Abstract:
A material fixation system is particularly adapted to improve the tendon-to-bone fixation of hamstring autografts, as well as other soft tissue ACL reconstruction techniques. The system is easy to use, requires no additional accessories, uses only a single drill hole, and can be implanted by one person. Additionally, it replicates the native ACL by compressing the tendons against the aperture of the tibial tunnel, which leads to a shorter graft and increased graft stiffness. It is adapted to accommodate single or double tendon bundle autografts or allografts. It also provides pull out strength measured to be greater than 1000 N.

Description:
[0001]     This application claims the benefit under 35 U.S.C. 119(e) of the filing date of Provisional U.S. Application Ser. No. 60/784,422, entitled Method and Apparatus for Attaching Soft Tissues to Bone, filed on Mar. 20, 2006, and of the filing date of Provisional U.S. Application Ser. No. 60/854,178, entitled Methods and Systems for Material Fixation, filed on Oct. 24, 2006, and of the filing date of Provisional U.S. Application Ser. No. 60/898,946, entitled Devices, Systems and Methods for Material Fixation, filed on Jan. 31, 2007. All of these prior provisional applications are expressly incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to devices, systems and methods for material fixation. More specifically, the present invention relates to a soft tissue or bone-to-bone fixation system that permits a practitioner to repair many soft tissue injuries, such as an Anterior Cruciate Ligament (ACL) injury.  
         [0003]     One of the most common needs in orthopedic surgery is the fixation of tendon to bone. The fixation of diseased tendons into a modified position is called tenodesis and is commonly required in patients with injury to the long head of the biceps tendon in the shoulder. In addition, tendons which are torn from their insertion site into bone also frequently require repair. This includes distal biceps tendon tears, rotator cuff tears, and torn flexor tendons in the hand. Tendons are also frequently used in the reconstruction of unstable joints. Common examples include anterior cruciate ligament and collateral ligament reconstructions of the knee, medial and lateral elbow collateral ligament reconstructions, ankle collateral ligament reconstruction, finger and hand collateral ligament reconstructions and the like.  
         [0004]     Traditional techniques that are used to fix tendon to bone suffer from a number of limitations as a result of the methodology used, including the use of a “keyhole” tenodesis, pull-out sutures, bone tunnels, and interference screw fixation. The “keyhole” tenodesis requires the creation of a bone tunnel in the shape of a keyhole, which allows a knotted tendon to be inserted into the upper portion, and subsequently wedged into the lower narrower portion of the tunnel where inherent traction on the tendon holds it in place. This technique is challenging as it is often difficult to sculpt the keyhole site and insert the tendon into the tunnel. In addition, if the tendon knot unravels in the postoperative period, the tendon will slide out of the keyhole, losing fixation.  
         [0005]     Another traditional form of tendon fixation is the use of the “pull-out stitch.” With this technique, sutures attached to the tendon end are passed through bone tunnels and tied over a post or button on the opposite side of the joint. This technique has lost favor in recent years due to a host of associated complications, which include wound problems, weak fixation strength, and potential injury to adjacent structures.  
         [0006]     The most common method of fixation of tendon to bone is the use of bone tunnels with either suture fixation, or interference screw fixation. The creation of bone tunnels is relatively complicated, often requiring an extensive exposure to identify the margins of the tunnels. Drill holes placed at right angles are connected using small curettes. This tedious process is time-consuming and fraught with complications, which include poor tunnel placement and fracture of the overlying bone bridge. Graft isometry, which is easy to determine with single point fixation, is difficult to achieve because the tendon exits the bone from two points. After creation of tunnels, sutures must be passed through the tunnels to facilitate the passage of the tendon graft. Tunnels should be small enough to allow good tendon-bone contact, yet large enough to allow for graft passage without compromising the tendon. This portion of the procedure is often time-consuming and frustrating to a surgeon. Finally, the procedure can be compromised if the bone bridge above the tunnel breaks, resulting in loss of fixation. The technique restricts fixation to the strength of the sutures, and does not provide any direct tendon to bone compression.  
         [0007]     More recent advances in the field of tendon fixation involve the use of an internally deployed toggle button, for example, the ENDOBUTTON, and the use of interference screws to provide fixation. The ENDOBUTTON allows the fixation of tendon into a bone tunnel by creating an internally deployed post against a bony wall. While this technique eliminates the need for secondary incisions to place the post, the fixation strength is limited to suture strength alone. This technique does not provide direct tendon to bone compression; as such this technique may slow healing and lead to graft tunnel widening due to the “bungee effect” and “windshield wiper effect”. As a result, this technique has limited clinical applications and is used primarily for salvage when bone tunnels break or backup fixation is important.  
