Abstract:
A knotless interference or contact fixation anchor assembly comprising an anchor body with a removable inserter/driver that fully supports the suture anchor over its entire working length. The anchor body may also incorporating a suture trap that is housed within a proximal eyelet of the anchor body. The anchor body may include a plurality of elongate ribs extending longitudinally along at least a portion of the anchor body. The plurality of ribs comprise flexible barbs. When inserted into bone or a bone tunnel, the barbs compress slightly, resulting in less bone compression around the anchor due to the forces required for insertion, and increased pull-out resistance.

Description:
BACKGROUND 
       [0001]    Suture anchors are often used in surgical procedures. For example, a suture anchor may be used for securing soft tissue to bone by means of a suture attached to the suture anchor which is then inserted into the bone. A typical suture anchor is inserted into bone by pounding the suture anchor either into a bone tunnel or directly into the bone. Alternatively, the suture anchor can be configured for insertion by a screw mechanism. The suture anchor may be made of metal, plastic or bioreabsorbable material (which dissolves in the body over time). The suture anchor can include an eyelet that allows the suture to pass therethrough and link the suture anchor and the suture. Upon implantation into bone, the suture anchor engages the bone and resists further movement, providing an anchor point for the attached suture. 
         [0002]    In recent years, surgeons have been moving towards the use of smaller suture anchors in surgical repair operations. The use of smaller suture anchors may be less invasive, require less bone removal or disruption at the site of implantation, and allow for more rapid patient healing. In such cases, it is desirable for suture anchors to be as small as possible, without compromising anchor integrity or fixation strength. For “pound-in” type anchors, as the size of the anchor is reduced, the volume of material making up the anchor is also reduced, which can present challenges to the integrity of the anchor during its insertion into bone. In addition, reducing anchor size may present challenges in terms of suture retention within the anchor body, and thus the reliability of the repair. It is therefore desirable that both the integrity of the suture anchor and the reliability of suture retention during insertion is not compromised with the use of smaller suture anchors. 
         [0003]    Another desirable feature of suture anchors is increased fixation strength. The fixation strength of a suture anchor to bone is determined by the area of contact between the bone and the suture anchor and the normal force present there between (i.e., frictional sliding resistance). Assuming a constant normal force, as the contact area is increased, the fixation strength generally increases and vice versa. With the user of smaller suture anchors, however, less surface area is available for frictional engagement with the surrounding bone. Thus, lower fixation strength is observed in smaller suture anchors. As a consequence, such suture anchors may not be suitable for certain repair operations, where a certain level of fixation may be required. 
       SUMMARY 
       [0004]    Described herein are “pound-in” suture anchors that preserve and/or increase fixation strength with bone when implanted. Embodiments include knotless interference or contact fixation anchors that includes an anchor body having a plurality of external ribs orientated with the longitudinal axis of the anchor body. Multiple cuts are provided in one or more of the side walls, edges or corners of the ribs, creating flexible barbs. When inserted into bone or a bone tunnel, the barbs compress slightly. Advantageously, this results in less bone compression around the suture anchor due to the slightly reduced forces required for insertion, in comparison to anchor which does not include such barbs, making for an easier insertion and promotion of boney ingrowth and healing. In addition, the suture anchor described herein is more resistant to removal as the barbs engage with the inner wall of the bone tunnel and flex as the anchor is pulled proximally. As a result, fixation strength is enhanced as the collective resistance of the multiple barbs provide an appreciable gain in pull-out resistance. An additional advantage is that, in an uncompressed state, the suture anchor will have a slightly larger diameter than the bone tunnel, thereby permitting tunnels of smaller diameters to be used or, in the alternative, smaller diameter anchors. 
         [0005]    Embodiments of the suture anchor described herein also include a pound-in suture anchor with a removable inserter/driver that fully supports the suture anchor over its entire working length. The removable inserter advantageously provides support over the length of the suture anchor during installation and can be removed from the suture anchor once the suture or sutures have been loaded. 
