Patent Publication Number: US-2009234451-A1

Title: Method and system for graft ligament attachment

Description:
FIELD OF THE INVENTION 
     Generally, embodiments disclosed herein relate to systems and methods for attaching a graft ligament to a bone. 
     BACKGROUND OF THE INVENTION 
     Ligaments are strong fibrous bands of tissue that serve to connect the articular extremities of bones to each other. Ligaments are typically composed of coarse bundles of dense white fibrous tissue that are disposed in a parallel or closely interlaced manner. The fibrous tissue of ligaments is pliant and flexible, but not significantly extensible (stretchable). Ligaments may be torn or ruptured as a result of accidents. The ligaments of the knee joint are especially susceptible to injury and may be repaired using arthroscopic surgical procedures. 
     For example, in the human knee, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) extend between the top end (proximal end) of the tibia and the bottom end (distal end) of the femur. The ACL and PCL cooperate, together with other ligaments and soft tissue, to provide both static and dynamic stability to the knee. The rupture or tearing of the ACL is a common occurrence among participants of sports activities such as football and skiing. 
     A rupture of the ACL generally does not heal spontaneously. Loss of the function of this stabilizing ligament may cause mechanical damage to other intra-articular tissues and may impair daily living activities, such as walking and bending the knee. Various surgical procedures have been developed for reconstructing the ACL to restore normal function to the knee. ACL reconstruction may be done by intra-articular or extra-articular methods. The intra-articular method seeks to duplicate the anatomic position and function of the ACL. The extra-articular method seeks primarily to reduce harmful force normally restrained by an uninjured ACL. The ideal reconstruction should be minimally invasive and allow early mobilization to preserve knee motion and avoid stiffness without failure of the construct. Furthermore, the extra-articular cortex and adjacent soft tissues should not be disturbed to reduce the chance of stress-riser fractures, quadriceps weakness, and heterotopic bone-formation. 
     In many instances, the ACL may be reconstructed using an intra-articular method by replacing the ruptured ACL with a graft ligament. The graft ligament may be a ligament or tendon which is harvested from elsewhere in the patient, such as a hamstring muscle (or any of the Semitendinosus, Semimembranosus, and Biceps femoris muscles), or may be a synthetic device. To connect the graft ligament to the bones at issue, bone tunnels are typically formed in the top end of the tibia and the bottom end of the femur, with one end of the graft ligament being positioned in the femoral tunnel and the other end of the graft ligament being positioned in the tibial tunnel. The two ends of the graft ligament are anchored in place in various ways known in the art so that the graft ligament extends between the femur and the tibia in substantially the same way, and with substantially the same function, as the original ACL to restore normal function to the knee. 
     Various methods of anchoring a graft ligament to the bone are known, such as staples, suture over buttons, and interference screw fixation. Staples and suture buttons are disadvantageous because they often do not provide a sufficiently strong attachment to withstand the normal tensile loads to which they are normally subjected. For example, with suture button fixation, the strand of suture coupling the button and the graft ligament becomes the weakest link in the chain, and if the suture breaks, the graft ligament will detach. 
     Another method of anchoring a graft ligament to a bone includes inserting the graft ligament into the opening in the femur and looping the graft ligament over a post inserted transversely into the bone. However, this method requires two holes to be drilled into the bone and requires that a deep intermuscular incision be made through soft tissue down to the bone to insert the transverse post, which requires additional recovery time. Furthermore, wrapping a graft ligament around a conventional post may cause the graft ligament to slide back and forth on the post during movement of the joint in what is known as the “windshield wiper effect.” The windshield wiper effect may abrade the graft ligament due to friction against the post and may cause failure of the graft ligament. 
     Another method of anchoring a graft ligament to a bone uses an interference screw to pin the graft ligament against the side of the femoral tunnel. However, interference screws are known to abrade the graft ligament with the screw threads and risk failure of the graft ligament fixation by rupturing. The probability of failure is exacerbated by the use of a metal screw with graft ligaments. Furthermore, the use of a biodegradable screw in place of a metal screw does not sufficiently reduce the risk of failure because the very nature of interference fixation requires high friction and tight apposition of the hard screw threads against the graft ligament, which can themselves cause abrasive damage. Furthermore, biodegradable screws have a higher rate of drive failure under a torque force sufficient to produce effective interference fixation. 
