device shown in  FIG. 2 . 
         FIG. 9  is a top perspective view of a sleeve component of the anchor device shown in  FIG. 2 . 
         FIG. 10  is a side cross-sectional view of the sleeve shown in  FIG. 9 . 
         FIG. 11  is a longitudinal cross-sectional view of a fixture with holding pins for holding the position of the ball insert relative to the socket during engagement of the yoke. 
         FIG. 12  is a longitudinal cross-sectional view of the fixture with holding pins used to crimp or swage the threads of the yoke to fix the yoke to the ball insert. 
         FIG. 13  is a longitudinal elevational view of a cap with set screw of the anchor device of  FIG. 2 . 
         FIG. 14  is a top plan view of the cap shown in  FIG. 13 . 
         FIG. 15  is a cross-sectional view of the cap of  FIG. 14  taken along viewing line XV-XV. 
         FIG. 16  is a longitudinal cross-sectional view similar to  FIG. 5  showing forces generated to lock the components of the anchor device of  FIG. 2 . 
         FIG. 17  is a side elevational view of a fastener inserter tool for use with one embodiment of the anchor device of the present invention. 
         FIG. 18  is a longitudinal cross-sectional view of the fastener inserter tool shown in  FIG. 17  engaged to components of the anchor device shown in  FIG. 2 . 
         FIG. 19  is a longitudinal cross-sectional view of the lower end of a rod persuader tool engaged to a partially assembled anchor device of  FIG. 2 . 
         FIG. 20  is a longitudinal cross-sectional view of an anchor device according to an alternative embodiment of the invention. 
         FIG. 21  is a longitudinal cross-sectional view of a screw inserter tool engaged to a partially assembled anchor device, such as the alternative anchor device shown in  FIG. 20 . 
         FIG. 22  is a longitudinal side cross-sectional view of the lower end of the rod persuader tool engaged to a partially assembled modified anchor device, such as the alternative device shown in  FIG. 20 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
     The present invention contemplates a spinal fixation system, such as the system  10  depicted in  FIG. 1 . As is known in the art, the fixation system  10  spans between successive vertebrae of the spine. An elongated member, such as rod  12 , extends along the length of the spine and provides an anchor point for connecting each vertebra to the rod. The rod is typically contoured to approximate the normal curvature of the spine for the particular instrumented spinal segments. Anchor devices  15  are provided for connecting the vertebral segments to the rod. These anchor devices may include hooks, bolts, screws or other means for engaging a vertebra. For the purposes of the present discussion, the anchor device  15  includes a bone engaging fastener  20  which is a bone screw, as shown in  FIG. 2 . The bone screw  20  includes a threaded shank  22  configured for threaded engagement within a portion of a vertebra. In a specific example, the shank is configured for engagement within the pedicle of a vertebra. 
     The bone engaging fastener or screw  20  further includes a head  24  by which the screw, and ultimately the vertebra, is anchored to the spinal rod  12 . In accordance with one feature of the present invention, the head  24  defines a spherical socket  26  with a socket opening  28  facing the rod, as shown in  FIGS. 4-5 . The bone screw  20  further defines a central bore  30  intersecting the socket and extending part way into the threaded shank  22 . A transverse bore  32  extends through the head  24  and across the socket, as best seen in  FIG. 5 . The function of the bores  30  and  32  are discussed herein. The head  24  includes a spherical outer surface  34 . 
     It can be appreciated from considering  FIGS. 4-5  that the spherical head  24  of the bone screw is more than simply hemispherical. In other words, the spherical socket  28  subtends a spherical angle of greater than 180° so that socket opening  28  is defined at a chord of the spherical socket. The planar diameter of the opening  28  at the chord is less than the inner diameter of the socket. In a specific embodiment, the spherical head subtends a spherical angle of about 240 20  and the planar chordal diameter of the socket opening  28  is about 90% the spherical diameter of the socket. It can thus be appreciated that a ball element of about the same spherical diameter disposed within the socket will be retained within the socket, unable to pass through the socket opening. It will be appreciated from the following discussion that a smaller planar chordal diameter will reduce the range of angulation of the articulating components of the anchor device. 
