MX2008007519A - Endoprosthesis with intermediate part - Google Patents

Abstract

The invention relates to an endoprosthesis for replacement of a joint, wherein an intermediate part (2) forms slide surfaces that are of different contours and accordingly define planes of movement for a respective bearing (1). According to the invention, a tensioning band encloses the intermediate part and its side faces and is arranged thereon in such a way that it is arranged free from the planes of movement defined by the slide surfaces of different contours. In this way, it is also possible to reinforce endoprostheses of the kind that have complex biomechanics with a plurality of degrees of freedom.

Description

ENDOPROTESIS WITH INTERMEDIATE ELEMENT DESCRIPTION OF THE INVENTION The invention relates to a stent for replacing a joint comprising a joint component with the lower bone having a sliding surface in its upper part, a joint component with upper bone having a sliding surface on the lower side, and an element intermediate which has on its lower and upper side in each case a complementary sliding surface and which respectively forms a bearing with the sliding surfaces of the previously mentioned components. Endoprostheses of this type are used, by way of example, to replace the tibiotarsal joint (FR-A-2 676 917, O-A-03/075802, O-A-2005/030098). In these the components and the intermediate element act together through sliding surfaces that allow bending and stretching in a sagittal plane. The sagittal plane is in this a plane that is defined by the AP direction and the vertical e. The components in the splint and the intermediate element form sliding surfaces that act together and allow a rotation along the vertical axis. They can be made flat to allow compensation movements in the direction AP and LM (lateral-medial). For the articulation to offer the degrees of freedom corresponding to the natural model in terms of rotational, oscillating and / or pushing movements, the sliding surfaces have different contours, for example, a flat sliding surface is combined with a curved surface way of a cylindrical envelope. The stabilization is done by the natural tendon apparatus. The entire load of the endoprosthesis rests on the intermediate element. So absorb a very high load. In an endoprosthesis with an intermediate element of graphite material, a wrapping ring is provided as a reinforcement (EP 0 001 147 Al). In practice, it has been found that under high loads, a "work" can be present in the intermediate elements, usually made of polyethylene. Following this there is a risk that they may occur in the case of higher loads, such as may occur, for example, due to movement dynamics (in particular when climbing stairs or when jumping) there is an excessive load and with it an uncontrolled deformation of the intermediate element. This can cause excessive wear, cold flow or even the failure of the prosthesis due to the failure of the material. This is true in particular when the intermediate element is made thin, as is usually done as a result of the difference of contour of the sliding surfaces. The invention is based, therefore, on improving a stent of the type mentioned micially from the referred state of the art in the sense that it can withstand large loads, preserving multiple degrees of freedom. The solution is found in a prosthesis having the features of claim 1. Advantageous improvements are the subject of the dependent claims. Accordingly, in a stent to replace a joint comprising a connecting component with the lower bone that has a sliding surface in its upper part, a connecting component with upper bone having a sliding surface on the lower side, and an intermediate element having on its lower and upper side in each case a sliding surface which together with the sliding surfaces of the components mentioned above determine in each case a plane of movement for a bearing, where the surfaces of the intermediate element have different contours, is provided according to the invention that a tension ring, made with two zones, cinches the intermediate element by means of a cinching zone extending in the circumferential direction and an adjacent expansion protection zone on the side faces thereof and is arranged in the element Free intermediate of the planes of movement determined by the sliding surfaces having a different contour. The concept of the movement plane must be understood in a broad sense and also includes convex contours. Thanks to the tension ring, which offers a more resistant to tensile stress compared to polyethylene, the total modulus of elasticity resulting from the intermediate element is increased. Conveniently the tension ring is made of a material having a modulus of elasticity at least fifty times, preferably at least two hundred times greater than polyethylene. With the tension ring as a tensile-resistant cinching, an elastic or plastic deformation of the intermediate element is applied under load. By means of the zone of protection against expansion, an additional action is also taken against a diversion of the outer sections of the intermediate element under high bending load. The intermediate element can also withstand higher loads without deforming. Thanks to this it is possible to achieve the intermediate element with the inventive tension ring. Thanks to the inventive arrangement of the tension ring, the movement surfaces are free, as determined by the sliding surfaces of the upper and lower support. Thanks to this, the mobility of the endoprosthesis in all its articulation functions, notwithstanding the reinforcement by means of the tension ring. This is true not only for the area of normal movement of the endoprosthesis, but also for movements that exceed it, as they can occur in the case of bends (for example when bending the foot). It is known, of course, to reinforce polyethylene plates that function as a stent component of the knee joint because a metal plate is provided on its lower side resting on the tibia. In this way the polyethylene plate was reinforced from its rear face in such a way that it operates less under bending loads. But this known measure of EP-A-0829 243 is applicable only to prostheses that have a sliding surface on only one side. In the case of a stent of this type, however, the intermediate element has sliding surfaces on its upper face as well as on its lower face, so that the use of such a reinforcing plate is excluded. Something analogous is also true of a reinforcing ring as is known from US-A-5 766 256. This ring is also arranged on the underside which does not serve as a joint