Patent Publication Number: US-11039916-B2

Title: Flexible tibial sheath

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/334,865 filed Oct. 26, 2016 and entitled “Flexible Tibial Sheath,” which is a continuation of U.S. patent application Ser. No. 14/593,080 filed Jan. 9, 2015 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No. 9,486,307, which is a continuation of U.S. patent application Ser. No. 13/788,872 filed Mar. 7, 2013 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No. 8,956,410, which is a continuation of U.S. patent application Ser. No. 12/025,927 filed on Feb. 5, 2008 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No. 8,435,293, which is a divisional of U.S. patent application Ser. No. 11/935,653 filed on Nov. 6, 2007 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No. 7,699,893, which is a continuation of U.S. patent application Ser. No. 10/608,899 filed on Jun. 27, 2003 and entitled “Flexible Tibial Sheath,” now U.S. Pat. No. 7,309,355, which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to ligament fixation devices and methods, and more particularly to devices and methods for anchoring ligaments within a bone tunnel. 
     BACKGROUND OF THE INVENTION 
     Ligaments are tough bands of tissue which serve to connect the articular extremities of bones, or to support or retain organs in place within the body. Ligaments are typically composed of coarse bundles of dense white fibrous tissue which are disposed in a parallel or closely interlaced manner, with the fibrous tissue being pliant and flexible, but not significantly extensible. 
     In many cases, ligaments are torn or ruptured as a result of accidents or overexertion. Accordingly, various procedures have been developed to repair or replace such damaged ligaments. For example, in the human knee, the anterior and posterior cruciate ligaments (i.e., the ACL and PCL) extend between the top end of the tibia and the bottom 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. Often, the ACL is ruptured or torn as a result of, for example, a sports-related injury. Consequently, various surgical procedures have been developed for reconstructing the ACL so as to restore normal function to the knee. 
     In many instances, the ACL may be reconstructed by replacing the ruptured ACL with a graft ligament. More particularly, with such procedures, 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. This graft ligament then cooperates with the surrounding anatomical structures so as to restore normal function to the knee. 
     Despite the above-identified advances in the art, there remains a need for a graft ligament anchor which is simple, easy to install, and inexpensive to manufacture, while providing secure, trouble-free anchoring of the graft ligament. 
     SUMMARY OF THE INVENTION 
     The present invention generally provides a graft fixation device for fixing a graft member within a bone tunnel. In one embodiment, the device includes a bioabsorbable, radially expandable sheath having a substantially closed distal end with at least two sidewalls extending proximally therefrom and defining a central lumen. Each sidewall is at least partially separated by a longitudinally oriented slot extending from a proximal end along a substantial length of each sidewall and terminating at a position proximal to the distal end. The shape of the sidewalls can vary, but preferably each sidewall has a substantially concave outer surface adapted to seat a graft member. Each sidewall can optionally include surface features formed within the concave outer surface thereof. The device can also include a bioabsorbable sheath expander, e.g., a tapered screw, adapted to be disposed in the central lumen of the radially expandable sheath and configured to flex the sidewalls to radially expand the sheath so as to fix a graft member within a bone tunnel. In an exemplary embodiment, the sheath expander has a largest diameter that is greater than a largest inner diameter of the radially expandable sheath in an unexpanded state. 
     The radially expandable sheath can have a variety of configurations. In one embodiment, the distal portion of the radially expandable sheath, extending between a distal end of the longitudinally oriented slots and a distal end of the sheath, tapers to form a bullet-shaped distal tip. In another embodiment, at least two adjacent sidewalls are joined at a proximal end thereof by a stop member adapted to prevent over-insertion of the radially expandable sheath into a bone tunnel. 
     The configuration of the graft fixation device allows the device to be formed from a variety of materials, including materials having a low elasticity. In an exemplary embodiment, the graft fixation device is formed from one or more polymers or copolymers formed from monomers selected from the group consisting of lactic acid, glycolic acid, and caprolactone. In a more preferred embodiment, the material further includes tricalcium phosphate. 
