DE4425529A1 - System for constructing knee joint endoprosthesis - Google Patents

Abstract

At the lower end of the femur, joint components (10) are fixable with a convexly curved joint bearing surface and at the upper end of the tibia (11) are fixable bearing components (12). Between the femoral joint and tibial bearing components is a movably arranged meniscus (14) with upper and lower side slide surfaces which are formed complementarily to the joint bearing surface of the femoral joint component and the bearing surface of the tibial bearing component. The peripheral or sagittal outer limit (16) of the slide surfaces (15) of the meniscus facing the femoral joint components is higher than the central or sagittal inner limit.

Description

The invention relates to a system for training a Knee joint endoprosthesis with

• - Can be attached to the lower end of the thigh (femur) Joint elements with convexly curved joint bearing surface,
• - Can be attached to the upper end of the tibia (tibia) Bearing elements, and with
• - One each between the femoral and the tibial Bearing elements movably arranged on the meniscus Top and bottom trained sliding surfaces, which is complementary to the associated spherical bearing surface of the femoral joint element or the associated Storage area of the tibial bearing element are designed.

A knee joint endoprosthesis with the aforementioned features is known from DE 25 50 704 C2. The proposed one Knee-joint endoprosthesis is designed so that it Geometry of the "natural" knee joint and its Storage areas largely imitated. Between the joint and the bearing element is a movable meniscus element provided its arranged on the top and bottom Sliding surfaces a complementary design to the one hand Spherical bearing surface of the femoral joint element and on the other hand to the bearing surface of the tibial bearing element exhibit. With such a structure, the natural Game of muscles and ligaments disrupted and relatively little at the same time a relatively good distribution of the burden ensured. The joint and meniscus element on the one hand and the meniscus and the bearing element on the other hand, each can be relatively independent move from each other because the two are kind of mechanical  Partial joints connected in series are complementary have designed storage areas. In particular can the joint and the meniscus element relative to each other three orthogonal axes rotate, and the Meniscus element and the bearing element can be relative in two of these directions to each other and to each other around the third of these axes rotate. The resulting mobility between the joint and the bearing element therefore includes both a Rolling, sliding and twisting around them Combinations of these types of movement that you can do with natural Knee finds. The essentially convex and relatively flat Designs of the two storage areas can be so close to the Surface designs of the natural joint parts of the associated individual elements of the knee are adjusted that also the interaction with the muscles surrounding the joint and ligaments as in the natural knee. A shared one Execution of both the femoral joint elements and the tibial bearing elements enables the production of modular semi-prostheses fitted into the prosthesis system. To that extent the knee joint endoprosthesis differs according to the DE 25 50 704 C2 also advantageous from the knee Endoprosthesis according to EP 0 519 873 A2 or EP 0 519 872 A1.

A disadvantage of the known systems is that obliquely Load forces radiating outside into the tibial plateaus perpendicular to the implant can. Accordingly, the known ones stand out Constructions characterized in that the lateral stability only guaranteed by the sufficiency of the natural sidebands becomes.

The present invention is therefore based on the object to create a system of the type mentioned, in which radiating diagonally outwards into the tibia plateaus Effective loads with the simplest of means can be intercepted, so that the inherent stability of the In this regard, knee joint endoprosthesis compared to the state  technology is increased. At the same time, the mentioned Advantages of the knee joint endoprosthesis according to DE 25 50 704 C2 are maintained.

This object is achieved by the characterizing Features of claim 1 solved. Accordingly, the peripheral or sagittal outer limitation of the femoral Sliding surfaces of the Menisci facing joint elements higher as their central or sagittal inner limitation. These Design of the Meniscus sliding surfaces represents a clear one Moving away from previous practice with the effect that a significantly more stable knee joint is created.

Advantageous further developments of the invention are described in the subclaims, particularly the features of claims 2 and 5 should be emphasized, after which the bearing surfaces of the tibial bearing elements each are spherically concave. That way they are Menisci spherically defined on all sides, which means that not only one antero-posterior mobility, but also lateral Agility of the Menisci becomes possible. This is next to the above-mentioned increased stability of the invention Knee joint endoprosthesis also increased mobility the same in adaptation to a natural knee joint reached. In this double-spherical formation of the Menisci face this every time the knee is bent optimally relative to the associated femoral joint element.

