Patent Application
20070198093
This invention relates generally to improvements in spinal implants such as articulatory and fusion devices of the type designed for human implantation between adjacent spinal vertebrae. More particularly, this invention relates to a spinal implant having upwardly and downwardly projecting keels for improved implant fixation to adjacent spinal vertebrae, wherein these keels are vertically offset or misaligned to safeguard against undesired post-operative fracture of the adjacent vertebral bone structures.
Spinal implant devices are generally known in the art for surgical implantation between adjacent bony vertebral structures to correct or alleviate a number of clinical problems, such as degenerative disc disease, chronic back pain, spondylolisthesis, and others. In general terms, such spinal implants comprise a biocompatible construct formed from a biocompatible polymer, metal or ceramic, with a size and shape for intervertebral placement. In some designs, the implant has a porous region or regions adapted to receive autogenous or allogenous bone material, or otherwise defining a so-called porous bone ingrowth surface, for promoting bone ingrowth attachment of the implant device to the adjacent overlying and underlying vertebral structures. Such spinal implants are commonly used as fusion devices forming a substantially rigid interface between adjacent vertebral structures for alleviating specific patient symptoms. Alternative spinal implant designs comprise articulatory devices including upper and lower components adapted for respective fixation to overlying and underlying vertebral structures while defining an articulatory interface therebetween to maintain or restore normal patient movements.
Many state-of-the-art spinal implants include an upwardly projecting upper keel for relatively close-fit reception into a surgically prepared slot formed in the overlying vertebral structure, in combination with a downwardly projecting lower keel for similar close-fit reception into another surgically prepared slot formed in the underlying vertebral structure. See, for example, U.S. Pat. No. 6,740,118. Such upper and lower keels beneficially provide an improved mechanical interlock with the adjacent vertebrae, to achieve improved fixation of the spinal implant to patient bone. However, such keel structures have been formed on the implant in general vertical alignment with each other, whereby the upwardly and downwardly projecting keels cooperate to define a common stress line coupled with adjoining bone structures along a common line of relative weakness defined by the correspondingly vertically aligned slots cut into the overlying and underlying vertebral structures. As a result, the use of a spinal implant having these keel structures for improved mechanical interlock with patient bone is accompanied by an undesirably increased risk of bone fracture attributable to the common stress line running through the opposed keels and their associated bone slots.
There exists, therefore, a significant need for further improvements in and to spinal implants of the type having upwardly and downwardly projecting keels for improved interlock with adjoining vertebral structures, wherein the risk of post-operative bone fracture along a stress line running through the keel slots is substantially reduced or eliminated. The present invention fulfills these needs and provides further related advantages.
In accordance with the invention, an improved spinal implant is provided for implantation between adjacent overlying and underlying vertebral structures for maintaining intervertebral spacing and/or restoring substantially normal intervertebral articulatory function. The improved spinal implant includes at least one upper keel projecting upwardly for relatively close-fit reception into a surgically prepared slot formed in the overlying vertebral structure, and at least one lower keel projects downwardly for similar close-fit reception into a prepared slot formed in the underlying vertebral structure. The upper and lower keels are misaligned or offset from each other, as by vertical misalignment, to preclude creation of a common stress line extending therethrough, thereby significantly reducing or eliminating risk of vertebral fracture.
In one preferred form of the invention, the improved spinal implant includes an upwardly projecting upper keel disposed generally at a centered position on an upper or superior face of the implant, and wherein the upper keel is elongated generally in an anterior-posterior direction. This upper keel is sized and shaped for relatively close-fit reception into a matingly shaped slot formed in the overlying vertebrae as by cutting. The upper keel, when implanted, provides a mechanical interlock with the overlying vertebrae. Such mechanical interlock may be enhanced and/or combined with a porous bone ingrowth surface or holes formed on the upper face of the implant for appropriate fusion-ingrowth with the overlying vertebral structure.
A pair of lower keels project downwardly from a lower or inferior face of the implant, wherein these lower keels are also elongated generally in an anterior-posterior direction. These lower keels are disposed at positions offset from a centered position, or a center axis, of the implant, preferably by positioning these lower keels symmetrically relative to said center axis substantially at the laterally opposed sides of the implant. These lower keels provide a mechanical interlock with the underlying vertebrae, wherein this mechanical interlock may be enhanced and/or combined with a porous bone ingrowth surface or holes formed on the lower face of the implant for appropriate fusion-ingrowth with the underlying vertebral structure.
The thus-constructed spinal implant has the upper and lower keels positioned in offset or misaligned relation, thereby avoiding any common stress line running therebetween or otherwise running between the respective vertebral slots cut into the adjacent vertebral structures for receiving and supporting the keels. As a result, risk of vertebral stress fracture is substantially reduced or eliminated.
