Abstract:
An insertion instrument for a pair of sliding prostheses has, on a support plate, two clamps which are arranged at a predetermined spacing from one another and receive the sliding prostheses. The clamps are connected to the support plate via a releasable coupling and may be provided in any position in the lateromedial (LM) direction, i.e. the clamps can adopt different positions in the LM direction. They are clamped securely in the respectively chosen position by a fixing device.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   As is known, the human knee-joint comprises two pairs of tibio-femoral articular surfaces, namely a medial pair of articular surfaces and a lateral pair of articular surfaces, which are in each case formed by a femoral condyle and by a dish-shaped tibial articular surface interacting with said femoral condyle. It is known (EP-A-1099430) to replace the articular surfaces of the femoral condyles individually by a slide prosthesis and, for this purpose, to use an insertion instrument which guarantees the exact mutual alignment of the prostheses. The prostheses are held on the instrument by means of clamps which are secured on a support plate of the instrument via a releasable coupling. As soon as the prostheses have reached the desired seat in the operation, the coupling is released and the instrument is withdrawn with the support plate from the clamps which remain on the prostheses until the cement used for securing said prostheses has hardened. The releasable coupling consists of a plug on each clamp and of two receiving openings in the support plate, into which openings these plugs can be inserted with an exact fit. They are secured therein by means of screws which remain accessible during the operation so that they can be undone when the instrument is to be removed from the clamps. To be able to insert prostheses at a different predetermined spacing, different support plates are provided in which the receiving openings for the clamp plugs are at a different, predetermined spacing from one another. This has the disadvantage that a large number of different instruments with different support plates have to be kept in stock. Moreover, there are occasions when the operating surgeon wishes to change the spacing of the prostheses, on the insertion instrument being used, during the actual operation. For this reason, another known instrument was created (EP-A-1321116) in which a guide is provided in the support plate for the plugs provided on the clamps, in which guide the plugs and thus the clamps can be moved in the LM direction (LM=lateral-medial) by means of a threaded spindle. This achieved the objective of being able to adjust the spacing during the operation; however, it also had the disadvantage that the clamps are permanently anchored on the support plate by the threaded spindle and that the instrument can be removed from the implanted prostheses only if the clamps themselves are opened, which is not particularly easy. 
   SUMMARY OF THE INVENTION 
   Therefore, the object of the invention is to make available an insertion instrument for sliding prostheses which is of the type first mentioned above and which permits different spacings of the clamps and allows them to be separated from the instrument during the operation. 
   The solution according to the invention lies in the features of the invention as disclosed hereinafter. Accordingly, the coupling between the claims and the support plate is provided in any the clamps and the position in the LM direction, at least on one of the two clamps, so that the clamp in question can be set to a different spacing from the other clamp, and a fixing means is provided for fixing the clamp in the chosen position. 
   The coupling provided in any position in the LM direction can, like the known solutions, consist of a plug on the clamp and of an elongate hole arranged in the support plate and receiving the plug. The clamp is guided on the support plate in such a way that it is allowed only a lateral movement of adjustment (in the LM direction). 
   Both clamps are expediently connected to the support plate via in each case a coupling provided in any position in the LM direction and can be fixed in the chosen position. This is intended to ensure that the two clamps can each be arranged symmetrically with respect to the instrument axis. 
   For fixing the clamps in the chosen position, the invention provides for a fixing yoke whose ends, transverse to the adjustment direction and release direction of the couplings, press on a coupling part connected to the clamps, namely preferably on the coupling plug. The fixing yoke is connected centrally to the support plate by a pressing means, for example a screw, which can be actuated during the operation. In this way, it is possible to release the coupling during the operation and, if appropriate, close it again after changing the spacing of the clamps. It is not necessary for the fixing yoke to act on the coupling part, which is to be held, exactly perpendicular to the adjustment and release directions. However, this is generally advantageous. 
   The fixing yoke is, like the coupling, expediently provided in any position in the LM direction, in order to be able to secure the clamps at any desired spacing. If one wishes to permit only certain spacings, for example ones which are in accord with certain tibial prosthesis parts (see, for example, EP-A-1099430), the fixing yoke can also be designed such that it can act on the coupling parts of the clamp plugs only at the desired spacings. For example, the yoke can be provided with projections or indents which match recesses or projections on the plugs. It is also possible to provide several exchangeable fixing yokes, of which each one can interact with the associated clamp plugs only at a predetermined clamp spacing and thus acts as spacing gauge. Finally, there is also the possibility of providing a spacing gauge in addition to the fixing yoke provided in any position in the LM direction. The spacing gauge is on the one hand used for fixing on the support plate at a predetermined position. On the other hand, it has recesses or projections interacting with the clamp plugs or parts thereof to define position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in greater detail below with reference to the drawing which depicts an advantageous illustrative embodiment and in which: 
       FIG. 1  shows an overall view of the instrument, 
       FIG. 2  shows the main parts of the instrument head in a partially exploded view, 
       FIG. 3  shows the fixing yoke, and 
       FIG. 4  shows a partial section through the instrument. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The sliding prostheses  5  are shown obliquely from the dorsal-medial direction in  FIG. 1  and obliquely from the ventral direction in  FIG. 2 . The same direction designations are also applied below to the instrument. 
