Abstract:
Intervertebral disc prosthesis comprising an upper apposition plate appropriate to rest against the base plate of a vertebra, an apposition plate spaced apart from the apposition plate and suitable to rest against the upper plate of a vertebra, a plurality of elastic devices mounted between the two apposition plates and in their peripheral zones in a manner that the two apposition plates shall be displaceable in mutually resilient manner, and a central axis which runs essentially perpendicularly to the two apposition plates, the elastic devices being designed/configured in a manner that the intervertebral disc prosthesis as a whole exhibits asymmetrical stiffness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of International Application No. PCT/CH2003/00187, filed Mar. 24, 2003, the entirety of which is expressly incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a vertebral disc or intervertebral disc prosthesis, hereafter simply called intervertebral disc prosthesis. Such prostheses may be used as nucleus replacements, flexible cages or intervertebral disc prostheses, and are inserted posteriority (PLIF technique). When in the form of dynamic implants, they also may be emplaced between the dome extensions of adjacent vertebral discs.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Pat. No. 5,458,642 (Beer) discloses an intervertebral disc prosthesis comprising an upper and a lower kidneys-shaped plate, these plates being each peripherally connected (about a central core) to each other by a plurality of helical springs. The helical springs are configured in equidistant manner to each other and allow displacing said plates in three dimensions within given limits. However this known intervertebral disc prosthesis incurs the drawback that the stiffness of the implant is identical in every radial direction—at the same distance from the implant center, in other words, the implant is symmetrically stiff.  
       SUMMARY OF THE INVENTION  
       [0004]     The objective of the present invention is palliation. The present invention creates an intervertebral disc prosthesis exhibiting asymmetrical stiffness. Thereby the intervertebral disc prosthesis advantageously may be matched by design to physiological behavior so that for instance rearward spinal column extension shall be met by more stiffness than a forward lateral displacement. The present invention solves this problem by an intervertebral disc prosthesis.  
         [0005]     The advantages of the present invention substantially are that its prosthesis enables physiological behavior when the spinal column is stressed and that in particular the implant center of rotation may be controlled by the asymmetric change in stiffness. In a preferred embodiment mode, the number of elastic devices is between 4 and 12, in particular between 6 and 10.  
         [0006]     In another embodiment mode, the elastic devices are mutually identical but are configured radially unequally in the peripheral zones of the intervertebral disc prosthesis. This design offers the advantage that the rigidity, i.e. stiffness of said intervertebral disc prosthesis can be selectively set in production by using a variable number of identical elastic elements per unit peripheral angle or alternatively an irregular array (namely more or less dense) of elastically identical elements may be used in a manner that a different intervertebral disc prosthesis stiffness shall result depending on radial direction, said variable stiffness thereby better matching anatomical particulars than is the case for such conventional prostheses exhibiting symmetrical stiffness.  
         [0007]     In still another embodiment mode, at least a portion of the elastic devices is different from one another, said devices however preferably being configured radially uniformly in the peripheral zone of the intervertebral disc prosthesis.  
         [0008]     In a further embodiment mode of the present invention, at least one portion of the elastic devices is different, these elastic devices being configured in radially varying manner in the intervertebral disc prosthesis&#39; peripheral zones.  
         [0009]     In another further embodiment mode said intervertebral disc prosthesis exhibits higher stiffness in a sub-zone of a peripheral arc of 90° than in the complementary arc of 90°.  
         [0010]     In a further embodiment mode, at least a part of the elastic elements is made of materials of different stiffnesses.  
         [0011]     Appropriate materials are all known implant materials of a metallic or polymeric nature. Moreover the implant may be fitted with an HAC coating.  
         [0012]     Preferred implant materials are titanium, nitinol, titanium alloys and steel. The following are preferred material combinations: for apposition plates: titanium/titanium alloys—for intermediate plates: titanium/titanium alloys—for screws: titanium—for rings: nitinol, titanium or steel.  
         [0013]     The apposition plate geometry and surface shall appropriately match the natural end plates of the vertebras, the two apposition plates being optionally circular, rectangular, kidney-shaped, oval, spiral/helical in the various embodiment modes.  
         [0014]     In yet a further embodiment, the elastic devices are rings or partial rings, the ring plane of such elements optionally being such that: the ring plane intersects the central axis of the intervertebral disc prosthesis; the ring plane does not intersect the central axis of the intervertebral disc prosthesis; the ring plane is substantially perpendicular to the two apposition plates; or the ring plane is oblique to the two apposition plates.  
         [0015]     In another embodiment, at least part of the rings exhibit different stiffnesses, these rings preferably being configured in sequence with increasing respectively decreasing stiffness.  
         [0016]     In yet another embodiment, the rings are arrayed peripherally, thereby offering the advantage of several peripheral sub-zones of higher and of lower stiffness.  
