Abstract:
The present invention relates to a fastener assembly for a spinal fixation system and is particularly useful in treatment of pediatric and small-statured patients. The fastener assembly includes a fastener, an attachment member, and a locking member. The fastener has a lower portion for contacting a bone and a upper portion integral with the lower portion and having two open channels. Each channel is configured and dimensioned for receiving a portion of the longitudinal member along its circumference. The attachment member is positionable on the fastener and at least partially covers the channel that receives the longitudinal member. The attachment member is configured and dimensioned for receiving another portion of the longitudinal member along its circumference. The locking member is operatively associated with the upper portion of the fastener and secures the attachment member and longitudinal member to the fastener.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional application No. 60/221,518, filed Jul. 28, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a spinal fixation system, and in particular to a fastener assembly for securing a longitudinal support along a spinal column.  
           [0003]    1. Background of Invention  
           [0004]    Stabilization of the spine is often required following trauma, tumor, or degenerative pathologies. Each region of the spine presents unique clinical challenges as several vital neural and vascular structures including the vertebral arteries, nerve roots, and spinal cord must be avoided during surgery. The anatomy of pediatric and small-statured patients presents additional challenges that makes assisting such patients even more difficult. For example, because these patients are small in stature, lower profile systems are required. Also, it may be required to add fastener assemblies to an already assembled system, especially in pediatric patients. At the same time, it is desirous to keep inventory at a minimum and have an assembly that requires as few components as possible.  
           [0005]    Current methods of spinal fixation are not particularly well-suited for smaller statured patients. U.S. Pat. No. 5,737,685 to Hahn et al. discloses a bone screw that has a threaded shaft and a fork head. The fork head has two legs which define a single groove that opens in the vertical direction for receiving a corrective pin. The upper end of the fork head has an outside thread and a head nut is screwed onto the outer thread. The head nut has inner threads which a fastening screw is screwed to press onto the corrective pin. Because the groove opens in a vertical direction it is difficult to add additional screws to an already installed fixation system.  
           [0006]    U.S. Pat. No. 5,530,441 to Sherman et al. discloses an attachment plate configured for use with a spinal fixation element that has a posteriorly projecting central post. The attachment plate is generally L-shaped with an eyebolt engaging portion and a fixation element clamping portion. The fixation element has grooves in the lateral surfaces of the central post and one lateral surface of the post contacts the spinal rod when the rod extends through an aperture of an eyebolt assembly. A flange extends from the attachment plate and is configured to engage the opposite lateral surface of the post and the attachment plate includes a camming segment to provide a clamping force component directed toward the flange to clamp the spinal rod to the fixation element post when a nut is threaded onto the eyebolt threaded post. The attachment plate has a slot in the eyebolt engaging portion for receiving the posteriorly projecting threaded post of the eyebolt body. Because the eyebolt must be pre-assembled onto the spinal rod, it is difficult to add additional fixation elements to an already installed system.  
           [0007]    U.S. Pat. No. 4,653,481 to Howland et al. discloses a spinal support system that includes a plurality of screw clamp assemblies. Each screw clamp assembly has a threaded end for placement into the vertebra and a saddle assembly removably attached to the screw formed of upper and lower halves. Each of the upper and lower halves of the saddle assembly are provided with at least one pair of mating grooves for accepting a spinal rod. All embodiments shown and described in the Howland &#39;481 patent have a two-part saddle assembly that is removable from the screw clamp shaft member.  
           [0008]    As the discussion above illustrates, there is a need for an improved fixation apparatus for stabilizing the spine that can be assembled to an already installed fixation system and has minimal components.  
           [0009]    2. Summary of Invention  
           [0010]    The present invention relates to a fastener assembly for a spinal fixation system. The fastener assembly includes a fastener, an attachment member, and a locking member. The fastener has a lower portion for contacting a bone and a upper portion integral with the lower portion and having two open channels. Each channel is configured and dimensioned for receiving a portion of the longitudinal member along its circumference. The attachment member is positionable on the fastener and at least partially covers the channel that receives the longitudinal member. The attachment member is configured and dimensioned for receiving another portion of the longitudinal member along its circumference. The locking member is operatively associated with the upper portion of the fastener and secures the attachment member and longitudinal member to the fastener.  
           [0011]    The fastener can be a hook or a screw with the lower portion having a threaded end for engaging a vertebra. If the fastener is a hook, the hook can be provided with an arcuate portion and a flat portion for facilitating implantation. The arcuate portion can have a dimple on a posterior surface to further facilitate implantation. In one embodiment, the locking member is a nut and the upper portion of the fastener has a shaft with external threads to accept the locking member.  
           [0012]    The two channels of the fastener are preferably disposed on opposite sides of a central plane of the fastener with the two channels extending orthogonally with respect to the fastener longitudinal axis and equidistant from the proximal end of the fastener. A bore can be positioned transversely to the longitudinal axis and between the two channels.  
