EXTERNAL RETENTION DEVICE FOR BONE FRACTURES

Description

[0001] The invention relates to an external holding device for holding fragments of a bone after a bone fracture or, more generally, skeletal parts, with the features of the preamble of claim 1.

[0002] These fixation devices are also known as external fixators, and there are many different designs. What these devices have in common is that they allow two or more bone fragments or individual skeletal parts to be fixed together in a desired position relative to each other. The fragments are attached to each other in their original position within the unbroken bone so that they are held in the desired position. For this purpose, screws, pins, wires, or other fasteners, which are collectively referred to as pins below, are attached externally through the skin to or within the bone fragments. The pins are then attached to the fixation device, although they may or may not be integral parts of the fixation device.

[0003] Utility model DE 84 19 372 U1 discloses such a holding device with rods and joints connecting the rods. With the joints released, the rods can be rotated relative to each other about the axes of the joints and rigidly connected by locking the joints. The joints have two U-shaped clamping jaws with a slot open at one end, terminating in a round hole through which the rods can be inserted. Pins, which are fixed in fragments of a broken bone, can be attached to the rods—also via joints. The clamping jaws of the joints are connected by a screw that connects the clamping jaws perpendicular to the slot, at a distance from the round hole and near an open end of the slot, so that the clamping jaws can rotate around the screw. Tightening the screw clamps the rods firmly in the round holes of the clamping jaws and tensions the clamping jaws against each other.The clamping jaws feature serrated edges on their facing sides that enclose the screw. These serrations prevent the jaws from rotating when clamped together. To rotate the clamping jaws, the screw must be loosened until the serrated edges disengage. Furthermore, the clamping jaws can only be adjusted in angular increments determined by the serrations.

[0004] International patent application WO 2005 / 085 658 A1 discloses a joint for surgical fixation devices with two rod holders rotatable about an axis. A first rod holder has a truncated conical inner surface concentric to the axis as a first friction surface, which is located on an upper surface of the first rod holder facing a second rod holder. The second rod holder has a raised truncated conical surface as a second friction surface on an underside facing the first rod holder, which is concentric to the axis of the joint and opposite to the truncated conical inner surface of the first rod holder. The two rod holders and their truncated conical friction surfaces can be clamped together in a rotationally fixed manner by means of a clamping screw passing through the two rod holders and coaxial with the axis of the joint.

[0005] Patent US 2006 / 211 920 A1 describes an open surgical clamp with two rod holders rotatable relative to each other about an axis. The rod holders have recessed, frustoconical friction surfaces on their facing sides that are concentric to the axis. A ring with frustoconical friction surfaces opposite to those of the rod holders is arranged between the two rod holders. A clamping screw inserted through the rod holders and the ring allows the friction surfaces to be clamped together such that the two rod holders are rotationally fixed to each other via the friction surfaces of the ring. The clamping screw has a frustoconical friction surface that interacts with a corresponding, recessed, frustoconical friction surface on the facing side of one of the two rod holders.A frictional connection between the friction surface of the clamping screw and one of the rod holders counteracts a rotation of a rod rotatably arranged on the clamping screw relative to the one rod holder.

[0006] The object of the invention is to propose a holding device of the type described above, which facilitates the alignment of fragments of a broken bone into a desired position and has a high frictional force against twisting.

[0007] This problem is solved according to the invention by the features of claim 1. The external holding device according to the invention for holding fragments of a broken bone, which can also be called an external fixator, has two rods and a joint that rotatably connects the rods about an axis of the joint when it is released, and that fixes the rods to one another when the joint is locked. The joint has two rod holders to which the rods are attached or can be attached, and which are rotatable relative to each other about the axis of the joint when the joint is released.

[0008] According to the invention, the rod holders of the joint have frustoconical friction surfaces on their facing sides, which are concentric and opposite to the axis of the joint and can be clamped together with a clamping device.

[0009] "Fuscitate-conical" actually refers to the lateral surfaces of a truncated cone, where one axis of the truncated cone coincides with the axis of the joint. The clamped friction surfaces generate high friction due to a wedge effect, which prevents the two clamped rod holders of the joint from rotating. The friction depends on the clamping force with which the two friction surfaces are held together, the cone angle of the frustoconical friction surfaces, the material pairing, and the surface condition of the friction surfaces (dry, moist, lubricated, dirty, etc.). The cone angle is chosen to be acute enough to achieve high friction, preventing the rods from rotating around the axis of the joint, while avoiding self-locking, i.e., jamming of the clamped friction surfaces against each other even without clamping force.

