Abstract:
The invention is directed to a bone plate system. The bone plate system comprises 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.

Description:
BACKGROUND 
   The spinal column of vertebrates provides support to bear weight and protection to the delicate spinal cord and spinal nerves. The spinal column comprises a series of vertebrae stacked on top of each other. There are typically seven cervical (neck), twelve thoracic (chest), and five lumbar (low back) segments. Each vertebra has a cylindrical-shaped vertebral body in the anterior portion of the spine with an arch of bone to the posterior that covers the neural structures. Between each vertebral body is an intervertebral disk, a cartilaginous cushion to help absorb impact and dampen compressive forces on the spine. To the posterior, the laminar arch covers the neural structures of the spinal cord and nerves for protection. At the junction of the arch and anterior vertebral body are articulations to allow movement of the spine. 
   Various types of problems can affect the structure and function of the spinal column. These can be based on degenerative conditions of the intervertebral disk or the articulating joints, traumatic disruption of the disk, bone or ligaments supporting the spine, tumor or infection. In addition congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Anterior slippage of one vertebral body on another (spondylolisthesis) can cause compression of the spinal cord or nerves. Patients who suffer from one of more of these conditions often experience extreme and debilitating pain, and can sustain permanent neurologic damage if the conditions are not treated appropriately. 
   One technique of treating these disorders is known as surgical arthrodesis of the spine. This can be accomplished by removing the intervertebral disk and replacing it with bone and immobilizing the spine to allow the eventual fusion or growth of the bone across the disk space to connect the adjoining vertebral bodies together. The stabilization of the vertebra to allow fusion is often assisted by a surgically implanted device to hold the vertebral bodies in proper alignment and allow the bone to heal, much like placing a cast on a fractured bone. Such techniques have been effectively used to treat the above described conditions and in most cases are effective at reducing the patient&#39;s pain and preventing neurologic loss of function. However, there are disadvantages to the present stabilization devices. 
   The spinal fixation device needs to allow partial sharing of the weight of the vertebral bodies across the bone graft site. Bone will not heal if it is stress shielded from all weight bearing. The fixation device needs to allow for this weight sharing along with the micromotion that happens during weight sharing until the fusion is complete, often for a period of three to six months or longer, without breakage. The device must be strong enough to resist collapsing forces or abnormal angulation during the healing of the bone. Loss of alignment during the healing phase can cause a poor outcome for the patient. The device must be secure in its attachment to the spine to prevent migration of the implant or backout of the screws from the bone which could result in damage to the structures surrounding the spine, resulting in severe and potentially life threatening complications. The device must be safely and consistently implanted without damage to the patient. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a bone plate system that addresses one or more of the above drawbacks. In one embodiment, the bone plate system comprises 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. 

   
     DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a perspective view of a bone plate system according to the invention. 
       FIG. 2  is a top view of the bone plate system of FIG.  1 . 
       FIG. 3  is a side view of the bone plate system of FIG.  1 . 
       FIG. 4  is an end view of the bone plate system of FIG.  1 . 
       FIG. 5  is an end cross-sectional view of the bone screws inserted through the bone plate and covered by the interference device of a bone plate system as depicted in  FIGS. 1  to  4 . 
   

   DETAILED DESCRIPTION 
   An exemplary embodiment of a bone plate system according to the present invention is shown in  FIGS. 1  to  5 . The depicted bone plate system is particularly useful for stabilizing two or more vertebral bodies to allow fusion by holding the vertebral bodies in proper alignment, and thus allowing the bone to heal. The bone plate system of the invention is preferably weight-sharing over a period of at least about three months, and more preferably over a period of at least about six months. As used herein, the term “weight-sharing” as describing the system means that the system allows for partial sharing of the weight of one or more vertebral bodies across a bone graft site. 
   Generally, the bone plate system comprises a base plate  14 , bone screws  16 , and one or more interference devices  18 . The base plate  14  is a plate used to connect two or more bones. As used herein, the term “bones” is intended to include both bones and bone fragments or portions. The base plate  14  can be of any suitable shape or size for the desired application. 