         [0008]     The use of the interference screw is the most notable advance in the fixation of tendon to bone. The screw is inserted adjacent to a tendon in a bone tunnel, providing axial compression between the screw threads and the bony wall. Advantages include acceptable pull-out strength and relative ease of use. Aperture fixation, the ability to fix the tendon to bone at its entrance site, is a valuable adjunct to this technique as it minimizes graft motion and subsequent tunnel widening. Some disadvantages related to soft tissue interference screws are that they can be difficult to use, and can also cut or compromise the tendon during implantation.  
         [0009]     The newest generation interference screw allows the ability to provide tendon to bone fixation with limited exposure. For example, the BIO-TENODESIS SCREW (Arthrex, Inc.) allows the tensioning and insertion of tendon into bone, followed by insertion of an adjacent soft tissue interference screw. While this screw system provides advantages in the insertion of tendon into bone in cases when a pull through stitch is not available, it is still limited by the potential for tendon rotation or disruption as the screw compresses the tendon. The surgical technique is also complicated, typically requiring two or more hands for insertion, making it difficult to use the system without assistance during arthroscopic or open procedures. Finally, the use of the screw requires preparation of the tendon end, which can be difficult, time consuming, and can also require conversion of an arthroscopic procedure to open.  
         [0010]     Focusing particularly on the ACL, current ACL repairs utilizing soft tissue for the replacement graft are either difficult to perform or they result in less than favorable outcomes due to their relatively low tendon-to-bone fixation. Existing ACL reconstruction techniques that have acceptable outcomes (high tendon-to-bone fixation) involve extra operating room time and surgeon effort due to the requirement of multiple drill holes, external guides and fixtures for the drill holes, and multiple assistants. Moreover, these approaches to not closely replicate the native ACL in its anatomy or physiology.  
         [0011]     Two important factors in replicating the native ACL are aperture compression and tendon length. Compressing the tendons at the aperture of the femoral tunnel will improve the healing process by increasing the intimate contact between the tendon and the bone. A study shows that without intimate contact between the tendon and the bone, the result is a graft having less well organized fibrous tissue and lower pull-out strength. The stiffness of the repair is also important to replicate the native ACL. Graft stiffness is decreased by the length of tendon between the fixation points.  
         [0012]     Currently, two different sources are utilized for the tissue that replaces the injured native ACL. When the new tissue comes from the patient&#39;s own body, the new graft is referred to as an “autograft”, and when cadaveric tissue is used, the new graft is referred to as an “allograft”. The most common autograft ACL reconstruction performed currently is the bone-patellar-tendon-bone (BTB) graft. The BTB graft with an interference screw is used more often because it more accurately replicates the native ACL due to its aperture compression at the tibial tunnel aperture. However, BTB reconstructions result in an increased rate of anterior knee pain post-surgically for periods of up to three years after the reconstruction. Additionally, the harvest procedure for the BTB autograft is invasive and can be difficult to perform. Alternatively, the hamstring tendon autograft ACL reconstruction technique does not result in any significant post-surgical pain, and the harvest procedure is not minimally invasive. The reason that the hamstring tendon autograft procedure is not more frequently used in ACL reconstructions is that the fixation of the hamstring tendons to the femur an tibia, using prior art techniques, is not as strong as the fixation of the BTB autografts.  
         [0013]     Many systems have addressed some of the problems associated with ACL reconstruction using hamstring tendons, but there is not any system which addresses them all. The TriTis® system available from Scandius attempts to more accurately replicate the native ACL by adding material to take up space in the tibial tunnel, resulting in more intimate contact between the tendon and the bone. However, to insert the device into the femoral tunnel, the cross sectional area must be less than the cross sectional area of the hole. There is no real compression of tendon to bone. The TriTis system also requires additional drill holes, accessories, and people to perform the procedure.  
         [0014]     The IntraFix® system available from Mitek attempts to more accurately replicate the native ACL by using a screw to spread apart an integral four quadrant sheath. This acts to compress the four tendon strands against the bone. The system is easier to use than other alternatives, and does not need additional drill holes. However, it does require additional accessories, additional people to perform the procedure, and the four quadrant design does not accommodate certain allografts with two tendon strands, such as the tibialis.  
         [0015]     The WasherLoc™ system, available from Arthrotek, gives increased strength, compared to other prior art systems, but does not accurately replicate the native ACL. The tendons are sized to the hole, but not compressed to the walls. There is also a greater distance between fixation points with this system, which can decrease the stiffness of the repair.  
         [0016]     Interference screws such as the RCI™ Screw available from Smith &amp; Nephew are easy to use and provide compression of tendon to bone at the tibial tunnel aperture. However, the pull out strength and stiffness of the repair are significantly lower than is the case for other prior art systems.  
