FR2734148A1 - Spinal intervertebral disc replacement prosthesis - Google Patents

Abstract

The prosthesis consists of a pair of plates (1,2) with a space (3) of predetermined width between them housing an elastic connection such as a spring (5). The spring is formed as a spiral, whose stiffness increases exponentially in relation to a reduction of the distance (e) separating the two plates. The spring is pseudo-helical in configuration, with a frusto-conical shape about a longitudinal axis (7) perpendicular to the two plates. The wide end (8) of the spring is secured to the first plate, and the narrow end (9) to the second plate. The spring may be non-circular, e.g. elliptical, in plan view cross-section.

Description

Intervertebral prosthesis
The present invention relates to an intervertebral prosthesis intended to be implanted between two consecutive vertebrae of the spine of a human being or an animal.

 It is known that the spine of a human being or an animal comprises a plurality of vertebrae arranged one on the other, or one next to the other, being separated by intervertebral discs whose particular structure allows the spine to have some flexibility. Schematically, these discs comprise a core which essentially constitutes the point of rotation of one vertebra with respect to another and a fibrous sheath surrounding the core to absorb movements between two vertebrae.

 In general, the discs perform their function perfectly when they are in good condition. But they can be damaged, for example following an accident or trauma, or in the context of certain diseases. These deteriorations lead to often very intense suffering, and even disabilities.

 There are many methods which provide relief to patients. These procedures range from simple surgical intervention to eliminate the defect which appeared on the disc and which is the cause of the suffering, until the replacement, by a prosthesis, of the damaged disc.

 Many structures have already been produced for this type of prosthesis, but those which have been produced to date are not entirely satisfactory since they are relatively complex and difficult to install. Their total cost of manufacture and installation is therefore very high, which does not allow them to be accessible to the greatest possible number of people who need it.

 The object of the present invention is therefore to produce an intervertebral prosthesis which is of a very simple structure, inexpensive, giving it a short total cost considerably reduced compared to that of intervertebral prostheses known to date.

More specifically, the subject of the present invention is an intervertebral prosthesis comprising
two plates, which may be substantially circular or elliptical, delimiting a space, said plates being opposite one another at a determined distance, and
elastic means arranged in said space and mounted in cooperation with said plates to keep them substantially parallel to one another,
characterized by the fact that the elastic means consist of a spring whose stiffness increases substantially exponentially when said distance decreases when said plates move towards one another.

 According to another characteristic of the present invention, said spring consists of a pseudo-helical spring having a generally substantially frustoconical shape, which can be substantially of revolution, or as a variant with substantially elliptical bases, around an axis substantially perpendicular to said plates, one end of said pseudo-helical spring defining the large base of its frustoconical shape being secured to the first plate and its other end defining the small base of its frustoconical shape being secured to the second plate.

 According to another characteristic of the present invention, the pseudo-helical spring is constituted by a winding of a strip of a material having a certain elasticity, the cross section of said strip being of substantially rectangular shape and substantially identical over the entire length of said strip, the long sides of said rectangular section being substantially parallel to said axis.

 According to another characteristic of the present invention, the prosthesis further comprises means for limiting the compression of the pseudo-helical spring during the approaching of said plates and means for limiting the tightening of said pseudo-helical spring under the action of a rotation of one said plates relative to each other around a direction substantially parallel to said axis.

 Advantageously, the prosthesis includes means for limiting the tightening of said pseudo-helical spring under the action of a rotation of one of said plates relative to the other around a direction substantially parallel to said axis.

 In another aspect of the invention, the prosthesis further comprises elastic-viscous damping means mounted in cooperation with said plates. The elastic viscous damping means consist of a block of elastic viscous material interposed between said plates and secured to at least one of said plates. The prosthesis comprises a rod integral with one of said plates, said rod being at least partially embedded in said block of elastic-viscous material. The rod may have a substantially circular or substantially elliptical cross section. In all cases, it is centered on the axis and is substantially perpendicular to said plates. As a variant, the rod forms an angle with respect to axes perpendicular to said plates.

 Preferably, the first part includes a second through hole, said block of elastic-viscous material being at least partially in contact with the wall of this second hole. The rod is integral with the second plate and it is located in a part of the volumes delimited by said first and second through holes.

