WO1994004100A1

WO1994004100A1 - Intervertebral disk prosthesis

Info

Publication number: WO1994004100A1
Application number: PCT/FR1993/000825
Authority: WO
Inventors: Keyvan Mazda
Original assignee: Societe De Fabrication De Materiel Orthopedique - Sofamor
Priority date: 1992-08-24
Filing date: 1993-08-24
Publication date: 1994-03-03
Also published as: FR2694882B1; FR2694882A1; TR26927A; AU4964593A; CN1090485A; TW280767B; ZA936156B; MX9305071A

Abstract

Prosthesis comprising two plates (1, 2) secured to the corresponding vertebrae (L4, L5), and a ball joint linking the plates to one another, its centre of rotation (C), in an anterior-posterior plane (OX, OY) being offset rearwards in the posterior part of the plates. The ball joint with reference to a transversal plane (OX, OZ) is located substantially in the middle of the plates, and with reference to a vertical plane (OY, OZ) is located under the plate (1) of the lower vertebra (L5). The prosthesis also includes an elastic intercalary cushioning ring (10) having an opening (10a) for the passage of the joint (3) and being lodged in the space delimited between the plates and around the joint. This positioning of the centre of curvature (C) prevents straining of the articular apophyses during movements of the vertebrae, while the elastic ring (1) matches the prosthesis to the physiological lordosis of the vertebral column, cushioning the multiple stresses occurring during these movements.

Description

The subject of the present invention is an intervertebral disc prosthesis.

As is known, an intervertebral disc is in the form of a biconvex lens attached by its faces to the articular surfaces of the vertebral bodies. It consists of a hard peripheral part (annulus), formed of concentric fibrous lamellae, and a central part (nucleus pulposus), gelatinous and soft, consisting of thin fibrous bundles separated by spaces filled with mucous tissue. An intervertebral disc is a deformable element allowing all possible relative movements (6 degrees of freedom) but which limits the amplitudes of the latter, in particular in torsion, in association with the intervertebral ligaments and the bone buffers. The intervertebral disc is a viscoelastic damping structure, which contributes to the resistance and stability of the verticalized spine in a state of gravity.

An intervertebral disc can be damaged for a variety of reasons, including aging and degeneration, which can cause herniated discs.

Disc degeneration corresponds to functional and then anatomical destruction of the disc, which seems to result from the effect of mechanical stresses on a disc with structures in the process of disorganization. Disc degeneration modifies the mechanical behavior of the disc and results in a reduction in height of the intersomatic space, which leads to a disturbance of the functional disc-joint assembly. This results in instability which can cause annoying clinical consequences, in particular low back pain. Thus segmental instability leads to abnormal functioning of the articulars, inducing an arthritic reaction. as a source of pain and osteophytic processes.

It has therefore been proposed to replace the defective disc with an artificial disc, of which many types of embodiments have been envisaged. Thus, certain prostheses include articulations that do not suppress any degree of freedom, while others on the contrary completely neglect certain movements and only allow a maximum of one degree of freedom. The authors of the first prostheses consider that the artificial disc must in no way impose a movement, that the stability of the functional unit is ensured by the retention of the remaining ligament brakes and that an unconstrained joint guarantees a lasting bone / prosthesis connection. .

The authors of the second type of prosthesis consider physiologically useless certain degrees of freedom, and restore a degree of freedom considered essential. Thus, a prosthesis forming a hinge is known which allows rotation only around a transverse axis. The articulated prostheses can be extremely varied and include in particular materials having a certain flexibility, or else use mechanical means such as springs.

However, these known embodiments are not entirely satisfactory, for example because the friction at the level of the joint remains too high (prosthesis described in patent FR 2 659 226), or because an instability of the prosthesis in rotation. In addition, the prosthesis described in the aforementioned patent only allows a tilt and extension flexion limited to 10 degrees in all planes, a low value compared to the natural mobility of a vertebra.

It is still relatively difficult to define the indications for disc arthroplasty, which is proposed in particular for the treatment of low back pain and sciatica due to instability, low back sciatica post¬ aisectomy, and grade I spondylolisthesis.

