Abstract:
An elastic element which consists of a radiolucent material and is provided for a medical device. Said elastic element is produced from an unreinforced plastic material.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to an elastic element, produced from a radiolucent material for a medical device.  
       BACKGROUND OF THE INVENTION  
       [0002]     There is a large number of devices in medical technology, such as an external fixator, which must be brought one or more times into the beam path of a source of x-rays for the purpose of checking a position. Since such devices generally consist of metallic materials, which cast a shadow in an x-ray image, structures lying behind are hidden. This is a disadvantage. The proposal has therefore already been made to produce individual parts of such devices from radiolucent materials. However, such a replacement never extends to any elastic elements that may be present, such as springs, which continue to be made from metal. However, such springs affect the quality of x-ray photographs.  
       SUMMARY OF THE INVENTION  
       [0003]     It is an object of the invention to provide an elastic element, which is made from a radiolucent plastic and is in a position to fulfill all the mechanical functions of a metallic element, however, without casting a shadow on the x-ray image.  
         [0004]     The advantages, achieved by the invention, are essentially: 
        better transmittance of x-rays,     less weight     greater flexibility in comparison to elastic elements from reinforced plastic, which break easily.        
 
         [0008]     The radiolucent plastic may consist of polyoxymethylene (POM). Advisably, the plastic has a modulus of elasticity of 2600-3500 MPa and preferably of 2800 to 3300 MPa. The yield strain of the plastic advisably ranges from 20 to 24% and preferably from 21 to 23%. The nominal elongation at break of the plastic advisably ranges from 40 to 50% and preferably from 43% to 47%.  
         [0009]     For a special embodiment, the inventive element has the shape of a helical spring. The cross section of the spring spiral of the helical spring advantageously is out-of-round and preferably is quadrilateral. Advantageously, the cross section of the spring spiral tapers towards the outside of the helical spring, preferably in a trapezoidal manner.  
         [0010]     For a different embodiment, the inventive element is constructed as a disk spring. The disk spring preferably is configured in the shape of a hollow, truncated cone and tapers from the first to the second end ( 31 ;  32 ) and has several notches, which are distributed over the periphery and indented from the first end. The notches increase the elasticity of the disk spring and prevent breakage of the same. The conical angle of the disk spring in the shape of a hollow truncated cone may advantageously be between 70° and 140°, which leads to an optimum overall height.  
         [0011]     The invention relates to a medical device, all the components of which, with the exception of the elastic element, consist of a reinforced plastic. Such a device may be realized in the form of a clamp for fastening bone fixation means and/or longitudinal components. The elastic element permits bone fixation means and/or longitudinal components, introduced into the clamp, to be fastened temporarily. All components of the clamp, with the exception of the elastic element, consist of a fiber-reinforced plastic, which may, for example, be a polyamide.  
         [0012]     The invention furthermore relates to a bone fixation device for the mutual positioning of the longitudinal components, which comprises two medical devices with an inventive elastic element, which may be rotated with respect to one another about an axis of rotation. Each of the two clamps comprises at least two clamp jaws, which define a clamp opening located between then, it being possible to constrict or expand the clamp opening by the elastic deformation of the clamp. At least one jaw of each clamp has a borehole, which is coaxial with the axis of rotation and traverses the clamp jaw. The two clamps are mounted on a shaft and blocking means are provided in order to press the jaws of the two clamps coaxially against one another, in order alternatively to block the rotatability of the two clamps about the axis of rotation relative to one another.  
         [0013]     For a special embodiment, an axially inner clamp jaw is connected permanently with the shaft. The surfaces of the axially inner clamp jaw, directed against one another, may be provided with serrations, which can be brought into engagement with one another. There may also be a disk spring between the axially inner clamp jaws. The blocking means advisably are nuts, which can be screwed axially terminally onto at each end of the shaft and can be pressed against the axially outside clamp jaw. A helical spring may also be disposed between each nut and the adjacent outside clamp jaw.  
         [0014]     In the case of a further embodiment, a device to prevent twisting is provided between each axially outside and the adjacent axially inside clamp jaw. Advantageously, caulking material, which preferably is produced by thermal conversion, may be provided appropriately at one end of the shaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The invention and further developments of the invention are explained in even greater detail in the following by means of partially diagrammatic representations of several examples. In the drawing  
         [0016]      FIG. 1  shows a side view of an embodiment of the elastic element, constructed as a helical spring,  
         [0017]      FIG. 2  shows a cross-section along the line II-II in  FIG. 1 ,  
         [0018]      FIG. 3  shows a perspective view of an embodiment of the elastic element constructed as a disk spring,  
         [0019]      FIG. 4  shows a diametrical section through the disk spring shown in  FIG. 3 ,  
         [0020]      FIG. 5  shows a perspective view of an embodiment of the bone fixation device,  
         [0021]      FIG. 6  shows a side view of the embodiment of the bone fixation device, shown in  FIG. 5 ,  
         [0022]      FIG. 7  shows a cross-section along the line III-III in  FIG. 6  and  
         [0023]      FIG. 8  shows an enlargement of the section marked by the circle A in  FIG. 7 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIGS. 1 and 2  show an embodiment of the elastic element  1 , which has the shape of a helical spring  20  with flattened ends. The cross section  3  of the spring spiral  2  is trapezoidal and tapers towards the periphery of the helical spring  20 . A further embodiment of the elastic element is shown in  FIGS. 3 and 4 . The elastic element  1  is constructed as a disk spring  30  and has the configuration of a hollow truncated cone. The disk spring  30  tapers from its first end  31  to its second end  32  and has several notches  33 , which are distributed over the periphery and indented from the first and  31 . The conical angle α of the disk spring  30  in the shape of a hollow truncated cone is 70° in the embodiment shown here.  
