PROSTHESIS, IN PARTICULAR KNEE PROSTHESIS OR HIP SHAFT PROSTHESIS

Description

[0001] The invention relates to a prosthesis, in particular a knee prosthesis or a hip stem prosthesis, with a stem for arrangement and fixation in a long bone, wherein the stem extends along a stem axis between a proximal end and a distal end that can be inserted into the long bone, wherein the stem has a first, in particular solid, cross-section along a first partial length starting from the proximal end, and wherein the stem has a second cross-section that differs from the first cross-section along a second partial length, which includes the distal end of the stem, wherein the stem has stem sections extending parallel to the stem axis along the second partial length, which are separated from each other by a slot-shaped gap, wherein the stem sections can be deformed from an undeformed rest state by bending stress in such a way that the slot-shaped gap decreases.

[0002] Prostheses of the aforementioned type are known, for example, from EP 0 966 928 A2. To fix the stem in a long bone, the distal end of the stem is inserted into a bone marrow cavity of the long bone. The slot-shaped gap serves the purpose of allowing the stem sections to deform elastically and radially inward as they are inserted into the bone marrow cavity, thereby reducing the size of the slot-shaped gap. After complete insertion into the bone marrow cavity, the stem sections rest against a boundary of the bone marrow cavity under preload.

[0003] It has been found that, as a result of this pre-tension, at least some patients may experience so-called stem pain for an extended period after prosthesis implantation. Therefore, there is a need to provide stem sections that are deformable under low forces in order to reduce the elastic recoil forces. However, the problem also exists that excessively flexible stem sections could lead to undesirable plastic deformation or even stem failure.

[0004] Based on this, the present invention aims to provide a prosthesis that develops the lowest possible restoring forces, but at the same time has a sufficiently high stability.

[0005] This problem is solved according to the invention in a prosthesis of the type mentioned at the outset by a first shaft section having a first projection which is provided on a side of the first shaft section facing a second shaft section, forms a first narrowing of the space and is spaced apart from the first partial length of the shaft when viewed along the shaft axis, wherein the first projection has a first boundary surface for contact with the second shaft section, wherein the first boundary surface is spaced apart from the second shaft section in the undeformed rest state.

[0006] In the prosthesis according to the invention, at least one first shaft section is provided, the maximum bending deformation of which is limited by a projection having a first boundary surface that, in a maximally deformed state of the first shaft section, rests against the second shaft section. In an undeformed rest state, the cross-section of the slot-shaped space in the region of the first projection is narrowed. In this undeformed rest state of the first shaft section, the first boundary surface of the projection is spaced apart from the second shaft section. This allows the first shaft section to deform at the level of the first projection, even when subjected to a bending load along the shaft axis.However, the deformation path of the first shaft section at the level of the first projection is limited to a state in which the first boundary surface of the first projection of the first shaft section comes into contact with the second shaft section.

[0007] It is particularly possible that the first stem segment has a comparatively small area moment of inertia, meaning that it can be deformed with only low forces during surgical insertion of the prosthesis into the long bone and develops comparatively low restoring forces after insertion into the bone marrow space. Nevertheless, undesirable plastic deformation or even failure with fracture of the first stem segment can be prevented, since the first stem segment cannot deform further due to the contact of the first boundary surface with the second stem segment. The maximum deformation path of the first stem segment is limited by the degree to which the projection narrows the slot-shaped space. This degree is defined by the height of the first projection, which extends beyond a base plane of the first stem segment towards the second stem segment.

[0008] The prosthesis according to the invention enables the use of materials that have proven themselves in practice, such as titanium or steel alloys.

[0009] Particular advantages arise when the second shaft section has a second projection which is provided on a side of the second shaft section facing the first shaft section, forms a second narrowing of the space and is spaced apart from the first partial length of the shaft when viewed along the shaft axis, wherein the second projection has a second boundary surface for contact with the first shaft section, wherein the second boundary surface is spaced apart from the first shaft section in the undeformed rest state.

[0010] The first and second shaft sections are preferably arranged on opposite sides of a central shaft axis. If the second shaft section is also provided with a second projection, the deformation path of the second shaft section can also be limited by the fact that the second boundary surface of the second projection comes into contact with the first shaft section.

[0011] It is possible that the first projection and the second projection are arranged offset from each other when viewed along the shaft axis, so that under a bending load one of the two boundary surfaces first comes into contact with one of the two shaft sections and only subsequently the other boundary surface comes into contact with the other shaft section.