The underside of the tibial is also preferred Bearing elements spherically convex. The corresponding trained tibia implants are then with a so-called impactor or impact device lying cortically in desired inclination, whereby with cancellous Impaction the implant bed by itself the required Get fit without unnecessary bone removal.

Another disadvantage of the known systems is that the A particularly high implantation of the tibial bearing elements  Fitting accuracy is required when the swing load Implant anchoring surfaces should be avoided, which not quite with a single tibial implant can be eliminated. However, a correction is very common required, which is then either omitted or only accepting a larger tibial Loss of bone fragments is possible. Above all, they have to Bearing elements again, leaving large mortises be removed with the result that the bone significantly suffer from. It should be borne in mind that the stability of bones of older people is no longer too big. On Pry out and reinsert the implanted Bearing elements has a significant weakening anyway attacked bone. To solve this Problems are tibial according to claims 6 ff Sample storage elements with associated sample menisci proposed, at the bottom of the sample Bearing elements at least a small bone anchoring mandrel is provided. Accordingly, according to this requirement tibial sample bearing elements and sample menisci worked. The bone anchoring spike (s) on the underside of the Trial bearing elements should only be used for as long Keep sliding sideways in place until a final one Location of the sample bearing element and thus corresponding final bearing elements. Means A target device is attempted, the sample storage element with regard to its longitudinal and lateral inclination and height optimally set, for this purpose the trial Bearing element may be removed several times must to the bearing surface on the bone using a saw or to be able to rework the same tool until the correct position for the sample bearing element is found in which it did throughout the course of diffraction and The knee extension remains fully in place. The final one corresponds to the bearing element to be implanted Dimensions exactly the sample bearing element. If the right one for the sample bearing element Position relative to the assigned femoral  Joint element has been found, the trial Bearing element only through the final implant Bearing element to be replaced. That finally too implanting bearing element points much deeper into the Bone penetrating anchors. However, this is no further harmful, since the one to be finally implanted Bearing element only once in the previously by means of corresponding sample bearing element found optimal Position is implanted. The at the bottom of the sample Bearing anchoring spikes arranged in the bearing element penetrate not very deep in the bones. Accordingly low is the action on the bone, so that repeated Remove and reinsert a sample bearing element have no negative effects on the bones. Through the according to claim 5 proposed curvature of the underside of the tibial bearing elements receive an additional one Fixation at the proximal end of the tibia.

Because the tibial bearing of the menisci is divided in two the corresponding storage areas can be independent from each other. This applies to both their longitudinal and lateral inclination as well as their Distance from the left and right femoral joint element, these two joint elements also into a single one Double-skid femoral slides can be combined. Because of the lack of rigid connection between the two tibial implants also do not occur the undesirable "Rocking" the tibial bearing on like this one-piece design is very often the case. This Rocking also loosens the tibial Implant with the result that this is often relative must be replaced again in a short time.

The system according to the invention is also characterized through intercompatibility between femoral joint elements, tibial bearing elements and the Menisci from preferably Polyethylene or the like material. The described System is suitable for all levels of care, i. H. for one  femoral-tibial or femoral-patellar restoration, in particular a so-called partial or semi-prosthesis, Full denture and also revision denture.

The training of one is of particular importance groove-like sliding guide for the trial or final used Menisci on the top of the sample bearing elements as well as on the top of the final implanted tibial bearing elements. The groove-like is preferably Slideway around the longitudinal axis of the shin or transverse axis of the knee bent around. This "trough or bowl-shaped" recording The sliding guide for the Menisci has the consequence that the Menisci can be built relatively high across the entire width can. Accordingly, there is quite a lot of polyethylene Material for supporting the femoral joint elements for Available. For the rest, the Menisci are on the storage area the tibial bearing elements through a groove-like slide guide held. This sliding guide is preferably according to claim 7 or 8 designed.

The sample menisci can be just like the final one implanted menisci have different heights. Accordingly, depending on the degree of wear different distances between the tibial Bearing elements and the associated femoral joint elements be balanced.

The final one corresponding to the sample bearing elements implanted bearing element each have on their underside at least one cone or tab-shaped, preferably sleeve-shaped anchoring element with at least one, in particular two cross bores arranged one above the other on. The aforementioned anchoring element extends to Purposes of better anchoring the bearing element advantageously obliquely to the central axis of the same, namely preferably at an angle of 5 to 20 °. During training of two anchoring elements on the underside of the finally implanted bearing element can this  preferably extend parallel to each other. It is also from Advantage if the sleeve wall of the anchoring elements after Kind of a cutting edge is formed. The system described is therefore immanent that the mentioned sample menisci and sample Bearing elements corresponding menisci and bearing elements for final implantation are provided.

