Patent Application
20090275990
1. Field of the Invention
This invention relates to an anterior cervical plate, and, in particular, it relates to such a plate with a new and improved locking element.
2. Description of the Related Art
The present invention relates to cervical plates used for fusing cervical vertebrae in the treatment of spinal disorders, and, more particularly, to components for locking the fasteners for such plates.
An increasingly accepted procedure for treating spinal disorders involves using substantially rigid plates to hold vertebrae in desired spatial relationships and orientations relative to each other. The upper cervical spine can be approached anteriorly or posteriorly, although anterior approaches are of more interest in connection with this invention. In either case, holes are drilled and tapped in at least two of the vertebrae. These holes then receive screws or other fasteners used to secure the plate.
The holes are accurately positioned with reference to openings formed through the cervical plate. In some cases, the screws may be self-tapping. Typically, the plate is curved about its longitudinal axis to facilitate contiguous surface engagement of the plates with the vertebrae. With the plate maintained against the vertebrae, the fasteners are secure within the holes. As a result, the plate maintains the attached vertebrae in a desired spacing and orientation with respect to each other.
One of the problems associated with this technique is the tendency of the screws or other fasteners to gradually work loose after fixation. Slight shock or vibration of the vertebrae, due to walking, climbing stairs, or more vigorous activity by the patient following treatment increases this tendency, jeopardizing the integrity of fixation. Moreover, as the fasteners work loose, the outward protrusion of the heads over other components of the fasteners can be a source of discomfort and presents the risk of trauma to surrounding soft tissues.
Many cervical plates of the present type are known, each having various arrangements for securing the bone screws. Notwithstanding the development of the art to date, a need exists for improvements in arrangements for holding the bone screws in place after the bone screws have secured the cervical plate onto the adjacent vertebrae.
Anterior cervical plates that are fabricated from bioabsorbable materials such as polylactide offer a number of advantages relative to metallic plates. They do not interfere with techniques such as X-ray imaging or magnetic resonance imaging (MRI). Such plates also possess inherent material flexibility, thereby allowing load sharing with the graft. Because these plates are not metallic, there is no risk of corrosion. Furthermore, when heated, it is easy to shape the plates to match the contours of the vertebrae.
Anterior cervical plates can be used to stabilize, immobilize and align the cervical spine. For example, an intermediate disc or discs can be removed and replaced by a cage, a mesh, a bone graph/plug, or an allograft/autograft. In addition to removal of the discs, the intermediate vertebrae can also be removed. The cervical plate can also be used to correct instability of the cervical spine caused by trauma, tumors, advanced degenerative disc disease, cervical deformities caused by lordosis, kyphosis, or other conditions.
While the above approaches yield favorable results in certain circumstances, there remains a need for greater flexibility in positioning and orienting the bone screws or fasteners, and for a simpler, more reliable means of counteracting the tendency of the bone screws to work loose after cervical plate fixation.
Therefore, there is a need for a cervical plate and fixation system in which the bone screws or other fasteners are more securely retained and less likely to work loose, without the need for auxiliary screws or other additional fixtures. There is a further need for a locking element that has a tracking geometry similar to the outside surface of the screw head and fits snugly into the screw head track, thereby locking the bone screws head inside of the anterior cervical plate screw hole and preventing the retraction of the bone screws from the vertebrae. In addition, there is a need for a system including cervical plates with locking features for retaining fastener heads, in combination with tools for conveniently manipulating the locking features to selectively retain or release the fasteners.
The foregoing objects and advantages of the invention are illustrative of those that can be achieved by the various exemplary embodiments and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the various exemplary embodiments will be apparent from the description herein or can be learned from practicing the various exemplary embodiments, both as embodied herein or as modified in view of any variation which may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel methods, arrangements, combinations, and improvements herein shown and described in various exemplary embodiments.
In light of the present need for an anterior cervical plate with an improved locking element, a brief summary of various exemplary embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit its scope. Detailed descriptions of preferred exemplary embodiments adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in later sections.
