FLEXIBLE PROSTHETIC LINER AND PROSTHETIC SHAFT SYSTEM

Abstract

A flexible prosthetic liner having a proximal access opening for a stump of a limb and a distal end region as well as a flexible tension member which is arranged in the distal 5 end region on the prosthetic liner. The tension member is detachably fastened to the prosthetic liner by means of a closure system that can be actuated without a tool. A prosthetic shaft system may be formed by such a prosthetic liner and a prosthetic shaft.

Claims

1. A flexible prosthetic liner comprising: a proximal access opening for a stump of a limb; a distal end region; a flexible tensioning member arranged on the prosthetic liner in the distal end region, wherein the tensioning member is releasably fastened to the prosthetic liner via a closure system, the closure system being actuable without a tool.

2. The prosthetic liner as claimed in claim 1, wherein the tensioning member is fastened to the prosthetic liner in at least one of a form-fitting and a force-fitting manner.

3. The prosthetic liner as claimed in claim 1, wherein a first form-fitting element is arranged on the tensioning member and enters into engagement with a second form-fitting element arranged on the prosthetic liner.

4. The prosthetic liner as claimed in claim 3, wherein the first and second form-fitting elements are in the form of at least one of a snap-action connection, a hook and loop fastener, a press stud, a sliding closure and a quarter-turn fastener.

5. The prosthetic liner as claimed in claim 3, wherein the closure system blocks the tensioning member counter to an insertion direction of the prosthetic liner from a prosthesis socket.

6. The prosthetic liner as claimed in claim 3, wherein the closure system blocks the tensioning member in at least one direction perpendicular to a pulling-out direction of the prosthetic liner from the prosthesis socket.

7. The prosthetic liner as claimed in claim 5, wherein an additional securing element for securing the prosthetic liner on the prosthesis socket is arranged at the distal end region of the prosthetic liner.

8. The prosthetic liner as claimed in claim 1, wherein the closure system has a magnetic securing feature.

9. A prosthesis socket system with a prosthetic liner as claimed in claim 1 and a prosthesis socket, the prosthesis socket having a proximal access opening, a side wall at least partially circumferentially surrounding the prosthetic liner, a distal closing region, devices for connecting a prosthetic component, and a pass-through opening for the tensioning member in the distal closing region.

10. The prosthesis socket system as claimed in claim 9, wherein a dimensionally stable supporting device with a recess for the closure system is arranged at the distal closing region.

11. The prosthesis socket system as claimed in claim 9, wherein a fixing device for the tensioning member or at least one deflecting element for the tensioning member, on which devices for securing same on itself are arranged, is arranged on an outer side of the side wall of the prosthesis socket.

12. The prosthesis socket system as claimed in claim 9, wherein a window is formed in a proximal region of the side wall for a second tensioning member or for securing a deflecting element.

13. The prosthesis socket system as claimed in claim 12, wherein the tensioning member and the second tensioning member have fastening devices for securing same to each other.

14. The prosthesis socket system as claimed in claim 9, wherein the pass-through opening is oriented perpendicular to the insertion direction of the prosthetic liner such that the first tensioning member can be brought laterally out of the prosthesis socket.

15. A flexible prosthetic liner comprising: a flexible base body comprising: an internal cavity; a proximal access opening sized to receive a stump of a limb into the internal cavity; a closed distal end region; a flexible tensioning member arranged on the flexible base body in the distal end region; a closure system releasably connecting the tensioning member to the flexible base body, the closure system being actuable without a tool.

16. The prosthetic liner as claimed in claim 15, wherein the tensioning member is fastened to the prosthetic liner in at least one of a form-fitting manner and a force-fitting manner.

17. The prosthetic liner as claimed in claim 15, wherein a first form-fitting element is arranged on the tensioning member and enters into engagement with a second form-fitting element arranged on the flexible base body.

18. The prosthetic liner as claimed in claim 17, wherein the first and second form-fitting elements are in the form of at least one of a snap-action connection, a hook and loop fastener, a press stud, a sliding closure and a quarter-turn fastener.

19. The prosthetic liner as claimed in claim 17, wherein the closure system blocks the tensioning member counter to an insertion direction of the prosthetic liner into a prosthesis socket.

20. The prosthetic liner as claimed in claim 17, wherein the closure system blocks the tensioning member in at least one direction perpendicular to a pulling-out direction of the prosthetic liner from a prosthesis socket.

