FR2809304A1 - Intervertebral stabiliser comprises implant between adjacent vertebrae and movement damper to rear of spine - Google Patents

Abstract

The stabiliser consists of an implant (18) inserted at least partially between two adjacent vertebrae (2, 2') and permitting at least one degree of rotational freedom, and at least one extra-discal component (20) situated to the rear of the intervertebral space and designed to damp any movement between the vertebrae, especially in the direction of flexion. The damper is connected to the two vertebrae by pedicular screws (22, 22'), and the implant has an elastic core, partially covered by a rigid material to for a partial disc prosthesis.

Description

The present invention relates to an intervertebral stabilization device.

Usually, such devices are intended to replace all or part of an intervertebral disc, when the latter has been destroyed by surgery or disease.

A first type of bralged intervertebral stabilization device consists of a rigid cage, which may be an example of a rectangular cross-section, in which perforations adapted to receive grafts intended to ensure a satisfactory attachment of this cage to the two vertebrae between which are formed. they must be inserted. These rigid cages, which are implanted in particular by impact or screwing, have a disadvantage in that they lead to complete blockage of the two adjacent vertebrae, which limits the freedom of movement of the patient.

EP-A-0 346 269 also discloses an intervertebral stabilization device formed by a semi-rigid cage, namely it comprises a core of viscoelastic material and two rigid cover plates made of metal. This core interposed between these two plates, forming enroba ges intended to be in contact with the surface of the vertebrae, once cage implanted. Such a semi-rigid cage however has certain disadvantages, in that it induces an excessive mechanical stress of the vertebrae adjacent to those between which this cage is disposed. This leads to premature fragilization and degradation of these vertebrae.

 In order to overcome these various drawbacks, the invention proposes to provide a stabilizing device which, while ensuring a satisfactory freedom of movement between the vertebrae which are adjacent to it, induces only low mechanical stresses at the level of the whole vertebral chain. .

For this purpose, it relates to an intervertebral stabilization device, comprising a cage intended to be inserted between two adjacent vertebrae, said cage comprising an elastic core, in particular made of a silicone polymer or an elastomer, the core being covered, on a portion of its periphery, by a coating made of a rigid material and intended to be in contact with said two adjacent vertebrae, characterized in that the vertical dimension of said stand increases towards its anterior part in the vertical anatomical position of rest , in that said cage has means of preferential bending forwards and in that said device further comprises extra-discal means, arranged at the rear of the intervertebral space, suitable for damping a displacement between said vertebrae at least in extension.

According to other features of the invention - the forward preferential bending means comprise an anterior part of said core extending at a distance from walls facing said coating, so as to form at least one prior fférentiel volume; - The preferred forward bending means comprise an anterior element of said core, made of a first material more compressible than a second material constituting a rear element of said core; - said cage comprises pivot means for said bending forward; - The pivot means comprises an intermediate portion of the core extending said front portion, said intermediate portion being disposed in contact with the walls of said coating; said walls surround said intermediate portion over a substantial portion of its perimeter; said cage also has means for bending said cage towards the rear; the extra-discal damping means are capable of also damping a displacement in compression of said vertebrae; the length of the damping means varies in extension and, optionally in compression, in an asymptotic manner as a function of the force applied to them; - The damping means, the movement of the vertebrae comprise at least two separate damping devices and is further provided means for securing said damping devices together, at least in a vertical direction; said cage is provided with means for reducing anteroposterior shear; said cage is provided with lateral shear reduction means.

The invention will be described below, with reference to the accompanying drawings, given solely by way of non-limiting examples and in which - Figure 1 is a schematic side view, illustrating two neighboring vertebrae between which is placed a device of stabilization according to the invention; FIG. 2 is an exploded perspective view illustrating an intervertebral cage belonging to the stabilization device of FIG. 1; - Figures 3 and 4 are views in vertical section along lines III- and IV-IV in Figure 2; - The figure is a horizontal sectional view along the line V-V of Figure 3; - The figure is a schematic perspective view, with parts broken away, of a laying tool for the cage shown in Figures 2 to 5; FIG. 7 is a perspective view of a damping device belonging to the stabilization device of FIG. 1; the figure is a curve illustrating, as a function of the force exerted on it, the variations in the length of the damping device of FIG. 6; FIGS. 9 to 11 are schematic diagrams illustrating the operation of the connecting device of FIG. 1, in successive positions of rest, of forward bending and of extension towards the rear of a patient; - Figure 12 is a sectional view similar to Figure 3, a cage according to a second embodiment of the invention; and - the figure is a schematic rear view, illustrating a further variant of the invention.