     What is needed is a method and system to attach a graft ligament to bone without the drawbacks of the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The various embodiments described herein provide methods and apparatus for achieving the secure and stable fixation of a graft ligament within a bone tunnel. 
     One embodiment described herein provides a screw including a cylindrical wall having a leading end and a trailing end, threads arranged on an outside surface of the cylindrical wall, and a plurality of portals arranged in the cylindrical wall and adapted to receive a plurality of prongs of a screw driver. In other embodiments, the screw may include a cannula that extends from a leading edge to a trailing edge of the screw. 
     In another embodiment, the screw further includes a crossbar arranged at the leading end of the screw. Embodiments of the crossbar include a handle having a first end and a second end, and end portions connected to the first end and the second end of the handle, where the end portions are wider than the handle. 
     Another embodiment described herein provides a screwdriver adapted to drive the screws. The screwdriver includes a handle, a shaft, and a plurality of prongs adapted to fit the plurality of portals of the screws. 
     Another embodiment described herein provides a hood adapted to cover at least a portion of the prongs of the screwdriver. The hood includes an opening and is formed of a material that is perforated when the plurality prongs of the screwdriver are inserted into the plurality of portals of the screw. 
     Another embodiment described herein provides a method of attaching a graft ligament to a bone and includes forming a tunnel in the bone, providing a screw comprising, a cylindrical wall having an outer surface and an inner surface, a cannula extending from a leading end and a trailing end of the screw, threads arranged on an outside surface of the cylindrical wall, a plurality of portals arranged in the cylindrical wall and adapted to receive a plurality of prongs of a screw driver, and a crossbar arranged at the leading end of the screw, arranging a graft ligament through the cannula of the screw and around the crossbar, and screwing the screw and the graft ligament into the tunnel in the bone. 
     Another embodiment described herein provides a method of attaching a graft ligament to a bone and includes forming a tunnel in the bone, providing a screw comprising a cylindrical wall having an outer surface and an inner surface, threads arranged on an outside surface of the cylindrical wall, and a plurality of portals arranged in the cylindrical wall and adapted to receive a plurality of prongs of a screw driver, providing a graft ligament in the tunnel and screwing the screw into the tunnel to fix the graft ligament into the tunnel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram of a cannulated screw according to an embodiment described herein. 
         FIG. 1B  is a diagram of a cannulated screw according to an embodiment described herein. 
         FIG. 2  is a diagram of a screwdriver according to an embodiment described herein. 
         FIG. 3  is a diagram of a cannulated screw according to an embodiment described herein. 
         FIG. 4  is a diagram of a screwdriver according to an embodiment described herein. 
         FIG. 5  is a diagram of the screwdriver covered by a purse stringed hood according to an embodiment described herein. 
         FIG. 6A  is a diagram of a crossbar according to an embodiment described herein. 
         FIG. 6B  is a diagram of a crossbar according to an embodiment described herein. 
         FIG. 6C  is a diagram of a crossbar according to an embodiment described herein. 
         FIG. 7  is a diagram of a cannulated screw according to an embodiment described herein. 
         FIG. 8A  is a diagram of a cannulated screw having a crossbar and a graft ligament threaded through the cannulated screw and around the crossbar according to an embodiment of a method describe herein. 
         FIG. 8B  is a diagram of a cannulated screw having a crossbar and a graft ligament threaded through the cannulated screw and around the crossbar according to an embodiment of a method describe herein. 
         FIG. 8C  is a diagram of a cannulated screw having a crossbar and a graft ligament threaded through the cannulated screw and around the crossbar according to an embodiment of a method describe herein. 
         FIG. 8D  is a diagram of a cannulated screw having an allograft bone and a graft ligament threaded through the cannulated screw according to an embodiment of a method describe herein. 
         FIG. 9  is diagram of a human knee prepared for insertion of a cannulated screw according to an embodiment of a method described herein. 
