Abstract:
A rod connector for transversely connecting two vertebral column rods has two clamps and a transverse bridge that fixes the clamps to each other. The clamps have a spring device with a spring force that is low enough to allow the clamp to be manually snapped onto the vertebral column rod, to be slid onto the same in the installed state, and to be released from same. An attaching device for the clamp, which is independent of the spring device, generates a retaining force that is high enough to solidly attach the clamp on the vertebral column rod in the installed state. The transverse bridge has a connection part with a frame for receiving a first branch connected to one clamp and with a ball rotatably mounted in the frame and having a second branch pass therethrough that connects to another clamp.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation, under 35 U.S.C. §120, of copending international application No. PCT/EP2010/068969, filed Dec. 6, 2010, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2009 056 890.5, filed Dec. 10, 2009; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention  
       [0002]    The invention relates to a rod connector for the transverse connection of two vertebral column rods. 
         [0003]    Orthopedic operations are performed on patients in order, among other things, to correct the profile of the vertebral column. To do this, two vertebral column rods are generally placed to the left and right of the vertebral column during an operation and are secured, with the aid of various hooks or screws, to the vertebral column or to the vertebrae. In order also to fix both vertebral column rods securely in relation to each other, so-called rod connectors (also called rod-to-rod connectors) are known. 
         [0004]    U.S. Pat. No. 7,029,474 B2 describes a rod connector which is fixed on the rods in such a way that a clamping jaw is pressed against the rod with the aid of a screw. 
         [0005]    U. S. Pat. No. 7,066,938 B2 describes an alternative rod connector which is snapped onto the rod in the manner of an expandable bracket. 
       SUMMARY OF THE INVENTION 
       [0006]    It is accordingly an object of the invention to provide a rod connector which overcomes various disadvantages of the heretofore-known devices and methods of this general type and which provides for an improved rod connector. 
         [0007]    With the foregoing and other objects in view there is provided, in accordance with the invention, a rod connector for a transverse connection of two vertebral column rods, the rod connector comprising: 
         [0008]    first and second clamps each for partially enclosing and securing a respective vertebral column rod in an assembly state thereof; 
         [0009]    a transverse bridge for fixing the clamps relative to each other, the transverse bridge including a first branch mounted on the first clamp, a second branch mounted on the second clamp, and a connection part connecting the first and second branches; 
         [0010]    the connection part having a frame for receiving the first branch, a ball rotatably mounted in the frame and having the second branch pass therethrough, and a locking device acting on the first and second branches; 
         [0011]    a spring mechanism for each the clamp, the spring mechanism having a spring force with a value that is low enough to enable the clamp to be snapped manually onto the vertebral column rod, to be displaced thereon in the assembly state, and to be released therefrom; and 
         [0012]    a fixing mechanism for each the clamp, the fixing mechanism being capable of actuation independently of the spring mechanism and having a holding force high enough to solidly fix the clamp on the vertebral column rod in the assembly state. 
         [0013]    In other words, the object of the invention is achieved by a rod connector for the transverse connection of two vertebral column rods where the rod connector comprises two clamps, each partially enclosing and securing a respective vertebral column rod in an assembly state. The assembly state designates the state when the rod connector is mounted on the vertebral column rods; the first clamp then encloses the first vertebral column rod, the second clamp the second vertebral column rod. The clamps in this case only partially enclose the vertebral column rods, i.e. not around the entire circumference, but nevertheless to such an extent that the clamps are secured safely on the vertebral column rod. 
         [0014]    The rod connector also comprises a transverse bridge, which fixes the location of the clamps relative to each other. The transverse bridge can, for example, be of a rigid construction, i.e. the clamps have a fixed orientation to each other in relation to the vertebral column rods that are to be enclosed, for example with the clamps being oriented with respect to each other in such a way that enclosed vertebral column rods always run parallel in the assembly state. Generally, however, the transverse bridge will be adjustable in terms of the relative position of the clamps and will be able to be locked in any desired relative position. Thus, by means of one and the same rod connector, vertebral column rods can be fixed in relation to each other in different relative positions. 