         [0006]    Embodiments of the suture anchor described herein also include a pound-in suture anchor incorporating a suture trap that is housed within a proximal eyelet of the suture anchor. In a pre-implanted state, the suture trap is located at the distal end of the eyelet. When a force is applied, either by contact with bone during implantation, or removal of the metal inserter, the suture trap is driven toward the proximal end of the eyelet. Advantageously, the suture is then impinged within the eyelet by cooperating locking mechanisms on the suture trap and within the eyelet. 
         [0007]    In various embodiments of the suture anchor described herein, the suture anchor may include an elongated anchor body having a proximal end and a distal end, and a longitudinal axis extending between the proximal and distal ends and a plurality of elongate ribs extending longitudinally along at least a portion of the anchor body, with at least one of the plurality of ribs including at least one compressible barb. The anchor body may further include a transverse bore and a pair of channels extending proximally from the transverse bore, the channels suitable for receiving suture or tape. The anchor may include a tapered tip formed at the distal end of the anchor body. The distal end of the tapered tip may have a conical shape, and may be rounded or pointed. 
         [0008]    In further embodiments, at least one the plurality of ribs extends from the proximal end of the anchor body to a position proximal to the distal end or to a selection position within the tip. At least one of the plurality of ribs may include a plurality of barbs. At least a portion of the plurality of elongate ribs may be tapered. Each of the plurality of ribs may include at least one barb or a plurality of barbs. Each of the plurality of ribs may include a pair of sidewalls, with the at least one barb or plurality of barbs located in the sidewalls. Each of the plurality of ribs may include a pair of sidewalls and a top surface, with the at least one barb or plurality of barbs located in the sidewalls, the top surface, or both the sidewalls and the top surface. A portion of the plurality of elongate ribs may be tapered. 
         [0009]    In still further embodiments, a length of the at least one compressible barb or plurality of barbs may be uniform or vary along the length of the rib. The at least one barb or plurality of barbs may be located in an edge of the ribs, between the sidewalls or between the sidewalls and the top surface. The at least one barb or plurality of barbs may comprise a first material selected from the group including poly(lactic-co-glycolic) acid (PLGA), β-Tricalcium phosphate (β-TCP) and calcium sulfate, poly-L-lactic acid-hydroxyapatite (PLLA-HA), poly-D-lactide (PDLA), polyether ether ketone (PEEK) or variants thereof, and bioabsorbable materials. 
         [0010]    In still further embodiments, the suture anchor may comprise a tapered tip formed at the distal end of the anchor body. The tapered tip may comprise a distal tip end formed from a second material that is harder than the first material. The second material has a hardness within the range between about 40 Shore D to about 85 Shore D. The suture anchor according to claim  12 , wherein at least one of the plurality of ribs extends from the proximal end of the anchor body to a position proximal to the distal tip end or to a selection position within the tapered tip. The anchor body may further comprise an opening transverse to the longitudinal axis configured for the passage of a suture. A suture trap may be housed within the opening and configured to impinge a suture within the opening. 
         [0011]    In still further embodiments, a suture anchor assembly may include a suture anchor having an elongated anchor body having a proximal end and a distal end, and a longitudinal axis extending between the proximal and distal ends, and a plurality of elongate ribs extending longitudinally along at least a portion of the anchor body, at least one of the plurality of ribs comprising at least one compressible barb, and an inserter removably coupled to the suture anchor body. The inserter may further comprise a slot defined by first and second prongs, the slot in communication with an opening transverse to the longitudinal axis of the anchor body and configured for the passage of a suture, the slot allowing for passage of the suture between the first and second prongs when the inserter is removed from the anchor body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing and other objects, features and advantages will be apparent from the following more particular description of the examples, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the examples. 