         FIG. 10  is a diagram of a cannulated screw, a crossbar, and a graft ligament being inserted into a femur using a screwdriver according to an embodiment of a method described herein. 
         FIG. 11  is a diagram of a cannulated screw, a crossbar, and a graft ligament inserted into a femur according to an embodiment of a method described herein. 
         FIG. 12A  is a diagram of an interference screw according to an embodiment described herein. 
         FIG. 12B  is a diagram of an interference screw according to an embodiment described herein. 
         FIG. 13  is a diagram of an interference screw according to an embodiment described herein. 
         FIG. 14  is diagram of a tibia prepared for insertion of an interference screw according to an embodiment of a method described herein. 
         FIG. 15  is a diagram of two corticocancellous bone grafts. 
         FIG. 16  is a diagram of two corticocancellous bone grafts inserted into a tibia according to an embodiment of a method described herein. 
         FIG. 17  is a diagram of an interference screw being inserted into a tibia using a screwdriver according to an embodiment of a method described herein. 
         FIG. 18  is a diagram of an interference screw inserted into a tibia according to an embodiment of a method described herein. 
         FIG. 19  is a diagram of an interference screw and injectable or particulate graft inserted into a tibia according to an embodiment of a method described herein. 
         FIG. 20  is a diagram of a chisel according to an embodiment described herein that may be used to remove a screw from a bone. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration of specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them, and it is to be understood that structural, logical or procedural changes may be made to the specific embodiments disclosed herein. 
       FIG. 1A  is a side view and  1 B is a trailing end view of a cannulated screw  100  having a cannula  110  extending from a leading end  102  to a trailing end  104  of the cannulated screw  100  through a cylindrical wall  150  having an inner surface  152  and an outer surface  154 , two prong portals  120 , threads  130 , and optional perforations  140 . The cannulated screw  100  may be formed of metal, such as stainless steel or titanium, a biodegradable material, such as a biodegradable polymer, or a combination of the two forgoing materials. 
     The cannulated screw  100  may be sized to a width and length to best fit a particular bone for a particular application. In one embodiment, the cannulated screw  100  may have an outer diameter between about 6 to about 15 mm, and more particularly, between about 8 to about 12 mm. In another embodiment, the cannulated screw  100  may have a length of about 15 to 30 mm, and more particularly between about 18 to 22 mm. 
     The threads  130  are wrapped around the cylindrical wall  150  and are sized to affix the cannulated screw  100  firmly into cancellous bone in a bone tunnel. In one embodiment, the threads  130  may be wrapped around the cylindrical wall  150  about 5 to about 15 times. The trailing end  104  of the cannulated screw  100  may be smooth, i.e., may lack threads  130 , to prevent abrasion to the proximal intra-articular portion of the graft ligament that may come in contact with the trailing end  104  of the cannulated screw  100 . 
     The leading end  102  and the trailing end  104  of the cannulated screw  100  should lack sharp edges that might cut through a graft ligament. In one embodiment, the leading end  102  and trailing end  104  may be curved so that they lack sharp edges. In another embodiment, the leading end  102  and trailing end  104  may be rounded. 
     The perforations  140  are optionally arranged in the cannulated screw  100  to allow osteo-integration of the soft tissue graft within the femoral tunnel. The number, size, shape, placement of the perforations  140  may be varied, but should be arranged in such a way so as to allow osteo-integration of the soft tissue graft without significantly weakening the integrity of the screw. 
     As shown in  FIG. 1B , the prong portals  120  are arranged at the trailing end  104  of the cannulated screw  100  to accept prongs from a screwdriver ( FIG. 2 ) to drive the cannulated screw  100  into a tunnel formed in a bone. The prong portals  120  extend from the trailing end  104  of the cannulated screw  100  into the cylindrical wall  150 . In various embodiments, the prong portals  120  may extend to varying depths within the cylindrical wall  150 . In one embodiment, the prong portals  120  do not have an exit at the leading end  102  of the cannulated screw  100 . In another embodiment, the prong portals  120  do have an exit at the leading end  102  of the cannulated screw  100 . 