In fact, the purpose of the implant constituted by the disc prosthesis targeted by the invention is threefold:

a) Be sized to be able to restore a normal height of the intersomatic space. Indeed, disc degeneration, modifying the mechanical behavior of the disc and resulting in a reduction in height of the intersomatic space, leads to a disturbance of the functional disc-joint assembly. This reduction in height causes a mechanical overload on the articular facets, arthrogenic and possible source of low back pain. It can cause an upward and forward displacement of the upper articular of the lower vertebra, leading to a narrowing of the conjugation hole, a possible cause of sciatica. b) Allow physiological mobility between the two instrumented vertebrae.

Instability can be consecutive to disc degeneration or induced by surgery. The limits between normal and abnormal mobility have not yet been precisely defined. Segmental instability leads to abnormal functioning of the articulars, inducing as already indicated an arthritic reaction. c) Possess intrinsic stability and allow interbody movement.

The arthroplasty is intended for cases where, until now, arthrodesis is indicated. It can be an interesting alternative to this intervention, not without consequences on the adjacent floors. In fact, all lumbar arthrodeses involve in the segments adjacent an increase in stresses and a displacement of the centers of rotation, which can lead to hypermobility.

Long-term monitoring of the areas adjacent to arthrodesis shows constant disc pinching from the fifteenth year, and especially a sliding or angular displacement whose frequency increases with hindsight. We can also observe a significant osteoarthritis, a herniated disc or an acquired spondylolysis of this neocharnière.

The disc prosthesis targeted by the invention comprises two plates fitted with means for fixing to the corresponding vertebrae, as well as a device for articulating the plates one on the other. According to the invention, the articulation device is a ball joint whose center of rotation, in an anteroposterior plane is offset backwards in the posterior part of the plates and of the vertebral body, in a transverse plane is located substantially in the middle of the plates and the vertebral body, and in a vertical plane is located under the plate of the lower vertebra; this prosthesis further comprises an intermediate cushioning ring, made of a flexible material, pierced with an opening for passage of the patella and dimensioned to be housed in the space delimited between the plates and around the patella by substantially matching the contour of the plates.

The positioning of the center of curvature of the patella as defined according to the invention has the advantage of avoiding constraining the articular processes, as in natural vertebrae, which had not been obtained until now with known prostheses.

In addition, the prosthesis according to the invention has everything both the necessary mechanical resistance and reduced friction at the joint, thanks to the appropriate dimensioning of the ball joint system. In fact, the center of rotation is neither too far from the joint, nor too close to it, and thus makes it possible to reproduce mobility close to natural mobility.

Other features and advantages of the invention will appear in the course of the description which follows, given with reference to the accompanying drawings which illustrate an embodiment thereof by way of non-limiting example.

Figure 1 is an exploded perspective view of a pair of vertebrae equipped with the constituent elements of a disc prosthesis according to an embodiment of the invention.

Figure 2 is a top view of the lower plate of the disc prosthesis of Fig.l.

Figure 3 is a longitudinal sectional view along 3/3 of Fig.2 of the prosthesis according to the invention in a position where the two plates are parallel.

Figure 4 is an elevational view of the prosthesis of Fig.l to 3 in a position where the two plates are inclined relative to each other.

Figure 5 is a longitudinal sectional view along 5-5 of Fig.6 of a second embodiment of the invention.

Figure 6 is a top view of the prosthesis of Fig.5.

Figure 7 is a longitudinal elevational view of the prosthesis of Fig.5 and 6 with its parallel plates.

Figure 8 is a view similar to Fig.7 with the plates forming an angle therebetween.

Figures 9, 10, 11 and 12 are perspective views. pective of four alternative embodiments of the prosthesis according to the invention.

We see in Fig.l two adjacent vertebrae, for example the lumbar vertebrae L5 and L4, which can be connected by a disc prosthesis. This prosthesis comprises two plates 1 and 2, fixed respectively to the lower vertebra L5 and to the upper vertebra L4, a ball joint 3 of the two plates 1 and 2 one on the other, and a flexible ring 10 d damping inserted between the two plates 1 and 2 and traversed by the ball 3.

