Patent Application
20130261673
A lateral access approach is frequently selected to deliver interbody fusion cages to the lumbar spine. In comparison with conventional anterior or posterior approaches to the lumbar spine, the lateral approach is thought to minimize posterior and/or anterior tissue damage as well as reduce surgery time, associated blood loss, vascular damage and infection risk.
In general, it is known in the art to mount a lateral fusion cage with a plate that secures the cage to the sides of adjacent vertebral bodies.
One problem associated with these designs is that the plate often has a greater width than the associated cage, thereby requiring the use of a larger diameter tube to pass them from the patient's skin to the patient's spine. This detracts from the goal of providing minimally invasive access to the spine.
U.S. Pat. No. 7,594,931 (Louis) discloses an intervertebral arthrodesis implant for insertion in an intervertebral space separating opposite faces of two adjacent vertebrae. The implant has a ring-shaped intervertebral cage having a bar that extends perpendicular to the axis of the spine. The bar has a height less than the rest of the cage. A surface of the cage contacting the vertebrae has an undulating shape for limiting sliding of the cage in a plane parallel to the vertebral faces.
PCT Published Patent Application WO2011-080535 (Dinville) discloses anchoring devices, anchoring systems for intervertebral implants, intervertebral implants, and instruments and methods for implanting the implants. In preferred configurations, these various objects share the feature of comprising or cooperating with an anchoring device having a body comprising at least one curved plate elongated along a longitudinal axis. The plate is designed to be inserted through a passage crossing at least a part of the implant in order to penetrate into at least one vertebral endplate and attach this implant onto this vertebral endplate by means of at least one stop retaining the implant. The body of the anchoring device comprises at least one longitudinal rib on at least a part of at least one of its faces, the rib being designed to cooperate with a groove made in a passage of implant.
In one type of intervertebral device suited for the lateral approach, the fusion cage is mounted with a plate that secures the cage to the adjacent vertebral bodies. In particular, US Published Patent Application 2010-0004747 (Lin) discloses a spinal fixation device comprising a trans-vertebral and intra-vertebral plate and a rectangular cage with a slot for the plate for neutralizing intervertebral movement in spinal interbody fusion. The rectangular cage with a vertical or oblique slot is inserted into the intervertebral space from the lateral or anterior side of the spinal column. The plate is then inserted through the slot of the cage and hammered into and buried inside the two adjacent vertebral bodies to achieve three-dimensional intervertebral fixation.
U.S. Pat. No. 8,002,808 (Morrison) discloses a device for supporting and/or assisting in bone fusion, particularly in the spine. A plate member is provided, along with two or more attachment members that are anchorable to bones. In one embodiment, the plate member has a slot near one end and an aperture at another end. The attachment members include threaded posts for connection to the plate member via the latter's slot(s) and aperture(s). Alternatively, attachment members need not have a threaded post, and attachment members may be connected to the plate member via a bone bolt or similar fixation member. The slot(s) allow a single plate member to be used for a variety of operative situations and anatomies. A device for repositioning bones and a method for using the disclosed devices is also described.
U.S. Pat. No. 7,985,223 (Khodadadyan-Klostermann) discloses a ventral bone-plate assembly has a U-shaped base having a pair of legs bridged by a bight and each formed with an inner hole and an outer hole. A brace bar extends between outer ends of the legs and is formed with end holes alignable with the outer holes. Respective inner bone screws extend through the inner holes, and respective outer bone screws each extend through a respective one of the outer holes and a respective one of the end holes.
PCT Published Application No. WO2004-098448 (RSB Spine LLC) discloses bone plate system comprising a base plate having two generally parallel elongated screw slots extending therethrough. Two bone screws are provided that are capable of securing the base plate to a bone by insertion through the screw slots into the bone. Each bone screw has a screw head and a threaded portion extending therefrom. An interference device is attached to the base plate and retains the bone screws while permitting the bone screws to toggle and to controllably slide in the screw slots of the base plate. This design is particularly useful for joining adjacent vertebral bodies, as it permits controlled settling of the vertebral bodies, thereby enhancing the healing process.
U.S. Pat. No. 7,341,590 (Ferree) discloses an anterior thoracic/lumbar system comprising a thin plate and fasteners for securing the plate to vertebrae or other osseous material. The plate may be hinged along the central axis, with a pair of collinear holes on each portion of the plate. Each of the holes accommodates a bolt which is screwed into the vertebrae and secured to the plate using a nut.
