Abstract:
An anterior cervical plate system consists of a base plate and a connecting plate movably connected to the base plate. The base plate can be inserted into any number of cervical vertebral bodies in any one construct. The base plate contains holes for two unicortical bone screws and a third hole to accommodate a rather large diameter, but short screw that movably secures the connecting plate, and raised middle portion to strengthen screw purchase and fit of the connecting plate. The connecting plate has a central trough opening to accommodate this screw.

Description:
RELATED APPLICATION 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/560,415, filed Apr. 27, 2000 abandoned, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/131,629 filed on Apr. 28, 1999, the contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a modular anterior cervical plate designed to provide internal stabilization (temporary strengthening) to the spine in the cervical region (neck) during surgical repairs through an anterior approach to the neck. 
     This device is designed to improve healing and make it more likely that surgical fusion will be followed by bony union, and reduce the need for external braces following surgery. This is not the first anterior cervical plate ever designed, but it has several novel features that will facilitate decompressive aspects of cervical spine surgery, and will facilitate compression and distraction during cervical fusion and will allow dynamic settling if necessary in the first several weeks after surgery. 
     Devices presently on the market are basically thin (less than about 2.5 mm thick) molded metal plates that bridge gaps in the front of the cervical spine caused by surgery. (Examples are the Orion plate by Sofomor-Danke, the Codman plate by Johnson and Johnson, the Morsher plate by Synthes Inc., the Acromed plate, and others.) They stabilize the spine when screws are inserted through holes in the plate into bone above and below the surgical gap in the spine. As such, all plates on the market today are basically a single unit design. 
     SUMMARY OF THE INVENTION 
     The present invention provides an anterior cervical plate comprising modular parts: a pair of base plates and a connecting plate. The connecting plate is connected to the base plates so as to allow a controlled movement between the base plates and the connecting plate. The base plate design has two advantages. First, because of its small size, it does not obscure surface landmarks on the spine, reducing instances of errant screw insertion and surgical complications. When surface landmarks are obscured, chances for errant screw insertion and surgical complications increase. Secondly, the base plate can be used with distracting instruments to facilitate distraction during dicectomy or other decompressions, which is not a feature of any other anterior plate design. Distracting instruments, such as distracting forceps, stretch the vertebrae of the spine by bearing against distraction-compression portions on the base plates. The base plate may also be designed to interface with retractor blades. 
     The anterior cervical plate of the invention facilitates decompressive aspects of cervical spine surgery, and facilitates distraction (i.e. stretching of the spine) during cervical fusion. The base plate and connecting plate combination allows for insertion of fusion bone with distraction or compression, finely manipulated by the surgeon. No other plate design allows for this. With all other plates, one must rely on the tightness of fit obtained with the fusion bone (the degree to which the bone achieves a proper fit) to maximize conditions for fusion. The tighter the fit, the more likely fusion is to take place. Since this plate can maximize compression forces beyond what can be obtained with traditional plate designs, fusion rates should be higher. Finally, the connecting plate to base plate interface can be modified to allow dynamic settling of the spine over several weeks time in the saggital and coronal planes to the spine. Particularly when large surgical gaps are created, this kind of settling caused by gravity is felt to promote and enhance fusion. This function is available in only one other plate design on the market (Acromed), but by a different mechanism. 
     These advantages are derived from the modular design. With the development of a thickened midsection of the base plate, a lock screw has been shown in pullout tests to hold the two plates together very tightly, while allowing a controlled movement at the interface. While a controlled movement at the interface between the connecting plate and the base plate is allowed, the connecting plate is constrained from rotational movement by raised distraction and compression knobs at the lateral margins of the base plate. Therefore, the design will not fail under expected mechanical stresses, yet the unique advantages of the modular design will remain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the connecting plate of the invention; 
         FIG. 2  is a perspective view of the base plate of the invention; 
         FIG. 3  is a perspective view of the connecting plate and two base plates, as they would be assembled in use; 
         FIG. 4  is a cross sectional view of the assembled device according to one embodiment; 
         FIG. 5  is a schematic side view of the assembly of the invention attached to the spine; 
         FIG. 6  is a cross sectional view of the assembled device according to another embodiment, using lock screws to provide a movable interface between the base plates and the connecting plate; 
         FIG. 7  is a schematic plan view of the assembly of the invention attached to the spine, with a distraction tool attached; 
         FIG. 8  is a cross sectional view of the assembled device attached to the spine according to an embodiment of the invention, with a retractor blade attached. 
         FIG. 9  is a top view of a base plate, illustrating the process of inserting a retractor blade into a retractor knob of the base plate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an anterior cervical plate designed to facilitate settling of the spine in the saggital and coronal planes by allowing a controlled movement between a base plate assembly and a connecting plate. The invention will be described below relative to an illustrative embodiment. One skilled in the art will recognize that the invention is not limited to the illustrative embodiment and that variations may be made in accordance with the teachings and scope of the invention. 
