FR2700260A1 - Artificial patella component for knee prosthesis - Google Patents

Abstract

The patella component is designed to interact with the curved recess (6) between the trochlea surfaces (4, 5) of the femoral component. It comprises a base (10) with a connector (20) for the patella ligaments, a recess (11) and a button (12).The button is fitted into the recess so that it can rotate about a central point (14), and it has two concave curved surfaces (16, 17) which interact with matching surfaces on the femoral component. The two surfaces (16, 17) are spherical in shape and have a radius of curvature which is slightly smaller than that of adjoining curved surfaces on the base.

Description

 The present invention relates to artificial patellar parts for knee prostheses, that is to say the elements which restore the knee joint between the femur and the tibia, and more particularly the part called "patella".

 It is recalled that a knee prosthesis essentially comprises a femoral piece capable of being fixed on the femur and substantially taking the shape of a "U", the bottom and one of the sides of the "U" defining two separate condylar surfaces. by an empty space for the passage of ligaments, the other side of the "U" defining a trochlea, a tibial plateau capable of being fixed on the tibia, and means for coupling the two condylar surfaces with the tibial plateau.

 In fact, to make prostheses of any joint, the technicians try to copy the components of this joint when it is completely healthy. This is for example the case for the hip prosthesis in the mode of a ball joint of the male-female type, the reconstruction of which is relatively easy to obtain mechanically because the movements of the hip joint are practically only rotations around of a point. It is not the same for a knee prosthesis which is much more difficult to produce due to the complexity of the movements of the tibia compared to the femur which are combinations of rotations and translations.

 The Applicants have already produced a knee prosthesis which gives them satisfaction. This prosthesis essentially comprises a femoral part capable of being fixed on the femur, the femoral part substantially taking the shape of a "U", the bottom and one of the sides of said "U" defining two condylar surfaces, these two surfaces being separated by an empty space for the passage of ligaments, the other side of said "U" defining two trochlea surfaces, a tibial plateau capable of being fixed on the tibia, means for coupling the two said condylar surfaces with said tibial plateau, the tibial plateau being constituted by two plates and by means for mounting in rotation these two plates one on the other around an axis substantially parallel to the axis of the tibia, a first plate being able to be fixed on the tibia , the other second plate being turned towards the femur, means for coupling the two condylar surfaces with the tibial plateau comprising two pads disposed respectively between the two condylar surfaces and the second plate of the tibial plateau, the two pads being slidably mounted in translation along, respectively, two rectilinear and parallel axes, each pad having a sliding sliding surface for the condylar surface which is associated with it.

 In some cases, it is possible to keep the initial patella which can cooperate with the knee prosthesis mentioned above. On the other hand, if the patella is itself damaged, it is of course necessary to restore it, too, to obtain a functioning of the knee as similar as possible to that of the natural joint.

 The achievements of artificial kneecaps that have been developed to date do not give complete satisfaction because they do not allow to obtain all the original movements of the natural patella.

 The present invention thus aims to produce a patellar part for knee prosthesis, which allows to restore the movements of an original knee joint without creating discomfort for the patient on which it is intended to be implanted, and which is d 'an easy mechanical production, in particular by comprising only surfaces whose definition is relatively simple and therefore easily machinable.

More specifically, the present invention relates to an artificial patellar part for knee prosthesis comprising a femoral part capable of being fixed on the femur, said femoral part substantially taking the shape of a "U", the bottom and one of the sides of said "U" defining two condylar surfaces separated by an empty space for the passage of ligaments, the other side of said "U" defining two trochlea surfaces, the two said trochlea surfaces being separated by a hollow patellar surface comprising a bottom surface bordered by surfaces of substantially spherical shape, said artificial patellar part being characterized in that it comprises:
- a base body,
a recess a recess produced in said base body to constitute the female element of a socket,
- Ull button, and
means for mounting said button in rotation in said housing around a first point so that part of said button emerges from said housing,
- Said part of said button emerging from said housing comprising two first and second concave spherical surfaces respectively substantially complementary to said spherical surfaces bordering and belonging to said patellar surface, these two first and second concave spherical surfaces having substantially the same value of radius of curvature and being symmetrical from each other with respect to a first plane passing through said first point of rotation.