The sample bearing elements show in the area of their meniscus Sliding surfaces each have a hole to guide one Bone processing tool, especially one Cylindrical gouge, on. Through this hole, the corresponding mounting hole for the at the bottom of the finally implanted bearing element Anchoring element are formed. In this regard too the trial bearing element is special for the surgeon advantageous. The aforementioned guide hole extends preferably in an inclination that corresponds to the inclination of the Anchoring element at the bottom of the final implanted bearing element corresponds.

Instead of the femoral joint elements, if necessary a patella plain bearing are used.

The system described is basically real spheroidal unicompartmental slide prosthesis that not only move in one plane, but move several times Menisci, defined by appropriate polyethylene inlays are also undesirable eccentric loads on the Avoided knee joint. The Menisci pose in everyone Bend the knee optimally to the assigned femoral Joint element on the one hand and tibial bearing element on the other hand, without the risk of a lateral Evasion exists. The all-round is particularly advantageous Compatibility of the implants, which in later Wear of the opposite compartment an addition to the Fully articulated allowed, without implant parts still sitting well need to remove.  

Furthermore, it is preferably ensured that not only the knee-typical sagittal envelopes, but all Curvatures are circular sectors. This is production-related really important. This creates a high degree of mobility on the one hand, but an equally high stability of the knee on the other others.

An embodiment of a System according to the invention with reference to the accompanying drawing explained in more detail. It shows

Figure 1 is a hemi-knee endoprosthesis in front view.

FIG. 2 the side view of the hemi prosthesis according to FIG. 1;

Fig. 3, the hemi-prosthesis according to Fig 2 in comparison with FIG 2 modified relative position between femuralem joint member and associated meniscus..;

FIG. 4 shows the hemi prosthesis corresponding to FIG. 3 in a front view;

Figure 5 shows the tibial bearing element together with the meniscus corresponding to Figure 1 in plan view..;

Fig. 6 shows the meniscus 1 for the embodiment according to FIG front view.

Fig. 7 is a femurales joint part together with associated patella sliding bearing in a side view, and the patella sliding bearing additionally in front view;

FIG. 8 the patella plain bearing according to FIG. 7 in plan view;

Figure 9 is a double-runner-femoral patella sliding bearing with in association with two separate Tibiahalbimplantaten in front view.

FIG. 10 is a patella sliding bearing in a front view with the knee;

Fig. 11, the patella sliding bearing according to Fig 10 with associated patella spheroid in front view and partly in cross-section with the knee flexed.

FIG. 12 shows the arrangement according to FIG. 11 in side view and partly in longitudinal section;

. Fig. 13, the implanted patella spheroid Figures 11 and 12 stretched in a bottom view in accordance with the knee;

14 is a double-runner-femoral patella spheroid with a front view.

FIG. 15a is a patella spheroid in bottom view with the knee;

Fig. 15b a PE-Inlay for a patella spheroid of FIG 15a in bottom view.

FIG. 16a, a patella spheroid partly in cross section and partly in view knee in flexion;

Fig. 16b a PE-Inlay for a patella spheroid of FIG 16a in view corresponding to Figure 16a..;

17 is a spherical trial pivot slide bearing with locking ring for PE inlay in cross-section. and

Fig. 18a, 18b, a longitudinal section and a side view of the patella spheroid or PE-inlays as shown in FIGS. 16a, 16b.

