Abstract:
A flexible intervertebral linking device ( 1 ) is provided. The device ( 1 ) utilizes two sets of structures. A first structure ( 11 ) is a rigid structure ( 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 structure ( 12 ) is a flexible or damping structure ( 121  and  122 ) made of biocompatible viscoelastic materials, permitting repeated elastic deformations. The combination of the two structures providing the device with both resistance and mechanical dampening of forces to which it is subjected, to compensate for any deficiency in the flexibility of certain anatomical links of the human body.

Description:
FIELD 
     The invention relates to a posterior flexible vertebral linking device which works in tension, compression and flexion, and which dampens all mechanical stresses. This device has operational advantages that will be described. 
     BACKGROUND 
     Many posterior vertebral attachment units exist which rigidize a certain number of vertebrae by depriving them of any mobility, thus containing all mechanical stresses. However, the first vertebra adjacent to this rigidized block of vertebrae remains mobile and, consequently, there is an abrupt discontinuity in movement between the rigid block and this free vertebra which very often generates a very high stress in the linking elements. The result is an acceleration of the degeneration of this level (the interface between the adjacent vertebrae and the vertebrae comprising part of the rigidized block). 
     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 prior art systems present primarily one of two disadvantages, either they work only in tension or they work in compression with a thrust in tension. 
     As for those that work only in tension: this is the case of all the devices based on artificial ligaments. These systems have little elasticity and leave the care to regulate, in particular, the tension, to the skill of the operator, thus making the mechanical characteristics in the operating mode of interest (tension/compression) haphazard. 
     Further, those devices that work in compression with a thrust in tension are ineffective when dealing with displacements in tension. 
     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 is subjected. 
     Patent Application EP 0576 379 A1 proposes a shock absorber which seems to approach most closely at least from the point of view of the general concept of this invention. This patent describes 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. 
     The invention of EP 0576379 A1 deals with the exponential limitation of the displacement, which is a completely different problem as that of the present invention. 
     French Patent application No. 0,012,998, describes and claims a flexible and cast solid vertebral linking device functioning in a multidirectional way. This prior art reference does not solve exactly the same problem as that of the present invention due to its different means and functions. 
     SUMMARY 
     The invention concerns a flexible intervertebral linking device that meets the needs identified above. The device ( 1 ) utilizes two sets of structures. A first structure ( 11 ) is a rigid structure ( 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 structure ( 12 ) is a flexible or damping structure ( 121  and  122 ) made of biocompatible viscoelastic materials, permitting repeated elastic deformations, the combination of the two structures providing the device with both resistance and mechanical stress damping of forces to which it is subjected, with the purpose of compensating for any deficiency in the flexibility of certain anatomical links of the human body. 
     In an advantage, the surgeon can choose in a precise way the desired working method: tension/compression or flexion, or the combination of the two working methods, so as to avoid any contact between the articular facets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are perspective views (two alternative embodiments) of the device operating in tension, compression and flexion. 
         FIGS. 2A and 2B  are longitudinal cross-sectional views of two alternative embodiments of the invention. 
         FIG. 3  is an exploded view of the invention. 
         FIG. 4  is a perspective view of the invention for operation only in tension/compression. 
         FIG. 5  is a cross-sectional view of the invention operating only in tension/compression. 
         FIGS. 6 to 11  are perspective views depicting the individual parts of the invention. 
         FIG. 12  is a perspective view of another component for operation in the tension/compression mode. 
         FIG. 13  is a side, cross-sectional view of an alternative of the device working along two axes. 
         FIGS. 14 to 17  are perspective views of four forms of the mobile end of other embodiments of the invention. 
         FIG. 18  is a side, cutaway view of the invention in place in a patient. 
     
