Patent Publication Number: US-2012029576-A1

Title: Bone Fixation Device

Description:
The present invention relates to a bone fixation device. More particularly, but not exclusively, the present invention relates to a bone fixation device having at least one aperture extending therethrough, and a bone fixation screw. The screw and the aperture each comprise a threaded portion and a smooth portion, the smooth portion of the screw being adapted to radially abut the smooth portion of the aperture as the respective threaded portions are threaded into interengagement. 
     Bone fixation devices having apertures extending therethrough for receiving bone fixation screws are known. U.S. Pat. No. 5,336,224 discloses a bone fixation device comprising a plurality of screw receiving members connected together in a line by connecting arms. Extending through each screw receiving member is a screw receiving aperture. Each aperture comprises a threaded portion and a bowl shaped smooth portion. In use a screw is threaded into engagement with the screw receiving aperture. As the screw head is received in the bowl, the head urges the bowl forwards in the direction of the axis of the screw until the bone fixation device abuts the bone. The head is then tightened in the bowl. 
     Such a bone fixation device must be perfectly matched to the profile of the bone. If the two are not perfectly matched then as one screw is tightened urging the bone fixation device into contact with the bone then displacement of the bone fixation device may pull a previously tightened screw out of engagement with the bone. This can be a particular problem for elderly patients or patients with low bone density. 
     The bone fixation device according to the invention seeks to overcome the problems of the prior art. 
     Accordingly, in a first aspect, the present invention provides a bone fixation device comprising 
     a plastically deformable bone fixation body having at least one screw receiving aperture extending therethrough;
 
the shape of the aperture being defined by a side wall, the side wall comprising a first threaded portion, a second smooth walled portion extending from the first threaded portion and a third smooth walled portion extending from the second smooth walled portion;
 
the aperture having a step change in cross section between the second and third smooth walled portions defining a lip for abutment with the head of the screw.
 
     As a bone fixation screw is threaded into engagement with the screw receiving aperture of the bone fixation device according to the invention the head of the screw abuts the lip. The combination of threaded engagement of screw receiving aperture and screw and abutment of screw receiving aperture and screw head firmly locks the two together. The bone fixation device may therefore be arranged remote from the bone and locked in place by bone fixation screws. The bone fixation device therefore does not need to be exactly the same profile as the bone, considerably simplifying the surgical procedure. 
     Each of the portions of the aperture can be cylindrical. 
     The diameter of the first threaded portion can be less than the diameter of the second smooth portion. 
     There can be a step change between the first threaded portion and second smooth portion. 
     The second smooth portion can comprise a conical portion extending from the first threaded portion to a region of constant diameter of the second smooth portion. 
     The bone fixation body can comprise at least two screw receiving members each having a screw receiving aperture extending therethrough, the screw receiving members being connected together by a deformable connecting arms. 
     The bone fixation body can comprise a plurality of screw receiving members connected together in a line by connecting arms. 
     The screw receiving members can be substantially spherical. 
     A portion of the substantially spherical member can be flattened, the flattened portion being centered about a mouth of the aperture and being in a plane normal to the axis of the aperture. 
     The bone fixation body can be a plate. 
     The bone fixation body can comprise first and second laminar portions connected together by a step. 
     The side walls of the aperture can extend from the body. 
     The bone fixation device can further comprise at least one bone fixation screw, the screw comprising a threaded shaft and a head, the shaft being adapted to engage with the first threaded portion of the aperture and the head being adapted to radially abut the lip. 
     In a further aspect of the invention there is provided a bone fixation device comprising 
     a plastically deformable bone fixation body having at least one screw receiving aperture extending therethrough;
 
the shape of the aperture being defined by a side wall, the side wall comprising a first threaded portion and a second smooth walled portion extending from the first threaded portion; and,
 
a bone fixation screw having a threaded shaft and a head, the shaft being adapted to engage with the first threaded portion to draw the head into the aperture;
 
the second smooth portion and the head of the screw being adapted such that as the screw head is drawn into the aperture the head radially abuts the first smooth portion.
 
