Abstract:
An apparatus for rigid fixation of individual vertebra to the stabilizing rod. It consists of an anchor secured to the vertebra and a connector with multiple degrees of freedom linking each anchor to a rod. Pressure from the rod against a connector engages internal constricting mechanism and eliminates movement within the connection. This results in rigid fixation between individual anchors and the rod.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates to a spinal fixation device. More specifically, the present invention relates to an implant bridging together two or more vertebra by anchoring to the posterior vertebral arch and pedicles.  
       BACKGROUND  
       [0002]     Spinal fusion is utilized to correct painful movement across degenerated or unstable spinal segment and is supplemented by fixation with screws. Screws are commonly placed through the posterior vertebral arch. Multiple fixation targets have been used in the past including facets, spinal processes, lamina, transverse processes, lateral masses and pedicles. Currently fixation through the pedicles offers the most robust purchase and is utilized when possible.  
         [0003]     Screws are implanted from the back to the front of the vertebra with one screw inserted per each side per vertebra. Individual screws have to be rigidly linked to eliminate movement between corresponding vertebras. (U.S. Pat. No. 5,474,555 Puno et al.). Anatomy of individual patient makes it difficult and often impossible to line up pedicle screws in one plane. Screws project with variable medial-lateral displacements and different angulations. This places significant challenges on the mechanical design of the linking system. These complex mechanisms with multiple joints have to be manipulated in a deep wound restricted by muscle and soft tissue.  
         [0004]     Most of the available systems can be broken into one of three groups: plating system (U.S. Pat. No. 4,611,581); top loading rod system (U.S. Pat. No. 4,805,602, U.S. Pat. No. 5,360,431) and side loading rod system (U.S. Pat. No. 5,261,909).  
         [0005]     The top loading rod systems utilizes a seat for the rod that projects along the screw trajectory and forms a ball and socket joint with the spherical screw head. It offers several degrees of freedom about the screw axis: (i) 30-40 degrees of angular deflection from the screw axis (ii) 360 degrees of rotation about the screw axis. Any further adjustments have to be gained by contouring the rod to fit into the rod receptacle of the seat. These systems offer advantages when the exposure is directly over the screw such as in muscle splitting or minimally invasive approaches.  
         [0006]     In a standard midline approach screws project out into the soft tissue at the lateral extreme of the exposure. Top loaded systems require manipulation at this lateral extreme and side loading rod system can offer advantages in these circumstances. In side loading rod systems connectors run perpendicular to the screw trajectory and the rod is placed on the side of the screw toward the center of exposure U.S. Pat. No. 5,261,909).  
       SUMMARY OF THE INVENTION  
       [0007]     The claimed invention offers a number of significant advantages over existing art. 
        1. Screw-rod articulation functions as a rotating and pivoting joint and not a ball and socket joint characteristic of top loading systems. It allows much greater angular deflection (over 90 degrees) while still matching full 360 degree rotation.     2. Clamping mechanism allows substantial vertical travel along the screw trajectory contributing to additional degrees of freedom. This adds substantial flexibility particularly when more then 3 screws need to be connected.     3. Connector can be snapped onto the screw head after screw is placed into the bone. This keeps hardware profile minimal and improves exposure for delivery of graft material. It also allows for all the connectors and rods to be assembled outside of the wound. This is particularly valuable in small direct exposures characteristic of the minimally invasive approach.     4. This system can be utilized both as a top loading and side loading system which can not be matched by the currently available art.        
 
         [0012]     In accordance with an embodiment of the present invention, there is provided device for fixation of at least one spinal segment comprising a rod, at least two bone screws and at least two connectors said connectors having a rod receiver, screw clamp, compression spacer and a tightening nut. The rod receiver has at one end a channel with threaded walls accepting the rod and the tightening nut, with the opposite end sliding over the screw clamp. The screw clamp has the articulating end opposite to the sliding portion and fitting over the bone screw said bone screw having a threaded portion imbedded into the bone with protruding articulating portion coupled with the screw clamp and allowing for a pivoting and rotational motion in uncompressed state but immobilized by compression from the compression spacer. The compression spacer is housed within rod receiver, engaged by the rod pressed into the rod receiver and clamping on the screw clamp which in turn clamps on articulation between screw clamp and bone screw  
       DETAILED DESCRIPTION  
       [0013]     In its preferred embodiment device consists of a screw, screw clamp, rod receiver, compression spacer, rod and nut ( FIG. 1 ).  
