Abstract:
A polyaxial pedicle screw assembly incorporates a concave portion on a receiver which mates with a convex surface on a head of the screw to form a ball joint. The radius of at least a portion of the concave surface is less than a radius of the mating convex portion whereby to create an interference fit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/043,550 filed Jan. 11, 2002 now U.S. Pat. No. 6,869,433, which is hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to a polyaxial pedicle screw. 
   BACKGROUND 
   Polyaxial pedicle screws such as disclosed in Biedermann et al.&#39;s U.S. Pat. No. 5,443,467, incorporated herein by reference, are used for connecting vertebrae to rods in spinal surgery. They incorporate a ball joint at the connection to the rod to allow the surgeon some flexibility in placing the screws. Tightening a nut on the screw compresses the ball joint components to lock the angular position of the ball joint. 
   SUMMARY OF THE INVENTION 
   The present invention, improves the locking force achieved when locking the ball joint. 
   A pedicle screw assembly according to the present invention comprises a screw having a head with a convex portion and a receiver receiving the head. The receiver also receives an elongate member, such as a spinal fixation rod. The receiver has a concave portion which has a radius of curvature which is less than a radius of curvature of the convex portion of the head whereby to create an interference fit between the convex portion of the head and the concave portion of the receiver. 
   Preferably, a nut on the receiver compresses the convex portion of the head into the concave portion of the receiver. In one convenient orientation, the receiver comprises a U-shaped portion for receiving the elongated member. Preferably, the concave portion of the receiver is formed of titanium. Although other shapes may be employed, in one preferred orientation each of the concave portion and convex portion have a spherical shape. Any shapes which allow rotational freedom of the head and receiver prior to engagement of the surfaces would suffice. 
   In one preferred embodiment the screw comprises an elongated shank having bone threads thereon and the head located at one end thereof and the receiver comprises a body having an aperture therethrough for receiving the shank and having the concave portion located at the aperture. The receiver further comprises a channel therethrough opposite the aperture, the channel receiving the elongate member. 
   The pedicle screw can further comprises a compression member between the elongate member and the head; the head having a second convex portion facing the compression member and the compression member having a second concave portion facing the head, the second concave portion having a radius of curvature less than a radius of curvature of the second convex portion whereby to create an interference fit between the head and the pressure member. 
   The difference in the radius of curvature between the convex and concave portions in one embodiment is about 0.05 mm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cutaway view of a pedicle screw according to the present invention; 
       FIG. 2  is an additional cutaway view of the screw of  FIG. 1  with rod and locking nuts removed for clarity; and 
       FIG. 3  is a detailed cutaway view of a portion of the receiver of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIGS. 1 and 2  illustrates a polyaxial screw assembly  10  according to the present invention. It comprises a screw  12  having cancellous threads  14  for insertion into the cancellous bone of a vertebra, especially through the pedicle. A spherically shaped head  16  has a convex surface  18  and a tool recess  20  for receiving a hex driver or other tool (not shown). The head  16  is received within a tubular receiver  22  having an internal concave surface  24  and an adjacent opening  26 . The convex surface  18  of the head  16  mates with the concave surface  24 . The opening  26  is smaller than the head  16  so that the screw  12  can project out of the opening  26  without falling out of the receiver  22 . 
   A pressure disk  28  sits atop the head  26  and has a surface  30  of mating shape to that of the head  26 . The receiver also has a U-shaped portion  32  which receives an elongated rod  34 . The rod  34  is used to connect adjoining vertebrae as is known in the art. An internal nut  36  and external nut  38  compress the rod  34  against the pressure disk  28  which in turn compresses the head convex portion  18  into the receiver concave portion  24  and locks the angular position of the receiver  22  with respect to the screw  12 . 
   The pressure disk  28  preferably has a lateral indentation  40  into which a material on the receiver  22  is swaged  42  to hold the pressure disk  28  within the receiver  22  but allow some movement therein. 
     FIG. 3  illustrates the feature which improves the locking of the receiver  22  with respect to the screw  12  over prior similar screw assemblies. The concave surface  26  has a slightly smaller radius of curvature than does the convex surface  18  so that when the two are compressed together, the material deforms somewhat to allow the surfaces to mate in an interference fit and thus enhances the grip between the surfaces. 
   Tests of the deflection of the screw  12  under a torque load versus a prior screw show a significant decrease in deflection versus the prior screw, thus less slippage and better locking. Tables 1 and show the results of tests of screws with and without the interference fit. The seven screws in Table 1 were formed of stainless steel and the seven screws in Table 2 of titanium. The screws labeled Magnum contain the interference fit and the others did not. The screws are of similar dimensions; the numbers listed after the screw refer to the rod size. The tests consist of locking the screws to a uniform torque and then applying a lateral force to the screw  12  to induce a torque at the head  16 . The load at an offset of 0.5 mm and the stiffness were assessed for each sample. The screws with the interference fit of the present invention exhibited gains in both parameters. 
   
