Abstract:
An intervertebral disc prosthesis includes two opposing plates and a flexible seal extending between the two plates for forming a closed chamber therebetween. The disc prosthesis also includes a liquid provided in the chamber and a body disposed in the liquid and the chamber, the body having a plurality of pores at an outer surface thereof and cavities extending from the pores into the body. The liquid and the body are selected from materials so that said liquid does not normally wet to said cavities and compressive energy must be applied to the liquid for forcing the liquid into the cavities.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns intervertebral disc prostheses. 
     An intervertebral disc prosthesis intended to replace an intervertebral disc is known, for example, from the document EP-O,277,282. It comprises two plates which are able to bear against the vertebral plates of the vertebrae adjacent to the disc which is to be replaced, and a cushion which is interposed between the plates. The cushion comprises a compressible body and a chamber enclosing a liquid. This prosthesis has the disadvantage that the compressible body is susceptible to wear, which in the long term modifies the mechanical behavior of the prosthesis. Moreover, its friction against the plates causes erosion of particles which can migrate within the patient&#39;s body. 
     U.S. Pat. No. 6,052,992 to Eroshenko discloses a structure for selectively accumulating and dissipating energy. Referring to FIG. 1, the structure includes a porous capillary solid matrix  11  and a liquid  13  that surrounds the porous capillary matrix. The matrix is lyophobic relative to the liquid, i.e., the surface tension is such that the liquid is non-wetting when it comes into contact with the surface of the matrix. The liquid  13  surrounding the porous capillary matrix  11  is selected to define a solid/liquid separation surface having an area that varies isothermally and reversably as a function of the external pressure to which the structure is subjected. As such, the liquid can only be forced into the pores or capillaries when sufficient compression is exerted on the structure. In FIG. 3 a , the liquid  13  is present at the entrance to a capillary passage  12  formed in the porous capillary solid matrix  11 . The liquid is unable to penetrate into the passage  12 , however, forming a meniscus  15 . Referring to FIG. 3 b , when compressive forces are applied to the structure, the liquid is forced to penetrate into the capillary passage  12 . When the pressure applied to the structure is released, spontaneous expansion is obtained to enable a return from the state shown in FIG. 3 b  to the state shown in FIG. 3 a.    
     SUMMARY OF THE INVENTION 
     It is an object of the invention to make available a prosthesis whose mechanical behavior is identical over a long period of time, and which reduces the risks of particles being released in the patient&#39;s body. 
     With a view to achieving this object, the invention provides an intervertebral disc prosthesis comprising a compressible cushion having a body made of a material, and a liquid which is able to come into contact with the body, in which the liquid and the material are such that the liquid does not wet the body. 
     Given that the liquid does not wet the body, the forced contact of the liquid with the body requires a certain energy. This energy is restored in its entirety when the liquid is allowed to separate again from the body. A sort of spring effect is produced in this way. This effect is all the more appreciable as the surface of contact of the liquid with the body is great. The prosthesis can thus take up very substantial stresses, deforming under compression or flexion, and can then produce the inverse deformation by restoring the stored energy. Such a prosthesis can store a considerable quantity of energy in a small volume. In addition, the movement of the liquid in contact with the body does not generate any wear: the prosthesis is thus able to present the same mechanical behavior over a very long period of time. Moreover, the absence of friction between solid parts practically eliminates any risk of release of particles in the body. 
     The spring effect will be very great in the presence of a porous body, even one of very limited volume. 
     If the body has cavities communicating with each other along very long networks, the path to be traveled by the liquid upon entry and exit is particularly long. The result of this is that the liquid dissipates some of the energy on account solely of its circulation in the body. Consequently, the whole arrangement can constitute a partial damper. The combination of the spring effect and the damping effect thus means that for the prosthesis the curve representing the compression stress as a function of the variation in length of the prosthesis on its main axis has a hysteresis form, like a healthy natural intervertebral disc. The prosthesis thus comes very close to the mechanical behavior of a real disc. 
     The prosthesis advantageously comprises two plates which extend either side of the cushion, the body being fixed to at most one of the plates. 
     Other characteristics and advantages of the invention will become more apparent from the following description of a preferred embodiment which is given as a nonlimiting example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial section through a prosthesis according to the preferred embodiment of the invention; 
     FIG. 2 is the curve showing the variation in the compression force applied to the prosthesis as a function of the change in its height along its axis; and; 
     FIG. 3 is a perspective view of the prosthesis from FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 3, the prosthesis  2  comprises two plates  4  having a plane of a generally circular shape or preferably a bean shape with posterior hilum. The two plates are flat and extend parallel to one another, centered on a main axis  9  of the prosthesis perpendicular to the plates. 
