This application is a continuation of International Application No. PCT/EP2004/001346 filed on Feb. 13, 2004, which claims priority of German Application No. 103 10 004.0 filed on Feb. 27, 2003. The entire disclosures of these prior applications are considered as being part of the disclosure of this application and are hereby incorporated in their entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a surgical instrument for applying a bone plate fixing device comprising a first bone contacting element with a rod-shaped connecting member projecting therefrom and defining a longitudinal direction, and a second bone contacting element displaceable on the connecting member in a direction towards the first bone contacting element, with a first tool element positionable in a contacting position on the second bone contacting element, and a second tool element removable from the first tool element, with a transportation device for stepwise transportation of the connecting member with the second tool element in several transportation steps in a proximal direction away from the first tool element resting in the contacting position on the second bone contacting element. 
     An instrument of the kind described at the outset is known, for example, from DE 197 00 474 C2. With a second tool element formed by two clamping jaws the rod-shaped connecting member can be clamped in a clamp position and moved in the clamp position relative to the second bone contacting element. 
     A subsequent grasping of the connecting member with the clamping jaws is possible in the above-described manner. 
     With the known instrument, however, a defined transportation of the connecting member relative to the second bone contacting element is not clearly ensured. Moreover, it is difficult to securely grasp a smooth connecting member. With structured connecting members there is the problem that a structure of the connecting member may dig into the clamping jaws and cause damage to these. In any case, when high pulling forces act on the second tool element there is the danger that the clamping jaws will slide off the connecting member. Furthermore, the instrument is difficult to clean when the clamping jaws have been damaged by sharp-edged structures of the connecting members. 
     The object underlying the present invention is therefore to so improve a surgical instrument of the kind described at the outset that the bone contacting elements of the fixing device can be displaced relative to each other in a simple way and the handling of the instrument is simplified. 
     SUMMARY OF THE INVENTION 
     This object is accomplished in accordance with the invention with a surgical instrument of the kind described at the outset in that the second tool element has several receptacles for a projection protruding from the connecting member, in that with each transportation step the projection is at least partially engageable with a receptacle in an engagement position and is held therein immovably in longitudinal direction on the second tool element, and in that from one transportation step to a following transportation step the projection is engageable with a receptacle arranged in a more proximal direction on the second tool element. 
     The instrument according to the invention makes it possible to pass the projection of the connecting member in a defined manner and with a pre-determined size of step through the instrument. The formation of receptacles for receiving the projection allows the receptacles to be made of an appropriately large size, so that the second tool element and thus the entire instrument can be cleaned well. In the engagement position, movement of the connecting member relative to the second tool element in longitudinal direction is not possible. As a result, the second tool element cannot slide off the connecting member which is held by means of the projection in a receptacle. 
     It is advantageous for the second tool element to be engageable in a distal position relative to the first tool element with the projection in the engagement position, for the second tool element to be movable in the engagement position in proximal direction from the distal position into a proximal position more removed from the first tool element, and for the second tool element to be transferable in the proximal position from the engagement position to a release position in which the second tool element and the projection are disengaged. An instrument constructed in this way allows the projection to be gripped with the second tool element and moved in proximal direction, so that the second bone contacting element resting against the first tool element is moved relative to the projection of the connecting member. To subsequently grasp the projection, i. e., grasp it again, with the second tool element, the engagement position can be released, i. e., the second tool element and the projection are displaceable again relative to each other in longitudinal direction. This is only possible in the release position. 
     It is advantageous for the second tool element to be movable in the release position from the proximal position to the distal position. The projection then maintains its position relative to the second bone contacting element, whereas the second tool element can be moved past the projection into the distal position. In this way, a stepwise transportation of the projection is realized with the instrument in proximal direction. 
     A particularly secure connection is obtained in the engagement position when the projection is insertable into the receptacles with a positive fit. When the projection corresponds in design to a receptacle, damage to the second tool element is excluded. Moreover, the second tool element can be cleaned in a simple way when the projection and the receptacle are designed so as to be sufficiently large in size. 