         [0017]     Said elastic devices may be selected from the following materials in various embodiment modes: spiral/helical springs, elastic bellows, plastic cylinders, tapes/bands, wire mesh lattices, endless fibers, or plastic coated wires. Such designs offer the following advantages over the designs involving annular elastic devices: increased flexibility, simpler production know-how, easier handling; and visco-elastic behavior of the intervertebral disc prosthesis.  
         [0018]     In a further embodiment mode, the elastic devices are made of a wire rope which preferably is a unifilament.  
         [0019]     In further embodiments, the elastic devices include at least one spring element consisting of a spring wire designed as follows: the spring wire is fitted with serpentines, and/or the spring wire exhibits at least one loop.  
         [0020]     In still another embodiment, the intervertebral disc prosthesis comprises a plastic core. Such a feature offers the significant advantages that said prosthesis exhibits visco-elastic behavior and that the motions of the adjacent vertebras are better damped.  
         [0021]     In another embodiment of the present invention, the intervertebral disc prosthesis&#39; viscosity is at least 0.7 mm at the periphery, preferably at least 1.0 mm and at most 1.2 mm, preferably at most 3.5 mm.  
         [0022]     The elastic devices connected to the apposition plates may be geometrically locking, i.e. positively locking, or they may be frictionally locking.  
         [0023]     In still another embodiment of the present invention, the two apposition plates subtend between them an angle of 10° to 14°.  
         [0024]     In a further embodiment of the invention, the elastic devices are combined into one unit, the apposition plates being slipped onto or snapped onto said unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The present invention and further developments of it are elucidated below by means of several illustrative embodiments and in relation to the partly schematic Figures.  
         [0026]      FIG. 1  is a perspective view of an intervertebral disc prosthesis,  
         [0027]      FIG. 2  is a side view of the intervertebral disc prosthesis of  FIG. 1 ,  
         [0028]      FIG. 3  is a top view of the intervertebral disc prosthesis of  FIG. 1 ,  
         [0029]      FIG. 4   a  is a side view of another embodiment mode of the intervertebral disc prosthesis,  
         [0030]      FIG. 4   b  is a top view of the intervertebral disc prosthesis of  FIG. 4   a  less the upper apposition plate,  
         [0031]      FIG. 4   c  is a perspective view of the intervertebral disc prosthesis embodiment of  FIG. 4   a,    
         [0032]      FIG. 4   d  is a perspective of intervertebral disc prosthesis embodiment of  FIGS. 4   a  through  4   b  less the upper apposition plate,  
         [0033]      FIG. 5   a  is a front view of a further intervertebral disc prosthesis embodiment less the upper apposition plate,  
         [0034]      FIG. 5   b  is a top view of the intervertebral disc prosthesis of  FIG. 5   a,    
         [0035]      FIG. 5   c  is a side view of the intervertebral disc prosthesis of  FIGS. 5   a  and  5   b,    
         [0036]      FIG. 5   d  is a perspective view of the intervertebral disc prosthesis embodiment of  FIGS. 5   a  through  5   c,    
         [0037]      FIG. 6   a  is an elevation of another intervertebral disc prosthesis embodiment,  
         [0038]      FIG. 6   b  is a section B-B through the intervertebral disc prosthesis embodiment of  FIG. 6   a,    
         [0039]      FIG. 6   c  is a perspective view of the spring elements connected to the lower apposition plate of the intervertebral disc prosthesis embodiment of  FIGS. 6   a  and  6   b  less the upper apposition plate,  
         [0040]      FIG. 6   d  is a perspective of the intervertebral disc prosthesis of  FIGS. 6 through 6   c,    
         [0041]      FIG. 7   a  is a top view of a further intervertebral disc prosthesis embodiment,  
         [0042]      FIG. 7   b  is a section B-B of the intervertebral disc prosthesis embodiment of  FIG. 7   a,    
         [0043]      FIG. 7   c  is a side view of the intervertebral disc prosthesis embodiment of  FIGS. 7   a  and  7   b,    
         [0044]      FIG. 7   d  is a perspective view of the intervertebral disc prosthesis embodiment of  FIGS. 7   a  through  7   c,    
         [0045]      FIG. 8  schematically shows two intervertebral disc prostheses such as shown in  FIGS. 7   a  through  7   d  that are implanted between two vertebras,  
         [0046]      FIG. 9  is a top view parallel to the axis of the spinal column of the upper plate of a vertebra with two intervertebral disc prostheses such as shown in  FIGS. 7   a  through  7   d,    
         [0047]      FIG. 10  is an elevation of two intervertebral disc prostheses such as are shown in  FIGS. 7   a  through  7   d  implanted between two vertebras, and  
         [0048]      FIG. 11  is an elevation of another embodiment of the elastic devices of the intervertebral disc prosthesis of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0049]     The intervertebral disc prosthesis shown in  FIGS. 1 through 3  consists of an upper, circular apposition plate  1  suitable to come to rest against the base plate of a vertebra, further of a lower annular apposition plate  2  which is appropriate to come to rest against the cover plate of a vertebra, further two intermediate plates  6  which are configured between the two apposition plates  1 ,  2  and which are also circular, all plates being mounted perpendicularly to a common central axis  5 .  
