Abstract:
The invention relates to an orthopedic fixation device for securing a rod-like fixation element, with two clamping jaws which can be moved relative to one another and which, when brought together, clamp the fixation element between them. In order to permit space-saving configurations of a fixation device that can be introduced and operated through small orifices in the body, a cam body is provided which is mounted on the fixation device so as to be able to rotate next to the clamping jaws in the direction of tightening and which, when rotated, pushes one clamping jaw toward the other.

Description:
[0001]     This application is a continuation of international application number PCT/EP2002/009878 filed on Sep. 4, 2002.  
         [0002]     The present disclosure relates to the subject matter disclosed in international application No. PCT/EP2002/09878 of Sep. 4, 2002, which is incorporated herein by reference in its entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     The invention relates to an orthopedic fixation device for securing a rod-like fixation element, with two clamping jaws which can be moved relative to one another and which, when brought together, clamp the fixation element between them.  
         [0004]     To fix bones or bone parts relative to one another, for example in the region of the spinal column, it is known to use rod-like fixation elements which, at different locations, are clamped between clamping jaws, and these clamping jaws are then connected by bone screws or similar fixation means to the bones or bone fragments to be fixed (WO 97/06742; U.S. Pat. No. 5,741,255; U.S. Pat. No. 5,676,703; U.S. Pat. No. 5,024,213; U.S. Pat. No. 5,474,551). Very substantial forces arise in some cases, so that these fixation devices have to be of a very stable construction. This necessarily entails that some known devices have a complicated and awkwardly shaped structure. Particularly in procedures involving minimally invasive access, it is therefore difficult to implant these fixation devices and actuate the various locking screws and other types of clamping mechanisms. In some cases, a plurality of screws or locking nuts have to be tightened (U.S. Pat. No. 5,741,255), and, in other cases, the individual parts have to be joined together in the body because the direction of screwing and the direction of insertion of the rod-like fixation elements are different (WO 97/06742; U.S. Pat. No. 5,474,551).  
         [0005]     It is an object of the invention to construct a fixation device of the generic type in such a way that it can be easily implanted and fixed.  
       SUMMARY OF THE INVENTION  
       [0006]     In an orthopedic fixation device of the type described at the outset, this object is achieved, according to the invention, by a cam body which is mounted on the fixation device so as to be able to rotate next to the clamping jaws in the direction of tightening and which, when rotated, pushes one clamping jaw toward the other clamping jaw.  
         [0007]     With such a device, it is a simple matter to push the rod-like fixation element from above in between the two clamping jaws and then simply fix it by rotating the cam body. In this way, complicated screwing operations with a great many rotations are unnecessary, so that these procedures can also easily be performed through small openings in the body. Maneuvering of the cam body can additionally be made easier by the fact that the rotatability of the cam body is limited, for example by limit stops, in the tightening direction and/or in the release direction. On the one hand, this gives the operating surgeon an indication of whether a complete tightening or release of the cam body has taken place, and, on the other hand, this avoids overtightening, which could in some cases result in the device being damaged. This limiting action can be provided by limit stops.  
         [0008]     In a preferred embodiment, the cam body is mounted rotatably on a rotationally symmetrical mounting body on which the clamping jaws are also held. The rotation axis of the cam body can preferably extend parallel to the axis of the rotationally symmetrical mounting body, and the rotation axis of the cam body is in particular identical to the axis of the rotationally symmetrical mounting body.  
         [0009]     The rotationally symmetrical mounting body can, for example, have a circular cylinder configuration, but it is also possible for said mounting body to have, for example, a spherical configuration or a conical configuration. It is particularly advantageous if the mounting body is formed by the shaft of a bone screw or of a hook, since in this case it is then sufficient to push clamping jaws and cam body onto this shaft and rotate the cam body about the mounting body in order to assemble the fixation device and fix it. This fixing can constitute both a fixing against rotation and a fixing against axial movement.  
