Abstract:
A bone fixation device is disclosed that permits some movement of different portions of a fractured bone when the device is affixed thereto. Various embodiments are disclosed, including bone plates that have fixed holes and floating holes therethrough for receiving bone screws, and bone plates including oblong holes allowing for screw movement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/549,439 filed on Oct. 20, 2011, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to devices used in the fixation of fractures in bones. More specifically, the present invention relates to a flexible bone plating system that rigidly fixes adjacent ends of a fractured bone while still allowing axial dynamization of the bone to promote healing of the fracture. 
         [0003]    Typical fixation of a fracture of a long bone with a bone plate requires making an incision in the tissue, reducing the fracture, placing a bone plate on the fractured bone, and securing the bone plate to the bone with fixation elements such as screws. The bone plate immobilizes the fracture and keeps the bone in a correct position so as to allow the fracture to heal. In certain cases, the bone plate may facilitate the reduction of the fracture. 
         [0004]    Typically, bone plates have a bone contacting surface and an upper surface facing away from the bone with a plurality of holes or apertures extending between the two surfaces. These holes or apertures may be either threaded (for use with locking screws) or non-threaded (for use with regular screws) and may be circular or oblong in shape. In traditional fracture fixation, the plates are usually fixed to the bone parts by means of threaded screws, which are driven into the bone tissue after so-called pre-drilled or pilot-drilled holes have been generated in the bone tissue. These pre-drilled holes allow for a reliable screwing procedure whereby the risk of further destroying the bone with the screw is significantly reduced. These plates generally are rigid and resistant to bending or torsioning in order to stabilize the fractured bone. However, absolute rigidity is not always desirable and it can be advantageous when an implantable bone plate is capable of a degree of freedom along the main axis of the bone plate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    Various aspects of the present invention are achieved by bone fixation devices, screws, or drilling configurations that result in the capability of axial dynamization of the bone along the main axis of the bone while maintaining rigidity in other directions and restricting bending or torsioning. 
         [0006]    In one embodiment, a bone fixation device, such as a bone plate, for affixation to a bone has a first end, a second end remote from the first end, a bone contacting surface and an upper surface facing away from the bone contacting surface. The bone fixation device also has first and second holes extending between the bone contacting surface and the upper surface, with the first hole positioned on the first end, the second hole positioned on the second end, and each hole being capable of receiving a fastener. The bone fixation device also includes a nut with a top surface, a bottom surface, and a hole extending between the top surface and bottom surface. 
         [0007]    The first hole may be oblong with a major axis length and a minor axis length. The nut can have a width greater than the major axis length of the first hole. Alternatively, the nut can have a top portion and a bottom portion. The width of the top portion is smaller than the major axis length of the first hole and the width of the bottom portion is greater than the major axis length of the first hole. 
         [0008]    In another embodiment, a bone fixation device, such as a bone plate, for affixation to a bone includes a first end, a second end remote from the first end, a bone contacting surface, and an upper surface facing away from the bone contacting surface. The bone fixation device also includes first and second holes extending between the bone contacting surface and the upper surface, the first hole being positioned on the first end the second hole being positioned on the second end. Each hole is capable of receiving a fastener. The bone fixation device can also include a cap with a top surface, a bottom surface, a first side wall, and a second side wall. The bottom surface of the cap defines a recess therein. The first and second side walls of the cap can each include a flange extending generally parallel to the top surface toward the opposite side wall. 
         [0009]    In another embodiment of the invention, a method of affixing a bone fixation device to a bone with a fracture includes the step of providing the bone fixation device. The bone fixation device includes a first end, a second end remote from the first end, a bone contacting surface and an upper surface facing away from the bone contacting surface. The bone fixation device also includes first and second holes extending between the bone contacting surface and the upper surface, the first hole being positioned on the first end the second hole being positioned on the second end. Each hole is capable of receiving a fastener. The bone fixation device also includes a nut with a top surface, a bottom surface, and a hole extending between the top surface and bottom surface. This embodiment of the invention also includes the steps of placing the bone fixation device on the bone, inserting a first fastener through the first hole, through the hole in the nut, and into the bone on a first side of the fracture, and also inserting a second fastener through the second hole, and into the bone on a second side of the fracture. 