[0010] The clamping device may, for example, have a screw that passes through coaxial through-holes in the rod holders and that can be screwed into an internal thread in one of the clamping devices or onto which a nut is screwed.

[0011] "Counter-equal" means matching pairs of friction surfaces, one of which has the shape of an inner truncated cone and the other the shape of an outer truncated cone with the same cone angle and the same diameter.

[0012] Pins can be attached to the rods – either by means of joints according to the invention or otherwise – and these pins are attached to or can be attached to the fragments of the broken bone. Such pins are screws, rods, wires, or other fastening elements.

[0013] The holding device according to the invention can have more than two rods, each pair of which is connected by a joint in a way that allows for angular adjustment. Parallel rods can also be connected by more than one joint. Furthermore, the holding device can have pins or the like for attachment to or in the fragments of the broken bone, each pin being attached by a joint to one of the rods of the holding device. It is also conceivable to attach a pin with multiple joints to multiple rods.

[0014] The invention provides for a further friction surface, frustoconical in shape and concentric to the axis of the joint, on a side of the second rod holder of the joint facing away from the first rod holder. This friction surface interacts with a friction surface opposite to the further friction surface, which is rotationally fixed and axially movable to the first rod holder and is arranged on the side of the second rod holder facing away from the first rod holder and facing its friction surface. The pairs of friction surfaces on the side of the second rod holder facing away from the first rod holder are preferably clamped together using the same clamping device as the rod holders used to clamp the pairs of friction surfaces together on their facing sides. The pairs of friction surfaces on the side of the second rod holder facing away from the first rod holder double the frictional force between the rod holders of the joint to prevent rotation.This embodiment of the invention can have several frustoconical inner and frustoconical friction surfaces surrounding each other concentrically, which doubles or multiplies the frictional force against twisting.

[0015] To connect the friction surface, which is opposite to the further friction surface of the second rod holder, to the first rod holder in a rotationally fixed manner, the invention provides a shaft that is rotationally fixed to the first rod holder and coaxial with the axis of the joint. This shaft extends through a through-hole in the second rod holder, for example, which is coaxial with the axis of the joint. The friction surface, which is opposite to the further friction surface, is guided on this shaft in a rotationally fixed yet axially movable manner. The shaft can be part of the clamping device and transmit the axial clamping force for clamping all friction surfaces of the joint between the first rod holder and the friction surface opposite to the further friction surface. For example, the shaft can be formed by a threaded bolt that is rigidly connected to the first rod holder or by a screw whose screw head is rotationally and tensuantly connected to the first rod holder.The shaft, which is rotationally fixed to the first rod holder, also prevents the friction surface, which is opposite to the further friction surface, from rotating with the second rod holder when the rods connected by the joint are pivoted against each other, thus preventing the clamping force and the friction force from being changed by pivoting the rod holders with the rods.

[0016] The invention enables a high frictional force to prevent rotation of the joint's rod holders with a relatively low axial clamping force. A further advantage is the stepless angular adjustability of the rods. With only partial loosening of the joint, the rods can be pivoted relative to each other by hand by overcoming the friction between the rod holders, thereby aligning the bone fragments attached to the rods. The frictional force is sufficient to ensure that the fragments maintain their position without any external force. By adjusting the clamping force, the frictional force of the joints against rotation of the rods can be individually set to a desired level. Once the fragments are in the desired position, the joint is tightened, thus fixing the rods and the bone fragments attached to them in their relative positions.More than two, and in principle any number, rods can be connected to each other with a corresponding number of joints.