One aspect of the present invention is directed to a bone plate secured in the bone by both a screw and a post. The shaft of the screw is threaded. The distal portion of the shaft of the post is unthreaded while the proximal portion thereof is threaded. The post has a head that may directly (as a cam) or indirectly (through a lip on a connector plate) prevent screw backout. Preferably, the distal portion of the shaft of the post passes through its hole in the bone plate so that the distal unthreaded portion of the shaft substantially extends beyond the bone plate and the proximal threaded portion of its shaft substantially remains in its (threaded) through hole. The advantage of this embodiment is that the post not only prevents screw backout, it also aids in plate securement to the bone.
In some embodiments, the post directly prevents backout of the bone screw from the first screw throughhole, whereby the head of the post bears against the head of the screw. In other embodiments, the bone post indirectly prevents backout of the bone screw from the first screw throughhole, whereby the head of the post bears upon an intermediate member (such as a connector plate) which in turn bears upon the head of the screw.
In a first preferred aspect of the present invention, the conventional set screw component of a cam lock (which does not penetrate the bone) is replaced with a second bone fixation anchor that penetrates the bone. As this second anchor is inserted into the plate and bone, its proximal head bears against the head of the first anchor, thereby performing the function of a cam lock while adding to the overall securement of the assembly in the bone. This also allows dynamization of the first anchor with the plate while preventing backout of the first anchor. Preferably, this second fixation anchor is a post whose bone-penetrating distal portion of the shaft is unthreaded and whose proximal portion of the shaft is threaded only to the extent that it mates with its correspondingly-threaded hole in the plate. In some embodiments, a pair of camming screw-and-post combinations are seated in a single bone plate, thereby providing design simplicity.
Therefore, in accordance with the present invention, there is provided a bone fixation system comprising:
wherein the first screw is received in the first screw through hole and extends therethrough,
wherein the first post is received in the first post through hole and extends therethrough,
wherein the head of the first post bears against the head of the first screw,
wherein the axial length of the thread of the first post is substantially the same as the axial length of the second threadform of the first post through hole.
In some camming embodiments, the pair of screw-and-post combinations are seated in separate bone plates that are linked together by a connecting plate, wherein the head of each post acts as a cam that prevents backout of its respective screw. This embodiment holds particular advantage in that each plate (i.e., the two bone plates and the connecting plate) may be designed to be relatively long and thin, thereby allowing for their individual, minimally-invasive deliveries to the spinal site through an MIS portal.
Therefore, in accordance with the present invention, there is provided bone fixation system comprising:
In a second preferred aspect of the present invention, the locking is performed by a snap tab. The first anchor snaps into the head of a staple portion with a spherical socket. The second anchor is placed and at the same time approximates a connecting plate to bridge to an adjacent vertebral bone staple. The connecting plate prevents the tab from un-snapping, which keeps the first anchor in a locked position.
Therefore, in accordance with the present invention, there is provided bone fixation system comprising:
In preferred embodiments of this second aspect, the invention comprises a pair of screw-post-bone plate combinations linked together by a connecting plate that is fixed to each bone plate by the pair of posts. This embodiment holds particular advantage in that each plate (i.e., the two bone plates and the connecting plate) may be relatively long and thin, thereby allowing for their minimally invasive delivery to the spinal site through an MIS portal.
Therefore, in accordance with the present invention, there is provided bone fixation system comprising:
In a third aspect of the present invention, the head of the primary bone screw is cammed by the head of a secondary bone anchor having a machine thread. In this embodiment, the thread on the secondary bone anchor is more fine than the thread of the associated bone screw. The fineness of thread of the secondary bone anchor is an advantage because it provides a greater locking torque.
Therefore, in accordance with the present invention, there is provided bone fixation system comprising:
Now referring to
In this embodiment, the conventional cam is replaced by a bone post whose shaft has an unthreaded distal portion. Therefore, this embodiment provides additional securement of the plate to the bone without adding a “backout” failure mode.