surface. It is not necessary to provide the area of protection against expansion throughout the circumference, unless enough usually to foresee it on two opposite sides. Preferably the upper and lower edge of the ring are adopted to the contour of the respective adjacent sliding surface. As adopted, it is understood that, viewed from the side, the edge of the tension ring has a constant distance from the edge of the adjacent sliding surface. If a sliding surface is, by way of example, a plane, then its edge is a straight line and the edge of the tension ring is also a straight line; correspondingly, if the other sliding surface is bulged, then its upper edge has arc contour and the corresponding edge of the tensioning ring also a circular arc with a circular arc-shaped edge. Thanks to the upper and lower edges of the tension ring adopted, in each case, to the contour it is possible to achieve a reinforcement of the intermediate element particularly exposed to loads, even in the case of stents with a complex articulation function, such as those with sliding surfaces of different contour, and try, nevertheless, that the element that is responsible for the reinforcement, the tension ring, remains free of the complex sliding surfaces, of different contour. Conveniently, the tension ring has at its lower edge on at least two sides a chamfer which is configured in such a way that it vanishes without interruption in the intermediate element. The tension ring preferably has a projection in the form of a bulge which fits into a complementary recess in the intermediate element. In this way the tension ring is secured against undesirable displacements from its predetermined position. But it is also possible to envisage other fixing techniques such as gluing or tightening, in particular by clamping the tension ring on the intermediate element. A positive connection can also be provided, such as fixing with pins or screwing. In order to give the intermediate element a defined position, it has on its upper or lower face a flange on which the tension ring rests. This facilitates the assembly of the tension ring, since the intended position is clearly defined. This also ensures that the sliding surface of the intermediate element must not be reduced due to the tension ring. With this the load per area is not greater than in the conventional modalities of the intermediate element without the tension ring. Conveniently the tension ring is dimensioned in such a way that its upper edge and its lower edge have a distance of at least 1 mm, preferably between 1.5 and 2.5 mm, towards the edge of the respective sliding surface over its entire circumference. In this way it is made certain that undesirable contact between the Tension ring and the sliding surfaces of the components of the prosthesis, even in the case of large loads that cause a compression of the intermediate element, or in the case of wear of the intermediate element. According to a particularly preferred embodiment, which eventually deserves independent protection also for stents with sliding surfaces having the same contour, the tension ring has at least one outer side a bulging projection. With this projection it is caused that in a rotation, and also in a linear movement, of the joint and thus also of the intermediate element with everything and tension ring, the undesirable growth tissue material can be repressed. In this way it is possible to counteract or even inhibit the penetration of tissue matter, the so-called fibrosis. Thus, it is possible to counteract the risk of the joint losing mobility due to excessive fibrous formation. Thanks to the inventive improvement it is possible to avoid the pains, as they can usually occur in the case of such fibrosis due to tissue material that grows in the area of movement of the intermediate element. In this way, a surgical removal, if necessary, of this tissue material is not necessary, thanks to the inventive conditioning. A particular advantage of this improvement it is found that, by virtue of the tension ring, preferably consisting of metal, a contact of the bone or tissue material is in principle admissible, whereas in conventional intermediate elements, which usually consist of polyethylene material, a contact with the material was not desirable. the bone, respectively, the material matter, because this causes an undesirable wear of polyethylene. The inventive configuration takes advantage, therefore, of the tensor ring in two ways, namely its structure to form the bulging projection to repress the fibrosis and its material that, in the first place, allows the contact of the fibrosis tissue. The sphericity of the projection only needs to be of one dimension, so that an essentially cylindrical shape is produced; preferably, however, it is two-dimensional, so that it essentially results in a domed configuration, the curvature in the plane of the tensioning ring being able and in a vertical direction thereof may be different. Conveniently, the bulged projection extends over the entire length of the respective outer face. It is certainly possible to achieve the desired effect in principle also with a projection that extends only for a part of the length of an external face, but larger radii of curvature are given - and with it more favorable- in the case of an embodiment over the entire length. Particularly convenient is an arrangement of the bulged projection on a medial longitudinal side of the tension ring. In this region, for example, in the case of an implantation of the inventive endoprosthesis in the tibiotarsal joint, the malleol us medial i s. Precisely in this region undesirable fibrosis may occur whose harmful effects will be suppressed thanks to the inventive improvement. The arrangement extending over the entire length has the additional advantage that also in the case of a non-rotating movement, e.g., forward or rearward of the intermediate element, the desirable effect of repression can be achieved. Conveniently, the contour of the bulged projection is selected such that it has the shape of an arc of a circle seen from above. Such a contour is suitable for production and produces a uniform curvature of the projection without strong change of the curved contour. It is not necessary for this that the center of the circle on which the arc of the circle is based is in the center of the tension ring. Conveniently it is offset in the direction of the opposite lateral side. In this way, an eccentricity is produced which achieves a stronger repression of the fibrosis tissue material in the case of a greater rotary oscillation.

