Abstract:
A Spinal column implant for elastic stabilization of vertebrae includes an elastic rod that is anchored in a non-positive fit fashion in the receptacle of a pedicle screws by means of a mobile filling piece and a clamping element. The non-positive fit is supported additionally by a dynamic form-fit contribution upon exposure to load.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 60/738,759, filed Nov. 21, 2005, and claims priority from Swiss Patent Application 01488/05, filed Sep. 13, 2005, the entire disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present disclosure relates to a dynamic stabilizing system for spinal columns capable of stabilizing the spinal column without spinal fusion using elastic connection elements connected to bone or pedicle screws.  
         [0003]     A multitude of rod/screw connections are available for metallic rods and used mainly in fusion operations (spinal fusion). There are only a few elastic systems that only support and stabilize, but do not fuse, the spinal segments and, thus, there are only a few devices for attaching the elastic connection elements with the pedicle screws.  
         [0004]     As a matter of principle, rod/screw connections that are suitable for metallic rods are not necessarily also suitable for elastic connection elements since elastic rods made of plastic material, for example, possess different properties than rods made of metal that are stiffer by comparison. Accordingly, elastic rods made of plastic material cannot be simply clamped in a permanent fashion by means of non-positive fitting (force-fit, frictional-fit or press-fit) since they usually are capable of reducing the clamping force by flowing. Consequently, there is a need for connection concepts comprising, aside from a possible non-positive fit-type (basic) clamping, a contribution by positive fitting or, ideally, a dynamic positive fit that increases transiently with increasing load and lessens with decreasing load.  
         [0005]     WO 01/06939, WO 97/43974 and U.S. Pat. No. 4,987,892 include rotatable holding elements these serve the sole purpose of also connecting rods, which do not extend perpendicular to the screw axis, to the screw. Accordingly, these holding elements are rotatable during the attachment phase only and can no longer be moved after completion of the attachment phase.  
       SUMMARY OF THE INVENTION  
       [0006]     The present disclosure is based on the tasks to connect an elastic rod made of plastic material with a continuous smooth surface in discontinuity-free and secure fashion to a bone or pedicle screw and, in the process, transfer tensile and compressive as well as shearing and torsional forces between neighboring vertebrae.  
         [0007]     The solution to this task is characterized in that the connection is a combination of permanent (basic) non-positive fit and a dynamic form-fit contribution. The dynamic form-fit contribution is attained by local elastic or plastic deformation of the plastic rod. This dynamic form-fit contribution increases automatically with increasing axial force in the rod and decreases automatically thereafter with decreasing axial force in the rod.  
         [0008]     Accordingly, the non-positive fit rod/screw connection with a dynamic form-fit contribution allows a smooth elastic rod made of plastic material to be connected to the head of a bone or pedicle screw such that the expected forces can be transferred perma nently and securely due to its application as a dynamic stabilization of the lumbar spinal column. Thus, a non-positive fit connection, which can be positioned easily and continuously, is combined with the reliability of a form-fit contribution to the connection. In the process, the initial positive-fit is supported by a dynamic increase in the form-fit with increasing axial force, which decreases again with decreasing axial force. This takes care of the plastic rod and largely prevents the process of flowing under load.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     In the following, the present disclosure is illustrated in more detail by means of the appended drawings, in which exemplary embodiments are shown. In the figures, the following is shown schematically:  
         [0010]      FIG. 1   a  shows an elastic connection element in a (sectioned) pedicle screw with a filling piece in neutral position about a pivot point.  
         [0011]      FIG. 1   b  shows an elastic connection element exposed to tensile force with a filling piece in rotated position about a pivot point.  
         [0012]      FIG. 2   a  shows a side view of a, for example, round connection element in a pedicle screw with a filling piece, a pivot point with spherical surface, and a clamping element;  
         [0013]      FIG. 2   b  shows a frontal view of the elements shown in  FIG. 2   a.    
         [0014]      FIG. 2   c  shows a top view of the elements shown in  FIG. 2   a.    
         [0015]      FIG. 3   a  shows a side view of a, for example, round connection element in a pedicle screw with a filling piece, a pivot point in an axis, and a clamping element.  
