FR2730159A1 - Intervertebral disc prosthesis for reduced stress and natural movement - Google Patents

Abstract

The prosthesis comprises: (a) a lower plate (1) with a low friction zone (1b) on upper surface (1a), designed to interact with a corresponding low-friction surface (3b) on the underside of a lens-shaped spacer (3). The upper surface (3a) of the spacer is also low-friction and interacts with a similar surface (2b) on the underside of an upper plate (2). The low-friction surfaces of the spacer are convex in shape, while those on the two plates are concave.

Description

PROSTHESIS FOR INTERVERTEBRAL DISC
The present invention relates to a prosthesis for intervertebral discs intended to be substituted for the fibro-cartilaginous discs ensuring the connection between the vertebrae of the spine, in case of lumbosciatica
Low back pain is an infection that affects the central pivot of the intervertebral lingiculation formed by the intervertebral disc. This disc ensures both the stability and the spatial coherence of all the movements of the vertebrae between them. Its bankruptcy, which can result in compaction, deformation, displacement, or even simple wear, causes well-known lumbosciatric phenomena, which cause the patient intense pain and a certain discomfort.

 A therapeutic route consists in removing the defective disc and replacing it with a prosthesis generally consisting of a ball joint, of deformable but incompressible material serving as a spacer between two plates with corresponding spherical imprint, of non-deformable and incompressible material, anchored respectively in the lower, upper face, upper and lower vertebrae, constituting the deficient joint.

A first type of intervertebral disc prosthesis is disclosed in document DE-30 23 353. This prosthesis consists of three elements
- a lower plate comprising an upper cavity provided with a front centering shoulder,
- an upper plate identical to the lower plate and arranged symmetrically with respect thereto,
- a spacer hinged between the two cavities of the plates and taking the overall shape of a biconvex lens.

 This shape of the spacer, if it takes into account the shape of the organic disc, does not ensure the same articulatory functions. Indeed, the organic disc consists of a fibrous ring, filled by an ovoid mass, the nucleus pulposus, whose nature is mainly liquid since it contains about 80% of water. Thus, a relative inclination movement of the two vertebrae will cause a displacement of the liquid from the convexity towards the concavity of the movement, which will result in a reduction in the thickness of the disc towards the convexity of the movement, and an increase in the thickness of this same disc towards the concavity of the movement. Nevertheless, the average spacing of the centers of rotation of the adjacent lower and upper vertebrae will remain generally constant during the movement, due to the conservation of the liquid inside the disc.

This transfer of liquid makes it possible to respectively provide each of the adjacent vertebrae with a rotational movement about itself around a horizontal axis passing through its center of gravity, in opposite directions from one another. In the case of the prosthesis of the document DE3.023.353, the tilting of the plates on the surface of the biconvex lens will cause a relative rotation of said plates, and consequently of the adjacent vertebrae in opposite direction to one another, no longer around d horizontal axes passing through the centers of gravity of said vertebrae but around a common instantaneous axis situated towards the convexity of the movement, on the periphery of the biconvex lens and in the plane of the latter. It is understandable that this leads to an overall separation trays and therefore adjacent vertebrae; however, constant spacing between these plates is essential to solve the problems of impact of the prosthetic vertebral plates which are encountered regularly, in the long term, in existing prostheses. Indeed, if this spacing is not constant, the studs provided on the external surface of the plates and intended to be implanted in each of the adjacent vertebrae, will, during the flexing, escape from their imprints on the side of the concavity of the movement, while they remain well supported in their imprints, on the side of the convexity of the movement. This causes a gradual shift of the imprints when the sag is accompanied by a certain rotation.

 Another type of prosthesis making it possible to maintain a constant overall spacing of the vertebrae is disclosed in French patent FR-2,559,226. This document describes a prosthesis composed of two plates provided with anchoring fins and separated by a spacer in the form of a spherical cap with a cylindrical base articulated in the corresponding spherical cap of the upper plate. This assembly, which constitutes a ball joint nevertheless has an important drawback: it allows only a pure rotational movement, unlike the organic intervertebral disc which allows it a rotational-translational movement which, instead of a single center of rotation , has a multitude of centers of instantaneous rotation, constituting a geometrical place.