       FIG. 2  is a diagram of a screwdriver  200  according to an embodiment described herein and adapted to be used with the cannulated screw  100 . The screwdriver  200  includes a shaft  202 , a handle  206 , and two prongs  204 . The handle  206  of the screwdriver  200  may be of variable length, but should be long enough to seat a cannulated screw  100  into the end of a bone tunnel. The prongs  204  are adapted to fit into the prong portals  120  of the cannulated screw  100  to allow the cannulated screw  100  to be screwed into a bone tunnel. The length of the prongs  204  may match the depth of the prong portals  120  so that the prongs  204  may be fully seated within the prong portals  120 . 
     In various embodiments, the cannulated screw  100  may include more than two prong portals  120 , for example, three, four, five, six, or more prong portals  120 , to more evenly distribute the torque force from a screwdriver ( FIG. 4 ) needed to drive the cannulated screw  100  over a larger drive function surface.  FIG. 3  is a diagram of a trailing end of a cannulated screw  300  having a cylindrical wall  350  having an inner surface  352  and an outer surface  354 , a cannula  310  extending through the cannulated screw  300  from a leading end  302  to a trailing end  304 , four prong portals  320 , and threads  330 . 
       FIG. 4  is a diagram of a screwdriver  400  according to an embodiment described herein. The screwdriver  400  includes a shaft  402 , a handle  406 , and four prongs  404 . The four prongs  404  of the screwdriver  400  are adapted to fit into the four prong portals  320  of the cannulated screw  300 . By more evenly distributing the torque force from the screwdriver  400 , the cannulated screw  300  will be better able to tolerate the torque force applied to drive the cannulated screw  300  to interference fixation without failure. This is especially important for a cannulated screw formed of biodegradable material, because biodegradable material is know to structurally fail more readily than metal screws of the same type. 
       FIG. 5  is a diagram of the screwdriver  200  of  FIG. 2  covered by a purse-stringed hood  500 . The hood  500  includes an opening  512  and a purse-string  514  arranged around the opening  512  to cinch the hood  500  to the screwdriver  200 . The hood  500  is adapted to fit over the end of the screwdriver  200  to cover the prongs  204 . The hood  500  may be made of material of a type and thickness so that the hood is easily perforated by the prongs  204  of the screwdriver  200  when the prongs  204  are pushed into the pong portals  220 . For example, the hood  500  may be made of synthetic polymers, such as latex, or natural polymers, such as rubber. 
     If the screwdriver  200  is removed from a bone tunnel before finally seating the cannulated screw  100  in the bone, the screwdriver  200  may be replaced by covering it with the hood  500  and reinserting the screwdriver  200  into the tunnel. The hood  500  will prevent the prongs  204  from being entangled with soft tissue on reinsertion. By applying gentle force, the prongs  204  will perforate the hood  500  to allow penetration of the prongs  204  into the prong portals  120 . Alternately, the hooded screwdriver  200  may be reinserted into a bone tunnel, such as a femoral tunnel, from the anterior medial portal over a guidewire inserted into the cannulated screw  100  and/or femoral tunnel. 
       FIG. 6A  is a diagram of a crossbar  600  to be used in conjunction with the cannulated screw  100  according to an embodiment. The crossbar  600  includes a handle  610 , end portions  620  that are wider than the handle  610  arranged at both ends of the handle  610 , and optional apertures  630  arranged in the end portions  620 . The crossbar  600  may be arranged at the leading edge  102  of the cannulated screw  100  so that a graft ligament may be passed through the cannula  110  and wrapped around the handle  610  of the crossbar  600 . The crossbar  600  may be formed of metal, such as stainless steel or titanium, a biodegradable material, such as a biodegradable polymer, or a combination of the two forgoing materials. 
     In the embodiment shown in  FIG. 6A , the crossbar  600  is shaped much like a type of exercise equipment commonly known as a “dumbbell,” with the end portions  620  having a spherical shape and centered on the handle  610 . In other embodiments, the end portions  620  may have other shapes, such as oval, rectangular, and the like. The handle  610  of the crossbar  600  should be long enough so that the end portions  620  fit over the outer surface  154  of the cylindrical wall  150  so that the crossbar  600  does not move laterally with respect to the cannulated screw  100 . The optional apertures  630  may be used to fasten a graft ligament to the crossbar  600  using a suture. The crossbar  600  should not be longer than the outer diameter of the cannulated screw  100  and should not have sharp edges that may cut through a graft ligament. 