In
Now referring to
In
Now referring to
The utility of the design on
Now referring back to
In this embodiment, the camming of the head of the bone anchor upon the head of the screw acts to prevent screw backout, while the second threadform acts as a means of securing the bone anchor in the bone. The relatively fine nature of this second threadform provides an additional advantage to its backout prevention function by providing high torque locking
In some embodiments, each first threadform (of the bone screw holes) extends for less than one revolution.
In some embodiments, each screw through hole and its respective post hole overlap, as in
In some embodiments, at least a portion 303 of the plate surrounding each post hole is convex, as in
In some embodiments, the second threadform (which mates with the post) is more fine than the first threadform (which mates with the screw), as in
In some embodiments, the bone-contacting surfaces of a bone plate can comprise barbs 301 extending therefrom, as in
In some embodiments comprising a single plate, the plate comprises an intermediate portion 201 having a concave surface 203 defining a narrow width, as in
In some embodiments comprising a single plate, the plate comprises an intermediate portion 201 whose inner surface 205 is recessed so as to not contact bone, as in
In some embodiments comprising a single plate, the plate comprises a pair of endportions 202 flanking an intermediate portion 201, wherein the thickness of each end portion narrows towards the intermediate portion, as in
In some embodiments comprising a single plate, the plate comprises an additional central through hole 205 located in the intermediate portion of the plate, as in
In some embodiments, as in
In some embodiments, the head of each screw has a concave surface, the head of each post has a mating convex surface, and each convex surface bears against its respective concave surface.
In some embodiments, the screw extends through the plate at a non-orthogonal angle α, as in
In some embodiments such as
In some embodiments, the shaft of each post is tapered, as in
In some embodiments, at least 50% of the shaft of each post is unthreaded, as in
In some embodiments, the axial length of each post's thread is substantially the same as the plate thickness.
In some embodiments, the unthreaded portion of the shaft of the post has a feature adapted to promote bone securement. In some embodiments, the feature is a coating adapted to promote tissue ingrowth. In some embodiments, the coating is a hydroxyapatite coating. In some embodiments, the feature is porosity throughout the bulk of the post. In some embodiments, the feature is an osteoconductive layer. In some embodiments, the feature is an expandable post.
In some embodiments, the post (or, more generally, the camming anchor) extends from the plate at a non-orthogonal angle β, as in
In some embodiments, the head of the post (which acts as a cam) can be lobular.
After access to the affected spinal level is achieved and the appropriate interbody surgery is performed, the lateral/antero-lateral surfaces of adjacent vertebral bodies are prepared for the placement of the plate and anchors. For optimal plate application, the proximal bone contours intimately match the abutting distal plate surfaces. As a general rule, aggressive contouring of the bone or osteophyte debridement may potentially weaken the integrity of the superior and inferior anterior vertebral body cortical rims. Intact cortical rim structure is important for optimal plate positioning and optimal screw purchase. The preferred plate embodiment will generally match the natural contours of the bone rather than require the bone to be machined to match the plate.
Plates are available in a range of 15 to 40 mm and the appropriate matching plate is chosen. An inserter instrument is used to maneuver the plate into the optimum position and the surgeon holds the plate firmly against the vertebral bodies. The bone holes for the first pair of anchors are prepared through the plate holes by awling, drilling, and tapping (or if self drilling/tapping anchors are used, then they are merely awled)). The anchors are then inserted to definitively attach the plate to the bone. The holes for the second pair of anchors/posts are prepared and the second anchors/posts are inserted. The final rotations of the posts are performed with a torque limiting/measuring instrument and the posts are tightened to the appropriate level or until the orientation of a cam/lobe on the second anchor/post is at the desired angular interference position.
Two appropriate length bone screws are chosen and snapped into the spherical recess of respective plates. Bone holes are prepared in adjacent vertebral bodies of the spine. The short plates with screws are placed on a self retaining screw driver and threaded/tightened into the bone holes while assuring that the plates are oriented generally parallel to the vertebral body endplates. The plates are firmly lagged to the bones. A third (connector) plate, oriented generally parallel to a cranial-caudal direction, is held in place so that its' superior and inferior adjustment slots are directly over machined thread holes of the small plates. A pair of anchors/posts are introduced through the slots and into the holes such that the third plate is approximated to the two shorter plates. The anchors/posts are tightened and the machined threads are locked with high torque. The approximation of the third plate blocks the spherical recess snap of the two shorter plates resisting back out motion of the bone screws.
