Patent Publication Number: US-2003229346-A1

Title: Screw for fixing atlantoaxial joint

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to screws for fixing atlantoaxial joints used for spondylodesis, a medical treatment for spondylous patients such as atlantoaxial joint instability.  
       [0003] 2. Description of the Related Art  
       [0004] As shown in FIGS. 1A to  1 D, as an example of a method for fixing an atlantoaxial joint of an atlanto (the first cervical vertebra) and an axis (the second cervical vertebra), there is a method in which a pair of right and left screws  5  are screwed and embedded into lateral masses  4  of an atlanto  1  from inferior articular processes  3  of an axis  2 .  
       [0005] More specifically, after stomas  22  are bored in advance at the positions where the screws  5  are to be screwed and embedded into the first and the second cervical vertebrae to the axis  2  and the atlanto  1 , the screws  5  are screwed and embedded in the direction of R 1  in FIG. 1B. Ends  7  of the screws  5  used in this example have flat shapes, i.e. shapes which are cut close to perpendicular to a longitudinal direction of the screws  5 . Therefore, an attachment of the screws  5  to the atlanto  1  and the axis  2  is bad, and particularly when the screws  5  are screwed into a joint  8  where the atlanto  1  and the axis  2  are lapped over, there has been problems such as that the screws  5  are not screwed into articular surface stomas  9  of the atlanto  1 , the end faces in the traveling direction of the screws  5  abut the atlanto  1 , the atlanto  1  is pushed away from the axis  2  by component force F 2  of pressing force F 1  of the screws  5  (FIG. 1B).  
       [0006] Therefore, when the screws S are screwed and fixed, actions has been taken in such a way that, after formation of embedding parts  10  where the ends of the screws  5  are slightly embedded in the articular surface stomas  9  of the atlanto  1  is confirmed, the screws  5  are temporarily drawn back in the direction of an arrow R 2  in FIG. 1C. Thereafter, when an upper part of the atlanto  1  is returned to the original position by its own weight (FIG. 1C), the screws  5  are threaded again.  
       [0007] In the above method, the screws  5  should be pulled back and inserted, resulting in bad operating efficacy.  
       SUMMARY OF THE INVENTION  
       [0008] The invention is devised in consideration of the conventional problems as described above. According to the present invention, there is provided a screw for fixing an atlantoaxial comprising, a shaft body having a through-hole in a center of the shaft body from one end to the other end, a screw part provided on one end of the shaft body, and a cutting portion for screwing is provided on the one tip end of the screw part. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIGS. 1A to  1 D are explanatory drawings illustrating a conventional example of a method for fixing atlantoaxial joints.  
     [0010]FIGS. 2A to  2 C are explanatory drawings illustrating a usage example of screws for fixing atlantoaxial joints according to the invention.  
     [0011]FIGS. 3A to  3 C are explanatory drawings for the screws for fixing atlantoaxial joints according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0012] An embodiment of the present invention will be described below based on accompanying drawings.  
     [0013] As shown in FIGS. 2A to  2 C, a screw for fixing operation of an atlantoaxial joint  11  (hereinafter referred to as “screw 11”) used for the fusion of an atlantoaxial joint of an atlanto  1  and an axial  2  is diagonally screwed and embedded into a lateral mass  4  of the atlanto  1  from an inferior articular process  3  of the axis  2  as the conventional screws  5  (refer to FIG. 1A).  
     [0014] As shown in FIG. 3A, the screw  11  has a cutter portion  13  provided on one end side of a shaft body  12 , and a head with a large diameter  14  having a tool engaging hole  15 , which is provided on the other end side of the shaft body  12 .  
     [0015] On one end side of the shaft body  12 , a screw  16  is provided with the cutter portion  13 . In the center of the shaft body  12 , a through-hole  17  is provided from the head  14  to the cutter portion  13 .  
     [0016] As shown in FIGS. 3B and 3C, the cutter portion  13  has a pair of facing protrusions  20  provided by forming the first inclined plane  18  and the second inclined plane  19  in approximately symmetric figure at the end of the shaft body  12 . The clearance between inclined planes  18 ,  19  is getting small from the shaft body  12  to a tip portion thereof. Furthermore, on the outer peripheral surface on the end side of the protrusions  20 , taper faces  21  whose end sides are oriented inside are formed to acutely form ends  20 A of the protrusions  20 .  
     [0017] In other words, cutter blades  13 A are formed at crossing portions of the inclined planes  18 ,  19  and the taper faces  21 .  
     [0018] A method of fixing cervical vertebrae by using the screw  11  is described with reference to FIGS. 2A to  2 C as below:  
     [0019] In order to link and fix the axis  2  and the lateral mass  4  by the screw  11 , firstly, a stoma  22  is opened diagonally to the cervical vertebrae by a boring tool such as an air drill and the like from the inferior articular process  3  of the axis  2  to the lateral mass  4  of the atlanto  1 , and secondly, a guiding line  23  such as a wire (not shown), a guide pin (not shown) and the like is inserted in the through-hole  17  of the screw  11 , and then the screw  11  is screwed into the stoma  22  by using the guiding line  23  as a guide. In other words, the guiding line  23  is used as a guide by inserting it through the through-hole  17  of the screw  11 .  
     [0020] When the screw  11  is screwed into a stoma  6  which is bored in the lateral mass  4  of the atlanto  1  from the inferior articular process  3  of the axis  2 , as the screw  11  is pressured and fixed along with the rotation thereof, the cutter blades  13 A of the cutting portion  13  are screwed toward inside over the scraping inside of the stoma  22 .  
     [0021] As the ends  20 A of the both protrusions  20  can be elastically deformed so that they can close each other due to component forces which act inward on the cutter blades  13 A in cutting, screwing into the stoma  22  becomes easy.  
     [0022] According to the embodiment, at the start of the screwing, an end diameter of the screw  11  in the direction of screwing becomes small due to the component forces, so that screwing into the target is easy to perform. In the course of screwing, the screw  16  is surely embedded in the vertebrae due to reaction force which makes the diameter return to the original length against the component forces. By further continuing to screw the screw  16 , the atlantoaxial joint can be easily fixed.  
     [0023] Additionally, a trend that the screw  11  pushes away the atlanto  1  in screwing can be restrained, so that the conventional problem as mentioned above can be solved.  
     [0024] Therefore, as the atlanto  1  is never spaced from the axis  2  by the screw  11 , there is no need to take conventional actions such as that the screw  5  embedded in a embedding part  10  of the atlanto  1  is temporarily backed up, and the screw  5  is screwed again after the atlanto  1  returns to the original position, so that efficiency can be improved.  
     [0025] The entire content of a Japanese Application No. P2002-134261 with a filing date of May 9, 2002 is herein incorporated by reference.  
     [0026] Although the invention has been described above by reference to certain embodiments of the present invention, the invention is not limited to the embodiments described above and will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.