Patent Application
20090043341
The present invention relates generally to controlling kinematics of the body, and more specifically to a dynamic extension for an implant, and a method for installing a dynamic extension to an implant to control the kinematics of areas adjacent to the implant.
Anterior cervical discectomy and fusion (ACDF) is a common surgical procedure for treating nerve root or spinal cord compression caused by a degenerated or herniated disc. In the course of an ACDF procedure, the surgeon enters the intervertebral space and removes all or a portion of a degenerated disc. The intervertebral space is then filled with bone graft. The graft is intended to accelerate the biological fusion of adjacent vertebrae. The bone graft may be an autograft, allograft, synthetic bone substitute, or bone morphogenic protein (BMP). Alternatively, a spacer having a hollow center that is pre-filled with bone graft may be positioned in the intervertebral space.
The process of joining the vertebrae together with bone graft is commonly referred to as “fusion.” In many instances, a bone fixation plate (referred to herein as an osteosynthetic plate or plating system) is fastened to the vertebrae directly adjacent the fusion site with bone fasteners. The bone fixation plate stabilizes the vertebrae directly adjacent the fusion site to promote fusion of those vertebrae.
While the fusion may alleviate the pain in the fusion site, complications can develop. When vertebra are fused, the spine loses mobility at the fused location. As a result, vertebral discs that are in proximity to the fused location must make up for the lost mobility. In many cases, neighboring discs must provide a wider range of motion and withstand larger stresses than prior to the fusion. The added stress on a neighboring disc can lead to accelerated degeneration of the disc, causing additional pain and suffering for the patient. This can lead to further revision surgery to fuse more vertebrae adjacent to the original fusion site.
In the course of a common revision surgery, the previously implanted osteosynthetic plate is removed and replaced with a longer plate such that the fused vertebrae and the vertebrae adjacent to the fused vertebrae can be immobilized together by a single plate. To remove the previously implanted osteosynthetic plate, each bone fastener must be removed, some of which may be overgrown with bone. Explanting the old bone fasteners and implanting new bone fasteners is a highly invasive, risky procedure. In view of the foregoing, a need exists for an improved osteosynthetic plating system with improved interconnectivity such that revision surgery is less invasive and traumatic for the patient. In addition, a need exists to slow or even prevent the deterioration of discs that neighbor the fusion site, so as to reduce the need for revision surgery.
According to one aspect of the invention, an osteosynthetic plate assembly comprising a dynamic extension plate is provided. The dynamic extension plate includes a first end portion for connection to a first vertebrae, a second end portion for connection to a second vertebrae, and a dynamic flexible portion extending between the first and second end portions. The first end portion or second end portion comprises a connector for coupling to an adjacent plate component.
According to another aspect of the invention, an osteosynthetic plate assembly comprises a dynamic extension plate and a cervical fusion plate configured for connection to one of the first and the second end portions of the dynamic extension plate.
According to yet another aspect of the invention, an osteosynthetic plate assembly comprises a dynamic extension plate having a dynamic flexible portion extending between the first and second end portions. The dynamic flexible portion comprises at least one elongated flexible member for limiting relative motion of the first and second vertebrae.
According to still another aspect of the invention, a spinal surgical method is provided. The method comprises the step of engaging a connector of a dynamic extension plate with a coupling of a cervical fusion plate. The dynamic extension plate is mounted to the adjacent vertebrae.
According to another aspect of the invention, a spinal surgical method comprises the step of fastening a cervical fusion plate at a fusion site between a first vertebra and a second vertebra. A disc in proximity to the fusion site that is susceptible to accelerated disc degeneration as a result of the fusion site is identified, and the limited range of mobility of that disc is determined. A dynamic extension plate that provides the limited range of mobility for the disc is selected. The selected dynamic extension plate is implanted over the disc.
According to still another aspect of the invention, a spinal surgical method comprises the step of fastening a first plate between a first vertebra and a second vertebra, wherein the first plate provides resistance to motion in a plane relative to the spine. A second plate is fastened to the first plate, wherein the second plate spans between the second vertebra and a third vertebra adjacent to the second vertebra. The second plate provides less resistance to motion in the plane relative to the spine than the first plate.
According to still another aspect of the invention, an osteosynthetic plate assembly includes a spinal implant and a dynamic extension extending from the spinal implant, the spinal implant and dynamic extension forming a single one-piece body of unitary construction.
According to still another aspect of the invention, an assembly includes an interbody implant and a dynamic extension extending from the interbody implant. In one embodiment, the interbody implant is an intervertebral disc prosthesis.
The invention is best understood from the following detailed description when read in connection with the accompanying drawings, with like elements having the same reference numerals. Included in the drawings are the following figures:
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
The problems associated with spinal surgery and revision procedures are resolved in several respects by osteosynthetic plate assemblies and procedures for using the assemblies, all in accordance with the present invention. The applicants generally propose to protect discs that lie in proximity to fusion sites (“neighboring discs”) from accelerated degeneration by controlling the kinematics of those discs. The mobility of the neighboring discs is controlled without resorting to further fusion, which would sacrifice all mobility at those disc sites. Controlling the kinematics of neighboring discs protects the neighboring discs from damage caused by increased stress, while still preserving some mobility at those sites. A preferred way of controlling kinematics of neighboring discs in accordance with the invention is to provide a gradual change in mobility between the fused site and adjacent sites, rather than an abrupt change between the fused site and adjacent disc spaces.
