Patent Publication Number: US-2007123909-A1

Title: Extractor For Broken Bone Screws

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Application Ser. No. 60/730,605 filed Oct. 25, 2005 [the filing receipt has a date of Oct. 27, 2005 and a request for correction of that date has been made) and entitled Extractor for Broken Bone Screws. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to a device for removing bone screws which are embedded in bone and which have their heads detached.  
      Although primarily intended for use in removing broken bone screws, the device of the present invention can also be used for removing any embedded cylindrical object such as a drill bit, reamer, or tap; or similar devices which are frequently used in orthopedic surgery. The device is intended to remove a broken screw regardless of whether it stands proud of the bone surface.  
      Bone screws are often used in orthopedic surgery to secure bone sections to each other or to artificial joints, plates or other structural members to be retained in place. The screws, which usually have (i) a socket head but may have another type of head for receiving a turning tool, and (ii) a threaded shank (i.e. shaft) portion, are threaded into the bone. Due to various factors the implanted screws can come under sufficient stress to detach the head from the screw, i.e. to cause the screws to be broken.  
      Such broken screws cannot be removed using the device that was used to install the screw (hexagonal driver, torx driver, etc.) by rotating the screw head.  
      Various devices exist to remove such implanted (headless) broken screws from bone. These include hole saws sold by manufacturers such as Starrett and Lennox, and generic trephines from a variety of medical instrument manufacturers.  
      These prior art devices remove broken screws by coring the surrounding bone, after which the core is broken away from the bone at the bottom of the core section and removed. Such hole saw devices have the disadvantage of removing a relatively large core of host bone, resulting in considerable bone loss. In this prior art coring method for broken screw removal, the resulting bone cavity is typically filled with bone cement or a bone autograft or allograft.  
      In addition to the considerable loss of bone involved, the prior art technique for removing broken bone screws is a two step process. After coring completely around the entire length (or nearly the entire length) of the implanted screw, the distal end of the core remains attached to the host bone, and must be broken away using pliers, a pry bar, or a similar type instrument.  
      Accordingly, an object of the present invention is to provide a device for removing broken bone screws which can do so in a single step with reduced loss of host bone.  
     SUMMARY OF THE INVENTION  
      According to the invention, there is provided a generally cylindrical broken bone screw removal device which has a trephine assembly located at the distal end. The proximal end is adapted to be rotated by a tool such as a hand operated drill.  
      The trephine assembly has a trephine and a cam inside it which is preferably free to rotate independent of the trephine. The internal wall of the cam has a noncircular preferably D shaped cross-section which decreases in size from the distal toward the proximal end of the device.  
      The internal diameter of the trephine is preferably eccentric to its outer diameter, and the cross-section of the cam is preferably eccentric to the internal diameter of the trephine.  
      A broken screw is removed from bone by placing the trephine over the proximal end of the screw and rotating the device while applying pressure to cause the trephine to core bone around the screw, so that the device descends down the body of the screw until the screw to enters the interior of the cam to a depth where it engages the tapered walls of the cam and preferably causes the cam to rotate with respect to the adjacent internal wall of the trephine; and the body of the screw becomes wedged within the trephine to transmit torque from the device to the screw to unscrew it from the bone. 
    
    
     IN THE DRAWING  
       FIG. 1  is an isometric view of a device for removing bone screws which are embedded in bone and which have their heads detached, according to a preferred embodiment of the invention.  
       FIG. 2  is an exploded isometric view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 .  
       FIG. 3A  is a cross-sectional end view of the trephine end assembly of the device shown in  FIG. 1 .  
       FIG. 3B  is a cross-sectional elevation view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 .  
       FIG. 4 , is a cross-sectional elevation view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 , adjacent a broken bone screw embedded in bone.  
       FIG. 5  is a cross-sectional elevation view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 , after the trephine head assembly has cut in- to bone so that a portion of the shaft of a broken bone screw embedded in bone is disposed within the trephine end assembly.  
       FIG. 6A  is a cross-sectional view of the aforemen- tioned screw shaft portion in engagement with the inside wall of an element of the trephine end assembly.  
       FIG. 6B  is a cross-sectional elevation view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 , showing the aforementioned screw shaft portion adjacent the inside wall of an element of the trephine end assembly.  
       FIG. 7  is a cross-sectional elevation view of the trephine end assembly and adjacent end portion of the device shown in  FIG. 1 , showing the aforementioned screw shaft portion in engagement with the inside wall of an element of the trephine end assembly. 
    