         [0014]     Screw has a long threaded portion placed into the bone and a protruding cylindrical shaft with a spherical head ( FIG. 2 ). A compressible cylindrically shaped screw clamp of substantial thickness ( FIG. 3 ) has a spherical cut-out at one end such that it fits precisely over the screw head ( FIG. 4 ). It pivots about the screw head with angle of deflection greater then ninety degrees and 360 degrees of revolution through the entire range of deflection.  
         [0015]     Screw clamp ( FIG. 6 ) tightly fits into cylindrical assembly consisting of the rod receiver and compression spacer ( FIG. 7 ). Rod receiver has a travel tube with a tapered end which fits over the screw clamp. The other end forms the seat for the rod bounded by threaded walls ( FIG. 8 ). Up and down movement of the screw clamp within the rod receiver allows for a vertical adjustment. Compression clamp is a deformable component fitting within the travel tube of the rod receiver ( FIG. 5 ). It has a contact dome extending into the seat of the rod receiver. Pressure from the rod locked into the seat by a threaded nut, displaces the compression spacer into the tapered end of the travel tube. The resulting deformation of the compression spacer transfers compressive force onto the screw clamp. This fixes the screw head within the screw clamp and prevents any travel of the screw clamp within the travel tube. This process is repeated for each screw in the assembly.  
         [0016]     Alternative embodiments of the screw shaft and screw clamp articulation which enable pivoting and rotational motion are possible.  
         [0017]     ( FIG. 13 ,  FIG. 14 ). 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0018]      FIG. 1  Preferred embodiment of the invention showing screw ( 1 ), screw clamp ( 6 ), compression spacer ( 9 ), rod ( 17 ) and nut ( 18 )  
         [0019]      FIG. 1   a  Different orientation of the preferred embodiment of the invention showing screw ( 1 ), screw clamp ( 6 ), compression spacer ( 9 ), rod ( 17 ) and nut ( 18 ).  
         [0020]      FIG. 2  Bone screw consisting of threaded portion ( 2 ), protruding shaft ( 3 ) and spherical head ( 4 ). Please notice the channel ( 5 ) through the center of the screw.  
         [0021]      FIG. 3  Preferred embodiment of the screw clamp consisting of a cylindrical compressible shaft ( 8 ) and articulating end ( 7 ).  
         [0022]      FIG. 4  Articulation between screw head ( 4 ) and articulating end ( 7 ) of the screw clamp. Notice that this articulation allows only pivoting motion and is not a ball-socket joint.  
         [0023]      FIG. 5  One of the embodiments of the compression spacer ( 9 ) with contact dome ( 10 ) which is depressed by the rod ( 17 ) when its locked, and constricting tube ( 11 ) deformed by the tapered section of the rod receiver ( 16 ).  
         [0024]      FIG. 6  Preferred embodiment of the rod receiver ( 12 ) consisting of the seat ( 13 ) for the rod, threaded extensions ( 14 ) which accept a nut ( 18 ), travel tube ( 15 ) and tapered end ( 16 ).  
         [0025]      FIG. 7  Assembly consists of screw clamp ( 6 ), compression spacer ( 9 ) and rod receiver ( 12 ).  
         [0026]      FIG. 8  Preferred embodiment of the invention with compression spacer ( 9 ) and rod receiver ( 12 ) rendered transparent to demonstrate the entire assembly.  
         [0027]     Also note the screw clamp ( 6 ) and the screw ( 1 ). Note that if deformable compression spacer ( 9 ) is pressed into the rod receiver ( 12 ) the tapered configuration at the bottom of the rod receiver will constrict corresponding area of the compression spacer, which in turn will transmit compressive forces to the articulation between screw clamp ( 6 ) and a screw ( 1 ).  
         [0028]      FIG. 9  (Landscape orientation)  
         [0029]     Demonstrates 90 degrees deflection angle (potentially &gt;90 degrees) between screw ( 1 ) and screw clamp ( 6 ).  
         [0030]      FIG. 10  Assembly demonstrates vertical travel of screw clamp ( 6 ) within the rod receiver ( 12 ). Please compare to Pic  8 . Please note that compression by compression spacer ( 9 ) will be transferred to screw clamp ( 6 ) over a wide range of vertical travel.  
         [0031]      FIG. 11  Bone screw ( 1 ) with central channel ( 5 ).  
         [0032]      FIG. 12 . Bone screw ( 1 ) with off-center channel ( 5 ).  
         [0033]      FIG. 13  Alternative embodiment of the screw ( 1 ) and screw clamp ( 6 ) articulation with compressible spherical spacer ( 19 ) threaded over the cylindrical screw shaft( 3 ).  
         [0034]      FIG. 14  Alternative embodiment of the screw ( 1 ) and screw clamp articulation. Screw clamp would be modified to fit over the projections ( 20 ) of the compressible T-spacer ( 21 ).