Abstract:
The present invention is directed to a bone plate assembly including a bone plate, bone screws received in apertures in the assembly, and a screw retaining member fixed to the bone plate which covers at least a portion of the bone screws. When bone screws have been received by the bone plate and inserted into bone and/or tissue, the bone plate assembly can be used to fuse anatomical structures together, such as adjoining bones, or to heal a fracture in bone.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a bone plate assembly including a bone plate, bone screws received in apertures in the bone plate, and a screw retaining member attached to the bone plate which covers at least a portion of the bone screws. When bone screws have been received by the bone plate and inserted into bone and/or tissue, the bone plate assembly can be used to fuse anatomical structures together, such as adjoining bones, or to heal a fracture in bone. 
     BACKGROUND OF THE INVENTION 
     The bones and connective tissue of an adult human spinal column consist of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex. The complex consists of an anterior disc and two posterior facet joints. The anterior discs of adjacent bones are cushioned by cartilage spacers referred to as intervertebral discs. The bones of the spinal column are categorized as: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine is a sacral bones (including the coccyx). 
     The spinal column of bones is a highly complex anatomical structure as evidenced by the sophisticated interaction between the bones which comprise it. Furthermore, the spinal column houses and protects critical elements of the nervous system. Despite its complexity, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. 
     Various types of problems can affect the structure and function of the spinal column. Such problems can be based on degenerative conditions of the intervertebral disc or the articulating joints, or trauma to the disc, bone, or ligaments supporting the spine. Other problems include tumor or infection. In addition, congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Slippage (spondylolisthesis) anterior of one vertebral body on another can cause compression of the spinal cord or nerves. Patients who suffer from one or 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 pathrodisis of the spine. This can be accomplished by removing the intervertebral disc and replacing it with bone and immobilizing the spine to allow the eventual fusion or growth of the bone across the disc space to connect the adjoining vertebral bodies together. The stabilization of the vertebrae 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 these techniques are effective at reducing the patient&#39;s pain and preventing neurologic loss of function. However, there are disadvantages to the present stabilization devices and to the available tools to implant them. 
     The spinal fixation device should permit 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 healing can adversely affect the recovery. The device must be secure in its attachment to the spine to prevent migration of the implant or back out of the screws from the bone which could result in damage to the structures surrounding the spine, causing severe and potentially life threatening complications. The device must be safely and consistently implanted without damage to the patient. 
     The conventional method of installing bone screws entails drilling a hole, tapping the hole and threading the bone screw into the bone. To drill the hole a guide is held next to or attached to the plate. A drill is inserted into the guide and the hole drilled into the bone. The guide is removed and a tap is threaded through the hole attempting to follow the same angle as the drill hole. Caution must be used to prevent the sharp edges of the tap from damaging surrounding tissues or in creating too large a tap hole by toggling the handle of the tap. This will reduce the security of the screw bite into the bone and increases the likelihood of screw pullout. After tapping, the screw must be guided at the proper angle into the hole that has been created, as inadvertent misalignment can reduce pullout strength or result in damage to surrounding nerves or arteries. 
     Genetic or developmental irregularities, trauma, chronic stress, tumors and disease, however, can result in spinal pathologies which either limit this range of motion, or which threatens the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by use of a surgical implant. It is known that with cervical plates, the screw head may be provided with an arcuate shape, and the plate may be provided with a recess having a complimentary shape that receives the shape of the head. In this arrangement, the head and plate share load bearing responsibilities over an enlarged surface area. Further, since each of the head and recess have arcuate surfaces, the screw shaft is able to rotate in an arcuate path relative to its longitudinal axis. For instance, as shown in U.S. Pat. No. 5,534,027 at col. 4 lines 18-19 and FIG. 5, it is possible for “axis “A” of the screw 10 to be at an angle “AA” to axis “B” of the hole 31. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a bone plate assembly including a bone plate having apertures through which bone screws are received and a screw retaining member that covers at least a portion of the bone screws. The screw retaining member is provided with an aperture that receives a member such as a screw which fixes the screw retaining member to the bone plate. The bone plate assembly of the present invention can be fastened to at least two bones, or at least two portions of bones, in order to facilitate the healing process. The bone plate is provided with apertures through which bone screws are received and fitted into drill holes, in order to fasten the plate to bone. 