Description

[0020] Exemplary embodiments of the invention will be explained in more detail below with reference to the attached figures. The same reference signs denote identical components. In the figures:

[0021] FIG. 1 shows the components of a prosthesis socket system in a partially mounted state;

[0022] FIG. 2 shows a detailed view of a closure system;

[0023] FIG. 2a shows a partial view of a form-fitting element;

[0024] FIG. 3 shows a variant of the closure system;

[0025] FIG. 4 shows a detailed illustration of a prosthetic liner with tensioning means before assembly;

[0026] FIG. 5 shows a detailed view of FIG. 4;

[0027] FIG. 6 shows a top view of a closure system according to FIG. 5;

[0028] FIGS. 7 and 8 show variants of the fitted prosthesis socket system in a sectional view;

[0029] FIG. 9 shows a perspective illustration of a fitted prosthesis socket system; and

[0030] FIGS. 10a to 10e show individual illustrations of a tensioning means.

[0031] FIG. 1 shows a prosthesis socket system with a flexible prosthetic liner 10 which has a proximal access opening 11 for a stump of a limb, and also a distal end region 12. The prosthetic liner 10 is preferably formed in a closed manner in the distal end region 12, and also the side wall 14, which reaches from the distal end region 12 as far as the access opening 11, can be formed with a closed wall, and therefore the prosthetic liner 10 can be placed around the entire circumference of the stump (not illustrated) of a limb. The prosthetic liner 10 is preferably composed of a silicone or another flexible or optionally elastic polymer in the region of the side wall. In principle, it is also possible for a different material than the basic material to be applied or incorporated on or in the inner side or on or in the outer side of the prosthetic liner 10; a textile coating can also be arranged on one of the surfaces of the prosthetic liner 10.

[0032] A dimensionally stable closing cap 13 which is flexible to a limited extent and forms the distal closure of the prosthetic liner 10 is arranged at the distal end region 12, which is closed in the exemplary embodiment, of the prosthetic liner 10. The closing cap 13 serves for stabilizing the distal end region 12 and for forming a resting surface for the end of the stump. A second form-fitting element 42 is arranged, screwed in the exemplary embodiment shown, within the distal closing cap 13 and can be brought into engagement with a first form-fitting element 41, which is fastened to a flexible tensioning means 30, which is preferably rigid under tension, in the form of a strap or a cable.

[0033] The first form-fitting element 41 is secured to the tensioning means 30 in a form-fitting manner, but other types of fastening are also possible. The first form-fitting element 41 together with the second form-fitting element 42 forms a closure system 40, wherein the two form-fitting elements 41, 42 are separable from each other and securable to each other without a tool, and therefore the tensioning means 30 can be rapidly fastened to the prosthetic liner 10 or separated therefrom.

[0034] The tensioning means 30 is inserted in a prosthesis socket 20. The prosthesis socket 20 has a proximal access opening 21, a distal closing region 22 and an at least partially encircling side wall 23 via which a receiving space for the prosthetic liner 10 together with the stump is formed. The prosthesis socket 20 can be manufactured individually for the user, for example by taking impressions of the stump and adjusting a single-part prosthesis socket which is closed in the circumferential direction and follows the contour of the stump. The side wall 23 here can have a closed cross section, at least over most of the longitudinal extent or height of the prosthesis socket. Windows 28 can also be formed in the prosthesis socket 20 or the side wall 23. It is likewise possible for the prosthesis socket 20 to be composed of a plurality of segments which are integrally formed on or fastened to the distal closing region 22 and, fitted state, form a dimensionally stable prosthesis socket 20. The prosthesis socket 20 serves for receiving the prosthetic liner 10 with the stump and the arrangement and securing of further prosthetic components, for example prosthetic knee joints, prosthetic feet or prosthetic hands.

[0035] The prosthesis socket 20 in the example illustrated has a dimensionally stable supporting device 26 in the distal end region 22, said supporting device being arranged within the prosthesis socket 20. In principle, it is also possible to arrange the dimensionally stable supporting device 26 on the outer side. The supporting device 26 has a shell-like or cap-like shape and can have a cylindrical outer contour. It serves for supporting the liner 10 and the stump and/or for stabilizing the distal end of the prosthesis socket 20. In addition, a device 25 for connecting a prosthetic component is arranged at the distal end of the prosthesis socket 20. The device 25 can be in the form, for example, of a receptacle of a pyramid adapter or in the form of a pyramid adapter itself.

