TECHNICAL FIELD 
     This invention relates to closed loop suture for anchoring tissue grafts. 
     BACKGROUND 
     An increasing number of surgical techniques are now performed arthroscopically. One type of arthroscopic procedure reconstructs the anterior cruciate ligament (ACL) in the knee. Several ACL reconstruction techniques are described in U.S. Pat. Nos. 5,139,520, 5,306,301, and 5,769,894, incorporated herein by reference in their entirety. 
     When the ACL has ruptured and is nonrepairable, it can be replaced using a substitute graft harvested from the patient or from a donor. The substitute ACL graft may be a portion of a patellar tendon or a semitendinosus and/or gracilis tendon graft. Alternatively, an artificial graft formed from synthetic materials or from a combination of artificial and natural materials may be used and is sometimes referred to as a ligament augmentation device (LAD). The term “tissue graft” is used herein to encompass all of these tissue replacements. 
     In general, the replacement tissue graft is placed within a passage formed within the femur and a passage formed in the tibia, and secured to the femur and tibia. 
     SUMMARY 
     In one aspect, the invention features a suture loop comprising a braided length of filaments and a non-braided length of the filaments, where the braided length of the filaments and the non-braided length of the filaments form the loop. 
     One or more of the following features may also be included. The filaments are in the form of yarns, each yarn including multiple filaments. The braided length of the filaments comprises braided yarn. The non-braided length of the filaments comprises yarn twisted together. The yarns comprise monofilaments twisted together. The yarns comprise air-entangled monofilaments. Each yarn has a minimum denier of 198 and comprises monofilaments. The braided length of the filaments comprises a first section of braided filaments and a second section of braided filaments, and the non-braided length of the filaments is located between the first and second sections, wherein the first and second sections connect to form the suture loop. The second section defines a hollow conduit and a portion of the first section is inside the conduit to form the suture loop. The suture loop comprises at least one stitch securing the connection between the first and second sections of the braided filaments. 
     In another aspect, the invention features a method of manufacturing a suture loop. The method comprises connecting a first braided length of suture and a second braided length of suture to form the suture loop, the suture loop including a non-braided length of suture. 
     One or more of the following features may also be included. The method comprises threading the first braided length of suture through a button of a fixation device before connecting the first braided length of suture and the second braided length of suture to form the suture loop. Connecting the first braided length of suture and the second braided length of suture to form the suture loop comprises inserting the first braided length of suture into a hollow interior of the second braided length of suture. The method comprises stitching the connection between the first braided length of suture and the second braided length of suture. The method comprises before connecting the first braided length of suture and the second braided length of suture to form the suture loop, twisting the non-braided length of suture. 
     In another aspect, the invention features a method for securing a tissue graft within a bone passage. The method comprises forming a bone passage in a bone to accommodate the tissue graft. The method comprises coupling the tissue graft to a loop of suture, the loop of suture comprising a braided length of filaments and a non-braided length of the filaments. The tissue graft is positioned within the bone passage, and the tissue graft is secured in the bone passage. 
     One or more of the following features may also be included. The loop of suture is part of a fixation device and securing the tissue graft comprises placing a button of the fixation device across an opening of the bone passage. Positioning the tissue graft comprises pulling a filament attached to the button of the fixation device. The bone passage is formed in a femur. 
     In another aspect, a surgical device for securing a tissue graft within a bone passage comprises an elongated body and a loop of suture housed by the elongated body, wherein the loop of suture comprises a braided portion and a non-braided portion. 
     One or more of the following features may also be included. The elongated body defines a hole that receives a loop of suture. The elongated body defines a first hole and a second hole that receive the loop of suture. The elongated body defines a third hole and a fourth hole. A leading filament and a trailing filament are each received by one of the third and fourth holes. The first hole and the second hole are positioned centrally, the third hole is positioned on one side of the first and second holes and the fourth hole is positioned on an opposite side of the first and second holes. 
     Embodiments have one or more of the following advantages. The closed loop having both braided lengths of filaments and non-braided lengths of filaments provides added strength at the braided portions of the loop, and added compliance and improved wear at the non-braided portions of the loop. Fixing the braided portion of the loop to the graft fixation device strengthens the junction between the loop and the graft fixation device. Similarly, fixing the non-braided portion of the loop to the tissue graft allows for a more reliable junction with the tissue graft. The closed loop having both braided and non-braided lengths provides superior strength over loops that are tied or taped, or that do not feature both braided and non-braided portions. Moreover, the closed loop having both braided and non-braided lengths of filaments allows for added flexibility in the manufacturing of closed loop sutures, as the sutures can be manufactured independently of the graft fixation device and at varying lengths as required by the specific application. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a tissue graft implanted in a knee during an ACL reconstruction procedure and secured at one end by a graft fixation device including a closed loop suture. 