 The present invention aims in particular to overcome these drawbacks by providing an intervertebral disc prosthesis consisting of a lower plate having on its upper face a sliding surface on which is articulated a complementary sliding surface constituting the lower face of a spacer. generally in the form of a lens, the upper face of which has a sliding surface on which a complementary sliding surface provided on the lower face of an upper plate is articulated, a prosthesis characterized in that the sliding surfaces of the upper faces of the plate lower and the spacer are convex and the sliding surfaces of the lower faces of the spacer and the upper plate are concave, or vice versa.

 A prosthesis according to the invention therefore makes it possible, thanks to the combination of two sliding surfaces whose convexities are oriented in the same direction, to recreate a joint as close as possible to natural biomechanics, that is to say providing a multitude of centers of instantaneous rotation constituting a geometrical place while ensuring a constant spacing between the vertebrae, and this, whatever their relative position. In addition, the geometry of the prosthesis according to the invention brings, compared to a pure ball joint, a reduction in stresses by increasing the sliding surfaces.

An embodiment of the present invention will be described below, by way of non-limiting example, with reference to the attached drawing in which
FIG. 1 is an exploded perspective view of the prosthesis according to the invention,
FIG. 2 is a view in vertical longitudinal section of the upper plate of the prosthesis,
FIG. 3 is a view in vertical longitudinal section of the spacer,
FIG. 4 is a view in vertical longitudinal section of the lower plate,
FIG. 5 is a sectional view of the prosthesis according to the invention, assembled and in the rest position,
- Figure 6 is a sectional view of the prosthesis according to the invention, mounted and deflected.

 The prosthesis according to the invention is shown in exploded view in FIG. 1. It consists of a lower plate 1 on which an upper plate 2 is articulated by means of a lens-shaped spacer 3, one face of which is convex and the other concave. According to a preferred variant of the invention, the plates have the same general shape of ellipse, the short side of the ellipse measuring for example 28 millimeters, the long side 45 millimeters and the thickness of the plates 2 millimeters. According to the same variant, the spacer has a cylindrical peripheral surface with a radius measuring for example 25 millimeters.

 The lower plate 1, shown in FIG. 4, has on its upper face the a convex sliding surface 1b. On this sliding surface lb is articulated a sliding surface 3b provided on the underside of the spacer 3, shown in Figure 3. Said sliding surface 3b is complementary to the sliding surface ld. It is therefore a concave surface having the same radius of curvature as that of the sliding surface 1b.

 The upper face of the spacer 3 has a convex sliding surface 3a on which is articulated a complementary sliding surface 2b provided on the lower face 2a of the upper plate 2, shown in FIG. 2. Said sliding surface 2b has therefore the same radius of curvature as that of the sliding surface 3a.

 According to a preferred variant, the convex sliding surface 1b constitutes the bottom of a recess lc provided on the upper face 1a of the lower plate 1.

This recess lc is in the form of a cylindrical well centered on the ellipse constituting the lower plate 1 and whose radius measures for example 28 millimeters. Since the convex sliding surface 1b is larger than the concave sliding surface 3b of the underside of the spacer 3, said spacer 3 can slide by its sliding surface 3b on said sliding surface 1b constituting the bottom of the recess lc of the lower plate 1. This sliding movement of the spacer 3 on the upper face la of the lower plate 1 is limited by the edge ld that the recess lc provides at the periphery of the sliding surface lb.

 This border ld may be continuous over the entire periphery of the sliding surface ld or simply over a part of this periphery as shown in FIG. 1. It is sufficient for the interruption of this border ld to be less than the diameter of the periphery of the sliding surface 3b of the spacer 3 in order to prevent the latter from being able to leave, by sliding, the recess lc.

