Abstract:
An improved joint fusion screw for transiliac fixation has an elongate hollow shaft. The hollow shaft has an externally threaded end portion extending to a tip end and a non-externally threaded shank portion having openings. The tip end has at least two bone cutting flutes at the bottom of the shaft. Each bone cutting flute has a cutting edge on a circumferential exterior of the threaded tip to cut bone and direct the cut bone internally into the hollow shaft toward the shank.

Description:
TECHNICAL FIELD 
     The present invention relates to an improved spinal fixation screw for transiliac fixation and a method of use. 
     BACKGROUND OF THE INVENTION 
     Many complaints of lower back pain and leg pain have been attributed to herniated discs or other injuries to the spinal column. Extensive therapy and treatment has often been unsuccessful in alleviating such pain. It has been established that some of this lower back and leg pain can be attributed to symptomatic sacroiliac dysfunction or instability. Normally, the sacroiliac joint which spans between the sacrum bone and ilium bone has nutation of one to two degrees. “Nutation” is the medical term which describes the relative movement between the sacrum and ilium. A patient&#39;s sacroiliac joint can become damaged resulting in hypermobility of the joint. Because of the small range of motion in the sacroiliac joint, hypermobility is very difficult to diagnose. Therefore, lower back pain or leg pain caused by sacroiliac dysfunction often goes misdiagnosed or undiagnosed. 
     Accordingly, it is an objective of this invention to provide a device for correcting symptomatic sacroiliac dysfunction or instability. It is another aspect of this invention to provide a device which enhances stability and compression for purposes of immobilizing a joint, and for fusing two opposed bone structures across the joint. 
     SUMMARY OF THE INVENTION 
     An improved joint fusion screw for transiliac fixation has an elongate hollow shaft. The hollow shaft has an externally threaded end portion extending to a tip end and a non-externally threaded shank portion having a plurality of openings. The tip end has at least two bone cutting flutes at the bottom of the shaft. Each bone cutting flute has a cutting edge on a circumferential exterior of the threaded tip to cut bone and direct the cut bone internally into the hollow shaft toward the shank. Each cutting edge lies in a plane parallel to an axis of the elongate hollow shaft. 
     In each embodiment, the hollow shaft has a bone chamber for receiving the cut bone fragments. The bone chamber extends to at least the openings of the shank portion. Autograft cut bone fragments are directed to the openings to enhance new bone growth and rapid fusion of the fusion screw. Preferably, the openings of the shank portion are elongated slots. 
     The screw has an enlarged flat head affixed or integral to an end of the shank. The end of the shank portion has internal or female threads for receiving a threaded driver cap. The threaded driver cap has a cannulated opening or aperture for passing a guide wire and a torquing tool receiving cavity to thread the screw into the bone. The drive cap is affixed into the threaded end of the shank. The driver cap can be removably attached to allow bone packing material to be packed into the hollow shaft after screw insertion into the bone. The at least two cutting flutes are preferably diametrically opposed. 
     In one embodiment, each bone cutting flute has an arcuate ramp extending from the cutting edge toward an inside diameter of the hollow shaft. The cut bone fragments are directed internal along the arcuate ramps upon implantation of the screw into the hollow shaft. Each of the cutting edges form spiral cut autograft bone upon screw implantation. Each of the formed spiral cut autograft bone remains connected by tissue to increase osteoconductivity. 
     In a second embodiment, the at least two cutting flutes extend starting from the tip end longitudinally through two or more threads. 
     In a third embodiment, the tip end can have a web or bridge extending across the hollow shaft. The web or bridge has an aperture for receiving a guide wire. The aperture is coaxial with an axis of the screw and the aperture of the cap driver. 
     A method of transiliac fixation using the improved screw comprises the steps of pre-drilling an opening in the sacrum and the ilium bones to be fixed with a pilot hole opening and inserting a joint fixation screw with a hollow shaft onto the pre-drilled opening while cutting autograft bone fragments directed into the hollow shaft. The hollow shaft has a bone receiving chamber extending to a plurality of openings further in the hollow shaft and the step of threading of the screw directs the autograft bone fragments to the openings to enhance fusion. The screw can have apertures at the tip end and at the driver cap and the method may further comprise the steps of inserting a guide wire to create a drill path, inserting a cannulated drill over the guide wire to pre-drill the pilot hole, and then inserting the screw onto the guide wire to direct the path for insertion into the bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a first embodiment of the present invention. 
         FIG. 2  is a perspective view of a second embodiment of the present invention. 
         FIG. 3  is a perspective view of a third embodiment of the present invention. 
         FIG. 4  is an opposite end perspective view for each of the aforementioned embodiments. 
