Abstract:
An intervertebral disk prosthesis comprising two plates and a cushion interposed between the plates is contemplated. The cushion includes a compressible body having two ends in contact with the plates. At least one of the plates comprises at least one stud for stressing the body along a direction not parallel to a main access of the prosthesis and mobile relative to the body. Thus the prosthesis imitates and approximates the mechanical properties of a healthy natural intervertebral disk.

Description:
FIELD OF THE INVENTION 
     The invention concerns intervertebral disc prostheses. 
     BACKGROUND OF THE INVENTION 
     European Patent 0 356 112 discloses a prostheses comprising two plates and a body of compressible material interposed between and fixed to the plates. The prosthesis replaces the natural disc after the disc has been ablated, with the plates bearing against the vertebral plates of the adjacent vertebrae. With such prostheses it is possible to a great extent to reproduce the mechanical behavior of a healthy natural disc, in particular in compression or in torsion about any axis perpendicular to the longitudinal direction of the spine. However, the prostheses do not satisfactorily imitate two other movements: the relative rotation of the two plates about a main axis of the prosthesis or the axis of the spine, and the relative displacement of the two plates by shearing or sliding in a plane perpendicular to this axis. For these two movements, the known prostheses offer an insufficient mechanical reaction and are too rigid for the first movements. 
     It is therefore an object of the invention to more closely imitate the behavior of a healthy natural intervertebral disc. 
     The invention provides an invertebral disc prothesis comprising two plates and a deformable body interposed between the plates, where at least one of the plates comprises at least one contact stud which is able to stress the body in a direction not parallel to a main axis of the prosthesis and is movable relative to the body. 
     The invention also provides an intervertebral disc prosthesis comprising two plates and a deformable body interposed between the plates, where at least one of the plates comprises at least one contact stud which is movable in the body. 
     Thus, the contact stud offers mechanical resistance when the prosthesis is subjected to a rotational stress about its main axis or a shearing stress in a direction perpendicular to this axis. In addition, this resistance is variable depending on the relative position of the plates, for example depending on how close they are to each other and/or how inclined they are relative to each other. Indeed, the resistance to shearing and to rotation mentioned above will be greater the closer the contact stud is to the opposite plate and thus the more compressed the body is on the axis. Moreover, depending on the position of the contact stud, a relative inclination of the plates will bring the contact stud closer to the opposite plate and thus increase the resistance of the prosthesis locally near the contact stud. In contrast, the further away the contact stud is from the opposite plate the more the resistance is reduced. The prosthesis thus has a mechanical behavior which varies depending oh the relative position of the plates, which makes it similar to a healthy natural disc. If its dimensions are sufficiently great, the contact stud or each contact stud can additionally constitute an abutment limiting one of the relative movements of flexion or translation of the plates. Of course, what has just been described above for one contact stud is valid a fortiori when the prosthesis comprises a plurality of contact studs. 
     The contact stud is advantageously offcentered relative to the plate which bears it. 
     Thus, the resistance behavior of the prosthesis largely depends on the axis of the flexion and on the direction of this flexion. 
     The contact stud advantageously extends at a distance from the plate other than the one which bears it when the prosthesis is not stressed. 
     The contact stud advantageously has a length of between about 0.60 d and about 0.90 d, where d is a distance separating the two plates when the prosthesis is not stressed. 
     Advantageously, for the plate or each plate provided with at least one contact stud, the body is immobilized relative to the plate, in respect to a displacement parallel to the plate, only by virtue of the contact stud. 
     Thus, the body bears, without anchoring, on the plate or each plate comprising a contact stud. Assembly is thus carried out simply by stacking the plates and the body together. The body is, in particular, able to be separated from the plate under the effect of traction causing a displacement in the direction away from the plate, which is not conceivable in the normal conditions of use of the prosthesis. 
     The contact stud advantageously extends in a recess of the body opening into a lateral face of the body. 
     The contact stud advantageously has a flattened shape in a plane radial to the main axis of the prosthesis. 
