Abstract:
In an implant for fixing neighboring bone plates of the cranial bone comprising an inner bearing element which covers a spacing gap between the bone plates, an outer bearing element which covers the spacing gap and a connecting device which penetrates the spacing gap and which, when said bearing elements approach one another, connects the bearing elements together by means of a latching or a clamping connection in such a manner that they are no longer able to be moved apart, it is proposed that the two bearing elements be additionally connected by a thread-like tensioning element which is passed through the outer bearing element in displaceable manner and, when tensioned, moves the inner bearing element towards the outer bearing element in order to simplify the process of applying the implant and to ensure a secure connection of the bearing elements.

Description:
The present disclosure relates to the subject matter disclosed in international application PCT/EP02/05653 of May 23, 2002 and disclosed in German application No. 101 28 917.0 of Jun. 15, 2001, 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 neighbouring bone plates of the cranial bone, wherein said plates have an inner surface and an outer surface and the implant comprises an inner bearing element which covers a spacing gap between the bone plates, an outer bearing element which covers the spacing gap, and a connecting device which extends through the spacing gap and which, when the bearing elements approach one another, connects said bearing elements together by means of a latching or a clamping connection in such a manner that they are no longer able to be moved apart. 
     Implants of this type are known from the German utility model 29919090 for example. They are used in order to relocate bone plates in their original position within the cranial bone after they have been separated therefrom by means of a saw cut so that they can graft back in this position. 
     For this purpose, it is known to arrange plate-like bearing elements on both sides of the bone plates, which are only separated from one another by a spacing gap, and to connect them together by means of a central pin or a central locking strap. Moreover, rather than utilising latching or clamping means of this type, it is also known to simply draw the two plate-like bearing elements together by means of a thread-like tensioning element and to keep them clamped against one another by means of this thread-like tensioning element. 
     When using pin-like connecting means, it is necessary to clamp the two bearing elements together by means of a suitable tool which engages with the connecting device, which penetrates the outer bearing element and which simultaneously moves the outer bearing element along the connecting device in the direction of the inner bearing element. Such instruments are of complicated construction and are not always easy to manipulate. 
     If one uses a thread-like tensioning element for connecting the two bearing elements, then it is relatively easy to manipulate it when tensioning the bearing elements, but the result of this is that the connection between the bearing elements is very flexible and may sometimes give way in an undesirable manner in certain circumstances, for example, if the thread-like tensioning elements should work loose or if they dissolve in the case where a thread of resorbable material is being used. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an implant is provided with that, on the one hand, the application of the bearing elements and the process of clamping the bearing elements is executable in as simple a manner as possible whilst, on the other hand, it is intended that the two bearing elements should experience a reliable and durable connection when in the applied state. 
     In accordance with the invention, this is achieved in that the two bearing elements are additionally connected by means of a thread-like tensioning element which is passed through the outer bearing element in displaceable manner and, when tensioned, moves the inner bearing element towards the outer bearing element. 
     Thus, it is envisaged that the implant be provided with connecting means which connect the two bearing elements together in a durable and secure manner after they have been brought close together, and also thread-like tensioning elements, whose task consists essentially in tensioning the two bearing elements together during the application process so that the connecting means can thereby come into engagement. 
     The thread-like tensioning element can either be removed after the application process, or, it may be retained as an additional securing device for the two bearing elements, for example, the ends of such a thread-like tensioning element could be knotted together. 
     In a first preferred embodiment, provision is made for the tensioning element to be located at the inner bearing element so that the inner bearing element can then be pressed against the inner surfaces of the two bone plates by means of the tensioning element whilst the outer bearing element is simultaneously moved towards the inner bearing element in a latching or clamping manner. 
     Particularly preferred however is an exemplary embodiment wherein provision is made for the tensioning element to be passed through two openings in the inner bearing element and then be extended over the outer surface of the inner bearing element that is remote from the outer bearing element in the area between the openings. The tensioning element is thus guided in the form of a U-shaped loop, whereby the two ends of the tensioning element are passed through openings in the inner bearing element and, from there, extend through the spacing gap to the outer bearing element. It is expedient thereby, if the two ends of the tensioning element are passed through a respective opening in the outer bearing element. 
     It is expedient, if the two openings are spaced from one another by a distance which corresponds to at least half the elongation of the inner bearing element in the direction of interconnection. Consequently, the two ends of the tensioning element will engage with opposite outer surfaces of the inner bearing element, the bearing element thereby inevitably being aligned in such a way that the direction of connection of the two openings in the inner bearing element for the two ends of the tensioning element will extend in parallel with the direction of the spacing gap. 