     Of course, a ball element that is too large to pass through the opening  28  cannot be readily inserted into the socket  26 . The present invention addresses this matter by a ball insert element  40 , illustrated in detail in  FIGS. 6-7 . The ball insert  40  defines a central threaded bore  42  that is provided for connection to a yoke component  50 , as described in more detail herein. The ball insert is generally in the form of a truncated sphere, whereby the outer surface  44  of the ball insert includes a spherical surface  45  that is sized to closely approximate the spherical socket  26 , as shown in  FIG. 5 . Thus, spherical surface  45  defines an outer spherical diameter D 1 , that is slightly less than the interior diameter of the spherical socket  26 , but greater than the diameter of opening  28 . As seen more particularly in  FIG. 8   b , the ball insert  40  is further formed to have a cylindrical portion defined by curved surfaces  46 . The curved surfaces  46  of cylindrical portion define an outer diameter D 2  about axis A as depicted in  FIG. 8   b . Axis A in one arrangement is formed to be generally perpendicular to the axis of the central threaded bore  42 . In accordance with one aspect of the invention the maximum diameter D 2  is slightly less than the planar chordal diameter of socket opening  28  ( FIG. 8   a ) and defines an insert dimension for placing the ball insert  40  into the socket  26  as will be defined. While curved surfaces  46  are preferably formed to define a cylindrical insert dimension D 2 , it should be appreciated that other configurations may be considered, such as one or more flattened outer surfaces, provided that a maximum insert dimension such as diameter D 2  is formed less than the maximum dimension of the socket opening  28 . 
     The benefit of this configuration for the ball insert  40  can be readily appreciated upon consideration of the sequence in  FIGS. 8   a - 8   c  depicting insertion of the ball insert  40  into the socket  26  of the bone screw  20 . As shown in  FIG. 8   b , the ball insert  40  is rotated at least 90° so that the insert dimension D 2  with curved surfaces  46  is aligned to pass through planar chordal opening  28  and into the socket  26 . The insert dimension D 2  is oriented so that axis A of ball insert  40  is essentially aligned along the axis of the bone screw. The depth of the socket  26  is sufficient to fully receive the rotated ball insert  40  so that the spherical surface  45  exposed in the view of  FIG. 8   b  is within the socket. Then, in the final step shown in  FIG. 8   c , the ball insert  40  is further rotated at least 90° so that the threaded bore  42  faces upward through the socket opening  28 . In this position, the spherical surface  45  of the ball insert is juxtaposed with the interior of the spherical socket  26 , as shown in  FIG. 5 , and the ball insert  40  is captively retained in the socket  26  for swivel movement therewithin. 
     The ball insert  40  is further provided along axis A as seen in  FIG. 8   a  with a transverse bore  48  that may be aligned with the transverse bore  32  in the spherical head  24  of the bone screw, as shown in  FIG. 5  and  FIG. 8   a . As can be seen from the figures, the ball insert is truncated at the top and bottom of the insert. However, the ball insert in this arrangement is not symmetric—i.e., more of the top of the spherical ball is truncated than the bottom of the ball. Further, as a result of the formation of the curved cylindrical surfaces  46 , the lower truncated surface has indentations  49  as illustrated in  FIG. 7 . When the ball  40  is rotated as depicted in  FIG. 8   b , the indentations  49  may be directed toward the bottom of socket  26  and are not visible through the socket opening. 
     Returning to  FIGS. 4-5 , the anchor device  15  further includes a yoke  50  having a threaded stem  52  configured to engage the threaded bore  42  in the ball insert  40 . The stem is provided with a shoulder  53  that preferably abuts the ball insert  40  when the stem  52  is fully threaded into the bore  42  of the insert. The yoke  50  includes yoke arms  54   a ,  54   b  that define a yoke channel  55  therebetween. The gap between the arms  54   a ,  54   b , and consequently the width of the channel, is sized to closely fit the spinal rod  12 , as best seen in  FIG. 5 . The arms  54   a ,  54   b  define internal threads  56  at the upper open end of the yoke  50  for engaging a set screw  80 , as described below. A bore  57  passes through the threaded stem  52  that is aligned with the bore  30  in the bone screw when the yoke is mounted on the ball insert. 