The plates 1, 2 are similar, have dimensions substantially equal to those of the vertebrae L5, L4 (these vertebrae may be located on other stages than L5-L4, including non-lumbar stages) and approximate the contour associated joint surfaces. Each plate 1, 2 thus has a curvilinear edge 4, substantially elliptical or oval, and a rectilinear edge 5, intended to be contiguous to the spinal canal, connecting the two truncated ends of the ellipse 4. On the other hand, each plate 1, 2 is pierced, in its central part, with a through hole 6 into which is inserted a respective pad 7, 8, having a body sized to be able to engage in the corresponding hole 6. The latter can advantageously be conical, as can the complementary bases of the pellets 7, 8, which can thus be secured in a removable manner with the corresponding plates or plates 1, 2.

The pellets 7, 8 together constitute the articulation 3 with ball joint. The first patch 7, inserted in the lower plate 1, has a convex spherical surface 9, projecting from the corresponding face of the plate 1, while its opposite surface 11, flat, is flush with the opposite face, turned towards 1'exterior of the plate 1. The patch 8 of the upper plate 2 also protrudes from its face facing the plate 1, and has a concave spherical surface 12. The radius of curvature of the latter is equal to that (r) of the spherical surface 9, in order to be able to slide thereon by forming a ball joint. The opposite face 13 of the patch 8 is planar and is flush with the outside face of the plate 2.

The center of curvature or rotation C of the ball 3, distant from surfaces 9 and 12 of the radius of curvature r., Is located in the following manner:

- This center C is, in an anteroposterior plane (OX, OY), offset backwards in the posterior part of the plates 1, 2 and the vertebral bodies; - in a transverse plane (OX, OZ), the center of rotation C is located substantially in the middle of the plates 1,

2 and vertebral bodies;

- And in a vertical plane (OY, OZ), the center C is located just under the plate 1 covering the upper face of the lower vertebra, for example L5 in the example illustrated in Fig.l.

This positioning of the center C corresponds to a radius of curvature t comprised approximately between the height of the prosthesis when the plates 1, 2 are parallel, and half of this height.

In practice the center of rotation C can be located, below the lower surface SI of the plate 1 (Fig. 3), at a distance dl between Omm - that is to say being on the surface IF it - same - and about 5mm depending on the case.

On the other hand, the center of rotation C is placed from the middle M on the rectilinear side 5 of the plate 1, contiguous to the spinal canal, at a distance from the side 5 between about 1/3 and half the width L of the plate 1 from the middle M: in Fig.3 the center C is therefore located substantially between the distance d2 = 1 / 3.L and the distance d3 = 1 / 2.L. As a nonlimiting numerical example d2 = 4mm, d3 = 10mm and L = 20mm, so that C is in an area of approximately 6mm in width.

The prosthesis further comprises an intermediate cushioning ring 10, made of a suitable flexible material, pierced with an opening 10a for passage of the ball joint 3. The flexible ring 10 is sized to be able to be accommodated in space interlayer delimited between the plates 1, 2 and around the ball joint 3, closely matching the contour of the plates. In the uncompressed free state (FIG. 1), the flexible ring 10 has a variable thickness from its front part to its rear part. Its front part has a maximum thickness H, which decreases regularly, on either side of the center of the edge of this front part, up to the rear part of the ring 10, the thickness of which h is thus clearly less than H. The ring 10 is therefore in the form of an asymmetrical piece whose opposite faces are inclined relative to one another, approaching towards the rear part. Each plate 1, 2 comprises, on its face from which the corresponding pad 7, 8 projects, a central annular allowance 14, 15. The latter is contiguous to the associated pad 7, 8 which it surrounds by forming a reinforcement, coming of material with the rest of the plate. Preferably, as shown, the reinforcements 14 and 15 have a conical surface, the thickness of which decreases from the pad 7, 8.

In each plate 1, 2 are formed a set of holes 16, suitably distributed along the edges of the plate. These holes 16 are arranged to receive pins 17, fixed in the plate 1, 2 by any suitable means, such as screwing their cylindrical base 17a. Their points 17b, impacted in the subchondral bone of the vertebral plates, ensure the fixation of the plates 1, 2 to the vertebrae such as L5 L4, L5 ...

The pads 7, 8 forming the ball joint 3 are small compared to those of the plates 1, 2, and are generally chosen from a material having the best possible tribological characteristics (mechanical characteristics: static, dynamic and resistance wear), and of course biostable. Their coefficient of friction must be low and their lifespan, as well as that of the plates 1, 2, very long (for example 40 years).