         [0015]    The restriction to two clamps is not strictly necessary. It would also be conceivable to use rod connectors with three or more clamps, which are then each fixed in position relative to one another by the transverse bridge, and then either one vertebral column rod is intended to be locked particularly securely by two clamps being snapped onto it, or else a third vertebral column rod is fixed by a single rod connector relative to two other rods. 
         [0016]    The rod connector also comprises a spring mechanism for the clamp. In other words, the clamp has a resilient design, so as to be snapped onto the vertebral column rod or also removed again from the latter. For example, when the clamp is being mounted on the vertebral column rod, the jaws of the clamp must spring apart, and, when the clamp is in the fully mounted state, the jaws must spring closed again, enclosing the vertebral column rod. The clamp bears, in particular with the spring force, in a frictionally engaged manner on the vertebral column rod. According to the invention, the spring force is so low that the clamp can be snapped manually, e.g. by an operating surgeon, onto the vertebral column rod, can be easily displaced thereon in the assembly state, and can also be released manually from the vertebral column rod. The displaceability here denotes an axial displaceability and also a displaceability in the circumferential direction of the vertebral column rod, which is generally in the shape of a cylinder. 
         [0017]    According to the invention, the rod connector also additionally has a fixing mechanism for the clamp. This fixing mechanism can be actuated independently of the spring mechanism. When the fixing mechanism is actuated, it generates an additional holding force, which the clamp exerts on the vertebral column rod, which force is added to or replaces the spring force. The holding force is such that the clamp is fixed solidly on the vertebral column rod in the assembly state. In this context, fixed solidly means that a displacement or rotation of the clamp on the vertebral column rod is no longer possible; neither manually, nor later on through the use of the vertebral column rod in the patient. 
         [0018]    In other words, according to the invention, a rod connector with a dual function is provided. The first is an adjustment function: By virtue of the spring mechanism, the rod connector is designed such that, although it is snapped captively onto the vertebral column rods during an operation, it nevertheless permits adjustment through displacement, movement, possible unsnapping and then engagement at another position. However, the rod connector is already held with a certain force on the vertebral column rods, such that a safe, simple and reliable adjustment to a defined final position is possible. 
         [0019]    The second is a fixing function: Only when the desired final position is found is the fixing mechanism actuated and the vertebral column rod finally fixed in the position in question. In the case of an adjustable transverse bridge, the final fixing also concerns the final fixing of the transverse bridge, i.e. the final fixing of the relative position of the clamps to each other. 
         [0020]    In a preferred embodiment, the clamp has a main support, with a stationary clamping jaw, and also a spring jaw mounted resiliently on the main support. The resilient mounting is effected by the spring mechanism, which mounts the spring jaw resiliently with respect to the main support or clamping jaw. The fixing mechanism then locks the spring jaw against the clamping jaw upon actuation thereof, such that the spring jaw against the main support or clamping jaw can no longer spring away from a final position fixing the vertebral column rod. 
         [0021]    In a variant of this embodiment, the spring mechanism is a spring bridge connecting the spring jaw to the main support. The spring bridge thus has a dual function, namely, on the one hand, connecting the spring jaw to the main support, in order, on the other hand, to generate the above-described spring action of the clamp. A separate spring element is then no longer necessary for the spring-loading of a spring jaw, mounted for example in an articulated manner, on the main support. 
         [0022]    In addition, the following is thus possible: In another variant of this embodiment, spring jaw, spring bridge and main support are formed in one piece. The spring bridge, which connects a solid spring jaw to a solid main support, is adapted to the material of the clamp, for example made sufficiently thin. For example, the spring bridge is a connecting bridge between spring jaw and main support and has been cut out from a metal in the manner of a leaf spring. 
         [0023]    In another variant of this embodiment, the spring jaw and/or the spring bridge is cut out from the main support. The spring property of the spring part can be determined simply by the dimensions of the cutout. In addition, connection techniques for formerly individual parts are not needed in order to connect them, e.g. weld them, in one piece again. 
