Abstract:
In order to provide an implant for fixing adjacent bone plates, in particular cranial bone plates, which has an inner abutment element by means of which a separation gap between the bone plates can be overlapped at a bone plate inner side and has an outer abutment element for overlapping the separation gap at a bone plate outer side lying opposite the bone plate inner side and which is easy for a surgeon to use and by means of which the bone plates may be securely fixed, it is proposed that at least one tension band is guided displaceably through the outer abutment element, by means of which, when a tensile stress is exerted, the inner abutment element and the outer abutment element are mutually braceable, and that the at least one tension band is fixable on the outer abutment element.

Description:
The present disclosure relates to the subject matter disclosed in German application No. 101 28 918.9 of Jun. 15, 2001 and international application PCT/EP 02/05652 of May 23, 2002, which are incorporated herein by reference in their entirety and for all purposes. 
     BACKGROUND OF THE INVENTION 
     The invention relates to an implant for fixing adjacent bone plates, in particular cranial bone plates, having an inner abutment element by means of which a separation gap between the bone plates can be overlapped at a bone plate inner side and having an outer abutment element for overlapping the separation gap at a bone plate outer side lying opposite the bone plate inner side. 
     For brain surgery a craniotomy has to be performed in order to gain access to the brain tissue that is to be operated on. For this purpose, one or more bone plates are sawn out of the skull. On completion of the brain surgery these bone plates have to be reinserted and fixed to the bone plates of the rest of the skull. For this purpose, suitable implants are provided, which remain in the body of the patient. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an implant for fixing adjacent bone plates is provided, which is easy for the surgeon to use and by means of which the bone plates may be securely fixed. 
     This is achieved in accordance with the invention in that at least one tension band is guided displaceably through the outer abutment element, by means of which, when a tensile stress is exerted, the inner abutment element and the outer abutment element are mutually braceable, and that the at least one tension band is fixable to the outer abutment element. 
     Through the tightening of a tension band the inner abutment element and the outer abutment element may be moved towards one another and mutually braced so that the adjacent bone plates are fixed to one another. This tensioned position in turn may be secured by fixing the tension band to the outer abutment element. The bracing may therefore be effected easily by the surgeon without the requirement for an additional application instrument. 
     A tension band is easy for the surgeon to manipulate because it has a sufficiently large width for grasping and exerting tension. 
     A tension band moreover provides enough surface area to be able to effect fixing to the outer abutment element. In particular, for securing the fixed position of the bone plates hook elements are provided, which may penetrate the tension band and therefore secure this fixed position. 
     Furthermore, the implant according to the invention is also easy to manufacture since the inner abutment element and the outer abutment element may be manufactured separately from the tension band and the connection between tension band and abutment elements may easily be produced by threading the former through the latter. 
     The implant according to the invention is moreover insertable in an advantageous manner: during insertion of a bone plate into a cranial recess the inner abutment element may be held by means of the tension band until the desired relative positioning of the bone plates is achieved. It is then possible afterwards to guide the outer abutment element by means of the tension band and hence effect the bracing. 
     A tension band may in said case, depending on the application, be manufactured from an absorbable or non-absorbable material. Suitable absorbable materials are, in particular, synthetic materials or organic materials. Suitable non-absorbable materials are plastics materials such as PEEK or also metal materials such an titanium or other biologically compatible materials. 
     It is quite particularly advantageous when the width of the at least one tension band is greater than its height and in particular is substantially greater, e.g. at least five times greater. This then provides a corresponding surface area, which is easier for a surgeon to grasp and which allows easy fixing of the tension band to the outer abutment element by means of hook elements. 
     In said case, in order further to facilitate handling and obtain a secure fixing to the outer abutment element, the width of the at least one tension band is in the region of between 25% and 75% of a width dimension of an abutment element. In particular, in said case the width is in the region of ca. half of this width dimension, which in the case of a round disk-shaped abutment element is the diameter. 
     In particular, it is provided that the at least one tension band is of a flexibly bendable design so as to allow the two abutment elements to be drawn towards one another and hence braced relative to one another. 