     In order to ensure that the projection is holdable on the second tool element immovably in longitudinal direction, it is advantageous for the second tool element to be movable transversely to the longitudinal direction relative to the projection. In this way, it allows, as it were, a locking of the projection on the second tool element. 
     The construction of the instrument is particularly simple when the second tool element comprises a first and a second clamping jaw and when at least one of the two clamping jaws carries the receptacles. In this way, the projection can be held between the two clamping jaws. It is, of course, also possible to provide both clamping jaws with receptacles, so that the projection can be held on both sides by receptacles of the clamping jaws. 
     In accordance with a preferred embodiment of the invention it can be provided that the second tool element comprises a toothing having a plurality of teeth, and that the toothing comprises the receptacles. This results in a particularly simple design of the second tool element. 
     It is conceivable to construct the projection in the form of a head. However, in order to improve a connection between the second tool element and the projection, the projection may comprise a projection toothing having at least two teeth. It is thus possible for a tooth of the toothing of the second tool element to selectively engage between the at least two teeth of the projection toothing. It is also conceivable for the projection as a whole, i. e., also its projection toothing having at least two teeth, to be insertable into a single receptacle of the toothing of the second tool element. 
     For transportation of the projection away from the second bone contacting element in a defined manner, it may be advantageous for the projection to be transportable over at least one transportation path in proximal direction from one transportation step to a following transportation step, and for the transportation path to correspond to the smaller of the tooth spacings of the toothing and the projection toothing. This makes it possible to predetermine a defined smallest transportation path by the shape of the toothing or the projection toothing. An actual transportation path or stroke may, of course, correspond to an integral multiple of the smallest transportation path. 
     To facilitate cleaning of the second tool element, it may be provided that the toothing of the second tool element has a pitch which corresponds to an integral multiple of a pitch of the toothing of the projection toothing. This results in particularly large spacings of the teeth of the toothing of the second tool element. In particular, a pitch ratio may be 2:1 or 3:1. 
     It is advantageous for the projection to have a holding receptacle for receiving at least one tooth of the toothing. This has the advantage that, on the one hand, the projection as a whole is insertable into a receptacle of the second tool element and, on the other hand, a tooth of the toothing is insertable into the holding receptacle. A double connection can thus be realized, for example, in the form of two positively engaging teeth and gaps between two teeth, respectively. 
     The construction of the device becomes particularly simple when the projection toothing comprises the holding receptacle. 
     To avoid a relative movement in longitudinal direction in the engagement position between the projection and the second tool element, it may be advantageous for the at least one tooth of the toothing to be introducible into the holding receptacle transversely to the longitudinal direction. 
     It is advantageous for the holding receptacle to comprise a ring groove. This can be produced in a particularly simple way on the projection or directly on the connecting member. 
     To avoid damage to the second tool element or to the connecting member, it is advantageous for the receptacles to be of edge-free design. The edge-free design has the further advantage that when entering a receptacle, the projection is guided by advantageous roundings of the receptacles into the receptacles. 
     In order for the projection not to cause any damage to the second tool element, it is advantageous for the projection to be of edge-free design. In this way, it can slide even better into a receptacle of the second tool element. In this respect rounded shapes of the projection are helpful. 
     For simple handling of the instrument it is advantageous for the instrument to comprise a main body and at least one actuating element movably mounted on the main body, and for a pulling force to be transmittable to the second tool element in longitudinal direction away from the first tool element by a movement of the actuating element relative to the main body. The tool element can thus be moved in a simple way in longitudinal direction. 
     In accordance with a preferred embodiment of the invention, it may be provided that a holding force is transmittable to the second tool element transversely to the longitudinal direction by a movement of the actuating element relative to the main body. This allows a holding force and a pulling force to be simultaneously exerted on the second tool element by the movement of the actuating element. Therefore an operator only has to move the actuating element and can thereby move the projection away from the first tool element. 
     It is advantageous for a force deflecting element to be provided for deflecting a pulling force acting in longitudinal direction into the holding force acting transversely to the longitudinal direction. By exerting a pulling force, not only the second tool element is moved in the direction of the pulling force, but simultaneously a holding force can be exerted on the connecting member, in particular on the projection, with the second tool element. 
