Intervertebral implant whereof the parts can be spaced

English translation of the International Patent Application PCT/CH97/00293 "lntervertebral implant whereof parts can be spaced" in the name of Synthes AG Chur (Aebi et al.) INTERVERTEBRAL IMPLANT The present invention relates to an intervertebral implant. Such an intervertebral implant is described in European patent document A1 0,664,994. The drawback of this device is that the implant is integral with the expansion unit, and thereby can be inserted only as a unit into the intervertebral space, so that insertion must be by knocking the implant into its site. Moreover the prior intervertebral implant can be expanded only discretely, whereby the pre-surgical planning of the expansion angle can be carried out only inexactly. The objective of the invention is palliation. The invention addresses the problem of creating an intervertebral implant which in a first stage and in the absence of an expansion unit can be screwed into the intervertebral space and shall be expanded only in a second stage using an expansion unit insertable into the intervertebral space. This expansion shall be continuous and allow monitoring, whereby the expansion angle can be planned prior to surgery. According to the invention there is provided an intervertebral implant in the form of a support unit including a central yoke and legs integral with said yoke, said legs being configured in two essentially mutually parallel planes apart by the yoke and being abuttable as support surfaces against adjacent vertebrae and the yoke includes a guide cylinder running essentially parallel to and between the legs and including a thread, wherein A) the guide cylinder is configured rotationally stable at the central yoke; and B) the guide cylinder is configured hollow cylindrically at its end opposite to the yoke. The implant's guide cylinder is fitted with either an inside thread or an outside thread, or with both. The inside thread allows expanding the implant using an expansion unit comprising a spindle fitted with a thread matching the said inside thread. The guide cylinder's outside thread also allows expanding the implant when using an expansion unit fitted with a hollow-cylinder segment having a thread matching the said outer thread. Because of the larger diameter, the latter variation allows transmitting higher tensile forces. The preferred embodiment of the guide cylinder fitted with both an inside and an outside thread offers the additional advantage that surgery can be carried out in guided manner. A guide spindle affixed in the inside thread and over which the other instruments may be displaced can be used as an accessory guide element during all of surgery. Essentially surgery takes place in three stages: screwing-in the implant using a threaded case, filling the implant with bone chips, expanding the implant in the intervertebral space against the end plates. An additional stage is conceivable, namely to inject bone-growth stimulating materials in liquid form or as gel into the intervertebral space. Perforated screw-in, or bone-chipping, insertion instruments may be used for such a purpose. In a further preferred embodiment of the invention, the legs' outsides are fitted with guide ribs substantially running parallel to the guide cylinder to facilitate inserting the implant into the intervertebral space. Moreover the legs also may be fitted with perforations to enhance bone growth into them. For the same purpose, two legs also may be present in the two planes of the U-shaped support unit, the legs being affixed to the ends of the yoke and hence a free space being subtended in each plane between the two legs to allow bone growth into it. Preferably the implant yoke is mounted symmetrically to the legs. In a further preferred embodiment, the insides of the legs are fitted with grooves running transversely to the guide cylinder. This feature offers the further advantage that a defined angle (already specified before surgery) can be set when expanding the intervertebral implant. In another preferred embodiment of the invention, the intervertebral implant's legs comprise inward-pointing lips at their free front ends to prevent the bone chips filled into the implant from slipping out. Essentially the advantage of the invention are that thanks to its intervertebral implant: -- the patients natural lordotic spine radius can be restored to the patient and this angle can be determined before surgery; as a result optimal conditions for the ingrowth of the implant are present, -- an atraumatic insertion procedure