Figs. 1 to 4 show a hemi-knee-joint endoprosthesis in flexion (Fig. 1 and 2) or extended position (Fig. 3 and 4). Accordingly, a joint element 10 with a convexly curved joint bearing surface is arranged at the lower end of a thigh or femur, while a bearing element 12 with a spherically concave curved bearing surface 13 is fastened or anchored to the upper end of the shin or tibia 11 . Between the femoral joint element 10 and the tibial bearing element 12 , a meniscus 14 is arranged to be movable both laterally and antero-posteriorly. Each meniscus is provided on its top and underside with sliding surfaces which are designed to complement the curvature of the associated joint bearing surface of the femoral joint element 10 or the associated bearing surface 13 of the tibial bearing element 12 . The upper sliding surface of the meniscus 14 is identified by the reference number 15 . Meniscus 14 is made of polyethylene. The tibial bearing element 12 is anchored to the bone by sleeve-shaped anchoring elements 16 , 17 , each with a transverse bore. As already mentioned in the list of figures, FIGS . 1 and 2 show the femoral half-slide 10 in the flexion position, while in FIGS . 3 and 4 the half-slide is in the extended position. The femoral semi-prosthesis shown in FIGS . 1 to 4 can subsequently be replaced by a full prosthesis or revision prosthesis in the course of a later operation. In this case, the left and right femoral half slides 10 are connected to one another in one piece. This embodiment of a femoral slide is shown in Fig. 9 in association with two separate tibia half implants and there with the reference number 10 '.

As can Figs. 1, 4 and 6 very good recognize the peripheral or sagittal outer boundary of the femoral joint element 10 facing sliding surface 15 of the meniscus 14 is higher than the central or sagittal inner boundary. The peripheral or sagittal outer boundary is identified in FIGS . 1, 4 and 6 with the reference number 16 . As a result of this measure, the knee joint is supported laterally outwards and overall is more stable compared to the prior art.

As already briefly explained above, both the bearing surface of the tibial bearing elements 12 and the upper sliding surfaces 15 of the menisci 14 are each spherically concave. A high degree of mobility of the knee joint is achieved in all directions without losing its stability. On the contrary, the stability of the knee joint is also increased by these measures. Above all, the aforementioned construction allows the menisci to move not only antero-posterior, but also to the side. The menisci adjust optimally to the femoral joint element 10 on the one hand and the tibial bearing element 12 on the other hand in every relative position of the knee joint. In extreme cases, to prevent a meniscus from slipping out or lifting off from the bearing element 12 , the tibial bearing elements 12 have a groove-like sliding guide 17 on their upper side. The groove-like slide guide 17 is bent according to FIG. 5 in plan view around the longitudinal axis of the shin. This results in a relative rotation of the knee joint in accordance with the natural possibility of rotation. According to FIG. 5, the slide guide 17 extends around each meniscus 14 around and holds it on all sides on the bearing surface 13 of the associated bearing member 12. The groove-like Meniscus sliding guide 17 is limited in the illustrated embodiment upwards and to the side by on the tibial bearing element 12 on pluggable, in particular snap-on guide strips 18 . In the exemplary embodiment shown, the guide strips 18 are part of a ring which can be snapped over the lateral edge of the associated tibial bearing element 12 . In this regard, reference is made in particular to FIG. 5. Corresponding to the groove-like meniscus sliding guide 17 , the meniscus 14 has on its underside a laterally projecting edge 19 which extends around the meniscus in a plan view. This peripheral edge 19 extends into the complementary groove-like meniscus sliding guide 17 . In interaction of the peripheral edge 19 with the groove-like slide guide 17 , the meniscus 14 is held securely on the bearing element 12 , with not only the usual antero-posterior mobility, but also lateral mobility of the meniscus due to a corresponding lateral play between the peripheral edge 19 and the groove-like slide guide 17 is guaranteed.

Furthermore, FIGS. 1 to 4 show that the underside of the tibial bearing element 12 is also spherically convex, preferably in accordance with the top of the bearing element. This measure allows an implantation with minimal bone removal. The tibial bearing element is inserted cortically by an impactor in a predetermined inclination to the front or back and / or to the side, whereby a spherically curved implant bed is created by cancellous bone impaction, which is adapted to the underside of the bearing element. This implant bed can also be prepared using a separate impactor.

As stated at the beginning, in the system shown tibial trial bearing elements with trial menisci for use come, being on the bottom of the sample bearing elements at least one, especially two small ones Bone anchoring spikes are provided. Advantageously are on the underside of a tibial trial bearing element in each case close to the two sagittal longitudinal edges of the same Bone anchoring mandrels arranged in a row.

Both the trial and the finally implanted menisci 14 can have different heights. In addition, it makes sense for practice to provide at least three different sizes for both the sample bearing elements and the finally implanted bearing elements, including Menisci.

The sliding surfaces of the bearing elements are preferred polished.