         [0017]     Thus, although there are many conventional techniques used for the fixation of tendon to bone, each having some advantages, the disadvantages of each such technique presents a need in the art for a simple and universal technique to fixate tendon to bone such that the device is easy to use, the process is simple to follow, and the result is a firm and secure tendon to bone fixation with minimal negative effect on the tendon. Further, such device should be easy to manufacture, universally applied to different tendon to bone sites, and require minimal effort to understand and use in practice.  
       SUMMARY OF THE INVENTION  
       [0018]     The present invention is a system which is particularly adapted to improve the tendon-to-bone fixation of hamstring autografts, as well as other soft tissue ACL reconstruction techniques. The system is easy to use, requires no additional accessories, uses only a single drill hole, and can be implanted by one person. Additionally, it replicates the native ACL by compressing the tendons against the aperture of the tibial tunnel, which leads to a shorter graft and increased graft stiffness. It is adapted to accommodate single or double tendon bundle autografts or allografts. It also provides pull out strength measured to be greater than 1000 N, which is equivalent to or substantially higher than any of the high strength implants currently available on the market.  
         [0019]     More particularly, a material fixation system is provided, which comprises two sheath portions defining a space therebetween, and a hinge for attaching the sheath portions together along one side thereof. An insertion member, preferably a tapered screw, is insertable into the space for expanding the sheath portions laterally outwardly in order to urge a soft tissue graft against an adjacent bone surface. In a preferred embodiment, the hinge comprises a hinge protrusion disposed on a first of the sheath portions and a hinge slot disposed on a second of the sheath portions, wherein the hinge protrusion and the hinge slot engage one another. A second hinge protrusion is disposed on the second sheath portion and a second hinge slot is disposed on the first sheath portion, wherein the second hinge protrusion and the second hinge slot also engage one another.  
         [0020]     A driver is utilized for engaging and moving the insertion member. A hex opening is provided in the proximal end of the insertion member for engaging a distal end of the driver.  
         [0021]     Preferably, the screw has a bullnose screw head, and the two sheath portions are mirror images of one another.  
         [0022]     The invention is particularly advantageous, in that the system is adapted for use whether the soft tissue graft comprises an autograft, or an allograft. A distal end of the screw comprises a cut-out portion which permits the distal end of the screw to easily fit between the two sheath portions, thus permitting an operator to easily start rotation of the screw. The screw comprises external threads and the sheath portions comprise complementary internal threads. The screw further comprises a thread start to enable easier engagement of the screw threads and the sheath threads.  
         [0023]     At least one retaining rib is preferably disposed on at least one of the sheath portions. The rib protrudes outwardly to provide a small area of higher force between the sheath portion and the soft tissue graft. The sheath portions and the insertion member are preferably adapted for insertion into a bone tunnel in a patient&#39;s tibia, and the soft tissue graft comprises a tendon graft for making an ACL repair. A cortical hook is preferably disposed on one of the sheath portions for engaging hard cortical bone at the procedural site.  
         [0024]     One of the sheath halves preferably comprises a snap post and the other one of the sheath halves preferably comprises a complementary snap hole, wherein the snap post and the snap hole are engageable with one another to keep the two sheath halves from opening prematurely. In the preferred embodiment, a ramp is formed on one of the sheath portions for allowing a tip of the sheath portion to provide compression between the soft tissue graft and the bone at the aperture of bone tunnel in which the system is disposed. Flex grooves are disposed on one of the sheath portions, for permitting the sheath portion to flex and form around a tip of the insertion member. A bullnose sheath tip is provided on one of the sheath portions.  
         [0025]     In some embodiments, it is advantageous for the sheath portions to further comprise a loop for retaining a soft tissue graft along a laterally outer surface of the sheath portion.  
         [0026]     In another aspect of the invention, there is provided an implant system for promoting soft tissue to bone contact in order to promote good fixation of soft tissue to bone when making an orthopedic repair of a joint, wherein the implant system comprises a first implant adapted for receiving a tissue graft thereon and then being disposed in a first bone tunnel location, wherein ends of the tissue graft extend through a bone tunnel and out of a proximal end of the tunnel. A second implant is adapted for disposition in a second bone tunnel location, proximal to the first bone tunnel location, wherein the second implant comprises a plurality of sheaths having laterally outer surfaces and being adapted for advancing to the first bone tunnel location by sliding over the ends of the tissue graft, so that when the second implant is in the second bone tunnel location, the tissue grafts are disposed between the laterally outer surfaces of the plurality of sheaths and the bone defining the bone tunnel. An insertion member is insertable between the plurality of sheath members to laterally expand the sheath members toward the soft tissue grafts, thereby urging the soft tissue grafts into contact with the bone defining the bone tunnel.  