       FIG. 6B  is a diagram of a crossbar  700  according to another embodiment that includes a handle  710 , end portions  720 , and optional apertures  730  arranged in the end portions  720 . In the embodiment shown in  FIG. 7 , the crossbar is shaped like a dumbbell, with end portions  720  having a spherical shape and offset from the center of the handle  710 . The handle  710  is arranged to position the looped end of a graft ligament further from the leading end  102  of the cannulated screw  100  and closer to the end of a bone tunnel for osteo-integration. 
       FIG. 6C  is a diagram of a crossbar  800  according to another embodiment. The crossbar  800  includes a handle  810 , optional apertures  830 , and end portions  820  that are fixed to the leading end  102  of the cannulated screw  100 . The crossbar  800  will not slip or move with respect to the cannulated screw  100  because it is firmly affixed to the leading end  102 . The shape of the crossbar  800  may be modified so long as no sharp edges are present that may cut through a graft ligament. 
       FIG. 7  is a side view of a cannulated screw  100  having grooves  850  arranged in the cylindrical wall  150  at the leading edge  102  that are adapted to receive a crossbar  600 ,  700 . The grooves  850  will prevent the crossbar  600 ,  700  from moving in a transverse direction with respect to the cannulated screw  100 . 
     A method of installing a graft ligament within a femoral tunnel using the cannulated screws and crossbars of the various embodiments is described below.  FIG. 8A  is a diagram of a cannulated screw  100  having a crossbar  700  arranged transversely to the length of the cannulated screw  100 . A looped end of a graft ligament  801  is threaded through the cannula  110  at the trailing end  104  of the cannulated screw  100  and out the leading end  102 . The crossbar  700  is inserted through the looped graft ligament  801  at the leading end  102  of the cannulated screw  100  so that the graft ligament  801  rests on the handle  710  of the crossbar  700 . The crossbar  700  entraps the graft ligament  801  and prevents it from pulling back through the cannula  110 . The handle  710  of the crossbar  700  is of a length so that the end portions  720  extend over the edges of the cannulated screw  100  to keep the crossbar  700  from moving transversely to the length of the cannulated screw  100 . 
       FIG. 8B  is a diagram of a cannulated screw  100  having grooves  850  arranged in the cylindrical wall  150  at the leading edge  102  and a crossbar  700  positioned in the grooves  850 . The grooves  850  prevent the crossbar  700  from moving in a transverse direction with respect to the cannulated screw  100 . A looped end of a graft ligament  801  is threaded through the cannula  110  of the cannulated screw  100  and around the crossbar  700 . 
       FIG. 8C  is a diagram of a cannulated screw  100  including a crossbar  800  having end portions  820  that are fixed to the leading end  102  of the cannulated screw  100 . A looped end of a graft ligament  801  is threaded through the cannula  110  of the cannulated screw  100  and around the crossbar  800 . 
       FIG. 8D  is a diagram of a cannulated screw  100  including an allograft bone  870  having a ligament  801  attached. The allograft bone  870  may take the place of a crossbar in fixing a ligament  801  to the cannulated screw  100 . The allograft bone  870  is larger than the cannula  110  and prevents the graft ligament  801  from pulling back through the cannulated screw  100 . 
       FIG. 9  is diagram of a knee  900  prepared for insertion of the cannulated screw  100 , crossbar  700 , and graft ligament  801 . The knee  900  includes a tibia  902  and a femur  904  surrounded by soft tissue  905 . Incisions are made in the soft tissue  905  surrounding the tibia  902  to expose the tibia  902 . Next, a suitable tool, such as a pneumatic or electric drill or a “reamer” tool, or other such equivalent medical device, is used to drill a tibial tunnel  906  in the tibia  902 . Next, a femoral tunnel  908  is drilled into the femur  904  so that the femoral tunnel  908  and the tibial tunnel  906  are aligned. The femoral tunnel  908  is a blind tunnel, which terminates below the surface of the femur  904 . 
