Patent Publication Number: US-10307196-B2

Title: Bone fixation assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional continuation of U.S. Ser. No. 13/019,907, filed Feb. 2, 2011 (now US U.S. Pat. No. 9,084,644, issued Jul. 21, 2015), the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     In order to provide access to certain internal anatomy, such as the heart during an open heart procedures, midline sternotomies are typically performed. A midline sternotomy creates a cut substantially along the midline of the sternum, thereby dividing the ribcage into two halves and allowing the surgeon to move the ribcage so as to provide access to the heart. Upon completion of the open heart procedure, it is desired to approximate and compress the sternum, and rigidly maintain the sternal halves in their approximated position relative to each other so that the sterna halves are prevented from moving with respect to each other to promote bone fusion in the weeks following the surgical procedure. 
     During normal anatomical function, for instance during respiration, body movement, and carrying of objects, forces can be generated that act on the sternum. One conventional system sternal fixation assembly includes stainless steel wires that are placed either parasternally (around the sternum) or transsternally (through the sternum bone) using a cutting needle that is attached to the wire, and subsequently twisted to tighten the wire against the sternum. However, the twisting generates tensile forces onto the wires that tend to weaken the wire, which can result in breakage both during the closure or post-operatively. Furthermore, this type of system relies on the experience of the surgeon when tightening the wires. If the wires are not tightened enough, the sternal compression can be compromised. If the wires are tightened too much, the wire can cut into or through the sternum and/or can break. 
     SUMMARY 
     In accordance with one embodiment, a bone fixation instrument is configured to apply tension to a bone fixation member so as to tighten the bone fixation member about a target bone. The bone fixation instrument includes a body that defines a front end and an opposed rear end, a grip configured to secure a free end of the fixation member to the fixation instrument, a traveler that is connected to the grip such that the grip moves rearward along with the traveler so as to increase tension in the bone fixation member, and an actuator operatively coupled to the traveler. The actuator can be configured to move from an initial position toward a tension position in response to an applied force, thereby biasing the traveler to move rearward. The bone fixation instrument can further include a tension limiter connected between the actuator and the traveler. The tension limiter allows the traveler to move rearward when the tension in the bone fixation member is less than the selected tension, and prevents the traveler from moving rearward when the tension in the bone fixation member reaches the selected tension. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the preferred embodiments of the application, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the present disclosure, there is shown in the drawings preferred embodiments. It should be understood, however, that the application is not limited to the specific embodiments and methods disclosed, and reference is made to the claims for that purpose. In the drawings: 
         FIG. 1  is a perspective view of a bone fixation instrument constructed in accordance with one embodiment; 
         FIG. 2A  is a perspective view of a bone fixation member including a body and a locking member, showing the bone fixation member in an initial configuration; 
         FIG. 2B  is an enlarged perspective view of a portion of the bone fixation member illustrated in  FIG. 2A , showing the body inserted through the locking member so as to secure the bone fixation member about an underlying bone; 
         FIG. 3A  is a perspective view of a plurality of the bone fixation members illustrated in  FIG. 2A  shown tightened about a target bone and cut; 
         FIG. 3B  is a perspective view of the bone fixation instrument illustrated in  FIG. 1  operatively coupled to and tightening one of a plurality of the bone fixation members illustrated in  FIG. 3A ; 
         FIG. 3C  is a perspective view of the bone fixation instrument illustrated in  FIG. 1  operatively coupled to and cutting one of the tightened bone fixation members illustrated in  FIG. 3B ; 
         FIG. 4  is a perspective view of the bone fixation instrument illustrated in  FIG. 1 , with portions removed to illustrate internal components including a tension assembly and a cutter assembly; 
         FIG. 5A  is a perspective view of the bone fixation instrument illustrated in  FIG. 4 , showing the cutter assembly in a cutting position; 
         FIG. 5B  is a perspective view of a front end of the bone fixation instrument as illustrated in  FIG. 5A ; 
         FIG. 6  is a perspective view of a rear end of the bone fixation instrument illustrated in  FIG. 4 ; 
         FIG. 7A  is a perspective view of the front end of the bone fixation instrument illustrated in  FIG. 4 , showing the tension assembly in a disengaged position; 
         FIG. 7B  is a perspective view of the distal end of the bone fixation instrument illustrated in  FIG. 7A  having portions removed and showing the tension assembly in an engaged position; 
         FIG. 8A  is a side elevation view of the bone fixation system, including the bone fixation instrument illustrated in  FIG. 4  showing the bone fixation member as illustrated in  FIG. 3A , with the target bone removed for the purposes of illustration, shown loosely received in the bone fixation instrument with the tension assembly in the disengaged position; 
