Abstract:
An intervertebral cage of the present invention includes main body defined by an upper surface, a lower surface, and a pair of side surfaces. A withdrawal prevention portion is provided on the upper and/or the lower surfaces of the main body and asymmetrically with respect to the side surfaces in a top or bottom plan view. The withdrawal prevention portion regulates an insertion direction of the intervertebral cage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2002-239095 filed on Aug. 20, 2002; the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an intervertebral cage that can be inserted between upper and lower vertebrae in longitudinal and slating directions after an intervertebral disk is removed. 
     2. Description of the Related Art 
     There have been known intervertebral cages inserted between the vertebrae the intervertebral disk of which is removed. 
       FIGS. 1 and 2  show an intervertebral cage of a related art of this invention (Japanese Unexamined Patent Publication No. 9-503416). In the related art, an intervertebral cage  50  is comprised of a pair of left and right semicircular lateral spacers  51 A,  51 B; front and rear central spacers  53 A,  53 B are integrally fixed to each other by left and right fixing screws  55 . This intervertebral cage  50  is inserted between upper and lower vertebrae  59 U,  59 L after an intervertebral disk is removed. The central spacers  53 A,  53 B and lateral spacers  51 A,  51 B define a cavity  70 . 
     The related art has a problem that 1) since the intervertebral cage  50  is comprised of a large number of components and has a complex structure, and 2) does not have protrusions for preventing itself from coming off, it can not sufficiently be fixed between the vertebrae after it is inserted between the vertebrae. 
     Further, in the related art, it is premised that the intervertebral cage  50  is inserted between the upper and lower vertebrae from an anterior side but is not inserted from longitudinal and slanting anterior sides. Thus, an improved intervertebral cage has been desired. 
     SUMMARY OF THE INVENTION 
     This invention has been made to solve the above-mentioned problems. According to an aspect of the invention, there is provided an intervertebral cage inserted between vertebrae of a spine comprising: a main body defined by a pair of upper and lower surfaces and a pair of side surfaces connected thereto; and withdrawal prevention means formed on the upper and/or lower surfaces of the main body and asymmetrically in a sectional side view, wherein the withdrawal prevention means (e.g. withdrawal prevention portions) regulates an insertion direction of the intervertebral cage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a conventional intervertebral cage. 
         FIG. 2  is a side view of a conventional intervertebral cage inserted between the intervertbrae. 
         FIG. 3  is a perspective view of an intervertebral cage of the present invention. 
         FIG. 4A  is a plan (top plan) view of the intervertebral cage of the present invention. 
         FIG. 4B  is a plan (bottom) view of the intervertebral cage of the present invention. 
         FIG. 5  is a sectional side view of the intervertebral cage of the present invention. 
         FIG. 6  is a rear view of the intervertebral cage of the present invention. 
         FIG. 7  is a partially enlarged view of  FIG. 3 . 
         FIGS. 8A and 8B  show an insertion direction of the intervertebral cage to vertebrae. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment according to the present invention will be described with reference to  FIGS. 3 to 8B . 
     An intervertebral cage  41  includes a hollow main body  43  and withdrawal prevention means integrally formed on the upper and lower surfaces of the main body  43 . 
     As shown in  FIGS. 4A and 4B , the main body  43  is roughly formed in a deformed hexagon, and each corner is formed in a circular arc. Further, as shown in  FIG. 5 , the thickness H 2  of the rear end side of the main body  43  is larger than the thickness H 1  of the front end side (tip end side) of the main body  43 . 
     A front hole  45 , a center hole  47  and a rear hole  49  each having a length L 1  are respectively formed in the front side ( 43 F side), the central portion and the rear side portion of the main body  43  from an upper surface  43 E of the main body  43  to a lower surface  43 D in the longitudinal direction with respect to the direction in which the intervertebral cage  41  is inserted. Further, a hole  410  (see  FIG. 5 ) that is formed in the cross direction and made nearly equal in length in the longitudinal direction to the respective holes  45 ,  47  and  49  (that is, the length L 1  shown in  FIG. 4A  is nearly equal to the length L 2  shown in  FIG. 5 ) is formed in the main body  43  from a front surface  43 F to the rear hole  49 . 