           [0013]      FIG. 1  is an illustration of an embodiment of the suture anchor assembly of the present disclosure in an assembled state; 
           [0014]      FIGS. 2 a - b    are illustrations of another embodiment of the suture anchor assembly in a disassembled state; 
           [0015]      FIGS. 3-4  are cross-sectional illustrations of other embodiments of the suture anchor assembly; and 
           [0016]      FIG. 5  is an illustration of a section view of an embodiment of the suture anchor assembly 
           [0017]      FIG. 6  is an illustration of an embodiment of the suture anchor of the present disclosure in a pre-loaded, pre-inserted state; 
           [0018]      FIG. 7  is a cross-section of the embodiment of  FIG. 6  assembled with the suture anchor; 
           [0019]      FIG. 8  is an illustration of another embodiment of the suture anchor in a post-inserted state; 
           [0020]      FIGS. 9 a - d    are schematic illustrations of embodiments of the suture trap of the present disclosure; 
           [0021]      FIGS. 10 a  and 10 b    are schematic illustrations of alternative embodiments of the suture trap of the present disclosure; 
           [0022]      FIG. 11  is close-up isometric partial view of the ribs of the embodiment of  FIG. 1  barbs shown; and 
           [0023]      FIGS. 12-18  are illustrations of further examples of the barbs of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the description that follows, like components have been given the same reference numerals, regardless of whether they are shown in different examples. To illustrate an example(s) in a clear and concise manner, the drawings may not necessarily be to scale and certain features may be shown in somewhat schematic form. Features that are described and/or illustrated with respect to one example may be used in the same way or in a similar way in one or more other examples and/or in combination with or instead of the features of the other examples. 
         [0025]    Referring now to  FIG. 1 , an embodiment of the suture anchor assembly  100  of this disclosure having full anchor axial support is shown in a transparent view and in an assembled state. The suture anchor assembly  100  may be generally comprised of an inserter  102  and an anchor body  104 . The anchor body  104  is shown in  FIG. 1  as a tapered anchor body  104 , but other anchors (including non-tapered anchors) are also possible. The anchor body  104  may be tubular, possessing a circular or elliptical cross-section. In alternative examples, the cross-section of the anchor body  104  may adopt different closed shapes. The anchor body  104  may include a tapered tip  114  with a distal tip end  117 . An eyelet  106  may be located at the proximal end of the anchor body  104 . It may be noted that, because of the body taper as shown in the anchor body  104  of  FIG. 1 , inclusion of distal eyelet of sufficient size to receive sutures is not feasible. However, because of the wider body shown at the proximal end of the anchor body  104 , a sufficiently large eyelet  106  can be placed to accept large suture loads which may be doubled-over in combination with a passer. The anchor body  104  may also include an internal cannulation  116 . The cannulation  116  is primarily for receiving part of the inserter  102  so that the anchor body  104  may be mounted on the inserter  102  prior to its insertion in bone. This arrangement also permits the anchor body  104  to be delivered more precisely, and helps to prevent rotational movement of the anchor body  104  as it is inserted into bone. 
         [0026]    The anchor body  104  may be partially or entirely formed from a first material selected from a formulation of poly(lactic-co-glycolic) acid (PLGA), β-Tricalcium phosphate (β-TCP) and calcium sulfate, poly-L-lactic acid-hydroxyapatite (PLLA-HA), poly-D-lactide (PDLA), polyether ether ketone (PEEK) or variants thereof. Biocomposite examples of the anchor body  104  made from a combination of PLGA, β-TCP, and calcium sulfate are absorbable by the body, which is beneficial to natural healing. An example formulation of PLGA, β-TCP, and calcium sulfate is described in U.S. Pat. No. 8,545,866, the entirety of which is herein incorporated by reference. A copolymer of polyglycolic acid (PGA) and polytrimethylene carbonate (TMC) is another example of a bioreabsorbable material. Other commonly used materials for implants are also contemplated by this disclosure. In any case, the anchor body  104  comprises a material that is capable of providing the strength needed to set the fixation device into position and to hold the suture and tissue in position while bone-to-tissue in-growth occurs. 