     It is provided that the at least one tension band is held on the inner abutment element in order during the bracing to provide a fixed point, i.e. ensure that the tension band no longer moves relative to the inner abutment element. 
     It may then in principle be provided that the at least one tension band is fastened to the inner abutment element, i.e. is fixed invariably with regard to its holding point thereon. 
     It is however quite particularly advantageous when a tension band is passed through the inner abutment element and in said case the tension band is held on the inner abutment element by means of a tension band bend. Thus, the tension band may be fixed on the inner abutment element, wherein however said fixing is variable. Thus, on the one hand, manufacture is facilitated and, on the other hand, because of the holding by means of a tension band bend a high degree of certainty that the inner abutment element and the tension band will not detach from one another is achieved. 
     From the point of view of manufacture, it is advantageous when the inner abutment element has two spaced-apart openings for passing the tension band through. Between the openings a kind of bridge is then formed, which holds, i.e. forms a bearing surface for, the tension band bend. 
     In said case, the openings are advantageously so disposed and designed, i.e. the bridge element is also correspondingly so disposed and designed, that a first tension band region and a second tension band region, between which a tension band bend is formed, during penetration of the separation gap are alignable substantially parallel to one another. This prevents the tension band bends from possibly exerting transverse forces upon the bone plates and, when tension is exerted upon the at least one tension band, possibly shifting the bone plates towards one another. 
     In order to provide a uniform bearing surface for the adjacent bone plates, the openings are advantageously disposed substantially mirror-symmetrically relative to a center of the inner abutment element. 
     It is in said case advantageous when the spacing of the openings is less than ca. an eighth of a width dimension of the inner abutment element. In the case of a circular abutment element, this width dimension is the diameter. It is thereby possible to ensure that the separation gap, through which the tension band has to be passed, does not become too large and, on the other hand, achieve parallel guidance of appropriate tension band regions. 
     In said case, edges of the openings are advantageously rounded off to prevent damage to the tension band. 
     It is further advantageous when the outer abutment element has one or more openings, through which in each case one longitudinal end of a tension band is passable. Thus, by means of the at least one tension band the outer abutment element may be fixed relative to the inner abutment element and so, in turn, adjacent bone plates may be fixed between the two abutment elements. 
     In order by tightening the tension band to be able to effect a bracing of the two abutment elements and in addition move the outer abutment element relative to the inner abutment element, an opening has a deflection edge for deflecting a tension band, so that a tensile force is exertable upon the tension band transversely relative to a direction of spacing between inner abutment element and outer abutment element. In particular, in said case the deflection edge is rounded off in order to guarantee good guidance of the tension band in the opening and, on the other hand, prevent damage to the tension band. 
     Advantageously in said case the opening or openings are disposed and designed in such a way that the at least one tension band is positioned substantially at right angles to the abutment element in the separation gap in order thereby to prevent the tension band from exerting transverse forces upon the bone plates and to be able to keep the size of the separation gap small. 
     A fixation of the bone plates between the abutment elements is easily achievable when a tensile force with a transverse component in a first direction is exertable upon a first tension band end and a tensile force with a transverse component in an opposite direction is exertable upon a second tension band end. By a relative pulling of the two tension band ends apart from one another, the outer abutment element is then displaced in the direction of the inner abutment element and a bracing and hence fixation of the bone plates between the two abutment elements is effected. 
     An implant according to the invention may easily be manufactured when the first tension band end and the second tension band end are formed on the same tension band, i.e. when the tension band is looped through the inner abutment element and then the respective ends are conveyed in opposite directions. 
     The fixation of the bone plates may easily be secured when the at least one tension band may be hooked in relative to the outer abutment element. The tension band provides a large surface area for the engagement and in particular for the penetration of hook elements in order thereby to enable the fixing of the tension band relative to the outer abutment element. 
