Abstract:
A receiving part for a retaining component of a vertebral column implant, having a fork-shaped section for receiving the retaining component, with the inner wall of the fork-shaped section including inner threaded portions into which there may be screwed a screw which, either directly or via a pressing element, acts on the retaining component in the fork-shaped section. The inner threaded portions and the outer thread of the screw comprises a saw tooth thread of a shape that the steep flanks of the inner threaded portions face away from the opening of the fork-shaped section.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a receiving part for a retaining component of a vertebral column implant. 
     2. Description of Prior Art 
     With vertebral column implants there exists often the object of supporting certain vertebra of the vertebral column, of creating a tension between the vertebra or a compression in order to retain or change a relative position. For this purpose, suitable means are connected to the vertebrae which are subsequently described as retaining components and may be connected to one another by connecting means. As retaining components, there may be e.g. so-called pedicle screws which are screwed into the pedicle of the vertebrae. The head of the pedicle screw is either annular or fork-shaped. With annular pedicle screw heads e.g. a distraction rod is guided through and is fixed on both sides of the head with the help of a nut. With fork-shaped heads, it is known in the inside to provide threaded sections for a set screw with which a previously inserted distraction rod or likewise may be fastened. 
     A similar receiving part for a distraction rod or likewise is also known for hook-like retaining components, for example with so-called lamina hooks or pedicle hooks which are hooked into the corresponding vertebrae parts. 
     With fork-shaped receiving parts, there is the danger that on exerting a suitable pressing force on the distraction rod or likewise the arms of the fork-shaped part are bent apart by which means the fixation is compromised or even eliminated. It is known, for example, to counteract this in that a lifting eye bolt or nut is screwed on the outside around the receiving part in order to prevent the arms from bending apart. A so-called cap bolt has a similar effect, this bolt overlaps the receiving part at the open end and includes a centrally located peg-like section projecting into the recess of the fork-like part for fixation e.g. of a distraction rod. 
     From U.S. Pat. No. 4,946,458, it is also known to equip the end, on the head side, of a shank of a pedicle screw with a spherical bearing surface which is fastened in a corresponding bearing section of the head. By way of this, the shank of the pedicle screw may assume any angle with respect to the screw head. It has, however, been shown that it causes problems in securely fixing the relative position of the pedicle screw on the one hand and the receiving part or the head on the other hand. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a receiving part for a retaining component of a vertebral column implant with which the retaining component may be securely fixed without the fork-shaped section being deformed. 
     With the receiving part according to the invention which may be the slotted head of a pedicle screw or a section of a lamina or pedicle hook, inner threaded sections are formed as a saw tooth thread. The steep flank of the saw tooth thread of the receiving part faces away form the opening of the fork-shaped section. It is to be understood that the thread of the set screw is complementary, i.e., the steep shank is aligned oppositely. Such a design of the thread causes essentially only a tension force on the arm sections of the fork-shaped section when the screw is pressed against the retaining component to be fastened. A bending apart of the tongs or arms does not occur. 
     By saw tooth thread, it is meant the tooth shape of FIG. 1 in which the flanks facing the opening at the top of FIG. 1 are angled at about 30° with respect to the horizontal and the flanks facing the bottom of FIG. 1 are steep and, as shown, generally horizontal. 
     The thread within the receiving part may be spaced from the free end. In this way, it is achieved that the set screw cannot be screwed in skewed. Also, the set screw at one end may have a smooth section in order to simplify the introduction of the screw particularly into the smooth initial section in the receiving part. 
     An advantage of the receiving part according to the invention also lies in the fact that it may be dimensioned smaller, since the bending forces on the fork-shaped head on account of the fastening of the retaining component with the help of a set screw largely do not occur. 