         [0027]    The distal tip end  117  may have a conical, round, pointed or other suitable shape. The distal tip end  117  may be formed from a second material, different from the first material. The second material is harder than the first material, reflecting the fact that the distal tip end  117  is responsible for displacing a majority of the bone volume occupied by the anchor body  104 , including both the hard, outer cortical bone layer and the underlying cancellous bone. For example, the distal tip end  117  may be formed from a material having a hardness within the range between about 40 Shore D to about 85 Shore D. In further embodiments, examples of the second material may include, but are not limited to, stainless steels, titanium, titanium alloys, cobalt-chromium alloys, platinum alloys, and palladium alloys, carbon-reinforced polyether ether ketone (PEEK), and glass-reinforced PEEK. 
         [0028]    The anchor body  104  also includes a plurality of longitudinal ribs  115  extending from the distal tip end  117  of the anchor body  104  to the proximal end of the anchor body  104 . The plurality of ribs  115  are formed on the outer surface of the suture anchor body  104 , circumferentially spaced and extending radially outward therefrom. The plurality of ribs  115  are generally elongate and extend longitudinally along the anchor body  104 . In the example of  FIG. 1 , the plurality of ribs  115  extends substantially from the proximal end of the anchor body  104  to a position proximal to the distal end or to a selected point within the tip  114 . Furthermore, the portion of the ribs  115  formed within the tip  114  are configured to follow the taper of the tip  114 . In alternative embodiments, not shown, the ribs  115  formed with the tip  114  are not aligned with the ribs  115  of the anchor body  104 , and the tip  114  may comprise a greater number of ribs  115  than the anchor body  104 , or fewer ribs, some of which may or may not be aligned with the ribs  115  of the anchor body  104 . 
         [0029]    In further alternative examples, not shown, the plurality of ribs  115  may be include breaks along their length, and the anchor body  104  may comprise both ribs  115  which include breaks along their length and ribs  115  without such breaks. In further examples, each of the plurality of ribs  115  may be straight-sided, without any curved portions. For example, in  FIG. 1 , the cross-sectional shape of the plurality of ribs  115  is shown as generally rectangular. In alternative embodiments, not shown, the ribs  115  have a triangular or rounded cross-section. Each of the plurality of ribs  115  may be laterally offset or spaced from one another by a distance, which may be same distance around the circumference of the anchor body  104  or may vary. Additional embodiments of the plurality of ribs  115  are shown with reference to  FIGS. 10-17  as further described below. 
         [0030]      FIG. 2 a    shows the inserter  102  disassembled from the anchor body  104  ( FIG. 2 b   ). The inserter  102  may comprise a main body  121  including first and second prongs  122 ,  124  extending from the main body  121  and capable of insertion into the anchor body  104  through the cannulation  116  in the anchor body  104 , thus providing structural support for the entire length of the anchor body  104 . The inserter  102  may also comprise an opening  118  which may at least partially line up with the eyelet  106  when the inserter  102  is inserted into the anchor body  104 . The size of the opening  118  is selected to allow for a large number of sutures to be loaded simultaneously while doubled over using a suture passer. Once the sutures (not shown) have been loaded into the anchor body  104  and the anchor body  104  has been inserted into bone, the inserter  102  can be removed because the loaded sutures move through a suture exit slot  120  in communication with the opening  118  and defined by first and second prongs  122 ,  124 . In  FIG. 2 a   , the suture exit slot  120  is shown as tapering distally from the opening  118  but other configurations are possible. Because the sutures have already been passed through the anchor eyelet  106  and tensioned prior to removal of the inserter  102 , the sutures can align and funnel in the suture exit slot  120  with relative ease. 
         [0031]      FIG. 3  shows a cross-section of another embodiment of the suture anchor assembly  100 , perpendicular to the anchor eyelet  106 . The distal end of the anchor body  104  may comprise a slot spacer  126  extending proximally into the cannulation  116  in a variety of lengths. In order to properly orient the inserter  102  to the anchor body  104  both radially and circumferentially, the slot spacer  126  may be configured to mate to the first and second prongs  122 ,  124  of the inserter  102 . The slot spacer  126  may also ensure that the opening  118  aligns with the anchor eyelet  106  when the inserter  102  is inserted into the anchor body  104 . The slot spacer  126  may further ensure that first and second prongs  122 ,  124  of the inserter  102  remain “expanded” circumferentially, thereby ensuring intimate contact between the inserter  102  and the anchor body  104 . Intimate contact between the inserter  102  and the anchor body  104  promotes the structural integrity of the suture anchor assembly  100 , particularly during the insertion process. 