[0036] A recess 27 is formed within the supporting device 26 in order to receive the closure system 40 in the mounted and fitted state such that no excessive pressure loadings occur at the end of the stump because of a central, punctiform introduction of force via the closure system 40. The closing cap 13 rests extensively on the beveled upper edge of the supporting device 26 and brings about a uniform distribution of pressure to the end of the stump.

[0037] A pass-through opening 24 for the tensioning means 30 is formed in the supporting device 26 and in the side wall 23 in the distal closing region 22 of the prosthesis socket. A seal 29 is arranged on the outer side of the pass-through opening 24 in order as far as possible to ensure fluid-tight sealing of the interior of the prosthesis socket 20. A blocking element 31 is arranged at the free end of the tensioning means 30 and is fastened releasably to the tensioning means 30 in order, after the tensioning means 30 is passed through the passage opening 24, to avoid said tensioning means being inadvertently pulled out during removing of the stump from the prosthesis socket 20.

[0038] A deflecting element 51 is arranged on the outer side of the side wall 23. The deflecting element 51 is in the form of an eyelet which is fastened to a mounting part which is arranged in a window 28 in the side wall 23 of the prosthesis socket. The free end of the tensioning means 30 is pulled through the deflecting element 51 when the fitted prosthetic liner 10 is coupled with the tensioning means 30 via the closure system 40. During entry into the prosthesis socket 20, the end of the tensioning means is at the same time pulled through the passage opening 24 such that the prosthetic liner 10 is pulled into the prosthesis socket 20. After the free end of the tensioning means 30 is passed through the deflecting element 51, the tensioning means 30 can be fixed on itself via fastening elements (not illustrated), for example hook and loop fasteners, loops or the like, and therefore, in the fully introduced state, the closure element 40 is subjected to a tensile stress which is applied by the tensioned tensioning means 30. Instead of a deflecting element, a second tensioning means or a fastening element can also be secured to the side wall and can be connected to the first tensioning means 30 in order to fix the first tensioning means 30 in the desired tensioned position.

[0039] In the mounted state, the closure system 40 is approximately level with the horizontally oriented passage opening 24, which is directed toward the lateral side, or is slightly above said passage opening, and therefore, when the tensioning means 30 is sufficiently tensioned, fixing of the prosthetic liner 10 within the socket 20 is ensured. Neither a lateral movement in the horizontal plane nor a pulling-out movement counter to the insertion direction of the prosthetic liner 10 is then possible. The closure system 40, which provides form-fitting locking by means of the two form-fitting elements 41, 42, blocks a lateral relative movement of the two form-fitting elements 41, 42 in the horizontal plane and also shifting in or counter to the insertion direction, and therefore only a rotational degree of freedom is available because of the spherical-head-shaped configuration of the second form-fitting element 42. All other degrees of freedom are blocked.

[0040] Alternatively to securing the tensioning means 30 on itself and deflecting same via a deflecting element 51, the tensioning means 30 can also be secured directly to the outer side of the side wall 23.

[0041] For removing the stump from the prosthesis socket 20, the fixing of the tensioning means 30 is released and the prosthetic liner 10 together with the stump is guided out of the prosthesis socket 20. The tensioning means 30 is also pulled out at the same time from the interior of the prosthesis socket 20. As soon as the prosthetic liner 10 is located outside the prosthesis socket 20, the tensioning means 30 can be released from the prosthetic liner 10, by release of the form-fitting elements 41, 42 from each other, for example by pressing a spring lock, and therefore the prosthesis socket 20 together with the remaining prosthesis components, which are fastened thereto, and the tensioning means 30 is separated from the prosthetic liner 10.