     As shown in  FIGS. 8   d - 8   e , a sleeve  60  is interposed between the yoke  50  and the head  24  of the bone screw  20 . As further shown in  FIGS. 9 and 10 , the sleeve  60  defines a lower cavity  62  that has a spherical configuration to substantially match the spherical outer surface  34  of the screw head  24 . Sleeve  60  sits on the outer surface  34  for sliding movement thereon, and serves as a clamping element for the rod  12  relative to the yoke as will be described. The sleeve further defines an upper cavity  64  that generally parallels the outer surface of the yoke arms  54   a ,  54   b , as seen in  FIG. 5 . The upper face of the sleeve  60  defines opposite rod grooves  66  sized to receive the spinal rod  12  therein. The lower face of the sleeve defines opposite notches  68  that are oriented 90° from the rod grooves  66 . The notches  68  are arranged to align with the transverse bores  32  and  48  when the anchor device is assembled. The notches and bores are sized to receive retaining pins  155  ( FIG. 11 ) as described in more detail herein. In a preferred arrangement, sleeve  60 , is provided with opposing recessed surfaces  63  that engage the arms  54   a ,  54   b  of the yoke  50  to key the sleeve  60  to yoke  50  in a manner that allows common swivel movement of the yoke  50  and sleeve  60  relative to the screw head  24 . 
     As depicted in  FIG. 8   d - 8   e , after the ball insert  40  is properly aligned and captively retained as shown in  FIG. 8   c , the yoke  50  may engage the insert  40  to form an assembly therewith. In accordance with the preferred manner of assembly of the anchor device  15 , the threaded stem  52  of the yoke is extended through the sleeve  60  with the sleeve keying surfaces  63  aligned with the yoke arms  54   a ,  54   b . The threaded stem  52  is then threaded into engagement with the threaded bore  42  of the ball insert. In order to achieve this threaded engagement it is necessary to hold the ball insert  40  as the stem  52  of the yoke is threaded into the bore  42 . Thus, in one aspect of the invention, the ball insert  40  is oriented within the spherical socket  26  so that the transverse bores  48  in the insert are aligned with the transverse bores  32  in the screw head. When the bores are aligned, pins  155  may be pushed therethrough, taking care that the pins do not extend into the threaded bore  42 , as illustrated in  FIG. 11 . Arms  157  of a forceps-like tool may be used to introduce the pins into the bores. 
     With the pins  155  in position, the sleeve  60  may be placed over the head of the bone screw with the notches  68  aligned with the pins  155 . The yoke is then extended through the sleeve with the stem engaging the threaded bore  42  of the ball insert. The pins  155  resist rotation of the ball insert  40  as the stem is threaded into the bore. The yoke  50  is threaded into the ball insert until the shoulder  53  contacts the upper face of the ball insert  40  as shown in  FIGS. 4 and 5 . 
     In an additional feature, the pins  155  may be used to crimp, swage or deform the threads of the stem  52  of the yoke  50 . Thus, the tool arms  157  may be pressed toward each other so that the pins  155  contact the threaded stem  52 , as shown in  FIG. 12 . When the threads are distorted the stem  52  of the yoke cannot back out or unthread from the ball insert  40 . Once the yoke and ball insert have been locked together, the pins  155  can be removed. It is understood that this initial assembly of the anchor device, namely the steps shown in  FIGS. 8   a - 8   e , occur prior to introduction of the anchor device  15  into the spine, preferably by the supplier. It can also be appreciated that once the yoke  50  is locked with ball insert  40 , the ball insert  40  is free to swivel within the fastener socket  26  allowing the yoke attached thereto to freely angulate in multiple directions. Since sleeve  60  is keyed to yoke  50  it likewise freely slides on outer surface  34  of fastener head  24  as the yoke  50  moves, until the anchor device components are locked in use. Furthermore, even though the ball insert  40  is free to swivel within socket  26 , once the yoke  50  is attached the insert  40  remains captively retained since the insert  40  will not be able to move to a position where its insert dimension L 1  is aligned with the socket opening  28 . 