     As has been previously mentioned, the invention may be applied everywhere where, in a fork-shaped section, retaining components of vertebral column implants are to be fastened. The retaining component may according to one embodiment of the invention be a rod-like element, preferably a distraction rod. The retaining component may, however, also be formed by the shank of the pedicle screw, whose end near to the head is accommodated in a fork-shaped section. So that such a pedicle screw permits an angular position between the screw shank and the head. One embodiment of the invention provides that the end of the shank near to the head, which is broadened in diameter, comprises a spherical bearing surface which cooperates with a spherical bearing surface in the fork-shaped head. The end surface of the shank is convex, and between the screw and the inner surface there is arranged a disk which on the side facing the convex end surface may be plane or concave. Preferably, the curvatures of the end surface of the screw shank and of the facing side of the disk are formed with an approximately equal radius. On the opposite side, the disk according to a further embodiment of the invention may be planar. A set screw which cooperates with the fork-shaped head via a saw tooth thread then presses onto the disk which for its part fastens the shank of he pedicle screw at any angular position. It has been shown that such a design in the head region of the pedicle screw has the result of higher retaining forces so that the once assumed relative position of the shank to the head remains fixed even with larger forces. It is to be understood that between the set screw and the disk a distraction rod or likewise may be inserted through which the pressing force is transmitted to the disk, wherein the pedicle screw shank is fastened in the head of the pedicle screw assembly. 
     The disk may comprise an outer thread and be screwed into the head of the pedicle screw, wherein of course, the inner sides of the recess must be provided with a threaded section. Below this threaded section, there may be present a non-threaded section allowing free rotation which is reached when the disk is completely screwed in, bearing on the crowned surface of the end of the screw shank on the head side. 
     The disk may be provided on the periphery with radial slots so that a better bearing on the spherical end of the pedicle screw is made possible. The disk may further be provided centrally with an opening. Via the opening, a tool may engage with the facing end of the head of the shank of the pedicle screw and turn the shank. Moreover, the opening may be so dimensioned together with the concave surface of the disk, that a rod accommodated by the receiving part bears against the end, near to the head, of the pedicle screw shank when it is pressed with the set screw in the receiving part. 
     With the conventional pedicle screws, it is necessary that the fork-shaped head is pushed over the shank before the shank is rotated in. The handling of such a unit is, however, complicated. For this reason, one embodiment of the invention provides for the fork-shaped head to laterally comprise a slot, which is dimensioned such that the broadened end of the pedicle screw can be laterally introduced into the receiving space of the head. In this manner, the shank alone may be screwed into a vertebrae. The fork-shaped head may subsequently laterally be put onto the end of the pedicle screw shank which is near the head. Although the slot weakens the corresponding arm region of the head, but since the saw tooth thread is used, as has been cited several times, this prevents a bending apart of the head section, and the weakening is tolerable. 
     Another embodiment of the invention provides that on the outer side of the arms of the fork-shaped head diametrical recesses are formed. The recesses serve for accommodating a fork-shaped section which is part of a rotating or tensioning tool. With the help of the tool which can be applied in the recesses, the pedicle screw may be rotated. The tool may also laterally grasp the head so that a simultaneous access from above is still possible. It is to be understood that the recesses may be advantageous also with pedicle screws with a one-piece, formed-on head. 
     In order to securely fix a pedicle screw relative to the receiving part at any angle, one embodiment of the invention provides that the receiving part or head accommodates an annular cage. The cage comprises several slots which are formed by the lower side parallel to the axis. Furthermore, the inside the cage has a spherical concave bearing surface which cooperates with a convex end of the shank of the pedicle screw. The outer side of the cage has a conical surface which cooperates with a complementary conical surface in the receiving part. If a force is exerted on the cage in the direction of the pedicle screw, the cage is pressed radially together and by way of this exerts a pressing force onto the spherical surface of the screw shank. This pressure is sufficient to obtain a secure fastening of the screw shank relative to the receiving part. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in more detail by way of the drawings. 