         [0027]     In still another aspect of the invention, there is provided a material fixation system, which comprises a plurality of sheath portions defining a space therebetween, wherein the sheath portions are initially engaged with one another in an undeployed orientation. An insertion member is insertable into the space for expanding the sheath portions laterally outwardly to a fully deployed orientation in order to urge a soft tissue graft against an adjacent bone surface. As the sheath portions expand outwardly to the aforementioned fully deployed orientation, they become detached from one another.  
         [0028]     In yet another aspect of the invention, there is disclosed a method of making an orthopedic repair by fixing a soft tissue graft to bone. The disclosed method comprises a step of creating a tunnel within a desired bone site, wherein the tunnel extends through a first bone member and comprises a blind hole in a second bone member. A soft tissue graft is placed on an implant. The implant is secured within the blind hole, such that a plurality of ends of the soft tissue graft extend from the implant and substantially entirely through the tunnel in the first bone member. A second implant is then slid along the soft tissue graft ends into the tunnel in the first bone member, to a predetermined location. The second implant is then expanded outwardly to compress the soft tissue graft ends against the bony wall of the bone tunnel.  
         [0029]     The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying illustrative drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a view of the femur and tibia of a patient&#39;s leg;  
         [0031]      FIG. 2  is a view similar to  FIG. 1 , showing the use of a drill bit to make an access tunnel in the femur; and a corresponding blind hole in the tibia;  
         [0032]      FIG. 3  is a view similar to  FIGS. 1 and 2  showing the femur and tibia after the drilling step has been completed;  
         [0033]      FIG. 4  is a view similar to  FIGS. 1-3 , after a femoral anchor has been installed into the femur access hole, illustrating graft tendon bundles extend from the femoral anchor through the tibia access tunnel;  
         [0034]      FIG. 5  is a view showing a soft tissue graft pre-loaded onto a femoral implant for use in a graft procedure performed in accordance with the principles of the present invention;  
         [0035]      FIG. 6  is a view showing the femoral implant being inserted into the femoral socket and deployed;  
         [0036]      FIG. 7  is a view showing the femoral implant inserted being disengaged from the deployed femoral implant;  
         [0037]      FIG. 8  is an isometric view of an embodiment of a tibial implant constructed in accordance with the principles of the present invention;  
         [0038]      FIG. 9  is a perspective view of a screw portion of the tibial implant of  FIG. 8 ;  
         [0039]      FIG. 10  is a cross-sectional view of the screw portion of  FIG. 9 ;  
         [0040]      FIG. 11  is a driver used to deploy the tibial implant of the present invention;  
         [0041]      FIG. 12  is a perspective view of an assembled tibial implant according to the present invention;  
         [0042]      FIG. 13  is a perspective view of a disassembled tibial implant according to the present invention;  
         [0043]      FIG. 14  is a perspective view of an exterior surface of one tibial sheath which forms a part of the tibial implant of the present invention;  
         [0044]      FIG. 15  is a perspective view of an interior surface of the sheath of  FIG. 14 ;  
         [0045]      FIG. 16  is a perspective view of an assembled tibial sheath according to the present invention, shown from a first side thereof;  
         [0046]      FIG. 17  is a perspective view of the sheath of  FIG. 16  shown from a second side thereof;  
         [0047]      FIG. 18  is a top view of the sheath of the present invention;  
         [0048]      FIGS. 19-21  show the undeployed tibial implant in the tibia;  
         [0049]      FIG. 22  shows the undeployed tibial implant;  
         [0050]      FIG. 23  shows the undeployed screw and sheath combination;  
         [0051]      FIG. 24  shows the tibial implant as it is in the process of being deployed;  
         [0052]      FIG. 25  shows the screw rotated to cause the sheaths to start to deploy and rotate on their hinges as shown in  FIG. 24 ;  
         [0053]      FIG. 26  shows the fully deployed tibial implant;  
         [0054]      FIG. 27  shows the fully inserted screw, with the sheaths separated and fully deployed, as shown in  FIG. 26 ;  
         [0055]      FIG. 28  is a view showing the fully deployed implant in the tibia;  
         [0056]      FIG. 29  is a table showing measured pull out forces for an implant of the present invention, compared with the much lower pull out forces measured in a state of the art prior art tibial implant;  
         [0057]      FIG. 30  is a perspective view showing a modified embodiment of a sheath which forms a part of a tibial anchor device constructed in accordance with another embodiment of the present invention;  
         [0058]      FIG. 31  is a perspective view of an anchor portion of the tibial anchor device of  FIG. 30 ;  
         [0059]      FIG. 32  is a perspective view of a screw of the tibial anchor device of  FIGS. 30 and 31 ;  