         [0024]    In another embodiment of the invention, main support and spring jaw each have a form-fit element. The two form-fit elements have play between each other in respect of the spring mechanism. However, the form-fit elements can be moved into each other with a form fit by the fixing mechanism. In other words, the form-fit elements are not to be brought into engagement with each other in respect of the spring properties of the clamp, i.e. by simple spring-back of the clamp. Until the clamp is finally fixed, the form-fit elements do not engage in each other. A free spring movement of the clamp is possible. When the clamp is finally fixed, the form-fit elements engage in each other and ensure a secure and permanent hold of the clamp on the vertebral column rod. 
         [0025]    In a variant of this embodiment, the form-fit elements, on their faces directed toward each other, have an at least approximately S-shaped matching form. In other words, a gap present between main support and spring jaw in the area of the form-fit elements is S-shaped. By deformation or movement of the spring jaw with the aid of the fixing mechanism, both structural parts are moved toward each other such that the form-fit elements touch. The S-shaped matching form here affords the possibilities of form-fit engagement of the respective two U-shaped or arc-shaped parts of the S. 
         [0026]    In a preferred embodiment of the invention, the fixing mechanism comprises a form-fit element that prevents the pivoting-open of the clamp. By means of a corresponding form-fit element, a movement of the clamp in respect of its pivoting open is in fact prevented by a form fit, such that the clamp remains finally fixed on the vertebral column rod. In other words, a form fit then prevents the play of the clamp otherwise provided by the spring element. In conjunction with the abovementioned design of a main support with a spring jaw, a possible example of a form-fit element would be a form-fit element that supports the spring jaw relative to the main support with a form fit. The form-fit element is preferably held captively on the clamp by a loss prevention device. 
         [0027]    In a variant of this embodiment, the form-fit element is a threaded element acting on the clamp. In other words, by actuation of the threaded element, a form fit and in particular a high pressing force of the clamp on the vertebral column rod is generated. For example, the threaded element, starting from a position in which it is at a distance from the spring jaw and thus permits the spring movement of the latter, is screwed in and placed or pressed against the spring jaw, such that the previous spring movement is no longer possible. A threaded element is, for example, a screw, or a threaded pin which is mounted in the main support, bears on the main support and presses against the spring jaw. For example, according to the above, the screw is then held on the main support by a loss prevention device, such that it cannot come loose from the rod connector, even in the state when not locking the clamp. 
         [0028]    In a preferred embodiment of the invention, the transverse bridge comprises two branches, each of the branches being mounted respectively on one of the clamps. The transverse bridge also comprises a connection part, through which both branches extend and which has a locking device acting on the two branches. In the case of branches that overlap each other in their axial direction, a degree of freedom can be achieved here for the placing of the connection part. The latter is displaceable in the overlap area of the two branches. The connection part can thus be displaced to a desired position relative to the patient. 
         [0029]    In a variant of this embodiment, the locking device is one that generates a locking frictional engagement between branches and connection part. In other words, the two branches here are jammed in the connection part with such a high clamping force that this produces a permanent relative fixing of the two branches and, therefore, of the clamps. 
         [0030]    In another variant of the embodiment, the connection part comprises a frame for receiving the first branch, and a ball which is mounted rotatably in the frame and through which the second branch passes. The first branch is here mounted in the frame fixedly, for example, or so as to be axially displaceable along the longitudinal axis of the branch. By means of the second branch being mounted in the ball and the latter being rotatable, the second branch can be tilted, twisted and rotated relative to the first branch, within the range of rotation possibilities of the ball. Generally, an axial displaceability of the second branch in the ball is also possible. The locking device then also locks the ball in the frame and the respective position of the branches in the frame and in the ball. By means of the ball, the second branch can thus rotate, relative to the first branch and to the frame, about the center point of the ball. 
         [0031]    In another variant of this embodiment, the ball has a compressible gap for locking the second branch extending through the ball. In other words, the ball is compressed at the gap by the locking device, such that the ball closes firmly with frictional engagement around the second branch passing through it and locks this second branch. In other words, the branch sits between two ball parts separated by the gap, which ball parts are movable toward each other in order to lock the branch with a clamping action. 
