Abstract:
A variable angle spinal fixation system is disclosed. The system includes a longitudinal member positionable along a spinal column, a fastener having a threaded end for engaging a vertebra, and a connector member for connecting the fastener and the longitudinal member. The connector member has a channel extending through side surfaces of the connector member for receiving the longitudinal member, an opening laterally displaced from the channel and extending through top and bottom surfaces of the connector member for receiving the fastener, and a fastener clamping element for securing the fastener in the opening at a surgeon selected angle relative to the connector member and longitudinal member. The system according to the present invention allows angulation of the fastener to accommodate complex pathologies.

Description:
This application claims benefit to U.S. provisional application Serial No. 60/099,976, filed Sep. 11, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a spinal fixation system, and in particular one which includes a variable angle spinal fixation device. 
     BACKGROUND OF THE INVENTION 
     Stabilization of the spine is often required following trauma, tumor, or degenerative pathologies. Although each region of the spine presents unique clinical challenges, posterior fixation of the cervical spine is particularly troublesome. The anatomy of the cervical spine makes it a technically challenging area to instrument. Specifically, several vital neural and vascular structures including the vertebral arteries, nerve roots, and spinal cord must be avoided during surgery. 
     Current methods of posterior cervical stabilization include the use of metallic wire or cable and plate/screw systems. Both wire and plating systems suffer from limitations. For example, in certain clinical applications such as occipital-cervical and cervical-thoracic pathologies, the fixed location of the plate screw holes makes alignment of the screws difficult and may comprise the achieved stabilization. 
     In order to alleviate the problems associated with fixed screw hole location, devices allowing variability in screw placement are available. For example, U.S. Pat. No. 5,735,852 discloses a clamp that can be placed anywhere along the length of a rod. One problem with this and similar designs is that the clamp has a screw hole surface oriented at a fixed angle with respect to the rod. As a result, these systems often do not provide sufficient angulation of the screw to accommodate complex degenerative pathologies. 
     Variable angle screw systems are also currently available. U.S. Pat. No. 5,549,608 discloses a polyaxial locking screw and coupling element device for use with a rod fixation apparatus. Because the rod is located on top of the screw after the device is assembled, the device disclosed in the &#39;608 patent has a high profile and does not allow independent screw and rod fixation. Without independent screw and rod fixation, some adjustability is lost along with the ability to compress or distract along the rod. Furthermore, should one of the components loosen, the entire construct also loosens. The osteosynthetic fixation device disclosed in U.S. Pat. No. 5,501,684 requires that the rod be threaded through a channel rather than simply slipping the rod through an opening on the side or top of the device. It should also be noted that the devices of the &#39;684 and &#39;608 patents were designed for use in the lumbar and sacral regions of the spine. 
     U.S. Pat. No. 5,643,259 discloses spine fixation instrumentation for the cervical spine. Because the instrumentation disclosed in the &#39;259 patent relies on a sleeve mechanism to secure the rod and the screw to the connector, it is difficult to align both sleeves with the connector. Furthermore, when several connectors are used, alignment is even more difficult and unintended distraction between levels may occur when joining the rod sleeves to the connectors. Due to the limited space between adjacent vertebrae in the cervical region of the spine, there may not be sufficient room between connectors to allow placement of the sleeves. In addition, optimal orientation of the &#39;259 patent system is limited by the inability to rotate the screw about an axis parallel to the rod, i.e. the system only provides for two degrees of freedom, possibly resulting in the need to bend the rod in order for it to fit within the connector. Since the system of the &#39;259 patent depends on the ability to slide the sleeve mechanism along the rod when securing the rod to the connector, the fact that the rod may have to be bent to account for the missing third degree of freedom may prohibit the proper functioning of this sleeve mechanism, and the rod may not lock securely. Finally, the assembly and disassembly processes are tedious and require complicated instruments. 
     As the discussion above illustrates, there is a need for an improved fixation apparatus for stabilizing the cervical spine. 
     SUMMARY OF THE INVENTION 
     The spinal fixation system according to the present invention comprises a longitudinal member positionable along a spinal column; a fastener having a threaded end for engaging a vertebra; and a connector member for connecting the fastener and the longitudinal member. The connector member includes a channel extending through side surfaces of the connector member for receiving the longitudinal member; an opening laterally displaced from the channel and extending through top and bottom surfaces of the connector member for receiving the fastener; and a fastener clamping element for securing the fastener in the opening at a surgeon selected angle relative to the connector member and longitudinal member. Preferably, the system further comprises a longitudinal member clamping element for securing the longitudinal member in the channel. 