These pellets 7, 8 can be made of a suitable material such as a ceramic, in particular a sintered oxide such as alumina or zirconia, a synthetic stone, a metal alloy with surface treatment (deposit of diamond, etc.).

The flexible damping ring 10 is preferably advantageously made of an elastomer of suitable hardness.

The plates 1, 2 are provided in standard number, suitable for all possible morphotypes of the vertebral bodies. The ball joints 3 are on the other hand in greater number, adapted to the morphology of each individual and of each vertebral stage. We can thus produce a range of parts (plates 1, 2, ball joints 3, flexible rings 10) adapted to all the disc heights of the patients and allowing combinations in infinite number, adapted to each particular case.

In addition to the technical advantages already mentioned, the disc prosthesis according to the invention has the following advantages. Thanks to the appropriate dimensioning of the ball joint 3 and to the position of its articulation center C, defined above, the friction surfaces are reduced and the friction caused by the tilting of the plates 1, 2 one relative to the the other (Fig. 4) remains weak. This results in reduced wear of the pellets 7, 8. Furthermore, the surfaces 9, 12 of the articulation have, however, a radius of curvature sufficient not to cause excessive instability of the prosthesis, while ensuring it a suitable three-dimensional mobility, practically identical to that of natural vertebrae.

During the three-dimensional movements of the plates 1, 2 and the corresponding vertebrae, the flexible ring 10 provides a function of damping of multiple stresses which limits the movements, the opposite faces of the ring 10 always remaining in contact with the corresponding faces of the plates 1. The variation in height, or thickness of the ring 10 from front to back has the advantage of adapting it to physiologic lordosis of the lumbar spine. The H / h ratio thus adapts to the spinal static profile, depending on the disc floor concerned. The inclination of the faces of the ring to one another may vary depending on the floor and the morphology of the patient. For example, it can reach 15 ° in L5 / S1. It should also be observed that the fact that the thickness of the front part of the ring 10 is greater than that of its rear part allows the flexible material made up to be brought into play earlier. guardian of this ring. This therefore results in the production of a damping effect upon the appearance of a movement of the vertebral bodies associated with the prosthesis. On the other hand, the suitable choice of a flexible material for the ring 10, allows obtaining an optimal stiffness to reproduce the slopes of the rotations of a healthy disc in the diver¬ its stresses. Finally, the prosthesis according to the invention makes it possible to obtain a stiffness and a damping very close to or even identical to those of a natural disc. The implantation by the surgeon of the prosthesis which has just been described is done as follows.

First of all, the surgeon places on the lower vertebra, for example L5, the corresponding plate 1, the center suitably with respect to the vertebral body, and fixes it by pressing the pins 17 into the vertebra.

Then the surgeon places the upper plate 2 on the upper vertebra, for example L4, by positioning it appropriately relative to the lower plate 1.

The surgeon then places between the plates 1, 2 a composite "sandwich" constituted by the assembly of the flexible ring 10 and the ball joint 3, formed by the pellets 7, 8. He places this assembly in the central zone between the plates 1, 2, with the pellets 7, 8 opposite the conical holes 6, then releases the distraction imposed hitherto on the vertebral bodies receiving the plates 1, 2. This results in an impaction which causes the entry of the conical bases pellets 7, 8 in the holes 6, and their joining by force with the plates 1, 2.

The values of the angular amplitudes of the possible movements with the prosthesis according to the invention are close to the amplitudes below of a healthy disc:

L5 / S1 18 ° ≤14 °

4 ^th

The prosthesis according to the invention guarantees, by its solidity, the restoration of the intersomatic space of height H, while having intrinsic stability, that is to say not allowing non-physiological mobility, especially in translation.

The materials capable of being used for the constitution of the prosthesis are of course biocompatible and must be biologically neutral, that is to say non-toxic, insensitive to corrosion. In addition, they must not be pro-inflammatory, their mechanical and biological qualities not to be altered by the mode of sterilization chosen. By way of example, the plates 1, 2 can be made of stainless steel, titanium stellite or titanium alloy.

The titanium alloy has the advantage of having advantageous radiological and magnetic characteristics for iconographic monitoring of spinal pathology, as well as excellent biotolerance and high mechanical characteristics.