     Thus, in the event of shearing or torsion about the main axis of the prosthesis, the bearing surface between the contact stud and the body is considerable, resulting in good distribution of the load, although the volume of the contact stud can be relatively small. 
     The contact stud advantageously has a flattened shape in a plane tangential to a direction circumferential to the main axis of the prosthesis. 
     The contact stud advantageously has a cylindrical lateral face, and the body has a cylindrical face bearing on the cylindrical face of the contact stud. 
     The contact stud advantageously has a lateral face extending outside the body. 
     Thus, the contact stud encroaches moderately on the volume of the body. 
     The lateral face of the contact stud advantageously extends in the continuation of an outer lateral face of the body. 
     Advantageously, for the contact stud or each contact stud, the plate other than the one bearing the contact stud has a recessed zone constituting the part facing the contact stud when the prosthesis is at test. 
     Thus, in the event of flexion or of the plates coming extremely close to each other, the contact stud or each contact stud does not come into abutment against the opposite plate, so that the deformable body taking up the stresses continues to impose the mechanical behavior of the prosthesis. 
     The plate advantageously comprises at least two contact studs arranged symmetrically about a center of the plate. 
     Each plate advantageously comprises at least one contact stud, the contact studs overlapping in a direction parallel to the main axis of the prosthesis when the prosthesis is not stressed. 
     The anti-shearing effect is increased in this way. 
     The contact studs advantageously overlap by a length of between about 0.35 d and about 0.65 d, where d is a distance separating the two plates when the prosthesis is not stressed. 
     Advantageously, when the prosthesis is not stressed, the contact studs overlap by a length of between about 0.45 h and about 0.85 h, where h is a height of the contact studs parallel to the main axis of the prosthesis. 
     Each plate advantageously comprises at least two contact studs, the contact studs being arranged alternatingly around the main axis of the prosthesis. 
     Other characteristics and advantages of the invention will also become apparent from the following description of two preferred embodiments given as nonlimiting examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a prosthesis according to a first preferred embodiment, without its bellows; 
     FIG. 2 is a side view of the prosthesis from FIG. 1, partially cut away in the area of the bellows; 
     FIG. 3 is a perspective view of an alternative to the prosthesis of the first embodiment; 
     FIGS. 4 and 5 are perspective views of a prosthesis according to a second preferred embodiment, in the assembled state and in an exploded representation, respectively; and 
     FIGS. 6 and 7 are perspective and plan views, respectively, of the body of the prosthesis in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, an intervertebral disc prosthesis  2  is shown. The intervertebral disc prosthesis  2  is intended for the lumbar region of the spine. 
     The prosthesis  2  comprises two plates  4  which are identical to each other. Each plate  4  has a generally planar disc shape. In the present embodiment, each plate has a slightly raised peripheral edge  6  projecting from a plane central inner face  8 , thus giving it the appearance of a saucer. 
     Each plate comprises contact studs  10 , preferably two identical contact studs. Each contact stud  10  has the general shape of a rectangular pipe with one edge in contact with the plate, and the other edge rounded. The height and width of the contact stud are about equal, but the thickness is substantially smaller. The contact studs  10  project perpendicularly from the inner face  8  and are arranged symmetrically with respect to a main axis  12  of the prosthesis passing through the center of the face  8 . The contact studs  10  are substantially perpendicular to the face  8  when not stressed. The thickness of the contact studs  10  extends in a plane radial to the axis  12  so that the two contact studs are substantially in the same radial plane, either side of the axis  12 . On a plane outer face  14 , the plate has reliefs making it easier to fix the plate  4  to a vertebral plate, for example, by means of a hydroxyapatite covering. The plates can be made of metal or composite material. 