Other features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
The accompanying drawings illustrate the invention. In such drawings:
As shown in the exemplary drawings, an improved spinal implant referred to generally in
The illustrative drawings show the improved spinal implant 10 in one preferred form to comprise an articulatory device having an implant body defined by an upper component 16 adapted for fixation to an overlying vertebral structure, and a lower component 18 adapted for fixation to an underlying vertebral structure. These upper and lower implant components 16, 18 cooperatively define a matingly shaped articulatory interface referred to generally by arrow 20, wherein this articulatory interface is shaped to accommodate a range of inter-component motion consistent with substantially natural intervertebral articulatory function. While the specific geometry of this articulatory interface 20 may vary, preferred configurations are shown and described in U.S. Publi. 2004/0133281, which is incorporated by reference herein. In addition, while the exemplary drawings show the spinal implant 10 in the form of an articulatory device including the upper and lower components 16 and 18, persons skilled in the art will recognize and appreciate that the vertically offset upper and lower keels 12, 14 may be incorporated in a non-articulatory spinal implant of the type designed for intervertebral fusion wherein the upper and lower components 16, 18 are formed as a unitary body.
The upper and lower implant components 16, 18 are constructed from a suitable biocompatible material, such as a titanium-based or a cobalt chrome-based metal, a relatively high strength ceramic, or a biocompatible polymer-based material. Preferred high strength ceramic materials are disclosed in U.S. Pat. No. 6,881,229, which is also incorporated by reference herein. Such ceramic implant may incorporate relatively dense and comparatively porous regions which respectively mimic the physical characteristics of natural cortical and cancellous bone, and wherein the porous region enhances bone ingrowth attachment with the adjoining vertebral structure, as shown and described in U.S. Pat. Nos. 6,790,233 and 6,846,327, which are also incorporated by reference herein.
The lower component 18 of the spinal implant 10 includes a lower or inferior face 30 for engaging the underlying vertebral structure when the implant is placed into the intervertebral space. As shown, this lower face 30 spans or extends a generally centered region of the lower component 18, between a pair of downwardly protruding lower keels 14 disposed generally at the opposite, or medial and lateral sides of the implant component. These lower keels 14 are also elongated in a fore-aft or anterior-posterior direction, but are disposed in substantially offset or substantially misaligned positions relative to the centered upper keel 12 (and center axis 24) on the upper implant component 16. In the preferred form, the lower keels 14 are positioned in symmetric relation at opposite sides of the center axis 24. The lower face 30 may further incorporate a porous bone ingrowth region 32 (
While the illustrative drawings show a pair of the spinal implants 10 installed between adjacent pairs of vertebral structures, persons skilled in the art will understand that one or more of the spinal implants 10 may be employed according to the condition of each specific patient.
Post-surgically, each spinal implant 10 maintains appropriate intervertebral spacing while, in the case of the illustrative articulatory implant device, restoring substantially normal patient movement function. Importantly, the upper and lower keels 12, 14 of each spinal implant 10 are vertically out-of-alignment, so that there is no common vertical stress line extending between the overlying and underlying vertebral structures. Similarly, when multiple implants 10 are placed between adjacent pairs of vertebrae, the lower keels 14 of an upper implant are also vertically misaligned with the upper keel 12 of a lower implant, thereby also avoiding any common vertical stress line extending through any single vertebral structure such as the intermediate vertebrae 34 shown in
More particularly,
Post-surgically, each modified spinal implant 110 maintains appropriate intervertebral spacing while, in the case of the illustrative articulatory implant device, restoring substantially normal patient movement function. Importantly, the upper and lower keels 112, 114 of each spinal implant 110 are, once again, positioned in vertical misalignment, so that there is no common vertical stress line extending between the overlying and underlying vertebral structures. Similarly, when multiple implants 110 are placed between adjacent pairs of vertebrae, the lower keels 114 of an upper implant are also vertically misaligned with the upper keel 112 of a lower implant, thereby also avoiding any common vertical stress line extending through any single vertebral structure such as the intermediate vertebrae 134 shown in
A variety of further modifications and improvements in and to the improved spinal implant of the present invention will be apparent to those persons skilled in the art. In this regard, while the invention is shown and described herein with at least one upper keel in vertical misalignment with at least one lower keel, persons skilled in the art will appreciate that the orientation terms “upper”, “lower”, and “vertical” are exemplary only and that alternative keel misalignment orientations including but not limited to fore-aft or medial-lateral misalignment are within the scope of the disclosed invention. Accordingly, no limitation on the invention is intended by way of the foregoing description and accompanying drawings, except as set forth in the appended claims.