 In another aspect of the invention, the prosthesis includes a sleeve connecting the edges of said plates to close said space. The sleeve is preferably a bellows.

 The plates can be substantially parallel, or even form an acute angle between them, preferably between 5 and 10 degrees, when the prosthesis is at rest.

Other characteristics and advantages of the present invention will appear during the following description of several embodiments given with reference to the drawings annexed by way of illustration, but in no way limitative, in which
- Figure 1 shows a schematic sectional view in a sagittal plane of a first embodiment of an intervertebral prosthesis according to the invention
- Figure 2 shows a diagram of the stiffness of elastic means as a function of the distance between two plates in the intervertebral prosthesis according to the invention;
- Figure 3 is a perspective view with parts removed from a second embodiment of the intervertebral prosthesis according to the invention; and
- Figure 4 is a schematic sectional view in an anteroposterior plane of a third embodiment of an intervertebral prosthesis according to the invention.

 FIG. 1 represents a schematic sectional view of a first embodiment of an intervertebral prosthesis making it possible to solve certain problems like those which have been mentioned in the preamble of the present description.

 The intervertebral prosthesis has a general shape of a small cylinder of revolution around an axis of symmetry 7.

 More particularly, the intervertebral prosthesis comprises two plates 1 and 2 which are in the general form of two plates in the form of a circular pancake or the like made of a rigid and solid material, for example titanium or the like. These two plates delimit a space 3 and are arranged opposite one another at a determined distance "e" which is the intervertebral distance defined between two consecutive vertebrae of a spine. Practitioners who use such prostheses know the possible values of this distance "e" and it is therefore not necessary to specify them here, all the more since they vary according to the morphology of each individual.

 Between the two plates 1 and 2, in the space 3 defined above, are arranged elastic means 4 which are mounted in cooperation with the two plates to keep them substantially parallel to each other, but still leaving them a certain freedom. of rotation around the axis 7 and around axes perpendicular to the axis 7. When the prosthesis is inserted between two consecutive vertebrae, the two plates are substantially horizontal and respectively integral with the two consecutive vertebrae, the axis 7 is substantially vertical and the trays follow the relative movements of these two vertebrae when the spine folds normally.

 According to an important characteristic of the present invention, these elastic means are constituted by a spring 5 whose stiffness "E" increases substantially exponentially when the distance "e" between the two plates decreases, that is to say during a bringing together of the two plates 1 and 2 towards one another. For example, the distance "e" can vary from 7 to 10 mm.

 FIG. 2 represents a diagram of the stiffness "E" of the spring 5 as a function of the distance "e" separating the two plates. On this diagram, it can be seen that the stiffness "E" of the spring increases very rapidly when the distance "e" decreases, that is to say when the two plates tend to approach. In other words, the stiffness "E" of the spring 5 increases from a minimum stiffness "Em" to a maximum stiffness "" when the distance "e" decreases from a maximum distance "eM" to a minimum distance "em ". The stiffness "E" reaches very important values "EM" to avoid that, even under high loads, the two plates do not get too close to each other. This characteristic is mainly intended to avoid accidents which could happen if the spring did not play its role of shock absorber according to the function defined above.

 In an advantageous embodiment as illustrated in FIG. 1, this spring 5 is constituted by a pseudo-helical spring 6, also known to technicians under the name "conical volute spring". The turns of the spring 6 form an envelope of a generally substantially frustoconical shape, substantially of revolution around the axis 7 substantially perpendicular to the two plates 1 and 2 when these are in normal position called "rest", that is ie when they are parallel to each other at a normal distance "eM" from each other. One end 8 of the pseudo-helical spring defines the large base of the frustoconical shape and is secured to the plate 1 which will be designated below by the first plate. The other end 9 of the pseudo-helical spring 6 defines the small base of the frustoconical shape and is secured to the other plate 2 which will be referred to below as a second plate.

 Advantageously, to give a certain transverse rigidity to the pseudo-helical spring 6, that is to say in a direction perpendicular to the axis 7 defined above, the spring 6 is constituted by a winding of a strip 10 of a material having a certain elasticity, for example titanium, steel or the like. The cross section of this strip is advantageously substantially rectangular and identical over the entire length of the strip, the long sides 11 of this rectangular section being substantially parallel to the axis 7.