         [0032]    In another variant of this embodiment, the locking device comprises a clamping element that bears on the frame and that presses the first branch against the ball. Thus, all parts of the locking device and of the transverse bridge are fixed relative to one another by a single clamping element, since, on the one hand, the clamping element presses the first branch against the ball and fixes these relative to each other, and at the same time the first branch is jammed in the ball, as a result of the compression of the latter, and is held in this way. In addition, the ball is also finally pressed against the frame, and all the parts are thus fixed relative to one another. In other words, therefore, the two branches, the ball and the frame are pressed against one another and fixed in the connection part by the clamping element. 
         [0033]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0034]    Although the invention is illustrated and described herein as embodied in a rod connector, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0035]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying schematic illustrations in the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0036]      FIG. 1  shows a perspective view of a rod connector mounted on two vertebral column rods; 
           [0037]      FIG. 2  shows a perspective view of the rod connector from  FIG. 1 , in a centrally cut longitudinal section; 
           [0038]      FIG. 3  shows a perspective view of the branch of the unassembled bracket from  FIG. 1  in transverse section along the line III-III; 
           [0039]      FIG. 4  shows a perspective view of the unassembled frame from  FIG. 1 ; 
           [0040]      FIG. 5  shows the front view of the frame from  FIG. 4  in the direction of the arrow V; 
           [0041]      FIG. 6  shows the plan view of the frame from  FIG. 4  in the direction of the arrow VI; 
           [0042]      FIG. 7  shows a perspective view of the unassembled ball from  FIG. 1 ; 
           [0043]      FIG. 8  shows a section through the ball from  FIG. 7  in the direction of the arrow VIII-VIII in  FIG. 9 ; 
           [0044]      FIG. 9  shows the front view of the ball from  FIG. 7  in the direction of the arrow IX; 
           [0045]      FIG. 10  shows a perspective view of the ball and frame in the assembly position; 
           [0046]      FIG. 11  shows the ball pushed into the frame, in the direction of the arrow XI in  FIG. 10 ; 
           [0047]      FIG. 12  shows the situation from  FIG. 11  looking in the direction of the arrow XII; 
           [0048]      FIG. 13  shows the ball lowered into the inner surface compared to the situation illustrated in  FIG. 12 , and in a view according to  FIG. 12 ; and 
           [0049]      FIG. 14  shows the ball rotated compared to the situation from  FIG. 13 , and in a view according to  FIG. 12 ; and 
           [0050]      FIG. 15  is a perspective view of the loss prevention device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1 and 2  thereof, there is shown a rod connector  2  or rod-to-rod connector  2 , which is mounted on two vertebral column rods  4   a,b.  The rod connector is made principally in three parts and comprises two brackets  6   a,b  and a connection part  8 . By means of the latter, the brackets  6   a,b  are variable, in terms of their spatial position relative to each other, in several degrees of freedom and can be fixed relative to each other in a desired spatial position. Each of the brackets  6   a,b  has a longitudinally extending branch  10 , on the respective one end  12   a  of which a clamp  14  is arranged. The respective other end  12   b  of the branches  10  lies beyond the connection part  8 , i.e. the branches  10  pass through the latter. In other words, the branches  10  are directed toward each other and overlap each other. The branches  10  thus form, together with the connection part  8 , a transverse bridge  11 , which fixes the location of the two clamps relative to each other. 
         [0052]    Each clamp  14  is made up of a clamping jaw  16  and of a spring jaw  18 , which is connected resiliently to the clamping jaw  16  and is formed integrally thereon. In other words, the clamping jaw  16  is part of a main support  19 , on which the spring jaw  18  is mounted and the branch  10  is securely adjoined. 
         [0053]    The connection part  8  comprises a frame  20  and a ball  22  mounted in the latter in the manner of a ball joint. The ball  22  has an outer surface  36 , which forms a part of a sphere and whose center point  24  is the ball center point. The frame  20  has, in its interior or in an insert opening  58  (see also  FIG. 4 ), an inner surface  38 , which receives the outer surface  36  in the manner of a slide bearing and which is likewise part of a ball surface with the same radius. The center point  25  of the inner surface  38  is once again the corresponding ball center point thereof. Since, as can be seen from  FIG. 2  for example, the ball  22  lies in the inner surface  38 , in this situation the ball center points  24 ,  25  coincide concentrically. Outer surface  36  and inner surface  38  thus interact in the sense of a slide pairing and form a ball joint that is rotatable in principle in all directions. 
