Method and apparatus for providing proper vertebral spacing

An expandable osteosynthesis implant has branches ( 5 ) each connected at one end to a seat ( 7 ) which is pierced by an orifice ( 8 ), suitable for being slid from a posterior direction between the facing faces of two consecutive vertebrae in order to hold them a given distance apart and restore stability of the spinal column. According to the invention, said branches ( 5 ) and said seat ( 7 ) define a hollow cage ( 1 ) which, in a “rest” position, has an outside general shape that is a cylinder of circular section, and a portion at least of the inside volume ( 9 ) of the cage ( 1 ) towards the distal ends of said branches ( 5 ) is in the form of a circular truncated cone whose large base is towards said seat ( 7 ), which implant has at least three branches ( 5 ) and, inside said inside volume ( 9 ) at least one spacer ( 2 ) suitable for passing through said orifice ( 8 ) and the large base of the truncated cone.

Whatever the embodiment, the expandable osteosynthesis implant comprises in conventional manner branches 5 , each connected at one end to a seat 7 pierced by an orifice 8 . According to an essential characteristic of the invention, said branches 5 and the seat 7 constitutes a hollow cage 1 which, in a “rest” position as shown for the embodiments of FIGS. 1, 3A , 4 A, and 5 A, and of FIGS. 9, 10 , and 11 , is of general outside shape that is cylindrical or quasi-cylindrical having a cross-section which is also the director curve of said cylinder that is circular or quasi-circular, with the generator line which engages said director curve and which generates the cylinder or quasi-cylinder by moving around its axis of symmetry XX′ being either a straight line or a convex circular arc of large radius: this provides either a circularly-symmetrical right cylinder as shown in solid lines 20 , in FIG. 3 A, or else a pseudo-cylinder referred to in the present specification as being “oval” or “ovoid”, i.e. being of slightly bulging outside shape, as shown in long and short dashed lines 20 2 in FIG. 3A . At least a portion of the inside volume 9 of the cage 1 towards the distal ends of the branches 5 is in the form of a truncated cone that is quasi-circularly symmetrical with its larger base being closer to said seat 7 , which implant has at least four branches 5 and at least one spacer 2 suitable for passing through said orifice 8 and via the large base of the truncated cone into said inside volume 9 . In FIGS. 2, 3B , 4 B, 5 B, and 3 C, i.e. in the “active” position, said end spacer 2 spreads apart said branches 5 said inside volume 9 then tending towards a circular cylinder while the outside shape of the cage 1 tends towards an approximate truncated cone. In FIG. 3 B, for example, solid lines 20 1 show the slightly concave shape obtained from a regular base cylinder in the rest position while short and long dashed lines 20 2 shows the straighter shape obtained from a cylinder that was initially ovoid, as shown in FIG. 3A . Once the spacer 2 has been placed in the desired position, no internal part that has been used for putting the implant and the spacers in place remains inside the space defined by said branches 5 , the spacer 2 , and the orifice 8 . Whatever the embodiment, at least a portion of the outside surface of said branches 5 is threaded with a thread profile 11 having projecting ridges, as shown in detail in FIG. 7 . In particular, by way of example, for a cage having a length L of about 20 mm to 25 mm, the length l of the threaded portion of the branches 5 may lie in the range 13 mm to 16 mm with a pitch p of 1.5 mm to 2 mm, the outside diameter D of the circular cylinder of the cage being 9 mm to 16 mm and the height of a tooth 11 1 of the thread being of the order of 0.7 mm to 0.9 mm for a thread angle &bgr; between successive teeth being about 60°, and with an inside profile 11 2 of the thread having a radius of about 0.4 mm. Said rounded shape of the profile minimizes stress concentrations, thus making it possible to withstand large forces and impacts. Such an outside thread with projecting ridges thus facilitates installation since it is not traumatizing, given that there is no impact shock while inserting said implant since it is screwed into a hole previously bored by any tool compatible with the orifice 8 of the implant, and once in place such a thread also provides anchoring in the bone, thereby avoiding any subsequent migration. The seat 7 may include at least two flats 6 that are parallel or slightly inclined relative to each other to fit the profile of vertebrae more closely, each disposed between two successive branches 5 as shown in the embodiments of FIGS. 1, 2 , 9 , 10 , and 11 . Alternatively, the seat 7 may have four flats forming a square or pseudo-square section, as shown in the embodiment of FIGS. 3, 4 , and 5 . In addition to the seat, the branches 5 themselves may also have respective flats at least in line with those of the seat, as shown in the embodiment of FIGS. 3 to 5 , particularly if the section of the implant is quasi-circular, or alternatively may have no flats at all, as in the embodiment of FIGS. 1, 2 and 9 to 11 . Such flats may be replaced or at least associated with a longitudinal implant section that is slightly oval or ovoid in shape, as mentioned above. The orifice 8 of the seat 7 can be threaded with a is thread profile 15 as shown by way of example in FIG. 6 , with a thread pitch that is rounded both at its ridges and in its furrows, and for association with the dimensions given above