     In one embodiment, the screw holes in the bone plate are not provided with uniform dimensions. For example, in one embodiment, the size of a dimension D 1  of the apertures is greater than the size of a dimension D 2  of the apertures. In another embodiment, D 1  corresponds to the length dimension of the aperture and D 2  corresponds to the width dimension of the aperture. In yet another embodiment, length dimension D 1  of the aperture runs in the same direction as length dimension D 1  of the bone plate. The bone screws which are inserted into the apertures, which have a screw head, and a shaft, possess dimensions that permit the screw to move in the D 1  dimension. 
     In yet a further embodiment, the aperture of the screw retaining member is not provided with uniform dimensions. For example, in one embodiment, the size of a dimension D 1  of the apertures in the screw retaining members is greater than the size of a dimension D 2  of the apertures in the screw retaining members. In another embodiment, D 1  corresponds to the length dimension of the apertures and D 2  corresponds to the width dimension of the apertures. In yet another embodiment, length dimension D 1  of the apertures in the screw retaining members runs in the same direction as length dimension D 1  of the apertures that receive the bone screws. In yet another embodiment, length dimension D 1  of the apertures in the screw retaining members runs in the same direction as length dimension D 1  of the of the apertures that receive the bone screws, and in the same direction as length dimension D 1  of the bone plate. The screws which are inserted into the apertures in the screw retaining members and fix it to the bone plate possess a smaller dimension in the D 1  direction the screw retaining member is able to move with respect to the D 1  dimension of its aperture. 
     In a further embodiment, the screw retaining member is in contact with the bone screw when the screw retaining member is fixed to the bone plate. In another embodiment, the screw retaining member, or a portion thereof, resides in a position that permits it to contact the bone screw when the bone screw moves within the bone plate. Accordingly, in these embodiments, the present invention permits the screws and screw retaining members to move in at least one direction. This is desirable, as it allows the locations of the screws and screw retaining members to shift in accordance with the shifting loads placed upon the vertebrae to which the plate is anchored. In yet another embodiment, the screw retaining member is spaced away from the bone plate in a position prevents the screw retaining member from backing out of the bone plate, which could happen if the screws unloosened from the bone in which they were inserted. 
     Yet another embodiment of the present invention is directed to a bone plate assembly for implantation in an anatomical body having a bone plate having apertures and bone screws received in the apertures, a screw retaining member having an aperture for receiving a member that fixes the screw retaining member to the bone plate, the bone plate having a location for receiving the member that fixes the screw retaining member to the bone plate, the screw retaining member covering at least a portion of the bone screw, wherein when the screw retaining member is fixed to the bone plate, a gap is present between the bone plate and screw retaining member in the region between the apertures for the bone screws and the location for receiving the member that fixes the screw retaining member to the bone plate, and further, the screw retaining member rests upon the bone plate in at least one bone plate location. In yet another embodiment, the screw retaining member rests upon the bone plate at a sidewall of the apertures for receiving the bone screws. In yet a further embodiment, the bone plate is provided with a relatively flat region between the apertures for the bone screws and the location for receiving the member that fixes the screw retaining member to the bone plate. In yet a further embodiment, the screw retaining member is provided with an arcuate region between the apertures for the bone screws and the location for receiving the member that fixes the screw retaining member to the bone plate. In a yet further embodiment, the screw retaining member is provided with an arcuate region between the apertures for the bone screws and the location for receiving the member that fixes the screw retaining member to the bone plate, and the bone plate is provided with a relatively flat region between the apertures for the bone screws and the location for receiving the member that fixes the screw retaining member to the bone plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an embodiment of the invention. 
     FIG. 2 is an end view of an embodiment of the present invention. 
     FIG. 3 is a top plan view of an embodiment of the present invention. 
     FIG. 4A is a top plan view of an screw retaining member. 
     FIG. 4B is a top plan view of a portion of a bone plate. 
     FIG. 5 is a further top plan view of an embodiment of the present invention. 
     FIG. 6 is a side elevational view of an bone screw used in the embodiments of the present invention. 
     FIG. 7 shows an additional embodiment of the screw retaining member. 
     FIG. 8 shows an embodiment of an assembly showing the screw retaining member of FIG.  7 . 
     FIG. 9 is an end view of another embodiment of the present invention. 
     FIG. 10 is a top plan view of another embodiment of the present invention. 
     FIG. 11 shows an additional embodiment of the screw retaining member. 
     FIG. 12 shows an end view of yet another embodiment of the present invention. 
     FIG. 13 is a perspective view of features of the FIG. 12 embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 presents a perspective view of the structure  10  of the present invention in which bone plate  12  is shown with bone screws  14  inserted into the apertures  16  within the plate  12 . The bone plate  12  is further provided with apertures  18  located between the apertures  16 , which receive a screw in order to fix the screw retaining member  22  in place. 
