Abstract:
Provided is a novel system that includes a low profile anterior vertebral body plate and taper lock screws for the fixation and stabilization of the cervical spine, the anterior vertebral plate having a novel screw locking mechanism attached to the screw during the manufacturing thereof and providing a taper lock fit with the anterior vertebral plate. Also provided is a method of stabilizing cervical vertebrae using the disclosed device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to devices and methods for use in orthopedic spine surgery. In particular, the present invention relates to a system that provides a low profile anterior vertebral body plate and taper lock screws for the fixation and stabilization of the cervical spine, the anterior vertebral plate having a novel screw locking mechanism attached to the screw during the manufacturing thereof and providing a taper lock fit with the anterior vertebral plate. 
         [0003]    2. Background Art 
         [0004]    Disease, the effects of aging, or physical trauma resulting in damage to the spine has been treated in many instances by fixation or stabilization of the effected vertebra. The use of plates and screws for fixation and stabilization of the cervical vertebra has been widely accepted as a reliable practice and has proven to be highly successful clinically. 
         [0005]    The various plates, which are attached to the anterior vertebral bodies of the spinal column by bone screws have some common features such as relatively planar body profiles that define multiple holes or slots through which the screws fit and are threaded into the bone. Various means have been used to prevent the screws from becoming loose or detached from their necessary secured or locked attachment to the vertebral plate. Among the differences between the conventionally used plates and screws is the manner in which the screws are locked into place in the hole or slot of the plate after the screws have been secured to the bone. 
         [0006]    These conventional devices can be generally grouped into three basic categories with regard to how the screws are captured or secured in the plates. 
         [0007]    Early plate designs were standard bone plates having holes through which screws were passed and screwed into the bone. These plates had no special provision for attaching the screws to the plate and as such were susceptible to having the screws back out of the plate over time. There have been clinically reported instances of screws backing out of these type plates with resulting surgical complications. Due to the potential and actual unreliable performance of such plates, the need for secure fixation of the screw to the plate as well as to the bone is now considered a basic requirement for vertebral plates. Due to the lack of predictable security of the screw to the plate, plates which do not secure the screw relative to the plate have fallen out of favor and virtually disappeared from use. 
         [0008]    Efforts have been made to secure the screws relative to the plates. In one design the screw head contains a threaded hole configured to receive a set screw. After the screw has been driven into bone and the head is seated in the plate hole, the set screw is inserted into the receiving hole of the screw head. The set screw is tapered to cause the screw head to expand and frictionally engage the wall of the plate hole, thereby resisting forces which tend to cause the screw to back out. While such mechanisms have worked to some degree, the addition of a small additional part, the set screw, at the time of surgery presents the potential hazard of dropping the set screw into the surgical field or otherwise misapplying the set screw to the screw head, for example, cross threading. 
         [0009]    An alternative approach has been to provide features in the plate, which are specifically designed to hold the screw in position once the screw is inserted through the plate and screwed into the bone. One direction taken in this effort has been to design plates that incorporate or attach individual retaining rings or snap features associated with each plate hole configured to hold the inserted screw in place relative to the plate. These plates are very common and widely used; however, an inherent problem associated with such plates is the use of the additional very small retaining elements that can become disengaged from the plate and migrate into the surrounding soft tissues. Further, difficulty experienced in accessing and disengaging the small locking elements and removing the screws from this type of plate has caused some concern for the continued use of such plates. A similar approach involves individual cams associated with each plate hole, which when rotated apply friction pressure to the screw head in an attempt to resist back out. 
         [0010]    Another approach is to add a cover to the plate after the screws have been placed. Such a design undesirably adds steps to the surgical procedure, thickness or height to the overall construct, and is susceptible to misapplication. Yet another direction taken in this effort to provide plates with locking elements is to provide dedicated overlying features, which are attached to the top side of the vertebral plate for the purpose of covering at least a portion of the screw head and thereby holding the screw in a seated and locked position. Generally these plates are designed to provide a variety of screw covering features that are pre-attached to the plate, and which can be selectively slid or rotated into position once it has been inserted. In some devices, such covering plates cover multiple screw heads. These plates typically require an increase in the overall composite thickness of the plate in order to accommodate the additional locking feature attached to the top side of the plate. This is a particularly unacceptable condition due to the use of such plates in an area of the spine where a thin, smooth surfaced profile for the plate assembly is preferred. Another major problem with such plates is that the overlying locking element cannot always be properly positioned over the screw head if the screw shaft was, due to anatomical necessity, positioned through the plate and into the bone at an angle such that the screw head does not fully seat in the plate recess provided on the top side of the plate. Further, when one of the overlying locking elements of such a plate loosens or becomes disengaged it is then free to float away from the top side of the plate and migrate into the soft tissue adjacent to the top side of the vertebral plate. 