         [0010]     It is advantageous if a support piece, which carries the two clamping jaws, is held on the rotationally symmetrical mounting body.  
         [0011]     In a first preferred embodiment, the support piece is rigidly connected to the mounting body, and, in such a configuration, the clamping jaws can be moved relative to one another substantially only in the clamping direction.  
         [0012]     In a preferred embodiment, however, the support piece is held on the rotationally symmetrical mounting body so as to be rotatable about the axis of symmetry of said mounting body. This affords a further possibility of adjustment, that is to say the clamping jaws can not only be drawn toward one another, but can also be rotated together about the axis of the mounting body. In this configuration, when the cam body is rotated on the mounting body, this necessarily leads not only to a clamping of the rod-like fixation element between the clamping jaws, but in addition to a securing of the support piece mounted rotatably on the mounting body, because the cam body engages against the mounting body and thus clamps the support piece relative to the mounting body.  
         [0013]     The support piece can be pushed onto the rotationally symmetrical mounting body in the direction of the rotation axis, thus ensuring particularly simple assembly of the implant. For example, in the case of a mounting body formed by the shaft of a bone screw, it then suffices to push the support piece from above onto the bone screw, and then introduce, in the same push-on direction, a tool with which the cam body is adjusted, and only a very small opening in the body is needed for this.  
         [0014]     In a preferred embodiment, one of the two clamping jaws is configured as a stationary clamping jaw whose spacing from the rotation axis of the cam body is substantially fixed, whereas the other one is configured as a movable clamping jaw which can be moved by the cam body toward the stationary clamping jaw.  
         [0015]     In principle, the stationary clamping jaw can be connected rigidly to the support piece. A configuration of particular advantage, however, is one in which the clamping jaws are connected to the support piece so as to be able to pivot about a rotation axis extending parallel to the clamping direction of the clamping jaws. This affords a further possibility of adjustment, such that the direction of the rod-like fixation elements relative to the mounting body can also be adjusted. It is then advantageous if a limit stop for the stationary clamping jaw is disposed on the support piece and limits a movement of the stationary clamping jaw in the direction of the rotation axis of the clamping jaw and thus limits a movement of the stationary clamping jaw away from the rotation axis of the cam body. In such an embodiment, the clamping jaws are freely rotatable relative to the support piece about the rotation axis extending parallel to the clamping direction as long as the cam body is not tensioned. However, when the cam body is tensioned, the clamping jaws are pushed jointly against the limit stop and are thus fixed against further rotation about the rotation axis extending parallel to the clamping direction; the movable clamping jaw is then pivoted further toward the clamping jaw now fixed by the limit stop and additionally clamps the rod-like fixation element between the clamping jaws. It is thus possible, simply by tensioning the cam body, to eliminate all the degrees of freedom that the device has when the cam body is released.  
         [0016]     The rotation axis of the clamping jaws and the rotation axis of the cam body can be located in one plane, and, in a modified embodiment, provision can also be made for the rotation axis of the clamping jaws and the rotation axis of the cam body to be laterally offset from one another. By means of these different configurations, it is possible for the implant to be adapted to the local conditions.  
         [0017]     It is advantageous if both clamping jaws are formed by the side walls of a clamping body which has a U-shaped cross-section and whose side walls can be bent resiliently relative to one another. An especially advantageous configuration is obtained if, with the cam body released, the clamping jaws can be moved resiliently apart from one another so far that a rod-like fixation element can be pushed in between them. This allows the fixation element to be snapped in elastically between the two clamping jaws as long as the cam body has not yet been tensioned, and yet the resilient clamping jaws will fix the inserted fixation element in a provisional way, so that, although the connection does not immediately come loose, it can nevertheless still be adjusted.  
         [0018]     It is advantageous if the pivot connection between the clamping jaws and support piece is releasable.  