     As best viewed in FIG. 2, the bone plate has an accurate shape which mirrors the shape of the bone structure against which it is placed. In this Figure and in FIG. 7, the screw retaining member  22  has hemispherical surfaces  50  at the edges  25  that contact the heads of the bone screws. With this arrangement, the screw retaining member  22  shares some of the load borne by the bone screws, which possibly may reduce or eliminate the occurrence of load shear upon the screws. Alternatively, the hemispherical surfaces can be omitted. See FIG.  11 . 
     FIG. 9 shows an arrangement in which the screw head and screw retaining member are not in contact with each other. The screw retaining member is situated over the bone screw, sufficiently close thereto to prevent the screw from backing out of the bone in which it is inserted. In this arrangement the tapered shape of the screw retaining member, which will be described below, allows the bone screw to be inserted at variety of angles, including perpendicular to the surface of the bone and at, for example, angles of 5 degrees and 10 degrees to the shaft of the screw when the screw is inserted perpendicular to the surface of the bone. 
     Turning now to FIG. 3 which shows a top plan view of the bone plate, it can be seen that regions  24  in which the bone screws  16  are inserted is sized greater in dimension D 2  than in the size of corresponding dimension D 2 ′ of the intermediate regions  26 . 
     The plate  12  is provided with a first dimension D 1  , a second dimension D 2  (FIG.  3 ), and a third dimension D 3  (FIG.  2 ). While as shown in the disclosed embodiments, D 1  corresponds to length dimension L, D 2  corresponds to width dimension W, and D 3  corresponds to depth dimension, or thickness T, this may not always be the case. That is, for example, D 1  may not always correspond to L, it may in other cases correspond to some other dimension, such as W. For this reason, the present disclosure will hereinafter use only the generic terminology with the understanding that what the terminology denotes is determined on a case-by-case basis. 
     Referring to FIG. 4B, dimension D 1  of the apertures  16  in the bone plate  12  are sized greater than dimension D 2  of the apertures  16  . The size of at least one of the corresponding dimensions D 1 , D 2  of the shaft of the bone screw and/or screw head are smaller than the dimension D 1  of the aperture  14 . (It should be understood that with respect to screws, often there is symmetry, in which case D 1 =D 2 , further in which case both dimensions are smaller than the dimension D 1  of the aperture  16 .) Therefore, when screws are within the apertures  16 , open space is present within the aperture on at least in the direction in which D 1  extends. This enables the screw to move in the direction of the D 1  dimension. In one embodiment D 1  runs in the direction of the length dimension of the bone plate. This arrangement is shown in FIG.  5 . 
     The intermediate regions are provided with apertures  28 . These apertures provide a view of the underlying bone structure, and further, provide a location for the bone graft. 
     As best seen in FIG. 2, the sidewalls  30  defining the aperture  16  are provided with a tapered profile, in which the cross sectional area of aperture  16  at the bottom  12   a  of the plate  12  is smaller than the cross sectional area of the aperture  16  at the top  12   b  of the plate  12 . Over dimension D 3 , the cross sectional area of the aperture  16  gradually increases from the bottom plate side  12   a  to the top plate side  12   b . This can be effected by gradually increasing the size of dimension D 1  and/or the size of dimension D 2  over the traversal of dimension D 3  from the bottom of  12   a  to the top of plate  12   b.    
     A screw suitable for use in conjunction with the bone plate  12  is shown in FIG.  6 . Screw  14  is shown having a head  30  connected to a shaft  32 . The shaft is provided with threads that permit it to be inserted into an anatomical body, such as bone or tissue, by rotating it. The screw head is provided with a groove or slot  36  in its top surface  38 , which may be key shaped or hex shaped, The screw head  36  receives the head of a screwdriver, drill, hex driver, or other device used to drive the screw into bone. These screws, as well as the bone plate  12  and screw retaining member  22  may be constructed of any material known to be suited for constructing surgical implants. To name just a few merely for exemplary purposes, such materials include titanium, cobalt chromium alloy, stainless steel, plastic materials, and bioabsorsbable materials. 
     A suitable screw used in joining the screw retaining member to the bone plate may be the Spiralock®, available from Spiralock Corporation, Madison Hills, Mich., USA. 