     The prosthesis comprises a body  16  of general cylindrical shape, having a cylindrical outer face  18  and two opposite plane end faces  20 . The body  16  is here made of an elastic material such as an elastomer. The body defines channels or recesses  22 , here four in number. The channels are profiled in a direction parallel to the axis  12  of the body  16 . The profile is identical to the profile of the contact studs  10  in a plane transverse to the height of the contact studs  10  or the axis  12 . Each channel  22  thus has a generally plane shape and extends in a radial plane to the axis  12 . Each channel  22  extends at a distance from the axis  12  and opens out on the cylindrical face  18  across its entire height and on each of the two end faces  20 . Viewed from the end, the body thus has a cross or cloverleaf shape. 
     For assembly, the body  16  is arranged between the two plates  4  which have their inner faces  8  and their contact studs  10  facing each other, the two plates being mutually offset by a quarter turn about the axis  12 , the four channels  22  being in line with the four contact studs. The two plates  4  are moved toward each other until their inner faces  8  come into surface contact with the end faces  20  of the body, the four contact studs  10  penetrating the four respective channels  22 . The four contact studs thus belong to the two plates alternately about the axis  12 . The plates  4  bear on the body  16  without any anchoring other than the contact studs. The body  16  is immobilized by bearing on the inner faces  8  and the contact studs  10 , the latter by themselves prohibiting its displacement in translation in a plane perpendicular to the axis  12 . 
     The prosthesis comprises a compressible bellows  26  in the form of a sleeve which has a corrugated profile and is fixed to the edges  6  of the two plates in order to isolate the space to the inside of the bellows, including the body  16 , from the outside. In this case, the bellows has ten convolutions, which creates nine crests in addition to the crests fixed to the edges  6 . The bellows and the plates can be made of titanium or titanium alloy. 
     In FIG. 2, the prosthesis is illustrated in the unstressed state. Each contact stud  10  has, on the axis  12 , a height h of between about 0.60 d and about 0.90 d, where d is a distance, taken between the edges  6 , separating the two plates when the prosthesis is in the unstressed state. In the present case, h is equal to about 0.75 d. 
     In one embodiment, the contact studs  10  of one plate partially overlap the contact studs  10  of the other plate as shown in FIG.  2 . Thus, any cut through the prosthesis transverse to the axis  12  intercepts two contact studs  10  of the same plate  4  in the vicinity of the end faces  20 , and the four contact studs in a median portion of the body. The length of overlap r, measured parallel to the axis  12 , can be between about 0.35 d and about 0.65 d or between about 0.45 h and about 0.85 h. Here, r is equal to about 0.66 h and about 0.5 d. 
     Once fitted, the prosthesis behaves as follows. 
     If the prosthesis is subjected to rotation about the axis  12 , the contact studs  10  cooperate with the body  16  in order to take up a large part of the generated stresses, which locally are shearing stresses. 
     If the prosthesis is compressed on its axis  12 , the four contact studs  10  each penetrate further into their channel  22 , moving in the direction of the opposite plate. The resistance of the prosthesis to shearing perpendicular to the axis  12  or to rotation about this axis is therefore greater. 
     If the prosthesis is subjected to flexion about an axis perpendicular to the axis  12 , the two plates  4  incline relative to each other, which corresponds locally to a compression on certain parts of the body  16  and to a traction on other parts of the body. The resistance to shearing is thus increased in the former and reduced in the latter. 
     Although the body  16  is deformable, the movement of each contact stud  10  in its channel  22  is on the whole similar to a sliding movement. 
     Referring to FIG. 3, it is possible for each plate  4  to have two lugs  30  projecting from the outer face  14  of the plate  4  perpendicular to the plane of the plate. Each lug  30  has an orifice  32  passing right through it in the direction of the center of the plate and an identification of spherical shape on a face of the lug directed away from the plate. The orifices  32  can receive a bone screw  34  having a head whose lower face has a male spherical shape interacting with the female indentation of the lug  30  in order to permit free orientation of the screw relative to the associated lug. 
     For short-term anchoring of the disc prosthesis in the column, the screws  34  can be anchored in the body of the vertebrae adjacent to the disc which is to be replaced. 