[0017] A further development of the invention provides that each rod holder has two or more frustoconical friction surfaces concentric with each other and with respect to the axis of the joint, which enclose one another. Preferably, each rod holder has an inner frustoconical friction surface and an outer frustoconical friction surface, which either concentrically encloses the inner frustoconical friction surface or is concentrically enclosed by the inner frustoconical friction surface. A second rod holder of the joint has frustoconical friction surfaces that are opposite to the two friction surfaces of a first rod holder of the joint. The friction surfaces are arranged on opposite sides of the rod holders so that they can be clamped together with the clamping device.For example, one rod holder has a circular rib concentric to the axis of the joint with an inverted V-section on one side facing the other rod holder of the same joint, and the other rod holder has a similarly circular groove with a V-section, also concentric to the axis of the joint. "Inverted" means that the apex of the rib's V-section faces the groove. The rib and the groove have the same diameters and opposite cross-sections. The rib's side faces and the groove's cheeks form the frustoconical friction surfaces. The apex of the rib's V-section is preferably flattened so that it does not rest on the groove's bottom, which may also be flattened. The friction surfaces of a rod holder can have the same or different cone angles; corresponding opposite friction surfaces of the two rod holders of a joint have the same cone angles and the same diameters.The doubling or multiplication of the friction surfaces achieved with this embodiment of the invention doubles or multiplies the friction between the rod holders of the joint at the same axial clamping force, so that the clamped rod holders are reliably held against rotation relative to each other.

[0018] Another advantage of the numerous friction surfaces is the low surface pressure, which improves the sliding of the friction surfaces against each other when the clamping device is loosened to adjust the joint. This allows the rod holders, and with them the rods, to rotate smoothly against each other.

[0019] One embodiment of the invention provides a push-off spring that pushes the friction surfaces apart when the clamping force is reduced or released. The push-off spring counteracts self-locking, i.e., clamping of the frustoconical friction surfaces against each other, and enables smooth angular adjustment of the rod holders with the rods with adjustable friction force by reducing the clamping force.

[0020] To compensate for manufacturing tolerances, one embodiment of the invention provides radially elastic friction surfaces, whereby it is sufficient if a part of the friction surfaces is radially elastic.

[0021] One embodiment of the invention provides releasable clamps for securing the rods and / or pins. The rod holders can, for example, have pairs of clamping jaws between which the rods and / or pins are arranged and clamped. The pairs of clamping jaws can be clamped together with the clamping device that also clamps the friction surfaces. The pairs of clamping jaws can also be clamped together in another way. For example, the pairs of clamping jaws can be elastically connected to each other in one piece or be made of two or more pieces.

[0022] One embodiment of the invention provides that one, several, or all clamps have clamping devices for rods with different cross-sections. For example, the clamping jaw pairs of the clamps can have opposing grooves with different cross-sections in which the rods or pins can be arranged and clamped.

[0023] All features mentioned in the description and / or illustrated in the drawing can be implemented individually or in any combination in embodiments of the invention. Embodiments of the invention that do not include all, but only some, of the features of a claim, including an independent claim, are possible.

[0024] The invention is explained in more detail below with reference to an embodiment illustrated in the drawing. The drawing shows: Figure 1 shows the use of an external fixator according to the invention for holding fragments of a broken bone; Figure 2 shows a perspective view of two rods and a joint connecting the rods of the fixator from Figure 1; Figure 3 shows a perspective axial sectional view of the joint from Figure 2 Figure 4 shows a perspective exploded view of individual parts of the joint. Figure 3; and Figure 5, a part of the joint made of Figure 2 in a modified embodiment of the invention in perspective view.

[0025] The external holding device 1 shown in Figure 1, according to the invention, serves to join fragments 2 of a broken bone or, more generally, to hold skeletal parts in a predetermined position relative to one another. The external holding device 1 is also referred to as an external fixator.

[0026] To attach the holding device 1 to the fragments 2 of the fractured bone, or conversely, to attach the fragments 2 to the holding device 1, one or more bone screws are screwed into each fragment 2 from the outside through the patient's skin. Instead of bone screws, pins, wires, or other pin- or otherwise shaped fastening elements, currently available or developed in the future, can also be attached to or in the fragments 2 of the bone. These fastening elements are referred to here as pins 3, regardless of whether they have the shape of a pin.

[0027] A rod 5 is attached to the pins 3 of each fragment 2 of the broken bone using joints 4, and the rods 5 are connected to each other using joints 4. The joints 4 and their individual parts show the Figures 2 to 4 .

[0028] Each joint 4 has two rod holders 6, 7, which are rotatable relative to each other about an axis and can be clamped together in a rotationally fixed manner. The rods 5 and the pins 3 can be attached to or in the rod holders. The rod holders 6, 7 are designed as clamps and have clamping jaws 8 between which the pins 3 and the rods 5 can be clamped. The axis about which the rod holders 6, 7 are rotatable relative to each other forms an axis of the joint 4. Each clamping jaw 8 can have clamping devices, for example, grooves, for one, two, or more different pins 3 and / or rods 5.