     In yet another embodiment, a graft fixation device for fixing a graft member within a bone tunnel is provided. The device includes a bioabsorbable, radially expandable sheath having a substantially closed distal end with at least two sidewalls extending proximally therefrom and defining a central lumen. Each sidewall has a substantially concave outer surface adapted to seat a graft member, and each side wall is at least partially separated by a longitudinally oriented slot extending from a proximal end along a substantial length of each sidewall and terminating at a position proximal to the distal end. 
     In other aspects, a graft fixation kit for fixing a graft member within a bone tunnel is provided. The kit includes a bioabsorbable expandable sheath having proximal and distal ends with at least two sidewalls extending therebetween and defining a central lumen. Each sidewall is at least partially separated by a longitudinally oriented slot extending from the proximal end and terminating at a position just proximal to the distal end, and each sidewall has an outer surface adapted to seat a graft member. The kit further includes a plurality of sheath expanders of varying sizes, each being disposable in the central lumen of the expandable sheath and configured to flex the sidewalls to radially expand the sheath so as to fix at least one graft member within a bone tunnel. 
     Methods for fixing a ligament graft in a bone tunnel are also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
         FIG. 1  is a perspective view of a radially expandable sheath and a sheath expander in accordance with one embodiment of the invention; 
         FIG. 2  is a perspective view of the radially expandable sheath shown in  FIG. 1 ; 
         FIG. 3A  is a side view of the radially expandable sheath shown in  FIGS. 1 and 2 ; 
         FIG. 3B  is a cross-sectional view of the radially expandable sheath shown in  FIG. 3A  taken across line  3 B- 3 B; 
         FIG. 4A  is an illustration of a bone tunnel having four segments of a ligament graft and a radially expandable sheath disposed therein in an unexpanded position; and 
         FIG. 4B  is an illustration of the bone tunnel, ligament segments, and radially expandable sheath shown in  FIG. 4A  with the radially expandable sheath in the expanded position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , the present invention generally provides a radially expandable sheath  10  for attaching a ligament graft to bone. In general, the expandable sheath has a substantially closed distal end with at least two sidewalls ( FIG. 1  illustrates four sidewalls  14 ,  16 ,  18 ,  20 ) extending proximally therefrom and defining an inner lumen  34 . Each sidewall  14 ,  16 ,  18 ,  20  can have a substantially concave outer surface adapted to seat a graft member, and each sidewall  14 ,  16 ,  18 ,  20  is at least partially separated by a longitudinally oriented slot  22 ,  24 ,  26 ,  28  that extends from a proximal end  10   a  along a substantial length of each sidewall  14 ,  16 ,  18 ,  20 . The slots  22 ,  24 ,  26 ,  28  preferably terminate at a position just proximal to the distal end  10   b  of the sheath  10 . The device can also optionally include a sheath expander  12  that is adapted to be disposed in the central lumen  34  of the radially expandable sheath  10  and that is configured to flex the sidewalls  14 ,  16 ,  18 ,  20  to radially expand the sheath  10  so as to fix a graft member within a bone tunnel. 
     A person skilled in the art will appreciate that a variety of implants having virtually any configuration can be used to expand the expandable sheath, and that sheath expander  12  is merely one embodiment of an implant that can be used with the expandable sheath. Moreover, the expandable sheath  10  can be used to attach a variety of materials to bone in a variety of medical procedures. Accordingly, the terms “graft” or “ligament graft,” as used herein, are intended to include any number of natural and/or synthetic graft and/or tendon materials. 
     The expandable sheath  10  can have a variety of configurations, shapes, and sizes, but it should be adapted to expand within a bone tunnel to attach a ligament graft to bone.  FIGS. 1-3B  illustrate an exemplary embodiment of an expandable sheath  10  having proximal and distal ends  10   a,    10   b  with four sidewalls  14 ,  16 ,  18 ,  20  extending therebetween. While four sidewalls  14 ,  16 ,  18 ,  20  are shown, a person skilled in the art will appreciate that the sheath  10  can have any number of sidewalls. The sheath  10  should, however, have at least two sidewalls to allow the sheath  10  to expand upon insertion of a sheath expander therein. Each sidewall  14 ,  16 ,  18 ,  20  preferably tapers from a proximal end  10   a  of the sheath to a distal end  10   b  of the sheath to form a sheath  10  having a bullet-shaped profile. As a result, the proximal end  10   a  of the sheath  10  defines the largest outer diameter or circumference Ds 1  of the sheath, and the distal end  10   b  defines the smallest outer diameter or circumference Ds 2  of the sheath, as shown in  FIG. 3A . The sheath  10  also includes an inner diameter x which is the largest at its proximal end  10   a  and decreases towards its distal end  10   b.    