     The size of the tibial tunnel  906  and the femoral tunnel  908  depends upon the size of the bones and the size of the graft ligament to be implanted. The tibial tunnel  906  and femoral tunnel  908  may be drilled to any required diameter, but are generally between about 5 and 18 millimeters. 
     As shown in  FIG. 10 , the cannulated screw  100 , crossbar  700 , and entrapped graft ligament  801  are inserted into the knee  900  using a screwdriver  200 . The ratio of the diameter of the femoral tunnel  908  to the outside diameter of the cannulated screw  100  may vary according to a doctor&#39;s preference of tightness of fit. In one embodiment, the ratio may be 1:1. In another embodiment, the ratio may be 9:10. 
     The prongs  204  of the screwdriver  200  are inserted into the prong portals  120  of the cannulated screw  100 . The cannulated screw  100  is then inserted through the tibial tunnel  906  into the femoral tunnel  908 . In one embodiment, the tibial tunnel  906  may be drilled wider than the femoral tunnel so that the cannulated screw  100  may be pushed through the tibial tunnel  906  without turning the cannulated screw  100 . The cannulated screw  100  is then screwed into the femoral tunnel  908  to seat the cannulated screw  100  into the femoral tunnel  908 . In another embodiment, the tibial tunnel  906  and the femoral tunnel  908  may be the same size and the cannulated screw  100  is screwed through the tibial tunnel  906  and into the femoral tunnel  908 . In other embodiments, the screwdriver  200  may be inserted into the femur  904  from another suitable portal. 
     As shown in  FIG. 11 , the screwdriver  200  is removed and the cannulated screw  100 , crossbar  700 , and graft ligament  801  remain implanted in the femoral tunnel  908 . In one embodiment, the cannulated screw  100  may be seated so that it fills up most of the femoral tunnel  908 . In another embodiment, the cannulated screw  100  may protrude from the end of the femoral tunnel  908 . 
     The exposed graft ligament  801  above the crossbar  700  may be pressed against the cancellous bone inside the femoral tunnel  908  to allow osteo-integration. The graft ligament  801  is thus protected within the cannulated screw  100 . The cannulated screw  100  will support the interior wall of the femoral tunnel  908  to increase the stiffness of the overall construct while simultaneously allowing osteo-integration of the graft ligament  801 . 
     The graft ligament  801  will contact the inner wall of the femoral tunnel  908  to help to maximize the tunnel fill and improve biological activity within the femoral tunnel  908 . These features are designed to reduce axial and longitudinal motion, e.g., the windshield wiper effect, of the graft ligament  801  within the femoral tunnel  908 , especially after cycling forces performed in the postoperative rehabilitation period, thereby reducing lysis and tunnel enlargement. 
     As discussed above, the trailing end  104  of the cannulated screw  100  is smooth and acts as a grommet or “bushing” to prevent abrasion to the proximal intra-articular portion of the graft ligament  801  in contact with the trailing end  102  of the cannulated screw  100 . This type of fixation improves the strength of the fixation and may reduce slippage to cyclical loading in the post-operative rehabilitation period. 
     The embodiments described above provide an improvement in fixation of ACL graft construct, and may allow early and accelerated rehabilitation, in part due to the fact that there is no need to make a transverse hole through the femur  904  to insert a crossbar, which requires incisions to be made through the nearby external soft tissue. The embodiments described above also allow the use of a biodegradable or metal cannulated screw  100  to attach a graft ligament  801  without concern for drive failure or graft abrasion until biologic incorporation, i.e., osteo-integration of the graft ligament  801  within the femoral tunnel  908 , is complete. The ability to use a metal cannulated screw  100  without the fear of graft abrasion may reduce later failure due to degraded debris and lysis within the femoral tunnel  908 . 