[0012] However, it is also possible that the first and second projections are arranged at the same height along the shaft axis and that bending deformation of the shaft sections in the region of the first and second projections is limited by contact with the first and second boundary surfaces. This is particularly advantageous if the shaft sections have cross-sections that are mirror images of each other with respect to the shaft axis. This results in the two shaft sections deforming symmetrically with respect to the shaft axis when the distal end is inserted into a long bone. In the implanted state of the prosthesis, these two sections exert opposing and equal-magnitude restoring forces on adjacent sections of the bone marrow space boundary.

[0013] It is further preferred if, in the undeformed rest state of the shaft sections or in a deformed state of the shaft sections, the first and second boundary surfaces are oriented parallel to each other. With a parallel orientation of the boundary surfaces in the undeformed rest state of the shaft sections and with deformation of the shaft sections, the two boundary surfaces initially only come into contact along a line of contact. With further deformation, this line of contact extends until the boundary surfaces are in contact across their entire length (i.e., across their surface). In this state, the two shaft sections can be stably supported against each other, thus avoiding locally increased surface pressures.This applies in particular if the boundary surfaces are arranged at a slight angle to each other in an undeformed rest state, the angle being chosen such that the two boundary surfaces come into contact with each other simultaneously and over their entire extent in the maximally deformed state of the shaft sections.

[0014] It is particularly preferred that the first boundary surface and / or the second boundary surface extends along the shaft axis to the distal end of the shaft. In this way, the deformation path of a shaft section or sections can be limited in a region of the shaft sections that exhibits maximum deformation under bending stress. Furthermore, it is advantageous that the slot-shaped space at the distal end of the shaft has a narrowed cross-section due to the projections, which makes it particularly easy to insert the distal end into a bone marrow cavity.

[0015] It is further proposed that the first boundary surface and / or the second boundary surface, viewed along the shaft axis, has a boundary surface length which is between 5% and 45%, in particular between 15% and 35%, of the second partial length. In other words, 95% to 55%, in particular 85% to 65%, of the second partial length of the shaft sections preferably has no projections, i.e., a slot-shaped gap that is not narrowed by at least one projection. This "projection-free" second partial length of the shaft sections allows for a large-volume gap, which is accompanied by a corresponding reduction in the cross-section of the shaft sections, resulting in low deformation resistance.

[0016] The shaft sections, in a region adjoining the first shaft section and in the undeformed rest state, exhibit a shaft section spacing corresponding to the width of the slot-shaped gap. This ("protrusion-free") shaft section spacing is preferably at least 20%, and particularly at least 25%, of a maximum diameter of a circumferential surface of the shaft. This maximum diameter is, for example, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, 20 mm, 22 mm, or 24 mm. For the example of a shaft with a diameter of 10 mm, it is therefore preferred that the aforementioned shaft section spacing is at least 2 mm, so that the radial extent of the shaft sections, in a symmetrical design, is a maximum of 4 mm relative to the shaft axis.

[0017] The first projection has a first height, measured perpendicular to the first boundary surface, by which the first projection extends beyond a base plane of the first shaft section. If present, the second projection has a second height, measured perpendicular to the second boundary surface, by which the second projection extends beyond a base plane of the second shaft section. These base planes form mutually facing boundary surfaces of the slotted space, viewed along the shaft axis in an area where no projection is located. It is proposed that the first height and / or the second height be at least 0.5 mm.This means that the slot-shaped gap in the area of ​​at least one projection is narrowed by at least 0.5 mm deformation path, in comparison to a prosthesis known from the prior art, which only had mutually facing base planes and no projection or projections according to the invention.

[0018] If two projections are provided which are arranged at the same height along the shaft axis, the slot-shaped gap is narrowed by at least 1 mm compared to the prior art, thus shortening the common deformation path of the shaft section by at least 1 mm compared to the prior art.

[0019] In particular, it is preferred if the ratio between the first height or the second height, or between the sum of the first height or the second height, or between the sum of the first height and the second height, on the one hand, and the ("projection-free") shaft section spacing, on the other hand, is at least 10%, and in particular at least 20%. This results in a reduction of the deformation path – relative to the "projection-free" prior art – by at least 10%, and in particular by at least 20%.

[0020] It is further advantageous if the stem has a circumferential surface with longitudinal grooves extending parallel to the stem axis. Such a design of a circumferentially closed surface of a stem can also be described by the stem having a "Wagner profile." The longitudinal grooves support reliable anchorage to a boundary of the bone marrow space, but in themselves create local weakenings in the cross-section of the stem sections. The prosthesis according to the invention enables compensation for this weakening.

[0021] Further features and advantages of the invention are the subject of the following description of a preferred embodiment.

[0022] The drawing shows: Fig. 1 a sectional view of a prosthesis known from the prior art; Fig. 2 a side view of an embodiment of a prosthesis according to the invention; Fig. 3 a in Fig. 2Section marked III in enlarged view.