     Each sidewall  14 ,  16 ,  18 ,  20  of the sheath  10  is preferably separated from an adjacent sidewall by a longitudinally oriented slot  22 ,  24 ,  26 ,  28  extending therebetween. Each slot  22 ,  24 ,  26 ,  28  can have the same length ls, or alternatively the length of each slot  22 ,  24 ,  26 ,  28  can vary with respect to each other. In an exemplary embodiment, each slot  22 ,  24 ,  26 ,  28  has the same length is and originates at the proximal end  10   a  of the sheath  10  and extends along a substantial length Ls of the sheath  10  to allow the sidewalls  14 ,  16 ,  18 ,  20  to flex with respect to each other. Each slot  22 ,  24 ,  26 ,  28  preferably terminates at the same position P just proximal to the distal end  10   b  of the sheath to provide a slot-free distal tip  30 . This termination point P defines the area at which each sidewall  14 ,  16 ,  18 ,  20  will bend during expansion of the sheath  10  by a sheath expander. Thus, while the termination point P can vary, the distance between the termination point P at the end of each slot  22 ,  24 ,  26 ,  28  and the distal end  10   b  of the sheath should be sufficient to provide structural integrity to the device such that the sidewalls  14 ,  16 ,  18 ,  20  do not break apart from one another or from the distal tip  30  during expansion. 
     Each sidewall  14 ,  16 ,  18 ,  20  of the sheath  10  can also have a variety of shapes and sizes. In an exemplary embodiment, each sidewall  14 ,  16 ,  18 ,  20  has a substantially concave outer surface that is adapted to seat a graft. The concave surface preferably extends along the length is of each sidewall  14 ,  16 ,  18 ,  20 . The proximal-most portion of each sidewall  14 ,  16 ,  18 ,  20 , however, can include a flared region  32  ( FIG. 2 ) to provide an enlarged opening to the inner lumen  34  to facilitate insertion of a sheath expander therein. Each sidewall  14 ,  16 ,  18 ,  20  can also include one or more surface features  36  formed on the concave surface to facilitate engagement of a graft between the sidewalls  14 ,  16 ,  18 ,  20  and the bone tunnel when the sheath  10  is implanted. The surface features  36  can have a variety of configurations, and can be formed on all or a portion of one or more of the sidewalls  14 ,  16 ,  18 ,  20 . As shown in  FIGS. 1-3A , the surface features  36  are formed from a series of transversely-oriented ridges  36  formed along a substantially portion of each sidewall  14 ,  16 ,  18 ,  20 . The ridges  36  are effective to engage or grip the graft to prevent sliding movement of the graft with respect to the sheath  10 . A person skilled in the art will appreciate that the sheath  10  can include a variety of different features to facilitate engagement between the sheath  10  and the graft. 
     Each sidewall  14 ,  16 ,  18 ,  20  can also include one or more longitudinal flexion regions to allow each sidewall  14 ,  16 ,  18 ,  20  to expand radially outward upon insertion of a sheath expander into the lumen  34  of the sheath  10 .  FIG. 2  illustrates flexion regions  38   a,    38   b  on sidewall  16 . As shown, the flexion regions  38   a,    38   b  are formed from substantially parallel edges that extend along the length Ls of the sheath  10  and that define the concave shape of each sidewall  14 ,  16 ,  18 ,  20 . The flexion regions can optionally be formed by thinning the material that forms each sidewall  14 ,  16 ,  18 ,  20  longitudinally in the region of desired flexion, and in one embodiment, may be one or more longitudinal grooves cut into each sidewall  14 ,  16 ,  18 ,  20 . In use, the expansion of the sidewalls  14 ,  16 ,  18 ,  20  at the flexion regions  38   a,    38   b  will retain the graft material disposed within the sidewall  14 ,  16 ,  18 ,  20  by an interference fit between the expanded sidewall  14 ,  16 ,  18 ,  20  and the bone tunnel wall. 