     In one embodiment, the channel has a substantially oval-shaped cross section to allow positioning of the longitudinal member in the channel at discrete locations. In another embodiment, the channel is open on top to allow insertion of the longitudinal member in the channel. Alternatively, the channel is open on a side to allow insertion of the longitudinal member in the channel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a first embodiment of the spinal fixation system according to the present invention; 
     FIG. 2 is an exploded side view of the system of FIG. 1; 
     FIG. 3 is a side view of the connector member of FIGS. 1 and 2; 
     FIG. 4 is a side view of a connector member that allows a rod to be placed at two different positions in the connector member; 
     FIG. 5 is a side view of a top loading connector member; 
     FIG. 6 is a cross sectional view of another embodiment of the spinal fixation system according to the present invention; 
     FIG. 7 is a side view of a different embodiment of the spinal fixation system according to the present invention; 
     FIG. 8 is a cross sectional view of the system of FIG. 7; 
     FIG. 9 is a side view of another embodiment of the spinal fixation system according to the present invention with a partial cross section; 
     FIG. 10 is a plan view of the system of FIG. 9; 
     FIG. 11 is an exploded plan view of another embodiment of the spinal fixation system according to the present invention; 
     FIG. 12 is a side view of a side loading connector member; 
     FIG. 13 is an exploded side view of the connector member of FIG. 12; 
     FIG. 14 is a plan view of a final embodiment of the spinal fixation system according to the present invention; and 
     FIG. 15 is another plan view of the system of FIG.  14 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a first embodiment of the spinal fixation system according to the present invention. Although the discussion of this and the other embodiments focuses on cervical fixation, it should be noted that the present invention can be used in other areas of the spine. System  10  includes a connector member  12  for connecting a fastener  14  to a rod  16  (seen best in FIG.  10 ). Usually at least two fasteners  14  and at least two corresponding connector members  12  are used with one rod  16 . Fastener  14  has a stem  18  with a plurality of threads  20  and a hemispherical head  22  with a coupling  24  for attachment to a driver or other device for screwing fastener  14  into a vertebra. As described in more detail below, fastener  14  can be placed in the bone before the other components of system  10 . This results in independent and optimal placement of fastener  14  according to the clinical situation, patient anatomy, and surgeon preference. 
     Connector member  12  has a channel  26  configured and dimensioned to receive rod  16 . A threaded hole  28  (FIG. 3) intersects channel  26  so that when a set screw  30  is screwed into threaded hole  28 , rod  16  is secured in channel  26 . Connector member  12  has a tapered opening  32  (FIG. 3) for receiving cap  34 . Cap  34  has a skirt  36  that has an exterior surface  38  tapered to closely match in size and shape tapered opening  32 . An interior surface  40  of cap  34  is hemispherical to closely match in size and shape head  22  of fastener  14 . Exterior surface  38  includes a plurality of slots  42  so that when head  22  is inserted into skirt  36 , skirt  36  can flex outward until head  22  is flush with interior surface  40  at which point skirt  36  flexes back. Typically, correct insertion of head  22  into skirt  36  is determined by audible (i.e. a “click”) and/or tactile feedback. Once head  22  is seated in skirt  36 , head  22  and interior surface  40  function like a ball and socket joint so that fastener  14  can rotate about skirt  36 . 
     The rotation freedom of fastener  14  allows for three dimensional variability of fastener  14  with respect to connector member  12  and rod  16 . Thus, system  10  can be placed in any orientation that the surgeon desires to conform system  10  to patient anatomy. Another advantage of the rotation freedom is that fasteners  14  and connector members  12  can be connected without the need to contour, i.e. bend, rod  16 . This simplifies the surgical procedure, reduces operating time, and prevents undue stress or damage to rod  16  caused by the bending. 
     Cap  34  also has a threaded segment  44 . When a nut  46  is screwed onto threaded segment  44 , cap  34  is drawn into tapered opening  32 . As cap  34  is drawn into tapered opening  32 , the size of slots  42  is reduced and skirt  36  is compressed. The compression of skirt  36  secures head  22  in skirt  36  at a fixed position. Because fastener  14  and rod  16  are secured to connector member  12  by two separate mechanisms, fastener  14  can first be fixed to the vertebra at the desired location and angulation and then secured to connector member  12  before rod  16  is secured to connector member  12 . The tightening of fastener  14  to connector member  12  prior to securing rod  16  allows compression or distraction of the vertebrae along rod  16  without sacrificing the optimal orientation of connector member  12  and fastener  14  already achieved. The separate tightening of fastener  14  to connector member  12  and rod  16  to connector member  12  also adds significant safety to system  10 . This is due to the fact that both set screw  30  and nut  46  would have to loosen to completely destabilize system  10 . In designs in which the rod sits on top of the screw, loosening of any of the stacked elements can result in failure of the entire implant. In further comparison to designs with the rod situated superior to the screw, laterally displacing fastener  14  with respect to rod  16  reduces the profile of system  10 . 
