Intersomatic cage with unified grafts

[0001] The present invention relates to an intervertebral cage designed to be placed between two vertebrae to restore and/or maintain the intervertebral space, to replace the intervertebral disk. After the cage or implant has been placed, the intervertebral space is filled with autologous spongy bone or adapted bone substitutes. This invention also relates to instrumentation for placement of the intervertebral cage. Finally, it also relates to a pair of intervertebral cages of this type.

[0002] Prior art has already described intervertebral cages, for example Stryker cages. These cages are parallelepiped-shaped, the bottom and the top of the cage being completely open, the side and top openings will be placed facing the two vertebrae that are to be kept at a distance from each other. Spongy bone is then compacted inside the cage to finally cause fusion of the bone (or arthrodesis) of the two vertebrae separated by a suitable disk space.

[0003] For posterior placement, there are usually two cages adjacent to each other at a distance from each other in the intervertebral space, and the graft is firstly compacted inside the two cages and secondly the space between the two cages is filled in by spongy bone or bone substitute.

[0004] It is found that the growths of the three independent grafts (two inside the cages and one between the cages) vary with respect to each other. In particular, the graft placed between the cages contributes more quickly to the fusion than the compacted grafts inside the cages. Obviously, this is a disadvantage, particularly with regard to the stability of the grafts and the two vertebrae held in place by these grafts, and concerning the time necessary to obtain good stability.

[0005] Document WO/98/55052 also discloses intervertebral cages.

[0006] In the first case, they are composed of a main very thick curved part particularly with a thickness that increases from the middle towards the ends. Therefore, these cages are complicated to make due to their curved shape.

[0007] In the second case, they are composed of a straight main part and return parts each being prolonged by an end part that together form a small interval less than 25% of the longitudinal extension of the internal housing of the cage.

[0008] The present invention overcomes the disadvantages mentioned above by proposing an intervertebral cage that enables good growth of the graft or grafts placed between the vertebrae and is each to make, particularly due to the fact that it has straight longitudinal walls, and has good resistance to compression forces applied by the vertebral plates, particularly in the case of posterior implantation in which two intervertebral cages are placed side by side at a distance from each other.

[0009] According to the invention, the intervertebral cage is composed of a ring, with a thin main wall prolonged by two return parts, the thin main wall having an approximately constant thickness and each return part being extended by an end part, the end parts facing each other with a gap between them, the wall, the return parts and the end part delimiting an inner cavity, the extension of the clear gap parallel to the straight wall being between 50% and 100% of the largest dimension of the cavity parallel to the wall, and preferably between 70% and 90%.

[0010] Thus, according to the invention, the intervertebral cage comprises only three walls, namely the straight main wall and the two return parts prolonged by the end parts, such that the graft that was compacted on the inside of each cage before the cage was placed (in the case of posterior placement in which two cages are used arranged side by side and at a spacing from each other), and the graft that is made to grow between the two cages are all three in mutual contact, such that there is actually only a single graft which will therefore grow homogeneously to obtain an optimum contact between the vertebral plategraftvertebral plate, in other words a good homogeneous and stable contact, also making the cage more resistant even though it only has one straight main wall and a large gap between the end parts.

[0011] The applicant realized before anyone else that in the case in which there is a main part in the form of a straight wall, it is still possible to significantly move the end parts away from the return parts without endangering the stability and the strength of the intervertebral cages between the vertebral plates. The applicant believes that the fact that the central graft and the compacted grafts in the internal parts of the two cages arranged with their openings facing each other fuse together well and helps to make the two cages more resistant to the tension forces exerted by the vertebral plates, and it is therefore possible to increase the air gap between the end parts, although the main part is only a thin single straight wall; but this is only a hypothesis that the applicant cannot confirm.

[0012] According to one preferred embodiment of the invention, the thin main wall is a straight wall.

[0013] According to one advantageous embodiment, the thickness of the end parts is the same as the thickness of the thin main wall.

[0014] According to one advantageous embodiment, the thickness of the thin main wall is between 1 mm and 5 mm and preferably between 2 mm and 4 mm, for example 3 mm.

[0015] According to one advantageous embodiment, the end part that projects from the largest return part is provided with at least one hole, and preferably two holes on the front face facing the other end part, and the other return part comprises a threaded hole on its part external to the ring, these holes being designed to cooperate with pins associated with placement instrumentation for placement of the implant or the intervertebral cage in the intervertebral space.

[0016] The present invention also relates to instrumentation for placement of an intervertebral cage, particularly by posterior placement.

[0017] According to the invention, the instrumentation for placement of an intervertebral cage between two vertebrae in the disk space comprises a rod that will cooperate with associated coupling means formed in an outer surface of the intervertebral cage.

[0018] According to one improvement of the invention, the instrumentation comprises a prolonged part in the form of a rectangular or slightly curved wall, on the distal end of the rod, the dimensions and position of which are such that the wall closes off the clear gap between the two end parts, when the rod is in a position to cooperate with the coupling means.

[0019] The rectangular wall replaces the missing wall of the cage during placement, firstly to retain the graft that was compacted inside the cage before the cage was put into position, and secondly to make the assembly stiffer to facilitate its placement in the intervertebral space.

[0020] For example, coupling means are composed of a hole formed in the outer surface of the cage, the hole being sized such that the rod can be inserted inside it.