     While each sidewall  14 ,  16 ,  18 ,  20  is described as being separated by a longitudinally oriented slot, two or more sidewalls adjacent to one another can optionally include a stop member extending therebetween and adapted to prevent over-insertion of the sheath  10  into a bone tunnel. While the stop member can have a variety of configurations,  FIGS. 1-3B  illustrate an exemplary embodiment of a stop member  40  formed on the proximal-most end  10   a  of the sheath  10  and extending between two adjacent sidewalls, e.g., sidewalls  14  and  20 , to connect the sidewalls  14 ,  20 . The stop member  40  can have a variety of configurations, and in one embodiment, as shown, is a tab-like protrusion that extends outward from the circumference of the proximal end  10   a  of the sheath  10 . As a result, the stop member  40  will abut the edge of a bone tunnel during insertion of the sheath  10  into the bone tunnel, thereby preventing over-insertion of the sheath  10 . 
     While the stop member  40  connects two adjacent sidewalls, the stop member  40  can optionally be adapted to break upon insertion of a sheath expander into the sheath  10  to allow the sidewalls  14 ,  16 ,  18 ,  20  to expand. To ensure that breakage occurs at the proper location, the stop member  40  can include a weakened portion (not shown) formed at the desired breakage point. A person skilled in the art will appreciate that a variety of techniques can be used to achieve the desired breakage. 
     The distal tip  30  of the sheath can also have a variety of configurations, shapes and sizes. Since the distal tip  30  connects the four sidewalls  14 ,  16 ,  18 ,  20  to one another to provide structural integrity to the sheath  10 , the distal tip  30  is preferably slot-free, and also preferably does not include any surface features  36  formed thereon. While the shape of the distal tip  30  can vary, the distal tip  30  preferably originates adjacent to the termination point P of each longitudinal slot  22 ,  24 ,  26 ,  8 , and tapers toward the distal end  10   b  of the sheath  10 . The distal tip  30  can optionally include a flattened distal-most surface  42  ( FIG. 3A ) that joins a distal-most end of each sidewall  14 ,  16 ,  18 ,  20  to one another. The edges (not shown) that connect the flattened surface  42  to each sidewall  14 ,  16 ,  18 ,  20  are preferably rounded to form a substantially rounded distal tip  30 . The distal tip  30  can also optionally include a bore  44  ( FIG. 3B ) extending through the flattened surface  42  for receiving a guide wire to facilitate implantation of the device. A person skilled in the art will appreciate that the distal tip  30  of the sheath  10  can have virtually any shape and size, and can optionally be substantially open or closed. 
     Referring back to  FIG. 1 , a sheath expander  12  can be used to expand the expandable sheath  10  once the sheath  10  is inserted into a bone tunnel. While the sheath expander  12  can have virtually any configuration,  FIG. 1  illustrates an exemplary embodiment of a sheath expander  12  in the form of a tapered screw. The screw  12  includes a proximal end  12   a  defining the largest diameter ds 1  of the screw, and a distal end  12   b  defining the smallest diameter ds 2  of the screw  12 . Threads  50  are formed around the screw  12  and extend from the proximal end  12   a  to the distal end  12   b.  In use, the screw  12  is adapted to expand the expandable sheath  10 , thus the largest diameter ds 2  of the screw  12  is preferably larger than the largest inner diameter x ( FIG. 1 ) of the sheath  10 , and more preferably it is at least as large as the largest outer diameter Ds 1  of the sheath  10 . The expander screw  12  also preferably includes a socket  52  formed in the proximal end  12   a  thereof for receiving a driver tool, such as a hex wrench, that is effective to drive the screw  12  into the sheath  10 . The expander screw  12  can also include a lumen (not shown) extending therethrough for receiving a guide wire to facilitate insertion of the screw  12  into the sheath  10 . As previously stated, a person skilled in the art will appreciate the sheath expander  12  can have a variety of configurations, shapes, and sizes. 