         [0050]     A total of eight elastic devices  3  in the form of rings  3   a - 3   h  are configured between the two circular apposition plates  1 ,  2  and said devices run radially to the central axis  5 , their annular plane being perpendicular to the apposition plates  1 ,  2 .  
         [0051]     In order that the intervertebral disc prosthesis be held together, the upper apposition plate  1  is connected to the adjacent intermediate plate by a total of eight screws  7  and in turn the lower apposition plate  2  is connected to its adjacent intermediate plate  6  by a total of eight screws  7 . Accordingly the apposition plates  1 ,  2  comprise corresponding boreholes  9  receiving the screw heads and the intermediate plates  6  are fitted with corresponding threaded boreholes receiving the screw shanks. The screws  7  are always configured between two adjacent rings.  
         [0052]     The individual rings  3   a - 3   h  are diametrically captured by the above described pair of plates  1 ,  6  and by the lower pair of plates  2 ,  6  in a manner that the two pairs of plates  1 ,  6  and  2 ,  6  are connected to each other by the rings  3   a - 3   h  and that, thanks to the elasticity of these rings  3   a - 3   h,  they may be moved from the parallel state when unstressed into a mutually slanted state within given limits (compressibility of about 1.0 to 1.5 mm at the periphery). As a result the two apposition plates  1 ,  2  may subtend between them an angle approximately of 12°.  
         [0053]     In order to minimize the height of the intervertebral disc prosthesis, the apposition plates  1 ,  2  and the intermediate plates  6  may comprise clearances  8  matching the contours of the rings  3   a - 3   h  in the regions of the ring crossings.  
         [0054]     The elastic rings  3   a - 3   h  are peripherally apart at regularly equidistant angles of 45° but exhibit different elasticities and stiffnesses, this feature being attained by using different materials, different ring geometries or different ring cross-sections (solid, hollow, round, rectangular). Consequently higher stiffness is attained in the zone of the rings  3   b,    3   c  and  3   d  than in the zone of rings  3   f,    3   g  and  3   h.  When a given force is applied perpendicularly to the apposition plate  1  to the zone of the ring  3   c,  then less compression shall be incurred (reduction of the distance between the two apposition plates  1 ,  2  than when the same force is applied to the zone of the ring  3   g.  This asymmetrical behavior results in improved physiological behavior of the intervertebral disc prosthesis in that, when the spinal column is bent forward, compression of the rings  3   f,    3   g  and  3   h  shall be larger than the compression of the rings  3   b,    3   c  and  3   d  if the spinal column were bent backward.  
         [0055]     The spring constants of the individual rings may appropriately vary between 50 and 100%. Thus the spring constants may vary between 300 N/mm and 1,000 N/mm.  
         [0056]      FIGS. 4   a  through  4   d  illustrate an embodiment mode wherein the elastic devices  3  are a helical spring  10  of which the longitudinal axis  11  is circular in a plane orthogonal to the central axis  5 , as a result of which the helical spring  10  encloses the implant central axis  5  by an angle of 360°. In this design the helical spring  10  has been shifted by such a distance toward the periphery of the circular apposition plates  1 ,  2  that it does slightly project beyond said periphery. The helical spring  10  comprises two arcuate portions exhibiting opposite pitches/number of turns per unit length of the spring wire. The turns of the helical spring  10  between said two portions are connected to each other by a loop  12 ,  13 . In this embodiment the loops  12 ,  13  are configured at those circumferential segments of the helical spring  10  which point toward the upper apposition plate  1 . This design of the helical spring  10  exhibiting two arcuate portions of opposite turn pitches allows controlling the implant&#39;s impedance to torsion. Circular elevations  19 ,  20  concentric with the central axis  5  are present at the mutually opposite inner surfaces of the apposition plates  1 ,  2 . For each turn, the spring wire passes once through boreholes in each of the two circular elevations  19 ,  20 , and as a result the apposition plates  1 ,  2  and the helical spring  10  are firmly held together. Moreover the two loops  12 ,  13  are different form one another regarding the space they occupy between the two adjoining spring wire turns. In each arcuate portion of the helical spring  10 , the turns exhibit a constant pitch, as a result of which the implant spring constant differs in value only at the junctions of the two arcuate portions of the helical spring  10 . By means of the design of the two portions of opposite turn pitches of the helical spring  10 , the invention offers equal implant torsion impedance in both directions of rotation.  