     It is particularly advantageous, if the connecting device is arranged between the two sections of the tensioning element extending between the bearing elements. Due to the fact that the two ends of the tensioning element are fed between the two bearing elements such that they are mutually spaced, sufficient space will remain therebetween for arranging the connecting device there, for example, centrally. 
     Furthermore, it is advantageous if the dimensions of the connecting device in a direction transverse to the plane spanned by the sections of the tensioning element extending between the two bearing elements is small so as to enable said connecting means to pass through the spacing gap. The connecting device and the tensioning means are thus located essentially in one plane and extend only in this plane and as little as possible in the direction transverse to this plane so that the elements described can easily be passed through even a very narrow spacing gap, it thus being possible to move the bone plates that are to be connected relatively close to one another. 
     In a particularly preferred embodiment, provision is made for the tensioning element to be fed through the inner bearing element from the outer surface thereof in the form of a loop which extends through two mutually spaced openings in the outer bearing element so that, between these openings, the loop will rest upon the outer surface of the outer bearing element which is remote from the inner bearing element. As a result of this form of guidance of the thread, there will be a total of four sections of thread between the two bearing elements, i.e. a kind of pulley-block will be formed so that a displacement of the bearing elements corresponds to a shortening of the thread-like tensioning element, this accordingly being effected in a step-down ratio, although with greater power. Hereby, provision may be made for the openings for the loop and the openings for the ends of the tensioning element in the outer bearing element to lie along a straight line, and in particular, these openings are arranged to be mutually equidistant. 
     It is also advantageous hereby, if the loop extends through two mutually spaced openings in the inner bearing element. These openings too could also lie along a straight line together with the openings for the ends of the tensioning element and, in particular, be arranged such that they are mutually equidistant. 
     A particularly expedient arrangement, is one wherein the sections of the tensioning element extending between the bearing elements, the loop and the connecting device are arranged in one plane. 
     In a particularly preferred embodiment, the tensioning element may be freely displaceable in relation to both bearing elements and therefore adapted to be withdrawn from both bearing elements after they have been connected together by the connecting device. 
     It is also expedient, if the tensioning element consists of a material which is absorbable in the body. 
     In a preferred embodiment of the invention, provision is made for the connecting device to be arranged in the area between the bearing elements and not to project outwardly therethrough. Due to the use of the flexible clamping means, it then becomes possible to reliably clamp the bearing elements together even when the connecting device does not project outwardly through the outer bearing element. 
     For example, latching projections may be arranged on both bearing elements on the inner surfaces thereof facing the other bearing element, said latching projections engaging behind one another in latching manner when the bearing elements approach one another. 
     It is advantageous thereby, if two latching elements are arranged on a bearing element such as to be mutually adjacent, and if a latching element on the other bearing element engages between said adjacent latching elements in latching manner when the bearing elements approach one another. 
     A step-by-step latching process and thus adaptation to the thickness of the bone plates that are to be fixed together thereby become possible if at least one of the latching elements carries a plurality of latching projections. 
     In the case of another preferred embodiment, provision is made for at least two mutually adjacent connecting devices to be arranged on the bearing elements, said connecting devices having very small dimensions in a direction transverse to the plane defined thereby. 
     Thus here, in contrast to the known bearing elements which generally use a central connecting device, at least two such neighbouring connecting devices are used on the bearing elements, said connecting devices spanning a plane, and this plane passing through the spacing gap when the bearing elements have been applied. Due to the use of two connecting devices, the bearing element will be oriented relative to the bone plates in such a manner that this plane will extend in parallel relative to the spacing gap, it also being possible to fix the bearing elements in a non-parallel arrangement should this be necessary, i.e. in the event that this should be required due to different thicknesses of the bone plates that are to be connected. 
     Such an implant comprising two bearing elements that are connected to one another by two mutually adjacent connecting devices can also be used without the previously described thread-like tensioning means, the scope of protection extending expressly also to such an embodiment. 
     It is expedient, if the connecting devices comprise latching or clamping pins which are fixed to the inner bearing element and penetrate latching or clamping openings in the outer bearing element. 
     These latching or clamping openings may carry latching projections which co-operate with latching projections on the latching or clamping pins. 
     In another embodiment, provision is made for the latching or clamping openings to comprise resiliently deformable clamping members which co-operate with the surface of the latching or clamping pins. For example, these clamping members may be springy tongues which are bent out from the plane of the bearing elements when the latching or clamping pins are pushed through the latching or clamping openings, and the edges thereof rest on the surface of the latching or clamping pins in such a way that a relative displacement of the latching or clamping pins with respect to the bearing element is only possible in one direction. 
     The bearing elements and the connecting means are preferably manufactured in one-piece from a synthetic material. 
