Abstract:
Plastic rod anchorage for the surgical treatment of the spine where the plastic rod ( 1 ) has at least two plane-parallel surfaces ( 1   a ) in order to uniformly clamp the rod ( 1 ) on its entire circumference.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §371 National Phase conversion of PCT/CH2010/000168, filed Jul. 1, 2010, which claims benefit of Swiss Application No. 1113/09, filed Jul. 16, 2009, the disclosure of which is incorporated herein by reference. The PCT International Application was published in the English language. 
     FIELD OF THE INVENTION 
     The present invention relates to an anchorage arrangement for a connecting rod for stabilizing a spine. 
     BACKGROUND OF THE INVENTION 
     For stabilizing the spine, pedicle screws are inserted in the spine and their heads are interconnected by a rod. In order to maintain a dynamic behavior, elastic rods are known as an alternative to rigid metallic rods. 
     Since elastic rods are often manufactured from plastic materials, and plastic materials are generally susceptible to creep, plastic rods cannot simply be clamped by a press-fit as this is easily possible, for example, with metal rods. 
     Therefore, for a reliable long-term clamping of plastic rods (i.e. made of polymeric material), particular devices are required. 
     Only few methods to this end are known in the art so far. These will be explained below and their disadvantages as compared to the present invention will be pointed out. 
     The inventions according to patent specifications EP 1 364 622 B1 (Freudiger) and EP 1 527 742 B1 (Freudiger) show positive or form-fit anchorages. The rod and its seat in the screw head are provided with geometrically mutually fitting grooves. However, positioning the grooved surfaces requires precise introduction to avoid canting. Furthermore, grooved surfaces do not allow continuous positioning. 
     The invention according to patent specification EP 1 759 646 B1 (Freudiger) shows a frictional anchorage with an indirect form-fit. The connecting rod is smooth and is clamped between two members, the screw head and a laterally guided spacer, whose contour nominally deviates from the rod contour in the area of contact. Therefore, this nominal contour deviation locally penetrates into the rod such that high stress intensities result in the clamped areas and relative movements due to shear deformation in the non-clamped areas. Furthermore, rod material may flow off between the clamping areas if they do not form a closed cage. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to propose an anchorage arrangement for a connecting rod exerting less stress on the rod while maintaining positioning accuracy. 
     Such an anchorage arrangement includes the connecting plastic rod including at least two plane-parallel surfaces; and one or more bone screws, each bone screw including sidewalls defining an opening positioned and configured to receive the connecting plastic rod; and a clamping device comprising a clamping element and a spacer together positioned and configured to provide a clamping action in a direction parallel to the at least two plane-parallel surfaces so as to clamp an entire circumference of the rod, such that a distance between the sidewalls is adapted to a distance of the plane-parallel surfaces in an area of contact so that the rod is snugly slideable in the opening. The invention will be further explained by means of preferred exemplary embodiments with reference to the Figures. They show schematically: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A round rod between two semicircles (prior art). 
         FIG. 2   a  An exemplary cross-section having 4 sides. 
         FIG. 2   b  An exemplary cross-section having 5 sides. 
         FIG. 2   c  An exemplary cross-section having 6 sides. 
         FIG. 3   a  A typical cross-section having 5 sides and a taper. 
         FIG. 3   b  The opening between the screw head and the spacer whose contour is congruent with the rod. 
         FIG. 3   c  The rod in the mounted condition in the variant having an external nut and a separate spacer. 
         FIG. 4   a  The rod opening with a circumferential clamping ridge. 
         FIG. 4   b  A spacer with a clamping ridge. 
         FIG. 5   a  The rod opening with two circumferential clamping ridges. 
         FIG. 5   b  A spacer with two clamping ridges. 
         FIG. 6  A variant using an internal nut and a separate spacer. 
         FIG. 7  A variant using an external nut and an integrated spacer. 
         FIG. 8  A variant using an internal nut and an integrated spacer. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A plastic rod  1  is anchored in a pedicle screw  2  such that it can be introduced in a continuous manner without creating stress peaks in the anchoring area due to local clamping as a result of deviating contours and without any alteration of the rod cross-section by flowing of the material. Furthermore, the anchorage is able to transmit both tensile and compressive as well as shearing and torsional forces from the rod to the screw. 
     This is accomplished by clamping action that is uniform peripherally (cf.  FIGS. 2   a - 2   c ), i.e. around the entire cross-section, due to the fact that the contour of the rod cross-section nominally exactly coincides with the contour of the screw head opening and of the spacer in the area of contact, i.e. is congruent therewith. More specifically, the encircling clamping action is intended to act as a cage and to produce hydrostatic properties in the clamping plane, which can only be achieved with a cross-section having at least two plane-parallel surfaces. 