 In an advantageous embodiment, to give even more security to the operation of the prosthesis and in particular to avoid irreversible accidents when the pseudo-helical spring 6 undergoes very high compression, the prosthesis includes means 15 for limiting the compression of the pseudo-helical spring 6 during approximation of the two plates 1 and 2, and thus prevent them from approaching beyond the minimum tolerable distance for the organism.

 Similarly, it is known that two consecutive vertebrae can undergo rotations around the axis of the spinal column, that is to say substantially around the axis 7 defined above, of course within given limits. Consequently, since the two plates 1 and 2 are intended to be respectively secured to two consecutive vertebrae, the prosthesis also includes means 16 for limiting the tightening of the pseudo-helical spring 6 under the action of a rotation of one of the two plates 1, 2 relative to each other around a direction substantially parallel to the axis 7, that is to say under the action of a rotation during which the spring tends to tighten.

 In a preferred embodiment, the two means 15 and 16 defined above are constituted by a first part 20 substantially in the form of a frustoconical cone, the external frustoconical surface 21 of which comprises a first notch 22 in the form of substantially a first dihedron substantially straight whose corner 23 follows a line of pseudo-helical shape substantially identical to that of the pseudo-helical spring 6. The large base of the first part 20 is mounted integral with the first plate 1, for example by embedding and welding in the first plate 1, so that on the one hand that the first part 20 is partially surrounded externally by the pseudo-helical spring 6, and on the other hand that at least the ends of the turns of the pseudo-helical spring 6 opposite the first plate 1 are located in the first notch pseudo-helical 22.

 The role of the first part 20 is as follows.

When the frusto-conical pseudo-helical spring 6 is subjected to a depression, it is known that it is first of all the turns of larger section which begin to nest into each other. To limit the compression of this spring 6, the first notch 22 is produced and positioned in the first part 20 so that the edge of the strip 10 formed by the short side of the rectangular section of the turn of larger diameter facing the first plate 1 comes to abut, in the turn of larger diameter of the notch 22, against the plane of the first dihedral which is substantially perpendicular to the axis 7. The notch 22 is also advantageously designed so that, as and as the turns of the pseudo-helical spring 6 fit into each other, the small inner side of the spring band 10 comes to abut, in the turn corresponding to it in the notch 22, against the plane of this first dihedral which is parallel to axis 7.

 This first part 20 thus limits the tightening of the pseudo-helical spring 6 during a rotation of one of the two plates 1, 2 relative to the other around a direction parallel to the axis 7, in the direction where the diameters of spring coils tend to decrease. Indeed, during such a rotation, the inner edge of the spring strip 10 corresponding to the long side 11 of the rectangular section which is closest to the axis 7 abuts against the plane of the first dihedron which is substantially parallel to the axis 7, which prevents this edge from approaching the axis 7 beyond a certain limit. Therefore, this part of the notch 22 limits the tightening of the spring 6 and consequently limits the rotation of the two plates 1 and 2 relative to each other in the direction which produces this tightening.

 The limitation of the compression and the tightening of the spring 6 is preferably progressive and increases rapidly when the distance "e" between the plates 1 and 2 decreases, in correspondence with the diagram of FIG. 2.

Thus for a pseudo-helical spring of the "conical volute spring" type having regularly spaced turns and of constant height at rest, the height of the pseudo-helical notch 22 taken parallel to the axis 7 and the width of the notch 22 taken perpendicularly to axis 7 gradually decrease, from the large turn to the small turn of the notch, that is to say from the first plate 1 to the second plate 2 according to FIG. 1.

By choosing the heights of the turns of the notch 22, but also their widths, the surgeon will adapt the prosthesis to the surrounding physiological constraints which differ depending on whether the prosthesis is for example introduced between two lumbar vertebrae or two dorsal vertebrae.

 In addition, it is also necessary to limit the rotation of the two plates 1 and 2 with respect to each other in the opposite direction to the previous one, that is to say in the direction which produces the loosening of the pseudo-helical spring 6.