     Returning again to  FIGS. 4-5 , the assembly of the rod  12  to the fastener  20  is shown. The rod  12  is initially placed between the arms of the yoke  50  to rest on the rod grooves  66  of the sleeve  60 . The yoke channel  55  may then be closed, securing the rod within. In accordance with a further feature of the invention, a cap  70  is fitted over the top of the yoke arms  54   a ,  54   b . The cap  70  as further detailed in  FIGS. 13-15 , includes a generally cylindrical skirt  74  that fits snugly around the arms  54   a ,  54   b  to prevent the arms from splaying outward as set screw  80  is threaded into the arms. The skirt  74  is preferably provided with diametrically opposed flats  75  that correspond to the transverse opening of the yoke channel  55 , as best seen in  FIG. 8   f . The flats  75  define rod grooves  72  that align with, but do not contact, the rod  12  when it is situated within the yoke channel  55 . 
     The cap  70  includes an upper boss  76  that defines an enlarged circumferential interior groove  78 . This groove is sized to receive a retaining ring or snap ring  90  therein, as seen in  FIG. 5  and  FIG. 15 . The groove is axially enlarged or lengthened so that the snap ring  90  may translate up and down within the boss  76  for reasons explained below. 
     The set screw  80  is provided with a threaded stem  82  that is configured to engage the internal threads  56  of the yoke arms  54   a ,  54   b . Preferably the threaded engagement between set screw and yoke are in the form of buttress threads, as depicted in  FIGS. 4-5 . The buttress threads minimize the outward force produced as the set screw is threaded into the yoke. Thus, the use of buttress threads help minimize any splaying of the yoke arms that might otherwise occur when the set screw  80  is threaded tightly into the yoke  50 . In addition as shown in  FIG. 15 , the bottom of the set screw is recessed upwardly of the bottom of the skirt  74  of cap  70 . Thus, when cap  70  is placed over the arms  54   a ,  54   b  of yoke  50 , not only does the close fit of the skirt  74  relative thereto prevent splaying as noted, but skirt  74  also serves as a guide to align the threads  82  of set screw  80  into the threads  56  of the yoke  50 , thereby also reducing the risk of disadvantageous cross-threading. 
     The set screw  80  includes a pressure face  83  that contacts and exerts a securing force against the spinal rod  12 . The pressure face  83  as well as the rod surface may exhibit surface features intended to enhance the fixation between set screw and rod, as is known in the art. In particular, a surface roughness may be provided that becomes deformed or cold formed when the set screw is tightened onto the rod. This feature helps prevent the rod from slipping axially (along its length) within the anchor device  15 . 
     The set screw  80  defines a bore  84  therethrough. The upper portion  86  of the bore may be configured to receive a driving tool, such as with hex or TORX surfaces. 
     Like the cap  70 , the set screw  80  defines a circumferential groove  88  ( FIG. 4 ) configured to receive the retaining ring  90  therein. However, unlike the cap groove  78 , the groove  88  in the set screw is preferably sized to closely fit the snap ring. Thus, while the snap ring  90  is held by the set screw, the snap ring is free to translate within the elongated cap groove  78 . The elongated groove  78  is thus intended to allow the set screw  80  to fully engage the rod  12  while the cap  70  essentially floats by virtue of the snap ring  90  translating within groove  78 . Thus, the cap  70  effectively exerts no force on the rod  12  or on the top surface of the yoke  50 , even if some contact is made. 
     The set screw  80  generates the force that locks the ball insert  40  within the spherical socket  26  at the desired angular orientation, and that further locks the spinal rod  12  within the anchor device  15 . In particular, once the anchor device  15  has been fully assembled about the rod  12 , as best seen in  FIG. 16 , the set screw  80  is tightened within the yoke  50 . As the screw is tightened, it presses against the rod  12 , clamping it between the pressure face  83  of the set screw and the rod grooves  66  in the sleeve  60 . As the set screw is driven further into the internal threads  56  of the yoke  50 , the set screw pushes the rod  12  downwardly until the lower cavity  62  of the sleeve  60  is firmly engaged to the outer surface  34  of the head  24  of the bone screw generating locking force, F 1 . At this point the sleeve  60  and rod  12  can move no further toward the bone screw  20 . Therefore, any further tightening of the set screw is reacted by the yoke itself. As the set screw is driven further into the yoke internal threads (i.e., advancing toward the head of the bone screw) this reaction force pulls the yoke upward. While the yoke is pulled upward with continued rotation of the set screw, the stem  52  of the yoke pulls the ball insert  40  upward, owing to the fixed engagement between the yoke stem and the ball insert. As the ball insert is pulled upward, it bears forcefully against the upper face of the spherical socket  26 , with a force F 2  clamping the socket wall between the sleeve  60  and the ball insert  40  and thereby locking the ball insert  40  and yoke  50  relative to fastener  20 . Any tendency of the socket  26  to attempt to gap at the socket opening  28  is resisted by the sleeve  60  that is already in firm engagement about the outer surface  34  of the screw head. 