       FIG. 12A  is a side view and  FIG. 12B  is a trailing end view of an interference screw  1200  having a cylindrical wall  1250  having an outer surface  1254 , an optional cannula  1210  having an inner surface  1252  extending through the interference screw  1200  from a leading end  1202  to a trailing end  1204 , threads  1230  arranged on the outer surface  1254  of the cylindrical wall  1250 , two prong portals  1220 , optional perforations  1240 , and optional threads  1260  arranged on the inner surface  1252  at the trailing end  1204  of the cylindrical wall  1250 . The interference screw  1200  may be formed of metal, such as stainless steel or titanium, or may be formed of a biodegradable material, such as a biodegradable polymer, or a combination of the two foregoing materials. 
     The interference screw  1200  may be sized to a width and length to best fit a particular bone for a particular application. In one embodiment, the interference screw  1200  may have an outer diameter between about 4 mm to about 16 mm, and more particularly, between about 8 mm to about 12 mm. In another embodiment, the cannulated screw  100  may have a length of about 15 to 25 mm and more particularly between about 18 to 22 mm. 
     The threads  1230  are wrapped around the cylindrical wall  1250  and are sized to affix the interference screw  1200  firmly into cancellous bone in a bone tunnel. In one embodiment, the threads  1230  may be wrapped around the cylindrical wall  1250  about 5 to about 15 times. The outer surface  1264  of the trailing end  1204  may be smooth to prevent abrasion to the proximal intra-articular portion of the graft ligament  801  that may come in contact with the trailing end  1204  of the interference screw  1200 . 
     The leading end  1202  and the trailing end  1204  of the interference screw  1200  should lack sharp edges that might cut through a graft ligament. In one embodiment, the leading end  1202  and trailing end  1204  may be curved so that they lack sharp edges. In another embodiment, the leading end  1202  and trailing end  1204  may be rounded. 
     The perforations  1240  are optionally arranged in the interference screw  1200  to allow osteo-integration of a graft ligament within a bone tunnel. The number, size, shape, placement of the perforations  1240  may be varied, but should be arranged in such a way so as to allow osteo-integration of the soft tissue graft without significantly weakening the integrity of the interference screw  1200 . The interference screw  1200  may optionally embody a whole or partial cannula or lumen  1210 , or may have a solid core. 
     As shown in  FIG. 12B , the prong portals  1220  are arranged to accept prongs from a screwdriver, such as the screwdriver  200  in  FIG. 2 , to drive the interference screw  1200  into a bone tunnel. The prongs  204  of the screwdriver  200  are adapted to fit into the prong portals  1220  of the interference screw  1200 . In various embodiments, the prong portals  1220  may extend to varying depths within the cylindrical wall  1250 . In one embodiment, the prong portals  1220  do not have an exit at the leading end  1202  of the interference screw  1200 . In another embodiment, the prong portals  1220  do have an exit at the leading end  1202  of the interference screw  1200 . 
     In various embodiments, the interference screw  1200  may include more than two prong portals  1220 , for example, three, four, five, six, or more prong portals  1220 , to more evenly distribute the torque force from a screwdriver, such as the screwdriver  400  of  FIG. 4 , needed to drive the interference screw  1200  over a larger drive function surface.  FIG. 13  is a diagram of an interference screw  1300  having a wall  1360  having an outer surface  1364 , an optional cannula  1310  having an inner surface  1362  extending through the interference screw  1300  from a leading end  1302  to a trailing end  1304 , threads  1330 , optional perforations  1340 , and optional threads  1350  arranged on the inner surface  1362  at the trailing end  1304  of the wall  1360 . The prongs  404  of the screwdriver  400  are adapted to fit into the prong portals  1320  of the interference screw  1300 . By more evenly distributing the torque force from a screwdriver  400 , the interference screw  1300  will be better able to tolerate the torque force applied to drive the interference screw  1300  to interference fixation without failure. In one embodiment, one screwdriver  200 ,  400  may be adapted to fit the prong portals  120 ,  320  of a cannulated screw  100 ,  300  and the prong portals  1220 ,  1320  of an interference screw  1200 ,  1300 . 
     A method of installing a graft ligament within a tibial tunnel using an interference screw of the various embodiments is described below. After a graft ligament  801  has been attached to the femur  904 , for example, by the method described above or by other methods, the graft ligament  801  will extend from the femoral tunnel  908  through the tibial tunnel  906 , as shown in  FIG. 14 , and must be attached to the interior of the tibial tunnel  906  until biologic incorporation, i.e., osteo-integration of the graft ligament  801  within the tibial tunnel  906 , is complete. 