         [0054]    The ball  22  has a receiving space  64  (see  FIG. 7 ) through which the branch  10  of the bracket  6   b  passes. In the receiving space  64 , the branch  10  is guided axially displaceably in the manner of a slide guide only in the direction of the arrow  40 . The branch  10  of the bracket  6   a  is located between ball  22  and frame  20  in a further insert opening or in a receiving space  76  (see  FIG. 13 ) formed between ball  22  and frame  20 . The branches  10  and the ball  22  can be fixed relative to each other in terms of their spatial position by tightening of a clamping element  26  in the form of a threaded pin, which is part of the connection part  8 . A tool is generally used for this purpose. The clamping element  26  thus forms a locking mechanism  31  for fixing the branches  10  and the connection part  8  in a desired position relative to one another. 
         [0055]    The clamping element  26  has a collar  27  at its inner end, i.e. the external thread of the clamping element  26  is not formed or milled all the way to the inner end. In the position shown in  FIG. 2 , the collar  27  abuts against the inner end  29  of the internal thread formed in a threaded opening  56 . Therefore, during assembly of the connection part  8 , the clamping element  26  can be screwed into the threaded opening  56  only from the inside, that is to say from the direction of the insert opening  58 . After complete assembly of the connection part  8 , the clamping element  26  is thus held in such a way that it cannot be lost. The clamping element  26 , should it come loose, cannot therefore be lost, either during the handling of the rod connector  2  during an operation or in the state when fitted in the patient. 
         [0056]    The force-fit engagement for the fixing is obtained in detail as follows: The clamping element  26  engaging in the frame  20  presses the branch  10  of the bracket  6   a  against the outer surface  36  of the ball  22  and compresses the latter on account of a gap  66  provided therein (see  FIG. 7 ), such that this presses against the inner branch  10  of the bracket  6   b,  and the latter in turn clamps the opposite side of the ball  22  against the inner surface  38  of the frame  20 . 
         [0057]    Since the ball  22  is rotatable about the common center point  24 ,  25 , the following degrees of freedom are obtained for the adjustment of the rod connector  2 : The branches  10  are axially movable along the arrows  40  in relation to the connection part  8 . In this way, it is possible to compensate for different distances between the vertebral column rods  4   a,b.  By means of a rotation of the ball  22  in the frame  20  about the center point  24 ,  25  according to the arrows  42 , the longitudinal axes of the branches  10  can be tilted relative to each other between a parallel orientation and a skew orientation. Moreover, by rotation of the ball  22  in the frame  20  about the longitudinal axis of the branch  10  of the bracket  6   b  in the direction of the arrow  44 , it is possible to twist the two brackets  6   a,b  relative to each other. In this way, it is possible to compensate for all the spatial positions of the vertebral column rods that are conceivable during an operation on a patient or for all the deviations of the vertebral column rods from a parallel arrangement. 
         [0058]    The connection part  8  can be moved freely in the direction of the arrows  40  in the overlapping area of the branches  10 . If, for example, when the rod connector  2  is fitted in place in an operation, the branches  10  extend between two closely bearing spinous processes of vertebrae, the connection part  8  can be moved from the center toward the left or right side of the vertebral column, so as not to touch or impede the spinous processes. 
         [0059]    When the clamp  14  is fitted onto the vertebral column rod  4   a,b,  the spring jaw  18  pivots about its bearing axis  28  away from the clamping jaw  16  in the direction of the arrow  30 , in order to receive the vertebral column rod  4   a,b  in the clamp  14 . As soon as the vertebral column rod  4   a,b  lies therein, the spring jaw  18  pivots counter to the direction of the arrow  30  back into the position shown. A further component of the brackets  6   a,b,  namely a fixing element  32  in the form of a threaded pin, is then screwed into the clamp  14 , in order to press against the spring jaw  18  and press this against the vertebral column rod  4   a,b  and press the latter against the clamping jaw  16  and finally fix it. Similarly to the above, the fixing element  32  is also equipped with a loss prevention device  33 , such that it cannot come loose from the rest of the rod connector  2  during handling or after being placed in the patient (see also  FIG. 15 ). For the sake of clarity, the loss prevention device is not shown in  FIG. 2 . For this purpose, the fixing element  32  carries a radially protruding collar  37   a,  and, after introduction of the fixing element  32 , a stop pin  37   b  is introduced, e.g. welded, into the main support  17 . When the fixing element  32  is unscrewed, the collar  37   a  strikes against the stop pin  37   b  and prevents release of the fixing element  32  from the respective bracket  6   a,b.    