         FIG. 8B  is a side elevation view of the bone fixation system illustrated in  FIG. 8A , showing the tension assembly in the engaged position such that the bone fixation instrument is secured to the bone fixation member; 
         FIG. 8C  is a side elevation view of the bone fixation system illustrated in  FIG. 8B , showing the tension assembly in a tightened position; 
         FIG. 8D  is a side elevation view of the bone fixation system illustrated in  FIG. 8C , showing the tension assembly in a disengaged position after having been previously in the tightening position  FIG. 8C ; 
         FIG. 8E  is a side elevation view of the bone fixation system illustrated in  FIG. 8D , showing the operation of a tension limiter whereby actuation of the tension assembly toward the tightened position does not tighten the received bone fixation member; 
         FIG. 9  is a side elevation view of the bone fixation system illustrated in  FIG. 8D , but showing actuation of the cutter assembly; and 
         FIG. 10  is a perspective view of a bone fixation instrument similar to the bone fixation instrument illustrated in  FIG. 4 , but constructed in accordance with an alternative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “proximally” and “distally” refer to directions toward and away from, respectively, the surgeon using the surgical instrument. The words, “anterior”, “posterior”, “superior”, “inferior” and related words and/or phrases designate preferred positions and orientations in the human body to which reference is made and are not meant to be limiting. The terminology includes the above-listed words, derivatives thereof and words of similar import. 
     Referring to  FIGS. 1-3B , a bone fixation assembly  20  includes a bone fixation instrument  22  and at least one bone fixation member  24  such as a plurality of bone fixation members  24  that are configured to secure a first and second bone segments  26   a  and  26   b  of a target bone  28 , such as a sternum, that are separated at a fracture location  30  together in a compressed approximated position. 
     In accordance with the illustrated embodiment, each bone fixation member  24  can be substantially configured as a cable tie, and can include a flexible strap  32  defines a strap body  33  and has first end  34   a  and a second end  34   b  opposite the first end  34   a  along the length of the strap  32 , a needle tip  36  that extends from the first end  34   a , and a locking mechanism  38  that extends from the second end  34   b . The strap  32  can be made from any suitable biocompatible material as desired, such as PEEK. 
     Each bone fixation member  24  can further include a first initiation region  40  that extends from the first end  34   a  toward the second end  34   b  along a portion of a length of the strap  32  (for instance, approximately ⅓ the length of the strap  32 ) and a second locking region  42  that extends between the first initiation region  40  and the second end  34   b . In accordance with the illustrated embodiment, the second locking region  42  extends from the first initiation region  40  to the second end  34   b . The first initiation region  40  can include a plurality of small protrusions that extend out from the strap body  33  and alternate with recessed regions disposed between adjacent protrusions. Alternatively, the initiation region  40  can be substantially smooth and devoid of protrusions or teeth. The second locking region  42  can include a plurality of locking teeth  48  that extend out from the strap body  33  a distance greater than the protrusions and are separated by recessed regions  51  disposed between adjacent locking teeth. It should be appreciated that the locking region  42  can extend along any portion up to all of the strap body  33  as desired. 
     The locking mechanism  38  includes a housing  50  a strap receiving slot  52  that extends through the housing  50  and is configured to receive the first end  34   a  of the strap  32 . In accordance with the illustrated embodiment, the first end  34   a  is inserted through the slot  52  so as to define a loop  55  about the target bone  28 . The locking mechanism  38  is configured to allow the strap  32  to translate unidirectionally through the slot  52  along the direction of Arrow A so as to reduce the size of the loop  55  about the first and second segments  26   a  and  26   b  of the target bone  28 . For instance, the needle tip  36  can be inserted through the slot  52  and subsequently removed, for instance by cutting a neck  56  of the strap body  33  that defines reduced thickness at a location adjacent the needle tip  36 , such that the strap  32  remains in the slot  52 . In accordance with the illustrated embodiment, the locking mechanism  38  includes a locking member such as a tongue  54  that is connected to the housing  50  and includes at least one complementary tooth such as a plurality of locking teeth  58  that extend into the slot  52 . The locking teeth  58  define a beveled leading edge  60  that that is configured to cam over complementary beveled leading edges  49  of the locking teeth  48  when the strap  32  is translated through the slot  52  along the direction of Arrow A. The locking teeth  58  and  48  further define trailing edges  62  and  47  that are sloped less than the beveled leading edges  60 , such that the trailing edges  62  and  47  engage to prevent the strap  32  from translating through the slot  52  along the direction opposite Arrow A, which would increase the size of the loop  55 . 