FIG. 1 shows two upper and lower vertebrae 2 'which are connected via a stabilizing device according to the invention. Each vertebra comprises a vertebral body 4, 4 'extended by a pedicle, 6', an upper articular 8, 8 'and a lower articular 10, 10'. The intervertebral space is designated by 12 as 14 and 14 'as the opposite articular facets and as 6 and 16' as the articular capsules.

The two vertebrae 2, 2 'are interconnected by means of a stabilization device, comprising a cage 18, constituting an interbody implant, housed in the intervertebral space 12, and a damping device, designated as a whole by the reference 20, the two ends are fixed on the corresponding vertebrae via pedicle screws 22, 22 '. It is possible to use, between these two vertebrae 2, 2 ', two cages 18 and two damping devices 20.

The intervertebral cage 18 is shown more precisely in FIGS. 2 to 5. This cage comprises a core 24 made of biocompatible elastic material and whose outer surface is partially covered by means of a coating formed by two shells 26A, 2613. These latter, made of a biocompatible rigid material, are subject to the core 24, for example by means of a silicone glue.

The cross section of the core 24 consists of two end portions 28 whose outer periphery describes an arc and which are connected by two flats 30 forming a central portion 32.

Each shell 26 comprises an inner cap 34, internally cylindrical, intended to come into contact with the core 24. This cap 34 is connected to a peripheral cap 36 made in the form of a profile having, in cross section, a shape of arc. The outer surface of these caps is provided with a threading 38 intended to facilitate the implantation of the cage 18.

The cap 34 and the cap 36 are connected by two longitudinal tabs 42. The latter define, in the vicinity of each edge of the shell, two longitudinal guides or grooves 44, in T-section. These tabs 42 laterally delimit a through opening 48 to section in the form of a circular arc, separating the cap from the cap.

There are four components, 46A front flaps and 46B rear flaps for the 26A shell and 47A front flaps and 47B rear flaps for the 26B shell. Each flap 46, 47 extends from one of the shells 26, towards the opposite one, substantially perpendicular to the main axis of the prosthesis.

These shutters are arranged asymmetrically. Thus, the front flap 46A of the first shell 26A and the opposite flap, namely the rear flap 47B of the other shell 26B lies, according to their main dimension, so as to form a covering zone ZR (Figure 3) . Projections, on the same plane, along the main axis A of the prosthesis, these opposite flaps 46A and 47B have a common region, form the recovery zone ZR. The presence of the latter is such as to reduce the anteroposterior shear to which the cage 18 is subjected once implanted.

Each shell 26 is also provided with side skirts 50 extending from each longitudinal edge of the cap. Each skirt 50 has, from the side, a variable height, namely that it extends in a generally sinusoidal corrugated profile. However, each skirt may also include at least one recess. The adjacent skirts, namely 50A and 50C on the one hand, and 50B and 50D on the other hand, which are provided with two different shells, have substantially conjugate profiles, so that they are fit to come into mutual nesting.

In contrast, two opposite skirts, namely either 50A and 50B, or 50C and 50D, are arranged asymmetrically. There are therefore overlapping zones ZR '(FIG. 4), on the one hand between the opposite skirts 50A and 50D, and on the other hand between the opposite skirts 50B and 50C. Projections on the same plane, along an axis perpendicular to the main axis of the prosthesis, of each pair of opposite skirts have common regions which form these overlapping zones. These help to reduce the lateral shearing effects of the prosthesis. With particular reference to Figures 2 and, the height of the flats 30 increases towards the anterior part of the cage, once the latter implanted. The height, or vertical dimension, of this cage therefore increases forward. The transverse dimension, or width, of the end portions 28 is not constant all along the web 24. In fact, this core 24 has an intermediate portion 24A, disposed in the vicinity of the rear third of the soul, whose transverse dimensions are maximum. This intermediate portion 24A extended by a rear thinned portion 24B, thus a thinned portion before 24C.

Once the cage has been implanted and in the case of a patient in a standing position, lying neither in flexion nor in extension (FIG. 5), only the intermediate portion 24A extends in contact with the inner wall of the skirts 50, which surround this portion 24A substantially over its entire periphery.

On the other hand, the rear portions 24B and 24C of the core 24 extend away from these skirts 50, which contributes to creating differential volumes before V and rear V '. It should be noted that the differential volume before V is substantially larger than the rear differential volume V ', so as to initiate preferential bending initiation of the cage 18, towards the front part thereof.

The bending of the cage 18 forwards or backwards, occur around a pivot point belonging to the intermediate portion 24A, substantially incompressible. The latter guarantees the maintenance of the intervertebral spacing. This pivot point is movable from back to front, depending on the movements of the patient.