     In particular, it is in said case advantageous when a hook element has an inclined flank and a steep flank, wherein the steep flank is arranged facing a pulling end of the at least one tension band. In said case, the steep flank is in particular designed in such a way that, when the tension band is hooked in, the steep flank is substantially at right angles to the tension band. A hook element then has a substantially triangular cross section, wherein the steep flank is the one at a steeper angles relative to a vertical direction of the triangle. Given a corresponding arrangement of the steep flank facing the pulling end, it is guaranteed that during the hooking-in operation the tension is not reduced, since hooking-in of the hook-connection face of a hook element is effected in a region of the tension band situated closer to the inner abutment element, on which the tension band is held, and so the tensioning force is maintained while the hooking-in operation is effected. On the other hand, however, it is thereby also ensured that the forces acting upon the tension band do not increase, with the result that e.g. a relative positioning of the bone plates desired by the surgeon is not destroyed by increased forces during the hooking-in operation. 
     In a variant of a form of construction the hook element or elements are disposed on the outer abutment element. The surgeon may then easily effect the fixed position of the bone plates by hooking-in of the tension band on the outer abutment element. In particular, in said case a row of spaced-apart hook elements is provided for effecting a fixation over a large surface area. 
     It may in said case be provided that the hook element or elements are disposed on an outer surface of the outer abutment element. In particular, in said case hook tips are directed away from an outer surface of the outer abutment element. 
     It may alternatively or additionally be provided that the hook element or elements are disposed in an opening for passing the at least one tension band through. In this variant, fixing of the tension band is effected in the opening. Hook tips are then orientated transversely of a direction of spacing between inner abutment element and outer abutment element. 
     In a further form of construction a fixation cap is provided for mounting on the outer abutment element, wherein the tension band is fixable between the outer abutment element and the fixation cap. It is then possible to effect fixation of the bone plates and secure this position by mounting the fixation cap. 
     It is in said case quite particularly advantageous when the fixation cap comprises a bridge element, which is insertable into the separation gap. This bridge element then effects an additional fixation in the separation gap and, on the other hand, the separation gap may be filled by the bridge element. 
     The bridge element is advantageously insertable between opposite tension band regions into the separation gap, thereby achieving an additional securing of the tensioned position of the tension band. 
     In a variant of a form of construction there are formed on the bridge element transverse tabs, which are elastically movable at right angles to the direction of spacing between inner abutment element and outer abutment element. 
     Thus, in the inserted state of the bridge element, when this is suitably adapted to the outer abutment element, an elastic force may be exerted by the bridge element upon the abutment element, which effects an additional fixing of the fixation cap on the outer abutment element, so that in turn a good securing of the fixation of the bone plates may be achieved. 
     In order to fix the tension band between the outer abutment element and the fixation cap, the fixation cap and/or the outer abutment element is provided with one or more hook elements and the outer abutment element and/or the fixation cap is provided with corresponding openings for receiving the hook element or elements. It may thereby be guaranteed that the hook elements penetrate and hence securely hold the tension band. The openings then ensure that the hook elements, which project relative to the tension band, are received. Furthermore, an additional fixing may be achieved by the engagement of the hook elements into the openings. 
     The following description of preferred forms of construction is used in connection with the drawings to explain the invention in detail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a side sectional view of a first embodiment of an implant according to the invention for fixing adjacent bone plates, wherein a fixed position is shown; 
         FIG. 2  a sectional view in direction A of the implant according to  FIG. 1 ; 
         FIG. 3  a variant of the embodiment according to  FIGS. 1 and 2  in a sectional view in the direction A; 
         FIG. 4  a sectional view of a second embodiment of an implant according to the invention for fixing adjacent bone plates; 
         FIG. 5  a sectional view of a third embodiment of an implant according to the invention for fixing adjacent bone plates and 
         FIG. 6  a plan view in direction B of the implant according to  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     By means of an implant according to the invention adjacent bone plates  10 ,  12  may be fixed. For example, for brain surgery the cranium is opened by removing one or more pieces of bone. At the end of surgery the corresponding bone pieces are reinserted, for which purpose the adjacent bone plates  10  and  12  then have to be positioned relative to one another and fixed in an appropriate final position. In said case, between the bone plates  10  and  12  a separation gap  14  is formed, through which connecting means  16  for the connection between an inner abutment element  18  and an outer abutment element  20  may be passed. If, for example, the bone plates  10  and  12  are cranial bone plates, then the inner abutment element  18  is positioned on bone plate inner sides  22  facing the cranium interior and the outer abutment element  20  is positioned on an opposite bone plate outer side  24 . 