    
     DETAILED DESCRIPTION  
      As seen in  FIG. 1 , the broken bone screw removal device  1  has a proximal end  11  for receiving a rotating tool such as a drill chuck, an elongated shaft portion  12 , and a trephine end assembly  2 .  
      The trephine assembly  2  includes an internal cam  4  and a trephine  5  which is attached to the end portion of the device  1 , as shown in  FIGS. 1, 2  and  3 B. The end assembly has a distal end  13  comprising a cutting tip with sharp edged teeth  14 .  
      As shown in  FIGS. 2 and 3 B, the trephine end assembly  2  includes an alignment pin  3  which extends between aligned holes in the trephine  5  and adjacent end portion of the device  1  to axially align the same with each other.  
      A portion of the alignment pin  3 , and a cam  4  are housed inside the trephine  5 , which is rigidly attached to the end portion of the device  1  as shown in  FIG. 3B .  
      The cam  4  is free to rotate within the trephine  5 . As shown in  FIGS. 3A and 3B , the cam  4  is in the form of an annular, i.e. cylindrical shell having a generally D shaped cross-section  6 . This D shaped cross-section decreases in size, i.e. is tapered, from the distal toward the proximal end of the device  1 .  
      The internal diameter of the trephine  5  is eccentric to its outer diameter, and the D shaped cross-section of the cam  4  is eccentric to the internal diameter of the trephine  5 , as shown in  FIG. 3A .  
      In use, a rotating tool such as a hand operated drill (not shown) has its chuck tightened to engage the proximal end  11  of the device  1 . The device  1  is then positioned so that its trephine assembly  5  is directly over the shaft end of a broken screw  7  embedded in bone  8  ( FIG. 4 ). The tool is then rotated to turn the device  1  in a desired direction, which would be counterclockwise to extract the usual (right-handed thread) type of screw which is threaded so as to tighten when rotated clockwise; and downward pressure is applied as the device  1  is rotated.  
      As the device  1  rotates counterclockwise (that is, opposite the clockwise bone screw insertion direction), the cutting tip  14  removes bone from an annular region immediately surrounding the screw  7 , causing the proximal end of the screw to enter the longitudinal hole in the trephine assembly  2 .  
      After the proximal end of the screw enters the longitudinal hole in the trephine assembly as shown in  FIG. 5 , the screw end maintains the alignment of the device  1  with the remaining portion of the screw. In that way, using the major diameter of the screw itself as a guide, the trephine assembly tightly “hugs” the outer diameter of the screw as the cutting tip  14  advances into the host bone, expelling bone chips, until the proximal end of the screw has advanced a predetermined distance into the device longitudinal hole to enter the “capture cone” of the cam  4  and engage and wedge against the inner wall thereof as shown in  FIG. 7 .  
      That is, the screw shank, the threads of which are progressively exposed as the cutter advances, advances into the tapered capture cone of the eccentric cam  4 . This in turn causes the screw to effectively wedge itself between the tapered flat D-section of the cam and an inside portion of the trephine. The taper of the cam is made such that ample clamping force is generated, causing the screw to start rotating and continue rotating in the same (counterclockwise) direction as the device  1  is being rotated.  
      Thereafter the device  1  continues to be rotated until the screw backs completely out of the host bone. Then the screw, which is wedged in and thus releasably retained in the trephine assembly  2 , is removed by using pliers or a similar device.  
      Therefore the device  1  removes the broken screw with less loss of host bone than prior art devices and without the need to break a core of host bone from the adjacent bone, by rotating the device  1  and continuing to rotate the device until the screw is completely removed.