With the help of the bone anchoring mandrels that do not penetrate very deeply into the bone, a sample bearing element can be held at this stop without damaging the bone until the correct position of the bearing element and associated meniscus is found relative to the associated femoral joint element. For this purpose, it may be necessary to remove the sample bearing element several times in order to rework the bone support surface and to reinsert the sample bearing element until the correct longitudinal and transverse inclination of the bearing element and the correct height of the meniscus have finally been found . Then a tibial receiving bore is formed through a bore of a sample meniscus and the sample bearing element formed in the meniscus sliding surface, preferably by means of a hollow hollow chisel. This tibial bore serves to receive an anchoring element 16 or 17 arranged on the underside of the finally implanted bearing element, as indicated in FIGS. 1 to 4. The sample bearing element and the associated sample menisci are essentially designed in exactly the same way as the finally implanted parts. The sample bearing element differs from the finally implanted bearing element essentially only by the smaller bone anchoring spikes on the underside in comparison to the anchoring elements penetrating deeper into the bone in the finally implanted bearing element and possibly also by a flat contact surface.

In Fig. 7, a femoral double skid 10 'is shown in side view. At the rear proximal end of a patella sliding bearing can be connected to 20, which is shown in Fig. 7 above and in side view in Fig. 7 on the bottom right in front view. In Fig. 8, the patella plain bearing 20 is shown in plan view. In this respect, the described knee joint endoprosthesis system contains a compatible femuropatellar joint prosthesis.

An embodiment of a patella plain bearing prosthesis as part of the system described above is explained in more detail with reference to FIGS . 10-18b. Accordingly, the patellar supplemented system comprises a patella slide bearing 20 between the two femoral half slides 10 (see also the corresponding explanations for FIGS. 7 and 8) and a so-called patella spheroid 21 , which lies between the femoral patella slide bearing 20 and the osseous one Patella (patella) 24 is placed, in a fixed assignment on the underside or inside of the patella 24 . The patella spheroid 21 itself consists of a spherical or spherical rotary slide bearing 22 for a support and slide element in the form of a polyethylene (PE) inlay 23 , by means of which the patella 24 is supported on the femoral patella slide bearing 20 . In this regard, reference is made in particular to FIGS. 11-14. In Figs. 12 and 13 also the patellar tendon 24 are shown associated, namely the quadriceps tendon and the patellar ligament 25 26th The PE inlay 23 is held or secured on the spherically curved rotary slide bearing 22 by a snap ring 27 in such a way that the required rotary slide movement of the inlay 23 relative to the rotary slide bearing 22 is not impeded. The rotary sliding surface of the rotary slide bearing 22 is through a spherically curved metal plate, in particular plate z. B. made of titanium alloy. Bone anchoring elements corresponding to the tibial anchoring elements 16 , 17 described above are provided on the side facing away from the rotary sliding surface or facing the patella 24 .

The femoral patella plain bearing 20 is anchored by a femoral anchoring pin 29 (see FIGS. 7 and 12). The rotational sliding movement of the PE inlay 23 is indicated in FIGS. 13, 15a, 15b by the rotary double arrow 31 .

In Fig. 17 a spherical probe swivel slide bearing 28 is also shown with a snap ring 27. This trial bearing differs from the finally implanted rotary slide bearing 22 in that it has relatively small bone anchoring pins 30 which are just sufficient to temporarily secure the trial bearing 28 on the patella 24 . Furthermore, the contact surface facing the patella is flat, that is to say is not convexly curved, as is preferably the case with the finally implanted rotary slide bearing 22 . The convexly curved bone support surface of the rotary slide bearing 22 corresponds to the anatomical shape of the residual patella 24 , so that no major modifications of the biomechanical lever arms occur.

The freedom of rotation of the PE inlay 23 in accordance with the double arrow 31 permits a congruent fit on the femoral patella slide bearing 20 even in the positions in which the pulling directions patella quadriceps and patella tibia (ligamentum patellae) are not parallel to one another and also do not extend parallel to the furrow of the femoral patella plain bearing 20 .

The patella spheroid, in particular the spherical rotary slide bearing 22, is preferably implanted by pressing it into the remaining patella 24 .

In analogy to the unicompartmental tibial implants, the trial rotary slide bearing 28 serves for lateral centering and for adaptation of the implantation inclination in the sagittal plane, as well as corresponding bores for receiving a cylindrical hollow chisel with regard to the anchoring elements on the finally implanted patella bearing element.