         [0032]      FIG. 4  shows a cross-section of another embodiment of the suture anchor assembly  100  cut through the anchor eyelet  106 . In this view, two possible additional features of the suture anchor assembly  100  are shown: a suture bridge  128  and a suture feed slot  130 . The suture bridge  128  may be a portion of the anchor body  104  that is sufficiently thick for sutures to bear upon without “cheese wiring” the anchor body  104  when the sutures are pulled on post-implantation. The suture bridge  128  may also serve to impinge sutures against surrounding bone, fixing or locking them in place. The suture bridge  128  may be present on one or both sides of the anchor body  104 . The suture feed slot  130  may allow sutures to be tensioned and provide unimpeded ingress into the anchor body  104  during the final stages of inserting the suture anchor assembly  100  into bone. 
         [0033]      FIG. 5  illustrates an example of how sutures may be loaded into the suture anchor assembly  100 .  FIG. 5  shows the side of the suture anchor assembly  100  that may contain a suture feed slot  130 , 180 degrees opposite from the suture bridge  128  ( FIG. 4 ).  FIG. 5  also depicts a typical cross-section of the distal portion of the suture anchor assembly  100 . As stated above, in order for the suture to properly feed into the anchor body  104  during pound-in insertion, a suture feed slot  130  may be present in the anchor body  104 . During use, the suture bridge side of the anchor body  104  (not shown) may be the side through which the sutures enter the suture anchor assembly  100 . The side of the suture anchor assembly  100  containing the suture feed slot  130  may be the side of the suture anchor assembly  100  from which the sutures exit. The suture feed slot  130  may allow the sutures to be better tensioned during surgery and allow the free end of the sutures to flow into the suture anchor assembly  100  when the proximal end of the inserter  102  is pounded into bone. 
         [0034]    Referring now to  FIG. 6 , another embodiment of the suture anchor assembly  200  of this disclosure, having a suture capture mechanism, is shown. In  FIG. 6 , the suture anchor  204  is shown as assembled with inserter  202 . An eyelet  206  is located at the proximal end of the suture anchor  200  and transverse to the longitudinal axis of the suture anchor  200 . A suture trap  210  (described in greater detail with reference to  FIGS. 9 a - c   ) is located at the distal end of eyelet  206 . The suture trap  210  may be fabricated out of materials such as PEEK, PLLA-HA, implantable metals, bioreabsorbable or other suitable materials. As shown in  FIG. 6 , one or more sutures  212  may be passed through the proximal, non-occluded portion of the eyelet  206  with a standard suture passer or by other means. In this state, the one or more sutures  212  are proximal to the suture trap  210 . 
         [0035]      FIG. 7  shows a cross-section of the inserter  202  assembled with the suture anchor  204  of  FIG. 6  with suture trap  210  located at the distal portion of the eyelet  206  and the sutures  212  passed therethrough. In use, with the one or more sutures  212  passed through the eyelet  206  of the suture anchor  204 , the suture anchor  204  is partially inserted into bone (not shown). With the distal tip end  217  embedded in bone, the sutures  212  passing through the suture anchor  204  are tensioned. 
         [0036]    Turning now to  FIG. 8 , with the sutures properly tensioned, the suture anchor  204  is fully pounded into the bone. Since a portion of the suture trap  210  extends outside of eyelet  206 , as the exposed portion of the suture trap  210  encounters the bone surface, the suture trap  210  is driven toward the proximal end of eyelet  206  where it encounters the suture  212  that was passed though the eyelet  206 . Once the suture anchor  204  is fully counter-sunk in the bone, inserter  202  can be removed. When inserter  202  is removed, the prongs  222 ,  224  ( FIG. 7 ) of inserter  202  drag over the suture trap  210 , further impinging the suture trap  210  against the suture  212  at the proximal end of the eyelet  206 . This mode of action is particularly useful in softer bone situations where impact between bone and the exposed portion of the suture trap  210  is not sufficient to generate full seating of the suture trap  210  at its proximal travel limit. Alternatively, pre-tensioning of suture  212  prior to final insertion of the suture anchor  204  could be used to seat the suture trap  210  proximally if the suture  212  were routed through or around suture trap  212 . It is also contemplated by this disclosure that the most lateral aspects of the suture trap  210  could be made to fold onto the suture  212 , impinging the suture  212  between the suture anchor  204  and the suture trap  210  after the suture trap  210  is inserted into bone. 