         [0060]      FIG. 33  is a perspective view of the opposing side of the sheath shown in  FIG. 30 ;  
         [0061]      FIG. 34  is a perspective view of an opposing side of the anchor portion of  FIG. 31 ;  
         [0062]      FIG. 35  is view similar to  FIGS. 1-4 , with portions of the bone removed in order to show the tibial anchor device of  FIGS. 30-34 , wherein the tendon bundles have been pulled through the sheaths of the anchor device;  
         [0063]      FIG. 36  is a view similar to  FIG. 35  wherein the sheaths and anchor have been slid up along the tendons into the hole until the anchor bottoms out against an angular surface within the hole;  
         [0064]      FIG. 37  is a view similar to  FIG. 36 , wherein tension has been applied to the tendons and the screw of  FIG. 32  has been inserted into the anchor;  
         [0065]      FIG. 38  is a view similar to  FIG. 37 , wherein the screw has been tightened until it bottoms out against the anchor;  
         [0066]      FIG. 39  is a view similar to  FIG. 38  wherein the tendon bundles have been trimmed flush with the face of the anchor;  
         [0067]      FIG. 40  is a view similar to  FIG. 39 , except that the removed portions of the bone have been restored in order to shown the patient&#39;s knee after the inventive repair procedure has been completed;  
         [0068]      FIG. 41  is a perspective view showing a right anchor portion of another embodiment of a tibial anchor device constructed in accordance with the principles of the present invention;  
         [0069]      FIG. 42  is a perspective view similar to  FIG. 41  showing a left anchor portion of the embodiment of the tibial anchor device of  FIG. 41 ;  
         [0070]      FIG. 43  is a view of a screw retention cup of the tibial anchor device of  FIG. 41 ;  
         [0071]      FIG. 44  is a perspective view of a screw for use with the tibial anchor device of  FIG. 41 ;  
         [0072]      FIG. 45  is a perspective view similar to  FIG. 41  of the opposing side of the right anchor portion;  
         [0073]      FIG. 46  is a perspective view similar to  FIG. 42  of the opposing side of the left anchor portion;  
         [0074]      FIG. 47  is a view similar to  FIG. 43  of the opposing side of the screw retention cup;  
         [0075]      FIG. 48  is a view of a patient&#39;s femur and tibia, with portions of the bone removed for ready visualization, showing the tibial anchor of  FIGS. 41-47  being installed, by pulling tendon bundles through the left and right anchors;  
         [0076]      FIG. 49  is a view similar to  FIG. 48 , wherein the anchor portions are slid up the tendons into the bone hole until the anchor bottoms out against an angular surface in the hole;  
         [0077]      FIG. 50  is a view similar to  FIG. 49 , wherein the tendons have been tensioned, and the screw and retainer cup tightened;  
         [0078]      FIG. 51  is a view similar to  FIG. 50 , wherein the tendon bundles have been trimmed flush with the face of the anchor;  
         [0079]      FIG. 52  is a view similar to  FIG. 51 , except that the removed portions of the bone have been restored in order to shown the patient&#39;s knee after the inventive repair procedure has been completed;  
         [0080]      FIG. 53  is a perspective view of a tibial anchor device similar to that shown in  FIGS. 41-47 , in an assembled configuration;  
         [0081]      FIG. 54  is another view of the anchor device of  FIG. 53 ;  
         [0082]      FIG. 55  is still another view of the anchor device of  FIGS. 53 and 54 ;  
         [0083]      FIG. 56  is yet another view of the anchor device of  FIGS. 53-55 ;  
         [0084]      FIG. 57  is a view of the anchor device of  FIG. 54-56 , wherein some of the anchor portions have been removed for visibility;  
         [0085]      FIG. 58  is a perspective view of yet another tibial anchor embodiment in accordance with the principles of the present invention;  
         [0086]      FIG. 59  is another view of the embodiment of  FIG. 58 ; and  
         [0087]      FIG. 60  is still another view of the embodiment of  FIGS. 58-59 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0088]     Referring now more particularly to the drawings, procedures and anchoring devices for repairing a patient&#39;s knee, by securing a graft of soft tissue therein, connected between the patient&#39;s femur and tibia, are illustrated. There is shown in  FIG. 1 a  view of a femur  10  and a tibia  12  of a patient&#39;s knee.  FIG. 2  illustrates the same knee structure, wherein a drill bit  14  is utilized to drill a tunnel in the tibia  12 , and a blind hole corresponding to the tunnel in the femur  10 . The tibial tunnel  16  and femur blind hole  18  are shown in  FIG. 3 .  
         [0089]     As shown in  FIG. 5 , a tendon bundle  20  is pre-loaded onto a femoral implant  22 . In a preferred embodiment, the tendon bundle  20  is comprised of a soft tissue graft comprising a portion of a hamstring (such as pre-harvested semitendinosus and gracilis grafts), but any soft tissue may be used. Details of a presently preferred femoral implant are disclosed in co-pending provisional patent application Ser. No. 60/854,178, which has already been expressly incorporated herein by reference. However, the invention may be utilized with any suitable femoral implant.  