     In the first embodiment of an implant according to the invention for fixing adjacent bone plates  10 ,  12 , which is illustrated in  FIG. 1  and denoted there as a whole by  26 , the inner abutment element  18  is of a disk-shaped design, wherein it has in particular a circular cross section. It is made of a plastics material well-tolerated by the body, an absorbable plastics material or a metal. An outer surface  28  of the inner abutment element is curved in a convex manner. An opposite surface  30  to this outer surface  28  is provided with a toothed wheel ring  32 , which is used for an improved grip on the bone plates  10  and  12 . 
     By means of the inner abutment element  18  the separation gap  14  is overlappable, so that the toothed wheel ring  32  lies in a sub-region  34   a  against the bone plate  10  and in an, in particular, opposite sub-region  34   b  against the bone plate  12 . 
     Outside of the toothed wheel ring  32  the surface  30  is set back relative to tooth tips. 
     Formed in the inner abutment element  18  in a central region are two spaced-apart slot-shaped through-openings  36 ,  38 , which extend between the surfaces  30  and  28 . They are separated by a bridge element  40 . This bridge element  40  has rounded-off edges  42  in particular facing the outer surface  28 . 
     The outer abutment element  20  is likewise of a disk-shaped design and has in particular a circular cross section, wherein the outer dimensions of the outer abutment element  20  substantially correspond to those of the inner abutment element  18 . It is, like the inner abutment element  18 , made of a plastics material well-tolerated by the body, an absorbable plastics material or metal. It likewise has a convex outer surface  44  and an opposite surface  46 , which is provided with a toothed wheel ring  48  corresponding to the toothed wheel ring  32  of the inner abutment element  18 . 
     Formed in the inner abutment element in a central region is a slot-shaped through-opening  50 , through which the connecting means  16  are passable. 
     In the outer surface  44  there is provided adjacent to the opening  50  a square recess  52 , which is used to receive a fixation cap  54 . A bridge element  56  is in said case disposed in a central region and, in particular, integrally on the fixation cap  54  and is of a wedge-shaped design at its front end  58 . This bridge element  56  may be inserted into and hence at least partially fill the separation gap  14  between the bone plates  10  and  12  and the connecting means  16  may be fixed relative to the separation gap  14  in the manner described in greater detail below. 
     The fixation cap  54  has, symmetrically relative to the bridge element  56 , opposing hook elements  60 ,  62 , which may engage into corresponding through-openings  64 ,  66  of the outer abutment element  20 . A hook element  60 ,  62  in said case has a steep flank  68  and an inclined flank  70 , so that a hook element  60 ,  62  is of a triangular design. Thus, once the fixation cap  54  has been inserted into the recess  52 , a tip  72  of a hook element  60 ,  62  points in the direction from the outer surface  44  to the surface  46 , wherein the inclined flank  70  faces the central region of the outer abutment element  20 , i.e. in particular faces the bridge element  56 , while the steep flank  68  is disposed remote from this central region. 
     The connecting means  16  are formed by a tension band  74 , which is elastically bendable. This tension band may be made of an absorbable or non-absorbable material. At the inner abutment element  18  the tension band is held in the openings  36 ,  38  on the bridge element  40  by means of a tension band bend  76 , which is adjoined in each case by a first tension band region  78  and a second tension band region  80 , which penetrate the separation gap  14 . The first tension band region  78  is deflected at the opening  50 , which for this purpose has in particular rounded-off edges, and extends substantially parallel to the outer surface  44  of the outer abutment element  20 . In a corresponding manner the second tension band region  80  is likewise deflected and likewise extends parallel to the outer surface  44  of the outer abutment element  20 . Corresponding ends of the tension band  74 , which are associated with the first tension band region  78  and the second tension band region  80 , are in said case situated remote from one another and the bridge element  56  is situated between these ends. 
     In the separation gap  14  the two tension band regions  78  and  80  are aligned substantially parallel to one another. 