         [0057]     The embodiment mode shown in  FIGS. 5   a  through  5   d  differs from that of  FIGS. 4   a  through  4   d  merely in that the apposition plates  1 ,  2  (only apposition plate  2  being shown) are oval and as a result the elastic devices  3  comprise 4 arcuate but separate helical spring elements  15 ,  16 ,  17 ,  18  configured along an oval longitudinal axis  14 . Every two mutually diametrically opposite helical spring elements  15 ,  16 ,  17 ,  18  are mirror-symmetrical, two helical spring elements  15 ,  16  exhibiting turns of opposite pitches and the other two mirror-symmetrically configured helical spring elements  17 ,  18  each comprising at their midsts a loop  12 ,  13  entail a change in the direction of rotation of the turns. Furthermore the pitches of the two pairs of mirror-symmetrically configured helical spring elements  15 ,  16 ,  17 ,  18  are different, whereby the spring constants of the elastic devices  3  will be different depending on the position of the axis of rotation between the two vertebras  34 ,  35  ( FIG. 8 ) adjoining the two apposition plates  1 ,  2 .  
         [0058]      FIGS. 6   a  through  6   d  show an embodiment mode of which the design of the elastic devices  3  differs from that of the embodiment mode shown in  FIGS. 4 and 5  merely in that it comprises two spring elements  22 ,  23  concentric with the central axis  5 , each spring element comprising a spring wire  25  exhibiting several serpentines  24 . The spring elements  22 ,  23  are in the form of partly toroidal surfaces, the junction between the loops  12 ,  13  of the serpentines  24  running obliquely to the torus meridians. The angles between the torus meridians and the junctions between the loops  12 ,  13  of the serpentines  24  of the two spring elements  22 ,  23  are opposite and of equal magnitude. Also each apposition plate  1 ,  2  comprises two elevations  19 ′,  19 ″,  20 ′,  20 ″ concentric with the central axis  5 . Similarly to the case of the embodiments of  FIG. 4 , each loop  12  of a serpentine  24  passes through two boreholes in one of the circular elevations  19  at the upper apposition plate  1 , whereas the other loop  13  of the serpentine  24  passes through two boreholes in one of the circular elevations  20  at the lower apposition plate  2 , as a result of which the two apposition plates  1 ,  2  and the elastic devices  3  are held together. The loops  12 ,  13  of the inner spring element  22  pass through boreholes in the inner elevations  19 ′,  20 ′ and the loops  12 ,  13  of the outer spring element  23  pass through the boreholes in the outer elevations  19 ″,  20 ″.  
         [0059]      FIGS. 7   a  through  7   d  show an embodiment mode comprising an upper and a lower apposition plate  1 ,  2  fitted with rectangular surfaces transverse to the central axis  5 . The longitudinal axis  11  of two, in-series helical spring elements  15 ,  16  between the apposition plates  1 ,  2  is parallel to the long axes of the rectangular apposition plates  1 ,  2 . The two helical spring elements  15 ,  16  exhibit mutually opposite turn pitches. Elevations  19 ,  20  parallel to the long axes moreover are configured at the inside surfaces of the apposition faces  1 ,  2  and are fitted with boreholes running transversely to the said long axes. The turns of the helical screw elements  15 ,  16  passing through said boreholes therefore hold together the two apposition plates  1 ,  2 .  
         [0060]      FIGS. 8 through 10  illustrate how to use two intervertebral disc prostheses such as are shown in  FIGS. 7   a  through  7   d.  The two intervertebral disc prostheses are inserted into intervertebral space of two mutually adjoining vertebras  34 ,  35  in a manner that the longitudinal axes  11  of the helical spring elements  15 ,  16  run from anterior to posterior, each intervertebral prosthesis being mounted laterally to the longitudinal axis of the spinal column. This configuration of the intervertebral disc prostheses offer differential spring constants of the elastic devices  3  for the flexion/extension and lateral bending of the spinal column.  
         [0061]      FIG. 11  shows an embodiment mode of the elastic devices  3  including a spring wire  25  wound into loops  26 . The loops  26  on the spring wire  25  may be so designed on one hand that similarly to the case of the rings of  FIG. 1  they shall be closed and constitute individual spring elements which, in a desired manner, shall be distributed between the apposition plates  1 ,  2 . On the other hand and similarly to the design shown in  FIG. 4 , the loops  26  may constitute the turns of a kind of helical-spring element. The anchoring of the spring wire  25  onto the apposition plates  1 ,  2  may be carried out in the manner of any embodiment shown in  FIGS. 1 through 10 . Also, the design of the apposition plates  1 ,  2  as well as the distribution of the elastic devices  3  is carried out in the manner of any embodiment mode illustrated in  FIGS. 1 through 10 .