         [0090]     In  FIG. 6 , a femoral implant inserter  24  is utilized to insert the femoral implant into the femoral socket, wherein it is deployed. Following this, as shown in  FIG. 7 , the femoral implant inserter  24  is disengaged from the deployed implant  22 , and withdrawn.  
         [0091]     In  FIG. 4 , the femoral anchor (not shown) has already been inserted into the femur blind hole  18  for securing the tendon bundle  20  therein, as shown. As is illustrated, the tendon bundles  20  extend from the femoral anchor in the femur hole  18  down through the tibial tunnel  16 .  
         [0092]     Referring now to  FIGS. 8-10  and  12 - 18 , there is shown a first, and presently preferred, embodiment of a tibial implant  26  constructed in accordance with the principles of the present invention. As shown in  FIG. 8 , the implant  26  comprises a tapered screw  28  and two sheath portions, or halves  30 . The two sheath halves  30  are preferably mirror images of one another.  
         [0093]     The tapered screw  28 , shown particularly in  FIGS. 9 and 10 , has several key features. The tapered design, tapering from a relative wide proximal end  32  to a relatively narrow distal end  34 , distributes the pressure between the tendon and the sheath halves  30  throughout the length of the screw  28 , increasing the pull out force of the system. The screw has an easy start feature  36 , which comprises a cut-out that allows the tip of the screw to fit between the sheath halves  30 . With the tip between the sheaths, a thread start  38  ( FIG. 10 ) easily engages thread  40  on the screw with an internal thread  41  of the sheath ( FIG. 15 ) as the screw is rotated clockwise. This minimizes the force required to start the screw by reducing the distance the sheath halves  30  must be spread apart in order to start the screw. This feature also prevents the user from needing to dilate the hole between the tendon bundles. A tapered hex  42  ( FIG. 10 ) engages with a driver  44  ( FIG. 11 ) in order to transmit the torque required in order to deploy the screw. A bullnose screw head  46  at the proximal end  32  of the screw  28  leaves a smooth completed repair.  
         [0094]     The sheath halves  30  have many key features as well. It is first noted that having two sheath halves  30  allows for the use of either a double or a single tendon bundle loop  20 . There is no need to separate four separate ends of a double tendon bundle loop into four separate quadrants. With a double bundle loop, the implant has two free ends on either side of the sheath assembly. With a single bundle loop, one free end is in place on either side of the sheath assembly. The internal thread  41  ( FIG. 15 ) on each sheath half  30  prevents the screw from backing out of the sheath assembly during and after deployment. The interlocking threads  40 ,  41  between the screw and the sheaths allow the screw to be pulled between the sheath halves  30 , thus providing easier deployment. Retaining ribs  48  provide small areas of higher force between the implant and the tendon, thereby increasing the pull out force of the system.  
         [0095]     A cortical hook  50  functions to grab the hard cortical bone of the tibia, which assists in keeping the implant in place during loading and also increases the pull out force of the system. Each sheath half  30  comprises a hinge  52  and a hinge slot  54 . The hinge  52  on one sheath half  30  is placed in the hinge slot  54  of the opposing sheath half  30 . This feature permits the sheath to consistently open up in one direction, as shown in  FIGS. 22, 24 , and  26 , thus providing a repeatable deployment mode. One sheath half  30  has two snap posts  56 , and the opposing sheath half  30  has a snap hole  58 . These features keep the sheath halves  30  from opening prematurely. A screw ramp  60  ( FIG. 18 ) allows for the tip of the sheath to provide compression between the tendon and the bone at the aperture of the tibial tunnel. A bullnose sheath tip  62  provides for a smooth transition between the implant system and the exit of the tibial tunnel. This reduces any stress concentrations that could sever the tendon bundle  20 .  
         [0096]     Another feature that reduces stress concentrations at the tip of the sheath halves  30  are flex grooves  64 . These grooves  64  allow the sheath halves  30  to flex and form around the tip of the screw  28 .  
         [0097]     Now with reference to  FIGS. 19-28 , the deployment of the implant  26  will be described. The sheath halves  30  of the tibial implant  26  are disposed between the tendon bundles  20  in the tibial tunnel  16 , which extend proximally through the tibial tunnel  16  from the femoral implant. The sheath halves  30  are advanced distally through the tunnel  16  until the cortical hook  50  is flush with the cortical surface of the tibia. The hook is aligned to the top of the tibial tunnel. The graft is then tensioned by pulling the tendons  20  taut, using manual traction, tensioning pulleys, or other suitable means. Again, it is noted that the primary objective with respect to the tibial anchoring solution is to ensure that good aperture fixation is achieved, and to ensure that cancellous bone fixation is not entirely relied upon. Some type of cortical fixation or backup is required to ensure a good and permanent result.  