[0042] FIG. 2 shows a detailed illustration of the two form-fitting elements 41, 42 according to FIG. 1 in the locked state. The first form-fitting element 41 is fixed to the strap-like tensioning means 30. A spherical head is formed on the second form-fitting element 42, at the opposite end of which spherical head a screw thread is arranged, and therefore the second form-fitting element 42 is fastenable to the prosthetic liner 10 with a tool. The first form-fitting element 41 has a recess corresponding to the second form-fitting element 42, in which the spherical head can be snapped in order to lock the two form-fitting elements 41, 42 in a form-fitting manner to each other. For the mounting, the first form-fitting element 41 is pressed perpendicularly onto the spherical head, the material of the first form-fitting element 41 expands, or resilient components or slots are arranged therein and are shifted, and therefore, after the mounting, the spherical head is accommodated in the first form-fitting element 41. In the mounted state, only rotation of the first form-fitting element 41 about the longitudinal extent of the second form-fitting element 42 is possible; all other rotational or translational shifting is blocked.

[0043] In order to open the closure system and therefore in order to release the tensioning means 30 from the liner 10 (not illustrated), the first form-fitting element 40 is pulled off from the second form-fitting element 42 or separated by bending. Tools are not necessary for this.

[0044] A variant of the first form-fitting element 41 is shown in a side view and partial sectional illustration in FIG. 2a. The form-fitting element 41 has a disengaging mechanism 45 with which it is possible to disengage locking elements within the form-fitting element 41 by means of a pull on the outer collar, and therefore the second form-fitting element, which corresponds to a recess and is not illustrated, can be inserted without pressure or only under slight pressure into the recess of the first form-fitting element 41 and locked there. For the release, corresponding unlocking is undertaken by pulling on the collar 45.

[0045] FIG. 3 shows a variant of the closure system with the two form-fitting elements 41, 42 in the locked state. Instead of a spherical-head-shaped form-fitting element, locking is realized via a in the truncated-cone-like projection with an undercut and a corresponding recess in the first form-fitting element 41. In addition to the pure form-fitting lockings, magnetic securing 43 is arranged in the first form-fitting element 41 and pulls the second form-fitting element 42 into the end position and holds same therein or at least applies an additional holding force. In the locked state, all of the translational degrees of freedom are also blocked here. In the case of a truncated-cone-like configuration, a rotational degree of freedom is free, and the latter can also be blocked in the case of a configuration which is not rotationally symmetrical.

[0046] A further variant of the invention is illustrated schematically in FIG. 4. Instead of locking by placing on and pressing in the insertion direction of the prosthetic liner in the prosthesis socket, i.e. in the longitudinal extent of the prosthetic liner, in the variant according to FIG. 4 the locking takes place by pushing the first form-fitting element 41 into the second form-fitting element 42 in a horizontal plane, i.e. perpendicularly to the insertion direction of the prosthetic liner 10 into the prosthesis socket 20. After the insertion and displacing as far as an end stop, locking against an unintentional moving out can take place as a result of a resilient snap-action connection, a rotational movement or an additional form-fitting element or turning a lock.

[0047] After the first form-fitting element 41 is inserted by being pushed in the arrow direction into the second form-fitting element 42, form-fitting locking takes place, and therefore the tensioning means 30 is fixed to the prosthetic liner 10 and separating perpendicularly to the insertion direction of the prosthetic liner into the prosthesis socket is not possible. By means of the configuration of the second form-fitting element 42 with a solid side wall as end stop, further shifting after the end stop is reached is prevented.

[0048] FIG. 5 shows an enlarged detailed illustration of the closure system 40 with the two form-fitting elements 41, 42 and the locking direction. For unlocking purposes, pressure can be exerted on the first form-fitting element 41, as a result of which the corresponding form-fitting element is expanded and release counter to the insertion direction is made possible.

[0049] FIG. 6 shows a top view of the form-fitting elements 41, 42 according to FIG. 5. An insertion channel with an undercut is formed in the second form-fitting element 42. When the first form-fitting element 41 is inserted into the insertion channel, spring tongues are expanded and, after reaching the end position, surround a pin-shaped projection again and prevent unintentional unlocking, but permit pushing out by application of an oppositely directed force, optionally after actuation of an unblocking device.

[0050] Alternatively to the form-fitting elements shown, the latter can also be in the form of a hook and loop fastener. There are highly effective variants of hook and loop fasteners, with which it is possible to achieve a strong connection. Such high-strength hook and loop fasteners can also be used on account of the fact that a peeling-off movement is possible after at least partially removing the stump from the prosthesis socket 20. Pressure closures, pure sliding closures and/or quarter-turn fasteners, i.e. a sliding and rotating closure device can likewise be provided in order to connect the prosthetic liner 10 to the tensioning means 30 without a tool.