         [0060]    Alternatively, in a manner not shown, the loss prevention device  33  can also conceivably comprise a resilient latching lug, inserted into the clamp  14 , and an abutment on the fixing element  32 . The latching connection then snaps into engagement the first time the fixing element  32  is screwed in, such that the latter is then no longer removable. 
         [0061]    The clamps  14  for securing on the vertebral column rod  4   a,b  thus have a dual function. The first is a spring function as follows: A partial cutout  46  in part of the one-piece clamp  14  creates the movable spring jaw  18 . The remaining part of the clamp  14  forms the stationary clamping jaw  16 . By virtue of the spring jaw  18 , the clamp  14  can be snapped onto a vertebral column rod  4   a,b  and can also be released from the latter again. A spring force F, with which the clamp  14  acts on the vertebral column rod  4   a,b,  is so low that it is sufficient, for example, for a rough or displaceable mounting of the rod connector  2  on the vertebral column rods  4   a,b,  such that said rod connector  2  cannot at first slip out of position or slide off the vertebral column rod  4   a,b.  However, the spring force F still allows the clamp  14  to be held on the vertebral column rod  4   a,b  such that it can be twisted or displaced manually or can even be removed again. No tools are therefore necessary for this purpose. 
         [0062]    The clamps  14  bear on the vertebral column rods  4   a,b  via in each case three radially inwardly protruding beads  15  that extend parallel thereto. These beads  15  provide a linear bearing of the clamp  14  on the vertebral column rods  4   a,b,  such that the surface pressure forces that occur upon final fixation are kept as low as possible. Vertebral column rods  4   a,b  made of titanium are extremely pressure-sensitive, for example, and can be easily damaged when a clamping screw bears directly on them, as is known in the prior art. This situation is thus avoided. 
         [0063]    The spring jaw  18  is connected to the clamp via a spring bridge  48  and a hinge  49 . Both parts are produced in one piece with the clamp  14  by means of the cutout  46 . The hinge  49  is formed by two form-fit elements  55 , which lie opposite each other and are separated by a gap  57 , and serves for the final fixation, namely when the hinge parts thereof are pressed with a form fit into each other by tightening of the fixing element  32 . The spring bridge  48  serves for the resiliency of the spring jaw  18 , such that the latter, during the snapping of the clamp onto the vertebral column rod  4   a,b,  springs back, and, in the snapped-on position, fixes the vertebral column rod  4   a,b  between itself and the clamping jaw  16  with a spring force F. The spring bridge  48  thus forms a spring mechanism  47 . The spring bridge  48  has an undulating shape and is therefore also extensible in its longitudinal direction. Thus, when the fixing element  32  is tightened, the displacement of the spring jaw  18  is taken up, if the hinge  49  slides into the form fit, i.e. a stretching or breaking-off of the spring bridge  48  is prevented. 
         [0064]    At their respective ends  12   b,  the branches  10  are provided with a pin  34  which, after the assembly of the rod connector  2 , is pressed into the branches  10 , welded to the upper face  35  thereof and then trimmed. Because of the pins  34 , it is then no longer possible to remove the branches  10  from the connection part  8 . This avoids the rod connector  2  breaking up into individual parts during handling. 
         [0065]    The position of the bearing axes  28  is at the end of the cutout  46 . The final fixing on the vertebral column rod  4   a,b  represents the second of the dual functions of the clamps  14 . Manual displacement or release is then no longer possible. In the final fixing, i.e. when the fixing element  32  is tightened using a tool (not shown), the spring jaw  18  is forced in the direction of the arrow  51 , as a result of which the hinge  49  comes into abutment or form fit. In other words, in the area of the hinge  49 , the spring jaw  18  is pressed on in the body of the clamp  14 . The undulating structure of the spring bridge  48  has the effect that this stretches and provides the required length compensation for the displacement of the spring jaw  18  in the direction of the arrow  51 . As a result of this, the clamp is pressed with a holding force H against the vertebral column rod  4   a,b,  and this prevents release and displacement. Fixing element  32  and hinge  49  thus form a fixing device  53  for the final fixing. 