     FIG. 1 shows a receiving part according to the invention, in section; 
     FIG. 2 shows a part of the receiving part according FIG. 1 together with a screw; 
     FIG. 3 perspectively shows a pedicle hook with a receiving part according to the invention; 
     FIG. 4 show perspectively a lamina hook with a receiving part according to the invention; 
     FIG. 5 shows the arrangement of two hooks for forming a bridge with a receiving part according to the invention; 
     FIG. 6 shows a two-part pedicle screw with a receiving part according to the invention, in section; 
     FIG. 7 shows a receiving part similar to that according to claim 6, but with additional features; 
     FIG. 8 schematically shows a possible modification of the receiving part according to FIG. 6; 
     FIG. 9 show s schematically a disk modified with respect to FIG. 6; 
     FIG. 10 shows a pedicle screw with a modified receiving part; 
     FIG. 11 shows a section through the representation according to FIG. 10 taken along the line 11--11; 
     FIG. 12 shows in a larger scale the receiving part of the embodiment form according to FIGS. 10 and 11; 
     FIG. 13 shows perspectively the cage of the embodiment form according to FIGS. 10 and 11; and 
     FIG. 14 shows a section through the cage according to FIG. 13. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, there is represented a receiving part 10 which is fork-shaped with arm sections 12, 14. The receiving part 10 may be the head of a pedicle screw or a section of a hook for a vertebral column implant. As can be recognized on the inner side of the arm sections 12, 14, there are formed threaded sections 16, 18 with a saw tooth shape. It is essential that the steep flank 20 of the saw tooth thread faces away from the open side of the receiving part 10. From the open side toward the corresponding recess of the receiving part 10, a distraction rod may be placed in as is shown dashed at 22. 
     In FIG. 2, only the arm section 12 of the receiving part 10 is to be recognized. Further, a set screw 24 is to be seen which for its part comprises a saw tooth thread 26 which cooperates with the threaded sections 16, 18. The distraction rod 2 may be fixed in the receiving part 10 with the help of the set screw 24. Due to the shape of the thread on the arm sections 12, 14 with an axial force on the sect screw 24 essentially a tensional loading is exerted so that by way of this it is prevented that the arm sections 12, 14 are bent apart. 
     FIG. 3 shows a pedicle hook 28 with a fork-shaped receiving part 10a which comprises threaded sections 16a, 18a in the shape of a saw tooth thread. A set screw 24a with a hexagon socket 30 comprises a corresponding saw tooth thread 26a. 
     A lamina hook 32 comprises a receiving part 10b with inner threaded sections 16b, 18b for receiving a set screw 24b with a saw tooth thread 26b. In both cases, in the receiving parts of the hooks 28 or 32, a distraction rod may be fixed. 
     In FIG. 5, there are shown two hooks 34, 36 which receive rod-like elements 38, 40. In the receiving parts of the hooks 34, 36, there is accommodated a distraction rod 42 which is channeled or provided with a thread and which bears against the rods 38, 40. With the help of set screws 44 or 46, the device shown, which forms a type of bridge, may be fixed. The set screws 44, 46 or the receiving parts of the hooks 34, 36 are in turn provided with a saw tooth thread of the above-mentioned type and manner. 
     In FIG. 6, there is represented a pedicle screw 50 with a shank 52 and a head 54. The shank 52 comprises a normal thread 54. The shank 52, is, at the end near to the head, formed with a broadened section 56 which on the lower side comprises an annular spherical bearing surface 58. Furthermore, the end surface of the broadened section is shaped spherically convex, as is shown at 60. The broadening has a hexagon socket 62 for receiving a corresponding rotational tool. 
     The head 54 is fork-shaped similar to the receiving part 10 and at the end facing the shank 52 is provided with an opening 64 through which the shank 52 extends. A part of the wall of the opening 64 comprises an annular spherical bearing surface 66 which cooperates with the spherical bearing surface 58. The shank 52 may therefore assume in a limited spacial angle, any angular position to the head 54. 
     In the receiving space of the head 54, there is arranged a disk 68 which on the upper side is planar but on the lower side comprises a concavely curved surface with a curvature which corresponds to the end surface 60 of the shank 52. The disk 68 is inserted approximately fittingly into the inner space of the end 54 so that it is radially secured as shown in FIG. 6. The disk 68 may be provided with an outer thread, and the inner thread of the head 54 may extend until shortly above the opening 64, 66. In this way, the disk 68 may be screwed in until it reaches free rotation and here by way of a corresponding contact pressure can be pressed into engagement with the crowned end of the shank 52. The outer thread of the disk 68 and the inner threaded sections in the inside of the head 54 are not provided with reference numerals. 
     Referring to FIG. 6, a set screw 72 is threaded into inner threaded sections of the head 54, wherein the threaded sections again comprise a saw tooth thread. A shank 74 of the screw 72 cooperates with the planar surface of the disk 68 and presses this against the end surface 60 of the shank 52. In this manner, the angular position between the head 54 and the shank 52 can be effectively fixed. 
