Abstract:
An intervertebral disc 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 ends is freely displaceable relative to the plate in a parallel direction. Thus, the prosthesis imitates and approximates the mechanical properties of a healthy natural intervertebral disc.

Description:
FIELD OF THE INVENTION 
     The invention relates to intervertebral disk prostheses. 
     European Patent No. 277 282-Al discloses an intervertebral disk prosthesis, comprising two plates and a cushion interposed between them. The cushion comprises a compressible body delimiting a cavity filled with an incompressible fluid. This prosthesis is essentially incompressible in the axial direction and allows only a relative inclination of the plates. This behavior is different from that of a healthy, natural intervertebral disk. 
     An object of the invention is to provide a disk prosthesis which more closely imitates and approximates the mechanical properties of a healthy, natural intervertebral disk. 
     SUMMARY OF THE INVENTION 
     With a view to achieving this object, according to the invention an intervertebral disk prosthesis is envisaged comprising two plates and a cushion interposed between the plates, the cushion comprising a compressible body and containing a fluid, which is compressible. 
     Thus the compression of the cushion affects the compression of the body and the fluid. Since the compression properties of the body and the fluid can be different, their combination allows a very close approximation to the mechanical properties of a healthy, natural intervertebral disk. More particularly, when the body is made of suitable material, the curve of the mechanical reaction to a compression of the cushion as a function of a variation in a dimension of the cushion in the direction of compression can be obtained, having a hysteresis shape close to that associated with a healthy, natural disk. 
     Advantageously, the fluid has a pressure such that it is more compressible than the body. 
     This difference can thus be utilized to approximate as closely as possible the mechanical properties of the healthy, natural disk. 
     Advantageously, the fluid comprises a gas. 
     Advantageously, the cushion is arranged such that a fluid pressure is applied directly to the plates. 
     Advantageously, the fluid extends around the periphery of the body. 
     Advantageously, the body comprises a viscoelastic material, preferably silicone. 
     The aforementioned curve, thus has a highly pronounced hysteresis shape can thus be obtained. 
     Advantageously, the body is in contact with the plates. 
     Advantageously, the body has at least one end having a contact zone with one of the plates, the prosthesis being arranged such that the contact zone has a surface area which increases whenever a stressing of the plate in the direction of the body is increased. 
     For the lowest compression values, the mechanical reaction of the prosthesis upon the compression of the body thus varies very little as a function of the dimensional change in the cushion in the direction of compression. In other words, the aforementioned curve is little inclined relative to the horizontal for low compression values and little force is provided in the initial operation. This property reproduces that of a healthy, natural disk. 
     Advantageously, the contact zone is defined by a face of the plate and an end face of the body, one of the two faces, typically the face of the body, being curved and convex and the other face being flat. 
     Advantageously, the contact zone is defined by a face of the plate and an end face of the body, the two faces being curved in at least one common direction and being respectively concave and convex, the concave face having at least one radius of curvature greater than a corresponding radius of curvature of the convex face. 
     As a result of this configuration, the variations in mechanical reaction, previously mentioned, can be effected. Moreover, when the body is free to shift laterally relative to the plate, as will be seen later, this configuration guarantees the relative centering of the two faces. For example, after the two faces have been mutually offset, these curvatures enable them to re-center automatically. 
     Advantageously, the body has at least one end in contact with one of the plates, this end being free to move relative to the plate in a direction parallel to the plate. 
     Advantageously, the end is accommodated in a recess of the plate and forms a lateral abutment for the body. 
     The lateral displacements of the body relative to the plates can thus be limited, or even barred. 
     Advantageously, the cushion comprises a shell containing the fluid and arranged such that it has a cross-sectional area parallel to the plates which is essentially invariable when variation occurs in a compression of the cushion between the plates. 
     Advantageously, the cushion comprises a chamber containing the fluid and extending around the periphery of the body at a distance. 
     The erosion of the body by the chamber in the course of its movement and the dispersion of particles of the body are thus prevented. 
     Advantageously, the chamber forms a spring, especially a compression spring. 
     The chamber thus influences the reaction of the cushion whenever this is compressed. 
     Advantageously, the cushion is arranged to exhibit a hysteresis-shaped curve of mechanical reaction to a compression as a function of a variation in a dimension of the cushion in the direction of the compression. 
     Advantageously, the cushion is arranged such that the reaction to the compression grows less markedly for relatively low reaction values than for relatively high reaction values. 
     Advantageously, the cushion is arranged such that the reaction to the compression diminishes more markedly for relatively high reaction values than for relatively low reaction values. 
     Advantageously, the cushion is arranged such that the reaction to the compression has higher values when it grows than when it diminishes. 