         [0019]     For example, the pivot connection can comprise two mounting members which engage behind one another in a bayonet fashion but which, in a defined angular position, do not engage behind one another and are then axially movable relative to one another. If, for example, support piece and clamping jaws are rotated through 90° relative to one another, they could be axially movable relative to one another, whereas in a parallel orientation both mounting members are secured on one another in the axial direction so that a rotation connection is then produced which cannot be released over a certain angle range, but only when the two mounting members are pivoted back into the starting position extending perpendicular to one another.  
         [0020]     The cam body can engage directly on the movable clamping jaw, but in some cases it is advantageous if, between the cam body and the clamping jaws, there is an intermediate member which, when the cam body is rotated, is moved to a greater or lesser extent against the clamping jaws. On the one hand, this permits an adjustment in dimensions, and, on the other hand, this intermediate member can be configured in such a way that, in the case of clamping jaws that are rotatable relative to the support piece, it is possible to ensure that, in different angular positions of the clamping jaws, the cam body can transmit a tensioning or loading force to the movable clamping jaw via the intermediate member.  
         [0021]     The intermediate member can be held movably on the support piece, and it is advantageous in particular if the intermediate member is held on the support piece via a resilient connection.  
         [0022]     The cam body can be formed as a ring.  
         [0023]     It is advantageous if the cam body has recesses for accommodating a rotary tool, for example indentations provided on the edges and into which the projections of a rotary tool can engage with a positive fit.  
         [0024]     As has already been mentioned, the mounting body can be part of a bone screw or of a hook; for example, the mounting body can be formed by a cylindrical shaft of a bone screw or of a hook.  
         [0025]     A particularly advantageous configuration is one in which the mounting body is connected to a bone screw via a transverse support, the longitudinal axis of the bone screw extending parallel to the longitudinal axis of the mounting body and laterally offset from this longitudinal axis. Compared to a bone screw screwed into the bone, a transverse support of this kind provides an additional possibility of varying the position of the mounting body, that is to say the transverse support can be rotated on the bone screw and point in different angular positions or can be moved also in the axial direction, so that optimal adaptation to the anatomical circumstances is possible. It is advantageous in particular if the bone screw is rotatable about its longitudinal axis in the transverse support and can be secured in a defined angular position and/or in a defined axial position. This securing can preferably be effected by clamping.  
         [0026]     According to a preferred embodiment of the invention, clamping in the transverse support can be provided for by a clamp piece which can be moved against the bone screw.  
         [0027]     A particularly simple construction is obtained if, in order to move the clamp piece in the transverse support, a cam is mounted so that it engages against the clamp piece and is rotatable about a rotation axis. When this cam is rotated, it pushes the clamp piece into the clamping position, and the bone screw and the transverse support are no longer able to rotate relative to one another, but instead are fixedly connected to one another. Here too, the rotation movement of the cam can be limited, in particular by limit stops, so that overtightening is avoided and, in addition, maneuvering is made easier for the operating surgeon.  
         [0028]     The rotation axis of the cam is preferably perpendicular to the plane of the transverse support.  
         [0029]     The rotation axes of the cam in the transverse support and of the cam body can extend parallel to one another; in particular, these rotation axes can coincide.  
         [0030]     The rotation axis of the cam and the longitudinal axis of the bone screw can be disposed parallel to one another, but it is also possible, in one particular embodiment, for the bone screw to be connected pivotably to the transverse support via a spherical connection, and for the clamp piece to be moved in the direction of a spherical engagement surface of the bone screw at the time of fixing. In this case, therefore, the bone screw can be pivoted relative to the rotation axis of the cam.  
         [0031]     A particularly simple configuration is obtained if the transverse support has an elongate hole in which the bone screw, the clamp piece and the cam are accommodated. By rotating the cam, the latter and the bone screw are pressed against the edge of the elongate hole and clamped.  
         [0032]     In a particularly preferred embodiment, the rotation axis of the cam and the longitudinal axis of the mounting body coincide.  