In various exemplary embodiments, an anterior cervical plate may stabilize, immobilize, and align vertebrae. This plate has a new and improved arrangement for securing the bone screws in place after the cervical plate has been attached to the vertebrae.
Thus, in various exemplary embodiments, an anterior cervical plate may include an exterior surface, an interior surface, and a number of bone screw holes for attaching the cervical plate to the vertebrae. Specifically, in accordance with the present invention, there is provided at least one pair of adjacent bone screw holes, preferably transversely aligned, wherein the cervical plate has a locking element housing that is perpendicularly mounted between the adjacent bone screw holes and movable between a first, open position, where the locking element is not in contact within the two bone screw holes, thereby allowing insertion or removal of the bone screws, and a second, locking position wherein the locking element is in contact with at least a portion of each of the two bone screw holes, thereby locking those bone screws in place. The locking element of the present invention is intended essentially to prevent the screws from backing out.
In various exemplary embodiments, the locking element may be resilient, ductile, and movable from an open position to a closed position by application of an external force above the elastic limit of the locking element. The locking element may comprise an elastically deformable body that is capable of locking the head of the bone screw inside of the anterior cervical plate's screw hole, thereby preventing retraction of the bone screws from the vertebrae.
In various exemplary embodiments, an elongated locking element may be mounted into the locking element housing of the cervical plate between two adjacent bone screw holes. This locking element is movable to a first position wherein it is completely out of contact with the two adjacent holes so as to permit the insertion of and removal of the bone screws. The locking element is then arranged to be partially in contact with the two bone screw holes when the bone screws are secured in place. The screw head, when maintained in the aperture against the interior surface region, is advantageously contained, so that the head does not protrude outwardly beyond the plate or other structural element.
In various exemplary embodiments, the aperture may be sized with respect to the shank to allow a pivoting of the screw with respect to the structural member about at least one transverse axis. More preferably, the fastener is pivotable about all transverse axes passing through a given point in the first aperture, thereby defining a conical volume within which the fastener is selectively positionable.
In various exemplary embodiments, locking and releasing tools may be provided. A locking tool includes a shaft with a locking end that can be attached to the proximal surface of the locking element. This tool is capable of inserting the locking element into the locking element housing of the cervical plate and allowing the deformation of the locking element body against the screw head into a closed position, thereby preventing the retraction of the bone screws from the vertebrae. A releasing tool, having a shaft with a locking end that can be attached to the proximal surface of the locking element in a closed position, applies a tension force to the proximal surface of the locking element, thereby advancing the locking element out of the housing to an open position. This movement allows withdrawal of the bone screws.
In various exemplary embodiments, a system including the structural member and screws may further include tools for securing and removing the structural member. In particular, the heads of the screws can include non-circular recesses. A drive tool with a drive shaft can be provided. One end of the shaft has a non-circular profile that corresponds to the profile of the recess. The screw shanks, in this approach, are externally threaded, so that rotation of the drive tool turns the screws. The drive tool can incorporate a sleeve that is coaxial with the drive shaft and flexure members that grip the fastener turned by the drive tool.
In various exemplary embodiments, anterior cervical plates may have a virtually limitless number of configurations. Cervical plates are generally referred to by the number of levels that they overlie, wherein the word “level” refers to the number of intervening intervertebral spaces that are spanned. Thus, for example, a three level cervical plate would span the four vertebrae and three intervening intervertebral spaces. Such a plate can be connected by a single central bone screw through a central bone screw hole instead of by two adjacent bone screw holes. In various configurations, an opening will be provided between adjacent vertebrae for viewing the intervening intervertebral space.
Thus, in various exemplary embodiments, cervical plates and other structural members may be secured to vertebrae or other osseous material in a manner that more reliably prevents fasteners from working loose in response to shock or vibration. The locking elements are movable, through either plastic or elastic deformation, to close positions that allow insertion and removal of fasteners, and alternatively are positioned to prevent fasteners from working free of their respective apertures in the cervical plate or other structural members.
In order to better understand various exemplary embodiments, reference is made to the accompanying drawings, wherein:
Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments.