     Again, as the bone fixation screw is threaded into the screw receiving aperture the screw head abuts the side wall of the aperture locking the bone screw and bone fixation device together. 
     The screw receiving aperture can be cylindrical. 
     The diameter of the aperture can be constant along its length. 
     The diameter of the smooth portion can be larger than the diameter of the threaded portion. 
     The bone fixation device can further comprise a tapering portion between smooth portion and threaded portion. 
     The screw head can be tapered with the diameter of the head increasing in a direction away from the threaded shaft. 
     The plastically deformable bone fixation body can comprise a plurality of screw receiving members connected together in a line by connecting arms. 
     The screw receiving members can be substantially spherical. 
     A portion of the substantially spherical member can be flattened, the flattened portion being centered about a mouth of the aperture and being in a plane normal to the axis of the aperture. 
     The bone fixation body can be a plate. 
     The bone fixation body can comprise first and second laminar portions connected together by a step. 
    
    
     
       The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which 
         FIG. 1  shows a known fixation plate before and after deformation; 
         FIG. 2  shows a bone fixation device according to the invention in perspective view; 
         FIG. 3  shows the embodiment of  FIG. 2  in combination with a screw; 
         FIG. 4  shows a further embodiment of a bone fixation device according to the invention in cross sectional view; 
         FIG. 5  shows the embodiment of  FIG. 4  after twisting; 
         FIG. 6  shows a further embodiment of a bone fixation device according to the invention in perspective view; 
         FIG. 7  shows a cross section through a screw receiving aperture of the device of  FIG. 6 ; 
         FIG. 8  shows a further embodiment of a bone fixation device according to the invention; and, 
         FIG. 9  shows a further embodiment of a bone fixation device according to the invention. 
     
    
    