In the second embodiment of the invention, illustrated in FIGS. 5 to 8, each pellet 21 and 22 is housed in a respective receiving bowl 23, 24 formed in the associated support plate 25, 26. The pads 21, 22 partially project from their housing bowls 23, 24, in which they are mounted freely, and not by force. Around each bowl and each pellet, the annular allowance 29, 31 is conical and has a height at the top IL substantially equal to the thickness d. from plate 25, 26.

The pellets 21, 22 are conical, like their receiving cups 23, 24, and have a taper angle of 1 to 7 ° approximately, and preferably 5 ^e .

Furthermore, the prosthesis illustrated in FIGS. 5 to 8 is similar to that of FIGS. 1 to 4, in particular for the positioning of the center of rotation C. The flexible damping ring 10 has not, however, been shown.

The arrangement of bowls 23, 24 for receiving the pellets 21, 22 whose external dimensions correspond to the internal dimensions of the bowls, in which they are not fixed and from which they can be easily removed if necessary after a certain time, without having to separate the plates 25, 26 from the vertebrae and replacing them has an appreciable advantage. In fact, it makes it possible to avoid local flexi¬ bility of the plates at the level of the through holes 6 of the previous embodiment, flexibility which is not desirable due to the localized clamping forces to which the pellets are subjected.

Of course these may alternatively be of cylindrical shape, as well as the walls of the corresponding bowls. But this geometry does not facilitate the installation and subsequently the possible removal of the pellets. It has in fact been observed that appreciable adhesion may occur from the walls of the pellets to the walls of the bowls, so as to hinder their removal. This is the reason why the slight taper of the pellets 21, 22 and their cups 23, 24 has been advantageously provided, because it avoids this drawback. The efforts exerted by the patient's body on the plates 25, 26 are reduced due to the presence of the bottoms 27, 28, of thickness e. sufficient, for example 3/4 of the thickness d. of each plate or even this d_ value. Thus the plates have no local mechanical weakness in bending.

The annular reinforcement or extra thickness 29, 31 of each plate 25, 26 also increases the surface of the faces in contact between the conical walls of the cups 23, 24 and the pellets 21, 22, by virtue of the increase in their height relative to the extra thicknesses 14, 15 of the previous embodiment. This increase is indeed required by the adjustment of funds 27, 28.

Such an arrangement makes it possible to produce a prosthesis whose total height can be adapted to the height desired by the surgeon, after consultation of the radiographs of the local spinal system of the patient.

To obtain the total height to be given to the prosthesis, the surgeon has a set of several ball joints 21, 22 the height of which is progressively increased, for example 11, 13 and 15 centimeters. For each of these sets, the radius of curvature of the contacting surfaces 9, 12 forming the patella is adapted so that it is approximately comprised between the height H1 of the prosthesis when the plates 25, 26 are parallel to each other (as shown in Fig. 7), and half this height.

Fig.8 illustrates the respective position of the plates 25, 26 in their maximum angular opening A, for example approximately 32 °. Figs. 9 to 12 illustrate various possible embodiments of the means for fixing the plates or plates to the vertebral bodies, replacing the pins 17. Thus Fig. 9 shows plates 32, 33 of which the surface 34 facing the vertebrae is knurled, FIG. 10 shows plates 35, 36 with corrugated surface 37, FIG. 11 shows plates 38, 39 comprising curved surfaces 41. FIG. 12 shows plates 42, 43, the bearing surfaces 44 of which support the vertebral bodies have undergone a suitable treatment: fixing of a metallic fabric in a manner known per se, projection of a metal or a ceramic, or bonding coating. This surface treatment can, moreover, be combined with the variants of FIGS. 9 to 11 to be applied to surfaces 34, 37, 41.

The pins 17 can also be replaced by screws or studs. On the other hand, fixing by cement (methylmethacrylate) is not possible, given the proximity of the nervous elements and the release of heat during polymerization. For non-lumbar stages, the flexible ring 10 may not have the asymmetry described and therefore have a uniform thickness.

It should be noted that the disc prosthesis according to the invention does not have a free part, unlike certain known disc prostheses. This has the advantage of eliminating any risk of ejection of such a free part against the aorta or towards the spinal canal.