is made possible, namely the implant, instead of being knocked into position, can be screwed-in gently and in monitored manner, and -- simplified and minimally invasive surgery is henceforth feasible, that is, a three-stage surgical technique being controlled by means of a guide spindle. The invention and its further developments are elucidated below in relation to several illustratively embodiments partly shown in diagrammatic form. Fig. 1 is a perspective of the intervertebral implant of the invention, Fig. 2 is a threaded bush for the intervertebral implant of Fig. 1, Fig. 3 is a perspective of an expansion unit with nut for the intervertebral implant of Fig. 1, Fig. 4 is a perspective of a modified expansion unit with screw for the intervertebral implant of Fig. 1, Fig. 5 is a section of the expansion unit of the invention with insertion of the threaded bush, Fig. 6 is a perspective of a modified threaded bush and hollow, cylindrical shank, Fig. 7 is a section of the intervertebral implant with the bush of Fig. 6 in the inserted position, Fig. 8 is a section of the intervertebral implant of the invention with a partly inserted bone-chip impactor to fill the support unit with bone chips, Fig. 9 is a partial perspective of the bone-chip impactor of Fig. 8, Fig. 10 is a cross-section of an intervertebral implant of Fig. 1 with an inserted expansion unit of Fig. 3 being screwed by an insertion instrument slipped over the guide spindle into the support unit, Fig. 11 is a partial perspective of the insertion instrument of Fig. 10, Fig. 12 is a longitudinal section of a modified intervertebral implant of the invention having a shortened guide cylinder and an expansion unit of Fig. 4 screwed into it, and Fig. 13 is a front view of the intervertebral implant of Fig. 12 with screwed-in expansion unit. The intervertebral implant shown in Fig. 1 assumes the shape of a support unit 1 comprising a central yoke 2 and legs 3, 4 integral with this yoke 2. The legs 3, 4 are configured in two substantially mutually parallel planes 5, 6 spaced apart by the yoke 2 relative to which they are symmetrical, said planes 5, 6 being abuttable as support surfaces against adjacent vertebrae. The yoke 2 comprises a hollow guide cylinder 7 fitted with an inside thread 8 and an outside thread 28 and situated between and essentially parallel to the legs 3, 4. The inside surfaces of 25 of the legs 3, 4 are fitted with grooves 30 running transversely to the guide cylinder 7. The outside surfaces 24 of the legs 3, 4 are fitted with guide ribs 12 running essentially parallel to the guide cylinder 7. Moreover the legs 3, 4 prise perforations 22 and an inward-pointing lip 29 at their free front ends. A pair of legs 3, 4 is present in each plane 5, 6 of the U-shaped support unit 1, situated at the ends of the yoke 2 to subtend a free space 23 in each plane 5, 6. Fig. 2 shows a bush 9 with an outer thread 10, a cavity 13 and a hexagonal socket 34, said bush being slippable onto the guide cylinder 7 of the support unit 1. Preferably the bush diameter is slightly larger than the height of the support unit 1 (spacing between planes 5 and 6). The bush 9 inserted into the support unit 1 can be rotated about the guide cylinder 7 using an appropriate instrument inserted into the hexagonal socket 34 and is used to insert the support unit 1 into the intervertebral space. Because the bush 9 slightly projects beyond the outsides 24 of the legs 3, 4, part of its outer thread 10 engages the bone material of the adjacent vertebra, and thereby the support unit 1 is easily rotated into the intervertebral space. Once the support 1 has been inserted into the intervertebral space, the bush 9 also is easily screwed out of and away from the guide cylinder 7. Figs. 3 and 4 show two embodiment variants for an expansion unit 11 expanding the support unit 1, each comprising an identical dilator element 18. The dilator element 18 assumes the shape of a right parallelipiped having a central borehole 19 and is fitted with two side cams 21 insertable between the legs 3, 4. The dilator element 18 furthermore comprises two lateral slots 55 to seize and handle the dilator element 18. The first variation of the expansion unit 11 is shown in Fig. 3 and comprises moreover a hollow, cylindrical nut 26 with an inside thread 27 matching the outside thread 28 of the guide cylinder 7. The geometry of the borehole 19 is such that the nut 26 can enter it as far as the stop The second.embodiment variation of the expansion unit 11 is shown in Fig. 4 and instead of the nut 26 comprises a screw 14 with a head 