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:
BACKGROUND OF THE INVENTION 
     The invention concerns intervertebral disc prostheses. 
     European Patent No. 0356112 discloses an intervertebral disc prosthesis comprising two plates and a cushion of compressible material interposed between and fixed to them. Such a prosthesis permits substantial mobility between the two adjacent vertebrae. It can in particular be subjected to movements of compression between the two vertebrae, and movements of torsion on three mutually perpendicular axes. Its mechanical behavior thus comes close to that of a healthy natural intervertebral disc. However, it has the particular disadvantage that the lateral stresses of each vertebra on the disc in a direction perpendicular to the longitudinal direction of the spine are transmitted in their entirety to the other vertebra, which is not the case in a real healthy disc. These shearing stresses are detrimental. 
     It is an object of the invention to make a disc prosthesis which more closely imitates and approximates the mechanical properties of a healthy natural intervertebral disc. 
     SUMMARY OF THE INVENTION 
     With a view to achieving this object, the invention provides an intervertebral disc prosthesis comprising two plates and a cushion interposed between the plates, the cushion comprising a compressible body which has ends in contact with the plates, where at least one of the ends is freely displaceable relative to the associated plate in a direction parallel to the plate. 
     By this means it is possible to avoid excessive lateral stresses between the two vertebrae in a direction perpendicular to the longitudinal direction of the spine. The behavior of a healthy natural disc is thus more faithfully reproduced. 
     The end or each end of the body is advantageously lodged in a recess which is provided in one of the plates and which can form a lateral abutment for this end. 
     It is thus possible to limit the relative lateral displacements between the body and the plate or plates. 
     The end or each end advantageously has a face having a zone of contact with a face of the associated plate, said faces being arranged in such a way that the zone of contact has a surface area which increases when a stressing of the plate in the direction of the body increases. 
     Thus, for the lowest compression values, the mechanical reaction of the prosthesis upon compression of the body varies very little as a function of the change in dimension of the body in the direction of compression. To put it another way, the curve of the mechanical reaction of the prosthesis upon compression as a function of the variation in height of the cushion is barely inclined relative to the horizontal for low values of compression, and little force is applied at the start. This property reproduces that of a healthy natural disc. 
     The end or each end advantageously has a face in contact with a face of the associated plate, the two faces being curved in at least one common direction and being respectively concave and convex. 
     Thus, after a relative lateral displacement of the body and of the plate, these two faces ensure automatic centering of these elements, repositioning them in a coaxial arrangement. When the concave face has at least one radius of curvature greater than a corresponding radius of curvature of the convex face, this additionally provides a zone of contact having a variable surface area, as mentioned above. 
     The body advantageously comprises a viscoelastic material, in particular silicone. 
     Thus, during compression, this material behaves such that the aforementioned curve has a hysteresis form, which brings the behavior of the prosthesis even closer to that of the healthy natural disc. 
     The prosthesis advantageously comprises a fluid interposed between the plates. 
     The fluid, particularly when it is compressible, further accentuates the hysteresis form. 
     The fluid is advantageously in contact with the plates. 
     The fluid advantageously extends about the periphery of the body. 
     The prosthesis advantageously comprises a chamber enclosing the fluid and arranged in such a way that it has a surface area of transverse section parallel to the plates which is substantially invariable when a stress moving the plates toward one another varies. 
     The prosthesis is advantageously intended for the lumbar region of the spine. 
     Other characteristics and advantages of the invention will become more apparent from the following description of the preferred embodiments given as nonlimiting examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prosthesis according to the invention; 
     FIG. 2 shows the prosthesis from FIG. 1 in axial section on the plane II—II; 
     FIG. 3 is a larger-scale view of a detail D from FIG. 2; 
     FIG. 4 is a curve indicating the compression force F exerted by the two plates on the cushion as a function of the variation in the distance separating them; 
     FIG. 5 is a sectional view of a detail of an alternative embodiment of the prosthesis; and 
     FIG. 6 is a simplified view analogous to FIG. 2, showing a second alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an intervertebral disc 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 receiving a seal. 
     The disc prosthesis  2  comprises a cushion or intermediate part  10  interposed between the two plates  4 . The cushion comprises a solid compressible body  12 , here made of viscoelastic material, for example silicone. This body has a Shore-A hardness advantageously between about 60 and 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  that it is movable relative to each of these plates in a direction parallel to the plates, or 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  12  thus rolls between the two plates. The two plates are thus displaced laterally relative to each other while remaining parallel, if necessary. This therefore avoids transmission of lateral stresses from one vertebra to the other. 
     The cushion  10  additionally comprises a bellows  22 . The bellows  22  coaxially surrounds the body  12  at a distance therefrom. The bellows  22  has a shape symmetrical in revolution about the axis  14 . Its wall profile comprises corrugations  24  which allow the length of the bellows  22  to be varied in the axial direction  14  without any appreciable variation to 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 be twisted about the axis  14  or 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 encloses a fluid, for example a gas such as air. The corrugations  24  nearest the body  12  extend at a distance from it in order to permit free circulation of the gas from one panel  6  to the other. 
     As shown, the bellows  22  has ten convolutions, with eight outer crests in addition to the two crests attached 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 loaded, measures about 10 mm. The wall of the bellows can be produced using one, two or three sheets each measuring about 0.1 mm in thickness. The sum of the thicknesses of the sheets forms the thicknesses of the wall. The bellows here has an inherent strength of about 1.6 N/mm. 
     Each border  8  includes 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  transversing through it in the direction of the center of the panel 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  so as 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 disc to be replaced. 
     A “long-term” anchorage may also be envisaged in which the surfaces 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. 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 said 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 the length l 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 starting from the zero origin being distinct from the curve Cd indicating the decrease in 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 compression F has, for the highest values of the force F, a portion Cd 1  with a pronounced slope and then, for the lowest values of the force F, a portion Cd 2  with less slope. The presence of a portion with slight slope near the origin for 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 , which means that the surface area of the zone of mutual contact between each plate and the body, generally of disc shape, increases when the force F increases. This increase takes place until the maximum surface area of the contact zone is reached, when the whole face  18  is touching the plate  4 . 
     The connection 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 their maximum. 
     The prosthesis can be configured such that the point Ja corresponds to a value Δ1 between 25% and 75% of the maximum variation in length envisaged for the prosthesis during use. 
     Referring to FIG. 5, in an alternative embodiment (otherwise having the other characteristics of the prosthesis in 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 abutment 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 alternative 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 greater, for each direction, 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 and 80 mm and between about 140 and 200 mm respectively. Such an arrangement makes the two faces to be centered automatically while at the same time permitting a relative lateral displacement of the body  12  with respect to the plate in any direction perpendicular to a longitudinal direction of the spine. 
     In the embodiment 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 alternative 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  19  with the associated plate and fixed relative to the latter. 
     Of course, many modifications can be made to the invention without departing from the scope thereof. 
     The fluid of the leaktight chamber can be a liquid, or even a mixture of a liquid and of a gas, the latter for example being weakly soluble in the liquid. 
     The body  12  can have an elliptic shape in cross section transverse to the axis  14 . 
     The inner face  20  of the plates  4  can be convex, the axial end face  18  of the body  12  being plane, 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 of the body can be convex. The curvature of the faces can be limited to a single plane. The characteristics relating to the bellows  22  (spring effect, distance from body  12 ) can be used 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.