         [0037]      FIGS. 9 a - c    illustrate embodiments of the suture trap  210  of this disclosure in greater detail.  FIG. 9 a    shows a cross-section of the interface of the suture trap  210  with the suture anchor  204  and inserter  202 . As shown in  FIGS. 9 b - c   , the suture trap  210  can include a middle spar  232  and four or more flexible elements  234  configured to engage with teeth  236  on a pair of racks  238  formed on an inner surface of the eyelet  206 . As the suture trap  210  moves proximally, it rides up over the teeth  236  located in the eyelet  206 . The flexible elements  234  and racks  238  ( FIG. 9 c   ) cause the suture trap  210  to remain parked distally in the eyelet  206  prior to installation of the suture anchor  204 . After installation of the suture anchor  204 , the flexible elements  234  and racks  238  cause the suture trap  210  to remain locked proximally in the eyelet  206 . This proximally locked position allows for suture impingement between the suture trap  210  and the proximal end of the eyelet  206  to be maintained. The flexible elements  234  and middle spar  232  are shown in more detail in  FIG. 9   d.    
         [0038]    Another embodiment of the suture anchor assembly  300  of this disclosure having a suture capture mechanism is shown in cross-section in  FIGS. 10 a  and 10 b   . The suture anchor assembly  300  is shown in both a pre-clamped ( FIG. 10 a   ) and post-clamped ( FIG. 10 b   ) state. The suture anchor assembly  300  generally includes an anchor body  304  and an inserter  302 . The anchor body  304  has a proximal end, a tapered distal end, and a longitudinal axis extending between the proximal and distal ends. An eyelet  306  is formed in the proximal end of the anchor body  304 , transverse to the longitudinal axis. A suture trap  310  is formed within the eyelet  306  and has at least one flexible member  334 , such as wings or flaps, extending radially from the eyelet  306 . The suture trap  310  may be fabricated out of materials such as PEEK, PLLA-HA, implantable metals, bioreabsorbable or other suitable materials. 
         [0039]    In use, a suture  312  is first routed through the proximal, non-occluded portion of the eyelet  306  ( FIG. 10 a   ) with a standard suture passer. In the original, pre-clamping state, the suture  312  is proximal to the suture trap  310 . With the suture  312  passed through the eyelet  306 , the anchor body  304  is partially hammered into bone (not shown) and the suture  312  is tensioned. Once the suture  312  is tensioned, the anchor body  304  is fully pounded into bone. As the flexible members  334  of the suture trap  310  encounters bone surface of sufficiently hard bone, the suture trap  310  is driven toward the proximal end of the anchor body  304  where it encounters the suture  312  ( FIG. 10 b   ) that was passed through the eyelet  306 . The inserter  302  is then removed. Removal of the inserter  302  causes a distal portion of the inserter  302  to drag over the middle spar  332  of the suture trap  310 , further locking the suture trap  310  against the suture  312  at the proximal end of the eyelet  306 . This mode of action is particularly useful in softer bone where impact between bone and the flexible members  334  of the suture trap  310  is not sufficient to generate full seating of the suture trap  310  at its proximal travel limit. Once the suture trap  310  is fully seated and locked, the suture  312  is trapped between the suture trap  310  and the proximal end of the eyelet  306 , as well as between the anchor body  304  and surrounding bone. 
