Abstract:
A knee prosthesis including a femoral component, a tibial component and an intermediate piece which connects these forming a flexion bearing with the femoral component and a rotation bearing with the tibial component, which rotation bearing can be locked relative to the tibial part. To allow the physician to decide during the operation whether to use a rotation-free prosthesis or a rotationally fixed prosthesis, the intermediate piece is alternatively provided with a projection which engages in a matching recess in the tibial component.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The poorer the state of preservation of the ligament apparatus of a knee which is to be provided with a knee prosthesis, the greater inherent stability demanded of the prosthesis. If it is a ligament apparatus of reduced stability, it may be necessary to use a prosthesis type in which the relative movement between femoral component and tibial component is limited by a stabilizing intermediate part to the flexion movement about a horizontal transverse axis and to a rotation movement about an axis substantially parallel to the tibia (DE-C-2 744 710). For this, it is required that the ligament apparatus can at least still control the rotation movements. If this ability too has been lost, the rotation movement must be excluded and the degree of freedom of the prosthesis must be limited to the flexion movement. Different prosthesis structures are normally used for these two cases of application. If the decision whether to choose a rotation-free or rotationally fixed prosthesis is to be made only during the operation, a prosthesis system can be used whose components are partly exchangeable (EP-B-539 654), with one matching femoral component and different tibial components being provided. Finally, it has also already been proposed (DE-A-2 636 816) to use for both cases a rotation-free prosthesis in which a fixing device can be arranged which excludes the rotation movement if so desired. This is intended to be done by drilling a hole through the bone and the rotation bearing after implantation and then introducing a fixing pin into the hole to prevent turning of the rotation bearing. This is obviously impracticable. Instead, a fixing pin can also be guided exclusively inside the prosthesis through a rotation axial bearing and prevent its turning. It has not been disclosed whether this proposal has ever been adopted in practice. In fact, there are a great many practical difficulties against it. One factor may have been that, compared to a prosthesis which exclusively includes a flexion hinge, it is relatively expensive to use a prosthesis additionally provided with a rotation bearing and then to lock this. 
   SUMMARY OF THE INVENTION 
   Nevertheless, in endeavouring to make available a prosthesis which permits a simple intraoperative decision between the rotation-free and the rotationally fixed version, the invention follows on from the latter proposal and makes it suitable in practical terms by means of the features of the invention of this application. Accordingly, the intermediate part which connects the femoral component to the tibial component, and forms the rotation bearing with the tibial component, is provided with a projection which engages in a matching recess in the tibial component. 
   This concept is advantageously realized by the fact that the prosthesis system comprises an alternative intermediate part (or component of the intermediate part) with or without projection, this projection being connected permanently to the intermediate part (or the component of the intermediate part). Thus, after implantation both of the femoral component and of the tibial component, the physician is given the possibility, by alternative selection of the projection-carrying part, of making a decision in favour of one or other prosthesis version and possibly also of trying out both versions. Instead of this, it is also possible to equip the intermediate part (or its component) with a connecting device for alternative connection to the projection. 
   If the tibial component has a support plate with at least one depression for receiving a tibial plateau, the projection is expediently designed to cooperate with this depression. Since this depression is of sizeable design, it permits a very stable rotational connection. The tibial component can be used without any adjustment in both versions of the prosthesis. In this embodiment, the projection is expediently formed by a plate which fits into the depression and which in itself and/or with the borders of the depression forms cutouts for receiving parts of a tibial plateau. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below with reference to the drawing which depicts an advantageous illustrative embodiment. In the drawing: 
       FIG. 1  shows a dorsal view of the prosthesis, 
       FIG. 2  shows a perspective view of the tibial component with the intermediate part, 
       FIG. 3  shows a perspective top view of the tibial plateau, 
       FIG. 4  shows a perspective view of the support plate of the tibial component, 
       FIG. 5  shows a perspective bottom view of a normal tibial plateau, 
       FIG. 6  shows a perspective view of a normal intermediate part, 
       FIG. 7  shows a perspective view of an intermediate part which prevents rotation, 
       FIG. 8  shows a perspective view of the tibial component with a fitted intermediate part which inhibits rotation, and 
       FIG. 9  shows a perspective bottom view of a tibial plateau to be used in connection with an intermediate part which prevents rotation. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The prosthesis has a femoral component  1  with a stem  2  intended for implantation in the femoral bone, and a tibial component with a stem  4  intended for implantation in the tibia. The femoral component has wing-like runners  5  lying on a tibial plateau  6  which promotes sliding and which is supported by a support plate  7  of the tibial component. To stabilize the two components relative to one another, an intermediate piece  10  is provided which, with the femoral component, forms a hinge indicated in  FIG. 1  by an axis line  11 .  FIG. 2  shows an axis stump  12  which protrudes from the intermediate piece  10  and which cooperates with bearing bores (not shown) in the femoral component  1 . It sits in a bore  13  of the intermediate part. 