[0029] The clamping jaws 8 have two clamping grooves 9 in mutually facing inner surfaces and a through hole 10 between the clamping grooves 9. The inner surfaces are defined as the mutually facing sides of the clamping jaws 8 of a rod holder 6, 7. The through holes 10 run perpendicular to the clamping grooves 9 or perpendicular to the inner surfaces of the clamping jaws 8 in which the clamping grooves 9 are located. The through holes 10 of all clamping jaws 8 of a joint 4 are aligned with each other. Each clamping groove 9 is designed such that a pin 3 can be inserted into it and clamped in place. The other clamping groove 9 is provided for inserting and clamping a rod 5. In the exemplary embodiment, the clamping grooves 9 have semicircular cross-sections with the diameter of the pins 3 or the rods 5, respectively. The clamping grooves 9 can also have other cross-sections, for example, V-shaped or trapezoidal cross-sections (not shown).

[0030] A screw 11 is inserted through the through holes 10 of the clamping jaws 8, which each pair form the two rod holders 6, 7. The shank of the screw forms a shaft 12, which rotatably connects the clamping jaws 8 and the rod holders 6, 7 about a longitudinal axis of the shaft 12. The longitudinal axis of the shaft 12 forms an axis of the joint 4.

[0031] The screw head 13 of the screw 11 has two parallel surfaces on its circumference and is seated in a complementary countersink in the outer surface of an outer clamping jaw 8 of the first of the two rod holders 6, in which the screw head 13 is secured against rotation and tensile forces, so that the shank of the screw, which forms the shaft 12, is connected against rotation to the outer clamping jaw 8 of the first rod holder 6. Screw heads 13 with other shapes, for example, 4- or 6-sided screw heads, which are seated against rotation in a complementary countersink in the outer surface of the outer clamping jaw 8, are also possible (not shown). The shaft 12 can also be connected to the outer clamping jaw 8 in another way, for example, by welding to it (not shown).

[0032] A nut 15 is screwed onto a thread 14 of the shaft of the screw 11, which forms the shaft 12. The nut 15 holds the rod holders 6, 7 on the shaft 12. The screw 11 and the nut 15 form a clamping device 16, with which the rod holders 6, 7 and the clamping jaws 8, respectively, can be clamped together. In the exemplary embodiment, the nut 15 has a hexagon 17 for attaching a wrench (not shown), such as an open-end or ring wrench, and a knurled section 18 on one circumference of the nut 15 for turning the nut 15 by hand.

[0033] An inner clamping jaw 8 of the first rod holder 6 has a circular rib 19 with a V-shaped cross-section on its outer surface, which concentrically surrounds the through-hole 10. The clamping jaw 8 of the first rod holder 6 that faces the other, i.e., the second, rod holder 7 of the joint 4 is referred to as the inner clamping jaw 8. Its outer surface also faces the other, i.e., the second, rod holder 7. The flanks of the rib 19 form two frustoconical friction surfaces 20, 21, wherein an inner frustoconical friction surface 20 is hereinafter also referred to as the first friction surface 20, and an outer friction surface 21, which has the shape of an outer frustoconical, is hereinafter also referred to as the second friction surface 21. Strictly speaking, the friction surfaces 20, 21 and also all other friction surfaces 23, 24, 25, 26, 30, 31 which are yet to be described have the shape of the lateral surfaces of inner or outer truncated cones.

[0034] Both clamping jaws 8 of the second rod holder 7 have circular grooves 22 on their opposite outer sides, which concentrically enclose the through holes 10 of the clamping jaws 8. The grooves 22 have the same diameter as the rib 19 and V-shaped cross-sections opposite to the rib 19. The flanks of the grooves 22 form friction surfaces 23, 24, 25, 26 opposite to the friction surfaces 20, 21 of the rib 19, which are subsequently also referred to as the third, fourth, fifth and sixth friction surfaces 23, 24, 25, 26.