     The following description of the preferred embodiments serves, in conjunction with the drawing, to provide a more detailed explanation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : a side view of a first preferred exemplary embodiment of an implant incorporating a U-shaped thread-like tensioning element and having latching projections directed towards the respective other bearing element prior to the bearing elements being brought together; 
         FIG. 2 : a sectional view along the line  2 — 2  in  FIG. 1 ; 
         FIG. 3 : a view of the implant of  FIG. 1  in the direction of the arrow A in  FIG. 1  after the application of the implant to two bone plates; 
         FIG. 4 : a view similar to  FIG. 1  of a further preferred exemplary embodiment of an implant comprising two mutually spaced connecting devices and a thread-like tensioning element in a multiple loop arrangement and 
         FIG. 5 : a view of the implant of  FIG. 4  in the direction of the arrow B in  FIG. 4  after the application of the implant to two bone plates. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The implant  1  illustrated in  FIGS. 1 to 3  incorporates two circular, plate-like bearing elements  2 ,  3  which comprise a serrated boundary region  5  on the flat, mutually facing inner faces  4  thereof and an external surface  6  which is slightly domed in the outward direction. 
     The inner bearing element  2  carries a central latching projection  7  which points towards the outer bearing element  3  and has a tip  8  with inclined glide surfaces  9 . This latching projection  7  has an elongated rectangular cross section and the narrow sides thereof carry a number of latches  10 , whilst the broad sides thereof are smoothly formed. 
     The outer bearing element  3  carries two mutually spaced latching projections  11  which point towards the inner bearing element  2  and are likewise provided with a number of latches  12  on the mutually facing narrow sides. These narrow sides incorporating the latches  12  are spaced from one another by a distance which corresponds to the width of the latching projections  7  on the inner bearing element  2 . When the two bearing elements  2 ,  3  are moved close to one another, the free ends of the latching projections  11  slide on the inclined glide surfaces  9  of the latching projections  7  and are slightly flexed apart so that the latches  12  can slide past the latches  10 . Hereby, the shaping of the latches  10  and  12  is selected in such a way that although it is possible for the bearing elements  2  and  3  to come close together, the two bearing elements  2 ,  3  will not be able to separate from each other following the engagement of the latches  10  and  12 . 
     The latching projections  11  are also of rectangular cross-section and are provided with latch-free broad sides and narrow sides which carry the latches  12  in like manner to the latching projection  7 . The latching projections  7  and  11  thus extend substantially in a plane which runs parallel to the broad sides of the latching projections  7  and  11 , but they are of very small dimensions in the transverse direction, as is clear from the illustration in  FIG. 3 . 
     In this plane spanned by the latching projections  7  and  11 , there are provided two openings  13  and  14  which penetrate the respective bearing elements, both the inner bearing element  2  and the outer bearing element  3 , and which are mutually spaced by a distance which corresponds to at least the radius of the bearing elements  2 ,  3  and which are arranged symmetrically relative to the two sides of the respective latching projections  7  and  11 . A thread-like tensioning element  15  is fed through these openings  13 ,  14  in such a manner that a central section  16  will rest against the outer surface  6  of the bearing element  2  between the openings  13  in the inner bearing element  2 , whilst two connecting sections  17  will extend in parallel with the latching projection  7  from each opening  13  in the inner bearing element  2  to each opening  14  in the outer bearing element  3 , the two ends  18  of the tensioning element  15  then extending through the openings  14  and continuing over the outer surface  6  of the outer bearing element  3  ( FIG. 1 ). 
     The two bearing elements  2  and  3  preferably consist of a body-compatible synthetic material, whilst the latching projections  7  and  10  are formed in one-piece with the respective bearing elements  2  and  3 . The thread-like tensioning element  15  may be a conventional surgical thread, it being expedient if it consists of an absorbable material which gradually decomposes after insertion into the body. 
     The described implant  1  serves for fixing two bone plates  19 ,  20  of the cranial bone which are to be fixed directly adjacent one another whilst leaving a spacing gap  21  therebetween. The spacing gap  21  is usually a saw-slot which is formed as a result of a bone plate being sawn out from the cranial bone. Consequently, one of the two bone plates is usually the solid bone plate of the skull, whilst the other bone plate is a sawn-out cranial bone which allowed access to be made to the brain. As is apparent from  FIG. 1 , for the purposes of fixing the two bone plates to one another, the implant  1  is firstly advanced towards a bone plate in such a way that the inner bearing element  2  will come to rest against the inner surface of the bone plate. The other bone plate is then introduced laterally into the intermediary space between the two bearing elements  2 ,  3  which are still spaced from one another, and the inner bearing element  2  is then clamped against the inner faces of the two bone plates  19 ,  20  with the help of the tensioning element  15 . When the inner bearing element  2  is fixed in position, the outer bearing element  3  is pushed towards the inner bearing element  2  until such time as the respective latches  10  and  12  of the two latching projections  7  and  11  come into engagement with one another. This process of advancing the outer bearing element  3  can either be effected manually or by utilising an instrument, whereby the inner bearing element  2  will be held in place on the inner surface of the bone plates with the help of the tensioning element  15 , or else by virtue of the two ends of the tensioning element  15  being drawn together, for example, by means of a known tensioning knot which is adapted to effect a shortening of the tensioning element  15  in the form of a loop extending through the openings  13  and  14 , but not however, a prolongation thereof. 