Other characteristics and advantages of the present invention will appear during the following description given with reference to the drawings annexed by way of illustration but in no way limitative, in which:
I.es Figures 1 and 2 show a first embodiment of a patellar part according to the invention in two sectional views along two perpendicular planes, the section according to Figure 1 being referenced II in Figure 2 and the section according to FIG. 2 being referenced Il-Il in FIG. 1,
FIGS. 3 to 5 represent, respectively seen from the front, from the side and from below, another embodiment of a patellar part according to the inventioll,
FIG. 6 schematically represents a human knee in order to better understand the aim of implementing the invention and the advantages of this invention, and
FIG. 7 represents, in schematic form, three possible positions of a patellar part according to the invention in association with a knee prosthesis.

 Although the figures do not represent the same embodiment of a patellar part according to the invention, for the sake of simplification, the same elements bear the same references therein, regardless of the figure in which they appear.

 I.es Figures 1 and 2 show a first embodiment of a patellar part according to the invention seen in section along two perpendicular planes.

 The artificial patellar part 1 is intended to cooperate, as will be explained below, with a knee prosthesis 2 comprising a femoral part 3 capable of being fixed on a femur. The femoral piece substantially follows the shape of a "U", the bottom and one of the sides of the "U" defining two condial surfaces separated by an empty space for the passage of ligaments, the other side of the "U" defining two trochlea surfaces 4, 5 of cylindrical shape and separated by a hollow patellar surface 6 having a bottom surface 7 bordered by surfaces 8, 9 of substantially spherical shape, these two surfaces joining the cylindrical trochlea surfaces 4, 5.

 The patellar part 1 according to FIGS. 1 and 2 comprises a base body i0 in which a recess 11 is formed, shaped to be able to constitute the female element of an interlocking. In this housing is arranged a button 12, knowing that this element 12 is designated by the term "button" essentially because of its shape, but not of its function which will be defined below.

 The base body 10 includes means 20 for associating it with the patellar ligaments 55. These means are known in themselves and will not be further developed.

 Means 13 are also provided for mounting the button 12 so that it pivots in rotation in the housing 11 around a first point 14 and that part 15 of this button 12 emerges from the housing 11.

 The part 15 of the button emerging from the housing 11 comprises two first 16 and second 17 concave spherical surfaces respectively complementary to the two spherical surfaces 8, 9 bordering and belonging to the patellar hollow surface G, these two first 16 and second 17 concave spherical surfaces having substantially the same value of radius of curvature and being symmetrical to each other with respect to a first plane 18 passing through the point of rotation 14.

 Advantageously, the button 12 also comprises a connection surface 19 substantially complementary to the bottom surface 7 of the patellar surface 6.

 The means 13 for mounting the button 12 in rotation in the housing there around the point 1 4 so that a part 1 5 of this button emerges from the housing are advantageously constituted, as illustrated in Figures 1 and 2, by the fact that the housing 11 is delimited by a cylindrical wall 40 of revolution about an axis 41 passing through the point 14, this wall 40 ending on an opening 22 in the form of a spherical zone 42 with two bases and having the point of curvature at the center of rotation 14, and by a bottom 47 comprising a part in the form of a spherical cap 43 whose center of curvature coincides with point 14 and whose radius of curvature is less than that of the spherical zone 42.

 The part 44 of the button 12 which is able to enter the housing 11 comprises a spherical segment 45 with two bases and complementary to the spherical zone 42, and a part 46 projecting from the spherical segment 45, this projecting part 46 being complementary of the spherical cap 43 of the bottom 47 of the housing 11. It has substantially the shape of a half-sphere of the same level as the spherical cap 43.

 Ia patellar part 1 described above with reference to Figures i and 2 provides results that those of the prior art do not give. These results will be more fully developed below, at the same time as those obtained with the embodiment which is illustrated in FIGS. 3 to 5.

 According to the embodiment illustrated in Figures 3 to 5, the patellar part comprises, as in the embodiment according to Figures 1 and 2, a base body 10 and a button 12. The latter being identical to that which is described with reference to Figures 1 and 2, its definition will therefore not be repeated here.

 the base body 10 also comprises means 20 for associating it with the patellar ligaments. These means are known in themselves and will not be developed here either.

 On the other hand, the base body 10 comprises, on its face 21 bordering the opening 22 of the housing 11, four spherical concave surfaces, respectively a third 23, a fourth 24, a fifth 25 and a sixth 26. These four concave surfaces spherical 23-2G have respectively points of curvature points 33-36 and are substantially symmetrical two by two with respect to two second and third perpendicular planes 27, 28 belonging to the base body 10 and passing through the first point of rotation 14 of the button 12 defined above. The value of the radii of curvature of these four spherical concave surfaces 23-26 is substantially equal to that of the radii of curvature of the surfaces 8, 9 belonging to the patellar surface 6 defined above (FIG. 2 ).