[0051] FIG. 7 shows a variant of the prosthesis socket system in the mounted state in a sectional illustration. The construction substantially corresponds to that of FIG. 1. It can be seen that the prosthetic liner 10 lies with its side wall 14 over substantially its entire longitudinal extent against the inner side of the side wall 23 of the prosthesis socket 20. A second tensioning means 35 is arranged instead of a deflecting element 51 within the window 28 in the side wall 23, said second tensioning means being able to be in the form of a strap, a lug or the like. For example, hook regions of a hook and loop fastener or components of identical effect are arranged on that side of the second tensioning means 35 which faces the socket 20 while the loop components or correspondingly acting fastening devices are arranged on the first tensioning means 30 on the outer side, i.e. the side facing away from the prosthesis socket 20. As a result, it is possible to connect the two tensioning means 30, 35 to each other in their end regions such that they can be secured to each other. Instead of a hook and loop fastener, other fastening devices 36 can also be arranged on the tensioning means 30, 35, for example a buckle, holes for hooks, bolts or other projections, press studs or clamping devices, with which the first tensioning means 30 can be secured to the second tensioning means 35.

[0052] FIG. 8 shows the variant according to FIG. 1 in the mounted state. A deflecting element 51 is fastened to the side wall 23 of the prosthesis socket 20. That end of the first tensioning means 30 which is led out of the prosthesis socket 20 is passed through the deflecting element 51. A component of a hook and loop fastener is arranged on the outer side of that portion of the tensioning means 30 which has not yet been passed through. The corresponding component of the hook and loop fastener is arranged on the turned-over side, which then points inward in the direction of the prosthesis socket, of the turned-over end of the first strap, and therefore, after the tensioning means 30 is turned over and deflected on itself, the tensioning means 30 in the form of a strap can be secured on itself via the devices 37 with the hook region and loop region.

[0053] A further variant of the invention is illustrated in FIG. 9, in which, in the exemplary embodiment illustrated, the tensioning means 30 is in turn fixed on itself, as shown in FIG. 8. In addition, a fixing device 50 in the form of a loop region or a hook region is arranged or formed on the outer side of the side wall 23 in order to be able to secure the tensioning means 30 on the outer wall either after deflection by the deflecting element 51, or else in order to be able to dispense with the deflecting element 51 and therefore also the window 28, and to fix the first tensioning means 30 directly to the side wall 23. Instead of fixing via a hook and loop fastener, said tensioning means can also be fixed via other form-fitting devices, for example by holes in the tensioning means 30 and one or more projections which latch into the holes, by buckles or by other form-fitting or force-fitting fixing devices 50.

[0054] FIG. 10a, in a detailed illustration, illustrates a tensioning means 30 which is secured in a form-fitting manner to the second form-fitting element 42 of a prosthetic liner (not illustrated specifically). In the exemplary embodiment illustrated, the tensioning means 30 is in the form of a strap which is turned back on itself and is in the form of two layers. An upper layer 33 and a lower layer 32 are connected at their free ends to a seam 392 via an end region. Alternatively or additionally, the two layers 32, 33 can be welded, adhesively bonded or connected to each other in some other way. In the front, turned-over region of the tensioning means 30, the two layers 32, 33 are likewise permanently connected to each other, for example via a seam 391, which can be in the form of an adhesive seam, welding seam or stitched seam. The connection of the two layers 32, 33 can be formed over the full surface area in the region of the free ends, and can be located over most of the full surface area or else basically also only at the circumference in the region of the turning-over point. A free region 38 in which the two layers 32, 33 are not connected to each other is formed between the two seam regions 391, 392, and therefore the two layers 32, 33 can be shifted away from each other in said central region.

[0055] A band 34 which can be in the form of a string, cable, cord, wire, strand or strap is fastened to the tensioning means 30. The band 34 projects beyond the turned-over end of the tensioning means 30 and, in the mounted state, surrounds the second form-fitting element 42 in such a manner that the tensioning means 30 cannot be released from the second form-fitting element either in the longitudinal extent of the undercut pin or transversely with respect thereto. The band 34 is fixed to the tensioning means 30 at two fastening points 341, 342, the first fastening point 341 is located in that region of the tensioning means 30 which lies section located on that side of the free region 38 which faces away from the second form-fitting element 42, while the second fastening point 342 is located in the region of the turning-over point of the tensioning means 30. It is also possible in principle for the second fastening point 342 to be located shifted further in the direction of the free region 38.