           [0013]    In an exemplary embodiment, the attachment member includes a cylinder having upper, lower, and side surfaces with a bore extending through the upper and lower surfaces and defining a longitudinal axis lying in a central plane; a slot extending through the cylinder offset from the central plane and parallel with the central plane; and a protrusion extending from the bottom surface on an opposite side of the central plane from the slot. The channels can define a seat for accepting the protrusion of the attachment member. The slot can be provided with serrations along the inner surface. The slot can also have an eccentric cross-sectional shape with a geometry substantially conforming to a diameter of the longitudinal member.  
           [0014]    The fastener can have a cavity extending longitudinally from the distal end for receiving a manipulation device. This manipulation device includes a ball detent mechanism and a groove extending transversely to the cavity and around its circumference for receiving the ball detent mechanism. Alternatively, the manipulation device has a threaded end and the cavity of the fastener is cylindrical and includes threads along the interior. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is an exploded perspective view of a fastener assembly according to the present invention;  
         [0016]    [0016]FIG. 2 is a side view of one embodiment of a fastener for the fastener assembly of FIG. 1;  
         [0017]    [0017]FIG. 3 is a front view of another embodiment of a fastener for the fastener assembly of FIG. 1;  
         [0018]    [0018]FIG. 4 is a cross-sectional view of the upper portion of a fastener of the fastener assembly of FIG. 1 taken along a central plane;  
         [0019]    [0019]FIG. 5 is a cross-sectional view of the attachment member of the fastener assembly of FIG. 1 taken along a central plane;  
         [0020]    [0020]FIG. 6 is a cross-sectional view of another embodiment of the attachment member of the fastener assembly of FIG. 1 taken along a central plane;  
         [0021]    [0021]FIG. 7 is a bottom view of the attachment member of FIG. 5;  
         [0022]    [0022]FIG. 8 is a side view of the fastener assembly of FIG. 1 showing the fastener as a hook;  
         [0023]    [0023]FIG. 9 is a front view of one embodiment of a manipulation device according to the present invention; FIG. 10 is a front view of another embodiment of a manipulation device according to the present invention;  
         [0024]    [0024]FIG. 11 is a cross-sectional view of the fastener of FIG. 2 taken along a central plane;  
         [0025]    [0025]FIG. 12 is a back view of the fastener of FIG. 2; and  
         [0026]    [0026]FIG. 13 is a perspective view of a transverse bar according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Referring to FIG. 1, the fastener assembly  10  according to the invention generally includes a fastener  12 , an attachment member  14  positionable on fastener  12  and a locking member  16  securable upon fastener  12 . The fastener assembly  10  may be used in a spinal fixation system to secure a longitudinal member  18 , such as a rod, along a spinal column. Fastener  12  has a lower portion  20  for engaging a vertebra, and an upper portion  22  integral with lower portion  20  for securing the longitudinal member  18  to fastener  12 .  
         [0028]    Referring to FIGS. 2 and 3, lower portion  20  of fastener  12  preferably comprises a hook or pedicle screw, and has a longitudinal central axis  24  extending from a proximal end  25  to a distal end  27  and lying in a central plane. In FIG. 2, lower portion  20  comprises a hook  28  and includes a curved hook body  29  with a first end  30  connected to the upper portion  22  and a second free end  31 . Hook body  29  is convex on its posterior side  32 . Free end  31  runs essentially perpendicular to the central axis  24  and is adapted to the spinal geometry. As can be seen in FIG. 3, in another embodiment, lower portion  20  comprises a pedicle screw  80 . Pedicle screw  80  comprises a screw body  82  with a screw point  84  and a thread  86  on the outside of the screw body  82  for engaging the vertebra. Preferably, the screw  80  is self-tapping and includes a blunt screw point  84 .  
         [0029]    Referring to FIGS.  1 - 4 , the upper portion  22  is generally in the form of a shaft  26  extending along central axis  24  and accepts the attachment member  14  and the locking member  16 . Upper portion  22  has a shoulder  33  at a first end adjacent the lower portion  20  and an external threaded portion  34  at the distal end  27  of fastener  12 . At least two channels  35  traverse the shaft  26 , and the channels  35  are preferably disposed on opposite sides of the central plane. The channels  35  extend orthogonal with respect to the central axis  24  and are preferably equidistant from the distal end  27 . Preferably a bore  36  runs between the channels  35  transverse to the central axis  24  to facilitate the machining of a cavity  37  in the distal end  27  of fastener  12 . The external threading  34  engages internal threading of locking member  16  so that locking member  16  is tightenable on the shaft  26 .  