   In the depicted embodiment, the base plate  14  is a generally rectangular plate having two relatively long, generally parallel sides  20 , a top end  22  and a bottom end  24 , with the top end and bottom end also being generally parallel and shorter than the sides, as shown in FIG.  2 . In the illustrated embodiment, each of the sides  20 , top end  22  and bottom end  24  includes a region that is scalloped or concave. The concave regions are preferably located along the midsections of each of the sides  20 , top end  22  and bottom end  24 . As discussed further below, this scalloped or concave design is particularly desirable to prevent the base plate  14 , when implanted, from interfering with the esophagus of the patient. 
   When viewed from a side  20 , the base plate  14  has a bottom surface that is slightly concave (along the length of the base plate), as best shown in FIG.  3 . Similarly, when viewed from the top end  22  or the bottom end  24 , the bottom surface of the base plate  14  is slightly concave (along the width of the base plate), as best shown in FIG.  4 . This design permits the base plate  14  to be curved in the sagittal and horizontal planes to provide a better fit when implanted in a patient. 
   In one embodiment, the sides  20  each having a length ranging from about 20 mm to about 40 mm, and the top and bottom ends  22  and  24  each having a length ranging from about 14 mm to about 20 mm. The dimensions of the base plate  14  can vary depending upon the size of the patient in whom the plate is to be introduced. Further, the base plate  14  can have any other suitable shape, such as an oval shape. 
   The base plate  14  can be made of any suitable material, and is preferably made of titanium or a titanium alloy. If desired, the base plate  14  can also be part of a larger device. 
   The base plate  14  contains at least two bone screw slots  30  for receiving the bone screws  16 . In the depicted embodiment, the base plate  14  contains four bone screw slots  30  arranged in two pairs. The bone screw slots  30   a  of the first pair are located nearer the top end  22  of the base plate  14 , and the bone screw slots  30   b  of the second pair are located nearer the plate&#39;s bottom end  24 . The bone screw slots  30   a  of the first pair are generally parallel to each other and to the sides  20  of the base plate  14 . Similarly, the bone screw slots  30   b  of the second pair are generally parallel to each other and to the sides  20  of the base plate  14 . The first pair of bone screw slots are  30   a  preferably generally aligned with the second pair of bone screw slots  30   b  and the pairs of slots positioned relative to the sides  20  and top and bottom ends  22  and  24  of the base plate  14  so as to create a generally symmetrical arrangement about the center of the base plate, as best shown in FIG.  2 . The distance D 1  between the pairs preferably ranges from about 6 mm to about 14 mm. The distance D 2  between the slots  30   a  of the first pair preferably ranges from about 4 mm to about 8 mm. 
   In the depicted embodiment, all of the slots  30  have the same dimensions. The length of each slot preferably ranges from about 4 mm to about 8 mm, more preferably from about 6 mm to about 7 mm, still more preferably about 6.6 mm. The width of each slot preferably ranges from about 3 mm to about 6 mm, more preferably from about 4 mm to about 5.5 mm, still more preferably about 4.8 mm. 
   The depicted base plate  14  is particularly designed for the portion of the plate nearer the top end  22  to be attached to one vertebral body and the portion of the plate nearer the bottom end  24  to be attached to another vertebral body so that the plate is mounted in a vertical arrangement. The plate  14  has a bottom surface  26  that, in use, is adjacent to one or more of the bone surfaces, and a top surface  28  that faces away from the bone surface(s). 