 The prosthesis therefore comprises means 30 for limiting the loosening of the pseudo-helical spring 6 under the action of a rotation of one of the two plates 1 and 2, relative to the other around a direction substantially parallel to the axis 7.

 The means 30 are constituted by a second part 31 comprising a first through-hole 32 in the form of a substantially frustoconical cone, the frustoconical surface 34 of which comprises a second notch 33 in the form of substantially a second dihedral, the corner 36 of which is substantially complementary to that 23 of the first notch 22 follows a line of pseudo-helical shape substantially identical to that of the pseudo-helical spring 6. The end of the second part 31 containing the small base of the first substantially frustoconical through-hole 32 is mounted integral with the second plate 2, by example by embedding and welding in the second plate 2, so, on the one hand that the second part 31 externally partially surrounds the pseudo-helical spring 6, and on the other hand that at least the ends of the turns of the pseudo-helical spring 6 opposite of the second plate 2 are located in the second pseudo-helical notch 33.

 Under these conditions, when the plates 1 and 2 tend to pivot relative to each other about an axis substantially parallel to the axis 7 and in the direction in which the spring 6 tends to loosen, the outer edge of the strip 10 corresponding to the long side 11 of the rectangular section which is furthest from the axis 7 abuts against the plane 37 of the second dihedral which is parallel to the axis 7. The loosening of the spring 6 and therefore the rotation of the two plates defined above are thus limited.

 Advantageously, in addition to the means described and defined above, the prosthesis comprises elastic-viscous damping means 40 mounted in cooperation with the two plates 1 and 2. In a preferred embodiment, these damping means elastic-viscous 40 are constituted by a hollow block 41 of elastic-viscous material, for example polyurethane or the like, interposed between the two plates 1 and 2 and integral with these.

 The prosthesis then further comprises a rod 42 integral with one of the two plates 1 and 2, for example the second plate 2 according to the embodiment illustrated in FIG. 1. The rod 42 is typically in the form of a mooring bite and protrudes perpendicularly from the plate 2 in the direction of the plate 1. The rod 42 is provided with a head 43 and is centered on the axis 7. The rod 42 is embedded and preferably glued at least partially in the hollow block 41 of elastic material— viscous. With such an embodiment, the block 41 provides additional damping of the forces parallel to the axis 7, in particular when the two plates move towards each other, but also, by the presence of the rod 42 , the damping of a rotation of one of the two plates relative to the other around a direction perpendicular to the axis 7. The rod 42 has a tendency, by its connection with the block 41, to return the two trays to their initial position which is where they are t substantially parallel to each other as shown in Figure 1.

 To improve the damping effect of the block 41, the first part 20 has a through breakthrough 50 hereinafter called "second breakthrough" and the block 41 of elastic-viscous material is arranged so that it is at least partially in contact with the wall of this second breakthrough 50, as shown in FIG. 1. This configuration is advantageous in that it retains in block 41 all of its damping effect by preventing it from tending, in its deformation when subjected to constraints, to compensate for its crushing by an increase in width, the latter being prevented by the wall of the second breakthrough 50. In the embodiment illustrated in FIG. 1, the rod 42 is integral with the second plate 2 and it is located in a part of the volumes delimited by the first and second through holes 32 and 50. The large base of the block 41 is preferably bonded in a central counterbore to the internal face of the plate 1.

 To prevent fibers or other elements living in the human body from interfering with the operation, in particular of the spring 6 and of the block 41, the prosthesis comprises a compressible sleeve 60 hermetically connecting the edges of the plates 1 and 2 in order to close the space 3 and make it waterproof. In an advantageous embodiment, this sleeve 60 is constituted by a bellows whose wall has, as illustrated, corrugations.

 The first embodiment of intervertebral prosthesis according to the invention is the easiest to manufacture, and therefore the least expensive, due to its axial symmetry around the axis 7. In addition, its placement by the surgeon in the body of the patient is also simplified thanks to the axial symmetry since its damping properties are identical in all directions radial to the axis 7. This first embodiment of prosthesis is suitable for a large number of pathologies.

 The first and second plates 1 and 2 can indifferently constitute lower and upper parts, or else upper and lower parts, between two consecutive vertebrae, respectively.