         [0030]    Each channel  35  is configured and dimensioned for receiving at least a portion of the longitudinal member  18 . In a preferred embodiment, longitudinal member  18  is an elongate rigid rod having a circular cross-section taken in a plane extending perpendicular to the longitudinal central axis of the rod. As best seen in FIG. 4, in accordance with this embodiment, channels  35  preferably have an arcuate section  40  centrally disposed between upper  42  and lower  44  legs opening substantially laterally with respect to the central axis  24  of shaft  26 . Arcuate section  40  has a diameter substantially conforming to the diameter of the rod. Preferably, the lower leg  44  extends further radially outward from the central axis  24  than upper leg  42  and defines a seat  46  for supporting a portion of the attachment member  14 . Numerous other embodiments are envisioned in which the longitudinal member may have varied cross-sections, such as a rectangular bar or elliptical wire. Accordingly, channels  35  can have numerous shapes corresponding to the particular geometry of the longitudinal member used.  
         [0031]    Referring again to FIG. 1, the attachment member  14  preferably has a generally cylindrical shape having upper  48 , lower  50  and side  52  surfaces with a bore  54  extending through the upper  48  and lower  50  surfaces along central axis  24 . Bore  54  extends through the attachment member  14  so that the attachment member  14  can be inserted over the external threading  34  to be positionable on the upper portion  22  of fastener  12 . A slot  56  extends transversely through the attachment member  14  and is orthogonal to central axis  24  and offset from the central axis  24 . Slot  56  is configured and dimensioned for receiving the longitudinal member  18  and locatable adjacent either of the channels  35  for securing the longitudinal member  18  to the upper portion  22  of fastener  12 . When the longitudinal member  18  is inserted in channel  35  and attachment member  14  is assembled upon the upper portion  22 , locking member  16  tightens attachment member  14  to secure the longitudinal member  18  in slot  56  and against one of the channels  35 .  
         [0032]    Locking member  16  substantially resembles a nut and has a generally disc-like shape with top  60 , bottom  61  and side  62  surfaces with a hole  63  extending centrally through the top  60  and bottom  61  surfaces. The hole  63  is provided with internal threading  64  which mates with external threading  34  provided on shaft  26  of upper portion  22 . As such, locking member  16  is tightenable on the shaft  26  and the bottom surface  61  contacts the upper surface  48  of the attachment member  14  to force the attachment member  14  downward toward the lower portion  20  as locking member  16  is tightened. Side surface  62  is preferably provided with a polygonal geometric configuration comprising twelve points to facilitate tightening.  
         [0033]    The slot  56  has a generally cylindrical cross-section and has a geometry substantially conforming to the diameter of the longitudinal member  18 . Preferably, slot  56  has an eccentrically shaped cross-section. Referring to FIG. 5, slot  56  comprises multiple circular sections having centers offset with respect to one another to define an inner surface  57  towards the central axis  24  connecting inner section  53  with outer section  55 . Inner and outer sections  53 ,  55  preferably have circular geometries. Inner section  53  has a center of curvature C i  offset inward towards central axis  24  with respect to a center of curvature C o  of outer section  55 . The eccentric shape of inner surface  57  is useful to allow the camming of longitudinal member  18  towards the channels  35  when the attachment member  14  is tightened onto upper portion  22  as inner section  53  allows for movement of the longitudinal member axially inward to forcibly engaging the longitudinal member  18  with either of the channels  35 . As can be seen in FIG. 6, in one embodiment of attachment member  14 , the slot  56  includes serrations  62  along inner surface  57  for engaging corresponding ridges  65  on a ridged longitudinal member, such as transverse bar  66  (FIG. 13). In this way, the ridged longitudinal member is less likely to rotate relative to fastener  12  when the attachment member  14  is secured by locking member  16 .  
         [0034]    Referring to FIGS.  5 - 7 , a protrusion  68  extends from lower surface  50  on an opposite side of the attachment member  14  from slot  56 . Protrusion  68  preferably extends generally perpendicular from lower surface  50  and is spaced radially inward from the side surface  52 . Also preferably, protrusion  68  runs arcuately along a portion of lower surface  50  opposite slot  56 . As such, when attachment member  14  is assembled upon the upper portion  22  of fastener  12 , the protrusion  68  preferably engages seat  46  for preventing the attachment member  14  from tilting when the attachment member  14  is tightened by locking member  16 .  
         [0035]    As shown in FIG. 8, upper portion  22  of fastener assembly  10  has a height H measured form the base of shoulder  33  to the top surface  60  of locking member  16 . Preferably height H is as minimal as possible and only slightly greater than the diameter of longitudinal member  18 . Preferably height H is less than 10.0 mm. Also, preferably the ratio of longitudinal member diameter D to height H is greater than 0.50 so that the upper portion  22  has only a slightly higher profile than the longitudinal member  18 . A notable advantage of maintaining such a low height H, or low profile, is that such a fastener assembly  10  is especially well suited for use in pediatric and small-statured patients.  