         [0066]    The branch  10  of the bracket  6   b  has a greater length l b  than the branch  10  of the bracket  6   a,  which has the length l a . Thus, when the branches  10  are moved along the arrows  40 , the greatest possible range of distance between connectable vertebral column rods  4   a,b  can be covered. For the achievable minimum distance, the ends  12   b,  on the one hand, lie in proximity to the fixing element  32  of the bracket  6   b  and, on the other hand, in proximity to the spring jaw  18  of the bracket  6   a.  Since the respective spring jaw  18  is farther than the fixing element  32  from the connection part  8 , the branch  10  of the bracket  6   b  can also be designed with a greater length l b . This additional length difference l b -l a  provides an extended maximum possible distance for vertebral column rods  4   a,b  if the branches  10  are drawn out until their pins  34  abut against the connection part  8 . 
         [0067]      FIG. 3  shows a cross section along the line III-III through the branch  10  of the bracket  6   a,  i.e. looking in the direction of the end  12   b  thereof. The underside  50  of the branch  10  directed toward the ball  22  or toward the outer surface  36  thereof has a cross section shaped as an arc of a circle, in order to bear on the spherical outer surface  36 . The circle radius of the underside  50  is chosen here to match the sphere radius of the ball  22 , in order to achieve a surface pressure between branch  10  and ball  22 . 
         [0068]      FIG. 3  shows the bracket  6   a  in the as yet unassembled state, i.e. before it has been inserted into the connection part  8 . Therefore, an opening  54  can still be seen at the end  12   b,  which opening  54  later serves to receive the pin  34 . 
         [0069]      FIGS. 4-6  show the frame  20 . In particular, they show the insert opening  58 , through which the ball  22  is later introduced into the interior of the frame  20  and to bear on the inner surface  38 . To be able to introduce the ball  22 , the insert opening  58  has a clear width w and a height h, in each case greater than corresponding dimensions of the ball  22  (see  FIG. 8 ). 
         [0070]    A part of the interior of the frame  20  or of the insert opening  58  is the inner surface  38 , formed as part of a sphere surface. The frame  20  has a threaded opening  56 , into which the clamping element  26  is screwed. 
         [0071]    On the outside of the frame  20 , the threaded opening  58  is surrounded by a bayonet catch  60 . The latter serves to grip the rod connector with the aid of a tool (not shown), e.g. a special holder, which engages on the bayonet catch  60 . 
         [0072]      FIGS. 7-9  show the ball  22  with its outer surface  36  shaped as part of a sphere. On two opposite sides, the ball  22  is in each case cut flat in the manner of a spherical layer, as a result of which two parallel flat surfaces  62  are obtained. These have a perpendicular distance to each other, i.e. the ball  22  has a width b. The width b is smaller than the abovementioned clear width w of the insert opening  58 , such that the ball  22  can be pushed in a corresponding direction into the insert opening  58 . 
         [0073]    The greatest dimension of the ball  22  is its diameter d, which is dimensioned such that it is smaller than the height h of the insert opening  58 . This is also necessary to ensure that the ball  22  can be introduced into the insert opening  58 . 
         [0074]    The ball  22  has a receiving space  64  into which, according to  FIGS. 1 and 2 , the branch  10  of the bracket  6   b  is inserted. In order to achieve the abovementioned clamping effect by pressure on the outer surface  36  in respect of the branch  10  lying inside the receiving space  64 , the receiving space  64  is interrupted at the side by a gap  66 . The two ball halves, which lie opposite each other in relation to a plane of symmetry of the ball  22  extending through the gap  66 , can thus move with a spring action toward each other, as a result of which, when external pressure is applied to the ball  22 , a clamping effect that fixes the branch  10  is obtained between the inner surfaces  68   a,b.    