by way of example, an opening 8 may have an inside diameter d of 7 mm to 10 mm, a pitch p′ of 1 mm to 1.5 mm, a thread depth of about 0.6 mm, and a thread angle &ggr; between the walls of the thread of about 30°. A plug 3 for closing the inside volume 9 is then screwed into said orifice 8 , either to serve as an anchor point for an implant-installing rod, or else after the implant has been put into place and the branches have been splayed apart by the end spacer 2 for the purpose of closing the inside volume 9 in order firstly to stiffen the cage and secondly to prevent any bone matter that may have been implanted inside the cage from escaping via said posterior end. Such a plug 3 is shown in face view in FIG. 14 A and in side view in FIG. 14 B, having a thread 15 2 of the type shown in FIG. 6 and a central orifice 17 of polygonal shape, being square, hexagonal, etc., or replaced by any means suitable for securing therein the end of a rod having a compatible end for the purpose of enabling the plug to be screwed and unscrewed. In addition, it is possible to form on the posterior face of the implant and at the periphery of the orifice 8 in the seat 7 , studs or grooves serving to secure a portion of the “ancillary” installation appliance around the rod, enabling the cage 1 , spacers, and/or the plug 3 to be driven so as to fix more securely the positioning of the implant while it is being put into place and so as to facilitate dismantling of the ancillary appliance without moving the implant. In the embodiments shown with four branches 5 , said cage 1 has four slots 10 forming inter-branch spaces as shown in FIG. 8 looking along arrow VIII of FIG. 9 , for example. Such slots serve firstly to improve fusion of the bone graft that may be housed in the inside volume 9 with the adjacent intervertebral disks, and secondly to obtain better deformation of the branches 5 during installation of the implant, the branches in this particular portion being of section that is smaller than at their ends. In addition, such slots can be oblong in shape with their ends situated towards the distal ends of the branches 5 being narrower than their opposite ends, as shown in FIG. 8 , and terminating in a narrow slit 10 1 , between the distal ends of adjacent pairs of said branches. Such a shape, at least for the main slot 10 , makes it possible to have a slot with parallel edges, once the cage has been expanded. In addition, choosing a profile or this shape instead of an initial slot of constant width as shown in FIG. 3 A, makes it possible to increase the bearing surface area between the spacer 2 and the distal ends of the branches 5 of the cage, thus providing greater strength. In the embodiments of FIGS. 1, 2 , 9 , 10 , and 11 , the inside surfaces of the branches 5 defining the inside volume 9 of the cage 1 are smooth, so the volume then includes at its distal end an axial housing 12 suitable for receiving said spacer 2 2 as shown in FIG. 12 , and for holding it by means of a shoulder 13 of greater diameter than that of the inside volume 9 of the cage in its active position, as shown in FIG. 1 . Said spacer 2 2 has a threaded axial bore 14 suitable for receiving a rod whose end at least is likewise threaded and compatible for the purpose of putting the spacer into place merely by applying thrust and by being moved in translation, with said installation rod being subsequently removable. In FIGS. 1 and 2 , in order to avoid any risk of the body of the cage 1 deforming at its anterior end near the distal ends of the branches 5 under large stresses due to pressure from adjacent vertebrae, which could have the effect of moving branches towards one another by sliding around the spacer 2 , it is possible to provide the spacer with at least two splines 18 or other means that are disposed symmetrically about the axis XX′ of the implant. Such a spacer is either put into place inside the cage 1 before the implant is put into place by being engaged therein from the ends of the branches, or else the seat 7 has at least two compatible grooves 19 allowing the said two splines 18 to slide therealong when the spacer 2 is put into place if it needs to be inserted via the seat, the width e of the splines being no greater than the distance and the spacing between the distal ends of two adjacent branches 5 . The device may include additional systems so that the branches do not deform after they have been expanded, for example an outside ring received in a groove at the ends of the branches and deformable by the branches. In the embodiment shown in FIGS. 3, 4 , and 5 , the inside surfaces of the branches 5 defining the inside volume 9 of the cage 1 are threaded with a pitch equivalent to that of the orifice 8 in the seat 7 , e.g. as shown in FIG. 6 , and said spacer 2 1 , is threaded in compatible manner, as shown in FIGS. 13A and 13B . In this embodiment with an inside thread, said cage 1 may also include at least one other intermediate spacer 4 threaded like the end spacer 2 1 and capable of being screwed behind it in order firstly to compress the bone matter that may have been inserted in the space defined between the two spacers, and secondly to stiffen the central portion of the cage. Said end and intermediate spacers 2 1 and 4 have respective polygonal axial orifices 16 suitable for receiving a removable rod having a male end of compatible shape so as to drive them while they are being installed. The frustoconical shape of the inside volume 9 of the cage 1 may have a half-angle of slope &agr; lying in the range 6°to 9°, for example.