The external surface of the bulged projection is preferably designed in smooth form. Preferably it can be polished. This produces a behavior that is favorable to sliding, in particular under the influence of tissue fluid. This effectively counteracts the risk of breaking or shearing tissue matter. The bulged projection can also be conveniently provided on the adjacent external faces. In the case of a design with corners, this means that such a bulged projection is formed on the anterior, posterior and medial side of the tension ring. The invention is explained below with reference to the accompanying drawing, in which an advantageous embodiment of the invention is shown. Shown is: Fig. 1 a sagittal section through a tibiotarsal joint equipped with an inventive prosthesis; Fig. 2 the prosthesis according to Fig. 1 in an exploded perspective view; Fig. 3 a) and 3 b) a front and side view of a tension ring of the prosthesis; Fig. 4 a partial cross-sectional representation of the tension ring with an intermediate element of the prosthesis; FIG. 5 a front view of a lower part of the tibia with a part of a variant of the endoprosthesis according to FIGS. 1 to 4; Fig. 6 a bottom view of the variant according to Fig. 5; and Fig. 7 a) and 7 b) a front and side view of the tension ring shown in Fig. 6. The illustrated embodiment of the inventive prosthesis is a prosthesis of the tibiotarsal joint. It is noted that the invention can find application also in other types of stents, for example, intervertebral stents. Essential is that the endoprosthesis has two supports whose planes of movement are determined by sliding surfaces of different contours. The stent according to the illustrated embodiment has essentially three components. The first component is a tibia component 1 which is configured to be disposed at the lower end of a tibia bone 91. It has a plate-shaped part 10 whose lower face forms a flat sliding surface 11. On the upper side of the plate-shaped part 10 there is an anchoring body 12 provided with projections which serves for the fixation of the tibia component 1 in corresponding resection notches of the tibia 91. The prosthesis also comprises a tarsal component 4. This saddle-shaped one has a convex convex sliding surface 44 on its upper face. This may have a cylinder shell shape, as illustrated; but it can also be made in the form of a cone. It is located in it a nerve guide 46 that is located in the AP direction. It serves as a guide in a movement of flexion or stretching of the tibiotarsal joint. Between the tibial component 1 and the tarsal component 4, an intermediate element 2 is arranged. It has on its upper face a flat sliding surface 21 which is configured in a complementary manner to the sliding surface 11 of the tarsal component 1. On its lower face the intermediate element 2 has a sliding surface 24 configured in a manner congruent to the sliding surface 44 of the tarsal component 4. It additionally has a groove 26 which is configured for the slidable housing in the longitudinal direction of the rib 46. In this way, the intermediate element 2 is guided laterally in relation to the tarsal component 4. This allows only bending and stretching movements. In contrast, flat sliding surfaces 11, 21 allow arbitrary movement in a horizontal plane, and namely, both longitudinal and transverse movements, as well as in particular a rotation on the vertical axis. The tibia component 1 and the tarsus component 4 conveniently consist of metal, e.g., an alloy of cobalt and chromium, which are provided on their respective outer face with a coating that promotes bone growth (e.g., calcium phosphate). The intermediate element 2, in contrast, preferably consists of a plastic material that favors sliding, in particular, polyethylene. However, the possibility of using other materials with sufficient strength and sliding capacity must not be excluded. In the implanted state the joint, and in particular the intermediate element 2, is exposed to a large axial load (symbolized by arrow 95) along the vertical axis. Due to the compression that arises in this, a horizontal force, oriented outwardly (in FIG. 1 symbolized by arrows 96) is generated in the polyethylene material of the intermediate element 2. Thanks to the convex configuration of the sliding surface 44 of the tarsal component 4, this diverging force is further increased. This can produce an undesirable deformation of the intermediate element 2 ba or a high load. In order to counteract this, it is foreseen a ring 3 inventive tensor. It consists of an alloy of cobalt and chromium with a modulus of elasticity that is approximately 400 times greater than that of the polyethylene material of the intermediate element 2. It is also possible to use titanium which has a modulus of elasticity approximately two hundred times greater. The ring 3 tensioner is made of a flat tape material. It has a thickness of, e.g., 1 mm. The ring 3 tensioner has in the horizontal section an outline that corresponds to the external contour of the intermediate element 2. In the illustrated embodiment, it is a square contour. But it is also possible to provide other contours, e.g., rounds for a modality as an intervertebral prosthesis. Their dimensions are selected in such a way that they enclose the intermediate element 2 in a tensioned manner. The tension ring 3 has a cinching zone 36 in its upper section. This counteracts a deformation of the intermediate element 2 ba or load in all lateral directions (in the longitudinal and lateral direction). Additionally, a zone 37 of protection against expansion follows the cinching zone 36 downwards. The expansion protection zone 37, which also includes the chamfer 32, additionally stabilizes the outer regions of the concave sliding surface 24 and thus acts particularly effectively against the component 96 of divergent force resulting from the convex configuration of the sliding surface 44. In this way, thanks to the inventive tension ring, the intermediate element 2 is reinforced. As a result, undesirable bending is counteracted also under a large load. The ring 3 tensioner is made with a straight outline on its upper edge 31. In this way a constant distance is produced towards the edge of the upper sliding surface 21. At its lower edge 34 the tension ring has on its longitudinal sides 33 (oriented parallel to the rib 46) an arched contour. It is made in such a way that a constant distance of the lower edge 34 of the tension ring 3 occurs at the edge of the lower sliding surface 24. On its transverse sides the tension ring 3 has a straight lower edge. This gives a constant distance again to the corresponding edge of the sliding surface 24. In the region of the transverse sides, the lower edge of the tension ring is stretched downwards in the shape of an apron and also has a chamfer 32. The latter is made in such a way that it forms a plane without interruption with the corresponding lateral surface 22 of the element 2 intermediate. Thanks to the chamfer an additional reinforcement is produced just in a region exposed to divergent forces (see arrow 96), and namely without this being accompanied by an undesirable restriction of mobility. The intermediate element 2 has in the region of its upper sliding surface 21 a flange 20. The ring 3 tensioner is pushed against the underside thereof to be flush, so that a transition occurs without interruption in the external part between the flange 20 and the outer face of the ring 3 tensioner. Thanks to the inventive configuration of the tension ring 3, the upper and lower sliding surfaces 21, 24 are free, so that they do not suffer any impairment in their support function. In Figs. 5 to 7, a variant of the tibiotarsal joint stent according to Figs. 1 to 4 is shown as an example of further embodiment. Fig. 5 shows the endoprosthesis at its implantation site provided at the distal end of the bone. tibia 91. For better clarity, only component 1 of the tibia, the intermediate element 2, and a ring 3 'of the endoprosthesis are shown. Parallel to the hollow of tibia 91 the fibular bone 90 extends. The tibia bone 91 forms at its distal end a flat area in which the tibia component 1 of the inventive endoprosthesis is disposed. This flat area is bounded on one side medially by an extension of the tibia bone 91, the so-called -ma lol us media l is 93, and laterally by a corresponding extension of the fibula bone 90, namely, the ma lol and the teralis 94. They enclose the flat area of the tibia bone. 91 in the form of a fork and with the tibial component 1 of the endoprosthesis. This can be seen well in Fig. 6. It has been shown that a time after implantation there is often a formation of tissue matter (fibrosis) 99 in the region between the badly wounded medium 93 and the intermediate element 2, respectively the ring 3 'tensioner arranged around it. This can cause pains that can not only be very unpleasant for the patient, but in many cases require a surgical intervention to eliminate the tissue material 99. To prevent, respectively, reducing the fibrous tissue material 99, a projection 39 has been formed on at least one longitudinal side 33 of the tensioning ring 3 ', conveniently on the medial side. The projection protrudes in relation to an outline congruent with the intermediate element 2. Such a projection, which protrudes from the congruent contour, can also be provided in stents that have sliding surfaces with an equal contour. The projection preferably has an external contour in the form of an arch, the arch extending over the entire length. The projection 39, conveniently, has a bulge shape in two dimensions, that is, it has a spherical surface shape (see Fig. 7a). The radii of curvature have in this different magnitudes, little curvature in the horizontal plane (as shown in Fig. 6) and a greater curvature in a frontal plane (as shown in Fig. 7a). In order to obtain a radius of curvature as great as possible, the center 30 of the circle defined by the radius of curvature is not located in the center of the tension ring 3 ', but is displaced eccentrically towards the side and preferably also frontal. The external face of the projection 39 has a smooth contour. In the embodiment shown, an option has been realized, namely also to provide the front side and the rear side of the tension ring 3 'in each case with a projection 39' respectively 39. They are conveniently configured analogously to the projection 39, but they may also have a different contour (for example, a cylinder envelope instead of a spherical one, as shown in Fig. 7b.) A transition with the same curvature between the projections 39, 39 ', 39"is not required. , but the geometries are conveniently selected in such a way that there is a free transition of steps. The lateral longitudinal side of the tension ring 3 'conveniently does not have a projection.