   In the depicted embodiment, the bottom and top surfaces  26  and  28  are generally parallel to one another over a center region of the base plate  14 , which encompasses the slots  30 , so that in the center region the base plate has a generally uniform thickness, although the plate need not have a uniform thickness in this region. The top and bottom ends  22  and  24  of the base plate  14  are generally tapered, being thicker closer to the slots  30  and thinner closer to the edge of the plate  14 , so that the bottom and top surfaces  26  and  28  are not parallel to one another at the top and bottom ends. Preferably the center region of the base plate  14 , which encompasses the slots  30 , has a thickness ranging from about 1 mm to about 4 mm, more preferably from about 2 mm to about 3 mm, still more preferably about 2.8 mm. The tapered top and bottom ends  22  and  24  each preferably taper to an edge thickness ranging from about 0.5 mm to about 2 mm, more preferably from about 0.5 mm to about 1.5 mm, still more preferably about 1 mm. The top and bottom ends  22  and  24  preferably taper at an angle ranging from about 20° to about 40°. This tapered end design is particularly beneficial in vertebral applications where the base plate  14  comes into contact with the esophagus. With the inventive tapered design, the esophagus is not positioned over a sharp corner, as in many plate designs, but instead is provided with a smooth transition surface as it passes over the inventive base plate, minimizing damage to the esophagus. In an alternative embodiment, only the top end  22  is provided with the tapered design, as the top end has the leading edge over which the esophagus interfaces. The tapered design in combination with the scalloped or concave design, discussed above, further enhances the ability of the plate system to avoid causing dysphagia. 
   The bone plate system of the invention includes a number of bones screws  16  that corresponds to the number of bone screw slots  30 , which in the depicted embodiment is four. In use, the bone screws  16  extend through the bone screw slots  30  in the base plate  14  and are screwed into the bone. The bone screws  16  can be made of any suitable material, and are preferably made of the same material as the base plate, such as titanium or a titanium alloy. The shape and function of the bone screws are described in more detail below. 
   One or more interference devices  18  are provided to retain the bone screws  16  that extend through the bone screw slots  30  in the base plate  14 . By “retain” is meant that a interference device  18  covers a sufficient portion of a bone screw  16  to prevent that bone screw from “backing out” out of the bone and base plate  14 . The terminology “retain” is not intended to require that the entire top surface, or even the majority of the top surface, of a bone screw  16  be covered. 
   In the depicted embodiment, the bone plate system includes two interference devices  18 , each one for retaining a corresponding one of the two pairs of bone screws  16 , so that one interference device  18   a  is provided nearer the bottom end  24  of the base plate  14  and the other interference device  18   b  is provided nearer the top end  22  of the base plate, as best shown in FIG.  1 . Each interference device  18  is generally rectangular having a rounded top edge. 
   The thickness of each interference device  18  preferably ranges from about 0.5 mm to about 2 mm, and more preferably from about 1 mm to about 1.5 mm. As shown best in  FIGS. 1 and 3 , the edge of each interference device  18  that is positioned nearest the corresponding top or bottom end  22  or  24  of the base plate  14  is tapered in a manner similar to the top and bottom ends of the base plate. 
   In use, the interference devices  18  are removably fixedly attached to the base plate  14 . As used herein, the term “fixedly attached” refers to the arrangement where the interference device  18 , while being removably attached to the base plate  14 , cannot slide relative to the base plate. The base plate  14  is preferably provided with recesses  34  to receive the interference devices  18  and retain them in the desired position. The interference devices  18  can be removably attached to the base plate  14  by any suitable mechanism. In the depicted embodiment, each interference device  18  is provided with a cover screw hole  36 . 
   The base plate  14  is provided with two cover screw apertures  38  that generally correspond is size and location to the cover screw holes  36  in the interference devices  18  when the interference devices are correctly positioned over the base plate. In the depicted embodiment, the cover screw apertures  38  are offset from the bone screw slots  30  in the base plate  14 , as best shown in FIG.  1 . By “offset” is meant that the cover screw aperture  38  is provided at a position that does not fall on a line that passes through the centers of the bone screw slots  30 , but instead that is offset from such a line, preferably in a direction closer to the center of the base plate  14 . 
   To removably attach an interference device  18  to a base plate  14 , a cover screw  39  is screwed into the cover screw hole  36  in the interference device and a corresponding cover screw aperture  38  in the base plate, as is generally known in the art. However, other mechanisms for attaching a interference device to a base plate known to those skilled in the art could also be used in accordance with the invention. If a different mechanism for attaching an interference device  18  is used, the point of attachment to the base plate  14  is preferably offset from the bone screw slots  30 . 