[0035] A circular disc 27 with a central hole 28 is arranged on the shaft of the screw 11, which forms the shaft 12, between an outer clamping jaw 8 of the second rod holder 7 and the nut 15. The central hole 28 has two parallel surfaces by which the disc 27 is rotationally fixed on the shaft 12. The shaft 12, in the area of ​​the thread 14, also has two parallel surfaces equidistant from each other, as are the parallel surfaces of the central hole 28 of the disc 27. Because the shaft 12 is rotationally fixed to the outer clamping jaw 8 of the first rod holder 6, the disc 27 is also rotationally fixed to the outer clamping jaw 8 of the first rod holder 6. The disc 27 is axially movable on the shaft 12.

[0036] On its side facing the outer clamping jaw 8 of the second rod holder 7, the disc 27 has a circular rib 29 that concentrically surrounds the central hole 28. This rib has the same diameter as the groove 22 in the clamping jaw 8 and a corresponding V-shaped cross-section. The flanks of the rib 29 form a seventh and an eighth friction surface 30, 31 of the joint 4.

[0037] By tightening the nut 15 on the screw 11, which together with the screw 11 forms the clamping device 16, the rod holders 6, 7, the clamping jaws 8, the washer 27 and the eight friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 can be clamped together, so that the rods 5 and pins 3 lying in the clamping grooves 9 of the clamping jaws 8 are firmly clamped between the clamping jaws 8 and the clamping jaws 8 or rod holders 6, 7 are held together by friction between the friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 which are in contact with each other and clamped against each other.The conical shape of the friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 creates a high frictional force between them due to a wedge effect. The eight friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 increase this frictional force eightfold, so that with a comparatively low clamping force, the rod holders 6, 7 are reliably held against rotation. The nut 15 is intended to be tightened with a wrench, such as an open-end or ring spanner. It may even be sufficient to tighten the nut 15 by hand. Alternatively, a quick-release fastener, such as those found on bicycles, or another eccentric lever could be used to clamp the clamping device 16 (not shown). Such a quick-release fastener is provided in particular on the screw head 13.

[0038] By loosening the nut 15, the rods 5 and pins 3 can be moved in the clamping grooves 9 of the clamping jaws 8, and the clamping jaws 8 or the rod holders 6, 7 can be rotated relative to each other. This allows the rods 5 and pins 3, and the bone fragments 2 or skeletal parts attached to them, to be positioned as desired relative to each other. The frictional force between the friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 can be finely adjusted by the clamping force generated by the nut 15 to align the rod holders 6, 7, clamping jaws 8, rods 5, and pins 3.

[0039] The tips of the ribs 19, 29 are flattened so that they do not rest on the base of the grooves 22.

[0040] In cylindrical recesses of the through holes 10 of the inner clamping jaws 8 of the two rod holders 6, 7, a helical compression spring is arranged as a push-off spring 35, which pushes the clamping jaws 8 and the friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 apart when they are not clamped together by the clamping device 16. This allows the frictional forces between the friction surfaces 20, 21, 23, 24, 25, 26, 30, 31 to be precisely controlled with the nut 15 for pivoting the rods 5 and the pins 3.

[0041] Figure 5Figure 27 shows a disc 27 with an alternatively designed rib 29 having a V-shaped cross-section, the flanks of which form the seventh and eighth friction surfaces 30, 31. The rib 29 is divided into an outer and an inner part by a cylindrical slot 32. Radial slots 33 further divide the rib 29 into several sectors. Additionally, a slot 34 is provided in a radial plane at one base of the rib 29, in this embodiment only on the outside. The slots 32, 33, 34 allow the friction surfaces 30, 31 to flex radially; they are radially elastic. This allows the friction surfaces 30, 31 to adapt to the corresponding friction surfaces 23, 24, 25, 26 in the grooves on the outer sides of the clamping jaws 8 of the second rod holder 7, thus compensating for manufacturing tolerances. The rib 19 on the outside of the inner clamping jaw 8 of the first rod holder 6 can also have such slots 32, 33, 34.