         [0011]    Yet another approach is to provide machine threads in the plate hole with corresponding threads on the screw head. Thus the screw has a first, bone engaging thread on its shaft and a second machine thread on the screw head. As the threaded shaft is screwed into bone the screw head approaches the plate hole and the machine thread engages the thread in the hole. Aside from the fact that there is nothing to prevent the same forces that urge the screw to back out of bone to have the same effect on the machine thread engagement, such an arrangement does not provide adequate clinical flexibility. First there is no assurance that the lead in thread of the machine thread will match up with the plate hole thread when the screw head reaches the hole, raising the possibility of cross threading. Second, the machine thread in the plate hole does not allow various angular positions between the screw and the plate, as the threads must match up and engage when the screw head reaches the hole. As to the latter point, one plate provides a threaded ring in the plate hole, which is intended to allow the head to assume a variety of angular positions. 
         [0012]    There is therefore, an unfulfilled need for an anterior cervical plate system that can maintain a relatively low profile, as found in the non-locking plates while providing the security of a locking plate system and doing so no matter how angulated the inserted screw may be relative to the plate. Further there is a need for a vertebral plate that does not have locking elements with the predictable problems of locking elements becoming disengaged from the plate and migrating away from the top side of the plate into the surrounding soft tissue. 
       SUMMARY OF THE DISCLOSURE 
       [0013]    The present invention meets the above identified need by providing a low profile anterior vertebral body plate, which is secured to the underlying bone using novel taper lock screws. 
         [0014]    Also provided is a low profile anterior vertebral body plate, which is secured to the underlying bone using novel taper lock screws having screw heads with circular or convex shaped lateral surfaces that correspond to the shape of the concavity of a circumferentially disposed tapered locking ring. 
         [0015]    Also provided is a low profile anterior vertebral body plate, which is secured to the underlying bone using novel taper lock screws, each of the screw heads being able to rotate within a respective tapered locking ring prior to the screw and locking ring being moved into a seated and locked position in the plate. 
         [0016]    Also provided is a low profile anterior vertebral body plate, which is secured to the underlying bone using novel taper lock screws, the screws being pre-assembled with a tapered locking ring. 
         [0017]    Also provided is a low profile anterior vertebral body plate, which is secured to the underlying bone using novel taper lock screws, the screws being pre-assembled with a tapered locking ring, the circumference of the locking ring being interrupted by a relief slot that permits limited expansion of the internal diameter of the tapered locking ring during pre-assembly with the screw head and also permitting limited compression of the internal diameter of the tapered locking ring as the tapered locking ring is fully engaged with the correspondingly tapered hole in the plate such that when fully seated in the hole, the taper locking ring securely locks the screw into position within the plate. 
         [0018]    Also provided is a method of stabilizing spinal vertebrae, the method including providing a low profile anterior vertebral body plate, which is securely attached to the underlying bone of adjacent vertebrae using novel taper lock screws so as to hold one attached vertebra in a fixed position relative to the adjacent attached vertebra. 
         [0019]    Also provided is a kit, which includes at least one low profile anterior vertebral body plate and a corresponding set of novel taper lock screws. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The foregoing and other features of the low profile anterior vertebral body plate and novel taper lock screws will become apparent to one skilled in the art to which the device relates upon consideration of the following description of exemplary embodiments with reference to the accompanying drawings, wherein: 
           [0021]      FIGS. 1  A-E show respectively a top view, isometric view, end view, side view, and cross-sectional end view of the plate with two taper lock screws fully seated and locked into the holes of the plate. 
           [0022]      FIGS. 2A-D  show respectively a top, isometric, first side, and alternate side view of the screw with tapered locking ring assembled.  FIG. 2C  shows a side view of the assembled screw and tapered locking ring with the relief slot showing on the tapered locking ring. 
           [0023]      FIGS. 2E-F  show respectively a side view and isometric view of the screw and the tapered locking ring prior to assembly of the two components. 
           [0024]      FIGS. 3A-D  show respectively a first side view, isometric view, an alternative side view, and a bottom view of the bone screw prior to assembly with the tapered locking ring component. 
           [0025]      FIGS. 4A-D  show respectively a top view, isometric view, first side view with the relief slot showing, and an alternative side view of the tapered locking ring prior to assembly with a screw head. 