 l) in an advantageous embodiment, the value of the curvature of the curvature of the first two 1G and second 17 concave surfaces of the button 12 is slightly less than the value of the radii of curvature of the third to sixth concave spherical surfaces 23-26 of the base body 10. This difference is more particularly visible in FIG. 4.

 the concave spherical surfaces 23-26 of the base body 10 are further produced so that the straight lines passing substantially through their center of curvature and their apex are substantially concurrent at a point 56 located on the axis 41 defined above and on the bottom side 47 of the housing there.

 l) e this way, when the two concave surfaces 1 6, 17 of the button 1 2 are in contact with the two spherical surfaces 8, 9 of the patellar surface 6 connecting the two trochlea surfaces of the femoral part of the knee prosthesis , the base body 10 can tilt around the button so that two of its four spherical concave surfaces 23-26 can alternately come into contact with these two surfaces 8, 9. For example, with reference to the mode of embodiment illustrated in Figures 3 to 5 and depending on the position of the button 12 relative to the base body 10 (Figure 5), when the two surfaces 23, 26 are in contact with the surfaces 8, 9, the two surfaces 24 , 25 are far from it, and vice versa. In this case, the tilting takes place around the plane 28. Of course the concave surfaces 16, 17 of the button 12 remain, they, constantly in contact with the spherical surfaces 8, 9 of the patellar surface 6.

 In an advantageous embodiment, the amplitude of the tilting of the base body relative to the button is generally about sixty degrees.

The patellar part according to the invention operates as follows
It must first be remembered that the patella is an essential element for the proper functioning of the knee joint by sliding on the patellar surface connecting the surfaces of the trochlea.

 Normally, the patella moves while being very strongly applied on the patellar surface by the quadriceps, and this, all the more that the flexion of the tibia 51 relative to the femur 52 is more accentuated. Figure G-a very schematically represents the force exerted on the patella when the knee is strongly bent. On the other hand, this coaptation force applied to the patella decreases during the leg extension movement, until it tends, when the knee is in hyper-extension as illustrated in figure 6-c, to reverse and detaching the patella from the trochlea connecting surface. In an intermediate position between the two preceding extreme positions, as represented on the figure 6-b, this force is relatively weak.More precisely, it exerts on the patella a set of three forces which, in such an intermediate position of the knee , substantially balance and give a result of low intensity represented by an arrow 57 of short length.

 The patellar part 1 according to the invention allows the movements of the patella described above and more particularly shown in FIG. 7 to be reconstructed in complete safety.

 The three positions respectively recalled above are illustrated on figure 7, the position according to figure 6-a being represented by position 5 () - A, that is to say in the most accentuated bending, the position 50-B representing an intermediate extension position and position 50-C corresponding to the position in hyper-e.xtension.

 The patellar part described above is put in place, for example by being integral with an original part of the patella which is integral with the ligaments 55 between the femur 52 and the tibia 51. It thus naturally comes into contact with the patellar surface 6.

 When the knee is slightly bent, position 50-B, only, the button 12 follows this patellar surface 6, by its two concave surfaces 16, 17, and the base body 10 is in equilibrium while pressing on this button, without that the third to sixth concave surfaces 23-2G are in contact with the patellar surface 6. It should be emphasized that, the button being mounted in rotation around the point 14, it occupies a perfectly congruent position from the bottom of the patellar surface 6 on its entire length regardless of the forces exerted on the base body 10.

 the patellar part according to the invention has advantages, compared to the anterior patellar parts, especially when the knee arrives in maximum flexion and in hyper-extension, that is to say in the positions illustrated in 50-A and 50- vs.

 When the knee reaches maximum flexion, position 50-A, the patellar part slides on the patellar surface 6 towards its bottom 7 and, under the action of the tensions exerted by the ligaments of attachment on the tibia 51, it tends to undergo a rotation about a transverse axis perpendicular to its direction of sliding on the patellar surface 6. Because the base body 10 is rotatable relative to the button 12 (and vice versa) but that the button remains in contact with the patellar surface G, the base body then also comes into contact with the two spherical surfaces 8, 9, FIG. 2, by its two concave spherical surfaces 24, 25 which slide over them, its two other surfaces 23, 26 in being distant.

 The slight difference between the values of the radii of curvature of the concave surfaces 23-2G of the base body 10 and the concave surfaces 16, 17 of the button 12 allows the latter to remain, at least partially by one of its sides 53 , in contact with the patellar surface G, as seen in position 50-A.

 The base body which is integral with the ligaments thus perfectly conducts the patellar part and maintains the ligaments so that they ensure their anatomical functions illustrated on the figure 6-a.

 when the knee arrives in hyper-extension, the patellar part slides on the patellar surface 6 towards a high position, represented in 50-C.