     Shown in  FIG. 1  is a known bone plate  1 . The bone plate  1  comprises a metal plate  2  having a plurality of apertures  3 . In use the bone plate  1  is placed against a bone (not shown). Bone fixation screws (not shown) are passed through the apertures  3  and then screwed into the bone. The screw heads typically engage with the plate  1  firmly fixing the plate  1  in position and preventing it from being displaced with respect to the bone. 
     A problem can arise however if the bone plate  1  needs to be deformed before it can be fixed to the bone. Deformation of the plate  1  deforms the apertures  3  in the plate  1  as shown. This prevents the screw heads from accurately engaging with the apertures  3  which may result in the plate  1  being free to wobble slightly with respect to the bone. This can reduce the effectiveness of the bone plate  1  as a support for the bone. 
     In addition, it can be difficult to remove such a known bone plate  1  from the bone when it is no longer required. The screw heads tend to cold weld to the bone plate  1  over time making the screws difficult to remove. It is often necessary to cut the bone plate  1  free which can result in damage to the bone. 
     Such a known bone plate  1  is also limited as to how it can be deformed. Whilst the plate  1  can be bent as shown in  FIG. 1  it is not a simple matter to twist it such that the apertures  3  lie in different planes. 
     In addition, such a known bone fixation plate  1  comprises relatively simple threaded apertures. The fixation plate is screwed in place by threading screws into engagement with the apertures. The heads of the screws urge the bone fixation plate into abutment with the bone. 
     In use a surgeon must deform the plate to exactly the same profile as the bone so that all of the screws can be fully tightened. This can be time consuming and difficult for the surgeon. If the curvature of the bone fixation plate does not match the bone exactly then as one screw is tightened the plate may move causing a screw further along the plate to be pulled out of engagement with the bone. 
     Shown in  FIG. 2  is a bone fixation device  8  according to the invention. The device comprises a plastically deformable bone fixation body which comprises a plurality of screw receiving members  9 . Apertures  10  extend through each of the screw receiving members  9  for receiving screws. A long axis  11  extends between each of the screw receiving members  9 . Plastically deformable connecting arms  12  extend along the long axis  11  between the screw receiving members  9  as shown. In this embodiment the long axis  11  passes through the apertures  10  of the screw receiving members  9 . 
     In use the bone fixation device  8  is gripped and bent to the required shape. The arms  12  are more pliable than the screw receiving members  9  and accordingly it is the arms  12  that bend when the force is applied, rather than the screw receiving members  9 . The apertures  10  therefore remain undistorted. In addition, in contrast to known bone plates  1  a torsional (twisting) force can be applied to the device  8  rotating one or more of the screw receiving members  9  about the long axis  11  of the device  8  if required. As the long axis  11  passes along the length of the connecting arm  12 , the connecting arm  12  twists about its length. The device  8  can therefore be twisted without significantly altering its dimensions. The device  8  can therefore be inserted into small apertures even after twisting. 
     In this embodiment of the invention each of the screw receiving members  9  is substantially spherical with the apertures  10  extending through the centers of the spheres  9 . Each aperture  10  intersects the sphere at mouths  13  on opposite sides of the sphere  9 . The sphere  9  comprises a slightly flattened portion  14  around one of the mouths. This reduces the profile of the device  8 . It also provides an extended contact area between the screw receiving members  9  and the bone. 
     The connecting arms  12  between the screw receiving members  9  are cylindrical. The interface  15  between the arms  12  and spherical screw receiving members  9  is chamfered so that any bending or torsional forces do not concentrate at this interface  15 . 
     The ends  16  of the device  8  are tapered as shown so that the device  8  can be placed between bone and soft tissue without surgically exposing the entire length of bone. 
     Each of the apertures  10  of this embodiment is cylindrical having a constant area along its length. A portion of the aperture  10  is threaded. The remainder of the aperture is smooth walled. 
     After bending and/or twisting to the correct shape the device  8  is positioned against the bone. Screws are inserted into the apparatus  10  through the smooth portions and into threaded engagement with the threaded portions. On further rotation of the screws they penetrate and grip the bone, fixing the device  8  to the bone. A significant advantage of the device  8  is that is can be bent/twisted to the correct shape, positioned correctly and then the screws inserted. This considerably simplifies the attachment procedure. As the screw receiving members  9  are aligned with the connection arms  12  along the long axis  11  the device  8  can be twisted about its length without any significant change in dimensions of the device  8 . This is particularly useful when inserting the device  8  into small apertures. 
     The device  8  is adapted to be used with a screw having two portions—a threaded portion for gripping the threaded portion of the aperture  10  and then the bone and a smooth portion extending from the threaded portion. The smooth portion has an outer face which is substantially cylindrical and of the same diameter as the threaded portion. The smooth portion is however slightly tapered with its diameter increasing in a direction away from the threaded portion. At its end the diameter of the smooth portion is slightly larger than the diameter of the aperture  10 . As the screw is turned and is drawn into the aperture  10  the smooth portion of the screw abuts the smooth portion of the aperture  10  so producing a press fit. This press fit firmly locks the bone fixation device  8  and screw together, even when the device  8  is remote from the bone. The bone fixation device  8  therefore does not need to exactly match the profile of the bone. This considerably simplifies the surgical procedure. 