         [0016]      FIG. 3   b  shows a frontal view of the elements of  FIG. 3   a.    
         [0017]      FIG. 3   c  shows a top view of the elements of  FIG. 3   a.    
         [0018]      FIG. 4   a  shows an elastic connection element in a (sectioned) pedicle screw with a filling piece, which includes ribs, in neutral position about the pivot point.  
         [0019]      FIG. 4   b  shows an elastic connection element exposed to tensile force with the filling piece, which includes ribs, in rotated position about the pivot point.  
         [0020]      FIG. 5  shows a top view of a, for example, round connection element in a pedicle screw with screw wings, with a filling piece, a pivot point with spherical surface, and a clamping element connected to the screw via a saw tooth thread.  
         [0021]      FIG. 6  shows a top view of a, for example, round connection element in a pedicle screw, with a filling piece, a pivot point in an axis and the same axis as clamping element.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]      FIG. 1   a  shows a connection element ( 2 ) in a pedicle screw ( 1 ) (shown in cross section) with a rotatable filling piece ( 3 ) in a neutral position about a pivot point ( 4 ). The connection element ( 2 ) may be a rod of any shape. For example, the connection element can have a round cross-sectional shape. The connection element ( 2 ) is elastic and is constructed from a plastic material. An example of such a material is Polycarbonate Urethane. A clamping element ( 5 ) (shown in  FIGS. 2   a - 2   c ,  3   a - 3   c ,  5  and  6 ) presses the filling piece ( 3 ) against the connection element ( 2 ) in a non-positive fashion, which is also referred to herein as a force-fit or a frictional fashion.  FIG. 1   b  shows how the filling piece ( 3 ) rotates as soon as the elastic connection element ( 2 ) is exposed to tensile force or extension. The filling piece ( 3 ) is arranged such that its edge opposite to the direction of force impinges upon the surface of the elastic connection element and generates a form-fit contribution that increases the anchoring force substantially.  
         [0023]      FIGS. 2   a ,  2   b , and  2   c  show a side, frontal, and top view, respectively, of the elastic connection element ( 2 ), pedicle screw ( 1 ), filling piece ( 3 ), and clamping element ( 5 ). The clamping element ( 5 ) directly presses the rotatable filling piece ( 3 ) against the connection element ( 2 ) via a spherical surface ( 4   a ).  
         [0024]      FIGS. 3   a ,  3   b , and  3   c  show a side, frontal, and top view, respectively, of the elastic connection element ( 2 ), pedicle screw ( 1 ), filling piece ( 3 ), and clamping element ( 5 ). The clamping element ( 5 ) presses the rotatable filling piece ( 3 ) against the connection element ( 2 ) via a pin ( 4   b ).  
         [0025]      FIG. 4   a  also shows an elastic connection element ( 2 ) in a pedicle screw ( 1 ) (shown in cross section) with a rotatable filling piece ( 3 ) in a neutral position about a pivot point ( 4 ).  FIG. 4   b  shows how the filling piece ( 3 ) rotates as soon as the elastic connection element ( 2 ) is exposed to tensile force or extension. In this embodiment, the filling piece ( 3 ) comprises ribs ( 6 ), which slightly impinge upon the surface of the elastic connection element ( 2 ) even in the neutral position. Thus, the ribs ( 6 ) increase the initial resistance on the filling piece ( 3 ) which supports its rotation in the direction of force.  
         [0026]      FIG. 5  shows a top view of, for example, a round connection element ( 2 ) in a pedicle screw ( 1 ) with a filling piece ( 3 ), a pivot point with spherical surface ( 4   a ), and a clamping element ( 5 ) that is connected to the screw via a saw tooth thread ( 7 ). The saw tooth thread prevents compression of the two screw wings (la) to possibly restrict the mobility of the filling piece ( 3 ).  
         [0027]      FIG. 6  shows a top view of a, for example, round connection element ( 2 ) in a pedicle screw ( 1 ), with a filling piece ( 3 ), a pivot point in a pin ( 4   b ), whereby this pin simultaneously serves as clamping element ( 5 ) and is positioned by elastic spreading of the screw wings ( 1   a ).  
         [0028]     While a particular form of the disclosure has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, it is not intended that the disclosure be limited, except as by the appended claims.