Under the action of the attachment ligaments on the femur 52, the patellar part then tends to undergo a rotation about a transverse axis perpendicular to its direction of sliding on the patellar surface 6. Because the base body 10 is rotatably mounted relative to the button i 2 (and vice versa) but the button remains in contact with this patellar surface 6, the base body also comes into contact with the two spherical surfaces 8, 9 (FIG. 2), but by its two spherical concave surfaces 24, 25 which slide over them, its two other surfaces 23, 26 while being spaced apart.

 The slight difference between the values of the radii of curvature of the concave surfaces 23-26 of the base body 10 and of the concave surfaces 16, 17 of the button 12 allows the latter to remain, at least partially by one of its sides 54 , in contact with the patellar surface 6, as seen in position 50-C.

 The base body which is secured to the ligaments therefore perfectly conducts the patellar part in the hyper-extension movement of the knee and maintains the ligaments so that they perform their anatomical functions illustrated in the figure 6-c.

 In the description above, it can be seen that the knee joint is almost perfectly restored and that the prosthesis allows all the original constituent elements of the knee which have been preserved during its installation, to function according to movements which are almost the same as the original movements, thus providing patients who wear such a prosthesis with greater comfort than that provided by knee prostheses of the prior art.

Claims (7)

 1. Artificial patellar part for knee prosthesis comprising a femoral part (3) capable of being fixed on the femur (52), said femoral part substantially taking the shape of a "U", the bottom and one of the sides of said "U" defining two condylar surfaces separated by an empty space for the passage of ligaments, the other side of said "U" defining two trochlea surfaces (4, 5), the two said trochlea surfaces being separated by a patellar surface in hollow (6) comprising a bottom surface (7) bordered by surfaces of substantially spherical shape (8, 9), said artificial patellar part being characterized in that it comprises:  - a base body (10),  - a recessed housing (11) produced in said base body to constitute the female element of a socket,  - a button (12), and  - means (13) for mounting said button in rotation in said housing around a point (14) so that a part (15) of said button emerges from said housing,  - Said part (15) of said button emerging from said housing comprising two first (16) and second (17) concave spherical surfaces respectively substantially complementary to said spherical surfaces (8, 9) bordering and belonging to said patellar surface (6), these two first and second concave spherical surfaces having substantially the same value of radius of curvature and being symmetrical to each other with respect to a first plane (18) passing through said point of rotation (14).  2. Part according to claim 1, characterized in that the button (12) has a connecting surface (19) substantially congruent with the bottom surface (7) of said patellar surface (6).  3. Part according to one of claims 1 and 2, characterized in that the face (21) of said base body (10) bordering said housing (11) comprises four third-sixth concave surfaces (23-26) of spherical, these said surfaces being substantially symmetrical in pairs with respect to two perpendicular planes (27, 28) passing through said point of rotation (14), the value of the radius of curvature of these third-sixth spherical concave surfaces being substantially equal to that of the radius of curvature of the spherical surfaces (8, 9) bordering said patellar surface (G).  4. Part according to claim 3, characterized in that the value of the radius of curvature of the two first (16) and second (17) spherical concave surfaces of the button (10) is slightly less than the value of the radius of curvature of the third -sixth surfaces (23-26).  5. Part according to one of claims 1 to 4, characterized in that said means (13) for mounting said button (12) in rotation around said point (14) in said housing (11) so that a part (15) of said button emerges from said housing, are constituted by the fact that  said housing (11) is delimited by a wall (40) of revolution about an axis (41) passing through said point of rotation (14), said wall ending on an opening (22) in the form of a spherical zone (42 ) at two bases and having for its center of curvature said point of rotation (14), and by a bottom (47) in the form of a spherical cap (43) whose center of curvature coincides with said point of rotation (14) and of which the radius of curvature is less than that of said spherical zone (42), and that  the part (44) of said button (12) able to be contained in said housing (11) comprises a spherical segment (45) with two bases complementary to said spherical zone (42) and a projecting part (46) on said spherical segment complementary to said spherical cap (43) of said bottom (47) of said housing.  6. Part according to one of claims 3 to 5, characterized in that said third-sixth spherical concave surfaces (23-26) of said base body (10) are made so that the straight lines passing substantially through their center of curvature (33-36) and their top are substantially concurrent at a point (56) located on the side of the bottom (47) of said housing (II).  7. Part according to one of claims 1 to G, characterized in that said base body (10) comprises means (20) for associating it with the patellar ligaments (55).