         [0075]      FIGS. 10 to 14  show the assembly of the ball  22  into the frame  20  in chronological order.  FIG. 10  shows the starting position for assembly, in which the ball  22  is oriented such that its flat surfaces  62  are oriented parallel to the insertion direction (direction of the arrow  70 ). To put it another way, the ball  22  lies such that the flat surfaces  62  are aligned with the direction of extension of the insert opening  58  in the frame  20 . Since the width b is smaller than the width w and the diameter d is smaller than the height h, it is possible to move the ball  22  through the insert opening  58 , in the direction of the arrow  70 , into the interior of the frame  20 . 
         [0076]      FIGS. 11 and 12  show the situation after completion of the insertion procedure, i.e. when the center point  24  of the ball  22  is located at the center (indicated by the center axis  72 ) of the frame  20 . 
         [0077]    It will be seen from  FIG. 12  that the ball  22  is inserted with its center point  24  slightly above the center point  25  of the sphere of the inner surface  38 . Insertion is only possible in this way, since otherwise the outer surface  36  of the ball  22  would collide with the inner surface  38 . Therefore, after complete insertion into the connection part  8 , the center point  24  of the ball  22  is still located, on the center axis  72 , above the center point  25  of the spherical inner surface  38 . 
         [0078]      FIG. 13  therefore shows the next assembly step: The ball  22  is lowered, in the direction of the arrow  74 , along the center axis  72 , until the outer surface  36  bears on the inner surface  38  on the underside  75  of the connection part  8 . The lowering movement is possible since, on account of the still present rotation position of the ball  22 , only a small area  73  of the outer surface  36  comes into contact with the inner surface  38 , and the rest of the ball  22  has a sufficient distance or movement clearance from the inner surface  38 . The center points  24  and  25  of ball  22  and inner surface  38  now coincide. Therefore, the ball  22  can now be rotated in any desired direction about the center point  24 ,  25 . In doing this, large parts of the outer surface  36  slide onto the inner surface  38 . In addition, above the ball  22 , this also creates the receiving space  76  for receiving the branch  10 , as shown in  FIGS. 1 and 2 . In the receiving space  76 , the branch  10  is guided axially displaceably, likewise in the manner of a slide bearing, only in the direction of the arrow  40 . Placing a branch  10  into the receiving space  76  with virtually no play also prevents the ball  22  moving counter to the direction of the arrow  74 . Thus, the form fit enforced by the inner surface  38  can no longer be overcome, and the ball  22  is fixed in the connection part  8 . 
         [0079]      FIG. 14  shows how in particular the ball  22  is rotated through approximately 90° about the center axis  72 , such that the flat surfaces  62  now lie approximately parallel to the cross-sectional surface of the insert opening  58 . Thus, the receiving space  64  is also accessible for receiving a first branch  10 . The possible movements of the ball  22  are again indicated by the arrows  42  and the arrow  44 .  FIG. 14  now shows a relative position of ball  22  and frame  20 , which position is rotated a further 90° in the direction of the arrow  44  compared to the position in  FIGS. 1 and 2 . 
         [0080]    By rotating the ball  22  between the positions shown in  FIGS. 13 and 14 , the ball  22  in  FIG. 14  is fixed in the inner surface  38 , such that it is no longer movable in the direction of the arrow  70 . To do so, it would also first have to be brought back to the position according to  FIG. 13 , i.e. with the flat surfaces  62  again oriented accordingly. 
         [0081]      FIG. 15  again shows the loss prevention device  33  in detail in the state before assembly. It also shows, by way of example, the bracket  6   a  from  FIG. 1 , which has an opening  78  for receiving the fixing element  32 , and an opening  82  for receiving the stop pin  37   b.  The figure shows in particular the collar  37   a  on the fixing element  32 , and the stop pin  37   b  interacting therewith. First, the fixing element is screwed into the opening  80 , until the collar  37   a  is recessed in the opening  78 . The stop pin  37   b  is then fixed in the opening  82 , e.g. by welding. The broken line  80  indicates that the stop pin  37   b  then covers the collar  37   a.  When the fixing element  32  is unscrewed, the collar  37   a  then strikes against the stop pin  37   b,  before the fixing element  32  can leave the opening  78 , and is thus permanently held there.