   The support plate  2  of the instrument head  3  is secured on a handle with grip  1 . The support plate  2  supports two clamps  4  for two sliding prostheses  5 , and two slide-surface supports  6 . 
   The slide-surface supports  6  consist of a threaded shaft, mounted in the support plate  2 , and of a head with an impact cushion of rigidly elastic plastic. By turning them, they can be adjusted so that their top face bears with a supporting action on the slide surface of the prosthesis part located in the associated clamp  4 . 
   Each clamp  4  comprises a first clamping jaw  10  and a second clamping jaw  11 . The foot  12  of the first clamping jaw  10  sits fully on the top face of the support plate  2  and is held by it in a manner which will be explained later. The second clamping jaw  11  is held by the foot  12  of the first clamping jaw  10  so as to be displaceable in parallel. The spacing of the clamping jaws  10 ,  11  from one another can be adjusted by means of a threaded spindle  16 . 
   Both clamping jaws  10 ,  11  carry projections which interact with corresponding recesses of the prostheses  5  when the latter are inserted into the clamps  4 . In this way, the prostheses are positioned in a defined manner relative to the instrument. Details of this design and of this mode of operation can be found in EP-A-1099430. 
   Protruding from the underside of each clamp foot  12  there is a plug  13  of rectangular profile which at its lower end carries a small cylindrical attachment  14 . The support plate  2  comprises two elongate holes  15  into which a rectangular plug  13  fits exactly in each case. The elongate holes  15  and the plugs  13  form interacting guides in the LM direction of the instrument. Each clamp  4  can therefore adopt different, positions along its line of displacement in the LM direction on the support plate  2 . The rectangular plugs  13  and the elongate holes  15  form the abovementioned releasable coupling between the clamps  4  and the support plate  2 . 
   In the assembled state, the plug  13  of rectangular profile is accommodated completely by the associated elongate hole  15 . Only the small cylindrical attachment  14  protrudes from the underside of the support plate  2 . It interacts with in each case one of two bores  17  in a spacing gauge  18  which can be secured on the underside of the support plate  2 . At one end, it has a plug  19  which can be inserted into a matching bore  20  of the support plate, and at the other end it has a laterally open bore  21  which interacts with a screw  23  in a threaded bore  24  of the support plate. Several spacing gauges  18  are made available whose symmetrically arranged bores  17  have a different spacing from one another. 
   On the dorsal face of the support plate  2 , the latter is provided with elongate cutouts  25  in the area of the elongate holes  15 . These cutouts  25  correspond to projections  26  on a fixing yoke  27  which is applied on the dorsal face of the support plate  2  with projections  26  protruding into the cutouts  25 . It is held thereon by means of a threaded screw  30  which passes through the bore  28  of the support plate and engages in the centrally arranged threaded bore  29  of the fixing yoke  27  and whose head is accessible on the ventral face of the support plate  2 . When the fixing yoke  27  is drawn onto the rear face of the supporting plate by means of the screw  30 , its projections  26  pass through the cutouts  25  and into the elongate holes  15  (see  FIG. 4 ) and press against the rectangular plugs  13  of the clamps  4  present there, by which means these clamps are fixed in their respective position. The rectangular plugs  13  are provided, on the dorsal face, with a transversely extending, shallow groove  31  into which the front of the projections  26  of the fixing yoke  27  engages. 
   The instrument is used in the following way. Depending on the desired prosthesis spacing, a spacing gauge  18  is selected and is connected to the underside of the support plate  2 . The fixing yoke  27  is loosely fastened on the rear face of the support plate  2  by means of the screw  30  so that its projections  26  pass into the cutouts  25  but not yet into the area of the elongate holes  15 . The clamps  4  are then fitted so that their rectangular plugs  13  lie in the elongate holes  15  and their small cylindrical attachments  14  lie in the bores  17  of the spacing gauge  18 . The fixing yoke  27  is then drawn tight with the screw  30 . The clamps are now held secure on the top face of the support plate and at the desired spacing. Thereafter, the prostheses  5  are inserted into the clamps  4 . If so desired, this can also be done before the clamps  4  are connected to the support plate  2 . 
   If one wishes to change the spacing of the prostheses, the spacing gauge  18  is removed and the screws  30  are carefully loosened so that the clamps  4  can be moved in the LM direction on the support plate  2 . If the projections  26  still engage in the groove  31 , they are secured against falling out. Another spacing gauge  18  can now be connected to the support plate, the clamps  4  at the same time being moved so that their cylindrical attachments  14  engage in the newly positioned holes  17  of the spacing gauge  18 . Thereafter, the fixing yoke  27  is tightened again. 
   After the prostheses have attained the desired position on the patient by means of the instrument, the coupling of the instrument between the clamps  4  and the support plate  2  is released. For this purpose, the screw  30  is loosened so that the instrument with the support plate  2  can be withdrawn from the clamps in the caudal direction. This can be done before any cement that is used has hardened.