Claims

1. External retaining device for retaining fragments of a broken bone or of skeletal parts in general, comprising two rods (5) and a joint (4) which connects the two rods (5) rotatable about an axis and which can be immobilized against rotation, the joint (4) comprising two rod holders (6, 7) to which the rods (5) are attached, the two rod holders (6, 7) being rotatable one with respect to the other about the axis, a first of the two rod holders (6) comprises a first, frustum-shaped friction surface (20) that is concentric with respect to the axis, and the second rod holder (7) comprises a mating, likewise frustum-shaped third friction surface (23) which faces the first friction surface (20) and is concentric with respect to the axis, the retaining device (1) comprises a tensioning device (16) by way of which the two friction surfaces (20, 23) can be tensioned one against the other so that the two rod holders (6, 7) are held against one another to prevent rotation by way of frictional engagement between the frustum-shaped friction surfaces (20, 23), the second rod holder (7) comprises a fifth frustum-shaped friction surface (25), which is concentric with respect to the axis and faces away from the first rod holder (6), and the joint (4) comprises a frustum-shaped seventh friction surface (30), which is concentric with respect to the axis and which is mating with the fifth friction surface (25), which faces the fifth friction surface (25) and is fixed against rotation with respect to the first rod holder (6), and that the tensioning device (16) simultaneously with the first and third friction surfaces (20, 23) also tensions the fifth and seventh friction surface (25, 30) with respect to one another, so that, in addition to the first and third friction surfaces (20, 23) also the fifth and seventh friction surfaces (25, 30) hold the two rod holders (6, 7) by frictional engagement between one another against rotation, characterized in that the joint (4) comprises a shaft (12), which is fixed against rotation with respect to the first rod holder (6) and is coaxial with respect to the axis and which extends through the first rod holder (6) and non-rotatably connects the seventh friction surface (30) to the first rod holder (6).

2. Retaining device according to claim 1, characterized in that the first rod holder (6) comprises a frustum-shaped second friction surface (21), which is concentric with respect to the axis and concentrically encloses the first friction surface (20), and the second rod holder (7) comprises a frustum-shaped fourth friction surface (24), which is the mirror opposite of the second friction surface (21), faces the second friction surface (21) and is concentric with respect to the axis and which concentrically encloses the third friction surface (23), and that the tensioning device (16) simultaneously with the first and third friction surfaces (20, 23) also tensions the second and fourth friction surfaces (21, 24) one against the other so that in addition to the first and third friction surfaces (20, 23), also the second and fourth friction surfaces (21, 24) hold the two rod holders (6, 7) between one another by way of frictional engagement to prevent rotation.

3. Retaining device according to claim 1, characterized in that the second rod holder (7) comprises a sixth, frustum-shaped friction surface (26), which is concentric with respect to the axis and faces away from the first rod holder (6) and which concentrically encloses the fifth friction surface (25), and that the joint (4) comprises a frustum-shaped eighth friction surface (31), which is concentric with respect to the axis, the mirror opposite of the sixth friction surface (26) and faces the sixth friction surface (26) and which cannot be rotated with respect to the first rod holder (6), and that the tensioning device (16), simultaneously with the first and third friction surfaces (20, 23) also tensions the sixth and eighth friction surfaces (26, 31) one against the other so that in addition to the other friction surfaces (20, 21, 23, 24, 25, 30) also the sixth and eighth friction surfaces (26, 31) hold the two rod holders (6, 7) between one another by frictional engagement to prevent rotation.

4. Retaining device according to claim 1, characterized in that the shaft (12) is part of the tensioning device (16) and that the shaft (12) transfers an axial tensioning force, used for tensioning the friction surfaces (20, 21, 23, 24) one against another from the second rod holder (7) to the seventh friction surface (30).

5. Retaining device according to one or more of the preceding claims, characterized in that the joint (4) comprises a push-off spring (35) which pushes the friction surfaces (20, 21, 23, 24, 25, 26, 30, 31) off one another when these are not tensioned one against another by the tensioning device (16) so that the two rod holders (6, 7) including the two rods (5) can be smoothly rotated one with respect to the other in an adjustable manner when the tensioning device (16) is released.

6. Retaining device according to one or more of the preceding claims, characterized in that the friction surfaces (20, 21, 23, 24, 25, 26, 30, 31) are radially elastic.

7. Retaining device according to one or more of the preceding claims, characterized in that the two rod holders (6, 7) comprise releasable clamps in which the two rods (2, 3) are clamped in place.

8. Retaining device according to claim 7, characterized in that at least one rod holder (6, 7) comprises a clamp comprising clamping devices for rods (2, 3) having differing rod cross-sections.