Abstract:
An intervertebral disc prosthesis comprising two opposing plates and a flexible seal extending between the two opposing plates for forming a closed chamber between the two plates, the flexible seal being adapted to enable the plates to move relative to one another. The disc prosthesis also includes a compressible body disposed in the closed chamber and located between the plates. The compressible body has a first surface in contact with a face of one of the plates, whereby an area of contact between the first surface of the compressible body and the face of the one of the plates increases when compressive forces are exerted upon the compressible body.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to intervertebral disc prostheses. The document EP-0 356 112 discloses an intervertebral disc prosthesis comprising two generally flat plates and an elastomeric body interposed between the plates and fixed to them by its flat end faces. The mechanical behavior of this prosthesis is quite similar to that of a natural healthy intervertebral disc, especially when the body is compressed between the two plates. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a disc prosthesis of a different type that more closely approximates to the mechanical properties of a natural healthy intervertebral disc. 
     To achieve this object, the invention provides an intervertebral disc prosthesis comprising two plates and a compressible body interposed between the plates, at least one end of which has an area of contact with one of the plates, in which prosthesis the body and the plate are such that the area of contact increases when a stress on the plate in the direction of the body is increased. 
     Thus, when the compressive stress on the body increases from a moderate value or zero, the height of the body, measured from one plate to the other, reduces relatively quickly. Subsequently, as the contact area increases, the reaction of the body becomes greater and a comparatively greater stress has to be applied to reduce the height by an equivalent amount. In other words, for the lowest compression values the mechanical reaction of the body during the compression varies very little as a function of the change of height. Consequently, the curve representing the applied stress as a function of the variation of height is inclined only a small amount from the horizontal for low compression values. At the beginning of the stroke, therefore, the load supplied is small. This property is also that of a natural healthy disc. The behavior of the body can therefore be adapted not only by the choice of material but also by the shape of the end face or faces with a contact area which varies so as to approximate as closely as possible to the mechanical properties of a natural healthy intervertebral disc. 
     Advantageously, the area of contact is defined by a face of the plate and a face of the end of the body, one of the faces of the plate or of the body, notably the face of the body, being curved and convex, and the other face being flat. 
     Advantageously, the area of contact is defined by a face of the plate and a face of the end of the body, both faces being curved in at least one common direction, and one being concave and the other convex, the concave face having at least one radius of curvature greater than a corresponding radius of curvature of the convex face. 
     This configuration therefore enables the variations of mechanical reaction to be introduced as indicated above. Furthermore, when the body is free to move sideways with respect to the plate, as will be seen later, this configuration ensures the relative centering of the two faces. For example, after the two faces have been displaced relative to each other, these curvatures enable them to recenter themselves automatically. 
     Advantageously, the body has at least one end in contact with one of the plates and free to move with respect to the plate in a direction parallel to the plate. 
     This arrangement therefore reduces the risk of excessive stress developing between the two vertebrae in a direction perpendicular to the longitudinal direction of the spine, that is to say in shear. 
     Advantageously, the body has an end housed in a depression in one of the plates that forms a lateral stop for this end. 
     Lateral displacements of the body relative to the plate can therefore be limited or even prevented. 
     Advantageously, the body comprises a viscoelastic material, notably silicone. 
     With such a material it is possible to give a hysteresis form to the curve representing the stress on the compressed body relative to its changing height. Since this curve is also that of a natural healthy disc, its mechanical properties are approximated to even more closely. 
     The prosthesis advantageously comprises a fluid interposed between the plates. 
     The addition of a fluid therefore increases the hysteresis form, especially when the fluid is compressible such as a gas or a mixture of a liquid and a gas which is partially soluble in the liquid. 
     Advantageously, the fluid is in contact with the plates. 
     Advantageously, the fluid is around the periphery of the body. 
     Advantageously, the prosthesis comprises an enclosure containing the fluid and constructed in such a way that it has a cross-sectional area parallel to the plates that is effectively invariable when a stress pushing the plates toward each other varies. 
     Advantageously, the prosthesis is intended for the lumbar region of the spine. 
     Other features and advantages of the invention will also appear in the following description of a preferred embodiment and two variants 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 view in axial section on plane II—II of the prosthesis shown in FIG. 1; 
     FIG. 3 is a view on a larger scale of a detail D indicated in FIG. 2; 
     FIG. 4 is a curve indicating the compressive force F exerted by the two plates on the cushion as a function of the variation of the distance between them; 
     FIG. 5 is a sectional view of a detail of a variant of the prosthesis; and 
     FIG. 6 is a simplified view similar to FIG. 2 showing a second variant. 