    
    
     DETAILED DESCRIPTION 
     The device  1  utilizes two sets of structures. A first structure  11  a rigid structure manufactured out of preferably metal, biocompatible material ensuring a good mechanical resistance of the device by completely transmitting the forces. 
     A second structure  12  is a flexible or damping structure manufactured out of viscoelastic biocompatible materials, supporting the repeated elastic strain. It is the combination of these two structure which makes the operation of the invention possible. 
     The first structure  11  includes four mechanical structures  110 ,  112 ,  114 ,  116  which have the function of transmitting stresses to which the device  1  is subjected without deformation. 
     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 rod  111  with a broad joining radius  113   a , and the assembly being able to slide in the cavity of the structure  114  which encloses a visco-elastic element  121 . 
     The mechanical structure  112  is a cap provided with a thread  117  allowing for the fixing of the structure  112  on structure  114 ; the structure  112  has a shoulder area  118  which encloses a viscoelastic-centering ring  121  between the plate  113   b.    
     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 mechanical structure  110  and  116  will be fixed on the vertebrae to permit the functioning of the device  1 . 
     The second structure  12  is made up of two viscoelastic components  121  and  122 . 
     The centering ring  121  preferably allows the rod  111  to slide in its center. 
     The second component  122  is a disc of viscoelastic material. These two centering rings  121  and  122  can undergo compressive stresses which may not be uniformly distributed. They are formed to resist many cyclic fatigue stresses without breaking. Compressive tests have been performed which verify that the components  121  and  122  are able to undergone these tests of elastic deformation as many times as necessary. 
     The selected material is preferably a biocompatible polyurethane; thanks to their integration inside mechanic components  110 ,  112 ,  114 ,  116 , the viscoelastic elements  121  and  122  are protected by the preceding mechanical structures from the aggressive environment of the human body, which avoids in particular the formation of fibers around these components which could deteriorate the viscoelastic properties of the material and consequently disturb the correct operation of device  1 . 
     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 component  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 component  114 ; the shoulder area  118  serves as a stop and maintains in its housing the viscoelastic element  121  thus constrained. 
     If one wishes to work in a uni-axial mode of tension/compression, component  112  is replaced by another component  115  equipped with threads  117 , which includes a cap  115   c , whose opening  119  is of a diameter which corresponds to the diameter of the rod  110  and which is elongated with a rod guide  115   a.    
     This device  1  is thus able to react dynamically to the applied stresses. Note that it is essential that structure  114  comprises a bore  114   a  to allow for low friction guidance of rod  110  in the aforementioned component  114 . 
     The adjustment of the diameter of the viscoelastic centering rings  121  and  122  must be selected with precision to enable them to be crushed freely until a stress threshold is reached, this threshold corresponding to a point of contact of the bore  114   a  of component  114 . 
     An alternative to the structure  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 ) having a lower threshold than that of the structures previously described (see  FIG. 2 ). 
     The rod  131  is fixed at its cap  130  by the intermediary of a thread located on shoulder  132  of the rod. 
     In this alternative embodiment, the possibilities of displacement of rod  110  subjected to the stresses in flexion are enabled by play  119  located between cap  130  and rod  110 . 
     For a uni-axial operation of device  1 , it is preferable to use components  110 , 112 ,  114 ,  116  which provide a better guidance of rod  110 . If small overall dimensions are needed, components  110 ,  130 ,  131  may be preferably used. 
     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. 
     The structure  12  is therefore comprised of two visco-elastic components  141  and  142 . Note that the rod  110  has a flange  110 ′ on its end. The component  141  is a cylinder of visco-elastic biocompatible material whose face in contact with the flange  110 ′ is inclined. The component  142  is a centering ring whose face in contact with the back of the flange  110 ′ is inclined. 
     The structure  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 allowed with the rod  110 . 
     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 the case in which the  119  orifice is eccentric and adjusted to the rod  116  or  131  (see  FIG. 14 ). 
     The rod  110 , forming an angle with respect to 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. (see  FIG. 15 ). 
     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  which, at rest, forms an angle with the rod  116  or the rod  131 , this also being the case where the component  119  is oblong or eccentric, ( FIG. 16 ). 
     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  (see  FIG. 17 ). 
     Multiple variations and modifications are possible in the embodiments of the invention described here. Although certain illustrative embodiments of the invention have been shown and described here, a wide range of modifications, changes, and substitutions is contemplated in the foregoing disclosure. In some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the foregoing description be construed broadly and understood as being given by way of illustration and example only, the spirit and scope of the invention being limited only by the appended claims.