         [0010]    In yet another embodiment of the invention, a method of affixing a bone fixation device to a bone with a fracture includes the step of providing the bone fixation device, which includes a first end, a second end remote from the first end, a bone contacting surface and an upper surface facing away from the bone contacting surface. The bone fixation device also includes first and second holes extending between the bone contacting surface and the upper surface, the first hole being positioned on the first end and the second hole being positioned on the second end, each hole being capable of receiving a fastener. The bone fixation device also includes a cap with a top surface, a bottom surface, first and second side walls, with the bottom surface of the cap defining a recess therein. This embodiment of the invention also includes the steps of placing the bone fixation device on the bone, inserting first and second fasteners through the first and second holes and into the bone on first and second sides of the fracture, respectively. This embodiment of the invention also includes the step of connecting the cap to the bone fixation device. 
         [0011]    The step of connecting the cap to the bone fixation device can also include placing the cap over the first hole such that at least a portion of the first fastener resides within the recess of the bottom surface of the cap. The step of connecting the cap to the bone fixation device can even further include engaging a flange on the first side wall of the cap with the bone contacting surface of the bone fixation device and engaging a flange on the second side wall of the cap with the bone contacting surface of the bone fixation device. 
         [0012]    Different embodiments of a bone fixation device include apertures into which screws are inserted to fix the bone fixation device to the bone on one side of a fracture site, and further include additional apertures that can receive screws on the opposing side of the fracture site. The additional apertures can have a means by which the screw and aperture can slightly move in a direction along the main axis of the bone while the screw remains fixed in the bone. This preferably allows for micromotion of the bone fragments on each side of the fracture to promote healing. 
         [0013]    Different embodiments of the means by which the screw and aperture can move are within the scope of the invention. These include, for example, a floating aperture element in the bone fixation device created by cutting out material from the bone fixation device, leaving the floating aperture connected by flexure joints or another type of spring mechanism. Other means contemplated herein include an aperture made within a sliding cart in the bone fixation device which allows the screw and the sliding cart to slide within a predefined area of the bone fixation device, while the screw engaged with the aperture remains fixed relative to the segment of bone into which it is inserted. 
         [0014]    Other embodiments of a bone fixation device to achieve the desired goals include a bone fixation device that includes flexible arm extensions, forming a cut-out section in the center, that connect opposing ends of the bone fixation device and allow the bone fixation device to flex in the direction of the main axis of the bone while maintaining rigidity in other directions. 
         [0015]    In another embodiment, the bone fixation device includes at least one oblong hole into which a bone screw can be inserted. The oblong shape of the hole allows movement of the screw along the major axis of the oblong hole, while tilting or movement in other directions is restricted by the minor axis of the oblong hole as well as other means. Examples of the other means contemplated herein include a nut, through which the screw is inserted, which at least partially contacts the underside of the bone fixation device to provide rigidity in the desired directions. Further means include a cap and screw assembly in which the screw can move with the bone in the direction of the main axis of the bone while still being restricted from movement in other directions by the cap. 
         [0016]    Still further embodiments of the invention are provided to achieve axial dynamization of a bone. These include screw configurations in which a top portion of the screw is narrower than a bottom portion of the screw, due to either the core diameter or the thread diameter changing along the axis of the screw. This geometry allows for reduced rigidity on the near cortex side of the bone after the screw is inserted through an aperture in a bone fixation device and further through the bone. The reduced rigidity allows the screw and bone to move along the main axis of the bone while restricting movements in other directions. Similarly, an embodiment of the invention achieves a similar result using a screw with a constant core and thread diameter, but instead creating a relatively large drill hole in the near cortex and relatively small drill hole in the far cortex of the bone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1A  illustrates a top view of one embodiment of a bone fixation device shown here as a bone plate. 
