Abstract:
The invention concerns a bone screw comprising a screw shaft and a forked head into which a correction rod can be fixed. The forked head bears a nut which is connected to the forked head by a bayonet joint.

Description:
This application claims Paris Convention priority of DE 101 08 065 filed Feb. 17, 2001 the complete disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a bone screw, in particular a pedicle screw, comprising a forked head with a groove for receiving a correction rod, a threaded screw shaft and a nut for the free ends of the forked head. 
     EP 0 699 055 B1 discloses an osteosynthesis device comprising a bone screw including a forked head into which a correction rod can be inserted. The correction rod is held by a head nut which is screwed onto a thread provided on the legs of the forked head. There are situations when screwing of the head nut can be difficult or e.g. present problems to a surgeon who cannot directly see the screw. 
     It is therefore the underlying purpose of the invention to further develop a bone screw of the above-mentioned type such that the nut and the forked head can be easily connected even in difficult situations. 
     SUMMARY OF THE INVENTION 
     This object is achieved with a bone screw of the above-mentioned type in accordance with the invention in that the free ends of the forked head and the nut form a bayonet joint. 
     In accordance with the inventive bone screw, the nut is disposed onto the free ends of the two legs of the forked head like a bayonet joint and slightly turned. This bayonet joint has the substantial advantage that the nut must no longer be mounted onto a thread but merely placed on the end of the forked head. This is easy to do even in difficult situations with insufficient sighting. 
     In a preferred further development, the free ends comprise radially outwardly extending projections. These projections extend through the peripheral length of the free ends of the forked head. This permits transmission of relatively large forces and the surface pressure is relatively small. 
     In accordance with the invention, the inner circumference of the nut comprises two radially inwardly projecting noses which each extend through 200 to 1200, preferably 850 of the inner periphery. The projections of the free ends of the forked head engage behind these noses to thereby form the bayonet joint. 
     In a further development, at least one free end of the forked head is provided with an axial extension. This axial extension defines the maximum turning of the nut to close the bayonet joint. Towards this end, the inner surface of the nut has a receiving groove for the extension extending through a portion of the inner circumference. When the nut is disposed onto the ends of the forked head, the extension engages into the receiving groove and the nut can be turned until the extension has reached the end of the receiving groove. 
     In another embodiment, the inner side of the nut is provided with a stop for the extension. In this embodiment, the stop engages the path of the extension such that the nut can be turned until the extension abuts the stop. 
     In a further development, the separation between the groove bottom of the forked head and the side of the nut facing the groove is larger than the inside diameter of the forked legs of the forked head, when the nut is disposed onto the forked head. This means that, when the correction rod has been inserted into the forked head and the nut is placed onto the free ends of the forked head, the nut is not supported on the correction rod. It can therefore be easily turned to close the bayonet joint. 
     In a further development, the nut is provided with a central threaded bore for a screw, in particular, a set screw. The correction rod is fixed in the forked head by this set screw which is screwed into the nut after closing the bayonet joint to press onto the correction rod. The screwed-in end of the set screw preferably comprises a tip or cupped gripping point which penetrates the surface of the correction rod. The set screw maintains the correction rod at the bottom of the forked head and secures the nut on the forked head to keep the bayonet joint in the closed position. 
     The screwing direction of the set screw preferably corresponds to the turning direction of the bayonet joint such that the nut must not be held when screwing in the set screw. Release of the nut is thereby relatively easy since the screwing direction of the set screw corresponds to the turning direction for opening the bayonet joint. 
     The outer circumference of the nut is provided with a tool engagement surface for turning the nut, in particular a hexagon, which permits easy movement of the nut in both turning directions to open and close the bayonet joint. 
     Further advantages, features and specifics of the invention can be extracted from the following detailed description of a particularly preferred embodiment with reference to the drawing. The features in the drawing, in the claims, and the description may be essential to the invention either individually or collectively in any arbitrary combination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a perspective view of the bone screw including inserted correction rod; 
     FIG. 2 shows a perspective view of the screw with removed nut; and 
     FIG. 3 shows a perspective view of the nut. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a particularly preferred embodiment in accordance with the invention, wherein the reference numeral  10  designates a bone screw. This bone screw  10  is connected to a correction rod  12 . The bone screw  10  comprises a shaft  14  having a thread for screwing the bone screw  10  into a bone, a vertebrae or the like. The shaft  14  is conical and merges at its upper end into a forked head  16  in which the correction rod  12  is disposed. The forked head  16  is provided with a nut  18  which partially surrounds and closes the forked head  16  and secures the two legs  20  and  22  of the forked head  16  from being forced apart (FIG.  2 ). The upper side of the nut  18  comprises a central threaded bore  24  into which a set screw  26  can be screwed, thereby securely fixing the correction rod  12  in the forked head  16 . 
     FIG. 2 clearly shows the groove  28  of the forked head  16  and several notches  32  extending in the longitudinal direction of the groove  28  are shown at the bottom  30  of the groove  28 . These notches  32  engage in longitudinal grooves  34  in the correction rod  12  and thereby fix the correction rod  12  to prevent rotation thereof. The free ends  36  of the two legs  20  and  22  comprise radially outwardly extending projections  38  which form part of a bayonet joint. Two extensions  40  are provided at diametrally opposed sides of the projections  38  which protrude axially. 
     One groove  42  is disposed below each of the projections  38  which extend, like the projections  38 , through the entire peripheral width of the legs  20  and  22 . 
     The outer circumference of the nut  18  shown in FIG. 3 comprises a tool engagement surface  44  which is formed as a hexagon. The inner side  46  of the nut  18  comprises two diametrally opposed noses  48  which project radially inwardly. These noses  48  extend over approximately 85° of the inner circumference. When the nut  18  is disposed on the forked head  16  (see FIG.  1 ), these noses  48  engage into associated grooves  42  below the projections  38  thereby forming a bayonet joint. A rotational limitation of the nut  18  is achieved via the extensions  40  which, when the nut  18  is disposed on the forked head  16 , engage into a receiving groove  50  which is also provided on the inner side  46  of the nut  18 . When the nut  18  is axially disposed onto the forked head  16 , the extensions  40  engage into this receiving groove  50  to permit rotation of the nut  18  in one rotational direction only until the extensions  40  abut the groove wall  52 . This ensures that the nut  18  and the bayonet joint can be closed merely in one rotational direction, namely in the mathematically “positive” direction. When the set screw  26  is screwed into the threaded bore  24  of the nut  18 , the nut is held in this closed position and does not require any external support.