   The intermediate piece  10  further forms a rotation bearing with the tibial component, namely by the rod  14  which is connected integrally to the intermediate part and which is mounted in a bearing bore  15  in the tibial component. The bearing bore  15  extends substantially parallel to the direction of the tibia which can be identified from the stem  4  of the prosthesis. The intermediate piece  10  thus limits the relative movement between the femoral component and tibial component to the flexion movement about the hinge axis  11  and to the rotation movement about the axis of the rotation bearing  14 ,  15 . The intermediate piece  10  can also be equipped with a collar  16  which, by cooperating with a step  17  of the tibial plateau  6 , prevents undesired lifting of the femoral component from the tibial component. 
   The tibial plateau  6  is made of polyethylene, for example, and on its upper side is shaped in a known manner in such a way that in the event of a rotation the femoral component is lifted slightly in relation to the tibial component, so that a restoring force is generated which seeks to return it to the neutral central position under the transmitted load. 
   The tibial plateau  6  is supported by the support plate  7  preferably over its entire surface. It has on the underside a horseshoe-shaped projection  20  whose height and contour are designed to match those of a horseshoe-shaped depression  21  in the support plate  7 . The correct setting of the tibial plateau on the support plate is ensured by the interaction of the projection  20  and of the depression  21 . By way of a screw hole  22  in the tibial plateau, the latter can be secured in the assembled position in the support plate by means of a screw (not shown) and a threaded bore  23 . The support plate  7  is provided with threaded bores  28  and the locking plate  25  is provided at corresponding positions with screw holes  29  which allow the locking plate to be screwed rigidly to the tibial component. 
   The intermediate piece in the normal embodiment has the configuration shown in  FIG. 6 . Alternatively, the intermediate piece  10 ′ according to  FIG. 7  can be used which differs from the normal intermediate piece  10  in that it is connected rigidly to a locking plate  25  whose contour is an exact match of that of the depression  21  in the support plate  7 . When the intermediate piece  10 ′ is fitted into the tibial component  3 , the locking plate  25  fills the depression  21  substantially completely. Its surface lies at the same height as the edge  24  of the support plate  7  surrounding the depression  21 . This results in a continuously plane contact surface for the tibial plateau  6 ′. In order to secure the latter in the correct position, it is provided with projections  20 ′ which take the place of the horseshoe-shaped projection  20  of the normal tibial plateau  6  and are designed to fit in cutouts  26  in the locking plate  25 . In the assembled state, the projections  20 ′ engage in the cutouts  26  and thereby secure the position of the tibial plateau. Moreover, like the normal tibial plateau, it can be secured on the threaded bore  23  of the support plate  7  by means of a screw passing through the bore  22  of the plateau and a bore  27  of the locking plate. 
   The locking plate  25  is secured on the intermediate piece  10 ′ at such a height that the distance between the locking plate  25  and the hinge axis  11 ′ of the intermediate piece exactly matches the position of the hinge axis  11  defined by the height of the runners  5  and of the plateau  6 . In other words, the flexion axis  11  set by the runners  5  and the plateau  6  is intended to exactly correspond to the axis  11 ′ of the intermediate part  10 ′. This is intended to ensure that most of the load is transmitted onwards via the large-area contact between runners  5  and the tibial plateau  6 . If it is desired that no force be transmitted via the intermediate piece, the locking plate  25  is connected to the intermediate piece  10 ′ not rigidly, but instead axially displaceably, although not pivotably. This can be done, for example, by providing the collar  16  with bores and by the locking plate  25  having pins which are parallel to the rod  4  and which are guided through these bores. The locking plate  25  can then move in the axial direction relative to the intermediate piece  10 ′, but is connected to it in a rotationally fixed manner via the pins and bores. 
   Further details of the prosthesis configured according to the invention are to be found in the applications filed at the same time by the same Applicant and bearing the administrative file references: LINO515PEP and LINO516PEP.