     The latching projections  7  and  10  lie in the same plane as that in which the connecting sections  17  of the tensioning element  15  also extend, the dimensions of each of the parts in a direction transverse to this plane being very small so that these parts will easily fit into even a very narrow spacing gap  21 , as is clear from the illustration in  FIG. 3 . At the same time, the latching projections  7  and  11  as well as the central section  17  of the tensioning element  15  will orient the two bearing elements  2 ,  3  in such a way that this plane runs parallel to the spacing gap  21 . 
     If the two bearing elements  2 ,  3  are clamped together in this way, then they are securely held in this seating position, which is illustrated in  FIG. 3 , by the engagement of the latches  10  and  12 . The tensioning element  15  can then either be removed by simply pulling it out of the openings  13 ,  14 , or, it may be left in place as an additional security device, whereby it is expedient to knot the two ends  18  to one another. 
     In the case of the implant illustrated in  FIGS. 1 to 3 , the connecting means of the two bearing elements  2 ,  3  formed by the latching projections  7  and  11  do not project therethrough, but rather, they merely extend through the space in the spacing gap  21 . 
     This is not the case in the exemplary embodiment of an implant illustrated in  FIGS. 4 and 5 , although this is of similar construction and the mutually corresponding parts thereof bear the same reference symbols. In the case of the implant in  FIGS. 4 and 5 , the inner bearing element  2  has two mutually spaced, parallel connecting pins  22  which extend symmetrically relative to the centre point and protrude through apertures  23  in the outer bearing element  3 . The connecting pins  22  carry peripheral latching ribs  24  which are bevelled at one side and can be pushed through the apertures  23  in one direction, but, in the opposite direction, the latching ribs engage and latch behind the edges of the apertures  23  in such a manner that it is no longer possible to pull the bearing elements  2  and  3  apart. The edges of the apertures  23  are thus latching projections which co-operate with the latching ribs  24 . 
     The connecting pins  22  are arranged in the plane which is defined by the openings  13  and  14  for the tensioning element  15 , and they are of very small diameter so that the connecting pins  22  exhibit only a very small dimension in a direction transverse to this plane and thus this implant  1  too, together with the tensioning element  15  and the two connecting pins  22 , can also be fitted into a very narrow spacing gap  21 . 
     Moreover, in the case of the exemplary embodiment of  FIGS. 4 and 5 , the central area of the tensioning element  15  is in the form of an additional loop  25 , thus deviating from the shape of the implant depicted in  FIGS. 1 to 3 . Namely, this loop  25  passes through two openings  26  located between the openings  13  in the inner bearing element  2  and through two openings  27  arranged between the openings  14  in the outer bearing element  3 , so that the central section  28  of this loop  25  rests on the outer surface  6  of the outer bearing element  3 . The openings  26  and  27  are thereby located closer to the centre of the respective bearing elements  2  and  3  than the apertures  23  so that two further connecting elements  29  now extend between the two connecting pins  22  in addition to the connecting sections  17  of the tensioning element  15 , these forming the legs of the loop  25  and lying in the same plane as the connecting sections  17  and the connecting pins  22 . This thereby results in a pulley-block-like guidance of the tensioning element  15  so that, when tensioning the latter, the displacement path of the bearing elements  2 ,  3  will be scaled down with respect to the tensioning path of the tensioning element  15 . 
     This guidance of the tensioning element  15  is particularly advantageous in an arrangement comprising two mutually spaced connecting means such as is formed by the connecting pins  22 , but use could also easily be made of differently constructed bearing elements  2 ,  3  wherein other forms of connecting means are provided. 
     It is also expedient in the case of the exemplary embodiment depicted in  FIGS. 4 and 5 , if the connecting pins  22  are formed in one-piece with the bearing elements  2 ,  3 , whereby these preferably consist of a body-compatible synthetic material. Here too, the tensioning element  15  may be made of an absorbable material.