         [0098]     The screw  28  is then placed on a distal end  66  of the hex driver  44  until it is fully seated. Next, the screw  28  is placed with the flat of the easy start feature  36  parallel with the midplane of the sheath halves  30 . With a force applied in a direction axial to the tibial tunnel, the screw is pushed distally between the sheaths. The implant  26 , in its undeployed state, is shown in  FIGS. 22 and 23 .  
         [0099]     While the axial force is being applied, and the easy start feature  36  is placed between the sheaths, the screw is rotated in a clockwise direction. This further separates the sheath halves  30  and presses the tendons  20  to the wall of the tibial tunnel  16 . The hinges  52 ,  54  along the same edge as the cortical hook are used to encourage the sheath halves to open in one direction, as shown in  FIGS. 24 and 25 . The screw  28  is rotated until it is fully seated when the bullnose screw head  46  is flush with the cortical surface of the tibia.  
         [0100]      FIGS. 26 and 27  show the screw in a fully inserted state, with the sheath halves  30  separated and fully deployed. In this state, the sheath halves  30  push the tendons  20  outwardly, into contact with the tibial tunnel walls. The fully deployed implant in the tibia is shown in  FIG. 28 .  
         [0101]     As shown in  FIG. 29 , verification testing of the embodiment shown in  FIG. 28  was completed by the inventors, relative to a prior art device which is presently considered to be state of the art. As can be seen from the table, the pull out forces for the inventive implant were significantly higher than those for the prior art device. The average pull out force for the inventive device for bovine bone was 1165.2 N, as opposed to 532.7 N for the prior art device.  
         [0102]     Now with reference to  FIGS. 30-34 , various components of another embodiment of a tibial sheath anchor are illustrated. In  FIGS. 30 and 33 , there is shown a sheath half  30  from two opposing sides thereof. An anchor  68  is shown in  FIGS. 31 and 34 , and comprises a pair of legs  70  and a disk  72 . A screw  28  is provided for actuating the anchor from an undeployed to a deployed configuration for securing the anchor and associated tendon bundle in place with respect to adjacent bone.  
         [0103]     Referring now to  FIG. 35 , the patient&#39;s femur  10  and tibia  12  are shown wherein a sheath anchor  74  is assembled and disposed for insertion into the tibial tunnel  16 . In accordance with the inventive procedure, the tendon bundles  20  are pulled through the sheath halves  30 , as shown, with portions of each sheath half serving to retain the tendon bundles in place adjacent to the and along the sheaths. In particular, in the illustrated embodiment, tendon loops  76  on each sheath half  30  are formed so that the tendon bundles slide lengthwise along the sheath half  30  beneath the loops  76  so that the loops perform a retention function. The anchor  74  and its legs  70  are placed between the sheath halves  30  so that square tabs  78  on the anchor legs  70  ( FIG. 31 ) are aligned with receptacle notches  36  on the rear of the sheath half  30  ( FIG. 33 ). The screw taper on the rear of the sheath half  30  is oriented toward the joint  82 , between the femur and the tibia.  
         [0104]     In  FIG. 36 , the sheath halves  30  and anchor  68  have been slid up along the tendon bundles  20  until the anchor bottoms out against an angular surface in the hole  16 . Then, as shown in  FIG. 37 , tension is applied to the tendon bundles  20 , and the screw  28  is inserted and tightened within the anchor body, using a suitable tool, such as a hex driver. As shown in  FIG. 38 , the screw  28  should be tightened until it bottoms out against the anchor  68 , approximately flush with or slightly recessed relative to the entrance to the tibial tunnel  16 . This is important in order to ensure that there are no protrusions from the tunnels  16  which could cause discomfort to the patient or possible later complications and wear. An important advantage of the present invention is that the distal end  84  of the sheath anchor  30 , as shown, for example, in  FIG. 38 , is disposed, once the anchor is fully inserted and deployed, so that it is in close proximity to the distal end (aperture)  86  ( FIG. 49 ) of the tibial tunnel  16 , at the joint  82 . This provides excellent aperture fixation for the tendon bundles  20 , in order to minimize wear on the tendon bundles over time due to the “windshield wiper” or “bungee” effects noted above in the Background of the Invention portion of the specification.  
         [0105]     Deployment of the anchor  68  occurs when the screw  28  is inserted into the anchor body. This insertion action causes the anchor legs  70  to splay laterally outwardly, thus forcing the sheath halves  30  and tendon bundles  20  against the bony wall forming the tibial tunnel  16 . As a result of this action, the tendon bundles  20  are clamped against the tibial bone  12  by the sheath halves  30 .  