[0021] According to the invention, it is also possible to use blocking means that will block mutual rotation of the cage and the instrumentation in the position in which the rod and the coupling means cooperate with each other.

[0022] For example, these blocking means may be composed such that the shape of the periphery of the hole is made hexagonal or to have at least one flat, and one end of the rod has a matching shape.

[0023] According to one improvement of the invention, attachment means designed to fix the rod to the coupling means may also be provided. These attachment means, which in particular can be released, are used to insert the cage such that it cannot accidentally stop cooperating with the rod.

[0024] For example, these attachment means may consist of tapping the hole and providing an associated thread at the distal end of the rod. In this case, the tapping in the hole and the thread on the rod also form blocking means.

[0025] According to one improvement to the invention, the outside distal edge of the wall comprises two pins that can penetrate into the holes formed in the face on the end part of the cage.

[0026] According to the invention, the wall is offset with respect to the centre line of the main rod, particularly by a distance equivalent to half of the dimension of the return part at the rod end.

[0027] The invention also relates to a pair of intervertebral cages according to the invention, in particular the two intervertebral cages of the pair being symmetric with each other like in a mirror.

[0028] The drawings, given solely as examples of one embodiment of an intervertebral cage according to the invention and instrumentation used for placement in the intervertebral space, are described.

[0029] In the drawings:

[0030] FIG. 1 is a perspective view of an intervertebral cage according to the invention;

[0031] FIG. 2 is a longitudinal sectional view of FIG. 1,

[0032] FIG. 3 is a top view of the intervertebral cage in FIG. 1,

[0033] FIG. 4 shows instrumentation for placement of an intervertebral cage according to the invention, also comprising the intervertebral cage in FIG. 1,

[0034] FIG. 5 is an enlarged view of part of FIG. 4.

[0035] FIG. 1 shows in intervertebral cage 1 according to the invention The intervertebral cage comprises a main wall 2 from which two return parts 3 and 4 project. The two return parts 3 and 4 are prolonged by a first end part 5 and a second end part 6 respectively The two end parts 5 and 6 are approximately parallel to the main wall 2. The wall 2, the return parts 3 and 4 and the end parts 5, 6 delimit an inner cavity 30 in the cage, by prolonging the two end parts towards each other.

[0036] The two end parts 5, 6, are at a distance from each other and there is a clear gap between them. The clear gap extends along the direction of the length parallel to the main wall 2 over a length equal to 78% of the longest length of the inner cavity 30.

[0037] The two end parts 5, 6 face each other through a first end face 7 and a second end face 8. The main wall 2 is 3 mm thick. The thickness of the end walls is the same as the thickness of the main wall 2. The two return parts 3 and 4 are approximately perpendicular to the main wall 2. The two end parts 5 and 6 extend from the return parts 3 and 4 that are approximately perpendicular to the end parts, parallel to the main wall 2, such that the shape of the ring 1 forming the intervertebral cage is approximately like a C with a straight back in the cross-sectional plane (see FIG. 2).

[0038] The upper edge 9 and the lower edge 10 of the ring forming the intervertebral cage have a toothed shape 11. These teeth or notches give good anchorage of the cage in the vertebral plates.

[0039] Two holes 12 are formed in face 8, facing the end part 5. A tapped hole 13 is formed in the outside edge of the return part 4. These holes 12 and 13 are designed to cooperate with the pins of an instrument 14 for the placement of an intervertebral cage of this type between the two vertebrae, particularly by posterior placement. In particular, this instrumentation 14 is shown in FIGS. 4 to 6.

[0040] It comprises a hollow rod 15 that is extended at its end 16 by a part 17 in the form of a wedge comprising a surface 18 perpendicular to the centre line of the rod 15 and a wall 19 forming an angle of about 75 from the surface 18. The shape of the wall 19 is rectangular. The angle between the wall 19 and the surface 18 depends on the extended length of the clear gap. It is designed such that the wall 19 completely fills the opening between the two end parts 5 and 6.

[0041] On its edge opposite the rod 15, the plate or wall 19 comprises two pins designed to cooperate with holes formed in the face 8 of the intervertebral cage 1. The instrumentation 14 also comprises a threaded rod (not shown in the figures) that will cooperate with the tapped hole formed in the outside part of the return part 4 of the intervertebral cage. This threaded rod extends inside the rod 15 and opens up at the end of this rod 15, at the surface 18 through a hole formed in it. The user screws and unscrews the threaded rod using the manoeuvring handle 22. Screwing the rod in firstly blocks the rod and the cage in rotation, and secondly fixes the rod to the cage.

[0042] The length of the plate 19 is approximately equivalent to the length of the main wall 2, such that when the threaded rod is screwed into the tapped hole 13, pins (not shown in the figures) penetrate into the holes 12. It is thus possible to insert the intervertebral cage between two vertebrae and then to remove the instrumentation. The two sides 7 and 8 facing the end parts 5 and 6 are offset from each other by a dimension corresponding to the thickness of the plate 19.

[0043] The wall 19 replaces the missing wall of the cage during placement, firstly to fix the position of the graft that was compacted in the cage before its placement, and secondly to make the assembly stiffer to facilitate its placement in the intervertebral space.

[0044] Finally, the cage may be bevelled in the longitudinal direction (see FIG. 3). This bevelled shape makes it possible to restore the lordosis.