           [0026]      FIGS. 5A-E  show respectively a top view, isometric view, end view, side view, and cross-sectional view of the low profile anterior vertebral body plate with multiple holes for receiving respective taper lock bone screws. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    Detailed embodiments of the present invention are disclosed herein; however, it is understood that the following description and each of the accompanying figures are provided as being exemplary of the invention, which may be embodied in various forms without departing from the scope of the claimed invention. Thus, the specific structural and functional details provided in the following description are non-limiting, but serve merely as a basis for the invention as defined by the claims provided herewith. The device described below can be modified as needed to conform to further development and improvement of materials without departing from the inventor&#39;s concept of the invention as claimed. 
         [0028]    The device, as generally shown at  10  in FIG&#39;s  1 A-E is a low profile anterior vertebral body plate  12  that, when implanted in a patient can be secured to the underlying bone using novel taper lock screw assemblies, which are generally shown at  14  in  FIGS. 1A-E  and  2 A-F and include a threaded bone screw  16  and a tapered locking ring  18 . The vertebral body plate  12 , as best shown in  FIGS. 1A-D  and  5 A-E can be provided as an elongated, low profile, plate structure that defines at least one and preferably multiple tapered screw holes  20 , which provide through passage for the threaded portion  22  of the threaded bone screw  16  from the plate upper surface  24  to the plate lower surface  26 . 
         [0029]    As shown in  FIGS. 1A-E  and  5 A-E, the plate  12  can be configured to be generally planar; however, the plate preferably will be formed to have arcuate upper and lower surfaces  24 ,  26 , arcing along both the longitudinal axis  28  as well as the transverse axis  30  of the plate  12 . This arcing of the plate surface provides a better conformational fit to the anterior surface of the vertebrae to which the plate is to be attached. Each of the screw holes  20 , which are defined as through passages in the plate  12 , is provided with a tapered side wall  32 . The degree of inward taper from upper to lower portion of the screw hole side wall  32  corresponds to the degree of inward taper from upper to lower portion of the outer wall  34  of the tapered locking ring  18 . Preferably, the taper is a Morse type taper; however other types of taper can be used without departing from the scope of the invention. 
         [0030]    The tapered locking ring  18  defines a through lumen  36 , which is formed to have a generally concave shaped inner wall  38  that is sized and configured to rotatably receive and hold the complimentary convex shaped outer side wall  40  of the head  42  of the bone screw  16 . As shown in  FIGS. 2A , B, C, E, and F, the tapered locking ring  18  is provided with a expansion/compression relief slot  44 , which serves to break the circumferential continuity of the tapered locking ring  18  such that if compressive forces are exerted inward about the circumference of the locking ring  18 , the relief slot  44  can decrease in size so as to enable the locking ring  18  to absorb those compressive forces and decrease in diameter, albeit doing so with an outward bias to return to the original larger dimensioned normal configuration. Similarly, if expansive forces are exerted outward against the concave shaped inner wall  38  of the tapered locking ring  18 , the relief slot  44  can accommodate those expansive forces and allow an increase in diameter of the locking ring  18 , albeit with an inward bias to return to the original smaller dimensioned normal configuration. 
         [0031]    The flexibility provided by the relief slot  44  is important to the function of assembling of the screw  16  to the tapered locking ring  18  to form the preassembled taper lock screw assembly  14 . The convex shaped outer wall  40  of the screw head  42  is sized and configured to be capable of being preassembled into the concavity formed by the inner wall  38  of the tapered locking ring  18 . This preassembly is easily achieved by forcing the convex shaped outer wall  40  of the screw head  42  into the concavity of the inner wall  38  of the tapered locking ring  18 . The joining and fit of the two components is a snap fit relationship in that the expansive forces created by the forcing of the screw head  42  into the concavity of the locking ring lumen  36  is absorbed by the relief slot  44  until the screw head  42  is in place within the locking ring  18 , at which time the locking ring yields to its bias to return to its normal smaller diameter size and configuration. Once preassembly of the taper lock screw assembly  14  is completed, the convex surface of the screw head  44  is free to rotate within the concavity of the locking ring  18  but is restrained from separating from within the locking ring lumen  36  due to the normal size of the locking ring lumen openings, which are sufficiently smaller than the diameter of the screw head  44 . This preassembly of the taper lock screw assembly  14  makes it possible in practice to insert the screw through the screw hole  20  of the plate  12  into the underlying bone and then lock the screw  16  into place without the need to attach and manipulate small additional locking elements or components as is commonly required with conventional screw locking plate systems. 