         FIG. 2  shows the tissue graft and the graft fixation device. 
         FIG. 3  shows the graft fixation device including the closed loop suture. 
         FIG. 4  is an isometric view of a button of the graft fixation device. 
         FIG. 5  depicts an example process for forming the closed loop suture. 
         FIG. 6  depicts an example process for forming the closed loop suture on the button of the graft fixation device. 
         FIG. 7  shows the tissue graft being implanted during an ACL reconstruction procedure. 
         FIGS. 8A and 8B  depict a process for seating the button of the graft fixation device against a bone to secure the tissue graft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a tissue graft  20  is shown implanted within a knee  11  during an anterior cruciate ligament (ACL) repair and reconstruction procedure. The tissue graft  20  is sized to fit within a femoral channel  14  formed in a femur  12  and a tibial channel  16  formed in a tibia  13 . The tissue graft  20  is secured to the femur  12  using a graft fixation device  22 , and is secured to the tibia  13  using a fastener  24  (for example, an interference screw, a post, or another fixation device  22 ). The graft fixation device  22  includes a button  100 , for example, an Endobutton available from Smith &amp; Nephew (Catalog Number: 013186), and a closed-loop suture  30 . Referring also to  FIG. 2 , the tissue graft  20  is attached to the graft fixation device  22  by looping the tissue graft through the closed-loop suture  30 . 
     Referring to  FIGS. 2-4 , the button  100  has an elongated body with a top surface  106 , a bottom surface  107 , and ends  110 ,  112 , and has a width, W, a length, L, and a thickness, T. The button  100  defines two central holes  103 ,  104 , a leading end hole  101 , and a trailing end hole  102  that pass through the button thickness, T. The central holes  103 ,  104  of the button  100  house the closed loop suture  30 , which passes through the hole  103  from the bottom surface  107  of the button  100  to the top surface  106  of the button  100 , over the top surface  106  of the button  100 , and through the hole  104  from the top surface  106  of the button  100  to the bottom surface  107  of the button  100 . 
     As described in further detail below, the closed loop suture  30  is a hybrid construct including a braided portion  31  of filaments located within the holes  103 ,  104  and passing over the top surface  106  of button  100 , and a non-braided section  32  of the filaments located under the bottom surface  107  of the button  100 . In some implementations, the braided portion  31  of filaments can extend below the holes  103 ,  104  and bottom surface  107  of the button  100 , as shown at  FIGS. 2 and 3 . The tissue graft  20  is looped through the closed loop suture  30  such that the tissue graft  20  is positioned against the non-braided section  32 . Such a configuration allows the stronger, braided length  31  of the closed loop suture  30  to form the connection to the button  100 , which is desirable for reliability purposes. The more compliant and less abrasive non-braided section  32  of the closed loop suture  30  forms the connection to the tissue graft  20 , which is desirable for pliability and to reduce wear on the tissue graft  20 . As described above, the tissue graft  20  may be a portion of a patellar tendon, semitendinosus and/or gracilis tendon graft, an artificial graft formed from synthetic materials, or a hybrid tissue graft formed from a combination of artificial and natural materials. 
     As illustrated in  FIG. 2 , the leading end hole  101  houses a leading suture  42  for pulling the graft fixation device  22  and the tissue graft  20  through the bone passages. The leading suture  42  can be any suitable suture, for example, Smith &amp; Nephew Size 5 ULTRABRAID™. The trailing end hole  102  houses a trailing suture  44  that can be used to manipulate the button  100 . Similar to the leading suture  42 , the trailing suture  44  can be any suitable suture, for example, Smith &amp; Nephew Size 2 ULTRABRAID™. 