     In one embodiment, the graft ligament  801  is secured in the tibial tunnel  906  using a bone-ligament-bone composite graft. As shown in  FIG. 15 , two corticocancellous bone grafts  1500  are obtained from either the patient or a donor. The corticocancellous bone grafts  1500  include softer cancellous bone  1502  on one side of the grafts  1500  and harder cortical bone  1504  on the other side of the grafts  1500 . In another embodiment, the bone grafts  1500  may be made from artificial material, such as hydroxyapatite, ceramics, coral, or other suitable materials. 
     As shown in  FIG. 16 , the bone grafts  1500  are placed into the tibial tunnel  906 , while the ends of the graft ligament  801  are held taut by stay sutures  1602 . The graft ligament  801  is sandwiched between the cancellous bone  1502  of the bone grafts  1500  and the cancellous bone  910  in the tibial tunnel. The interference screw  1200  is then screwed into the tibial tunnel  906  between the bone grafts  1500  using the screwdriver  200  to wedge the bone grafts  1500  and the graft ligament  801  tightly into the tibial tunnel  906 , as shown in  FIG. 17 . During the application of the interference screw  1200 , the knee joint  900  may be held in full extension or in varying degrees of flexion. 
     In another embodiment, the bone grafts  1500  may be omitted and the interference screw  1200  may be driven into the tibial tunnel  906  and between the graft ligaments  801  to force the graft ligaments  801  against the walls of the tibial tunnel  906 . 
       FIG. 18  shows the bone grafts  1500  and the interference screw  1200  seated in the tibial tunnel  906 . The bone grafts  1500  serve to protect the graft ligaments  801  from abrasion by the threads  1230  of the interference screw  1200 . The bone grafts  1500  should be at least as long as the threads  1230  of the interference screw  1200 . However, in various embodiments, the bone grafts  1500  and interference screw  1200  may be of varying lengths and diameters according to the preference of the practitioner of the ligament reconstruction. 
     Finally, as shown in  FIG. 19 , an injectable or particulate graft  1810  is introduced into the tibial tunnel  906  between the bone grafts  1500  to effect osteo-integration. If the interference screw  1200  includes a cannula  1210  and perforations  1240 , the injectable or particulate graft  1810  may be injected into the interference screw  1200  to be forced through the perforations  1240  and into contact with the bone grafts  1500 . The cannula  1210  of the interference screw  1200  may then be capped with a screw plug  1820  screwed into the threads  1250  arranged on the inner surface  1262  of the wall  1260 . 
       FIG. 20  is a diagram of a chisel  2000  that may be used to remove a screw from a bone tunnel. The chisel  2000  includes a handle  2006 , a shaft  2002 , and a cylindrical blade  2004  having a cannula  2010  and an edge  2008 . The edge  2008  may be sharpened and may optionally be serrated. In one embodiment, the cannula  2010  may be deep enough to hold a screw to be removed from a bone tunnel. In another embodiment, the cannula  2010  may have a diameter that is slightly larger than the outer diameter of a screw to be removed from a bone tunnel. In use, the chisel  2000  may be inserted into a bone tunnel to envelop an implanted screw by tapping or rotation. The edge  2008  of the blade  2004  is used to break through bone and soft tissue that may have integrated with the screw so that the screw may be removed. 
     While embodiments of the invention have been described in detail in connection with embodiments known at the time, it should be readily understood that they are not limited to the disclosed embodiments. Rather, they can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described. For example, while embodiments are described in connection with attaching a graft ligament to a femur and tibia in a procedure to reconstruct a damaged ACL, it should be understood that the embodiments described herein are not so limited and may be applied wherever surgical interference fixation is needed to fix soft tissue in a bone tunnel, for example, a PCL. Furthermore, while the various embodiments of methods are described as using a cannulated screw, crossbar, and/or interference screw of a particular embodiment, it should be understood than any of the cannulated screw, crossbar, and/or interference screw embodiments described herein may be used with the various methods.