     FIG. 5  shows how the bone screws  16  are positioned relative to a base plate  14  and interference device  18 . Each bone screw  16  comprises a screw head  15  and a threaded portion  17  extending from the screw head. Each bone screw head  15  has a generally rounded top surface  40 , optionally with a generally flat center region  42 , and a generally rounded bottom surface  44 . The center region  42  of the generally rounded top surface  40  includes a hexagonal or other shaped aperture  46 , as shown in  FIGS. 1 and 2 , for receiving the end of an insertion tool, as is generally known in the art. 
   Within the base plate  14 , each bone screw slot  30  has a generally rounded edge surface  48  that interfaces with the bottom surface  44  of a corresponding bone screw head  15 . An elongated axial opening  50  is provided along the bottom of each bone screw slot  30  to receive the threaded portion  17  of the bone screw  16  as the bone screw slides in the slot. The rounded edge surface  48  of the bone screw slot  30  joins the top surface  28  of the bone plate  14  to a top edge of the axial opening  50 . The axial opening  50  has a width less than the width of the top portion of the bone screw slot  30 , i.e., the portion of the bone screw slot receiving the screw head  15 . 
   The interference device  18  has two generally rounded edge surfaces  52  that interfaces with the edges of the generally rounded top surfaces  40  of the bone screw heads  15  while not substantially covering the center region  42  of the bone screw head, to thereby prevent the bone screw  16  from backing out of the base plate  14 . This design permits each bone screw  16  to toggle within its corresponding bone screw slot  30  while the interference device  18  still exerts sufficient force on the bone screw to control the degree to which the bone screw slides within the bone screw slot  30 . 
   In the depicted embodiment, the rounded top surface  40  of each bone screw  16  has a maximum outer diameter less than the maximum outer diameter of the screw&#39;s rounded bottom surface  44 . With this design, the rounded bottom surface  44  can be made sufficiently large to prevent the head of the bone screw  16  from passing through bone screw slot  30 , while the rounded top surface  40  is smaller to permit the interference device  18  to still lower on the base plate  14 , thus further reducing interference between the plate system and the esophagus. 
   In one embodiment, the interface of the top surface  40  of the bone screw head  15  and the edge surface  52  of the interference device  18  is provided to require a minimum force to cause the bone screw  16  to slide within the bone screw slot  30 , thus permitting the bone screws to controllably slide in the bone screw slots. The minimum force preferably ranges from about 15 to about 50 pounds, more preferably from about 30 to about 40 pounds. The system can be advantageously arranged to permit the bone screw  16  to both settle in a direct vertical plane of 1.5 to 2 mm and pivot under the interference device  18 . 
   The offset location of the anchor or attachment point (i.e., the cover screw) of the interference device  18  to the base plate  14  advantageously provides increased resistance to screw sliding and toggling as vertebral bodies joined by the plate begin to settle. Specifically, when the bone screws  16  are first introduced into vertebral bodies through the base plate  14 , they are introduced into the bone screw slots  30  closer to the top and bottom ends of the base plate (i.e., farther from the center of the base plate). In this location, the bone screws are permitted to toggle and slide fairly easily, which is desirable because it advantageous to permit the plate system to initially engage the vertebral bodies and permit the vertebral bodies to settle on the graft fairly quickly. After initial settling has occurred, the base plate  14  moves relative to the bone screws  16 , thereby positioning the bone screws closer to the center of the base plate. As the bone screws move toward the center of the base plate  14 , they also move nearer the cover screw or other point of attachment between the interference device and the base plate. At this point of attachment, the interference device exerts the greatest force on the bone screws  16 . Thus, as the bone screws  16  move toward the point of attachment, the incremental force required to further move the bone screws toward the point of attachment (and thus toward the center of the plate) increases. Thus, the plate system advantageously controls further settling, thereby preventing the vertebral bodies from collapsing too quickly. 
   The preceding description has been presented with reference to presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures may be practiced without meaningfully departing from the principal, spirit and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and illustrated in the accompanying drawings, but rather should be read consistent with and as support to the following claims which are to have their fullest and fair scope.