     FIG. 7 is a view on a larger scale of a detailed E indicated in FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     The intervertebral disc prosthesis  2  according to the invention is particularly intended in this case for the lumbar region of the vertebral column of the human body. It comprises two plates  4  in the general shape of a bean with the hilum on the posterior sides seen in plan view. Each plate  4  comprises a central circular dish  6  and a ring  8  extending around the periphery of the circular dish, in the plane of the circular dish. At rest, the two plates  4  lie parallel to each other at a distance facing each other so that their outlines coincide. On each plate  4 , the ring  8  and the circular dish  6  each have a groove  29  to house a seal  31 . 
     The disc prosthesis  2  comprises a cushion or intermediate part  10  interposed between the two plates  4 . The cushion comprises a compressible solid body  12 , in this case made of viscoelastic material, for example silicone. This body has a Shore A hardness of advantageously between  60  and  100 , in this case approximately  80 . The body  12  is a body of revolution about its minor axis  14 . It has a cylindrical lateral face  16  and two faces at the axial ends  18  which are generally perpendicular to the axis  14  and of slightly spherical convex shape. Each face  18  therefore has two identical curvatures in planes perpendicular to each other. The body  12  is arranged coaxially with the circular dishs  6 . Each circular dish  6  has a plane internal central face  20  perpendicular to the axis  14  and in contact with a respective axial end  18  of the body  12 . The convex spherical face  18  of the body therefore rests against the plane face  20  of the plate. The body  12  rests without mechanical connection against each of the plates  4  in such a way that it is able to move with respect to each of these plates in a direction parallel to the circular dishs, that is to say perpendicular to the minor axis  14 . In this way, lateral stresses are not transmitted from vertebra to vertebra. 
     The cushion  10  also includes a bellows  22 . The bellows surrounds the body  12  coaxially with it and at a distance from it. It is a symmetrical body of revolution about the axis  14 . Its wall in profile has corrugations  24  that enable the length of the bellows  22  to be varied along the axial direction  14 , without significantly varying the surface area of its cross section at right angles to the axis  14 . In the present case, the bellows, like the plates  4 , is made of titanium or titanium alloy, so that it has a certain axial rigidity and forms a compression spring. It can also be deformed in a direction perpendicular to the axis  14  or undergo torsion about the axis  14  or about any axis perpendicular thereto. 
     At both its axial ends, the bellows  22  has edges bonded to respective edges of the circular dishs  6  which project from the inside face  20 . The bond is made leaktight so that the bellows  22  defines with the two circular dishs  6  a sealed variable-volume enclosure extending around the body  12 . This enclosure contains a fluid, in this case a gas which in this case is air. The corrugations  24  closest to the body  12  are at a distance from it to allow free movement of gas from one to the other of the circular dishs  6 . 
     The bellows  22  in the present case has ten convolutions, i.e. eight external crests in addition to the two crests attached to the plates. It has here an outside diameter of about 30 mm and an inside diameter of about 17 mm. Its height, when the prosthesis is not under load, is 10 mm. The wall of the bellows can be made from one, two, or three sheets each 0.1 mm thick, the sum of the thicknesses forming the thickness of the wall. The stiffness of the bellows on its own here is approximately 1.6 N/mm. 
     Each annulus  8  comprises two lugs  25  projecting from an outside face of the plate  4  at right angles to the plane of the plate. Each lug  25  contains an orifice  27  passing all the way through it and directed toward the center of the circular dish and, on a face of the lug  25  turned away from the plate  4 , a spherical impression. The orifices  27  are to accommodate a bone screw  26  with a head  28  whose underside has a male spherical shape engaging with the female impression of the lug  25  so as to allow free orientation of the screw  26  with respect to the associated lug. 
     For short-term anchoring of the disc prosthesis  2  in the column, the screws  26  can be anchored in the spondyl of the vertebrae adjacent to the disc to be replaced. 
     However, a so-called long-term anchoring can be provided in which, in addition, the surfaces of the plates  4  in contact with the adjacent vertebrae are covered with hydroxyapatite, or any other substance known per se capable of stimulating bone growth. Before covering them, the said surfaces can be treated to obtain a more or less porous surface condition with anchor points for the bone tissue, in order to create a better interface with said bone tissue. 