 The concave sliding surface 2b provided on the lower face 2a of the upper plate 2 is located, according to a preferred variant, at the lower end of a cylinder 2c which extends downward and perpendicular to the lower face 2a of the plate upper 2. This cylinder is centered on the ellipse formed by the upper plate 2; it has a radius of 28 millimeters and a height of 4 millimeters.

 This double sliding interface between on the one hand the sliding surface ld and the sliding surface 3b and, on the other hand, the sliding surface 2b and the sliding surface 3a provides a mechanical joint very close to the biomechanical joint. natural. Indeed, instead of having a unique and constant center of rotation over time, as would be the case with a ball joint, we have a multitude of instantaneous centers of rotation. The double sliding ensures that, at all times, the articulation of the upper plate 2 on the lower plate 1 can be considered as a ball joint whose center moves over time. We are therefore no longer dealing with a single center of rotation but with a geometrical place of centers of rotation. This corresponds well to the reality of natural intervertebral articulation. Indeed, as has already been said previously, the organic intervertebral disc is spongy in nature, and therefore behaves substantially like a deformable but incompressible fluid. The reduction in volume which takes place in the convexity of the inclination movement of one vertebra with respect to the other is fully reflected in the form of an increase in volume at the other end, towards the concavity of the movement. The effect of this transfer is to give each of the vertebrae a rotational movement in itself, in the opposite direction from each other. In the case of a simple rotation which is considered to be performed in a vertical plane for a standing man, each vertebra will be rotated around an axis perpendicular to this plane, that is to say horizontal and passing substantially at the center of gravity of said vertebra, in opposite direction for each of the vertebrae. This double rotation, in opposite directions, around two axes linked to the vertebrae and fixed over time, can also be interpreted as a rotation of the two vertebrae, in opposite directions from one another, around a single axis mobile over time, therefore, ultimately like a rotation around a multitude of instantaneous axes parallel to each other and located in the convexity of the movement. This interpretation, limited, for reasons of clarity, to rotational movements in a plane around axes perpendicular to this plane, of course extends to rotations in space, around points.

 Thus, although the vertebrae approach each other by one of their end, and move away from their other end, the distance which separates their respective own axis of rotation remains constant. It is this peculiarity that makes it possible to recreate the prosthesis according to the invention and which is highlighted in FIGS. 5 and 6. In fact, when the upper 2 and lower 1 plates approach at one of their ends, the spacer 3 is gradually driven towards the other end, towards the concavity of the movement, thus mechanically carrying out the transfer of material which, in the real joint, is produced in the form of a transfer of fluid. The spacer 3 having been driven towards the concavity of the movement, the upper plates 2 and lower 1 have approached the side of the convexity of the movement, but the spacing d between the two respective axes of own rotation Ol of the lower plate 1 and 2 of the upper plate 2, has remained constant between the rest position, represented in FIG. 5 and the inclined position, represented in FIG. 6.

 It should be noted that all of the properties highlighted above are obviously retained if the sliding surfaces 1b, 3a respective to the upper faces of the lower plate 1 and of the spacer 3 are concave and the sliding surfaces 3b, 2b respective of the lower faces of the spacer 3 and of the upper plate 2, convex. This is why a prosthesis having such a geometry would not depart from the scope of the invention. However, for the sake of brevity and clarity of the description, it will be continued by placing itself in the first configuration which is also that shown in the figures.

 The prosthesis provides two rotary surfaces, one between the sliding surfaces 2b and 3a taking mainly into account the pure rotational movements, the other between the sliding surfaces 1b and 3b, taking mainly into account the components of horizontal translation of the complex movements of one vertebra with respect to the other. According to a particular variant of the invention, the radius of curvature of the sliding surfaces 1b and 3b is greater than the radius of curvature of the sliding surfaces 2b and 3a. This makes it possible to privilege the rotary component compared to the component of translation, which corresponds to the reality of the complex segmental movements of the intervertebral spaces L5-S1 to L1-L2 (Ln corresponds to the nth lumbar vertebra and Sn to the nth sacral vertebra ) of the lumbar spine.