         [0106]      FIGS. 39 and 40  illustrate the patient&#39;s knee joint once the inventive procedure has been completed.  FIG. 39  shows the joint with portions of the bone being removed or transparent so that the entire sheath anchor  30  is visible, while  FIG. 40  shows the same joint as it would appear naturally with all bone in place. The final step of the procedure is to trim the protruding ends  88  ( FIG. 38 ) of the tendon bundles  20  so that they are flush with the face of the sheath anchor  30 .  
         [0107]      FIGS. 41-47  illustrate components of a second inventive tibial anchor embodiment, which may be identified as a “cone anchor”.  FIGS. 41 and 45  illustrate opposing sides of a right anchor portion  90  and  FIGS. 42 and 46  illustrate opposing sides of a left anchor portion  92 . Opposing sides of a generally conical screw retention cup  94  are shown in  FIGS. 43 and 47 . A screw  28  is shown in  FIG. 44 . It is noted that, in this embodiment, all like elements to those shown in previous embodiments will bear identical reference numerals.  
         [0108]     The procedure for utilizing a cone anchor  96  of  FIGS. 41-47  ( FIG. 48 ) to repair a patient&#39;s joint  82  is initiated in the same manner as for the sheath anchor  30 . Thus, as shown in  FIGS. 1-4 , a femur hole  18  and tibial tunnel  16  are drilled, and a femoral anchor is inserted and deployed to anchor tendon bundles  20  in place within the femoral hole  18 , so that the tendon bundles  20  extend downwardly through the tibial tunnel  16 , as shown in  FIG. 4 . The reader is referred to the description above for further detail regarding this part of the procedure.  
         [0109]     Now, as shown in  FIG. 48 , the tendon bundles  20  are pulled through the cone anchor  96  in order to insert the tibial anchor into the tibial tunnel  16 . In this embodiment, the tendon bundles are secured against the anchor portions  90  or  92  because they are pulled through tendon loops  76 , which are formed in the proximal end of each anchor half  90 ,  92 , respectively. Then, as shown in  FIG. 49 , the anchor  96  is slid upwardly along the tendon bundles  20  until the anchor  96  bottoms out against the angular surface in the tibial hole  16 , as with the first embodiment. Again, as in the first embodiment, this positioning will cause the distal end  98  of the anchor  96  to be located in close proximity to the distal end  86  of the tibial tunnel  16  so that good aperture fixation will result. Then, as illustrated in  FIG. 50 , the tendons are appropriately tensioned and the screw  28  is inserted and tightened, together with the retainer cup  94 , until seated. This action of inserting and tightening the screw  28  and screw retainer cup  94  will cause the anchor portions  90 ,  92  to move laterally outwardly in order to engage the tendon bundles  20  between the anchor portions  90 ,  92  and adjacent tibial bone, as in the sheath anchor embodiment.  FIGS. 51 and 52  illustrate the anchor  96  in its fully installed condition, after the tendon ends  88  are trimmed flush and the procedure is otherwise completed.  
         [0110]      FIGS. 53-57  illustrate, in somewhat greater detail and in an assembled configuration, a cone anchor  96  of a type very similar to that illustrated in the embodiment of  FIGS. 41-52 . Like elements are denoted by like reference numbers.  
         [0111]     A modified tibial anchor embodiment  100  is illustrated in  FIGS. 58-60 . This embodiment is similar to prior disclosed embodiments to the extent that there are provided two opposing sheaths having tendon loops  76  disposed thereon. A screw  28  and associated screw retention disk or cup  94  are also provided. Thus, the basic procedural steps for utilizing this anchor  100  are similar to those already described in connection with the previous disclosed embodiments. What is different about this embodiment, in particular, is the provision of a distal wedge  101  which functions to provide positive aperture fixation by ensuring that the anchor will be stopped within the tibial tunnel at an appropriate point during the insertion step. Pivotable arms  102  connect the anchor body to the wedge  101 , wherein the arms  102  are pivotable outwardly about hinges  104 . Thus, when it is desired to lock the tibial anchor  100  in place within the tibial tunnel, insertion and tightening of the screw  28  within the anchor body actuates the arms  102  to pivot outwardly laterally about the hinges  104 , thereby functioning to expand the wedge and cause positive engagement of the wedge and arms  102  with the tendon bundles and adjacent tibial bone. As in prior embodiments, positive fixation is enhanced by the provision of spikes  106  or other suitable means for penetrating the tendon bundles and the bone to lock the tendon bundles and anchor in place.  
         [0112]     Accordingly, although an exemplary embodiment of the invention has been shown and described, it is to be understood that all the terms used herein are descriptive rather than limiting, and that many changes, modifications, and substitutions may be made by one having ordinary skill in the art without departing from the spirit and scope of the invention.