     At 76 there is shown in phantom a distraction rod via which a pressure may be exerted onto the disk 68, wherein the rod for its part may be fixed in its position. It is to be understood that the shank 74 of the screw 72 must be designed correspondingly shorter. It is still to be mentioned that the shank 74 at the lower end is convexly rounded. 
     In FIG. 7, the head of a pedicle screw similar to that according to FIG. 6, is shown. It, however, has some modifications. Thus, the head is indicated as 54a. One recognizes that the head 54a laterally in the region near the shank comprises a slot 78, via which the end, near the head, of the shank 52 may be inserted. Thus, the shank 52 firstly can be screwed into a pedicle of a vertebrae after which subsequently the head 54a is placed on. 
     On the outer side of the arm sections of the head 54a, there are formed slots 80, 82 for receiving the prongs of a hook-like tool section which laterally grasps the head on the outside in order to exert a tension effect or to rotate the head. It is to be understood that the slots 80, 82 may also be mounted on a head formed as one piece with the shank. 
     The embodiment form according to FIG. 8 differs from the receiving part or the head 54 according FIGS. 6 or 7 in that in the inside near to the free end there is provided a thread-free section 86 to which connects a threaded section 84. The screw 72b comprises at one end likewise a thread-free section 88. The outer diameter of the thread-free section 88 corresponds approximately to the inner diameter of the thread-free section 86. In this way, the screw 72b may be inserted straight into the head 54b and subsequently screwed. 
     The disk 68b according to FIG. 9 differs from the disk 68 according to FIG. 6 in that proceeding from the periphery a row of radial slots 90 s formed which are spaced in the circumferential direction. The slots permit an effective bearing of the disk 68b on the spherical convex surface 60 of the screw shank 52 according to FIG. 6. Moreover, the disk 68b comprises a passage opening 92. This permits the guiding through of a tool for engagement into the recess 62 of the screw shank 52 according to FIG. 6. 
     In the FIGS. 10 to 14, there is shown a further embodiment of the invention. The pedicle screw represented in FIGS. 10 and 11 comprises a screw shank 100 which is formed roughly as the screw shank 52 according to FIG. 6. At the upper end, the shank 100 comprises a spherical section 102 which from the free end is provided with a recess 104 for a rotational tool As can be recognized in FIG. 11, the section 102 is accommodated by a cage 106 which can be more clearly recognized in FIGS. 13 and 14. The cage is seated in a receiving part 108 which in the upper region comprises oppositely lying recesses and is thus fork-shaped, as can be recognized in FIG. 10. 
     The receiving part 108 which forms the head for the pedicle screw is provided with diametrically opposite threaded sections 110 which are formed corresponding to the threaded sections 16 according to FIGS. 1 and 2. In the part lying thereunder, the receiving part 108 is formed in the manner of an annular housing 112 which on the inner side comprises a conical annular surface 114 which converges downwardly. Via a lower opening 116, the cage 106 which is represented in the FIGS. 13 and 14, may be introduced. 
     Diametrically opposite on the outer side of the head 108, recesses 188 are formed for receiving a tool, not shown, for screwing in the pedicle screw. 
     The annular cage 106 comprises on the outer side a conical annular surface 120 which cooperates with the conical surface 114 of the receiving part 108 as can be recognized in FIG. 11. The cage 106 is furthermore provided with three slots 122 parallel to the axis, wherein one of the slots is continuous so that the annular cage 106 is parted or split in the circumferential direction. On the inner side, the cage 106 comprises a spherical bearing surface 124 which accommodates the spherical section 102 of the shank 100, as is shown in FIG. 11. 
     Into the threaded sections 110, there is screwed a screw 126 (FIG. 11) which centrally comprises a continuous hole 128 for leading through a tool with which the shank 100 can be rotated. With the help of the screw 126, there is fastened a distraction rod which is now shown and which is accommodated by the fork-shaped section of the head 108. The rod is pressed against the cage 106. Bay way of this, the cage 106 is moved downwards in the direction of the screw shank 100 and is radially pressed together. In this manner, the spherical section 102 is clamped in the cage 106 and the angular position between the shank 100 and the head or the receiving part may be fixed.