 According to a preferred variant of the prosthesis, the sliding surfaces 1b, 3a, 3b and 2b are produced in the form of portions of spheres. This arrangement which provides the same rotation characteristics in all directions of space is not the only one conceivable. It would also be possible, for example, to produce the sliding surfaces in the form of a portion of ellipsoid, which would make it possible for example to slow down the rotation of the upper plate 2 on the upper plate 1 in a horizontal plane. Indeed, the asymmetry of the complementary sliding surfaces would cause, during a relative rotation in a horizontal plane, a relative spacing of these complementary sliding surfaces. This spacing which would cause a spacing of the vertebrae, would obviously be braked in return by the couple resistant generated by the mass of the portion of the body resting on the upper vertebra.

 According to a particularly advantageous embodiment of the previous variant, the radius of the sphere carrying the sliding surface 3b of the spacer 3 and the sliding surface lb of the upper face 1a of the lower plate 1 is strictly greater than the radius of the circle circumscribed at an equilateral triangle, one vertex of which is located in the center of the patient's L4-L5 intervertebral disc and the other two vertices located respectively at the middle of the great trochanters of this same patient. Each large trochanter is an eminence of the upper end of the femur. The lumbopelvi-femoral triangle thus defined, after having been studied on a large number of patients, turns out to be an equilateral triangle, homothetic in patients, according to their brief or elongated morphology.

 The circle circumscribed to this triangle, the center of which is at the barycenter thereof measures on average 50 millimeters in radius over a large number of patients examined radiologically. This lumbopelvi-femoral triangle represents in a way the support polygon of the upper part of the individual in erect position since, on the individual represented in profile, it is directed in an oblique plane from top to bottom and d 'back in front, and that the line of gravity of the individual passes through the barycenter of this triangle.

All the movements of the lumbar spine are therefore related to this line of gravity, and consequently to this barycenter. This is why, in the case where the prosthesis according to the invention is used in the space L4-L5, it is necessary that the part of the lumbar spine situated above the spacer 3 can pivot around the latter. However, as we have just seen, the morphology requires that the spacer 3 of the prosthesis implanted between the vertebrae L4 and L5, be located on a circle circumscribed by the lumbopelvi-femoral triangle, and consequently on a sphere of the same radius centered on the barycenter of said lumbar-pelvi-femoral triangle. It is therefore necessary, to ensure the rotation of the part of the lumbar spine situated above the spacer 3, around the latter, to give the bearing sphere the sliding surfaces 3b and ld which constitute the rotary surface located just below, a radius greater than that of the sphere circumscribed in the lumbopelvi-femur> ral triangle and centered on the barycenter of the latter. This makes it possible to favor rotation of the upper part of the spine around the spacer 3 relative to the rotation of the spacer 3 on the sliding surface lb. Thus, in the particular case of implantation of the prosthesis in the space L4-L5, the simple fact of giving the upper rotary surface a radius of curvature greater than that of the lower rotary surface is no longer sufficient to ensure the predominance of the component of rotation on the component of translation, because of the nesting of this space L4-L5 within the lumbopelvi-femoral triangle. It is necessary in addition that the radius of curvature of the sliding surfaces lb and 3> is greater than the radius of the lumbar-pelvi-femoral triangle. It is possible, for example, to give the sphere carrying the sliding surfaces 3b and 1b a double value, namely a value close to 100 millimeters, which makes it possible to ensure an optimal distribution of the gravity forces transmitted by the spine and thus releasing the mobility of space L5-S1 in which translation is predominant over rotation.

 On the other hand, it has been determined experimentally, that a radius of about 22 millimeters represents an optimal value for the radius of the sphere carrying the sliding surfaces 3a and 2b. In addition, the spacer 3 is given an overall height of 8 millimeters, which, combined with the other dimensions already given in the description, results in an overall height of the prosthesis of approximately 10.75 millimeters. Nevertheless, it is quite clear that these are particular values and that a prosthesis dimensioned differently, but respecting the same geometry would not depart from the scope of the invention.