     As shown in  FIG. 5 , the closed loop suture  30  is formed, for example, from a continuous length of suture having two braided end sections  31   a ,  31   b  and a non-braided section  32  therebetween. The braided end sections  31   a ,  31   b  are tubular with hollow interiors. For example, the braided end sections  31   a ,  31   b  may be of a form similar to Smith &amp; Nephew Size 2 ULTRABRAID™ (Catalog Number 90505373). To form the closed loop, the two braided end sections  31   a ,  31   b  are connected. Each of the braided end sections  31   a ,  31   b  has a respective end  33   a ,  33   b . The braided end sections  31   a ,  31   b  are connected, for example, by inserting the end  33   b  of the braided end section  31   b  into the hollow conduit of the braided end section  31   a . To facilitate the insertion of the braided end section  31   b  into the hollow conduit of the braided end section  31   a , the diameter of the braided end section  31   a  can be increased by compressing the section  31   a  along its length. The diameter of the other braided end section  31   b  can be decreased by stretching the braided end section  31   b  along its length. The braided end section  31   b  is then inserted into the hollow conduit of the braided end section  31   a . The braided end sections  31   a ,  31   b  are then allowed to return to their original lengths, such that the diameters of both braided end sections  31   a ,  31   b  return to their original dimensions forming a “finger trap” connection between the two braided end sections  31   a ,  31   b , thereby completing the closed loop suture  30 . 
     In some implementations, the length of the overlap between the braided end sections  31   a ,  31   b  can be adjusted to increase or decrease the strength of the “finger trap” connection. Since the “finger trap” connection relies on friction between the overlapping braided end sections  31   a ,  31   b  to complete the closed loop suture  30 , a longer overlap increases the strength of the connection. Additionally or alternatively, the connection between the braided end sections  31   a ,  31   b  can be formed or reinforced using at least one stitch at a seam  35  or at one or more other locations along the length of the overlap between the braided end sections  31   a ,  31   b . In other examples, the braided end sections  31   a ,  31   b  can be connected to form the closed loop suture  30  by other means, for example, by tying, gluing, stapling, or otherwise fixing the end sections together. 
     Referring to  FIGS. 3 and 6 , a closed loop suture  30  can be formed on a button  100  by threading each of the ends  33   a ,  33   b  through one of the central holes  103 ,  104  of the button  100 . Once the braided end sections  31   a ,  31   b  are threaded through the central holes  103 ,  104  of the button  100 , the braided end sections  31   a ,  31   b  are connected as discussed above with respect to  FIG. 5  to form the closed loop suture  30 . 
     The button  100  is formed, for example, of a biocompatible material (such as titanium or PEEK), or a bioabsorbable material (such as polylactic acid, polyglycolic acid or a combination of absorbable polymers). The body of the graft fixation device  100  is sized such that the graft fixation device  100  can be pulled through the tibial channel  16 , the femoral channel  14 , and a passing channel  18  ( FIG. 1 ). Furthermore, the body of the graft fixation device  100  is sized such that once the graft fixation device  100  has been passed through each of the tibial channel  16 , the femoral channel  14 , and the passing channel  18 , the graft fixation device can be seated against the surface of the femur  12  to secure the tissue graft  20  within the bone passages  14 ,  16 ,  18 . 
     The four holes  101 - 104  may be distributed equally along the length of the button  100 , as shown in  FIG. 4 , or may be distributed in other ways along the graft fixation device  100 . In some implementations, each of the surfaces of the graft fixation device  100  are rounded, such that the body of the graft fixation device  100  is smooth and without sharp edges or angles. 
     The closed loop suture  30  is comprised of monofilament fibers that form the braided end sections  31   a ,  31   b  and the non-braided section  32 . In some instances, the braided end sections  31   a ,  31   b  and non-braided section  32  ( FIG. 5 ) can be manufactured, for example, using standard commercial braiding machinery, such as that described in U.S. Pat. No. 8,881,635, hereby incorporated by reference in its entirety. The braiding machine may be equipped with multiple spools of yarn that are each loaded on one of multiple carriers of the braiding machine. The yarn may be formed from twisted together monofilament fibers or air-entangled monofilament fibers, where the particular monofilaments used to form a particular yarn may be dependent upon the application. For example, yarns used to form the closed loop suture  30  may comprise monofilament fibers, where the yarns have a minimum denier of 198, to a maximum denier of 792. A braiding machine having at least 8 carriers, and up to 16 carriers, may be used to form the braided end sections  31   a ,  31   b  and non-braided section  32  of the closed loop suture  30 , where each carrier is equipped with at least one yarn. In some instances, multiple yarns may be loaded on a single carrier of a braiding machine to achieve a desired result. For example, a braiding machine having 12 carriers may be equipped with 2 yarns per carrier to achieve a desired braid strength or density, such as a braid having a linear mass density of 375 denier. Other configurations may be used depending upon the particular application or the capabilities of a particular braiding machine. The process of forming the closed loop suture  30  includes, for example, braiding the first end section  31  a using the braiding machine, stopping the braiding machine from braiding, while the braiding machine is stopped, pulling the multifilament yarns that are loaded on the carriers of the braiding machine through the braiding machine to form the non-braided portion  32  of the closed loop suture  30 , and then again braiding the yarns to form the second end section  32   b  using the braiding machine, as described in U.S. Pat. No. 5,147,400, which is hereby incorporated by reference in its entirety. By following this process, the resulting closed loop suture  30  comprising the braided end sections  31   a ,  31   b  and the non-braided section  32  is formed from continuous monofilament fibers. 