 With reference to FIG. 3, an intervertebral prosthesis according to a second embodiment of the invention has the general shape of a small cylinder whose directing curve is an ellipse.

 The intervertebral prosthesis comprises two plates 1a and 2a, the planar faces of which are substantially elliptical. The elliptical shape of the trays allows the vertebrae to sit on the prosthesis better suited to the average physiology of the patients. As an indication, the minor axis PA of the ellipse is between approximately 22 and 30 mm and the major axis GA between approximately 33 and 40 mm. A spring 6a is mounted between the two plates la and 2a. The spring 6a is pseudo-helical. The turns of the spring 6a form an envelope in the general shape of a cone with an elliptical base.

 The end 8a of the spring defining the large base of the frustoconical shape is secured to the lower plate 2a. The other end 9a of the spring defining the small base of the frustoconical shape is secured to the upper plate 1a. Thus, the spring 6a is reversed with respect to the spring 6 in FIG. 1.

 The prosthesis according to the second embodiment comprises means 15a and 16a similar to the means 15 and 16 of the first embodiment to limit the compression and the tightening of the spring 6a. These means 15a and 16a are constituted by a first part 20a substantially in the form of a truncated cone with an elliptical base. The outer frustoconical surface 21a of the first part 20a comprises a first notch 22a substantially in the form of a first dihedron whose corner 23a follows a line of elliptical pseudo-helical shape substantially identical to that of the spring 6a. The large base of the first part 20a is integral with the lower plate 2a. The first part 20a is surrounded by the spring 6a.

 The function of the first part 20a is similar to that of the part 20 according to the first embodiment. The first part 20a limits the compression and the tightening of the spring 6a.

 The prosthesis according to the second embodiment comprises means 30a for limiting the loosening of the spring 6 when one of the plates 1a and 2a rotates relative to the other substantially around the axis 7.

 The means 30a are constituted by a second part 3la comprising a first through-hole 32a substantially in the form of a truncated cone whose frustoconical surface comprises a second notch 33a substantially in the form of a second dihedral whose corner 36a substantially complementary to that 23a of the first notch 22a follows a line of pseudo-helical shape substantially identical to that of the pseudo-helical spring 6a. The end of the second part 3la containing the small base of the first substantially frustoconical through-hole 32a is mounted integral with the first plate 1a, on the one hand that the second part 31a partially externally surrounds the pseudo-helical spring 6a, and on the other hand that at least the ends of the turns of the pseudo-helical spring 6a facing the second plate 2a are located in the second pseudo-helical notch 33a.

 The prosthesis comprises a rod 42a secured to the upper plate 1a and projecting perpendicular to the plate 1a in the direction of the plate 2a. the rod 42a comprises a head 43a and is centered on the axis 7a. The rod 42a has a substantially elliptical cross section corresponding to the elliptical shape of the spring 6a.

 An elastic-viscous block 4la is interposed between the rod 42a, the internal face of the plate 2a and a central frustoconical through-hole 50a formed in the first part 20a.

 The ellipses of the faces of the plates 1a and 2a, the cross sections of the frustoconical spring 6a, of the frustoconical rod 42a and of the first and second frustoconical parts 20a and 3la are all oriented so that their minor axes are in the anteroposterior plane.

 As a variant, only the small axes of the faces of the plates 1a and 2a are in the anteroposterior plane. The minor axes of the elliptical cross sections of the spring, of the rod and of the first and second parts are not in the same plane as that containing the minor axes of the faces of the plates, and are for example in the sagittal plane orthogonal to the anterolateral plane. posterior.

 According to another variant, the faces of the plates are elliptical, and the spring, the rod and the first and second parts have circular sections transverse to the axis of symmetry.

 According to yet another variant, the faces of the plates are circular, and the spring, the rod and the first and second parts have elliptical cross sections.

 The elliptical, or oval, shape of the trays allows a good adaptation of the prosthesis to the vertebrae between which it is inserted.

 The elliptical, or oval, shape of the spring and of the rod, and consequently of the first and second parts, modifies the reaction of the prosthesis to torsion and to compression in the radial direction relative to the axis 7a.