     FIG. 4 shows another connector member  48  that can be used with system  10 . Connector member  48  has two channels  50  and  52 . Depending on the clinical application, rod  16  can be inserted in channel  50  or channel  52 . 
     FIG. 5 shows another connector member  54  that can be used with system  10 . A channel  56  on connector member  54  is open on the top so that rod  16  can be inserted into channel  56  from the top. As was the case with connector member  12 , a set screw (not shown) threaded into threaded hole  28  secures rod  16  to channel  56 . The top loading design of connector member  54  facilitates inserting rod  16  into channel  56  after connector member  54  and fastener  14  are properly positioned. Because rod  16  is not obstructing the area around connector member  54 , the top loading design also allows the surgeon to more easily pack the area around connector member  54  with bone graft or other osteoconductive material to enhance the formation of new bone. 
     FIG. 6 shows another embodiment of a system  58  according to the present invention. System  58  has a connector member  60  which, like connector member  54 , provides for top loading of rod  16 . Connector member  60  includes a yoke  62  sized to receive a sleeve  64 . As sleeve  64  is configured and dimensioned to cradle a portion of rod  16 , inserted sleeve  64  into yoke  62  secures rod  16  to connector member  60 . Any suitable sleeve design which securely fits in yoke  62  and tightly cradles rod  16  to connector member  60  can be used. An example of such a sleeve is the one disclosed in U.S. Pat. No. 5,643,259, the disclosure of which is incorporated herein by reference. A cap  66  of system  58  also differs from cap  34  of system  10 . Cap  66  has internal threads  68  (rather than threaded segment  44 ) that mate with screw top  70  (rather than nut  46 ) to draw cap  66  into tapered opening  32 . 
     FIGS. 7 and 8 show another embodiment of a top loading system  72  according to the present invention. A connector member  74  has a yoke  76  sized to accommodate rod  16 . In order to secure rod  16  within channel  26 , and in contrast to system  10  in which set screw  30  secures rod  16  in channel  26 , in this embodiment a threaded top  80  mates with threaded walls  82  of yoke  76  to push an apron  78  against rod  16 . 
     FIGS. 9 and 10 show another embodiment of a system  84  according to the present invention. System  84  is a side loading design, i.e. rod  16  slides into a connector member  86  from a side. Set screw  30  secures rod  16  to a yoke  88 . The side loading design of connector member  86  facilitates inserting rod  16  to yoke  88  after connector member  86  and fastener  14  are properly positioned. Because rod  16  is not obstructing the area around connector member  86 , the side loading design also allows the surgeon to more easily pack the area around connector member  86  with bone graft or other osteoconductive material to enhance the formation of new bone. 
     Another side loading system  90  with a connector member  92  is shown in FIG.  11 . Rod  16 , yoke  88 , and the manner in which set screw  30  secures rod  16  to connector member  92  are identical to system  84  of FIGS. 9 and 10. However, fastener  94  is secured to connector member  92  using a ball clamp mechanism. Such a ball clamp mechanism is disclosed in U.S. Pat. No. 5,501,684, the disclosure of which is incorporated herein by reference. Fastener  94  has a stem  18  with a plurality of threads  20 , and intermediate conical section  96 , and a threaded cylindrical head  98 . Ball clamp  100  has a conical bore hole  102  for receiving intermediate conical section  96  of fastener  94 . Ball clamp  100  is shaped like a spherical shell and is provided with slits  104  to allow bore hole  102  to compress against intermediate section  96  upon the application of a compressive force. As a result, when a nut  106  is tightened on the threads of cylindrical head  98  with ball clamp  100  and intermediate section  96  inserted in a through hole  108 , fastener  94  is secured to connector member  92 . 
     FIGS. 12 and 13 show another embodiment of a side loading system  110  according to the present invention. Connector member  112  is provided with a threaded stem  114  onto which an apron  116  slides to retain rod  16  in channel  26 . A nut  118  fixes apron  116  to threaded stem  114 . 
     In all the embodiments described above and shown in FIGS. 1-13, the channel for the rod runs perpendicular to the opening for the fastener. However, as shown in FIGS. 14 and 15, channel  26  for rod  16  can be oriented at a non-orthogonal angle to opening  32  for fastener  14 . This configuration is useful if angulation greater than that provided for by the fastener securing mechanism is desired. This is particularly useful for transarticular screw angulation requirements. 
     While it is apparent that the illustrative embodiments of the invention herein disclosed fulfil 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.