     Consequently, the invention relates to the rod cross-section with at least two plane-parallel sides, thereby allowing a uniform clamping action on the entire circumference, i.e. peripherally. Due to the uniform clamping action, maximal care is kept on the rod in the anchorage area, thereby allowing it to transmit the forces arising in the patient to the upper and lower pedicle screws safely and durably. Furthermore, the clamping action may be assisted by circumferential ridges in one or multiple planes. The clamping action is achieved either by an external or internal nut with a separate spacer or by an external or internal nut with an integrated spacer. 
     Another advantage of the plane-parallel cross-section is the variability of the stiffness in different directions. Thus, for example, the stiffness in the anterior/posterior direction can be increased in a dedicated manner with a minimum volume increase, which may be an essential advantage for patients suffering from vertebral slippage (spondylolisthesis). 
     Furthermore, the plane-parallel sides may most preferably be provided with a taper whose flanks each form an angle  37  of at least 10° with respect to the frontal plane of the rod in order to facilitate its introduction into pedicle screws that do not extend parallelly. 
       FIG. 1  illustrates the conflict that arises when a circular rod  30  is clamped between two semicircles  31 ,  32 . The semicircles  31 ,  32  would have to be able to interpenetrate. This would indeed be possible with complex comb-like tapering portions, but the plastic rod  30  would inevitably flow into the comb openings and be distorted. 
     By way of example,  FIG. 2   a  illustrates a cross-section having 4 sides of which at least two  35 ,  36  are plane-parallel. In this manner, a uniform peripheral fixation can be achieved by a clamping action (arrow  38 , clamping device  4 ) parallel to the plane-parallel lateral surfaces. For this purpose, the distance between the sidewalls  39  of the opening  3  is closely fitted to the distance between the lateral faces  35 ,  36  of the rod  1  so that the rod is tightly held between the sidewalls  39  substantially without any lateral play. Complementarily, the bottom side  41  of the opening  3  is complementarily shaped to the lower side  44  of rod  1  so that the all sides of the rod  1  opposed to sides  39 ,  41  of the opening  3  tightly and play-free abut against these sides once the rod  1  is inserted into the opening  3 . 
       FIG. 2   b  illustrates a second exemplary cross-section having 5 sides with the same properties as that of  FIG. 2   a , in particular two parallel sides  35 ,  36 . 
       FIG. 2   c  illustrates a third exemplary cross-section having 6 sides with the same properties as that of  FIG. 2   a.    
       FIG. 3   a  illustrates an exemplary rod  1  having 5 sides of which 2 sides  35 ,  36  are plane-parallel and 2 tapering flanks  42 ,  43  are bevelled. The bevel angle  37  of the tapered flanks  42 ,  43  is at least 10° with respect to the frontal plane  45  of the rod on either side. Hence, the frontal plane  45  is a virtual plane orthogonal to the parallel sides  35 ,  36 . 
       FIG. 3   b  illustrates a corresponding pedicle screw  2  with the opening contour  3  that is congruent with the rod  1  of  FIG. 3 , the screw head  51 , the screw thread  59 , the clamping device  4  including e.g. a spacer  5  and a clamping element  6 . 
       FIG. 3   c  illustrates the rod  1 , the screw  2 , the spacer  5 , and an external nut  7  as the clamping element  6 . The nut  7  is screwed on a thread  59  with the nut  7  exteriorly surrounding the head  51  in the assembled situation in a sectional view. 
       FIG. 4   a  illustrates an opening  3  in the screw head  51  with a central ridge  8 . 
       FIG. 4   b  illustrates the corresponding spacer  5  with side notches complementary to the ridge  8 . The spacer  5  is provided with a ridge  9  which completes the ridge  8  to a circumferential ridge once the spacer  5  is inserted in the opening  3 . 
       FIG. 5   a  illustrates an opening  3  in the screw head  51  with an upper ridge  10   a  and a lower ridge  10   b.    
       FIG. 5   b  illustrates the corresponding spacer  5  with upper side notches and lower side notches. The notches are complementarily shaped to the ridges  10 . As noted above, this spacer  5  is provided with an upper ridge  11   a  and a lower ridge  11   b  complementing the ridges  10   a/b  to a complete circumferential ridge. 
     In these variants, the circumferential ridge or ridges constitute the area of contact, the contour of which is substantially identical to the contour of the rod. 
       FIG. 6  illustrates a variant using an internal nut  12  with a separate spacer  5  as the clamping device  4 . Accordingly, the thread  63  for holding the clamping device  4  is machined inside the opening  3 . 
       FIG. 7  illustrates a variant using an external nut  13  with an integrated spacer  5  as the clamping device  4 . 
       FIG. 8  illustrates a variant using an internal nut  14  with an integrated spacer  5  as the clamping device  4 . 
     On the basis of the foregoing description, the one skilled in the art may conceive numerous variations of the invention without leaving the scope of protection which is defined by the claims. Such modifications may be:
         The bevel angle of the beveled flanks is different.   The part between the beveled flanks may be rounded (cf.  FIG. 3   a ), essentially constituted by an edge or more generally by a polygonal line, with or without part or all corners rounded.   The bevel angle is chosen in a range from 10° to 80°, and is preferably about 60°.