[0056] In the position according to FIG. 10a, the band portion which protrudes beyond the turned-over end of the tensioning means 30 is dimensioned in such a manner that the tensioning means 30 is pulled straight. The two layers 32, 33 also rest on each other in the free region 38, in which the two layers 32, 33 are not connected to each other via a seam, and the loop formed by the protruding band 34 substantially corresponds in respect of its circumference to the looped-around second form-fitting element 42 in order thus to fix the tensioning means 30 to the second form-fitting element 42.

[0057] The second form-fitting element 42 is in the form of a mushroom-shaped pin with a round cross section, and therefore the tensioning element 30 is movable about the longitudinal axis of the pin. In an alternative embodiment, it is possible to configure the pin to be non-circular or to provide same with a flattened portion in order not to permit any rotation in a plane perpendicular to the longitudinal axis of extent of the pin when the turned-over end of the tensioning means 30 lies against the flattened portion.

[0058] FIG. 10b illustrates the tensioning element 30 in a state in which the latter can be removed from the second form-fitting element 42. For this purpose, the front portion of the tensioning means 30, which portion is placed in the region of the turned-over point, moves in the direction of the rear portion with the free ends such that the tensioning means 30 is compressed. This results in compression in the free region 38, leading to two arches 382, 383 since the distance between the turned-over point of the tensioning means 30 and the free end is reduced. The band 34 is fixed at its rear end to the rear portion via the first fastening point 341 and in the front portion is guided in a longitudinally movable manner within the tensioning means 30 such that, during shifting of the turned-over point, the band 34 can slide out of the tensioning means 30, in particular when it is still fastened to the second form-fitting element 42. Only the front, turned-over end of the tensioning means 30 then has to be shifted in the arrow direction, as a result of which the loop which is formed by the band 34 outside the tensioning means 30 is enlarged. In the state according to FIG. 10b, it is possible to remove the loop formed by the band 34 in the region protruding beyond the turned-over point from the second form-fitting element 42. Alternatively to the embodiment illustrated, in which the band 34 is guided only along a channel or only with a portion in the front region of the double-layered tensioning means 30, it is also possible to design the band 34 as a tightening loop, in which the looping-around point of the tightening loop lies in the strap or tensioning means 30, namely on that side of the turned-over point between the upper layer 33 and the lower layer 32 which faces away from the second form-fitting element 42. The band 34 or the cable is led here out of the tensioning means 30 at the front end thereof and forms the loop which is placed around the second form-fitting element 42. Behind the front end of the tensioning means 30, the band 34 or the cable is placed in the form of a loop around that part of the band 34 or of the cable which is mounted movably in the front portion of the tensioning means 30, thus forming a tightening loop, in which, by pulling on the rear end of the tensioning means 30, the loop emerging from the tensioning means 30 is tightened. If, at the front end of the tensioning means 30, i.e. at the tensioning means portion in front of the free region 38, pulling is undertaken away from the second form-fitting element 42, the loop is enlarged and can be removed from the form-fitting element 42. Such a variant of the invention is shown in FIG. 10e.

[0059] The shortened, pushed-together state, as is reached after pulling back the front, turned-over end of the tensioning means 30, is shown in FIG. 10c. The arches 382, 383 are directed outward as free portions, and the band 34 forms a comparatively large loop which protrudes beyond the turned-over point. In order to reduce the loop diameter and therefore to secure the tensioning means 30 on the second form-fitting element 42 by the band 34 via the loop, the two ends of the tensioning means 30 are pulled apart again, which is shown in FIG. 10d and is indicated by the two arrows. By pulling apart the two ends of the tensioning means, the band 34 is also pulled through the front portion of the tensioning means and the free region 38, and therefore the loop diameter of the band 34 is reduced.

[0060] In an advantageous refinement, the tensioning means 30 is provided with resetting properties, for example by selection of material or by integrating resetting elements, such as plastics springs, struts, stiffening means or the like, such that automatic resetting into the state according to FIG. 10d takes place. As a result, a permanent prestressing force is applied to the band 34, and therefore the loop and the tensioning means 30 remain securely held on the second form-fitting element 42. Said prestressing force can be achieved by increased buckling stability of the material of the tensioning means.