     Shown in FIG. 4 is the curve C indicating the intensity of a compressive load F applied to the cushion  10  (that is to both plates  4 ), taking no account of their deformability (which is practically zero), in the axial direction  14 , as a function of the variation of the length 1 of the cushion in the axial direction  14  (or of 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. Furthermore, it exhibits hysteresis: the curve Ca indicating the increase in the compression F from the origin zero is separate from the curve Cd indicating the decrease in the compression F down to the origin, and is at all points above it. This pronounced hysteresis form is due principally to the viscoelastic material of the body and to a lesser extent to the combination in the cushion  10  of the body  12  and of the fluid. 
     Moreover, the curve Ca, relating to the increase in the compressive force F, exhibits beginning at the origin O a portion Ca 1  of shallow gradient followed by a portion Ca 2  of steeper gradient. The curve Cd illustrating the decrease in the compression F exhibits for the highest values of the force F a portion Cd 1  of steep gradient, followed by a portion Cd 2  of shallower gradient for the lowest values of the force F. The presence of a portion of shallow gradient in the vicinity of the origin in the case of the curves Ca and Cd is due principally to the conforming of the contacting faces  18 ,  20  of the body  12  and of the plates  4 , which has the effect that the area of mutual contact between each plate and the body, which is generally in the form of a disc, increases with the force F. This increase continues until the maximum surface of the contact area is reached, when the whole of the face  18  is touching the plate  4 . 
     The junction points Ja and Jd form the junctions between the curves Ca 1  and Ca 2 , and Cd 1  and Cd 2 , respectively. In the curve Ca, the point Ja corresponds to the load F at which the maximum contact areas have been reached between the plates and the body. Likewise in the curve Cd, the point Jd corresponds to the load at which these areas cease to be maximum. 
     The prosthesis can be configured in such a way that the point Ja corresponds to a value Δ 1  situated between 25% and 75% of the maximum variation of length envisioned for the prosthesis in use. 
     Referring to FIG. 5, it is possible in a variant (which in other respects has the other characteristics of the prosthesis shown in FIG.  1 ), for the face  20  of each plate  4  facing the body  12  to have a depression  32 , U-shaped in this case, forming a lateral stop, in which the corresponding axial end  18  of the body sits. Any sideways relative displacements of the body  12  with respect to each plate  4  are thus kept within a certain range, or even prevented altogether. 
     In the variant shown in FIG. 6, the face  20  of the plate may be curved and concave in one or two directions, as is the case here, and the face  18  can be curved and convex in the corresponding direction or directions, the radius of curvature of the face  20  being, for each direction, greater than that of the face  18  in the corresponding direction. Both faces  18 ,  20  here are spherical. The radii of curvature of the surfaces  18  and  20  will be for example between 70 and 80 mm, and 140 and 200 mm, respectively. Such an arrangement offers selfcentering of the two faces while permitting sideways relative displacement of the body  12  with respect to the plate in any direction perpendicular to a longitudinal direction of the spine. 
     In the embodiment shown in FIG. 2, both ends of the body  12  have a surface  18  in contact with the associated plate, the area of which is variable and which makes it able to move sideways with respect to the body. 
     By contrast, in the variant shown in FIG. 6, only one of the ends  18  of the body  12  has this property. The other end, which is the lower end in FIG. 6, has a plane circular shape whose area of contact with the associated plate is invariable and fixed with respect thereto. 
     FIG. 7 shows circular dish  6  having a first groove  29 A and ring  8  having a second groove  29 B. A seal  31  is disposed in the grooves  29 A,  29 B for providing a seal between the opposing edges of circular dish  6  and ring  8 . 
     It is of course possible to make numerous modifications to the invention without departing from its scope. 
     The fluid may be a liquid, or even a mixture of a liquid and a gas, the latter being for example slightly soluble in the liquid. 
     The body may be elliptical in cross section through the axis  14 . 
     The inside faces  20  of the plates  4  may be convex, the axial end face  18  of the body  12  being flat, or concave with a greater radius of curvature than the radius of the face  20  of the plate. The two contacting faces of the plate and of the body may be convex. 
     The curvature of the faces may be limited to a single plane. 
     The characteristics relating to the envelope  22  (spring, distance from the body  12 ) may be used independently of the other characteristics.