     The anterior cervical plate of an illustrative embodiment of the present invention includes a pair of base plates for engaging vertebral bodies and a connecting plate connecting the two base plates. The anterior cervical plate is used to stabilize the spine during spinal surgery by aligning and maintaining the vertebral bodies in selected positions and orientations relative to each other to promote and enhance spinal fusion of the vertebral bodies. 
     As shown in  FIG. 1 , the connecting plate  10  of the invention is a generally rectangular plate, preferably made from an alloy having good corrosion resistance, relatively low density and high strength and durability, such as titanium or a titanium alloy. One skilled in the art will recognize that any suitable materials may be used to form the plate. The connecting plate has screw slots  12  at either end, and at least two screw holes  14  in the middle for receiving bone graft screws or another suitable fastening element for connecting the connecting plate to a bone graft. One skilled in the art will recognize that the invention is not limited to bone graft screws and that any suitable fastening element, such as a pin, can be used. 
     In a preferred embodiment, the connecting plate is between about 1.9 mm and about 3.1 mm thick. Preferably, the connecting plate is about 2.02 mm thick. The length of the connecting plate may vary, for example, at 5 mm intervals, from about 20 mm to about 110 mm. The width of the illustrative connecting plate may be between about 15 mm and about 18 mm and is preferably about 16 mm. 
     The base plate  20 , as shown in  FIG. 2 , is generally rectangular, and preferably has a width of between about 8 mm and about 10 mm and preferably about 9 mm, a length of between about 18 mm and about 22 mm and preferably about 20 mm, and a maximum height, i.e. a raised central section  22 , of between about 1.5 mm and about 3 mm and preferably about 2.54 mm. One skilled in the art will recognize that the base plate is not limited to the illustrative dimensions and that variations in the width, length and thickness may be made. The base plate  20  may also be formed of titanium or a titanium alloy, though one skilled in the art will recognize that any suitable material may be used. 
     Each base plate  20  includes a raised central section  22  that is generally rectangular and insertable in one of the screw slots  12  of the connecting plate  10  (see FIG.  3 ). The base plate raised central section  22  has a threaded screw hole  24  for receiving a central screw  25 , preferably of titanium alloy, for securing the base plate  20  to the connecting plate  10 . One skilled in the art will recognize that other suitable materials may be used to form the screw  25 . The illustrative screw  25  has a 6/32 thread with 3 turns over about 2.0 mm, though the invention is not limited to these parameters. The threaded screw hole  24  may or may not extend through the base plate  10 . The screw slot  12  and the central screw  25  may form a controllably movable interface between the base plate  20  and the central plate  10 , to be described in detail below. 
     At either side of the base plate  20  are raised distraction-compression knobs or portions  26 , defining holes  28  oriented parallel to the spine, for the insertion of elements of distraction tools. 
     Between the raised central portion  22  of the base plate and the distraction-compression sections  26  at either end, are planar reduced thickness sections  28  (between about 0.25 mm and about 1 mm thick and preferably about 0.5 mm thick). These reduced thickness sections  28  of the base plate include two or more bone screw holes  30  for receiving unicortical bone screws, to attach the base plate to vertebral bodies. According to an illustrative embodiment, the bone screws  32  have an outside diameter of between about 3 mm and about 4.5 mm and preferably about 3.5 mm and a length between about 14 mm and about 18 mm. 
     To stabilize the spine using the anterior cervical plate of the present invention, retractor blades may be utilized to initially expose and provide access to the anterior vertebral column. As shown in FIG.  4  and  FIG. 5 , a pair of base plates  20  are secured to vertebral bodies  40  in the spine by bone screws  32  passing through the bone screw holes  30  of the base plate  20 . The connecting plate  10  is placed over the base plates  20  (see FIG.  3 ), and the central screw  25  is threaded into each base plate screw hole  24  to secure the connecting plate  10  to the base plates  20 . The width of the base plate central section  24 , and the distance between the base plate central section  24  and the distraction-compression knobs  26  are selected so that the connecting plate  10  fits snugly and angular movement of the connecting plate  10  relative to the base plate  20  is prevented. 
     According to one embodiment, shown in  FIG. 6 , the central screw  25  may comprise a lock screw to allow for a controlled movement between the base plates  20  and the connecting plate  10  during fusion of the spine. As shown, the perimeter of the screw slot  12  of the connecting plate  10  may comprise a beveled edge  120  that contacts the edge  251  of the screw head  250  to connect the base plate  20  to the connecting plate  10 . The amount of surface area contact between the beveled edge  120  of the slot  12  and the edge  251  of the screw head  250  determines the amount of friction holding the connecting plate to the base plate and may be varied to allow for controlled movement between the base plate and the connecting plate. For example, the length and configuration of the screw  25  and the angle of the edge  251  of the screw head  250  may be varied to provide varying amounts of pressure between the screw and the connecting plate. As shown in  FIG. 6 , the screw  25  may be configured to allow minimal surface area contact with the connecting plate  10 , resulting in less friction at the base plate to connecting plate interface, thereby allowing the base plate  20  to controllably slide in the slot  12  relative to the connecting plate  10 . Alternatively, the beveled edge  120  may slope at substantially the same angle as the edge  251  of the screw head  250  to increase the amount of contact between the edge  251  of the screw and the beveled edge  120  of the slot, thereby providing a relatively tighter fit. 