           [0018]      FIG. 1B  illustrates an enlarged view of a floating aperture element of the embodiment of the bone fixation device in  FIG. 1A . 
           [0019]      FIG. 2A  illustrates a top view of an alternate embodiment of a bone fixation device in the form of a bone plate. 
           [0020]      FIG. 2B  illustrates a cross section of a cart element of the embodiment of the bone fixation device in  FIG. 2A  along axis A-A. 
           [0021]      FIG. 2C  illustrates a perspective view of an alternate embodiment of a bone fixation device. 
           [0022]      FIG. 3  illustrates yet another embodiment of a bone fixation device, shown as a bone plate with flexible arm extensions. 
           [0023]      FIG. 4A  illustrates a top view of another embodiment of a bone fixation device, shown as a bone plate with an oblong screw hole. 
           [0024]      FIG. 4B  illustrates a cross-sectional side view of a screw engaged with a nut through the oblong screw hole of the bone fixation device shown in  FIG. 5A . 
           [0025]      FIG. 4C  illustrates a cross-sectional side perspective view of a screw engaged in the oblong screw hole of the bone fixation device shown in  FIG. 5A  along with a screw cap. 
           [0026]      FIG. 4D  illustrates a bottom perspective view of the screw cap shown in  FIG. 5C . 
           [0027]      FIG. 5  illustrates a cross sectional view, along the main axis of a bone, of a bone after different diameter holes are drilled through the bone. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    As used herein, when referring to bones or other parts of the body, the term “proximal” means closer to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means towards the head. The term “anterior” means towards the front part of the body or the face and the term “posterior” means towards the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
         [0029]    Referring to  FIG. 1A  there is shown a top view of one embodiment of bone fixation device  10 . Bone fixation device  10  includes a bone contacting surface (not shown) and a surface facing away from the bone (shown in  FIG. 1 ) with a plurality of holes or apertures  12  extending between the two surfaces. Holes or apertures  12  may be either threaded (for use with locking screws) or non-threaded (for use with non-locking or compression screws) and may be any suitable shape, such as circular or oblong. Bone fixation device  10  is implanted onto a bone across a fracture site in the bone such that at least one hole or aperture  12  is positioned on each opposing side of the fracture. Bone fixation device  10  also includes one or more floating aperture elements  14 , within a slot  11 , connected to bone fixation device  10  by flexure joints  16 , described in more detail below. 
         [0030]    Referring to  FIG. 1B  there is shown a close up view of floating aperture element  14  within a slot  11  of bone fixation device  10 . Each of the one or more slots  11  are created by removing material from the bone fixation device  10  around a hole or aperture  13 . The remaining material forms the one or more floating aperture elements  14 . The material is removed such that the floating aperture element  14  remains connected to the bone fixation device  10  by flexure joints  16  to allow the floating aperture element  14  to move only in the desired direction (e.g., along the main or longitudinal axis of the bone fixation device). As shown, the floating aperture element  14  is generally a rectangular shape that only contacts the bone fixation device  10  along the main axis of the device  10 . The flexure joints  16  are generally “x” shaped and integral with the bone fixation device  10  such that the floating aperture element  14  can move along the longitudinal axis of the bone fixation device  10 , even while the rest of the bone fixation device  10  is otherwise stationary. Thus, the flexure joints  16  allow the floating aperture element  14  to be stiff in some directions and flexible in others. By leveraging this asymmetric stiffness, a change in the distance between holes  12  and  13  is possible with minimal effect on the angle between the bone fixation device  10  and the bone to which it is attached. Additionally, there is minimal allowance for horizontal translation of a screw in aperture  13  as well as minimal allowance for translation of the screw in a direction normal to the bone fixation device  10 . 