 The elliptical shape of the plates and the elliptical shape of the spring and the rod are independent of each other and meet the specific needs of the patient.

 The prosthesis according to the second embodiment may include a compressible sleeve for hermetically connecting the edge of the plates 1a and 2a.

 Referring to Figure 4, a third embodiment of intervertebral prosthesis according to the invention differs from the first embodiment in that the outer faces of the plates 1b and 2b are not substantially parallel but form an acute angle between them at rest A preferably between 5 and 100 approximately.

 A rod 42b projects from the internal face of the plate 2b in the direction of the internal face of the plate lb. A longitudinal axis 7b of the rod 42b forms an angle A / 2 with the perpendicular to the internal face of the plate 2b. A block 41b cooperates with the rod 42b.

 The intervertebral prosthesis comprises a spring 6b similar to spring 6, with the difference that its large and small bases form the angle A between them.

 The prosthesis comprises first and second parts 20b and 31b similar to parts 20 and 31 respectively, except that they are shaped to adapt to the angle A.

 Similarly, a compressible sleeve 60b hermetically connects the plates 1b and 2b.

 The third prosthesis embodiment is more particularly intended to be implanted in parts of the spine where a natural curvature thereof must be respected. The angle A can correspond to a lordosis angle or a kyphosis angle.

 The prosthesis according to the invention is implanted like the prostheses of the prior art either by the anterior route, or by the posterolateral route. The operation of the prosthesis will not be described more fully here, since it can easily be deduced from the explanations given during the present description above. I1 will just be specified that the outer face of each plate 1, 2 which is capable of being secured to a vertebra is possibly covered with all products, known in themselves, which promote the joining of the bone of the vertebrae with the metallic material. or the like constituting the plates, in particular by osteosynthesis.

Claims (25)