All features disclosed in the registration documents are claimed as essential to the invention, insofar as they individually or in combination compared to the prior art are new.

Claims (17)

1. System for the formation of a knee joint endoprosthesis with

• - At the lower end of the thigh (femur) attachable joint elements ( 10 , 10 ') with a convex curved bearing surface,
• - At the upper end of the tibia (tibia) ( 11 ) attachable bearing elements ( 12 ), and with
• - Each between the femoral joint ( 10 , 10 ') and the tibial bearing elements ( 12 ) movably arranged meniscus ( 14 ) with sliding surfaces formed on the top and the bottom, which complementary to the associated joint bearing surface of the femoral joint element ( 10 , 10 ') Or the associated bearing surface of the tibial bearing element ( 12 ) are designed,

characterized in that the peripheral or sagittal outer boundary ( 16 ) of the sliding surfaces ( 15 ) of the menisci ( 14 ) facing the femoral joint elements ( 10 , 10 ′) is higher than their central or sagittal inner boundary. 2. System according to claim 1, characterized in that the bearing surface ( 13 ) of the tibial bearing elements ( 12 ) are each spherically concave. 3. System according to claim 1 or 2, characterized in that the tibial bearing elements ( 12 ) are provided on their upper side with a groove-like slide guide ( 17 ) for trial and also finally implanted menisci ( 14 ). 4. System according to claim 3, characterized in that the groove-like sliding guide ( 17 ) is bent in plan view around the longitudinal axis of the shin. 5. System according to one of claims 1 to 3, characterized in that the underside of the tibial bearing elements ( 12 ) is convex, in particular corresponding to the upper side ( 13 ) of the same spherical-convex. 6. System according to one of claims 1 to 5, marked by tibial sample bearing elements with sample menisci, whereby at the bottom of the sample bearing elements each at least one small bone anchoring mandrel is provided. 7. System according to claim 6, characterized, that at the bottom of the tibial trial bearing elements in each case close to the two outer or sagittal Longitudinal edges of bone anchoring mandrels are arranged. 8. System according to any one of claims 3 to 7, characterized in that the groove-like Meniscus sliding guide ( 17 ) upwards and to the side by on the tibial bearing element ( 12 ) attachable, in particular snap-on guide strips ( 18 ) is limited, with spherical-concave curved bearing surface ( 13 ) lateral play of the Menisci ( 14 ) is provided within the groove-like slide guide ( 17 ). 9. System according to claim 8, characterized in that the guide strips ( 18 ) of the groove-like Meniscus sliding guide ( 17 ) are part of a ring or the like element which can be snapped over the lateral edge of the associated tibial bearing element ( 12 ). 10. System according to one of claims 1 to 9, characterized in that the femoral joint elements are part of a one-piece femoral slide ( 10 '), or alternatively form two separate half-slides ( 10 ). 11. System according to one of claims 1 to 10, characterized in that in addition to the femoral joint elements ( 10 , 10 '), a patella plain bearing ( 20 ) is provided as part of the system, in particular in a predetermined association with the femoral joint elements ( 10 , 10 ′). 12. System according to any one of claims 1 to 11, characterized in that the sample as well as the finally implanted Menisci ( 14 ) have different heights. 13. System according to any one of claims 1 to 12, characterized in that the finally implanted bearing elements ( 12 ) corresponding to the sample bearing elements each have at least one sleeve-shaped, peg-shaped or tab-shaped anchoring element ( 16 , 17 ) on their underside. 14. System according to one of claims 6 to 13, characterized in that the menisci corresponding to the probe menisci ( 14 ) are provided for the final implantation. 15. System according to one of claims 11 to 14, characterized in that the patella plain bearing ( 20 ) has a patella spheroid ( 21 ) with a between the (bony) patella ( 24 ) and one on the femoral patella plain bearing ( 20 ) supporting sliding and support element, particularly PE-inlay (associated with 23) arranged spherical pivot sliding bearing (22), said sliding pivot bearing (22) is provided with sleeve-, journal or tab-shaped anchoring elements for anchoring to the patella (24). 16. The system of claim 15, characterized by a spherical trial pivot slide bearing (28), where the (bony) patella (24) a small bone anchoring thorn (30) is provided associated side at least, the just of temporary retention of the test bearing (28 ) on the patella ( 24 ) is sufficient.