         [0033]     For example, the mounting body can be a sleeve which is connected fixedly to the transverse support and through which a tool can be inserted for turning the cam.  
         [0034]     It is advantageous if the sleeve has seats for a stabilizing tool.  
         [0035]     The cam can, for example, be a ring which is rotatable in the transverse support and which is disposed at the transverse support end of the mounting body. In another embodiment, the cam is disposed on a core which is mounted rotatably in the sleeve and which, at least along part of its length, extends inside the sleeve.  
         [0036]     In another embodiment, the mounting body is rotatable about its longitudinal axis in the transverse support and itself carries the cam. In such a configuration, it is in any case necessary to secure the rotary connection between transverse support and bone screw, by tensioning the cam, before rotating the cam body, disposed rotatably on the mounting body itself, into the tensioning position in order to tighten the clamping jaws.  
         [0037]     The cam can be secured in the axial direction in the transverse support.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     The invention is explained in more detail in the following description of preferred embodiments and with reference to the drawings in which:  
         [0039]      FIG. 1  shows a perspective view of an orthopedic fixation device with a rod-like fixation element and a clamping device for securing this rod-like fixation element on the shaft of a bone screw;  
         [0040]      FIG. 2  shows a view similar to  FIG. 1 , with a transverse support between fixation device and bone screw;  
         [0041]      FIG. 3  shows a plan view of the clamp part of the fixation device shown in  FIG. 1 , with the clamping jaws not tightened relative to one another;  
         [0042]      FIG. 4  shows a sectional view along line  4 - 4  in  FIG. 3 ;  
         [0043]      FIG. 5  shows a side view of the clamping device from  FIG. 4 ;  
         [0044]      FIG. 6  shows a view similar to  FIG. 3 , with the clamping jaws tightened;  
         [0045]      FIG. 7  shows a sectional view along line  7 - 7  in  FIG. 6 ;  
         [0046]      FIG. 8  shows a perspective view of the fixation device from  FIG. 1  with the clamping jaws taken off and turned through 90° before being put back on;  
         [0047]      FIG. 9  shows a view similar to  FIG. 4  in a modified illustrative embodiment of a fixation device with a freely movable intermediate piece;  
         [0048]      FIG. 10  shows a sectional view along line  10 - 10  in  FIG. 9 ;  
         [0049]      FIG. 11  shows a plan view of the fixation device from  FIG. 9 ;  
         [0050]      FIG. 12  shows a perspective view of a modified illustrative embodiment of a fixation device with a pivot axis next to the eccentric axis;  
         [0051]      FIG. 13  shows a schematic sectional view through a further preferred illustrative embodiment of a fixation device with spherical pivotability;  
         [0052]      FIG. 14  shows a schematic longitudinal section through a further preferred illustrative embodiment of a fixation device with a cone-shaped limit stop for the clamping jaws;  
         [0053]      FIG. 15  shows a plan view of a transverse support for connection of fixation device and a bone screw, with a cam-type clamping means in the released state;  
         [0054]      FIG. 16  shows a view similar to  FIG. 15 , with the cam tensioned;  
         [0055]      FIG. 17  shows a sectional view along line  17 - 17  in  FIG. 16 ;  
         [0056]      FIG. 18  shows a sectional view along line  18 - 18  in  FIG. 16 ;  
         [0057]      FIG. 19  shows a view similar to  FIG. 17 , with the device mounted on a spherical bone screw head;  
         [0058]      FIG. 20  shows a view similar to  FIG. 19 , with the device mounted on a conical bone screw head;  
         [0059]      FIG. 21  shows a view similar to  FIG. 20  in another preferred illustrative embodiment with a cam in the form of a rotatable core and with a mounting body which surrounds this core and is fixedly connected to the transverse support, and  
         [0060]      FIG. 22  shows a view similar to  FIG. 1 , with a hook instead of a bone screw.  