16 fitted with a hexagonal socket 44 and a shank 20 fitted with an outer thread 15. The outer thread 15 matches the inner thread 8 of the guide cylinder 7. Similarly to the case of the nut 26 of Fig. 3, the head 16 of the screw 14 will abut the stop 45 within the central borehole 19. When using the second embodiment variation of the expansion unit 11 shown in Fig. 4, the guide cylinder 7 must be shortened in the manner shown in Fig. 12, that is, it must be approximately half the length of that shown in Fig. 1. Operation of intervertebral implant of the invention is discussed in further detail below in relation to Figs. 5 through 13. Upon removal of enough intervertebral-disk material, the U-shaped support unit 1 is inserted between the pertinent vertebrae. For that purpose and as shown in Fig. 5, bush 9 fitted with outer thread 10 is slipped over the guide cylinder 7 of said support ,1. The U-shaped support unit 1 together with the slipped-on bush 9 is lightly forced into prepared intervertebral space until the first turn of the outer thread 10 enters the bone material. Thereupon, by further rotating the bush 9, the U-shaped support unit 1 is screwed to the desired depth into the intervertebral space. The rotation of the bush 9 may be implemented by a tool inserted into the hexagonal socket 34. In a variation shown in Figs. 6 and 7, the bush 9 fitted with the outer thread 10 is affixed to a hollow, cylindrical shank 46 and forms a part of an insertion implement. For better guidance of the insertion implement, a guide spindle 31 (Fig. 7) fitted at its front end with an outer thread 32 (Fig. 10) can be previously screwed into the inside thread 8 of the guide cylinder 7 of the support unit 1 and can remain there during the full time of surgery. The bush 9 of Fig. 6 then can be easily slipped by its hollow, cylindrical shank 46 over the guide spindle 31 onto the guide cylinder 7 and be rotated when on latter. The guide ribs 12 present on the outer sides 24 of the legs 3, 4 of the support unit 1 prevent said support unit 1 from deviating off the desired direction during this screw-in procedure. Following screwing-in, the bush 9 is screwed-out of the support unit 1. Practically no friction existing between the surface of the guide cylinder 7 and the inside surface of the bush 9, the support unit 1 remains in place while the bush 9 is withdrawn. Optionally and as shown in Fig. 8, bone chips 47 may be introduced into the free space 23 of the support unity 1. For that purpose and using a bone-chip impactor 33, bone chips 47 are introduced into the free space 23 and compressed therein. The bone-chip impactor 33 shown in Fig. 8 consists of a flat base 48 which, as shown in Fig. 8, can be partly inserted between the legs 3, 4 and is affixed to a hollow, cylindrical shank 49. The bone-chip impactor 33 also can be axially guided on the guide spindle 31. Lastly, as shown in Fig. 10, the expansion unit 11 is introduced between the planes 5, 6 and legs 3, 4 configured therein of the support unit 1. Then the expansion unit 11 of Fig. 3 is inserted in its assembled state (that is with the nut 26 inserted into the borehole 19) by means of the insertion implement 40 and its two cams 21 between the legs 3, 4. The front end of the insertion implement 40 is shown in detailed manner in Fig. 11. At its front end, the rotatable bush 41 comprises four drive cams 50 for the nut 26; / r matching longitudinal grooves 56 are present for that purpose in the nut 26 to I it its rotation. The rotatable bush 41 is supported inside the holding stub 52 having a knurled ring 53 and is fitted at its front end with two holding pins 54 insertab!e into the lateral slots 55 (Fig. 3) of the cams 2! in order to hold the dilator element 18. When using an expansion unit 1! as shown in Fig. 4, which aside the identical dilator element !8 comprises a screw 14 (in lieu of the nut 26), the shank 20 of the screw !4 is inserted by its thread !5 into the inside thread 8 of the hollow guide cylinder 7 (Figs. !2 and !3). By screwing the screw !4 into the guide cylinder 7, the stop 45 for the screw head !6 in the borehole 19 wil! cause axial displacement of the dilator element !8 toward the guide cylinder 7, the cams 2! affixed to said element !8 entering the space between the !egs 3, 4 and spreading apart said !egs. By a continuous inward rotation of the screw 14, the legs 3, 4 can be spread apart so much that their planes 5, 6 subtend an angle cc of 10° to 12°. Said inward rotation of the screw 14 can be carried out using a tool inserted into the hexagonal socket 44 in the head 16.