Abstract:
The invention concerns a flexible intervertebral linking device ( 1 ) consisting of two sets of means. A first set of means ( 11 ) consisting of rigid means ( 110, 112, 114, 116 ) preferably made of biocompatible metallic materials providing the device with good mechanical resistance by integral load transmission without deformation. A second set of means ( 12 ) consisting of flexible or damping means ( 121  and  122 ) made of biocompatible viscoelastic materials, admitting repeated elastic deformations, the combination of said two sets of means providing it with both resistance and mechanical stress damping whereto it is subjected, to compensate for any deficiency of flexible anatomical links of the human body.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a posterior flexible vertebral linking device which works in tension, compression and flexion, and which damps all mechanical stresses. This device will have operational advantages that will be described.  
       PRIOR ART  
       [0002]     We know many posterior vertebral attachment units rigidifying a certain number of vertebrae by depriving them of any mobility, thus allowing the containment of all mechanical stresses. However, the first vertebra adjacent to this rigid block keeps all its mobility and this abrupt discontinuity between the rigid block and this free vertebra very often generates a very high stress of the linking elements. The result is an acceleration of the degeneration of this level.  
         [0003]     This problem was only partially solved by semi-rigid systems conceived to create an intermediate rigidity between the mobile vertebrae and the fixed vertebrae. These systems present the following disadvantages:  
         [0004]     Either: they work only in tension: this is the case of all the devices based on artificial ligaments. These systems are hardly elastic and leave with the discretion of the operator the care to regulate the tension in particular making thus random the mechanical characteristics in the operating mode tension/compression that concerns us.  
         [0005]     Or: they work in compression with a thrust in tension, which makes these devices ineffective once they must assist displacements in extension.  
         [0006]     In either case: none of the known devices entirely solves the problem which is posed, namely, damping the mechanical stresses existing in tension/compression and flexion to which a moving vertebra can be subjected  
         [0007]     We will name the first prior art: patent EP 0576 379 A1 which presents a shock absorber which seems to approach the most closely at least from the point of view of the general outline of this invention; claim  1  of this patent protects “a uni-axial shock absorber working only in compression while playing the part of an abutment which opposes any displacement of the piston beyond a given value.  
         [0008]     In this case the exponential limitation of the displacement solved by the prior art, is a problem which has nothing to do with that the person who wants to solve the present invention.  
         [0009]     We now quote a second prior art: the patent application Ser. No. 0,012,998 which describes and claims “a flexible and cast solid vertebral linking device functioning in a multidirectional way”. This anteriority does not solve exactly the same problem as the one that the present invention seeks to solve, this invention having different means and functions.  
         [0010]     In the present invention, one can choose in a precise way the desired working method: tension/compression or flexion, or the combination of the two working methods, this in order to avoid any contact between the articular facets.  
     
    
     DESCRIPTION  
       [0011]     We will list the drawings which help us understand the invention.  
         [0012]      FIG. 1  and  1  bis of sheet 1/6 presents perspective views (two alternative embodiments) of the device in the case of a working method combined in tension, compression and flexion.  
         [0013]     The  FIGS. 2 and 2  bis of sheet 1/6 are longitudinal cross-sections of two alternatives of the same device.  
         [0014]      FIG. 3  of sheet 2/6 is an exploded view of the device and its means.  
         [0015]      FIG. 4  of sheet 3/6 is a view in perspective of the device working only in tension/compression.  
         [0016]      FIG. 5  of sheet 3/6 is a cross-section of the device working only in tension/compression.  
         [0017]     FIGS.  6  to  11  of sheet 4/6 represent all the individual parts constituting the device.  
         [0018]      FIG. 12  of sheet 4/6 shows another specific means working according to the tension/compression mode.  
         [0019]      FIG. 13  of sheet 5/6 shows an alternative of the device working along two axes.  
         [0020]     FIGS.  14  to  17  of sheet 5/6 show four forms of the mobile end of another alternative of device  1 .  
         [0021]      FIG. 18  of sheet 6/6 shows the device in position. 
     
    
       [0022]     The device  1  consists of two sets of means: A first set of means  11  composed of rigid means manufactured out of preferably metal, biocompatible material ensuring a good mechanical resistance of the device by completely transmitting the forces.  
         [0023]     A second set of means  12  formed of flexible or damping means manufactured out of viscoelastic biocompatible materials, supporting the repeated elastic strain. It is the combination of these two sets of means which makes possible the functioning of the invention.  
         [0024]     The first set of means  11  includes four mechanical structures  110 ,  112 ,  114 ,  116  which have the function of transmitting the stresses, without becoming deformed, and to which device I is subjected.  
         [0025]     The mechanical structure  110  is made up of a mechanical rod  111 , one of its ends being surmounted by a circular plate  113   b  connected to the aforementioned rod  111  with a broad joining radius  113   a , the whole being able to slide in the hollow part of the structure  114  which encloses a visco-elastic element  121 .  