     It can thus be appreciated that the entire anchor device can be adjustably secured in a fixed relationship simply by rotation of the set screw  80 . As the set screw is threaded into the yoke threads it ensures solid clamping of the bone screw head  24  between the lower cavity  62  of the sleeve  60  and the spherical surface  45  of the ball insert  40 , regardless of the angular orientation of the yoke and rod relative to the screw. The rod itself is firmly clamped between the set screw and the lower sleeve. It can further be appreciated that the entire anchor device may be tightened by simply tightening the set screw. 
     In use, the bone screw and sleeve assembly of  FIG. 8   e  is provided together with one or more suitably sized rods  12  and a cap  70  so that a spinal fixation system  10  may be implanted into a patient. The surgeon may insert the bone screw assembly with a suitable screw inserter  140  as shown, for example, in  FIGS. 17-18 . The screw inserter  140  includes an outer sleeve  142  and an inner shaft  144  rotatably disposed within the sleeve. As shown in the view of  FIG. 18 , the end  146  of the outer sleeve  142  is configured to contact the proximal upper surface of the sleeve  60 . The outer sleeve  142  is fixed to a handle  150 , while the inner shaft is fastened to a tightening knob  152  that is rotatably supported on the handle. The inner shaft  144  includes a pin end  148  that is sized to extend through the bore  57  in the yoke  50  and into the bore  30  at the base of the spherical socket  26 . The pin end  148  ensures co-axial alignment of the driving tool  140  and the bone screw threaded shank  22 . The inner shaft further includes intermediate threads  149  axially offset from the pin end  148 . These threads  149  are arranged to engage the internal threads  56  of the yoke arms  54   a ,  54   b.    
     The threads  149  on the inner shaft  144  of the tool  140  operate similar to the set screw  80 . Specifically, as the threads are driven into the internal threads  56  of the yoke  50 , the pin end  148  reacts against the bottom of the bore  30  in the bone screw to generate an upward force on the yoke  50 . As the yoke is pushed upward, it pulls the ball insert  40  with it, thereby driving the insert into the spherical socket. When the inner shaft  144  has been fully tightened, the screw inserter tool  140 , yoke  50 , ball insert  40  and bone screw  20  form a rigid connection. The handle  150  of the outer sleeve  142  may then be used to drive the bone screw into the vertebral bone, either manually or with the assistance of an additional driving tool after a suitable hole has been drilled in the pedicle of a vertebra. 
     Once the bone screw  20  is threaded in position into the spine, the next step to completing the fixation system, such as system  10  shown in  FIG. 1 , is to introduce the rod  12  into the yoke  50  of the anchor device  15 . The rod may be contoured to match the normal curvature of the spine, either in lordosis or kyphosis depending upon the instrumented vertebral level. In some cases, the spine exhibits a lateral curvature, such as scoliosis, that is preferably corrected, at least partially, by the fixation system  10 . Thus, in certain cases, the rod  12  itself may be laterally offset from the position of the bone screw engaged within the underlying vertebra. In these cases, the variable angle capabilities of the anchor device of the present invention come into play. 