Locking element 40 fits into each mechanism 15, 30, and may be mounted perpendicularly between a first pair 11, 12 and a second pair 28, 29 of transversely aligned screw holes. When bone screws are fully inserted into the vertebrae through anterior cervical plate 10, locking element 40 prevents retraction of bone screws from the vertebrae. Locking element 40 is also elastically deformable due to its central hole 49. Locking element 40 may be fabricated from polylactide/polylactic acid (PLA), a copolymer of 70:30 poly L-lactide-co-D, L-lactide, or from other bioabsorbable materials.
When bone screw 50 is fully inserted into the vertebrae through anterior cervical plate 10, locking element 40 is forced to close mechanism 15. Longitudinal engagement tracks 43, 44 of locking element 40 fit snugly at aperture 20 into tracks 54, 55, 56 in head portion 53 of bone screw 50, thereby mechanically locking bone screw 50 inside screw hole 29 and preventing retraction of bone screw 50 from the vertebrae. Screw hole 29, previously described above in connection with
A screwdriver, inserted into bone screw socket 61, pushes screws 33, 34, 35, 50 into the vertebrae through screw holes 12, 11, 28, 29 of cervical plate 10. Each screw 33, 34, 35, 50 has an elongated, longitudinal, threaded shank 57 that matches the perimeters of screw holes 11, 12, 28, 29 in plate 10. After an appropriate amount of turning, screws 33, 34, 35, 50 penetrate into the vertebrae to a sufficient depth to engage exterior surface region 27 of plate 10, shown in
When screws 33, 34, 35, 50 are fully inserted into screw holes 11, 12, 28, 29, head portions 53 of screws 33, 34, 35, 50 engage interior surface region 23, thereby fixing the structural member to the vertebrae. Locking element 40 moves from an open position to a closed position, where it engages mechanism 15. Tracks 43, 44 of each locking element 40 fit snugly at aperture 20 into screw tracks 54, 55, 56 of head portion 53, thereby mechanically locking bone screws 33, 34, 35, 50 inside screw holes 11, 12, 28, 29 and preventing the retraction of screws 33, 34, 35, 50 from the vertebrae.
The preferred material for plate 10, bone screws 33, 34, 35, 50, and locking element 40 is polylactide, also known as polylactic acid (PLA). PLA is a copolymer of 70:30 poly L-lactide-co-D, L-lactide that is derived from lactic acid, a chemical that occurs naturally in the human body. The copolymer maintains its strength during the heating process and through hydrolysis slowly breaks down into lactic acid molecules. These molecules are subsequently metabolized into water and carbon dioxide, a gas that is released from the body through the lungs.
Plates made from such materials are easily contourable when heated to the shape of the vertebras. Other materials include polymer blends of glycolide and/or lactide homopolymer, copolymer and/or glycolide/lactide copolymer and polycaprolactone copolymers, and/or copolymers of glycolide, lactide, poly L-lactide-co-DL-lactide, caprolactonc, polyorthoesters, polydioxanone, trimethylene carbonate, and/or polyethylene oxide, or any other bioabsorbable material. Medical grade stainless steel, medical grade titanium, or hybrid construction may be applicable.
Further, the plate made from titanium has sufficient ductility to permit curving the plate about a longitudinal axis so that the cervical plate more readily conforms to the vertebrae. The ductility also plays a role in the use of retaining features that capture each fastener within its aperture, as will be explained. Certain stainless steels are suitable as alternatives to titanium in the plate and fastener construction.
Locking element 40 also may be fabricated from a pseudoelastic shape memory alloy. By way of example, one such pseudoelastic shape memory alloy might be a nickel titanium alloy such as Nitinol, which is available from Flexmedics of Minneapolis, Minn., among others. The use of such a material, in combination with the normal orientation of the barbs relative to the anchor body, permits the barbs to initially deflect inwardly to the extent required to permit the anchor to move forward in the bone tunnel, yet still resiliently “spring back” toward their normal, outwardly projecting position so as to prevent the anchor from withdrawing back out the bone tunnel.
Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments, and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only, and do not in any way limit the invention, which is defined only by the claims.