     
    
     DETAILED DESCRIPTION  
       [0061]     The orthopedic fixation device  1  shown in FIGS.  1  to  8  is, in the illustrative embodiment in  FIG. 1 , fitted onto the shaft of a bone screw  3  whereas, in the illustrative embodiment in  FIG. 2 , it is fitted onto a sleeve-shaped, cylindrical mounting body  4  which is disposed on the bone screw  3  in a laterally offset position by way of a transverse support  2 . In the following description, both the shaft of the bone screw  3  and the cylindrical mounting body  4  of the transverse support  2  are referred to jointly as mounting body  4 , although it will be appreciated that the described orthopedic fixation device  1  can be fitted onto a wide variety of pin-shaped, shaft-shaped or sleeve-shaped mounting bodies.  
         [0062]     A support piece  5  is pushed onto the mounting body  4  from above, tightly surrounds the mounting body  4  and is freely rotatable thereon. This support piece  5  has a mounting shaft  6  which protrudes from it in a direction transverse to the longitudinal direction of the mounting body  4  and which, along its length, has a peripheral groove  7  of arc-shaped cross-section and which, at its free end, terminates in an annular flange  8  flattened on opposite sides.  
         [0063]     A substantially U-shaped clamping body  9  is pushed onto this mounting shaft  6 , with two side walls which extend substantially parallel to one another and form two clamping jaws  10 ,  11 . For this purpose, the side walls, in the region above the mounting shaft  6 , are shell-shaped on the mutually facing sides, so that they can accommodate between them a rod-like fixation member  12  which is disposed transverse to the longitudinal direction of the mounting shaft  6 . The two clamping jaws  10  and  11  are made resilient with respect to one another by an incision  13  formed in the cross-piece  14  of the clamping body  9  connecting them, so that the fixation member  12  can be snapped in elastically from above between the two clamping jaws  10 ,  11 . In this snapped-in state, the fixation member dips into the peripheral groove  7  of the mounting shaft  6  ( FIG. 4 ).  
         [0064]     Extending through the clamping body  9  there is a continuous bore  15  which accommodates the mounting shaft  6  and thus bearingly supports the clamping body  9  so that it can rotate about the mounting shaft  6 . The annular flange  8  of the mounting shaft  6  engages against the outside of the clamping body  9  and thus prevents removal of the clamping body  9  from the mounting shaft  6 .  
         [0065]     The bore  15  is configured in such a way that, when the clamping body  9  is rotated through 90° relative to the support piece  5 , the clamping body  9  can be removed from the mounting shaft  6 , this being made possible by the lateral flattenings on the annular flange  8  of the mounting shaft  6 . A bayonet-like lock is thus obtained; by means of a deformation of the annular flange after attachment of the clamping body  9 , the clamping body  9  can be secured on the mounting shaft  6  in a way which is permanent and prevents removal.  
         [0066]     A ring surrounding the mounting body  4  is mounted rotatably on said mounting body  4  above the support piece  5 , its outer circumference being eccentric with respect to the rotation axis defined by the mounting body  4 , this ring thus forming a cam body  16 . This cam body  16  is provided at its upper edge with axial incisions  17  into which a rotary tool (not shown in the drawing) can engage so that the cam body  16  can be rotated about the mounting body  4 .  
         [0067]     Fitted between the cam body  16  and the adjoining clamping jaw  10  there is a substantially U-shaped intermediate piece  18  which engages, with the outside  19  of its connecting cross-piece  20 , on the clamping jaw  10 , while the two branches  21 ,  22  adjoining the connecting cross-piece  20  merge into resilient webs  23  which extend parallel to the mounting body  4  and connect the intermediate piece  18  to the support piece  5 . These resilient webs  23  permit a slight resilient movement of the intermediate piece  18  in the direction of the clamping jaws  10  and  11 .  
         [0068]     The branches  21  and  22  form a substantially semicircular internal engagement surface  24  surrounding the cam body  16 .  