     The tension band  74  is fixed relative to the inner abutment element  18  and the outer abutment element  20  in that the hook elements  60 ,  62  hook into the tension band  74  and these hook elements  60 ,  62  then engage further into the associated openings  64 ,  66  of the outer abutment element  20 . By said means a fixation of adjacent bone plates  10 ,  12  by means of the implant  26  may be achieved. 
     The implant  26  according to the invention operates in the following manner: 
     The tension band  74  is looped by means of the through-openings  36  and  38  through the inner abutment element  18 . The tension band in said case has a width B which is considerably greater than its height H. It is moreover looped through the outer abutment element  20  through the opening  50  thereof, in which case a surgeon may then grasp the respective ends of the tension band  74 . 
     The bone plates  10  and  12  and the abutment elements  18  and  20  are positioned relative to one another in such a way that, firstly, the tension band  74  with its first tension band region  78  and its second tension band region  80  penetrates the separation gap  14 , the inner abutment element  18  lies against the bone plate inner side  22  and overlaps the separation gap  14 , and the outer abutment element  20  lies against the bone plate outer side  24  and likewise overlaps the separation gap  14 . A surgeon then exerts tension upon each of the two ends of the tension band  74 , which are associated in each case with the tension band regions  78  and  80 , or holds one end fast and exerts tension upon the other end. In particular, for this purpose the tension band  74  is in each case folded over and conveyed parallel to the outer abutment element  20 , so that the surgeon may exert upon the respective ends tensile forces in opposing directions in opposite ends. By virtue of such a tightening of the band the abutment elements  18  and  20  are drawn towards one another so as to achieve a secure fixation of the bone plates  10  and  12  between the abutment elements  18  and  20 . The fixation cap  54  is then inserted into the recess  52 , wherein the bridge element  56  engages into the separation gap  14 . Thus, the tension band  74  is fixed in the separation gap  14  and the separation gap  14  is also at least partially filled by the bridge element  56 . 
     During mounting of the fixation cap  54  the hook elements  60 ,  62  simultaneously engage into the tension band  74  and form a hooked connection therewith. The hook elements  60  and  62  engage into the associated openings  64  and  66 . Consequently, the tension band  74  is fixed to the outer abutment element  20  and so, in turn, the fixed position of the bone plates  10  and  12  between the outer abutment element  20  and the inner abutment element  18  is secured. 
     The hook elements  60 ,  62  penetrate the tension band  74 , i.e. the steep flank  68  has a height that is greater than the height H of the tension band  74 . The designing of a hook element  60 ,  62  with the arrangement of the inclined flank  70  facing the opening  50  and the steep flank  68  facing the end of the tension band  74  in said case guarantees the forming of a hooked connection with simultaneous securing of the tensioned position. 
     The width B of the tension band is in the region of between ca. 25% and 75% of the diameter of the outer abutment element  20  and/or inner abutment element  18 . Firstly, this makes it easy for the surgeon to grasp the tension band. Secondly, a large surface are is provided for hooked connections by means of the hook elements  60  and  62 . 
     It may also be provided that, instead of a hook element  60  and/or  62 , a row of hook elements is provided. It may moreover alternatively be provided that the hook elements are formed on the outer abutment element  20  and the fixation cap  54  then has corresponding openings. 
     A variant of the first embodiment  26 , which is illustrated in  FIG. 3 , differs essentially in a different design of a fixation cap  82 . Otherwise, the corresponding implant is designed and operates in an identical manner to that described above. Identical elements of the implant according to  FIG. 3  therefore bear the same reference characters as for the implant  26 . 
     The fixation cap  82  comprises a round cover disk  84 , on which a bridge element  86  is integrally formed. This bridge element at its front end  88  is of a wedge-shaped design. 
     On the bridge element  86  opposite-lying transverse tabs  90 ,  92  are formed by respective slot-shaped recesses  94 ,  96 , which extend substantially at right angles to the cover disk  84 . On ends facing the cover disk  84  the transverse tabs  90 ,  92  are provided with retaining heads  98 ,  100 . 