The laying of the cage illustrated in FIGS. 2 to 5 is carried out by means of a tool described in FIG. 6. The latter, generally designated by reference numeral 52, comprises an elongate cylindrical handle 54, pierced with an orifice The handle 54 is terminated by two tabs 58, symmetrical to each other, capable of penetrating into the adjacent rails of the cage, namely on the one hand 44A and 44A on the other hand. 44C and the other 44B, 44D. The tool 52 is also provided with additional tabs not shown, able to penetrate into the openings 48 of which the shells 26 are provided.

The installation of the cage first consists of engaging the end of the tongues 58 in the rear end of the grooves 44. Then, for example, the cage front is compressed manually, which is possible at the same time. because of the elastic nature of the core 4 and the presence of differential volumes V, V '. Each tongue is then slid within a corresponding slideway 44 until the rear end of the cage comes into abutment against the end of the shaft 54 adjacent to the tabs 58.

The assembly composed of the compressed cage 18 and tongues 58 of the laying tool has a generally cylindrical shape. The cage is then screwed for assembly, taking advantage of the threads 38 with which it is provided. Then, once the cage is implanted in a suitable location, it is held axially by means of the rod 57 penetrating into the central orifice 56 of the handle 54. Then, the tabs 58 are slid backwards, so as to The cage then covers its original shape, as shown in FIG. 5, because of the nature of its prestressed constituent material.

FIG. 7 shows, more precisely, the damping device 20. The latter is in accordance with the teaching of FR-A-2 676 911. It is embodied in the form of a longitudinal body provided with a raised central portion 60 connected, by two necks 62, to two spherical ends 64.

For mounting this damping device 20, each pedicle screw 22 is extended by a cylindrical body terminated by a bowl 68 provided with a threaded inner wall and a concave bottom, of complementary shape with a spherical end 64 of the device. 20. The locking of the ends of the damper is effected by screwing a threaded end 70 inside the bowl 68.

Any suitable material may be adopted for the realization of the damping device 20, in particular a biocompatible elastomer.

Figure 8 shows a curve illustrating the variations in the principal dimension, or length, of the damping device 20 as a function of the intensity of the force applied to it, according to this principal dimension. In its rest position, that is to say when no force is applied to it, the device 20 has a length LR. In the extension phase, the variations in length as a function of the absolute value of the applied force have an approximately exponential, asymptote LN. The application of a compressive force on this device 20 leads to a variation of approximately exponential length, of asymptote Lm.

In order to achieve this damping device, it is possible to adopt a composite material which responds in different ways to extension and compression, as FR-A 676 911 also teaches.

can also use different damping devices, such as the one described in FR-A-2 751 864. Figures 9 to 11 show diagrams illustrating the mechanical principle of the device for inverted stabilization bral according to the invention.

The presence of the front differential volume contributes to creating a preferential flexion zone of the cage 18 towards the front, schematized by a spring R. The presence of the damping device 20, working in extension contributes to creating a resistance to spacing between vertebrae, schematized by a second spring R '. These springs R and R 'are sposés on both sides of a pivot point P, around which occur the physiological movements of flexion and extension. This pivot P is located in the intermediate portion 24A of the core 24.

The existence of a rear differential volume constitutes a bending initiation zone of the cage 18 towards the rear, which is schematized by a spring r. In addition, the damping imparted, in compression, by the device 20 contributes to creating additional resistance schematized by the reference r '. This figure 8 corresponds to a standing patient, in the vertical anatomical position of rest namely that the force induced by the weight of the body is exerted according to a component G passing substantially through the pivot P. When the patient leans forward as shown in FIG. 9, the force exerted on the vertebrae is indicated by the arrow G '. The cage 18 then flexes forward, which is facilitated by the presence of the differential volume before V. This bending is limited by 1 coming into abutment of the front portion 24C of the core 24 against the inner wall of the skirts 50. It is further controlled by the damping device 20, working in extension, because of its resistant component R '. Such bending induces a displacement of the pivot P towards the rear of the intermediate portion 24A.

When the patient leans backwards, as shown in FIG. 10, the cage 18 tends to bend backwards, under the effect of a force G ", which is permitted thanks to the presence of the This rearward flexion is controlled by the presence of the resilient component r 'of the damping device During such a movement, the pivot P tends to move towards the front of the intermediate portion 24A.