Claims

CLAIMS 1. Intervertebral disc prosthesis, comprising two plates (1, 2) equipped with means for fixing to the corresponding vertebrae (L4, L5), and a device (7, 8) for articulation of the plates one on the other, charac¬ terized in that the articulation device is a ball joint (3) whose center of rotation (C), in an anteroposterior plane (OX, OY) is offset backwards in the posterior part of the plates and of the vertebral body, in a transverse plane (OX, OZ) is located substantially in the middle of the plates and of the vertebral body, and in a vertical plane (0Y, 0Z) is located under the plate (1) of the lower vertebra (L5) , and in that this prosthesis further comprises an intermediate cushioning ring (10), made of a flexible material, pierced with an opening (10a) for passage of the ball joint (3) and dimensioned to be housed in the space delimited between the plates and around the patella by substantially following the contour of the plates. 2. Prosthesis according to claim 1, characterized in that the ball joint (3) consists of a first pad (7) with a convex spherical surface (9) and a second pad (8) with a concave spherical surface (12) receiving the convex spherical surface, these two spherical surfaces having the same radius of curvature (r) and the pellets being mounted in the central parts of the respective plates (1,

2), on the faces opposite which they project.

3. Prosthesis according to claim 2, characterized in that, each plate (1, 2; 25,26) having a rectilinear side (5) intended to be positioned close to the spinal canal and a complementary contour (4) substantially oval, or elliptical, the patch (7; 22) on the surface convex spherical (9) is mounted in the plate (1; 26) of the lower vertebra and its center of rotation (C) is located, starting from the middle (M) on the rectilinear side of the plate contiguous to the spinal canal, a distance from said side of between approximately one third (d2) and half (d3) of the width (L) of the plate from said medium and under the plate of the lower vertebra, the center of rotation is located at a distance ( dl) of the plate between 0 and about 5mm.

4. Prosthesis according to claim 2 or 3, characterized in that each plate (1,2; 25,26) comprises, on the face of which the pellet (7, 8; 21,22) projects, an additional thickness central annular (14,15; 29,31), contiguous to the patch which it surrounds forming a reinforcement.

5. Prosthesis according to claim 4, characterized in that the reinforcement (14,15; 29,31) has a conical surface, the thickness of which (hl) decreases from the patch (7, 8).

6. Prosthesis according to one of claims 2 to 5, characterized in that the pellets (7,8; 21,22) have a conical base adapted to conjugated conical holes formed in the plates (1,2; 25,26 ).

7. Prosthesis according to one of claims 1 to 6, characterized in that the means for fixing the plates

(1,2; 25,26 ...) to the vertebrae (L5, L4) include conical pins (17) screwed into holes (16) formed in the plates and suitably distributed thereon, or a knurling (34 ), or a corrugated surface (37), or a curved surface (41).

8. Prosthesis according to any one of claims 1 to 7, characterized in that the fixing of the plates (42, 43) to the vertebrae is ensured by a metallic fabric (44), a metal or ceramic projected onto the surface of the plate facing the vertebra, or by gluing, or else by combination of one of these means with either a knurled surface (34) or corrugated (37) or curved (41).

9. Prosthesis according to one of claims 1 to 8, characterized in that the plates (1,2; 25,26 ...) have identical dimensions and substantially equal to those of a vertebra, in particular lumbar.

10. Prosthesis according to one of claims 1 to 9, characterized in that the flexible ring (10) of damping has a front part of maximum thickness (H), greater than that (h) of its part posterior, said thickness (H) decreasing regularly from the center of the edge of the anterior part to the posterior part, and the opposite faces of the ring (10), in the uncompressed state, are correlatively inclined one with respect to The other.

11. Prosthesis according to one of claims 1 to 10, characterized in that the pellets (7,8; 21,22) of the ball joint (3) are made of one of the following materials: ceramic, in particular sintered oxide such as alumina or zirconia, synthetic stone, metal alloy with surface treatment.

12. Prosthesis according to one of claims 1 to 11, characterized in that the flexible ring (10) is made of an elastomer of suitable hardness.

13. Prosthesis according to one of claims 2 to 12, characterized in that each pellet (21, 22) is housed in a receiving bowl (23, 24) formed in the support plate (25, 26) of which it forms part projection and in which it is mounted freely.