This has the function of guaranteeing the free passage of the tension ring. It also has the advantage that this gives a univocal orientation of the ring 3 'tensor, which reduces the risk of an insertion in a wrong position. The tensioning ring 3 'preferably consists of a metal material, just like the ring 3 tensioner of the embodiment shown in FIGS. 1 to 4, in particular of titanium or of a cobalt and chromium alloy. This allows it to make contact with the fibrosis tissue material 99 without having to fear an impairment of the adjacent tissue. In the movement of the endoprosthesis, in particular in the rotation, but also in a movement in the longitudinal direction forwards or backwards, thanks to the projection 39, a repression of the fibrous tissue material 99 is achieved. It is thus effectively counteracted that the fibrosis tissue material 99 grows in the region of the stent. The projection 39 is usually made in one piece with the ring 3 'tensioner. However, the selection of a construction in several parts in which the projection 39 is made as a separate element and fixed in the tension ring 3 'by means of appropriate fixing means must not be excluded either. The latter offers the advantage that a material which is particularly favorable to sliding and for the sliding can be selected for the projection 39. contact with the material 99 of fiber fabric, regardless of its mechanical strength as a reinforcing element, which is decisive for the selection of the material for the ring 3 tensioner. It is noted, finally, that the conditioning of a tension ring with a projection 39 is not limited to a stent of the tibiotarsal joint.