     The expandable sheath  10  and sheath expander  12  can be used in a variety of medical procedures, but they are preferably used to anchor ligaments within a bone tunnel. In an exemplary embodiment, the device or system  10 ,  12  is used for tibial fixation of an anterior cruciate ligament graft. A bone tunnel is prepared in the patient&#39;s tibia, and the graft is separated into four tendon bundles, each of which is prepared by whip stitching a length of suture thereto. The graft is then passed through the tunnel and tensioned as necessary. While tensioning the graft, the expandable sheath  10  is inserted into the opening of the bone tunnel, preferably by sliding the sheath  10  along a guide wire extending through the tunnel. A mallet or other impacting device can be used to gently advance the sheath  10  into the tunnel. The stop member  40  will abut the opening of the tunnel when the sheath  10  is fully inserted. In this position, a graft bundle, which preferably includes four grafts  56 ,  58 ,  60 ,  62 , as shown in  FIG. 4A , is preferably seated within each sidewall  14 ,  16 ,  18 ,  20  of the expandable sheath  10 . The sheath expander, e.g., tapered expander screw  12 , is then slowly inserted into the inner lumen  34  of the sheath  10 , using a driver tool, to expand the concave sidewalls  14 ,  16 ,  18 ,  20  of the sheath  10 . As the sheath expander  12  is driven into the sheath  10 , the concave sidewalls  14 ,  16 ,  18 ,  20  of the sheath  10  will deform toward a circular geometry to conform with an outer diameter of the expander  12 , and thus to compress each tendon bundle  56 ,  58 ,  60 ,  62  against the bone tunnel wall  70 , as shown in  FIG. 4B . This will encourage rapid bony ingrowth into each bundle. 
     The sheath  10  of the present invention provides several advantages over prior art sheaths. In particular, the longitudinally oriented slots  22 ,  24 ,  26 ,  28  reduce or eliminate the risk of cracking or splitting during expansion. While some prior art sheaths provide a weakened portion adapted to rupture or break upon insertion, the sheath can crack at unintended portions. The slots  22 ,  24 ,  26 ,  28  of the sheath  10  of the present invention eliminate the need for the sheath  10  to crack during insertion of the sheath expander  12  therein since the longitudinal slots  22 ,  24 ,  26 ,  28  allow for expansion without cracking. The slots  22 ,  24 ,  26 ,  28  also allow the sheath  10  to be formed from a material having a relatively low elasticity, e.g., a brittle material. Since the sheath  10  does not need to be designed to break only at an intended location, the entire sheath  10  can be made from a brittle material. By way of non-limiting example, suitable materials include biocompatible, bioabsorbable polymers or copolymers formed of monomers selected from the group consisting of lactic acid, glycolic acid, and caprolactone. In a further embodiment, the material can also include tricalcium phosphate. The sheath  10  can also be formed from bioabsorbable glasses and ceramics (possibly containing calcium phosphates and other biocompatible metal oxides (i.e., CaO)). The sheath  10  can also be formed from metals, or it can comprise combinations of metals, bioabsorbable ceramics, glasses or polymers. In an exemplary embodiment, the sheath is formed from polylactic acid (PLA), tricalcium phosphate (TCP), and optionally polyglycolic acid (PGA), and/or polycaprolactone (PCL). More preferably, the sheath is formed from one of the following materials (all percentages are by volume unless otherwise indicated): 
     (1) 70% PLA+30% TCP 
     (2) 70% of a PGA/PLA mixture+30% TCP 
     (3) 70% of a PLA/PCL mixture+30% TCP 
     (4) 70% of a PGA/PCL/PLA mixture+30% TCP 
     Further, the longitudinal slots  22 ,  24 ,  26 ,  28  facilitate the use of expanders  12  having a wider variety of sizes, including the use of expanders  12  having an outer diameter ds 1  or circumference at least as large as the diameter Ds 1  or circumference of sheath  10 . In this way, a single sheath size may be stocked for a wide variety of procedures and intended bone tunnel sizes. In one embodiment, sheath  10  may be provided in a kit to surgeons in which a plurality of expanders having different sizes are provided for use with a single size sheath. 
     It is to be understood that the present invention is by no means limited to the particular constructions and methods herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.