     An alternative embodiment of the invention is shown in  FIG. 3 . In this embodiment the threaded portion of the aperture  10  is narrower in diameter than the smooth portion. The screw  17  has a narrow threaded portion  18  and a wider smooth tapered head portion  19  as shown. The narrow threaded portion  18  of the screw  17  engages with the narrow portion of the aperture  10  drawing the larger smooth head portion  19  into abutment with the smooth portion of the aperture  10 . 
     In the above embodiments the smooth portions are perfectly smooth. In alternative embodiments the smooth portions may comprise surface texture such as ripples or bevels. In a further embodiment of the invention a portion of the aperture  10  is slightly bevelled so that a standard bone screw head will press fit into engagement with the bevels as the screw is drawn into the aperture  10 . 
     Shown in  FIG. 4  in cross sectional view is a further embodiment of a device  8  according to the invention. The aperture  10  comprises a first narrow threaded section  20 . Extending from this is a second smooth walled section  21  of larger diameter. Extending from the second section  21  is a third smooth walled section  22  of slightly larger diameter than the second section  21 . The step change in diameter from the second section  21  to the third section  22  defines a lip  23 . In use a screw  24  is inserted into the aperture  10  with the threaded portion  25  of the screw  24  in threaded engagement with the narrower portion  21  of the aperture  10 . As the screw  24  is turned and drawn in to the aperture  10  the head  26  of the screw  24  abuts the lip  23  producing a tight fit. 
     Shown in  FIG. 5  is the embodiment of  FIG. 4  in perspective view. The device  8  has been twisted along its length such that the apertures  10  lie in different planes. As can be seen, the arms  12  have twisted before the screw receiving members  9  deform. 
     In a further embodiment of the invention the screw receiving members  9  are substantially elliptical. 
     The device  8  according to the invention can be used with any tool which grips the screw receiving members  9 . One preferred embodiment of such a tool comprises jaws having cut out sections which match the spherical components of the screw receiving members  9 . The tool also has cut outs which match the interface  15  between the spherical component  9  and connecting arm  12  so that the tool can apply a bending force throughout the length of the bone fixation device  8 . In an alternative embodiment the jaws have cut outs which match the flattened portions  14  of the screw receiving members  9 . 
     In use two of the tools are used to grip the screw receiving members  9 . The device  8  is then bent and/or twisted to the desired shape and then released. 
     Shown in  FIG. 6  is a further embodiment of a bone fixation device  8  according to the invention in perspective view. In this embodiment the plastically deformable bone fixation body comprises a thick rectangular plate  30 . Arranged along the plate  30  and extending through it is a plurality of screw receiving apertures  10  as shown. In use the plate  30  is deformed to approximately the same profile as the bone and then screwed to the bone by screws inserted through the apertures  10 . The bone screws are tightened until the screw head abuts the wall of the aperture  10  as described in further detail with reference to  FIG. 7 . 
     Shown in  FIG. 7  is a cross section through one of the screw receiving apertures  10  of the bone fixation device of  FIG. 6 . The aperture  10  comprises a first cylindrical threaded portion  31 . Extending from the first portion  31  is a smooth cylindrical second section  32 . The second section  32  comprises a conical portion  33  and then a uniform diameter portion  34 . Extending from the second section  32  is a third smooth walled section  35 . There is a step in cross section between the second and third sections  32 ,  35  defining a lip  36 . 
     In use a screw comprising a threaded shaft and a head is inserted into the aperture  10 . The threaded shaft is threaded into engagement with the first threaded portion  31  of the aperture  10 . As the screw is turned the head is drawn into the aperture  10 . The threaded shaft extends through the first threaded portion  31  and into the bone. As turning continues the head radially abuts the lip  36  locking the screw and bone fixation device  8  together. The head of the screw may be a domed head. Alternatively, the head of the screw may be frustoconical with the diameter of the head increasing in a direction away from the threaded shaft. In this embodiment the plate is rectangular. In other embodiments the plate can be other shapes depending upon the intended use of the plate. 
     Shown in  FIG. 8  is a further embodiment of a bone fixation device according to the invention. In this embodiment the plastically deformable bone fixation body comprises two laminar members  40 , 41  connected together by a step  42 . Each of the laminar members  40 , 41  comprises a plurality of screw receiving apertures  10 . 
     Shown in  FIG. 9  is a further embodiment of a bone fixation device  8  according to the invention. In this embodiment the bone fixation body is a laminar plate  50 . The plate  50  is substantially triangular in plan view having rounded corners. In this embodiment the side walls of the screw receiving apertures are upstanding from the plate  50  as shown such that the depth of the apertures  10  is greater than the thickness of the plate  50 . 
     Embodiments in which the bone fixation body is a combination of one or more fixation bodies as described above. For example the body fixation body can be a combination of a plate portion and a further portion comprising a plurality of screw receiving members connected together by arms.