     Advantageously, the prosthesis is intended for the lumbar region of the spine. 
     Other characteristics and advantages of the invention are yet to appear in the following description of the preferred embodiments, given by way of non-limiting examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prosthesis according to the invention; 
     FIG. 2 is a axial section along the plane II—II of the prosthesis of FIG. 1; 
     FIG. 3 is an enlarged scale view of a detail D of FIG. 2; 
     FIG. 4 is a curve indicating the compression force F applied by th o plates to the cushion as a function of the variation in the distance separating them; 
     FIG. 5 is a sectional view of a detail of an illustrative embodiment of the prosthesis; and 
     FIG. 6 is a simplified view analogous to FIG. 2 showing a second illustrative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an intervertebral disk prosthesis  2  according to the invention particularly intended for the lumbar region of the vertebral column of the human body. The prosthesis  2  comprises two flat plates  4  having the general shape of a bean with a posterior hilum in plan view. Each plate  4  comprises a central circular panel  6  and a border  8  extending about the periphery of the panel  6  in the plane thereof. At rest, the two plates  4  extend parallel to each other, at a distance facing each other with their contours in alignment. On each plate  4 , the border  8  and the panel  6  each have a groove  17  for the reception of a seal. 
     The disk prosthesis  2  comprises a cushion or intermediate part  10  interposed between the two plates  4 . The cushion comprises a compressible solid body  12 , here made of viscoelastic material, for example silicone. This body has a Shore-A hardness advantageously 60 to 100, in this case approximately 80. The body  12  has a shape of revolution about its main axis  14 . It has a cylindrical lateral face  16  and two axial end faces  18  generally perpendicular to the axis  14  and of slightly convex spherical shape. Each face  18  thus has two identical curvatures in mutually perpendicular planes. The body  12  is disposed coaxially with the panels  6 . Each panel  6  has a plane inner central face  20  perpendicular to the axis  14  and in contact with one of the respective axial ends  18  of the body  12 . Thus, the convex spherical face  18  of the body rests on the plane face  20  of the plate. The body  12  rests without anchorage on each of the plates  4  such that it is movable relative to each of these plates in a direction parallel to the plates, that is to say perpendicular to the main axis  14 . Given the compression of the body  12  exerted by the plates  4 , and the form of the faces of the plates and the body, the mobility in this direction is manifested by a rolling movement, optionally without sliding, of each axial end  18  of the body on the face  20  of the plate with which it is in contact. The body thus rolls between the two plates. The two plates are thus laterally displaced relative to each other while remaining parallel, if necessary. The transmission of lateral stresses from the one to the other of the vertebrae is thus prevented. 
     The cushion  10  additionally comprises a bellows  22 . The bellows coaxially surrounds the body  14  at a distance therefrom. The bellows  22  has a symmetrical shape in revolution about the axis  14 . Its wall profile comprises corrugations  24  allowing the length of the bellows  22  to be varied in the axial direction  14  without any appreciable variation in the surface area of its cross section transverse to the axis  14 . The bellows  22 , like the plates  4 , may be made of titanium or titanium alloy, so that it has a certain axial rigidity and forms a compression spring. The bellows can also be deformed in a direction perpendicular to the axis  14  or can be twisted about the axis  14  or about any axis perpendicular thereto. 
     At its two axial ends, the bellows  22  has edges bonded to respective edges of the panels  6  projecting from the inner face  20 . The bonding is leaktight so that the bellows  22  and the two panels  6  define a variable-volume, leaktight chamber extending around the body  12 . This chamber contains a fluid, for example a gas, such as air. The undulations  24  nearest to the body  12  extend at a distance from the latter to allow a free circulation of gas from the one to the other of the dishes  6 . 
     As shown, the bellows  22  has ten convolutions, with eight outer crests in addition to two crests for securement to the plates. The outer diameter is about 30 mm and the inner diameter is about 17 mm. Its height, when the prosthesis is not bonded, measures about 10 mm. The wall of the bellows can be produced by means of one, two or three sheets, each measuring about 0.1 mm thick. The sum of the thicknesses of the sheets forms the thickness of the wall. The bellows here has an inherent strength of around 1.6 N/mm. 
     Each border  8  comprises two lugs  25  projecting from an outer face of the plate  4  perpendicularly to the plane of the plate. Each lug  25  has an orifice  27  traversing through it in the direction of the center of the plane and, a spherical recess directed away from the plate  4  on one face of the lug  25 . The orifices  27  are able to receive a bone screw  26  having a head  28 , whose lower face has a male spherical shape cooperating with the female recess of the lug  25  to allow free orientation of the screw  26  relative to the associated lug. 