     A particularly compact design is obtained for the instrument when the at least one clamping jaw rests against the force deflecting element and is guidable thereon during a movement of the force deflecting element in longitudinal direction. This may be realized by, for example, inclined slide surfaces on the force deflecting element. Furthermore, a force may be transmitted directly from the force deflecting element onto the at least one clamping jaw; further parts are not required therefor. 
     In order that pulling forces may be transmitted from the force deflecting element, it is advantageous for a pulling force to be transmittable to the force deflecting element from the at least one actuating element. 
     To avoid recoil or kickback of the second tool element on the instrument, it may be provided that the at least one clamping jaw is resiliently supported on the force deflecting element in longitudinal direction. As a result, it is always held under bias on the force deflecting element, whereby a particularly gentle application of the instrument is made possible. 
     In order to additionally absorb recoil forces, should the at least one actuating element be released abruptly, the force deflecting element may be resiliently supported on the main body. 
     Damage to the instrument may be effectively avoided when a pulling force limiter is provided for limiting the pulling force in longitudinal direction. Irrespective of how large a force is exerted on the actuating element by an operator, a maximum pulling force is limited by the pulling force limiter. 
     For limited transmission of forces from the actuating element onto the force deflecting element, it is advantageous for a force initiated by the at least one actuating element to be transmittable to a limited extent onto the force deflecting element by the pulling force limiter. 
     Particularly good damping properties are obtainable for the instrument when the force deflecting element is resiliently supported on the pulling force limiter. 
     In order to further improve the damping properties of the instrument, the pulling force limiter may be resiliently supported on the main body. Recoil forces which may occur when the at least one actuating element is abruptly released are attenuated by the resilient support. 
     In order to achieve a separation of a lifting movement and a pulling movement of the second tool element, it is advantageous for the at least one clamping jaw to be mounted on a push-and-pull element which is mounted on the main body for displacement in longitudinal direction, and for a pulling force to be transmittable onto the push-and-pull element from the at least one actuating element. 
     The following description of a preferred embodiment of the invention serves in conjunction with the drawings for a further explanation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows tensioning pliers according to the invention with clamping jaws in a distal release position; 
         FIG. 2  shows the instrument of  FIG. 1  with the clamping jaws in a distal engagement position; 
         FIG. 3  shows the instrument of  FIG. 1  with the clamping jaws in a proximal pull position; 
         FIG. 4  shows the instrument of  FIG. 3  with operative pulling force limiter; 
         FIG. 5  shows a possible first engagement position of a projection of a connecting member on teeth of the clamping jaws; and 
         FIG. 6  shows a second possible engagement position of the projection on the teeth of the clamping jaws. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 to 4  show an instrument according to the invention in the form of surgical tensioning pliers. The tensioning pliers  10  serve to apply a rivet-like fixing element  12  comprising a first contacting element  14  with an elongated shaft  18  having retaining projections  16  and protruding from the first contacting element  14 , and a second contacting element  20  which is displaceable relative to the first contacting element  14  on the shaft  18  in the direction towards the first contacting element  14 . Displacement of the second contacting element  20  relative to the first contacting element  14  away from the first contacting element  14  is not possible owing to the retaining projections  16  acting in this direction. Between the contacting elements  14  and  20  two separate bone portions  22  and  24  forming, for example, parts of a human skull bone may be attached to each other by the two contacting elements  14  and  20  clamping the bone portions  22  and  24  on either side thereof between them. 
     Arranged at an end of the shaft  18  pointing away from the first contacting element  14  is a ring-shaped projection  26  with a ring-shaped constriction  28 . In this way, the projection  26  is, so to speak, provided with a toothing comprising two teeth  30  and  32 . 
     A relative movement between the two contacting elements  14  and  20  is realizable by means of the tensioning pliers  10 . For this purpose the tensioning pliers  10  comprise a first tool element in the form of a screw-in sleeve  34  which is provided with a longitudinal bore  36  and has a ring-shaped contact surface  38  pointing in distal direction for placement on the second contacting element  20 . The longitudinal bore  36  is of such dimensions that the shaft  18  can be passed with the projection  26  through the screw-in sleeve  34 . 