         FIG. 5  shows the optional anti-back out feature on the head. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1, 2 and 3 , three versions or embodiments of an improved joint fixation screw  10 A,  10 B and  10 C for a transiliac fixation are shown. Each embodiment has common features with variations on the cutting flutes and tip end design.  FIG. 4  shows the proximal head end opposite to the bone cutting tip end. The head  30  at this end is a common feature to all three embodiments. 
     Each screw  10 A,  10 B and  10 C has a hollow elongated shaft  20 . The shaft  20  has an externally threaded end portion  21  and a smooth shank portion  25 . The smooth shank portion  25  has a plurality of openings  24  open to a chamber  12  inside the hollow shaft  20 . At a proximal end of the screws  10 A,  10 B and  10 C is an enlarged head  30 . The center of the head  30  is a threaded opening  32  open to the chamber  12 . The threaded end portion  21  of the hollow shaft  20  has threads  29 . All these features are common to each screw  10 A,  10 B and  10 C. 
     In a first embodiment of  FIG. 1 , the screw  10 A has two cutting flutes  11 . Each flute  11  is diametrically opposed from the other and each has a cutting edge  13  formed from a leading thread  29  at the tip end  21 . The cutting edges  13  lie in a plane parallel to the axis of the screw shaft and each had an arcuate ramp  14  for directing bone fragments into the hollow chamber  12 . The cut fragments spiral into the chamber  12  along the ramped surfaces  14 . 
     In a second embodiment of  FIG. 2 , each of the cutting flutes  11  extend deeper across two or more threads  29  of the end portion  21  to a bottom  16 . The cutting edges  13  are still circumferentially in a plane parallel to the axis, but the deep longitudinally extending opening of the flutes  11  captures the cut bone fragments and directs them into the chamber  12  in pieces that are broken on threading. 
     With reference to  FIG. 3 , a third embodiment, a bridge or web  23  extends across the hollow shaft  20  at the tip end  21  defining a pair of openings  15  between the bridge or web  23  and the flutes  11 , as shown. The bridge or web  23  has an aperture  27  for receiving a guide wire. This third version screw  10 C has the same flutes  11  as shown in  FIG. 2 . 
       FIG. 4 , shows a threaded driver cap  40  inserted and threaded into the threads  32  of the enlarged head  30 . This driver cap  40  has a torque receiving cavity  41  with projections  42  to receive a torquing tool to implant the screw  10 A,  10 B or  10 C. Centrally, there is an aperture  47  to allow the screw  10 C to pass over a guide wire along a directional pre-drilled path. 
     As the screw  10 A,  10 B or  10 C is torqued into the pre-drilled pilot hole, the cutting flutes  11  create autograft bone fragments that are delivered directly into the chamber  12 . In this way, the patient&#39;s bone fragments are made available to enhance new bone growth to fuse the screw  10 A,  10 B or  10 C in place. 
     One purpose of this invention is to direct bone that is cut by the self-tapping threads and cutting edges  13  at the tip of the bone screw  10 A,  10 B or  10 C or otherwise gathered by the flutes  11  into the internal chamber  12  of the screw to serve as additional autograft material. Previously, this material would be compressed into the bone around the outside of the screw. The screw would be filled with previously harvested autograft material which could be packed into the screw from an opening in the head end of the screw. The screw is used to secure two bones together, in this case the sacrum and the ilium. When preparing the bone to accept the screw, a hole will be drilled and tapped to a size smaller than the actual screw. The screw can be packed with graft material prior to implantation. The self-tapping edge of the screw will cut additional autograft material as it is installed and the flute will direct this freshly cut autograft material to join the existing material in the inner chamber  12  of the screw. Some of this material will be pushed out of the plurality of openings  24  or fenestrations in the shaft  20  of the screw as it is tightened to aid in fusion around and into the body of the screw. Many variations of similar flute shapes will produce a similar result. The screw material can be anything hard and strong enough to cut and direct bone chips and withstand the biomechanical loads of the application, preferably titanium, stainless steel or alloys of these materials or metals will work satisfactorily. 
     The present invention SI (Sacro-iliac) screw described herein has shown several important features of this screw. These features include: the lagging where the head  30  is pulled down by the coarse threads  29  and the lagged portion is within the smooth shank portion  25  and not engaged by threads  29 ; the screw has a large bore or chamber  12  for inserting bone graft with the option to cap this bore which communicates to the SI space after insertion with the driver cap  40 ; the shaft  20  has only the single threaded end portion  21  to engage only the sacrum bone, the ilium bone is positioned on the smooth shank portion  25 ; an optional anti-back out feature under the head  30  in the form of a series of wedge-shaped teeth  33  to engage the iliac bone surface can be used as shown in  FIG. 5 . A wedge shaped washer (not shown) could be used under the screw head  30  to accommodate the surface angle of the ilium with respect to the screw axis and also employ the anti-back out feature shown in  FIG. 5 , but as a separate washer piece. 
     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described, which will be within the full intended scope of the invention as defined by the following appended claims.