         [0031]    Although the embodiment depicted in  FIGS. 1A and 1B  is discussed above, one skilled in the art would recognize that design variations in the fixation device are possible. For instance, floating aperture element  14  could be other shapes such as square or circular, and the flexure joints  16  could be any shape, such as a spring shape that allows translation in the axial direction while maintaining stiffness in other directions. Likewise, the fixation device can exhibit many different shapes for use in different situations, and can employ any number of floating aperture elements  14  and slots  11 . Still further, floating aperture elements  14  and flexure joints  16  can be configured to allow translation or movement of the screw in a number of different directions, as well as to allow more or less movement. Although discussed as being created through the removal of material from device  10 , floating apertures elements  14  could be separately formed and attached to device  10  through welding or another suitable process. 
         [0032]    Referring to  FIG. 2A  there is shown a top view of an alternate embodiment of a bone fixation device  20 . This embodiment is similar to that illustrated in  FIG. 1A  in many respects, with the main difference being the inclusion of a cart element  24 . Bone fixation device  20  includes a bone contacting surface and an upper surface facing away from the bone with a plurality of holes or apertures  22  extending between the two surfaces. Holes or apertures  22  may be either threaded (for use with locking screws) or non-threaded (for use with non-locking or compression screws) and may be any suitable shape, such as circular or oblong. Bone fixation device  20  is implanted onto a bone across a fracture site in the bone such that at least one hole or aperture  22  is positioned on each opposing side of the fracture. Bone fixation device  20  also includes one or more slots  21 . Each of the one or more slots  21  may include a cart element  24  guided by railing slots  26  of the bone fixation device  20 , described in more detail below. Each cart element  24  may be provided as a separate insert. This allows the surgeon to insert the cart element  24  into the respective slot  21  prior to or during surgery based on the surgeon&#39;s assessment of the most appropriate course of action at the time. 
         [0033]    Referring to  FIG. 2B , there is shown a cross section of the embodiment of bone fixation device  20  in  FIG. 2A  along axis A-A. As shown, each cart element  24  includes a hole or aperture  23  therein. Each cart element  24  also includes protrusions  28  that fit into railing slots  26  of the bone fixation device  20 . The fit between the railing slots  26  and protrusions  28  is such that the cart element  24  may slide along the length of the railing slots  26 . When a screw is fixed into a hole or aperture  22  on one side of a bone fracture and another screw is fixed in hole or aperture  23  of cart element  24 , a small degree of motion in the main or longitudinal axis of the bone fixation device  20  is possible due to the sliding capability of the cart element  24  without the introduction of shear or bending forces, which may be undesirable or detrimental to uniform bone growth. Although the cart element  24  is illustrated as having a smaller thickness than the bone fixation device  20 , the cart element may be thinner or thicker than illustrated. For example, the cart element  24  may alternately be approximately the same thickness as the bone fixation device  20 . 
         [0034]    Referring to  FIG. 2C , there is shown a perspective view of yet another embodiment of bone fixation device  20 ′. This embodiment of the bone fixation device  20 ′ has multiple cart elements  24 ′, each with a threaded hole or aperture  23 ′. As with other embodiments of the current invention, the holes or apertures may be threaded or unthreaded, and there may be one or more cart elements  24 ′ included with the bone fixation device. In this embodiment, as opposed to that shown in  FIGS. 2A-B , the cart element  24 ′ includes one or more cart slots  29  that each mate with a protruding railing  27  formed in slots  21 ′. Similar to the embodiment shown in  FIGS. 2A-B , the cart element  24 ′ can freely slide inside bone fixation device slot  21 ′, the cart element  24 ′ being guided by protruding railing  27  allowing motion only along the main or longitudinal axis of the bone fixation device  20 . As in the above embodiment pertaining to fixation device  10 , fixation device  20 ′ can vary from that shown, including, but not limited to, in its shape, the shape of its components and the direction in which its cart elements can slide. 