     By virtue of the recesses  94  and  96  the transverse tabs  90  and  92  respectively are movable transversely relative to the surface normal of the cover disk  84 , i.e. in the region of the transverse tabs  90 ,  92  the width of the bridge element  86  may be reduced. This reduction of the width requires an expenditure of force for elastic deformation of the bridge element  86 . The bridge element  86  may therefore be pushed through the opening  50  into the separation gap  14 , wherein however the transverse tabs  90 ,  92  are bent elastically in the direction of the centrical axis of the cover disk  84 . Once the cover disk  84  has been mounted and hooked in the tension band  74 , the transverse tabs  90 ,  92  in the opening  50  then in turn exert a force upon the outer abutment element  20 , wherein the retaining heads  98 ,  100  ensure reliable retention. The bridge element  86  and hence the fixation cap  82  are therefore additionally, besides the engagement of the hook elements  60 ,  62  into the associated openings  64 ,  66 , braced with the outer abutment element  20 . 
     In a second embodiment, which is denoted as a whole by  102  in  FIG. 4 , the inner abutment element  18  is in principle of an identical construction to that described above. Identical reference characters are therefore used. 
     In the implant  102  an outer abutment element  104  is provided, which has a convex outer surface  106  and, remote therefrom, a surface  108  for resting against a bone plate outer side  24 . This surface  108  is provided with a toothed wheel ring  110 . 
     In a central region the outer abutment element  104  has a slot-shaped through-opening  112 , through which the tension band  74  is passable. By means of the opening  112  deflection edges  114 ,  116  are formed, which are in particular rounded off, for changing the direction of the tension band from a direction substantially parallel to the separation gap  14  to a direction at right angles thereto. By exerting tension upon the respective ends  118  and  120  of the tension band  74  and/or by holding one end fast and exerting tension upon the other end, i.e. by tightening the tension band  74 , the inner abutment element  18  and the outer abutment element  104  are drawn towards one another and the bone plates  10  and  12  situated therebetween may be securely fixed. 
     On its outer surface  106  the outer abutment element  104  has hook elements  122 ,  124 , which are disposed in particular symmetrically relative to the opening  112  and are each associated with one end  118 ,  120  of the tension band  74 . A steep flank  126  of a hook element  122  and/or  124  is in said case directed away from the opening  112 , while an inclined flank  128  faces the opening  112 . 
     After tightening of the tension band, the tension band may then be hooked into the respective hook elements  122 ,  124  in order thereby to secure the fixation of the bone plates  10  and  12  by means of the abutment elements  18  and  104 . 
     In a third embodiment, which is denoted as a whole by  130  in  FIGS. 5 and 6 , the inner abutment element  18  is likewise in principle of an identical construction to that described above. An outer abutment element  132  has a convex outer surface  134 . Remote from said surface, a surface  136  of the outer abutment element  132  is provided with a toothed wheel ring  138  in the manner described above. 
     In the outer abutment element  132  two spaced-apart slot-shaped through-openings  140  and  142  are formed in a central region and separated by a bridge element  144 . The tension band  74  is passed through these openings  140  and  142 . 
     On the outer abutment element  132  a row of hook elements  146 ,  148  is formed in the openings  140  and  142 , in each case opposite the bridge element  144 . A hook  150  of such a row in said case has a steep flank facing the outer surface  134  of the outer abutment element  132  and an inclined flank arranged facing the surface  136 . 
     Through the openings  140  and  142  respective ends of the tension band  74  are threaded, and by tightening the tension band the inner abutment element  18  and the outer abutment element  132  may be drawn towards one another and so the bone plates  10  and  12  may be fixed relative to one another by means of the abutment elements  18  and  132 . This fixed position may be secured by hooking the tension band into the hooks  150  of the hook rows  146  and  148 . 
     Given the use according to the invention of a tension band, no application instrument is required for fixing the bone plates  10 ,  12  between inner and outer abutment element: by tightening the tension band  74  a surgeon may draw the abutment elements towards one another and hence achieve a secure fixation of the bone plates  10  and  12  relative to one another. This fixed position is then secured by hooking-in of the tension band  74 , wherein the corresponding hook elements penetrate the structure of the tension band  74 . In said case, depending on the application the tension band  74  may be made of an absorbable material or a non-absorbable material.