         [0069]     In a release position, the cam body  16  protrudes to the least extent in the direction of the clamping jaws  10 ,  11 . However, if the cam body  16  is turned, its periphery moves increasingly in the direction of the clamping jaws  10 ,  11  and in so doing moves the intermediate piece  18  in the direction of the clamping jaws  10 ,  11 . In this way, the clamping jaw  10 , designated below as the movable clamping jaw, is moved in the direction of the opposite clamping jaw  11 , designated below as the stationary clamping jaw. The movable clamping jaw  10  engages against the rod-like fixation member  12  and moves the latter in the direction of the stationary clamping jaw  11 , the clamping body  9  at the same time being pressed against the annular flange  8 . The cam body  16  thus engages against the mounting body  4  and, as a result of this tensioning or loading of the cam body  16 , all degrees of freedom of the described device are locked by clamping. As a result of the firm engagement on the annular flange  8 , the clamping body  9  is fixed against rotation about the axis of the mounting shaft  6 , the fixation member  12  is secured between the two mutually tightened clamping jaws  10 ,  11 , and the support piece  5  is secured against rotation about the mounting body  4  by the engagement of the cam body  16  on the mounting body  4 . Thus, simply by rotating the cam body  16 , it is possible to secure four different degrees of freedom of the described configuration. It is of importance in this respect that this securing can be done from above and, similarly, the fixation member  12  can be inserted by snapping it from above in between the as yet untightened clamping jaws  10 ,  11 . Thus, all the important steps involved in assembling and fixing the fixation device  1  can be performed by the operating surgeon from above through a very small access opening.  
         [0070]     In the illustrative embodiment in FIGS.  9  to  11 , a similar construction is chosen. Corresponding parts therefore carry the same reference numbers. In contrast to the illustrative embodiment in FIGS.  1  to  8 , the intermediate piece  18  is not connected by resilient webs to the support piece  5  and instead is completely separate from this. The intermediate piece  18 , however, engages with a flange-like widening  23   a  in a guide groove  23   b  extending parallel to the clamping direction of the clamping jaws  10 ,  11 , so that the intermediate piece  18  is guided longitudinally displaceably in the support piece  5 . In this way, greater movements of the intermediate piece can also be effected, which could possibly lead to excessive bending of the resilient webs  23 .  
         [0071]     In the illustrative embodiment in  FIG. 12 , a similar construction to the one in the illustrative embodiment in  FIG. 1  is chosen, but the mounting shaft  6  has here been shifted to a position laterally alongside the bone screw  3 , the cam body  16  and the intermediate piece  18 , and, in the illustrative embodiment shown, the mounting shaft  6  is not disposed on the support piece  5  but instead on the clamping body  9  and protrudes into a corresponding receiving bore in the support piece  5 . These are modifications which can be adopted individually or in combination and lead to different geometries of the overall configuration, but in principle do not change anything.  
         [0072]     In the illustrative embodiment in  FIG. 13 , which is represented only very schematically and in which parts corresponding to the above-described embodiments carry the same reference numbers, a spherical mounting body  4  is used, for example a spherical head on a bone screw. In this way, the whole fixation device is not only mounted on the bone screw to be rotatable about an axis, it can also be pivoted in other directions and then clamped on the spherical head of the mounting body  4  by the cam body  16 .  
         [0073]     In the illustrative embodiment in  FIG. 14 , in which corresponding parts once again carry the same reference numbers, the mounting shaft  6  has a conical configuration so that, when the cam body  16  is tensioned, the clamping body  9  is pressed against the conically increasingly thicker end of the mounting shaft  6  and is thus secured.  