     The screw-in sleeve  34  is provided with an outer threaded section  42  which corresponds with an inner threaded section  44  at a distal end of a main body  40  of the tensioning pliers  10 . At its proximal end the screw-in sleeve  34  has a conical surface  46  pointing in proximal direction. A tip of a cone defined by the conical surface  46  would lie on a longitudinal axis  48  of the tensioning pliers  10 , which simultaneously forms an axis of symmetry of the tensioning pliers  10  and the fixing element  12 . 
     The main body  40  is in the form of an elongated sleeve and has a ring-shaped contact section  50  adjoining the conical surface  46  for two elongated clamping jaws  52  and  54  arranged symmetrically in relation to the longitudinal axis  48 . At the distal end, the clamping jaws  52  and  54  are each provided with an inclined slide surface  56  and  58 , respectively, corresponding with the conical surface  46 . At the proximal end, free ends of the clamping jaws  52  and  54  are mounted both pivotably and displaceably on bearing lugs  60  and  62 , namely by a pin  68  and  70 , respectively, orientated in a rotationally fixed manner on the clamping jaws  52  and  54 , respectively, transversely to the longitudinal axis  48 , extending through a slot  64  and  66 , respectively, pointing in proximal direction at an incline from the longitudinal axis  48  on the bearing lugs  60  and  62 , respectively. At the distal end the bearing lugs  60  and  62  are arranged so as to protrude radially on a pulling sleeve  72 , which at the proximal end is connected to a bearing journal  74  formed rotationally symmetrically in relation to the longitudinal axis  48 . At the distal end, the bearing journal  74  is secured against rotation and axial displacement in a proximal end of a clamping sleeve  76  by means of a bolt  78  extending through both the bearing journal  74  and the clamping sleeve  76  transversely to the longitudinal axis  48 . 
     The clamping sleeve  76  is axially displaceably mounted in the main body  40  and secured against rotation relative to the main body  40  by a longitudinal groove  80 , which extends on the outside away from a proximal end of the clamping sleeve  76  and in which a securing pin  82  engages, which protrudes on the inside from the clamping sleeve  76  and points in the direction towards the longitudinal axis  48 . At the distal end, the clamping sleeve  76  has a decreasing inner diameter, whereby a deflection surface  84  is formed, which points at an incline in proximal direction towards the longitudinal axis  48 . The clamping jaws  52  and  54  have inclined slide surfaces  86  and  88 , respectively, which correspond with the deflection surface  84  and, in an initial position shown in  FIG. 1 , rest essentially in their entirety on the deflection surface  84 . 
     A spiral spring  90  surrounding the pulling sleeve  72  is supported, on the one hand, on the bearing lugs  60  and  62 , and, on the other hand, on the bearing journal  74 . The spiral spring  90  therefore presses the clamping jaws  52  and  54  in distal direction with their slide surfaces  86  and  88  against the deflection surface  84 , and the slide surfaces  56  and  58  against the conical surface  46 . 
     The bearing journal  74  has a central bore  92  in which a cylindrical elongated pulling bolt  94  is inserted and held rotationally fixedly and axially indisplaceably on the bearing journal  74  by means of the bolt  78 . At the distal end, the pulling bolt  94  is displaceably mounted in the pulling sleeve  72 , which has two guide slots  96  and  98 , which extend parallel to the longitudinal axis  48  and in which a guide pin  100  extending through the pulling bolt  94  transversely to the longitudinal axis  48  engages and thereby holds the pulling sleeve  72  on the pulling bolt  94  so that it is axially displaceable and secured against rotation. 
     At the proximal end, the pulling bolt  94  is connected to a pulling force limiter generally designated by reference numeral  102 . This comprises a bearing sleeve  104  which is axially displaceably guided for longitudinal displacement on a bearing ring  106  which is screwed into a proximal end of the main body  40 . The bearing sleeve  104  guides in its interior a ring-shaped head  108  which is rotationally fixedly connected to a proximal end of the pulling bolt  94 . At the distal end, the pulling bolt  94  is axially displaceably guided at a central axial sleeve bore  110 . 