Claims (13)

1. Endoprosthesis to replace a joint comprising a component that must be attached to a lower bone and that has a sliding surface on its lower face, a component that must be joined to an upper bone and that has a sliding surface on its upper face, and a intermediate element having a sliding surface respectively on its lower and upper face, which together with the sliding surfaces of the components referred to above determines in each case a plane of movement for a support, the sliding surfaces of the intermediate contour element being in this case different, characterized in that a tension ring, made with two zones, having a cinching zone extending around in the circumferential direction and a zone of protection against expansion adjacent to it encloses the intermediate element on its side faces and is arranged in the element free intermediate of the planes of movement determined by the sliding surfaces of different contour.

Endoprosthesis according to claim 1, characterized in that the upper and lower edges are adapted to the contour of the adjacent sliding surface in each case.

3. Endoprosthesis according to one of the preceding claims, characterized in that the intermediate element has on its upper or lower face a flange where the tension ring rests. Endoprothesis according to one of the preceding claims, characterized in that the tension ring has at its lower edge on two sides a chamfer which is configured in such a way that it passes smoothly towards the intermediate element. Endoprothesis according to one of the preceding claims, characterized in that the upper and lower edges of the tension ring have a distance of at least 1 mm, preferably 1.5 to 2.5 mm. Endoprothesis according to one of the preceding claims, characterized in that the tension ring has on its internal face a projection in the form of a bulge that fits into a complementary recess in the intermediate element. Endoprothesis according to one of the preceding claims, characterized in that the tension ring has a modulus of elasticity at least fifty times, preferably at least two hundred times greater than the intermediate element. Endoprothesis according to one of the preceding claims, characterized in that the tension ring has the except on one of its external faces a bulging projection. Endoprosthesis according to claim 8, characterized in that the bulged projection extends over the entire length of the respective external face. Endoprosthesis according to claim 8 or 9, characterized in that the projection is configured on a medial longitudinal side of the intermediate element. Endoprotesis according to one of claims 8 to 10, characterized in that the bulged projection has an arc-shaped contour, seen from above. Endoprosthesis according to claim 11, characterized in that the arc-shaped contour follows an arc of a circle whose center is displaced towards the opposite side of the tension ring. Endoprothesis according to one of claims 8 to 12, characterized in that additional cuff projections have been formed on an anterior side and a rear side of the tension ring.