[0023] A prosthesis known from the prior art (EP 0 966 928 A2) is in Figure 1 The prosthesis is shown and designated with reference numeral 10. It has a stem 12 which is connected at one end to a joint section 14, for example by means of an intermediate connecting part 16.

[0024] The shaft 12 extends along a shaft axis 18 between a proximal end facing the joint section 14 and a free distal end 22, which, when the shaft is arranged in a bone marrow cavity of a long bone, is first inserted into the bone marrow cavity.

[0025] Starting from the proximal end 20, the shaft 12 has a first, particularly solid, cross-section along a first partial length 24. Along a second partial length 26, which includes the distal end 22 of the shaft 12, the shaft 12 has a second cross-section that differs from the first.

[0026] Along the second partial length 26, a slot-shaped gap 28 extending parallel to the shaft axis 18 is provided. The gap 28 is bounded on opposite sides by a first shaft section 30 and a second shaft section 32 spaced apart from it.

[0027] For the following description of an embodiment of a prosthesis 10 according to the invention, reference is made to the preceding description. Figure 1 Reference is made to the prosthesis 10 according to... Figure 2can be designed, for example, as a knee prosthesis or hip stem prosthesis; it is understood that a joint section 14 (not shown) may also be provided for the prosthesis 10, possibly using an intermediate part 16.

[0028] The shaft 12 has a circumferential surface 34 which, viewed along the shaft axis 18, is provided at least partially with longitudinal grooves 36, which are distributed over the circumference of the shaft 12, in particular regularly distributed. For example, a total of ten longitudinal grooves 36 are provided, offset from one another in the circumferential direction.

[0029] The shaft 12 has a maximum diameter of 38, which is, for example, between 10 mm and 24 mm.

[0030] For further description, please refer to the Figure 3 Reference made to.

[0031] The first shaft section 30 has a base plane 40 facing the second shaft section 32 in a region adjoining the first partial length 24 of the shaft 12. The second shaft section 32 has a second base plane 42 facing the first base plane 40. The base planes 40 and 42 are spaced apart from each other by a shaft section distance 44 and form the boundaries of the slot-shaped space 28.

[0032] The first shaft section 30 has a projection 46 on its side facing the second shaft section 32, which has a first boundary surface 48 facing the second shaft section 32. The distance between the first boundary surface 48 and the base plane 40 corresponds to a height 50 of the first projection 46. The first projection 46 is formed integrally with the first shaft section 30.

[0033] The second shaft section 32 has a second projection 52 on its side facing the first shaft section 30, with a second boundary surface 54 facing the first shaft section 30. The distance of the second boundary surface 54 to the base plane 42 of the second shaft section 32 corresponds to a second height 56 ​​of the second projection 52. The second projection 52 is formed integrally with the first shaft section 30.

[0034] The first height 50 is at least 0.5 mm; the second height 56 ​​is at least 0.5 mm.

[0035] The projections 46 and 52 form respective constrictions of the slot-shaped space 28. In the area of ​​the boundary surfaces 46, 48, the slot-shaped space 28 is constricted by the sum of the heights 50 and 56 of the projections 46 and 52.

[0036] The boundary surfaces 48 and 54 are oriented parallel to each other, especially in an undeformed rest state of the shaft 12.

[0037] The boundary surfaces 48 and 54 are arranged on opposite sides of the shaft axis 18 with respect to the shaft axis 18 and are mirror-symmetrical to each other with respect to their dimensions and position with respect to the shaft axis 18.

[0038] The boundary surfaces 48 and 54 extend along the shaft axis 18 over a preferably identical boundary surface length 58.

[0039] Starting from the undeformed resting state of the shaft sections 30 and 32 shown in the drawing, the distal end 22 can be inserted into a bone marrow cavity of a long bone, in particular a tibia or a femur. The diameter 38 of the shaft 12 is selected in relation to the diameter of the bone marrow cavity such that the shaft can preferably be inserted into the bone marrow cavity over the extension of the longitudinal grooves 36, and that after this insertion the radially outwardly facing surfaces of the shaft sections 30 and 32 bear against the respective boundary sections of the bone marrow cavity under preload.

[0040] Starting from an undeformed initial state, see Figure 3The shaft sections 30 and 32 can be bent, whereby the boundary surfaces 48 and 54 move towards each other, until a state is reached in which the boundary surfaces 48 and 54 are in contact with each other, at least sectionally, preferably over their entire surface. This arrangement defines a maximum deformation path of the shaft sections 30 and 32 over the boundary surface length 58.