         [0074]     In the illustrative embodiment in  FIG. 2 , the fixation device is held on the bone screw  3  via a transverse support  2 . In a preferred illustrative embodiment of such a transverse support  2 , as is shown for example in FIGS.  15  to  18 , this transverse support  2  is configured as a flat, strip-shaped plate and has an oblong hole  25  extending along a large part of its length. The bone screw  3  is passed through this oblong hole, and, in addition, a cam  26  is mounted rotatably at the opposite end of said oblong hole  25 . Disposed between the bone screw  3  and the cam  26  there is a clamp piece  27  which is longitudinally displaceable in the oblong hole  25  and engages both against the bone screw  3  and also against the cam  26 .  
         [0075]     For the sake of clarity, the mounting body  4  is not shown in the illustration in FIGS.  15  to  18 ; this mounting body could, for example, be welded to the transverse support  2  and surround the cam  26 .  
         [0076]     In the released position, the periphery of the cam  26  is at a maximum spacing from the bone screw  3  and, when the cam  26  is rotated, this spacing becomes smaller, so that in this way the clamp piece  27  is moved in the direction of the bone screw  3  and finally braces the latter against the edge of the oblong hole  25  and thus connects the transverse support  2  fixedly to the bone screw  3 . With the cam  26  released, the operating surgeon can therefore turn the transverse support  2  on the bone screw  3  to the desired position, and this position can be fixed simply by tensioning the cam  26 .  
         [0077]     This therefore affords an additional possibility of bringing the orthopedic fixation device shown in  FIG. 2  into different positions relative to the bone screw  3 , so that an especially wide range of possible engagement positions is obtained. In this case too, the cam  26  is actuated at the same place where the cam body  16  is actuated, since they are disposed coaxially with respect to one another, so that here too the movement possibilities can be obtained through the same very small access opening.  
         [0078]     In the illustrative embodiment in FIGS.  15  to  18 , the bone screw  3  has a cylindrical outer shaft in the region of the transverse support  2 , whereas in the illustrative embodiment in  FIG. 19 , which is otherwise of the same construction, the bone screw  3  has a spherical configuration in this region and is held between the clamp piece  27  on the one hand and the transverse support  2  on the other hand so as to be axially immovable, but free to pivot. When the clamp piece  27  is moved against this spherical engagement surface  31  upon tensioning of the cam  26 , the different pivot direction of the bone screw  3  in the transverse support  2  is thus also permanently fixed.  
         [0079]     The illustrative embodiment in  FIG. 20  largely corresponds to that in FIGS.  15  to  18 , and corresponding parts therefore carry the same reference numbers. Whereas the cam  26  in the illustrative embodiment in FIGS.  15  to  18  has an annular configuration, the illustrative embodiment in  FIG. 20  shows a cam  16  in the form of a rotatable core  32  which, in its lower region disposed in the transverse support  2 , forms the cam  26 . The core  32  can be turned by a tool connected to it from above with a positive fit.  
         [0080]     The core  32  can itself form the mounting body  4  on which the fixation device  1  is secured, but provision can also be made for the mounting body  4  to be formed as a sleeve connected fixedly to the transverse support  2 , as is shown in the illustrative embodiment in  FIG. 21  which otherwise corresponds to the illustrative embodiment in  FIG. 20 . To rotate the core  32 , a rotary tool  30  can engage in the interior of the hollow mounting body  4 . It is also expedient if, in addition, a stabilizing tool  34  is provided, for example in the form of a sleeve which is fitted onto the mounting body  4  and which, with projections  35 , engages with a positive fit into corresponding recesses  33  of the mounting body  4 . In this way it is possible, without transmitting torques to the transverse support  2 , to rotate the core  32  relative to the mounting body  4  and thus bring the cam  26  into the tightening or tensioning position.  
         [0081]     The bone screw  3  in the illustrative embodiments described above can also be replaced by a hook  36  which engages round an osseous structure. By means of a shaft-like upper part, this hook can also carry a transverse support  2  or a support piece  5 , in the same way as the bone screw, the only difference being that the connection to the bone is in this case effected not by a thread, but instead by a hook-shaped end. This modification is possible in all of the illustrative embodiments described above.