     In one example, the first braided end section  31   a  of braided suture and the second braided end section  31   b  of braided suture can have similar dimensions, including a similar length, a similar outer diameter defined by the outer portion of the tubular braid, and a similar inner diameter, defined by the hollow conduit of the inner portion of the tubular braid. 
     In some instances the braided length  31  of the closed loop suture  30  and the non-braided length of the closed loop suture  30  can also have similar dimensions. For example, a closed loop suture  30  with a circumference of 20 mm can include a non-braided portion  32  that is 10 mm in length, and a braided portion that is also 10 mm in length, such that each of the sections  31   a ,  31   b  is more than 5 mm in length to allow for the overlapping of the sections  31   a ,  31   b  to form the “finger trap” connection between the two braided ends  31   a ,  31   b . In some examples, the length of the non-braided portion  32  is equal to, more, or less than half of the loop diameter. In some examples, the non-braided portion  32  is twisted, where twisting the non-braided length of suture  32  can increase the uniformity and deter fraying of the non-braided portion  32  of suture. For example, the non-braided portion  32  can be twisted one half rotation to deter fraying of the non-braided portion  32  of suture. The non-braided portion  32  may be twisted by other amounts, for example, by twisting the non-braided portion  32  a full rotation or multiple rotations. 
       FIG. 7  shows the tissue graft  20  being implanted during the ACL repair and reconstruction procedure using the graft fixation device  22 . The procedure begins by drilling the appropriately sized tibial channel  16 , the appropriately sized femoral channel  14 , and the appropriately sized passing channel  18 . The leading filament  42  and trailing filament  44  are removably attached to a passing pin  50 . The leading filament may function as a pulling suture used to draw the graft fixation device  22  that is attached to the tissue graft  20  through the bone channels  14 ,  16 . The passing pin  50  is inserted through an incision below the knee and advanced through tibial channel  16 , femoral channel  14 , and passing channel  18 , the quadriceps muscle surrounding the femur  12 , and the skin of the knee  11 . Ends of the leading filament  42  and trailing filament  44  are withdrawn beyond the skin of the knee  11  using the passing pin  50 . A surgeon can pull the graft fixation device  22  that is attached to the tissue graft  20  through the tibial channel  16 , femoral channel  14 , and passing channel  18  by pulling predominantly on the leading filament  42 . Once the button  100  emerges from the passing channel  18  ( FIG. 8A ), the surgeon positions the button  100  transversely to the passing channel  18  and across the opening  19  of the passing channel  18  by pulling predominantly on the trailing filament  44 , as illustrated in  FIG. 8B . In this way, the trailing filament  44  acts as a flipping suture to rotate the button  100  transverse to the passing channel  18 . The surgeon can then proceed to seat the graft fixation device against the surface of the femur  12 , tension the tissue graft  20 , and secure the tissue graft  20  within the femoral channel  14  and the tibial channel  16  by attaching the tissue graft  20  to the tibia  13  using, for example, an interference screw, a post, or another fixation device  22 . 
     The graft fixation device can include a closed loop suture  30  formed in a double loop and used with a bone-tendon-bone tissue graft, as described, for example, in U.S. Pat. No. 7,530,990, hereby incorporated by reference in its entirety. 
     In general, the graft fixation device  22  can be used to secure any suitable kind of graft, such as allografts, autografts, and xenografts and can be used in surgical soft tissue reconstruction procedures other than those related to ACL repair and reconstruction. For example, the graft fixation device  22  can be used for the fixation of tendons and ligaments during other orthopedic reconstruction procedures such as posterior cruciate ligament (PCL) repair and reconstruction, biceps tenodesis, distal biceps, and small joint procedures. 
     The closed loop suture  30  can be assembled independently of the button  100  using the techniques discussed with respect to  FIG. 5 .
A method and device for securing a tissue graft within a bone passage includes providing a graft fixation device that houses a closed loop having both braided and non-braided portions. A tissue graft is looped through the closed loop at the non-braided portion to attach the tissue graft to the graft fixation device. The graft fixation device is passed through the bone passage and seated against the surface of the bone to secure the tissue graft within the bone passage.