FR2742044A1 - Spinal inter vertebral bone prosthesis - Google Patents

Abstract

The prostheses has a cage formed on a surface of rotation and which can be screwed between two vertebral plates to penetrate into the bone substance. The cage has, at one end, a thread (8) and also has a central cut-out (9) which opens onto opposite sides to receive a bone graft. The cut out allows osseous fusion with adjacent vertebral tissue. The threads can be interrupted to produce a self cutting action. The surface of revolution has a domed surface over at least part of its length.

Description

 Vertebral intersomatic arthrodesis prosthesis.

 The present invention relates to vertebral intersomatic arthrodesis prostheses.

 These prostheses are intended to compensate for the effects due to trauma or degeneration of the intervertebral disc which leads to a pinching or a reduction of the intersomatic disc space leading to repercussions such as, in particular, compression of the nerve roots with its known consequences.

 In other words, the aim of these prostheses is to restore a suitable disc space and, in the impossibility, generally, to restore joint mobility on the prosthesis, to achieve bone fusion of the two vertebral bodies.

 We already know a certain number of these prostheses, some of which are marketed, which are in the form of cards intended to be inserted between two vertebral bodies, these cards being, generally, associated in pairs and most generally placed by posterior route. .

All these prostheses, often called intersomatic cages, have a certain number of characteristics which derive from the teaching of patent US-A3 848 601; This patent provides for practicing, in the two vertebral plates opposite and through the disc space, a channel of constant rectangular section in which is inserted a bone graft of corresponding shape. These common characteristics are the following
- cylindrical shape of constant section of the plug, with an anterior chamfer to facilitate insertion into the previously perforated cylindrical channel;
- presence of roughness or surface roughness to avoid ejection of the prosthesis
- hollow form or comprising slots or grooves to receive a bone graft, thus giving the plug the appearance of a cage;
- Presence, at the rear end, of temporary fixing means of an ancillary installation tool.

 Such implants are described in particular in patents US-A-4,501,269 and EP-A-O 307,241.

 Another type of prosthesis is based on the teaching of patent DE-A-3 505 567, which describes a bone implant of cylindrical shape and having a peripheral thread allowing its screwing in a space formed between the two adjacent vertebrae. Such implants, also made of a resistant material, have a cylindrical shape of revolution, a thread serving to ensure fixing by screwing and to prevent removal, slots or passages for receiving a bone graft or promoting bone regrowth, and , at one end, means for temporarily receiving the end of an ancillary tool for laying by screwing. Such implants are described for example in US-A-5,015,247.

 Although of different conceptions, the two types of implants mentioned above assume that they must necessarily have a surface of constant section, that is to say a cylindrical surface, extending transversely to the longitudinal axis of the spine, so as to provide the stability necessary for optimal bone consolidation.

 The present invention proposes to provide a new prosthesis in the form of plug-in cages, in particular in the form of a plurality of cages, preferably two cages, for vertebral intersomatic arthrodesis, making it possible to improve bone synthesis and consolidation.

 Another object of the invention is to provide such a prosthesis having increased stability minimizing the risks of expulsion or displacement of the prosthesis.

 Yet another objective of the invention is to produce such a prosthesis in the form of cages which are very easy to implant and to extract.

 The subject of the invention is a vertebral intersomatic arthrodesis prosthesis, in the form of a cage or plug inscribed in a surface of revolution and capable of being screwed between two vertebral plates facing each other while penetrating into the bone substance, and having, for this purpose, a thread, said cage also having at least one central recess open on opposite sides, so as to be able to receive a bone graft and allow bone fusion with the tissue of the adjacent vertebrae and to interrupt the threads for the make self-tapping, characterized in that said surface of revolution has a domed shape over at least part of its length.

 Thus at least one of the ends of the prosthesis, and in particular the anterior end, has a relatively tapered shape.

 In a particularly preferred embodiment, the front part of the curved cage is of smaller diameter than that of the rear part, so that the curved surface extends from an underlying frustoconical geometric surface. This arrangement also has the advantage of facilitating the reconstruction of a normal lumbar lordosis.

 The angle at the top of such a cone trunk is preferably between 3 and 70, and in particular 5.

 Where appropriate, the anterior and / or posterior ends of the plug cage may not be curved, being formed from a generatrix, preferably rectilinear, which then forms an anterior and / or posterior end of conical shape.

 The radius of curvature of the curved surface is preferably between 20 and 35 mm, and, in particular, of the order of 25 mm.

 The center of curvature is preferably spaced laterally from the axis of revolution of the cage by a distance of the order of 19 to 22 mm, depending on the size of the implant.

 Preferably, the cage of the prosthesis according to the invention has a single thread whose pitch allows easy screwing between the two adjacent vertebral bodies. The threads may preferably be triangular, symmetrical or not.