         [0025]     FIGS.  5  to  8  show an embodiment of the bone fixation device, which comprises two clamps  10 , which can be rotated about an axis of rotation  13  relative to one another. Each of the clamps  10  comprises two clamp jaws  11 , which are disposed one behind the other axially, a clamp opening  12  being disposed between the two jaws  11  of each clamp  10 . The clamp openings  12  are configured in the form of a channel and have a channel axis  17  each, which is orthogonal to the axis of rotation  13 . The clamp openings  12  are intended to accommodate longitudinal fastening elements  18 . The clamp openings  12  are open transversely to the channel axis  17  towards the periphery of the clamp  10 , so that a longitudinal fastening elements  18  can be introduced into the clamp opening  12  transversely to the axis of rotation  13 .  
         [0026]     A borehole  14  passes coaxially with the axis of rotation  13  through the axially outer jaw  11  of the above-disposed clamp  10  as well as the two jaws belonging to the clamp  10  disposed at the bottom. On the other hand, the inner jaw  11  of the clamp  10  disposed at the top is connected permanently with the shaft  15  passing through the borehole  14 . The jaws  11  of the clamp  10  disposed at the top, which can be moved rotationally and axially, enable the clamp  10 , which is disposed at the top, to be moved rotationally relative to the clamp  10  disposed at the bottom. At the two ends of the shaft  15 , external threads  10  are provided, over which the blocking agents  16 , configured as nuts  41 , can be screwed. The diameters of the borehole  14  and of the shaft  15  are such that the shaft  15  has clearance in the borehole  14 .  
         [0027]     The nuts  41  are disposed axially terminally at the shaft  15  and press axially on the respectively adjacent axially outer clamp jaw  11 , so that, by tightening the upper nut  41 , the two jaws  11  of the clamp  10 , which is disposed at the top, can be pressed against the inner jaw  11  of the clamp  10 , which is disposed at the bottom and fixed to the shaft  15 . With that, the two jaws  11  of the clamp  10  can also be pressed against one another. By tightening the nut  41 , which is disposed at the bottom, the axially outer clamp jaw  11  of the nut  41 , which is disposed at the bottom, is pressed against the clamp jaw  11 , which is connected permanently with the shaft  15  and is part of this clamp  10 . By these means, the longitudinal fastening elements  18 , which are inserted coaxially with the channel axes  17 , can be fixed between the clamp jaws  11 . Moreover, the mutually adjacent inner face surfaces of the axially inner clamp jaws  11  are provided with serrations  40 , which, when the upper nut  41  is tightened, engage one another and prevent relative rotation of the two clamps  10  about the axis of rotation  13 .  
         [0028]     Each of the clamp openings  12  can be expanded or constricted by elastic deformation of the helical springs  20 , disposed axially between the nuts  41  and the outer clamp jaws  11 . Furthermore, the axially inner clamp jaw  11  of the clamp  11  disposed at the top is pressed by means of a disk spring  30 , inserted between the two axially inner clamp jaws  11 , against the axially outer clamp jaw  11  of the clamp  10 , which is disposed at the top. By these means, the serrations  40  are disengaged and relative rotation of the two clamps  10  about the axis of rotation  13  is made possible.  
         [0029]     The elastic pre-stressing, attainable by the helical springs  20 , is achieved by constructing the helical spring  20  appropriately. Moreover, two clamp openings  12  are constructed transversely to the channel axes  17  in such a manner, that they have a constriction  25  at each of their openings, which are remote from the axis of rotation  13 . The clamp openings  12  are provided peripherally with expansions  26  to simplify the introduction of a longitudinal fastening element  18  into the clamp opening  12  in question.  
         [0030]     Rotation of the two jaws  11  of each clamp  10  about the axis of rotation  13  relative to one another is prevented by a device  42 . In the case of the clamp  10 , which is disposed at the top, the device  42  is realized by a pin  43 , which is disposed between the clamp jaws  11 . In the case of the clamp  10 , which is disposed at the bottom, the device  42  is realized by key seat  22  at the shaft  15  and a corresponding groove  23 .