     However, it will be possible to provide for long-term anchoring in which, in addition, the surfaces  14  of the plates  4  in contact with the adjacent vertebrae are covered with hydroxyapatite or any other substance known per se for stimulating bone growth. Before being covered, said surfaces can be treated to obtain a more or less porous surface condition, with anchoring points for the bone tissue, so as to ensure a better interface with said bone tissue. In FIG. 3, the plates have the shape of a bean with a posterior hilum. 
     FIGS. 4 to  7  illustrate a second embodiment in which the elements analogous to those of the first embodiment bear reference numbers increased by one hundred. 
     In this prosthesis  102 , each of the two plates  104  here bears three contact studs  110 . Each contact stud  110  is contiguous with the edge  106  of the plate and has a flat shape parallel to this edge, or substantially in a plane tangential to a direction circumferential to the axis  112 . At the location of the contact studs  110 , the edge  106  has a plane of substantially circular convex shape, each contact stud  110  has a convex cylindrical shape with an outer lateral face  111  intended to be set opposite the body  116  extending in the continuation of the edge  106 . Each contact stud  110  additionally has an inner lateral face  113 , also of convex cylindrical shape. The two cylindrical faces  111 ,  113  of each contact stud form two ridges parallel to the axis  112 . The ridges slightly pronounced and very rounded (in a plane perpendicular to the main axis) in order to ensure that they do not cut into the body. The same is true of the ridges forming the free end face of the contact studs. Here once again, the contact studs of the two plates are intended to partially overlap parallel to the axis  112  so that the contact studs of the two plates mesh. 
     The body  116  has a lateral face  118  of generally cylindrical shape, except that its plane has the shape of a bean with posterior hilum. This is also the shape of the plane of the overall prosthesis. The six channels or seats  122  for contact studs on the body  116  are here delimited by cylindrical and concave faces, respectively, with axes parallel to the axis  112  and opening in the lateral face  118 . The body  116  thus has laterally an alternating sequence of concave cylindrical faces  122  and convex cylindrical faces  118 . Each face  122  has the same radius and the same length as the inner lateral face  113  of the corresponding contact stud in order to ensure surface contact of one with the other. However, the cylindrical face  122  of the body is higher than the contact stud, parallel to the axis  112 . 
     Since the prosthesis is bean shape of the prosthesis and each contact stud is intended to be received between two contact studs of the other plate, the two plates are not strictly identical. 
     The prosthesis is assembled as before by bringing the two plates  104  toward one another with their contact studs  110  facing each other. The body  116  is interposed between the two plates  104 . Each contact stud  110  thus penetrates into its seat  122 , the lateral inner face  113  of the contact stud coming into contact with the face  122  of the channel. Once assembly has been completed, as in FIG. 4, each plate has its plane inner face bearing on the respective plane end face of the body. 
     The curvatures of the outer lateral faces  111  of the contact studs and the curvatures of the lateral face  118  of the body are chosen in such a way that the faces extend without any protruding or recessed ridges, giving the plane of the prosthesis a bean shape. 
     Each plate  104  has in a plane three recesses  115  in the edge  106 , each delimited by a concave circular edge of greater radius than that of the associated face  122 , as will be seen. Each recess  115  extends between two of the contact studs  110  of the plate so that said recess itself lies opposite a contact stud of the other plate. 
     Thus, in the assembled position, and at rest, as is shown in FIG. 4, each contact stud  110  extends opposite a part of the other plate formed by the recess  115 . Thus, in the event of considerable stress displacing at least one of the contact studs  110  in the direction of the other plate, the contact stud can extend into the recess  115  without coming into abutment against the plate. The radius or the depth of the recess  115  is greater than that of the associated channel  122 , which gives the assembly a step-shaped configuration locally and prevents any abutment of the contact stud against the opposite plate. This substantial depth of the recess increases the flexibility of the prosthesis in the event of torsion about an axis perpendicular to the main axis. 
     The dimensions r, h and d apply in this embodiment and have the same characteristics previously discussed. Also in this embodiment, it will be possible to provide a bellows or a screw attachment as in FIG.  3 . 