     
       
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Static Cantilever Beam Evaluation 
             
             
               Moss-Miami (no interference fit) vs. 
             
             
               Moss-Miami Magnum (interference fit) 
             
             
               Stainless Steel 
             
           
        
         
             
                 
                 
                 
                 
                 
                 
                 
                 
                 
               Std 
             
             
               Specimen 
               1 
               2 
               3 
               4 
               5 
               6 
               7 
               Avg 
               Dev 
             
             
                 
             
           
        
         
             
                 
               Load at Offset 0.5 mm (Kn) 
             
           
        
         
             
               Moss-Miami 6.0 
               0.2107 
               0.2188 
               0.2121 
               0.2926 
               0.2483 
               0.2349 
               0.3571 
               0.25 
               0.05 
             
             
               Moss-Miami Magnum 
               0.2470 
               0.3101 
               0.3678 
               0.2752 
               0.2926 
               0.3074 
               0.2618 
               0.29 
               0.04 
             
             
               6.34 
             
           
        
         
             
                 
               Stiffness N/mm 
             
           
        
         
             
               Moss-Miami 6.0 
               602.2 
               459.8 
               229.9 
               594.3 
               245.8 
               538.8 
               570.8 
               463.1 
               160.99 
             
             
               Moss-Miami Magnum 
               637.0 
               705.2 
               627.8 
               611.4 
               753.2 
               721.1 
               689.8 
               677.9 
               53.27 
             
             
               6.34 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
           
             
             
             
             
             
             
             
             
             
             
           
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Static Cantilever Beam Evaluation 
             
             
               Moss-Miami (no interference fit) vs. 
             
             
               Moss-Miami Magnum (interference fit) 
             
             
               Titanium 
             
           
        
         
             
                 
                 
                 
                 
                 
                 
                 
                 
                 
               Std 
             
             
               Specimen 
               1 
               2 
               3 
               4 
               5 
               6 
               7 
               Avg 
               Dev 
             
             
                 
             
           
        
         
             
                 
               Load at Offset 0.5 mm (Kn) 
             
           
        
         
             
               Moss-Miami 6.0 
               0.2859 
               0.3047 
               0.2389 
               0.3074 
               0.2959 
               0.2403 
               0.3302 
               0.28 
               0.04 
             
             
               Moss-Miami Magnum 
               0.3730 
               0.4495 
               0.4502 
               0.4929 
               0.5348 
               0.5342 
               0.5114 
               0.48 
               0.06 
             
             
               6.34 
             
           
        
         
             
                 
               Stiffness N/mm 
             
           
        
         
             
               Moss-Miami 6.0 
               451.9 
               404.9 
               293.3 
               467.7 
               404.1 
               316.9 
               396.4 
               390.7 
               64.58 
             
             
               Moss-Miami Magnum 
               707.4 
               572.1 
               573.9 
               526.6 
               580.0 
               578.7 
               517.1 
               584.5 
               58.38 
             
             
               6.34 
             
             
                 
             
           
        
       
     
   
   In one preferred embodiment, the head convex surface  18  would have a diameter of 6.995 mm and the mating receiver concave surface  24  would have a diameter of 6.88 mm. Similar interference dimensions could also be applied to the mating interface of the pressure disk surface  30  and the head  16 . 
   While the invention has been described with regard to a particular embodiment thereof, those skilled in the art will understand, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art, particularly in light of the foregoing teachings. Reasonable variation and modification are possible within the foregoing disclosure of the invention without the departing from the spirit of the invention.