         [0036]    Referring again to FIG. 4, fastener  12  consists of a cavity  37  in distal end  27  opening upward and having a hexagon socket  38  near the distal end and a cylindrical hole  39  running in the direction of longitudinal central axis  24 , for receiving a manipulation device  70  (FIG. 9) to facilitate the insertion of the fastener  12  into bone. Cylindrical hole  39  is preferably provided with internal threading that continues from the base of hexagon socket  38  in the direction of lower portion  20  for the positive acceptance of manipulation device  70  with corresponding external threading  71  (FIG. 9). Here, cavity  37  permits a rotationally-stable, releasable connection with manipulation device  70 . Internal threading can be designed with multiple threads to strongly shorten the time required for the connection process. For this embodiment, manipulation device  70 , shown in FIG. 9, consists of hollow cylindrical sheath  73  that has at its one end  74  lug  75 . From the other end  76  of sheath  73 , a cylindrical pin  77  can be inserted into sheath  73 . Cylindrical pin  77  carries at its one end external threading  71  that, after successful insertion, protrudes beyond end  74  of sheath  73 , and at its other end, grip  78  protrudes from sheath  73  and serves to tighten the connection.  
         [0037]    Preferably a generally rectangular groove  72  extends transverse to the hexagon socket  38  and around the circumference of the cavity  37 . As shown in FIG. 10, another embodiment of the manipulation device  70  is provided with bearings  79  in lug  75  for engaging groove  72 . When manipulation device  70  is introduced into cavity  37  and bearings  79  align with rectangular groove  72 , pin  77  can be pushed through the sheath  73  in manipulation device  70  to press bearings  79  into rectangular groove  72 . This facilitates the transfer of tensile, compressive and rotational forces via manipulation device  70  without the need to have a threaded coupling.  
         [0038]    Referring to FIG. 11, at convex posterior side  32  of hook body  29  a dimple  100  is provided to accept an installation device (not shown). Dimple  100  comprises a cylindrical hole extending along a longitudinal axis  101  at an angle with respect to the central axis  24 . Referring to FIG. 12, preferably dimple  100  is located centrally with respect to the posterior side  32 . In this way, dimple  100  provides an ideally suited structure for impaction to facilitate the insertion of flat end  31  into the vertebra.  
         [0039]    Referring to FIG. 13, a transverse bar  66  is shown which may be used to secure the longitudinal member  18  to a fastener  12  that is laterally displaced from the longitudinal member  18  to avoid excessively bending the longitudinal member  18 . The transverse bar  66  includes a hook  130  at one end  132 . A set screw  134  extends through the hook  130  and pins the longitudinal member  18  against the inner surface  136  of the hook  134  to secure the longitudinal member  18  to the transverse bar  66 . The second end  138  of the transverse bar  66  is a shaft that has ridges  65  around the circumference that mate with serrations  59  provided in attachment member  14  as described above.  
         [0040]    In use, a plurality of fastener assemblies  10  are inserted into the vertebra along the spine with the aid of the manipulation device  70  and the longitudinal member  18  is placed in one of the two channels  35  on each of the fasteners  12 . An attachment member  14  is placed onto each of the fasteners  12  so that the longitudinal member  18  is held in the channel  35  by the slot  56  of attachment member  14 . A locking member  16  is then used to secure the longitudinal member  18  in the channel  35 . Also, a transverse bar  66  can be secured to the longitudinal member  18  to couple the longitudinal member  18  to a laterally positioned fastener  12 . In that case, the hooked end  127  of the transverse bar  66  is held to the longitudinal member  18  by a set screw  135  and the opposite end  138  of the transverse bar  66  is secured to the fastener  12  by an attachment member  14  in the same fashion as described above for the longitudinal member  18 .  
         [0041]    Because of the dual-channel structure of the fastener  12 , a surgeon implementing a spinal fixation system may insert the longitudinal member  18  in either channel  35 . Also, because the slot  56  is offset with respect to central axis  24 , the attachment member  14  may be located on either side of fastener  12  and the longitudinal member  18  may be secured to either one of channels  35 . As a result, fastener assembly  10  may be assembled on either side of the spinal column using the identical components. Further, the integral nature of the upper and lower portions  22 ,  20  allows for easy assembly to an already installed spinal fixation system. Because the channels  35  open laterally fastener  12  can be anchored to a vertebra and longitudinal member  18  may be laterally received in upper portion  22  and attachment member  14  can be vertically assembled thereover to secure the longitudinal member in place, and no preassembly is required.  
         [0042]    While it is apparent that the illustrative embodiments of the invention herein disclosed fulfill the objectives stated above, it will be appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments which come within the spirit and scope of the present invention.