Figure 12 shows a cage <B> 118 </ B> according to a second embodiment of the invention. Its core comprises several distinct elements, namely a front element 124 'of greater vertical transverse dimension and a rear element 124' of restricted vertical transverse dimension, the front element being made of a more easily compressible material than the rear element . These elements are covered by coating consisting of two threaded shells 126, whose inner walls are shaped to maintain these front and rear elements. It is also possible to provide this cage with a coating similar to that covering the cage 18. The presence of the front element 124 'which is more easily compressible gives the cage 18 a preferential flexural component towards the front.

Figure 13 shows a further variant of the invention. In this figure, the two adjacent vertebrae 2 and 2 'are mutually connected by two damping devices 120, provided on either side of the articular elements 8, 10. These damping devices 120 have a metallic part are for example in accordance with the teaching of FR-A-2 <B> 751 </ B> 864. They are connected to each other by means of a transverse rod 128, extending substantially horizontally. The connection between the devices 120 and the rod 128 is rigid, and the example involves a weld. It is advantageously carried out at the middle portion of these damping devices. The presence of this rod, which provides a relative immobilization in a vertical direction of the damping devices 120, significantly reduces the horizontal intervertebral shear.

The invention is not limited to the examples described and shown. Thus, the intervertebral cage may have non-cylindrical end portions. it can also be realized so that it has, in cross section, a generally rectangular section, its height increasing towards its anterior part. Such a cage, of approximately pyramidal shape, may be introduced by impaction.

The invention makes it possible to achieve the objectives mentioned above. In fact, the use of a cage whose vertical dimension increases towards its anterior part makes it possible to respect the natural anatomical configuration. Providing this cage with an elastic core having preferential flexing means forward confers to the vertebrae between which is disposed this cage a relative movement satisfying from a physiological point of view. In addition, providing extra-discal damping means disposed at 1 rear of this cage to regulate the bending movement allowed by the latter. This allows the stabilization device according to the invention to ensure a relative movement between the two vertebrae that it connects, which is sufficiently close to the movement allowed by a natural vertebral disc, so that such a movement does not induce major dysfunction at the level of the entire vertebral chain.

Predicting differential volumes inducing preferential bending forward is advantageous. Indeed, the presence of these differential volumes guarantees particularly progressive movement, which is not stopped abruptly. This is favorable in terms of the physiological integrity of the patient.

Claims (12)

<U> CLAIMS </ U> 1. Intervertebral stabilization device, comprising a cage (18; 118) intended to be inserted between two adjacent vertebrae (2, 2 '), said cage comprising an elastic core (24; 124), in particular made of a polymer of silicone or an elastomer, the core being covered, over part of its periphery, by a coating (; 126) made of a rigid material and intended to be in contact with said two neighboring vertebrae, characterized in that the vertical dimension of said seated cage increases towards its anterior part in the vertical anatomical position of rest, in that said cage (18) has means (24C, 50; 124 ') of preferential bending towards the front and in that said device comprises furthermore extra-discal means (20) arranged at the rear of the intervertebral space, suitable for damping a displacement between said vertebrae at least in extension. 2. Device according to claim 1, characterized in that the preferred forward bending means comprise an anterior portion (24C) of said core (24) extending at a distance from walls (50) opposite said coating ( 26) so as to form at least one previous differential volume (V). 3. Device according to claim 1, characterized in that the preferred forward bending means comprises an anterior element (124 ') of said core (124), made of a first material more compressible than a second material constituting a rear member (12411) of said core. 4. Device according to one of claims 1 to 3, characterized in that said cage (18) comprises means (24A) pivoting for said bending forward. 5. Device according to claims 2 and 4 taken together, characterized in that the pivot means comprises an intermediate portion (24A) of the core (24) extending said front portion (24C), said intermediate portion (24A) being disposed in contact with the walls (50) of said coating (26). 6. Device according to claim 5, characterized in that said walls (50) surround said intermediate portion (24A) on a substantial portion of its perimeter. 7. Device according to one of claims 1 to 6, characterized in that said cage also has means (24B, bending of said cage rearwardly. 8. Device according to claim 7, characterized in that means (20) for damping extra-disc are also able to dampen a displacement in compression of said vertebrae (2, 2 '). 9. Device according to one of claims 1 to 8 characterized in that the length of the damping means (20) varies in extension and optionally in compression, asymptotically as a function of the force applied thereto. 10. Device according to one of claims 1 to characterized in that the damping means, vertebral displacement comprise at least two separate damping devices (120, 120 ') and in that it is further provided securing means (128) of said damping devices to one another at least in a vertical direction. 11. Device according to any one of the preceding claims, characterized in that said cage is provided means (46, 47) for reducing anteroposterior shear. 12. Device according to any one of the preceding claims, characterized in that said cage (18) is provided with means (50) for reducing lateral shear.