 The depth of the threads can be constant, so that the bottom of the threads are on a curved geometric surface parallel to the curved surface coinciding with the vertices of the threads. However, it is preferred that the depth of the threads be variable, so that their bottoms are on a geometric surface parallel to said surface, preferably frustoconical, and offset towards the axis of revolution of the cage.

 Preferably the depth of the threads is between 1 and 3 mm, and in particular, on average, 2 mm.

 The thread pitch is preferably between 1 and 3 mm.

 The hollow central part, or recess, of a cage opens by diametrically opposite openings preferably extending over a substantial length of the cage, so that the line of intersection between the surface of revolution, in which is inscribed the cage, and the surface generating the central recess, a line remaining curved.

 Preferably, this recess leads to longitudinal grooves extending over the entire length of the cage, all of whose threads are then self-tapping.

 It is also possible to provide transverse holes or openings connecting said central recess and the lateral surfaces, to allow lateral bone diffusion.

 Conventionally, the cage according to the invention may preferably comprise, at its rear end, means for temporarily receiving a fitting and screwing tool.

 The prosthesis cage according to the invention can be made of any material usually used for implants of this kind, for example metal or a synthetic material, preferably radiolucent.

The surface can be treated, for example with fine roughness or even be covered with a material facilitating bone growth, for example calcium hydroxyapatite.

 Indeed, thanks to a coating of this biomaterial on the external surface of the cup, it becomes possible, after having carried out a primary fixing by the mechanical effect of the threads, to carry out a "secondary" fixing of physicochemical nature, which reinforces the first fixation in the first months following the operation. This fixation is obtained by adsorption of the various constituents of the bone tissue on the deposit of calcium hydroxyapatite covering the external face of the cup.

 A fixation is thus carried out in two stages, first mechanical by the threads, the shape of the cage and the roughness of the hydroxyapatite coating, then physicochemical by adsorption, a few months later, of the bone constituents on the hydroxyapatite.

 For the installation of the prosthesis, the surgeon, after having practiced his approach first, preferably posterior, and, in this case, laterally separated the corresponding section of the neural axis, presents the cage in the form of a plug by its anterior end in the the intervertebral space then begins to screw the cage in a sagittal plane through the intervertebral space. The threads bite quickly on the cortices of the vertebral plates which face each other and gradually, the passage of the tapered anterior end of the cage causes the vertebral plates to move apart while the cage advances by coming, by its points of net, in contact with the cancellous bone of the vertebral bodies through the cortex. Once the cage is in place, its domed shape, at its lower and respectively upper parts, adapts substantially to the frankly concave shapes of the plateau of the upper vertebra and slightly concave of the plateau of the lower vertebra, which facilitates the distribution of forces and ensures good stability. Better bone tissue development and good cooperation with the bone graft previously introduced into the hollowed-out central part of the cage are also obtained, leading to good arthrodesis. Furthermore, an accidental withdrawal of the cage is considerably reduced, due to the fact that the central bulge of the plug prevents its extraction.

 Finally, contrary to the fears expressed in the prior art, which have led to the search for systematically cylindrical shapes to avoid the risks of bending movement of the spine on either side of the implanted plug, excellent stability of the junction is obtained. level of the implanted plug, including when trying to reduce lordosis.

Other advantages and characteristics of the invention will appear on reading the following description, given by way of nonlimiting example referring to the appended drawing in which
- Figure 1 shows a perspective view of a prosthesis cage according to the invention;
- Figure 2 shows an elevational view of the cage;
- Figure 3 shows a top view of the cage
- Figure 4 shows a front view of the cage
- Figure 5 shows a rear view of the cage.

 The cage shown in the drawing is made, for example of titanium, but it can however be made of any other biocompatible material of suitable strength. It has the shape of a curved peg or cage inscribed in a surface of revolution in the form of an elongated keg. The front face 1 of the front end 2 of the cage is flat and, from the end, the diameter of the cage gradually increases, giving the front part of the cage a tapered appearance. The diameter becomes maximum in the central part of the cage and then begins to decrease in the direction of the rear end 3, in the front face 4 of which is formed a recess intended to receive the end of a screwing tool, with a blind hole 5 and a diametrical groove 6. The shapes of this hole and of this groove are conventional and need not be described further.

 In the example shown, it can be seen, in particular in FIG. 2, that the anterior 2 and posterior 3 ends are located on a geometric, that is to say fictitious, truncated cone, 7, shown in broken lines, so that the curved portion extends radially outward from this underlying surface 7. It is understood, moreover, that towards the rear end 3, the curved part is less tapered than towards the rear end 3, the curved part is less tapered than towards the front end 2. The angle at the top of this cone in this example, geometric is of the order of 5.