         [0026]     The mechanical structure  112  is a cap provided with a thread  117  allowing for the fixing of the aforementioned structure  112  on structure  114 ; the means  112  has a shoulder area  118  which makes possible the enclosure of a viscoelastic-centering ring  121  between the plate  113   b  and itself.  
         [0027]     The mechanical structure  114  is made up of two hollow cylinders, one of which is tapped to allow the fixing of a rod  116  with a threaded end. The means  110  and  116  will be fixed on the vertebrae to allow the operation of the device  1 .  
         [0028]     The second set of means  12  is made up of two viscoelastic means  121  and  122 .  
         [0029]     The first means  121  is preferably a centering ring which lets the rod  111 . slide in its center  
         [0030]     The second means  122  is a disc full of viscoelastic material. These two centering rings  121  and  122  can undergo compressive stresses which may not be uniformly distributed, they were conceived to resist many cyclic fatigue stresses without breaking, tests were carried out in this direction, means  121  and  122  are able to undergone these tests of elastic deformation as many times as necessary.  
         [0031]     The selected material is preferably a biocompatible polyurethane; thanks to their integration inside mechanic means  110 , 112 ,  114 ,  116 , the viscoelastic means  121  and  122  are protected by the preceding mechanical structures of the aggressive environment of the human body, which avoids in particular the formation of fibers around these means which could deteriorate the viscoelastic properties of the material and consequently disturb the correct operation of device  1 .  
         [0032]     This device  1  makes possible the damping of the stresses in tension/compression and flexion which it undergoes by the intermediary of rods  110  and  116 . This function is assured owing to the fact that means  112  has a sufficiently broad opening  119  to allow a clearance of rod  111  and that there is a functional allowance between plate  113  and the hollow body of means  114 ; the shoulder area  118  serves as a stop and maintains in its housing the viscoelastic mass  121  thus locked up.  
         [0033]     If one wishes to work in a uni-axial mode of tension/compression, means  112  is replaced by another means  115  equipped with a threading  117 , which includes a cap  115   c , whose opening  119  is adjusted to the diameter of the rod  110  while being extended by a guiding rod  115   a.    
         [0034]     This device  1  is thus able to react dynamically to the stresses applied. It is essential that structure  114  comprises a bore  114   a  to allow a guidance without excessive friction of rod  110  in the aforementioned means  114 .  
         [0035]     The adjustment of the diameter of the viscoelastic centering rings  121  and  122  must be carried out with precision to enable them to be crushed freely until a stress threshold corresponding to a point of contact of the bore  114   a  of means  114 .  
         [0036]     An alternative of the set of means  11  includes metal structures having the same functions as the structures  110 ,  112 ,  114 ,  116 , but the assembly of these three parts ( 110 , 130 ,  131 ) being of a weaker barrier than that of the structures previously described ( FIG. 2 ).  
         [0037]     The rod  131  is fixed at its cap  130  by the intermediary of a threading located on shoulder  132  of the rod.  
         [0038]     In the case of this alternative, the possibilities of displacement of rod  110  subjected to the stresses in flexion are ensured by play  119  located between cap  130  and rod  110 .  
         [0039]     For a uni-axial operation of device  1 , it is preferable to use means  110 , 112 ,  114 ,  116  which provide a better guidance of rod  110 . If small overall dimensions are needed, means  110 ,  130 ,  131  may be preferably used.  
         [0040]     Device  1  is able to function with rods  110  and  131  moving on convergent axes ( FIG. 13 ) with a small angle of displacement and according to given clearances.  
         [0041]     The set of means  12  is therefore comprised of two visco-elastic means  141  and  142 . The means  141  is a cylinder full of visco-elastic biocompatible material, and whose face in contact with a plate is inclined. The means  142  is a centering ring whose face in contact with the back of a plate is inclined.  
         [0042]     The set of means  11  (rigid means) is identical to the previous one that is described above, the orifice  119  being however eccentric depending on the chosen angle. The shape of orifice  119  is defined depending on the clearances which are allowed to the rod  110 .  
         [0043]     The rod  110  is thus able, thanks to these new technical characteristics, to function in tension/compression with a given angle with respect to the rod  116  or the rod  131  in case in which the  119  orifice is eccentric and adjusted to the rod  116  or  131 ( FIG. 14 ).  
         [0044]     The rod  110  forming an angle as against the rod  116  or  131  (the case in which the  119  orifice is oblong and eccentric) can in this case function equally well in tension/compression as in lateral flexion. ( FIG. 15 ).  
         [0045]     The rod  110  can function in tension/compression and in flexion following a preferred axis which can be for instance in the sagital plane of the spinal column and this one on the one side and one the other side of a given position of the rod  110  forming at rest an angle with the rod  116  or the rod  131 , this also in the case where the means  119  is oblong or eccentric, ( FIG. 16 ).  
         [0046]     Finally, the rod  110  can function in tension/compression and in flexion in all directions, forming an angle, as against the rod  116  or  131  in case the orifice  119  is eccentric or larger than the diameter of the rod  110  ( FIG. 17 ).