     Since the front hole  45 , the center hole  47 , the rear hole  49  and the hole  410  are formed in the main body  43 , the main body  43  is formed in a hollow body. Transverse through holes  45 A,  47 A and  49 A are respectively formed at positions corresponding to the front hole  45 , the center hole  47  and the rear hole  49  in the both side surfaces  43 A and  43 B of the main body  43  (see  FIG. 3 ). 
     Further, a groove  430  formed in the V-shaped in a sectional side view is made in the rear end surface of the main body  43 . Upper and lower screw through holes  450 U and  450 L made through the upper surface  43 E and lower surface  43 D are formed in the groove  430  in the slanting and vertical direction. Each of these screw through holes  450 U and  450 L is formed in a hole elongated in the longitudinal direction. 
     The withdrawal prevention means according to the present invention will be described with reference to  FIG. 7 . 
     In a corner  43 C, a plurality of notches (claw portions  490 ) are formed in parallel to a line  470  perpendicular to a bisector of a vertical angle formed by the side surface  43 B and the front surface  43 F. Each of the plurality of claw portions  490  is nearly formed in a wedge shape, and one surface  490 A of the claw portions  490  is set at a length shorter than that of the other surface  490 B connected thereto. The intervals between the respective claw portions  490  can be set at appropriate values. Further, a plurality of claw portions  490  are formed in the same way also on the surface (lower surface  43 D shown in  FIG. 4B ) opposite to the surface (upper surface  43 E) shown in  FIG. 7  of the main body  43 . At this time, in the end portion on the side opposite to the corner  43 C of the upper surface  43 E, claw portions  490 A are formed in parallel to a line  470 A perpendicular to a bisector of a vertical angle formed by a side surface  43 A and the front surface  43 F. As a result, the claw portions  490  and  490 A formed on the upper surface  43 E and the lower surface  43 D are provided symmetrically with respect to a horizontal cut plane of the upper surface  43 E and the lower surface  43 D. 
     Since the claw portions  490  and  490 A are formed at a predetermined slanting angle with respect to the side surfaces  43 A and  43 B of the intervertebral cage  41 , the insertion direction of the intervertebral cage  41  is regulated. That is, the insertion direction is regulated in a direction B vertical to the cutting lines  470  and  470 A ( FIGS. 4A ,  4 B). Further, the intervertebral cage  41  can be inserted either in a left direction or in a right direction with respect to the vertebrae, depending on which surface of the upper and lower surfaces is faced upward. 
     When the intervertebral cage  41  is inserted between the upper and lower vertebrae after the intervertebral disk is removed, the main body  43  is held by engaging a tool such as a pair of forceps with the transverse holes  45 A,  47 A and  49 A and is inserted between the vertebrae BV from the left and front side of a spine V such that, as shown in  FIG. 8A , the one corner  43 C of the main body  43  goes ahead. By turning the intervertebral cage  41  upside down, as shown in  FIG. 8B , the main body  43  can be inserted between the vertebrae BV from the right and front side. 
     Thus, even in a case where, for example, an organ is positioned in front of the spine, the main body  43  can be inserted between the vertebrae of the spine while avoiding the organ. At this time, in the main body  43 , the rear end side is formed more thinly than the front end side, so that the main body  43  can be easily inserted between the vertebrae. 
     Further, after the main body  43  is inserted between the vertebrae, the cutting lines  470  and  470 A of the plurality of claw portions  490  and  490 A for preventing withdrawal, formed in the upper and lower surfaces  43 D and  43 E, bite into the end plates of the upper and lower vertebrae to thereby prevent the main body  43  from coming off between the vertebrae. Still further, by screwing implant screws S from the V-shaped groove  430  formed on the rear end surface of the main body  43  through the screw through holes  450 U and  450 L into the upper and lower vertebrae sandwiching the intervertebral cage  41 , the main body  43  can be fixed between the upper and lower vertebrae with reliability. At this time, since the screw through holes  450 U and  450 L are elongated in lateral direction, the position into which the implant screws are screwed can be shifted in the longitudinal direction in response to the state of the vertebrae. 
     As described above, after the main body  43  is fixed between the upper and lower vertebrae, bone grows and gets into the front vertical hole  45 , the center vertical hole  47  and the rear vertical hole  49 , which are formed in the upper and lower surfaces of the main body  43  to thereby promote bone fusion. Then, it is possible to judge the bone fusion by passing X-rays through the transverse through holes  45 A,  47 A and  49 A formed in correspondence to the respective vertical holes  45 ,  47  and  49  and taking an X-ray picture.