     Screwed onto a distal end of the bearing sleeve  104  on the outside is a stop ring  112 , which forms a stop surface  114  pointing in distal direction. In the initial position shown in  FIG. 1 , a proximal end  116  of the clamping sleeve  76  and a ring projection  118  of the bearing journal  74  lie against the contact surface  114 . A journal portion  120  which is of reduced diameter in relation to the ring projection  118  engages a corresponding cylindrical recess  122  of the bearing sleeve  104 , which is open in distal direction. A proximal end  126  of the bearing journal  74  abuts on a bottom  124  of the recess  122 , through which the sleeve bore  110  extends. 
     A plate spring block  128  surrounding the pulling bolt  94  is arranged in the bearing sleeve  104  and supported, on the one hand, on the bottom  124  and, on the other hand, on the head  108  and thereby holds the bearing journal  74  under bias in the recess  122 . A further spiral spring  130  surrounding the bearing sleeve at its distal end is arranged inside the main body  40  and supported, on the one hand, on the stop ring  112  and, on the other hand, on the bearing ring  106 . It therefore presses the bearing sleeve  104  in its entirety in distal direction. 
     At the proximal end, two bearing blocks  132  and  134  protruding radially are symmetrically arranged on the bearing sleeve  104 , and a rod-shaped link  136  and  138 , respectively, is pivotably mounted on each of these. The links  136  and  138  are also pivotably connected to a swivel grip  140  and  142 , respectively. The swivel grips  140  and  142  are pivotably held on bearing lugs  148  and  150  protruding radially from the main body  40  by means of two hinge bolts  144  and  146 , respectively, oriented transversely to the longitudinal axis  48 . 
     The clamping jaws  52  and  54  are each provided with a toothing  152  and  154  respectively, which each have a plurality of teeth  156  and  158  pointing in the direction towards the longitudinal axis  48 . Recesses constituting receptacles  157  and  159 , respectively, are formed, in each case, between two teeth  156  and  158 , respectively. The teeth  156  and  158  are all rounded. A spacing of the teeth  156  and  158  from each other is so selected that the projection  26  is introducible in its entirety between two teeth  156  and  158 . Such an engagement position is shown in  FIG. 5 . 
     However, the shape of a tip of the teeth  156  and  158  also corresponds substantially to the shape of the constriction  28  of the projection  26  on the shaft  18 , so that a tooth  156  and  158 , respectively, of the clamping jaws  52  and  54  can respectively engage the constriction  28 . Such an engagement position is shown in  FIG. 6 . The toothings  152  and  154  are so selected that the teeth  30  and  32  of the projection  26  are spaced half as far apart as two teeth  156  from each other and two teeth  158  from each other. Thus, a pitch of the toothings  152  and  154  corresponds to twice the pitch of the toothing  160  of the projection  26 . Engagement positions, which correspond to half of the spacing of the pitch of the toothings  152  and  154  can therefore be defined. Two such engagement positions separated from each other at such a spacing are shown in  FIGS. 5 and 6 . 
     In conjunction with  FIGS. 1 to 4  it will be explained in further detail hereinbelow how by means of the tensioning pliers  10  the second contacting element  20  may be displaced relative to the shaft  18  in the direction towards the first contacting element  14 . 
     The two contacting elements  14  and  20  are first placed on either side of the two bone portions  22  and  24  to be joined together against these, and the shaft  18  is passed through a gap  25  in the bone. The shaft  18  with the projection  26  is introduced through the screw-in sleeve  34 . The screw-in sleeve  34  is placed against the second contacting element  20 . This initial position is shown in  FIG. 1 . 