         [0035]    Referring to  FIG. 3  there is shown yet a further embodiment of a bone fixation device  30 . Bone fixation device  30  includes a bone contacting surface and an upper surface facing away from the bone with a plurality of holes or apertures  32  extending between the two surfaces. Holes or apertures  32  may be either threaded (for use with locking screws) or non-threaded (for use with non-locking or compression screws) and may be any suitable shape, such as circular or oblong. Bone fixation device  30  includes arm extensions  34  that connect opposing ends of the bone fixation device  30  and which form an opening  36 . The shape of opening  36  is dependent on the shape of the arm extensions  34  and could be, by way of example and not limitation, diamond (as shown) or “O” shaped. Screws are inserted through the holes or apertures  32  into a bone such that both the fracture and the opening  36  are flanked on each side by at least one screw. The arm extensions  34  and opening  36  provide for resistance against torsion or bending while allowing for axial dynamization along the main axis of the bone fixation device  30 . In other words, if screws are fixed into a bone through holes or apertures  32  on opposing sides of the opening  36 , the arm extensions  34  can flex and allow for the bone to move along the main or longitudinal axis of the bone fixation device  30  while the screws remain rigidly fixed in the bone through holes or apertures  32 . Preferably, the arm extensions  34  are constructed such that the two arm extensions  34  have a similar thickness as each other and as the rest of the body of the bone fixation device  30 . This allows for the same bending rigidity in a plane through the bone fixation device  30 . Again, it is to be understood that fixation device  30  may vary, including, but not limited to, in the shape of its components and its intended use. 
         [0036]    Referring to  FIG. 4A , there is shown another embodiment of a bone fixation device  40 . Bone fixation device  40  includes a bone contacting surface and an upper surface facing away from the bone with a plurality of holes or apertures  42  extending between the two surfaces. Holes or apertures  42  may be either threaded (for use with locking screws) or non-threaded (for use with non-locking or compression screws) and may be any suitable shape, such as circular or oblong. Bone fixation device  40  also includes at lease one oblong hole or aperture  46 . During implantation of the bone fixation device  40  onto a bone across a fracture site, at least one hole or aperture  42  has a screw inserted into it and through the bone, and a screw is inserted into at least one oblong hole or aperture  46  on the opposite side of the fracture site. The oblong shape of the hole or aperture  46  allows the bone and screw to move in the direction of the main or longitudinal axis of the bone fixation device  40 . Preferably, the screw includes a mechanism to limit the ability of the screw to tilt in the plate while still allowing the screw to slide within the oblong hole or aperture  46  in the direction of the main or longitudinal axis of the bone fixation device  40 , as will be described more fully below. 
         [0037]    For instance, referring to  FIG. 4B , there is shown a mechanism to allow a screw  47  to slide within the oblong hole or aperture  46  in the direction of the main or longitudinal axis of the bone fixation device  40 , while restricting tilting or movement of the screw  47  in other directions. In this illustrative embodiment, a screw  47  is inserted into the oblong hole or aperture  46  and through a nut  48  that at least partially sits under the bone fixation device  40 . Similar to the apertures  46 , the nut  48  may include an aperture that is either threaded or non-threaded. In the illustrated embodiment, the nut  48  has a top portion and a bottom portion. The bottom portion of the nut  48  is in the nature of flanges that have a width. The width of the flanges is greater than the major axis of the oblong aperture  46  such that the flanges extend beyond the aperture  46  of the bone fixation device  40  to limit the ability of screw  47  to tilt in relation to the bone fixation device. The top portion of nut  48  had a width. The width of the top portion of the nut is smaller than the major axis of the oblong aperture  46  such that the top portion fits within the oblong aperture, allowing for the screw  47  and nut  48  to move within the gap spaces  49  between the nut and the ends of oblong hole or aperture. This configuration allows for micromotion of the bone fragments in the direction of the main or longitudinal axis of the bone fixation device  40  to promote healing while restricting bending, torsion or movement in other directions which could hinder healing of the bone fracture. The bottom flanges of nut  48  may be various shapes or sizes and are largely a matter of design choice. For example, the bottom flanges of nut  48  are preferably thin relative to the thickness of the bone fixation device  40  to provide a more flush contact between the bone fixation device and the bone. In an alternate embodiment, the nut  48  can be a thin disc with a hole to accept a screw  47 . In this embodiment, the gap spaces  49  exist between the screw  47  and the bone fixation device  40 , rather than between the nut  48  and the bone fixation device. 