         [0032]    The flexibility provided by the relief slot  44  is also important to the function of locking the taper lock screw assembly  14  into position within the plate  12 . As shown in  FIG. 1E , the rotational relationship of the convex shaped screw head  40  with the concave shaped inner wall of the locking ring  38  allows the screw to be inserted into the bone through the screw hole  20  of the plate  12  at virtually any angle necessary. This polyaxial feature of the taper lock screw head assembly  14  in relation to the plane of the plate  12  is a tremendous advantage to providing the best possible connection to the bone. As shown in  FIGS. 1A , B, and E and  FIGS. 2A , B, and F, a tool receptacle  46  having tool gripping elements  48  can be defined in the upper surface  50  of the screw head  40 . The tool gripping elements can be of any configuration that is suitable for facilitating the gripping of the screw head by a correspondingly configured tightening and/or loosening tool. As the preassembled taper lock screw assembly  14  is rotated inward by the action of a tightening tool, the screw threads  16  engage the underlying bone drawing the taper lock screw assembly  14  down into a sliding engagement with the screw hole tapered side wall  32 . As the tapered locking ring  18  slidably engages the tapered side wall  32  of the screw hole  20 , the locking ring  18  is forced to move into the screw hole  20  with an alignment coincident with the taper of the hole  20 . This alignment of the tapered surfaces of the assembly  14  with the screw hole  20  necessarily causes the convex shaped screw head  40  to rotationally adjust within the concavity of the tapered locking ring  18  so as to accommodate the already well established axis of entry of the threaded portion  22  of the screw  16  in the bone. Thus, the taper lock screw assembly  14  interaction with the tapered surface of the screw hole  20  provides the polyaxial feature of the device  10 . As the screw  16  continues to be driven into the underlying bone, locking ring tapered outer wall  34  continues to engage and finally friction locks to the tapered side wall  32  of the screw hole  20 . This friction locking engagement exerts radial compressive force on the tapered locking ring  18 , which at least partially closes or narrows the normal space of the relief slot  44  thereby decreasing the diameter of the tapered locking ring and the space within the concavity of the locking ring lumen  36 . These compressive forces are transferred to the convex shaped screw head  42  so as to hold and lock the screw head  42  in position relative to the plate  12 . 
         [0033]    Thus, the device  10  as described herein advantageously permits the screw  16  to be inserted into bone at a variety of angles relative to the plane of the plate, for example, polyaxial insertion, and with continued insertion of the screw  16  into bone, taper lock screw assembly  14  locks the screw into position relative to the plate  12 . 
         [0034]    The foregoing method of use of the device  10  can be employed as a method of stabilizing or fixing an injured or diseased vertebra and if necessary, multiple devices or a device, which is elongated beyond the examples depicted herein, can be employed as necessary. A reversal of rotational torque on the screw head using a tool designed to generate sufficient torque to overcome the taper lock established between the taper lock screw assembly  14  and the plate  12  can serve to remove the screw from the plate and thus remove the plate from a patient if necessary. The amount of force necessary to overcome the taper lock is greater than that required to simply unscrew the threaded portion  22  of the screw  16  from the bone underlying the plate and is also greater than commonly experienced micro-motion or other forces which can act to cause a conventional screw to back out of the bone. 
         [0035]    While the device as described herein can be preferably used to attach to the anterior surface of cervical vertebrae and is configured to be capable of stabilizing cervical vertebrae, it is within the inventors&#39; understanding that the plate can be configured and adapted to conform to any implantable surgical plate requirement to provide a low profile plate capable of securing and stabilizing any injured or diseased bone. 
         [0036]    The device  10  can be manufactured as integral components by methods known in the art, to include, for example, molding, casting, forming or extruding, and machining processes. The components can be manufactured using materials having sufficient strength, resiliency and biocompatibility as is well known in the art for such devices. By way of example only, suitable materials can include implant grade metallic materials, such as titanium, cobalt chromium alloys, stainless steel, or other suitable materials for this purpose. It is also conceivable that some components of the device can be made from plastics, composite materials, and the like. 
         [0037]    It is also within the concept of the inventors to provide a kit, which includes at least one of the vertebral plate and taper lock screw systems disclosed herein. The kit can also include additional orthopedic devices and instruments; such as for example, instruments for tightening or loosening the bone screws, spinal rods, hooks or links and any additional instruments or tools associated therewith. Such a kit can be provided with sterile packaging to facilitate opening and immediate use in an operating room. 
         [0038]    Each of the embodiments described above are provided for illustrative purposes only and it is within the concept of the present invention to include modifications and varying configurations without departing from the scope of the invention that is limited only by the claims included herewith.