Abstract:
The invention relates to an intervertebral disc prosthesis having an upper plate, a lower plate having an essentially-flat support surface, and an intermediate element having a base which is equipped with a rim and which is topped with a spherical cap. The upper plate and the cap define a ball and socket joint therebetween. The base of the intermediate element comes into contact with a support surface of the lower plate. The lower plate is equipped with a guide having two edges which maintain the intermediate element therebetween. The rim of the base comes into contact with the edges, the rim being circular in shape and adapted to enable the intermediate element to roll along the length of the edges.

Description:
FIELD OF THE DISCLOSURE 
     The present invention relates to intervertebral disc prostheses. 
     More particularly, the invention relates to an intervertebral disc prosthesis comprising an upper plate, a lower plate having an essentially flat support surface and an intermediate element comprising a base having a rim and surmounted by a spherical cap, i.e. a volume forming a projection, the upper plate having an indentation complementary to the spherical cap and coming into contact with said cap so as to define a ball and socket joint between the upper plate and the intermediate element, the base of the intermediate element coming into contact with the support surface, the lower plate having a guide comprising two front and rear edges facing each other, maintaining between them an essentially constant distance and holding between them the intermediate element, the rim of the base coming into contact with at least one of the edges. 
     BACKGROUND OF THE DISCLOSURE 
     US 2004/0143332-A1 describes a prosthesis of the type described previously in which the intermediate element is moved along an arc-of-circle trajectory by sliding in a rail. 
     However the different elements constituting the prosthesis can be subjected to localized friction which can damage the optimal operation of the prosthesis in the long term. 
     DE 203 15 611 U relates to an intervertebral disc prosthesis, the cap of which is maintained centrally with respect to a recess and surrounded by shock absorbers. Only a so-called “anteroposterior” movement from front to rear is possible, without the possibility of movement by rolling of the intermediate element along the edges. The edges with which the intermediate element is in contact are the lateral edges of the prosthesis. 
     US-A-04/0002761 describes an intervertebral disc prosthesis comprising a concave element. Only an anteroposterior movement is possible and in particular not a movement by rolling of the intermediate element along the anterior and posterior edges of the prosthesis. 
     SUMMARY OF THE DISCLOSURE 
     The purpose of the present invention is in particular to propose a prosthesis having an improved service life compared with the prostheses of the prior art. 
     To this end, according to the invention, a disc prosthesis of the kind in question is characterized in that the rim is circular in form and is suited to allow the movement by rolling of the intermediate element along the edges. 
     By means of these arrangements, the prosthesis no longer has any localized points of wear, by replacing the contact by friction on the edges of the guide by rolling, which makes it possible to increase the duration of optimal use of the prosthesis. 
     In various embodiments of the prosthesis according to the invention, it is optionally possible also to use one and/or other of the following arrangements:
         the edges of the guide have a non-circular conic type shape, so as to guide the intermediate element along a conic-type trajectory;   the edges of the guide are parabolic in shape;   the intermediate element can be displaced, for each point on the trajectory, along an axis perpendicular to a tangent at a point of the trajectory, by a value comprised between 0.1 and 3 mm between the two edges of the guide;   the amplitude of movement of the intermediate element along the trajectory is 0.01 to 2 times the diameter of the base  22 ;   the guide is a recess made in the lower plate;   the base of the intermediate element has an outer peripheral circular flange comprising several slots, so as to absorb the shocks between the edges of the guide and the flange by deformation of said flange;   the flange is made of high molecular weight polyethylene material.       

     Other characteristics and advantages of the invention will become apparent on reading the following description of one of its embodiments, given as a non-limitative example, with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  represents a vertical cross-sectional view of the prosthesis according to the invention arranged between two vertebrae; 
         FIG. 2  represents an elevational view of part of a prosthesis according to a first embodiment variant; 
         FIG. 3  represents an elevational view of part of the prosthesis according to a second embodiment variant; 
         FIG. 4  represents an elevational view of the intermediate element according to a third embodiment variant. 
     
    
    