In one exemplary embodiment of this procedure, a neighboring disc that appears susceptible to accelerated degeneration is identified, and an appropriate range of motion for that disc that does not carry a risk of accelerated degeneration is selected. A dynamic restraint that provides the desired range of motion is then implanted at the site of the neighboring disc. The dynamic restraint limits the mobility of the neighboring disc so that the disc does not become susceptible to accelerated degeneration. This, in turn, reduces the potential for pain and discomfort in the neighboring disc, thereby avoiding further revision surgery. The procedure for adding a dynamic restraint at a vertebral disc adjacent or in proximity to a fused site may be referred to as “topping off.”
A number of osteosynthetic plate assemblies are contemplated in accordance with the present invention.
The cervical fusion plate 30 stabilizes vertebrae V2 and V3 to promote fusion of those vertebrae with spacer S2. While the fusion of V2 and V3 may alleviate the pain at the fusion spacer site S2, pressures along the spine are transferred onto one or more adjacent discs, such as D1, which may cause rapid deterioration those discs, and additional pain and suffering for the patient. For those reasons, dynamic extension plate 20 is mounted to plate 30 and vertebrae V1 and V2 to limit or prevent degeneration of the neighboring discs, such as D1.
Cervical fusion plate 30 is sufficiently rigid to prevent relative motion of vertebrae V2 and V3. In contrast, dynamic extension plate 20 is flexible to permit limited and controlled relative movement of vertebrae V1 and V2. By limiting relative motion of the adjacent vertebrae V1 and V2, the dynamic extension plate 20 limits stresses exerted on disc D1 and therefore slows or prevents degeneration of disc D1. The plate 20 is uniquely adapted to absorb pressure at disc D1 and re-distribute that pressure across a larger segment of the spine thereby limiting stress concentrations at one particular disc.
According to one exemplary use of the invention, the cervical fusion plate 30 is mounted to vertebrae V2 and V3 for fusing those vertebrae in an initial fusion surgery. In a follow-up revision surgery performed at some time after the initial fusion surgery, the dynamic extension plate 20 is mounted to plate 30 and adjacent vertebrae V1 and V2 to improve or prolong the condition of disc D1. The time span between initial surgery and the revision surgery may vary.
In the revision surgery, dynamic extension plate 20 may be coupled to plate 30 and fastened to V1 and V2 while plate 30 remains implanted in vertebrae V2 and V3, by virtue of the unique design of osteosynthetic plate assembly 10. Therefore, removal of the implanted plate 30 from vertebrae V2 and V3 is not required in the revision surgery. Accordingly, the disadvantages associated with the removal of a previously implanted cervical fusion plate are avoided, as described in the Background section.
According to another exemplary use of the invention, the entire osteosynthetic plate assembly 10 may be installed in the initial surgery for preventative measures. The plates 20 and 30 may be coupled together before or after plate 30 is implanted into the spine.
The manufacturing cost of a dynamic extension plate 20 is lower than the manufacturing cost of a large replacement plate for fusing V1, V2 and V3. The dynamic extension plate 20 is particularly advantageous in that it may be mounted to an implanted fully-constrained, semi-constrained, semi dynamic or fully dynamic cervical plate and fastened to the neighboring vertebrae.
The pair of flexible members 26 extend in a parallel arrangement between the end portions 22 and 24 of plate 20. The flexible members 26 are generally referred to herein as the ‘dynamic flexible portion’ of extension plate 20. The flexible members 26 of plate 20 are adapted to permit limited relative motion of adjacent vertebrae V1 and V2 along or about various axes of the spinal cord, while restricting relative motion of adjacent vertebrae V1 and V2 along or about other axes.
More specifically, and referring to the free body diagram illustrated in
The plate 20 may include any number of flexible members 26 having any shape, size, spatial arrangement and material composition, as best shown and described with reference to the alternative embodiments of
The plate 30 includes two couplings 34 that include “Z”-shaped channels 34a (one shown in
According to one exemplary method of assembling plates 20 and 30, flexible tab 29 of connector 27 is inserted into coupling 34 of plate 30 from the side of thin section 37 at an oblique angle, as best shown in
By virtue of the geometry of flexible tab 29 and “Z”-shaped channel 34a of coupling 34, plate 20 may be assembled with plate 30 regardless of whether plate 30 is already mounted to the spine. The connection between tab 29 and coupling 34 does not require the use of separate screws or fasteners, and can be made while plate 30 is mounted to the spine, with plate 30 detached from the spine or while the plate is coupled with other plates. The tab 29 is easily snap-fit into coupling 34 by virtue of the positive engagement between heel 19 and thin section 37.