     Each plate  4  has two lugs  6  which project from an outer face  8  of the plate  4  perpendicular to the plane of the plate. Each lug  6  has an orifice  10  passing right through it in the direction of the center of the plate and, on a face of the lug  6  remote from the plate  4 , a recess  12  of spherical shape. The orifices  10  can receive a bone screw  11  which has a head whose lower face has a male spherical shape interacting with the female recess  12  on the lug  6  in order to permit free orientation of the screw relative to the associate lug. In FIG. 1, the two lugs  6  of each plate have been brought into the same sectional plane, which is not in reality the case, as is shown in FIG.  3 . 
     For short-term anchoring of the disc prosthesis  2  in the column, the screws  11  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  8  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  8  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. 
     The prosthesis has a cushion  14  interposed between the plates. This cushion comprises a bellows  16 . It has a shape which is symmetrical in revolution about the axis  9 . Its wall profile comprises corrugations which make it possible to vary the length of the bellows  16  in the axial direction  9  without appreciably varying the surface area of its cross section transverse to the axis  9 . In this case the bellows, like the plates  4 , is made of titanium or titanium alloy so that it has a certain degree of axial strength and forms a compression spring. It can also be deformed in a direction perpendicular to the axis  9  or be subjected to torsion about the axis  9  or any axis perpendicular thereto. 
     At its two axial ends, the bellows  16  has edges which are bonded to respective edges of the plates  4  projecting for example, but in a manner not illustrated, from an inner face  18  of the plates. Said bonding is made leaktight so that the bellows  16  and the two plates  4  define a leaktight chamber of variable volume. 
     The bellows  16  has for example, in a manner not illustrated, ten convolutions, that is to say eight outer crests in addition to the two crests attached to the plates  4 . It has here an external diameter of about 30 mm and an internal diameter of about 17 mm. Its height, when the prosthesis is not loaded, is 10 mm. The wall of the bellows can be made using one, two or three sheets each measuring 0.1 mm in thickness and of which the sum of the thicknesses forms the thickness of the wall. The bellows here has an inherent strength of about 1.6 N/mm. 
     The chamber delimited by the bellows  16  and the two plates  4  is filled with a liquid  20 . The cushion  14  comprises a body  22  which is here anchored to the plane inner face  18  of one of the two plates, for example the lower plate  4  in FIG.  1 . For this purpose, a raised part  24  of the plate extends into the body. The shape of the body can be, for example, cylindrical about the axis  9 . The body will, for example, be sufficiently narrow perpendicular to the axis  9  so as never to come into contact with the bellows during the deformations of the prosthesis. Likewise, a sufficient free space will be formed between the free end, here the upper end, of the body  22  and the opposite plate so that they never come into mutual contact. The body  22  is surrounded by the liquid which is able to come into contact with it. 
     The liquid  20  and the material of the body  22  will preferably be biocompatible. They are chosen such that the liquid does not wet the body, that is to say that in the absence of stress for this purpose it tends to remain out of contact with the body. The liquid will be water, for example, and the material of the body will be a biocompatible material such as titanium or a titanium alloy. This material will preferably be porous in order to offer a large area of surface contact with the liquid. Moreover, the cavities of the body constituting its pores will be made to communicate with each other in fairly long networks in order to supply a damping effect in addition to the spring effect. In FIG. 1, the pores or cavities  26  of the body  22  have been illustrated diagrammatically for greater clarity. It goes without saying that these pores are invisible on the scale in FIG.  1  and infinitely more numerous than those illustrated. Thus, with the height h being measured from one plate to the other at the level of the axis  9 , the curve in FIG. 2 illustrating the variation in compression F of the prosthesis along its axis  9  as a function of its negative variation in height Δh will have a hysteresis form. Thus, the curve Ca indicating the change in force F when this increases is entirely above the curve Cd illustrating the decrease in force F. This form is due to the combination of the spring effect and the damper effect. 
     A pocket of gas could also be provided inside the chamber of liquid. 
     The prosthesis according to the invention is adapted in particular for the lumbar region of the spine. 
     Many modifications can of course be made to the invention without departing from the scope thereof. The body  22  can be free of any anchoring to the plates. It will thus be free to move spontaneously in relation to each of them. The prosthesis will then be able to be arranged in such a way that, in any position and in any state of stressing of the prosthesis, the body  22  is in contact with at most one of the plates. 
     The body  22  will be able to have an ellipsoid shape.