It is known that a damaged intervertebral disk can be removed, and that the resulting space can be filled with cortico-spongious bone.
In this method, the vertebral bodies are first stretched apart as much as possible by means of spreaders. A special technique consists in inserting wedge-shaped elements--so-called dilators--between the two vertebral bodies, so as to spread them apart step-by-step. Dilators, each with a 1 mm larger diameter, in alternation left and right, are here attached posteriorly. After the maximum possible spread has been reached, the dilators are replaced by the above-mentioned cortico-spongious bone.
This known technique has the disadvantage that the bone is difficult to handle and to bring into the proper position, corrections being nearly impossible. Another disadvantage of this technique is that a rectangular or cylindrical recess must be punched and/or milled out of the intervertebral space, so that the bone plugs can be inserted between the originally concave sides of the adjoining vertebral bodies. This is complicated and furthermore results in damage to the vertebral body.
WO89/12431 discloses an implant for the intervertebral space. This implant has the shape of a hollow cylinder and is perforated on all sides. It can be closed from the manipulation side by means of a screw cover. Bone mass which may possibly be inserted into the cylinder cavity cannot be compressed and, in addition, the small perforations--intended only for the bone to grow in--would make pressing out the bone mass impossible.
WO90/000037 discloses a square implant, perforated on all sides, for the intervertebral space. It has a screw/wedge mechanism mounted in its interior, by means of which four claws can be run out both on the top and on the bottom. So that bone mass can be filled into this implant pre-operatively, the screw/wedge mechanism first must be unmounted and then must be remounted again, which would be very complicated. But even in such a case, the bone mass could not exit from the implant because, on the one hand, the existing, relatively large slits would be closed by the run-out claws and, on the other hand, the small perforations are unsuited for this.
PCT/CH94/00184 to BECKERS describes an implant whose specific shape and mode of insertion makes possibly an extremely stable clamping between the vertebral bodies, without thereby damaging the surface of the bony cover plate of the vertebral bodies.
The starting point of this implant is a certain shape of the cover plates which belong to the two adjoining vertebra and which bound the intervertebral space. However, in reality the geometry of the cover plate varies from patient to patient. As a result, the contact between th e bone material pt into the implant and the cover plates is not always optimal. Depending on circumstances, this can delay bone healing. The invention provides a remedy for this.