         [0040]    It is contemplated by this disclosure that flexures and rack teeth, such as those shown in  FIGS. 9 a - c   , could be employed between the suture trap  310  and the eyelet  306 . The flexible members  334  could be orientated at a different angle so as to occlude less of the eyelet  306 . Pre-tensioning of the suture  312  prior to final insertion of the anchor body  304  could be used to seat the suture trap  310  proximally if the suture  312  were routed through or around the suture trap  310 . 
         [0041]    Referring now to  FIG. 11 , a partial view of a series of the plurality of anchor ribs  115 , such as those described with reference to  FIG. 1 , is illustrated. Various other features of the anchor body  104 , both optional and required, have been omitted from this and the subsequent figures for clarity. In  FIG. 11 , the plurality of ribs  115  are shown as rectangular with a top surface  140  and opposing sidewalls  142 . One or more cuts, slashes, slices or ingressions  144  may be made in the plurality of ribs  115  to provide at least one thin, flexible barb  146  formed integrally with the ribs  115 , therefore comprising the same material as the rib  115 . The barbs  146  may be overlapping, as shown in  FIG. 10 , or may form non-overlapping units (see  FIGS. 15 and 16 ). The barbs  146  may be formed in a process once the anchor body  104  has been formed, for example by a series of cuts with a laser, blade or hot wire. Alternatively, the barbs  146  can be formed in the anchor molding process. The barbs  146  may comprise a hinge  148  closer to the distal end of the anchor body  104  and a free (cantilevered) end  150  closer to the proximal end of the anchor body  104 . In alternative embodiments (see  FIG. 12 ), the barbs  146  are formed perpendicular to the anchor body  104 , like the teeth of a comb. The barbs  146  may be present in all of the ribs  115  or in just some of the ribs  115 . 
         [0042]    As illustrated in  FIG. 13 , when the distal end of anchor body  104  is inserted into a bone tunnel (not shown), the free ends  150  of the barbs  146  are moveable from a first position (A) that is further from the top surface  140  of the rib  115  to the second position (B) that closer to the top surface  140  of the rib  114 . Compression of the barbs  146  from the first to the second position (A, B) reduces the amount of insertion force required to insert the anchor body  104  into bone. Conversely, when a pull-out force is exerted on the anchor body  104  from a proximal direction, the barbs  146  flex outwardly from the second position (B) towards the first position (A), enhancing the interference fit of the anchor body  104  against the bone and thus increasing pull-out resistance. 
         [0043]    The length of each barb  146  can either be uniform or vary along the length of the rib  115 . Where varying lengths occur, the length of the barbs  146  can increase along the length of the rib  115  from the proximal end of the rib  115  to the distal end of the rib  115 , or vice versa. The depth to which the cuts  144  encroach into the ribs  115  (compare, for example,  FIGS. 11  and  14 ) may also vary from rib to rib, or could change along the length of the rib  115 . It is contemplated by this disclosure that the barbs  146  of one rib  115  may run in a parallel configuration to those of an adjacent rib  115  or the barbs  146  may form a helical or other pattern about the anchor body  104 . 
         [0044]    Further examples of the barbs  146  are shown in  FIGS. 15-18 . In  FIG. 15 , the barbs  146  are shown as formed in the upper surface  140  of the rib  115 . The width of each barb  146  is shown as equal in width to the top surface  140  of rib  115  but other widths are possible. In  FIGS. 16 and 17 , the barbs  146  are formed instead in the edges of the rib  115 ,  FIG. 17  being a section through the embodiment of  FIG. 16 . The barbs  146  may also be formed in either or both opposing sidewalls  142  of the ribs  115 , as shown in  FIG. 18 . It is contemplated by this disclosure that the barbs  146  can be formed in one or more surface as shown in  FIGS. 15-18  separately or together in one rib  115 . 
         [0045]    Although the barbs  146  are illustrated in  FIGS. 11-18  with reference to the anchor body  104  of  FIG. 1 , it is contemplated that the barbs  146  may be a feature of any variety of anchor, plug, shank, pin, tack or other surgical fixation device that may employ ribs on its surface to enhance fixation strength. 
         [0046]    These and other features and characteristics, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of claims.