         [0038]    Referring to  FIG. 4C-4D , there is shown another embodiment of a mechanism to allow a screw  47  to slide within the oblong hole or aperture  46  in the direction of the main or longitudinal axis of the bone fixation device  40 , while restricting tilting or movement of the screw  47  in other directions. In this illustrative embodiment, a screw  47  is inserted into the oblong hole or aperture  46  and a screw cap  44  is connected to the bone fixation device  40  over the screw  47 . The screw cap  44  can have a generally oblong shaped recess in the underside of the top of the screw cap  47  to further facilitate sliding of the screw  47  along the main axis of the bone fixation device  40 . The screw head and the top of the screw thread of screw  47  constrains motion of the screw  47  in the direction of the short axis of the bone fixation device  50  (i.e., in the direction of the minor axis of the oblong hole or aperture  46 ). The screw cap additionally prevents the screw  47  from tilting in other directions relative to the bone fixation device  40 . The screw cap  44  may lock or otherwise engage the bone fixation device  40 , for example by way of snapping bottom portions  43  of the screw cap  44  to the bone contacting surface of the bone fixation device  40 , which acts to further constrain the motion of the screw  47  in a direction normal to the bone fixation device  40 . The bottom portions  43  of the screw cap  44  may be flanges extending inwards from the side walls of the screw cap in the direction toward the aperture  46  of the bone fixation device  40 . In the illustrated embodiment, the screw cap  44  includes two side walls projecting down from the top of the screw cap, and each side wall has two bottom portions  43  in the nature of flanges that engage the bone contacting surface of the bone fixation device  40 . The flanges extend generally parallel to the plane of the bone fixation device  40  and the top surface of the screw cap  44 . The outer surface of each side wall in the illustrated embodiment is generally orthogonal to the top surface of the screw cap  44 . The inner surface of each side wall may be of varying thickness. In the illustrated embodiment, the center of each side wall is relatively thin, while the outer portions of each side wall are relatively thick. The inner surface of each side wall may be rounded or otherwise contoured to provide clearance for the head of a screw  47 . In alternate embodiments, the two side walls could each have a single bottom portion  43  in the nature of a flange or other supporting structure to mate with the bone fixation device  40 . For example, the bone contacting surface of the bone plate  40  could alternately include notches or grooves and the side walls of the screw cap  44  could include protrusions to mate with the notches or grooves. It is to be understood that different caps  44  can be provided to allow for different movement of the screws. For example, the size and shape of the recess in the screw cap  44  may be altered to change the ability of the screw  47  housed therein to move different magnitudes in different directions. Similarly, the screw cap  44  and recess formed therein could take the form of other shapes than oblong, such as rectangular or square shapes. The screw cap  44  can be formed of a metal, plastic, or other material suitable for implantation in the human body. 
         [0039]    Referring now to  FIG. 5 , there is shown a cross section of a bone  50  along the main or longitudinal axis of the bone after a drilling procedure has been performed. In the method, a bone hole extension  56  is created in the near cortex  52  by overdrilling, for example by milling or sonic drilling. In addition, a relatively small drill hole  58  is created in the far cortex  54  of the bone  50 . If a bone fixation device with an oblong aperture, such as the embodiment illustrated in  FIG. 4A , is implanted onto the bone  50  and a screw is inserted through the oblong hole of the bone fixation device and through the hole extension  56  and further through the relatively small drill hole  58 , the rigidity of the screw and bone  50  construct is reduced by allowing movement of the screw within the bone hole extension  56 . The screw, either not being in contact or being in reduced surface contact with the bone  50  on the near cortex  52  side, could thus bend elastically in the bone hole extension  56 . By creating the bone hole extension  56  in an oblong shape, a screw inserted into the bone hole extension  56  and further through the relatively small drill hole  58  could move in the direction of the main axis of the bone  50  while limiting the torsion, bending, or movement of the bone in other directions. 
         [0040]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.