 1 - Intervertebral prosthesis comprising  - two plates (1, 2) delimiting a space (3), said plates being opposite one another at a determined distance, and  - elastic means (4) arranged in said space and mounted in cooperation with said plates to keep them substantially parallel to each other,  characterized in that the elastic means consist of a spring (5) whose stiffness (E) increases substantially exponentially when said distance (e) decreases when said plates (1, 2) are brought together towards the other.  2 - Prosthesis according to claim 1, characterized in that the spring (5) is constituted by a pseudo-helical spring (6; 6a) having a general shape substantially frustoconical around an axis (7; 7a) substantially perpendicular to said plates ( 1, 2; la, 2a), one end (8 8a) of said pseudo-helical spring defining the large base of its frustoconical shape being secured to one of said plates (1; 2a) and its other end (9; 9a) defining the small base of its frustoconical shape being secured to a second of said plates (2; la).  3 - Prosthesis according to claim 2, characterized in that said pseudo-helical spring (6; 6a) is constituted by a winding of a strip (10) of a material having a given elasticity, the cross section of said strip being of substantially rectangular and substantially identical in shape over the entire length of said strip, the long sides (11) of said rectangular section being substantially parallel to said axis (7).  4 - Prosthesis according to claim 2 or 3, characterized in that the pseudo-helical spring (6) is substantially of revolution around the axis (7).  5 - Prosthesis according to claim 2 or 3, characterized in that the pseudo-helical spring (6a) is substantially elliptical.  6 - Prosthesis according to any one of claims 2 to 5, characterized in that it comprises means (15; 15a) for limiting the compression of said pseudo-helical spring (6; 6a) during the approximation of said plates (1, 2 la, 2a).  7 - Prosthesis according to any one of claims 2 to 6, characterized in that it comprises means (16; 16a) for limiting the tightening of said pseudo-helical spring (6 6a) under the action of a rotation of the 'one of said plates (1, 2; la, 2a) relative to the other around a direction substantially parallel to said axis (7 7a).  8 - Prosthesis according to claims 6 and 7, characterized in that the means (15; 15a) for limiting the compression of said pseuàohélicoïtlal spring (6; 6a) and the means (16; 16a) for limiting the tightening of said spring (6 ; 6a) under the action of a rotation of one of the two plates relative to the other about a direction substantially parallel to the axis (7; 7a) are constituted by a first part (20; 20a ) substantially in the form of a frustoconical cone, the outer frustoconical surface (21; 21a) of which has a first notch (22; 22a) substantially in the shape of a first dihedral, the corner of which (23; 23a) follows a line of pseudo-helical shape substantially identical to that of said pseudo-helical spring (6; 6a), the large base of said first part being mounted integral with said first plate (1; 2a) so, on the one hand that said first part is partially surrounded externally by said pseudo-helical spring , and on the other hand that at least the ends of the turns of said pseudo-helical spring are located in said first notch (22; 22a).  9 - Prosthesis according to any one of claims 2 to 8, characterized in that it comprises means (30; 30a) for limiting the loosening of said pseudo-helical spring (6 6a) under the action of a rotation of the 'one of said plates (1, 2; la, 2a) relative to the other around a direction substantially parallel to said axis (7 7a).  10 - Prosthesis according to claim 9, characterized in that the means (30; 30a) for limiting the loosening of said spring under the action of a rotation of one of said plates relative to the other around a direction substantially parallel to the axis (7; 7a) are constituted by a second part (31 3 31a) comprising a first through-hole (32; 32a) substantially in the form of a truncated cone whose frustoconical surface (34) comprises a second notch (33; 33a) substantially in the form of a second dihedral, the corner (36; 36a) of which follows a line of pseudo-helical shape substantially identical to that of said pseudo-helical spring (6; 6a), the end of said second part ( 31; 31a) containing the small base of the first substantially frustoconical through-hole being mounted integral with said second plate (2; la) so that, on the one hand, said second part partially externally surrounds said pseudo-helical spring ( 6; 6a), and on the other hand that at least the ends of the turns of said pseudo-helical spring are located in said second notch (33; 33a).  11 - Prosthesis according to any one of claims 1 to 10, characterized in that it further comprises elastic-viscous damping means (40) mounted in cooperation with said plates (1, 2).  12 - Prosthesis according to claim 11, characterized in that the elastic-viscous damping means consist of a block of elastic-viscous material (41; 41a) interposed between said plates (1, 2; la, 2a) and secured to at least one of said plates (1, 2; la, 2a).  13 - Prosthesis according to claim 12, characterized in that it comprises a rod (42; 42a) integral with one of said plates (1, 2 la, 2a), said rod being at least partially embedded in said block of elastic-viscous material (41; 41a).  14 - Prosthesis according to claim 13, characterized in that said rod (42, 42b) has a substantially circular cross section.  15 - Prosthesis according to claim 13, characterized in that said rod (42a) has a substantially elliptical cross section.  16 - Prosthesis according to any one of claims 13 to 15, characterized in that said rod (42, 42a) is centered on the axis (7) and is substantially perpendicular to said plates (1, 2; la, 2a) .  17 - Prosthesis according to any one of claims 13 to 15, characterized in that said rod (42b) is centered on the axis (7b) and forms an angle (A / 2) relative to axes perpendicular to said plates (lb, 2b).  18 - Prosthesis according to claim 8 and any one of claims 12 to 17, characterized in that the first part (20; 20a) comprises a second through hole (50; 50a), said block of elastic-viscous material ( 41; 41a) being at least partially in contact with the wall of this second breakthrough (50; 50a).  19 - Prosthesis according to claims 10, 13 and 18, characterized in that said rod (42 42a) is integral with the second plate (2; la) and that it is located in part of the volumes delimited by said first and second through holes (32, 50; 32a, 50a).  20 - Prosthesis according to any one of claims 1 to 19, characterized in that it comprises a sleeve (60) connecting the edges of said plates (1, 2) to close said space (3).  21 - Prosthesis according to claim 20, characterized in that said sleeve (60) is constituted by a bellows.  22 - Prosthesis according to any one of claims 1 to 21, characterized in that said plates (1, 2) are substantially circular.  23 - Prosthesis according to any one of claims 1 to 21, characterized in that said plates (la, 2a) are substantially elliptical.  24 - Prosthesis according to any one of claims 1 to 23, characterized in that said plates (1, 2; la, 2a) are substantially parallel.  25 - Prosthesis according to any one of claims 1 to 23, characterized in that said plates (lb, 2b) form between them an acute angle- (A), preferably between about 5 and 10 degrees.