The individual features of plate 120 may be modified to evoke a specific mechanical response. For example, the thickness dimension of the flexible portion 126 may be maintained less than a thickness dimension of end portions 122 and 124, as shown, for a greater range of flexion (arrow 102) and extension (arrow 104). Additionally, the size of aperture 160 may be increased for a greater range of compression and expansion (arrows 100). According to another alternative embodiment shown in
According to one exemplary use of plate 220, the plate 220 may be implanted in adjacent vertebrae in a compressed state. Once implanted, the pre-compressed flexible portion 226 expands to its original shape and urges the adjacent vertebrae apart, thereby decompressing the disc between the separated vertebrae.
The plate 220 may include any number of apertures 260 having any desired shape or size. Moreover, apertures 260 may be oriented in any desired direction for tailoring the direction of the force of expansion.
In sum, depending upon its material composition, the flexible portion 326 may be configured to resist or even prevent torsion (arrows 106), while permitting a limited range of flexion (arrows 102), compression and expansion (arrows 100), lateral motion (arrows 108) and extension (arrows 104) of adjacent vertebrae. Similar to the exemplary use of plate 220, the plate 320 may be implanted in adjacent vertebrae in a compressed state for decompressing the disc in the intervertebral space.
The flexible portion of dynamic extension plates 120, 220, 320, 620, 720, 820 and 920 is preferably formed from an elastic material that is configured to return to its original shape. By way of non-limiting example, the flexible portion of the plates 120, 220, 320, 620, 720, 820 and 920 may be formed from spring stainless steel, titanium, nickel titanium, or any other bio-compatible metallic material, or combination thereof, which undergoes stress induced martensitic phase transformation with a portion of recoverable strain. The flexible portion may also be formed from a bio-compatible synthetic material having a glass transition temperature below both body temperature (˜37° C. (98° F.)) and room temperature (˜21° C. (70° F.)), or any other bio-compatible material in its rubbery and transition state, as defined by a thermal-mechanical curve. All or a portion of the dynamic extension plates 20, 120, 200 and 320 may be formed from any of the aforementioned materials. The material that is selected may have a broad range of physical properties, the selection of which may depend on factors including but not limited to the desired amount of flexibility of the flexible portion. Preferably, the material has a modulus of elasticity of between about 10 kPa to about 200 GPa. In addition, the material preferably has a yield strength of between about 4 kPa and about 1200 MPa. Materials having properties outside of these ranges may also provide excellent performance, however.
A surgeon may opt to implant plate assembly 410 to suspend the deterioration of two discs (D1 and D3 of
The plate assembly 510 is adapted for promoting fusion at fusion spacer sites S1 and S3 and preventing stress concentrations on neighboring disc D2. It should be understood that vertebrae V1 and V2, and vertebrae V3 and V4 are fused together in this embodiment. Vertebrae V2 and V3 are not fused together.
While the fusion of vertebrae V1/V2 and vertebrae V3/V4 may alleviate pain at fusion spacer sites S1 and S3, pressures along the spine may be transferred onto disc D2 causing rapid degeneration of disc D2 in the absence of dynamic extension plate 520. The plate 520 is uniquely adapted to absorb pressure at disc D2 and re-distribute that pressure across a larger segment of the spine thereby limiting stress concentrations at disc D2.
According to one exemplary use of the plate assembly 510, the entire assembly 510 may be implanted in a single surgical procedure. Alternatively, plate 520 and a single cervical plate 30 may be coupled to an existing implanted cervical plate 30 in a revision surgery. As another alternative, plate 520 may be coupled between two implanted cervical fusion plates 30 in a revision surgery depending upon the elasticity of the plate 520.
Referring now to
It should be understood that any of the embodiments contemplated herein, either unitary or modular, may be compatible and used with one another in an implantation or revision procedure. Moreover, it should be understood that assemblies may include a series of fusion portions and/or dynamic extensions, connected together either modularly or unitarily, with each portion having unique physical properties suited to a particular location on the spine.
Dynamic extensions in accordance with the present invention may be utilized under any circumstances where the natural kinematics of joints are altered unfavorably by implants at neighboring joints. The incorporation of dynamic extensions may be applied in the spine or other areas of the body. In addition to topping off spinal fusion plates, the dynamic extensions in accordance with the present invention may be used to top off other kinds of implants, including but not limited to interbody implants. For example, dynamic extensions in accordance with the invention may be associated with intervertebral disc implants to provide dynamic restraint to discs that neighbor the disc replacement site. Dynamic extensions may be modularly connected to the disc implant using, for example, the connector tabs described previously, or may be formed integrally with the disc implant. The physical properties of the dynamic extensions are chosen so as to adjust the kinematics of neighboring vertebrae, which may be dramatically changed after the disc prosthesis is implanted.
Referring now to
While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention.
For example, the cervical fusion plate may include opposing flexible tabs, while the dynamic extension plates include “Z”-shaped channels for coupling with the connectors of the cervical fusion plate. Furthermore, the osteosynthetic plate assemblies described herein are not limited to the illustrations, as an osteosynthetic plate assembly may include any number or configuration of cervical fusion and dynamic extension plates.
Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.