     A suitable screw retaining member  22  is shown in FIG.  4 A. The screw retaining member  22  has a shape in which its size in the mid portion of the member  23  is greater than the size at the edges  25  of the member. That is, with reference to the previously defined orientation of the dimensions of the bone plate, the screw retaining member  22  has a dimension D 1 M sized greater in the mid portion  23  of the member  22  than the dimension D 1 E at the edges  25  of the member  23 . Dimension D 1  of the screw retaining member gradually decreases from the mid portion  23  to the edges  25  of the member, so that the screw retaining member has a tapered wing-like appearance. The edges cover at least a portion of the bone screws. In one embodiment, the edges  25  of the screw retaining member reside over the groove in the same plane as the top surface  40  of the bone screw  12 . In one embodiment, the edges  25  of the screw retaining member reside in the groove in the same plane as the top surface  40  of the bone screw  12 . In yet another embodiment, the screw retaining member is in contact with the bone screw  12 . 
     Still with reference to FIG. 4A, the screw retaining member  22  is provided with an aperture  40  located in its mid portion  23 . The aperture  40  of the screw retaining member  22  is not provided with uniform dimensions. Dimension D 1  of the apertures  40 ,in the screw retaining member  22  is shown as greater in size than dimension D 2  of the aperture  16 . The size of at least one of the corresponding dimensions D 1 , D 2  of the screw are smaller than the dimension D 1  of the aperture  40 . (It should be understood that with respect to screws, often there is symmetry, in which case D 1 =D 2 , further in which case both dimensions are smaller than the dimension D 1  of the aperture  40 .) Screws  20  are received within the apertures  40  and received within aperture  18  of the bone plate, thereby joining the screw retaining member to the bone plate  12 . 
     An additional embodiment of the screw retaining member are shown in FIG.  7 . Here, the edges  25  of the screw retaining member are provided with bulbous protrusions that engage with and reside in the groove or apertures of the bone screws. An embodiment of an assembly showing this arrangement is shown in FIG.  8 . When the screw received in aperture  40  of the screw retaining member is tightened, the screw retaining member can be brought into compression against the bone screws. 
     When the assembly is implanted in an anatomical body, such as when bone screws are inserted though the bone plate apertures and implanted in adjoining vertebrae, the bone screws can move linearly within the apertures in response to a load placed on the vertebrae. In other words, when the spinal column is compressed, the screws of the implant are capable of moving in a linear direction in response to the compression. The movement in a linear direction is effected by the screws  14  moving through the apertures  16  in the bone plate  12 . As the screws  14  move, the edges of the retaining member  22  catch the top surface  38  of the screw, and accordingly, the retaining member  22  moves linearly along with the screw  14 , since the aperture through which the screw is positioned has the same dimensional orientation as the dimensional orientation of the apertures through which the bone screws are placed (i.e., the longer dimensions D 1  of the apertures  16 ,  40  in the bone plate  12  and screw retaining member  22  are oriented with each other). In this arrangement, the retaining member shoulders part of the load of compression and helps the bone screws maintain a linear orientation under compression. 
     In a further embodiment, shown in FIGS. 9 and 10, the screw retaining member does not contact the screw head, and there is a gap between the screw retaining member and the top surface of the bone screw. The screw retaining member is situated over the bone screw, sufficiently close thereto to prevent the screw from backing out of the bone in which it is inserted. In this arrangement the tapered shape of the screw retaining member, which will be described below, allows the bone screw to be inserted at variety of angles, including perpendicular to the surface of the bone and at, for example, angles of 5 degrees and 10 degrees to the shaft of the screw when the screw is inserted perpendicular to the surface of the bone. In this arrangement, the aperture of the screw retaining member which receives the set screw can be dimensioned in a way that the set screw fills the entirety of the aperture in which the set screw is received, thereby not allowing for movement of the screw retaining member. 
     In yet a further embodiment, shown in FIGS. 12 and 13, the upper surface  52  of the bone plate in the areas between the apertures and  16  is flat, although the overall shape of the bone plate when viewed from its ends, is arcuate. The bottom surface  54  of the screw retaining member  22  in this area is arcuate, so that along the segment between the apertures  18  and  16  the screw retaining member  22  and the bone plate are in only at the sidewall  56  of the aperture  16 . 
     When set screw  20  is tightened, fixing the screw retaining member in place, the application of a downward force in the midportion of the screw retaining member causes the screw retaining member to move into the gap  58  between the bone plate and screw retaining member. Movement of the screw retaining member causes the edges of the screw retaining member to flex upward. With this arrangement, the set screw  20  is prevented from backing out. 
     Numerous modifications and variations of the present invention are possible in light of the above teachings. It is evident that variations on the present invention may be constructed, which, in accordance with controlling law, are still subject to the claims written in view of the preceding disclosure.