     For short-term anchoring of the disc prosthesis  2  in the spine, the screws  26  can be anchored in the spondylus of the vertebrae adjacent to the disk to be replaced. 
     A “long term” anchorage might be envisaged, in which the surfaces of the plates  4  in contact with the adjacent vertebrae are covered with hydroxyapatite, or with any other substance known per se for stimulating bone growth. Prior to being covered, the surfaces can be treated to obtain a more or less porous surface condition, with anchoring points for the bone tissue, in order to ensure a better interface with the bone tissue. 
     FIG. 4 shows the path of the curve C, indicating the intensity of a compression force F exerted on the cushion  10  (that is to say on the two plates  4 ), disregarding their deformability, which is virtually nil, in the axial direction  14 , as a function of the variation in length  1  of the cushion in the axial direction  14  (or in the distance between the two plates). This curve also represents the mechanical reaction R of the cushion  10  under the same conditions. 
     This curve C is not linear. Moreover, it has a hysteresis form: the curve Ca indicating the increase in the compression F from the zero origin being distinct from the curve Cd indicating the decrease in the compression F up to the origin, and extending entirely above it. This pronounced hysteresis form is due principally to the viscoelastic material of the body and secondarily to the combination of the body  12  and the fluid in the cushion  10 . 
     In addition, the curve Ca, relating to the increase in the compression force F, exhibits a gently sloping portion Ca 1  from the origin O, then a more heavily sloping portion Ca 2 . The curve Cd illustrating the decrease in the compression F exhibits for the highest values of the force F a markedly sloping portion Cd 1 , then for the lowest values of the force F a more gently sloping portion Cd 2 . The presence of a gently sloping portion in the vicinity of the origin for the curves Ca and Cd is due principally to the configuration of the contact faces  18 ,  20  of the body  12  and of the plates  4 , the effect of which is to increase the surface area of the mutual contact zone between each plate and the body, generally in the form of a disk, whenever the force F is increased. This increase occurs until the maximum surface area of the contact zone is reached, when the whole of the face  18  is touching the plate  4 . 
     The connecting points Ja and Jd respectively form the junction between the curves Ca 1  and Ca 2  and Cd 1  and Cd 2 . On the curve Ca, the point Ja corresponds to the force F at which the maximum contact surfaces between the plates and the body are reached. Likewise, on the curve Cd, the point Jd corresponds to the force at which these surfaces cease to be at a maximum. 
     The prosthesis can be configured such that the point Ja corresponds to a value of Δl between 25% and 75% of the maximum variation in length envisaged for the prosthesis during use. 
     Referring to FIG. 5, in an alternate embodiment, (otherwise having the other characteristics of the prosthesis of FIG. 1) the face  20  of each plate  4  opposite the body  12  has a recess  32 , in this case, a U-shaped recess, forming a lateral abatement, in which the corresponding axial end  18  of the body fits. The relative lateral displacements of the body  12  with respect to each plate  4  are thus limited to a certain range, or even totally barred. 
     In the alternate embodiment shown in FIG. 6, the face  20  can be curved and concave in one or both directions, as shown, and the face  18  can be curved and convex in the corresponding direction(s), the radius of curvature of the face  20  being, for each direction, greater than that of the face  18  in the corresponding direction. The two faces  18 ,  20  are spherical as shown. The radii of curvature of the surfaces  18  and  20  will, for example, be within the range of about 70 to about 80 mm, and between about 140 to about 200 mm respectively. Such an arrangement allows the two faces to be centered automatically, while at the same time permitting a relative lateral displacement of the body  12  relative to the plate in any direction whatsoever perpendicular to a longitudinal direction of the spine. 
     In the embodiment shown in FIG. 2, the two ends of the body  12  have a contact surface  18  with the associated plate of variable surface area, making it laterally movable relative to the body. 
     By contrast, in the alternate embodiment shown in FIG. 6, only one of the ends  18  of the body  12  exhibits this property. The other end, being the lower end in FIG. 6, has a plane circular shape with an invariable contact zone with the associated plate and fixed relative to the latter. 
     Of course, numerous modifications might be made to the invention without departing from the scope thereof. 
     The fluid might be a liquid, or even a mixture of a liquid and a gas, the latter being, for example, weakly soluble in the liquid. 
     The body might have an elliptical shape in cross section to the axis  14 . 
     The inner face  20  of the plates  4  might be convex, the axial end face  18  of the body  12  being flat, or concave with a greater radius of curvature than that of the face  20  of the plate. The two contacting faces of the plate and the body might be convex. 
     The curvature of the faces might be limited to a single plane. 
     The characteristics relating to the envelope  22  (spring effect, distance to the body  12 ) might be effected independently of the other characteristics. 
     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.