     In the different figures, the same references denote identical or similar elements. 
     DETAILED DESCRIPTION 
     The present invention relates to a disc prosthesis  10  which is intended to be arranged between two vertebrae of a vertebral column  12 ,  14 . For example  FIG. 1  illustrates a side view of two vertebrae  12 ,  14  of a vertebral column between which the prosthesis  10  according to the invention is arranged. This type of prosthesis  10  can be used to replace an intervertebral disc subject to degeneration resulting from trauma, diseases or age. 
     These kinds of degeneration can result in an alteration of the natural space between two vertebrae. The narrowing of this natural space can result in a pressure which is exerted on certain nerves, and consequently pain can occur. 
     The disc prosthesis according to the invention can therefore be used to maintain the natural space between two vertebrae. 
     It must also allow the vertebrae to be moved in relation to each other according to a natural movement. In particular, it must allow a movement of axial rotation which corresponds in part to the movement of rotation of the trunk or neck in the cervical region of the human body, an anteroposterior movement which corresponds to a movement of flexion or extension of the upper part of the body or the head in the cervical region, and a lateral movement which corresponds to an inclination of the upper part of the body, or the neck in the cervical region. 
     The disc prosthesis  10  according to the invention comprises an upper plate  16  and a lower plate  18  each having an external face  16   a ,  18   a  oriented towards an upper vertebra and a lower vertebra respectively. On each of the external faces  16   a ,  18   a , teeth are arranged, not shown, which allow the anchoring of the plates  16 ,  18  in the vertebrae. 
     The upper  16  and lower plates  18  also each comprise an internal surface  16   b ,  18   b . The internal surfaces extend laterally along a first anteroposterior axis X and extend longitudinally along a second axis Y perpendicular to the first axis, facing each other and they are in contact with an intermediate element  20 . 
     The intermediate element  20  has a circular base  22  having a circular rim  23 . The base  22  is surmounted by an upper spherical cap  24 . The base  22  is mounted in a mobile fashion on the internal surface  18   b  of the lower plate  18 , and the spherical cap  24  cooperates with an indentation  26  produced in the internal surface  16   b  of the upper plate  16 . This indentation  26  has a spherical profile, which makes it possible to obtain a ball and socket joint between the spherical cap  24  of the intermediate element  20  and the upper plate  16 . 
     The upper plate  16  is made of a material of the chromium-cobalt or titanium or stainless steel type and the intermediate element is made of a plastic material, of high density polyethylene type, having very good sliding characteristics. 
     The ball and socket joint between the upper plate  16  fixed to the upper vertebra  12  and the intermediate element  20  itself connected to the lower vertebra  14  by the lower plate  18  makes it possible to reproduce the movements of flexion or extension and the movements of inclination between two vertebrae  12 ,  14  of a vertebral column. 
       FIG. 3  represents an elevational view of the internal surface  18   b  of the lower plate  18  and of the intermediate element  20 . The internal face  18   b  of the lower plate  18  has a recess  28  which guides the movement of the intermediate element  20 . As represented in  FIG. 3 , the recess  28  is “kidney”-shaped, delimited by an essentially flat support surface  34 , a front edge  28   a , a rear edge  28   b  and two extreme edges  28   c ,  28   d , connecting the front edge  28   a  and the rear edge  28   b . The front edge  28   a  and the rear edge  28   b  are symmetrical with respect to the anteroposterior axis (X) and each essentially have a conic type shape, illustrated in  FIG. 3  by a parabolic shape open towards the rear along the anteroposterior axis. 
     It is also possible for the edges ( 28   a ,  28   b ,  28   c ,  28   d ) to form a succession of conics. 
     In  FIG. 3 , the anteroposterior axis X represented is the axis of symmetry of the parabolic trajectory. The parabolic trajectory is represented by the broken line which corresponds to the median line between the two edges  28   a ,  28   b.    
     This feature allows the intermediate element  20  to be moved according to a trajectory of conic type such as an arc of a circle, a portion of a parabola, a hyperbola or an ellipse, or of conic-succession type, such as several adjacent arcs of circles, which confers upon the intermediate element a movement closer to the real movement of rotation between two vertebrae. 
     In  FIG. 2 , the trajectory represented is an arc of a circle. 
     In  FIG. 3 , the trajectory represented is a parabola, the reduced Cartesian equation of which is of the type
 
y 2 =2px.
 
     Moreover, the amplitude of movement D along the conic-type trajectory, between two extreme edges, is of the order of 0.01 to 2 times the diameter of the base  22  of the intermediate element  20 , so as to respond to different pathological cases and allow the patient to recover the ability to achieve a movement close to the real movement. 
     Moreover, the recess  28  is made such that the intermediate element  20  can translate from front to rear in order to obtain a translation towards the rear of the intermediate element during a movement of flexion, and a translation towards the front of the intermediate element  20  during a movement of extension. 
     The amplitude A of these movements from the front towards the rear, along the trajectory, is advantageously comprised, depending on the pathological cases, between 0.1 and 3 mm, and preferably between 0.5 and 3 mm, for each point of the trajectory, along an axis perpendicular to a tangent at the point of the trajectory. 
     This feature of the recess  28  makes it possible to obtain a prosthesis capable of reproducing a natural intervertebral joint in particular during a combined movement of flexion and rotation. 
     When the base  22  of the intermediate element  20  is moved in the recess  28  produced on the internal surface  18   b  of the lower plate  18 , the rim  23  of the base  22  comes into contact with the edges  28   a ,  28   b  of the recess  28 . The movement of the intermediate element  20  is then obtained by rolling of the intermediate element  20  along the edges  28   a ,  28   b  delimiting the recess  28 , which avoids premature wear phenomena due to friction as during a contact by pure sliding. Sliding between the bottom of the base  22  and the support surface of the recess  28  can be obtained if the support surface of the recess  28  is treated in order to improve its sliding characteristics (for example a polymirror treatment, or nitride coating) and if the bottom of the base is made of high molecular weight polyethylene, in order to limit the friction during the movement of the intermediate element  20  and consequently premature wear. 
     For example if the edges of the recess have the shape of an arc of a circle, each point of the rim  23  of the base  22  will describe a cycloidal curve. 
     According to a variant of the invention represented in  FIG. 4 , the base of the intermediate element comprises an outer peripheral circular flange  30  which comprises several slots  32  passing through or not passing through, so as to absorb the shocks between the edges  28   a ,  28   b  of the recess  28  and the flange  30 . In fact, the flange  30  is made of a material of high molecular weight polyethylene type or any other bio-compatible material having good shock-absorbing characteristics, which with the presence of the slots  32  has a deformation capacity sufficient to absorb the contact when the intermediate element comes to a stop against the edges  28   a ,  28   b  of the recess  28 . This makes it possible to limit the joint pain experienced by the patient.