The prosthesis thus dimensioned is obviously usable for discs L4-L5, but also for the discs of the adjacent stages, namely L3-L4 and L5-S1.

 According to a particular variant of the invention, the lower plate 1 and the upper plate 2 respectively have on their lower surface the upper, 2d vertical positioning pins 4 ensuring respectively the anchoring on the lower, upper adjacent vertebra.

 According to a preferred embodiment of the invention, the lower 1 and upper 2 plates are made of titanium or a titanium alloy and the spacer 3 is made of high density polyethylene.

Claims (11)

 1 - Prosthesis for intervertebral disc consisting of a lower plate (1) having on its upper face (la) a sliding surface (lk) on which is articulated a sliding surface (3b) complementary constituting the lower face of a spacer (3) generally in the form of a lens, the upper face of which has a sliding surface (3a) on which a complementary sliding surface (2b) is articulated provided on the lower face (2a) of an upper plate (2 ) characterized in that the sliding surfaces (lk, 3a) of the upper faces of the lower plate (1) and of the spacer (3) are convex and the sliding surfaces (3b, 2k) of the lower faces of the spacer (3) and the upper plate (2) are concave.  2 - Prosthesis for intervertebral disc according to the preceding claim characterized in that the radius of curvature common to the sliding surface (3a) convex of the upper face of the spacer (3) and to the sliding surface (2b) concave of the lower face (2a) of the upper plate (2) is less than the radius of curvature common to the concave sliding surface (3k) of the lower face of the spacer (3) and to the convex sliding surface (lk) of the upper face (la) of the lower plate (1).  3 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the sliding surfaces (lk2kt3a3b) are portions of spheres.  4 - Prosthesis for intervertebral disc according to the preceding claim characterized in that the radius of the sphere carrying the sliding surface (3 ~) concave of the underside of the spacer (3) and the sliding surface (lk) convex the upper face (la) of the lower plate (1) is strictly greater than the radius of the circle circumscribed by an equilateral triangle whose apex is located in the center of the intervertebral disc L4-L5 of the patient to whom said prosthesis is to be implanted and the two other vertices are located respectively at the level of the middle part of the large trochanters of this same patient.  5 - Prosthesis for intervertebral disc according to the preceding claim characterized in that the radius of the sphere carrying the sliding surface (3k) concave of the underside of the spacer (3) and the sliding surface (1k) convex of the upper face (la) of the lower plate (1) is equal to twice the radius of the circle circumscribed to the equilateral triangle, one vertex of which is located in the center of the intervertebral disc L4-L5 and the other two vertices are located respectively at the level of the part mean of the large trochanters, namely is close to 100 millimeters and the radius of the sphere carrying the convex sliding surface (3a) of the upper face of the spacer (3) and the concave sliding surface (2k) of the face lower (2a) of the upper plate (2) is close to 22 millimeters.  6 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the sliding surface (lob) convex of the upper face (la) of the lower plate (1) is larger than the sliding surface (3k) concave of the underside of the spacer (3)  7 - Prosthesis for intervertebral disc according to any one of the preceding claims characterized in that the sliding surface (lk) convex of the upper face (la) of the lower plate (1) constitutes the bottom of a recess (lc) provided on said upper face (la) of the lower plate (1).  8 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the sliding surface (2b) concave of the lower face (2a) of the upper plate (2) is located at the lower end of a cylinder (2c) extending vertically to said lower face (2a) of the upper plate (2).  9 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the lower plate (1) and the upper plate (2) respectively have on their lower surface (the), upper (20) pins (4) vertical positioning ensuring respectively the anchoring on the adjacent lower, upper vertebra.  10 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the lower (1) and upper (2) plates are made of titanium or a titanium alloy.  11 - Prosthesis for intervertebral disc according to any one of the preceding claims, characterized in that the spacer (3) is made of high density polyethylene.