[0041] Assuming a state of the art (see Figure 1Assuming identical shaft section spacing 44, identical materials, identical dimensions for shaft sections 30 and 32, and an identical second section length 26, the deformation path of shaft sections 30 and 32 in the region of the distal end 28 is thus shortened by a factor corresponding to the sum of the heights 50 and 56 of the projections 46 and 52. This ensures that shaft sections 30 and 32 are bendable with low deformation forces and, accordingly, develop only low restoring forces after insertion into a long bone, while simultaneously preventing undesirable plastic deformation or failure with fracture of shaft sections 30 and 32.

Claims

1. Prosthesis (10), in particular a knee prosthesis or a hip stem prosthesis, comprising a shaft (12) to be arranged and fastened in a long bone, the shaft (12) extending along a shaft axis (18) between a proximal end (20) and a distal end (22) that can be inserted into the long bone, the shaft (12), along a first partial length (24) starting at the proximal end (20), having a first in particular solid cross section, and the shaft (12), along a second partial length (26) that includes the distal end (22) of the shaft (12), having a second cross section that differs from the first cross section, the shaft (12), along the second partial length (26), having shaft portions (30, 32) extending parallel to the shaft axis (18), which shaft portions are separated from each other by a slot-shaped interspace (28), it being possible for the shaft portions (30, 32), starting from an undeformed rest state, to be deformed by bending load in such a way that the slot-shaped interspace (28) decreases in size, characterized in that a first shaft portion (30) has a first projection (46) which is provided on a side of the first shaft portion (30) facing a second shaft portion (32), forms a first narrowing of the interspace (28) and is arranged spaced apart from the first partial length (24) of the shaft (12), considered along the shaft axis (18), the first projection (46) having a first boundary surface (48) for bearing against the second shaft portion (32), the first boundary surface (48) being spaced apart from the second shaft portion (32) in the undeformed rest state.

2. Prosthesis (10) according to claim 1, characterized in that the second shaft portion (32) has a second projection (52) which is provided on a side of the second shaft portion (32) facing the first shaft portion (30), forms a second narrowing of the interspace (28) and is arranged spaced apart from the first partial length (24) of the shaft (12), considered along the shaft axis (18), the second projection (52) having a second boundary surface (54) for bearing against the first shaft portion (30), the second boundary surface (54) being spaced apart from the first shaft portion (30) in the undeformed rest state.

3. Prosthesis (10) according to claim 2, characterized in that a bending deformation of the shaft portions (30, 32) in the region of the first projection (46) and of the second projection (52) is limited by virtue of said projections bearing against the first boundary surface (48) and the second boundary surface (54).

4. Prosthesis (10) according to claim 3, characterized in that in the undeformed rest state of the shaft portions (30, 32) or in a deformed state of the shaft portions (30, 32), the first boundary surface (48) and the second boundary surface (54) are oriented parallel to each other.

5. Prosthesis (10) according to any of the preceding claims, characterized in that the first boundary surface (48) and / or the second boundary surface (54) extend or extends to the distal end (22) of the shaft (12), considered along the shaft axis (18).

6. Prosthesis (10) according to any of the preceding claims, characterized in that the first boundary surface (48) and / or the second boundary surface (54) have or has a boundary surface length (58), considered along the shaft axis (18), of between 5% and 45%, in particular between 15% and 35%, of the second partial length (26).

7. Prosthesis (10) according to any of the preceding claims, characterized in that the shaft portions (30, 32), in a region of the second partial length (26) of the shaft (12) adjoining the first partial length (24) of the shaft (12) and in the undeformed rest state of the shaft portions (30, 32), have a shaft portion spacing (44) relative to each other which corresponds to the width of the slot-shaped interspace (28), the shaft portion spacing (44) preferably being at least 20%, in particular at least 25%, of a maximum diameter (38) of a circumferential surface (34) of the shaft (12).

8. Prosthesis (10) according to any of the preceding claims, characterized in that the first projection (46) has a first height (50) measured perpendicular to the first boundary surface (48), and / or in that the second projection (52) has a second height (56) measured perpendicular to the second boundary surface (54), the first height (50) and / or the second height (56) preferably being at least 0.5 mm.

9. Prosthesis (10) according to claim 8 when dependent on claim 7, characterized in that a ratio between the first height (50) or the second height (56) or between the sum of the first height (50) and the second height (56) on the one hand and the shaft portion spacing (44) on the other hand is at least 10%, in particular at least 20%.

10. Prosthesis (10) according to any of the preceding claims, characterized in that the shaft (12) has a circumferential surface (34) comprising longitudinal grooves (36) extending parallel to the shaft axis (18).