 Furthermore, it can be seen that the anterior 2 and posterior 3 ends are conical and external to the convex part of the cage, but it is understood that in another embodiment, the convex shape can extend as far as the front front faces 1 and / or rear 4.

 The cage has, over practically its entire length, a thread 8, having a pitch preferably of the order of 2 mm and threads of progressively increasing diameter from the end towards the center, these threads having a triangular section with an inclined face towards the front and towards the center and a rear face situated substantially in a diametral plane.

 It can be seen in FIG. 2 that the thread bottoms can advantageously be arranged on a frustoconical surface parallel to the surface 7, and offset with respect to the latter, towards the longitudinal axis of revolution of the cage.

 In the central part of the cage, a diametrical recess 9 is produced, generated by an oblong cylindrical surface whose generator is clearly seen in FIG. 3. This obviously, which crosses the cage right through, comes to cut the curved surface into shape of keg, in which is inscribed the cage, according to an advantageously curved line.

 The central oblong recess 9 opens at each of its ends, in a groove 10, the cross section of which is clearly seen in FIGS. 4 and 5. These grooves delimit, at the front and at the rear of the prosthesis, 11, 12. They interrupt the threads of the thread 8 which thus become self-tapping. In the example described, the bottom of each groove, materialized by the areas 11, 12, is planar, but this bottom could have any other shape.

 It is also possible not to provide grooves, so that only the threads intersected by the recess 9 would remain self-tapping.

 Advantageously, transverse diametrical holes 13 can pass through the cage right through, in directions perpendicular to the wall of the central recess 9, thus allowing transverse bone progression, once the recess 9 has been brought into its final sagital position by rotation of the implant.

 We now understand that the cage has a shape which is anatomically close to that of a normal disc, seen in silhouette in the vertical sagittal plane.

 Although the invention has been described with the aid of a particular embodiment, it is understood that it is in no way limited thereto and that it can make various modifications of shape or material.

Claims (17)

 1. Vertebral intersomatic arthrodesis prosthesis, in the form of a cage inscribed in a surface of revolution and capable of being screwed between two vertebral plates facing each other while penetrating into the bone substance, and having, for this purpose, a thread (8 ), said cage also having at least one central recess (9) open on opposite sides, so as to be able to receive a bone graft and allow bone fusion with the tissue of the adjacent vertebrae and to interrupt the threads to make them self-tapping , characterized in that said surface of revolution has a domed shape at least over part of its length.  2. Prosthesis according to claim 1, characterized in that one of its ends has a tapered shape.  3. Prosthesis according to claim 2, characterized in that the front part (1) of the curved cage is of smaller diameter than that of the rear part (2), so that said curved surface extends from an underlying geometric surface. frustoconical (7).  4. Prosthesis according to claim 3, characterized in that the angle at the top of said conical surface (7) is between 3 and 7, and in particular of the order of 5.  5. Prosthesis according to one of claims 1 to 4, characterized in that the radius of curvature of said bomb surface is between 20 and 35 mm, and in particular of the order of 25 mm.  6. Prosthesis according to one of claims 1 to 5, characterized in that the center of curvature is offset from the geometric axis of the cage by a distance of 19 to 22 mm.  7. Prosthesis according to one of claims 2 to 6, characterized in that the anterior (2) and / or posterior (3) ends of the cage are not curved.  8. Prosthesis according to claim 7, characterized in that said ends are frustoconical and aligned on said frustoconical surface (7).  9. Prosthesis according to one of claims 1 to 8, characterized in that the cage of the prosthesis has a single thread (8) whose pitch allows easy screwing between the two adjacent vertebral bodies.  10. Prosthesis according to one of claims 1 to 9, characterized in that the depth of the threads is between 1 and 3 mm, and in particular 2 mm.  11. Prosthesis according to one of claims 1 to 10, characterized in that the thread pitch (8) is between 1 and 3 mm.  12. Prosthesis according to claim 10, characterized in that the depth of the threads is variable and that the thread bottoms are on a frustoconical surface, coinciding with, or centrally offset from said frustoconical geometric surface (7).  13. Prosthesis according to one of claims 1 to 12, characterized in that the hollow central part, or recess (9), of a cage opens by diametrically opposite openings extending over a substantial length of the cage .  14. Prosthesis according to claim 13, characterized in that this recess (9) opens onto longitudinal grooves (10) extending throughout the length of the cage, all of whose threads are then self-tapping.  15. Prosthesis according to one of claims 1 to 14, characterized in that the cage has transverse holes (13) communicating with said central recess (9) for a transverse progression of bone growth.  16. Prosthesis according to one of claims 1 to 15, characterized in that its surface is rough and / or covered with a coating of calcium hydroxyapatite.  17. Prosthesis according to any one of claims 1 to 16, characterized in that the cage comprises, at its rear end, means (5,6) for temporary reception of a fitting and screwing tool.