     Over the course of healing and fusion of the vertebrae, different screws  25  may be inserted to vary the amount of contact between the connecting plate edge and the screw, which varies the amount of force holding the base plate to the connecting plate and allows a controlled movement of the base plate along the slot of the connecting plate. 
     According to another embodiment, the movable interface may be achieved by increasing the length of the body of the lock screw, i.e. the threaded portion, such that the threaded portion is longer than the length of the screw hole  24 . The increased length of the screw  25  relative to the screw hole  24  causes the screw head  250  to protrude from the screw hole  24  and the connector plate slot  12 . When the lock screw  25  is screwed into the threaded hole  24  of the base plate  20 , the bottom surface  260  of the screw  25  abuts the bottom surface  240  of the threaded hole  24 . When the bottom surface  260  abuts the bottom surface of the threaded hole  24 , the screw head  250  sits slightly above, and spaced from, the beveled edge  120  of the connecting plate  10 . The screw head thus forms a gap between the edge  251  of the screw head  250  and the beveled edge  120  on the connecting plate  10 . The gap between the screw head  250  and the connecting plate  10  may allow for a controlled movement of the connecting plate  10  relative to the base plate  20 . The lock screw  25  limits the amount of relative movement between the connecting plate  10  and the base plate  20 . 
     According to an alternate embodiment, the screw head  250  may have a flat edge, rather than a beveled edge, and/or the perimeter of the slot  12  may also be flat. The contact area between the edge of the screw head and the perimeter of the slot may be varied, allowing for a controlled movement of the connecting plate relative to the base plate. One skilled in the art will recognize that any suitable configuration for varying the amount of surface contact area between the screw head and the connecting plate or for forming a gap between the screw head and the slot may be utilized to allow for a controlled movement between the base plate and the connecting plate. 
     The length of the threaded hole  24  and the screw  25  and the configuration of the screw head  250  may be specifically selected to determine the contact area and control the amount of potential movement between the connecting plate and the base plate when anterior cervical plate is assembled. One skilled in the art will recognize that alternate means for providing a movable interface may be utilized according to the teachings of the invention. 
     The ability to provide a controlled movement between the base plate and the connecting plate allows for compression or distraction of the spine during the performance of a decompression and facilitates decompression or compression during subsequent fusion of the vertebrae. This controlled movement of the base plate and the connecting plate relative to each other allows gravitational settling to place further compression on a graft as the base plate moves along the slots in the connecting plate. The controlled movement thus promotes, facilitates and enhances a controlled amount of settling of the vertebrae, while preventing rotation, falling and flexing of the connecting plate, which significantly improves healing and fusion of the vertebrae in the cervical region of the spine. 
     As also shown in  FIG. 6 , the distraction-compression knobs  26 ′ may be configured to interface with retractor blades. The distraction-compression knobs  26 ′ include slots  270  configured for the insertion of retractor blades. The details of the retractor blade interface will be described in detail below. 
     As shown in  FIG. 7  a distraction tool  50  may be used with the assembly  10 . Pins  52  the distraction tool  50  are insertable into the holes  28  the distraction-compression knobs  26 . The assembly facilitates distraction by allowing for a controlled movement between the connecting plate and the base plates when a force is applied to the distraction-compression knobs  25  by the pins  52  of the distraction tool. In this manner, fusion of the spine is promoted, enhanced and accelerated. 
     As shown in FIG.  8  and  FIG. 9 , retractor blades may also be used with the assembly  10 . The retractor blades are used to hold open an incision in the skin  300  to expose and provide access to the spine  400 . As shown in  FIG. 8 , the retractor blades  60  are unattached to the base plate  20  and may be inserted into the slots  270  on knobs  26 ′ of the base plate  20 . As shown in  FIG. 9 , the retractor blade fits into an open end  271  of the slot  270 , which is formed at a first end of the knob  26 ′ in the base plate  20  and slides through the slot  270  to allow for retraction. The slot  270  may includes a closed end  272  to retain the retraction blade  60 . 
     Variations on the assembly are possible. For example, the connecting plate  10  may have screw notches in one or both of the screw slots  12 . 
     A preferred material to construct the modular anterior cervical plate assembly of the present invention is titanium, though one skilled in the art will recognize that alternate materials may be utilized. 
     Since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.