     During operation, the strap  32  is wrapped around the first and second segments  26   a  and  26   b  of the target bone  28 , and the needle tip  36  is inserted through the slot  52  and pulled through the slot  52  so as to cause the strap  32  to subsequently translate through the slot  52 . The needle tip  36  can be removed from the strap  32 , and the strap  32  can then be further pulled, for instance manually, through the slot  52 . As the strap  32  is translated through the locking mechanism  38  along the direction of Arrow A, the small protrusions of the initiation region  40  can slide through the slot  52  without engaging the locking teeth  58  of the locking mechanism  38 . As the locking region  42  of the strap  32  is translated through the slot  52  along the direction of Arrow A, the locking teeth  48  and  58  can engage to prevent the tension that is induced in the strap  32  from causing the strap  32  to back out of the slot  52  along a direction opposite Arrow A. For instance, as the strap  32  translates through the locking mechanism  38  along the direction of Arrow A, the size of the loop  55  about the target bone  28  decreases until tactile feedback indicates that tension has been induced in the strap  32 . 
     As illustrated in  FIG. 3B , the fixation instrument  22  includes a tension assembly  70  that is configured to secure the fixation instrument  22  to the strap  32 , and is further configured to further pull the strap  32  through the locking mechanism  38  thereby further inducing tension in the strap  32  until the strap  32  has securely compressed the bone first and second bone segments  26   a  and  26   b  of the target bone  28  together at the fracture location  30 . As illustrated in  FIG. 3C , the fixation instrument  22  further includes a cutter assembly  72  that is configured to cut a free end  35  of the strap  32  that has passed through the locking mechanism  38  once a desired tension has been induced in the strap  32  about the first and second segments  26   a  and  26   b  of the target bone  28 . For instance, the desired tension can be within a range defined by and between a lower end that can be approximately 50 Newtons or approximately 80 Newtons, and an upper end that can be approximately 150-160 Newtons or 200 Newtons. It should be appreciated that the desired tension can depend on the bone quality and the preference of the surgeon, and can for instance be any tension as desired that reliably secures the target bone  28  without overtightening the strap  32 . 
     Referring now to  FIGS. 1 and 4 , the fixation instrument  22  includes a body  76  that defines a front end  78   a  and an opposed rear end  78   b  spaced from the front end  78   a  along a longitudinal direction L and opposed sides  80  that are spaced along a lateral direction A that is substantially perpendicular with respect to the longitudinal direction L. The fixation instrument  22  further includes a handle  82  that is supported by the body  76 , and can extend down from the body  76  along a transverse direction that is substantially perpendicular with respect to both the longitudinal direction L and the lateral direction A. In accordance with the illustrated embodiment, the transverse direction T is oriented vertically, and the longitudinal and lateral directions L and A are oriented horizontally, though it should be appreciated that the orientation of the fixation instrument may vary during use. In accordance with the illustrated embodiment, the body  76  is elongate in the longitudinal direction L. 
     The fixation instrument  22  further includes a trigger  84  that extends down from the body  76  at a location spaced forward from the handle  82 , and a nose  86  disposed at the front end  78   a  of the body  76 . The handle  82 , the trigger  84 , and the nose  86  can be discreetly attached to the body  76  or integral with the body  76  as desired. The body  76  can include an outer housing  88  that includes a pair of housing members  90   a  and  90   b  that are laterally opposed and define respective outer sides  92  and can be joined together via fasteners such as screws  94  so as to support the various internal components of the fixation instrument  22 . 
     It should be appreciated that the body  76  of the fixation instrument  22  can be constructed in any suitable manner as desired. For instance, as illustrated in  FIG. 1 , the outer hosing members  90   a  and  90   b  can include a flange  93  that extends laterally inward toward each other from the sides  80  at a forward location of the body  76 . Alternatively, as illustrated in  FIG. 10 , the flange  93  can extend further rearward and terminate adjacent an actuator  100 . 