     By swiveling the swivel grips  140  and  142  in the direction towards the longitudinal axis  48 , the bearing sleeve  104  is pulled in proximal direction and presses the spiral spring  130  together. So long as the force exerted by the swivel grips  140  and  142  is smaller than the force exerted by the plate spring block  128 , the bearing journal  74  is held in the recess  122  of the bearing sleeve  104 . Together with the bearing journal  74  the clamping sleeve  76  is pulled in proximal direction, whereby the slide surfaces  86  and  88  of the clamping jaws  52  and  54  slide along the deflection surface  84  of the clamping sleeve  76 . The deflection surface  84  therefore acts as a deflection element by means of which a pulling force acting in the direction of the longitudinal direction  48  is deflected into a pushing force in the direction towards the longitudinal axis  48 . The clamping jaws  52  and  54  are moved with forced guidance in the direction towards the longitudinal axis  48 , and guidance is effected, on the one hand, by the slide surfaces  56  and  58  lying against the conical surface  46 , and, on the other hand, by the pins  68  and  70  guided in the slots  64  and  66 . 
     The clamping jaws  52  and  54  can be moved in the direction towards the longitudinal axis  48  until the toothings  152  and  154  enter into engagement with the projection  26 . For this purpose there are two engagement positions, which have already been explained in more detail in conjunction with  FIGS. 5 and 6 .  FIG. 2  shows the engagement position of the clamping jaws  52  and  54  on the projection  26  in a distal position thereof.  FIG. 5  corresponds to an enlarged detail from  FIG. 2 . 
     When the swivel grips  140  and  142  are swiveled further in the direction towards the longitudinal axis  48 , the clamping jaws  52  and  54  are taken along in proximal direction. The force of the spiral spring  90  is not sufficient to bias the clamping jaws  52  and  54  further in distal direction.  FIG. 3  shows a position of the tensioning pliers  10  in which relative to the second contacting element  20  the projection  26  was moved away from the second contacting element  20 , so that the second contacting element  20  already assumes a changed position in the direction towards the first contacting element  14 . 
     When the swivel grips  140  and  142  are swiveled further in the direction towards the longitudinal axis  48 , the pulling force limiter  102  begins to act. The pulling force exerted on the bearing sleeve  104  now exceeds the force exerted by the plate spring block  128 , whereby the plate spring block  128  is compressed. An axial position of the clamping sleeve  76  relative to the main body  40  thereby remains practically constant. On the other hand, the spiral spring  130  as well as the plate spring block  128  are further compressed. This position is shown in  FIG. 4 . 
     To grasp the projection  26  with the clamping jaws  52  and  54  again, the swivel grips  140  and  142  are swiveled away from the longitudinal axis  48  again. This may occur automatically, for example, by means of a leaf spring, which is not shown. With appropriately chosen spiral springs  90  and  130 , the arrangement of the tensioning pliers  10  makes it possible, in the pull position shown in  FIG. 3 , upon swiveling back the swivel grips  140  and  142  away from the longitudinal axis  48 , for the clamping jaws  52  and  54  to first be moved radially away from the longitudinal axis  48  and from the projection  26  when the pull acting on the bearing sleeve  104  is reduced. As a result, the clamping jaws  52  and  54  release the projection  26  on the shaft  18 . A further swiveling of the swivel grips  140  and  142  back into the initial position shown in  FIG. 1  results in the clamping jaws  52  and  54  being moved in distal direction, but not being in engagement with the projection  26 . Once the slide surfaces  56  and  58  come to rest against the conical surface  46  again, the projection  26  may be moved further in proximal direction in a further transportation step. 
     All in all, such a number of transportation steps are carried out in the above-described manner until the two bone portions  22  and  24  are held clamped between the two contacting elements  14  and  20 .
A bone plate fixing device comprises a first bone contacting element with a rod-shaped connecting member and a second bone contacting element displaceable on the connecting member. A surgical instrument for applying the bone plate fixing device comprises a first tool element positionable on the second bone contacting element, and a second tool element removable from the first tool element, with a transportation device for stepwise transportation of the connecting member with the second tool element. The second tool element has toothing comprising a plurality of teeth forming several receptacles for engaging toothing of the connecting member. The toothing of the connecting member may be on a projection protruding from the connecting member. The toothing of the second tool element may have a pitch which corresponds to an integral multiple of a pitch of the toothing of the connecting member.