     Here once again, each contact stud  110  is movable relative to the body and is able to stress the body in a direction not parallel to the axis  112 , this stress being variable depending on the position of the contact stud in the body in the radial direction and the direction parallel to the axis  112 . Although the contact studs penetrate less deeply into the body than in the preceding embodiment, they still ensure the transmission of the forces from the plates to the body. 
     The torisonal flexibility of the prosthesis can be regulated in particular by modifying the thickness of the three contact studs  110  since thicker contact studs make the prosthesis more rigid in torsion about the axis  112 . Moreover, since there is no attachment between the body  116  and each of the plates  104 , the latter can slide to a certain extent on the body during torsion. The body  116  arranged between the opposed contact studs  110  is compressed and pushed radially inward during torsion on account of the overall movement of the contact studs. 
     The axial rigidity of the prosthesis can be regulated in particular in the area of the contact zone between the body  116  and each plate  104 . The inner face of each plate and the associated end face of the body can in fact be configured in such a way that the contact between these faces is established over a zone which becomes ever greater as the compression stress on the axis  112  increases. It will be possible, for example, to retain the plane shape of the end face of the body and give the inner face of the opposite plate a slightly spherical convex shape. The axial rigidity is in particular a function of the radius of the spherical face. 
     To ensure a long-term fixation between the plates and the body, the body is arranged to fit to a large extent between the contact studs of the plates. Since the plates are not rigidly fixed to the body production and regulation of the height of the prosthesis is facilitated before or during the intervention. In fact, in order to modify the height of the prosthesis itself, it suffices to modify the height of the body on the axis  112 , for example by changing the body. 
     When the prosthesis is subjected to torsion about its main axis, the zones of the body situated between two successive contact studs are compressed in a circumferential direction. Since there are three contact studs here on each plate, there are three zones of compression. The torsional rigidity thus depends on the shape of the contact studs, the distance between the contact studs, and the extent of the axial overlap between the contact studs. 
     Each of the plates and also the body are made separately by injection molding. 
     The prosthesis tolerates prolonged cyclic loading without modification of its shape. 
     In some known prostheses, the joint between the plates and the body is fragile and risks rupturing. The prosthesis according to the invention eliminates this risk since the plates are designed to be movable by sliding on the body. 
     The contact studs limit the extension of the body beyond the contour of the plates, particularly during axial compression. 
     The body can have a spatially variable modulus of elasticity. 
     The body can be made of a number of materials. 
     When a homogeneous elastomer is used for the body, or an elastomer having the same modulus of elasticity everywhere, the ratio of the axial compression rigidity to the torsional rigidity is generally too high. To reduce the axial rigidity without reducing the torsional rigidity too much, one of the two following alternatives can be applied. First, the center of the body can be made of an elastomer which is less rigid than that of the periphery of the body. This reduces the compression rigidity without too much affecting the torsional rigidity since the latter is governed mainly by the periphery of the body. Second, a cavity can be formed in the inner surface of the plates in contact with the body, in order to reduce the surface of contact of the plates with the body. This reduces the volume of elastomer which is compressed in the event of axial compression, thereby diminishing the axial rigidity, but once again without too much affecting the torsional rigidity. 
     Many modifications can of course be made to the invention without departing from the scope thereof. 
     The contact stud  10 , if there is only one per plate, can be at the center of the plate which bears it. 
     The contact stud or each contact stud  10 ,  110  can be in contact with the opposite plate  4 ,  104  when the prosthesis is not stressed or only when the compression of the prosthesis in the area of the contact stud exceeds a certain limit: the contact stud thus forms a stop limiting certain types of movements. 
     It is conceivable that only one of the plates  4 ,  104  will have one or more contact studs  10 , 110 . 
     The body  16 ,  116  can be made of a viscoelastic material such as silicone. 
     The contact studs will be able to have another shape, for example a cylindrical shape with an axis parallel to the axis  12 ,  112 . 
     The contact stud will be able to extend in the body within a seat which does not open out on a lateral face of the body. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.