     To accomplish the introduction of the rod  12  into the yoke channel  55  of the yoke  50 , a rod persuader tool  185  is provided, as shown in  FIG. 19 . The rod persuader tool  185  includes an outer tube  186  and an inner tube  192  concentrically disposed within the outer tube for relative axial movement. The outer tube  186  defines a rod notch  189  at its bottom end  187 . The inner tube  192  defines a slot  193  that forms legs  194  at the distal end. The legs define an inner shoulder  195  that is configured to suitably engage the partially assembled anchor device. The inner shoulders  195  may engage a groove (not shown) in the outer surface  34  of fastener socket  26 . In another embodiment, the yoke  50  may be modified to have a groove (not shown) that may be engaged by the inner shoulders  195 . In either embodiment, the legs  194  are configured to partially encircle and firmly grasp the partially assembled anchor device, while the slot  193  accommodates the initial presence of the rod  12  within the yoke channel  55 . A guide pin  190  spans the diameter of the outer tube  186  and fits within the slot  193  to control the relative axial movement between the outer tube  186  and the inner tube  192 . A suitable mechanism is provided to move the outer tube  186  downward axially relative to inner tube  192 . As the outer tube  186  moves downward, it forces the rod  12  into the yoke channel  55  by lower notch  189  and into the rod groove  66  of the sleeve  60 . 
     With the rod  12  suitably placed into the yoke  50 , the spinal fixation device  10  may then be completed. Cap  70  as shown in  FIG. 8   f  is then assembled to the yoke  50 , as described above with reference to  FIGS. 4-5 , to lock the rod  12  relative to the yoke  50  and the yoke  50  relative to the bone fastener  20 . It should be appreciated that the spinal fixation device  10  as particularly described herein has the advantage of establishing a low profile, since the outer surface of the screw head  24  may be driven down relatively deeply into the pedicle of the vertebra, while still maintaining swivel movement of the yoke  50  until the set screw  80  is tightened. Furthermore, the relatively large surface area of spherical surface  45  of the ball insert  40  tightly pressed against the interior surface of the screw socket  26  provides for a very rigid construct for locking the polyaxial motion of the yoke  50  relative to the screw  20 . 
     Having described one particular embodiment of the anchor device for use in spinal fixation device  10 , it should be understood that alternative embodiments are also contemplated. One alternative as illustrated in  FIG. 20 , also uses the captively retained ball insert  40  described above. An alternative anchor device  100  shown in  FIG. 20  includes a bone engaging fastener  102  with a spherical head  103 . The head defines a spherical socket  105  like the bone screw  20  described above. The ball insert  40 , lower sleeve  60 , upper sleeve  70  and set screw  80  may be constructed as described above. The yoke  110  includes a threaded stem  111  and a shoulder  115  for threaded engagement with the ball insert. However, unlike the previously described yoke  50 , the yoke  115  includes an internal cavity  113  extending from the distal end  117 . This cavity corresponds to a dimple  107  formed in the base of the spherical socket  105 . 
     In accordance with this embodiment, a retention ball  120  is seated within the dimple and residing within the cavity  113 . A spring  121  is disposed within the cavity to exert a relatively slight force against the ball  120 . The ball and dimple serve as a releasable detent to maintain a proper orientation between the ball insert  40  and the screw head  103  for ease of screw insertion. The spring maintains pressure on the seating ball  120  and also exerts an upward force on the ball insert  40  to help engage the insert within the spherical cavity  113  of the bone screw head. The fixation of the anchor device  100  otherwise proceeds as outlined above by tightening the set screw  80 . 
     A suitable insertion tool  200  for inserting device  100  into a vertebra is shown in  FIG. 21 . Tool  200  has an inner tube  202  with shoulders  204  partially configured to engage a groove  206  formed in the outer surface of yoke  110 . Tool  200  further has a driver element  208  that is configured to fit within the channel of yoke  110  to thread the device  100  into a vertebra. 
     A rod persuader tool  220  for particular use with device  100  is shown in  FIG. 22 . Tool  220  is similar to the rod persuader  185  having an outer tube  186  axially movable with respect to an inner tube  192 . The shoulders  204  are configured to engage yoke groove  206  and hold the device  100  while an elongated rod  12  is pushed into the yoke  110  of the device  100 . The rod is pushed by notch  189  upon downward movement of outer tube  186 . Completion of the spinal fixation system takes place by assembly of the cap  70  over the yoke  110  and tightening of the set screw  80  against the rod  12  as described above. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.