     For instance, the housing  88  can support the tension assembly  70  that is configured to tighten the bone fixation member, thereby inducing tension in the bone fixation member  24 , such as the strap  32 , and can further support the cutter assembly  72  that is configured to remove a free end  35  of the bone fixation member  24  once the tension assembly  70  has induced a desired level of tension in the bone fixation member  24 . The tension assembly  70  includes a grip  96  that is movable between a disengaged position ( FIG. 7A ) whereby the grip  96  is configured to loosely receive the strap  32 , such as the portion of the strap that has passed through the locking mechanism  38 , and an engaged position ( FIG. 7B ) whereby the grip  96  is configured to be secured to the received strap  32 . The tension assembly  70  further includes a traveler  98  that is operably coupled to the grip  96  and extends rearward from the grip  96 , such that rearward movement of the traveler  98  causes the grip  96  to move rearward in the secured configuration, thereby inducing tension in the strap  32 . The tension assembly  70  can further include an actuator  100  such as the trigger  84  and a tension limiter  102  ( FIG. 6 ) connected between the trigger  84  and the traveler  98 . The tension assembly  70  can further include a force transfer member  104  that is connected between the trigger  84  and the tension limiter  102 , thereby operatively coupling the trigger  84  to the traveler  98 . 
     During operation, and as described in more detail below, the free end  35  of the strap  32  is received in the grip  96 , the nose  86  is placed against the housing  50  of the locking mechanism  38 , and the actuator  100  is moved from an first initial position to a second grip position that causes the grip  96  to iterate from the disengaged position to the engaged position, and is further moved from the second grip position to a third tension position that causes the traveler  98  to move rearward, thereby inducing tension in the strap  32  when the tension in the strap  32  is less than a select tension, which can be a desired maximum tension as determined by the tension limiter  102 . When the tension in the strap  32  reaches the maximum tension, the tension limiter  102  prevents the traveler  98  from moving rearward when the actuator  100  is moved to the tension position. 
     The cutter assembly  72  includes a cutter arm  106  movably supported by the body  76  and a cutter blade  108  that is carried by the cutter arm  106 . The cutter arm  106  is movable from a seated disengaged position (see  FIG. 4 ) whereby the cutter blade  108  is spaced from the free end  35  of the strap  32  that is received in the grip  96  to an engaged position (see  FIGS. 5A-B ) whereby the cutter blade  108  cooperates with a complementary cutter blade  87  of the nose  86  so as to cut the free end  35  of the strap  32  (see  FIG. 9 ). The fixation instrument  22 , and in particular the cutter assembly  72 , further includes a safety mechanism  110  that moves from a disengaged position, whereby the cutter arm  106  can move from the seated disengaged position toward the engaged position, and an engaged position that prevents the cutter arm  106  from moving from the seated disengaged position toward the engaged position. 
     Referring now to  FIGS. 4 and 7A -B, the traveler  98  of the fixation instrument  22  can be provided as a longitudinally extending traveler rod  111  that extends forward from the tension limiter  102  and supports the grip  96  at its front end. The traveler rod can define a rectangular cross-section as illustrated in  FIG. 4 , a substantially circular cross section as illustrated in  FIG. 10 , or any alternative size and shape as desired. The grip  96  includes a first lower grip member  112  and a second upper grip member  114  spaced from the lower grip member  112  so as to define a gap  116  disposed between the lower and upper grip members  112  and  114 . The lower grip member  112  defines first grip surface  118  that faces the upper grip member  114 , and can further define a plurality of teeth  120  that extend out from the first grip surface  118  toward the upper grip member  114 . The teeth  120  are configured to assist in reliably securing the grip  96  to the strap  32 . The upper grip member  114  is supported by the traveler rod  111  and defines a second grip surface  115  that faces the first grip surface  118 . The first and second grip surfaces  118  and  115  can be sized and shaped as desired. In accordance with the illustrated embodiment, the first grip surface  118  is curved and substantially arc-shaped in accordance with the illustrated embodiment, such that the first grip surface  118  is convex with respect to the upper grip member  114 . Furthermore in accordance with the illustrated embodiment, the second grip surface  115  is substantially flat. 
     The traveler rod  111  defines a forked first front end  117   a  that defines a pair of laterally spaced side walls  122 . The upper grip member  114  can be rigidly supported by the traveler rod  111 , and the lower grip member  112  can be pivotally coupled to the traveler rod  111  at a pivot location  113  about a laterally extending pivot pin  115  that defines a lateral pivot axis, and can further be disposed between the side walls  122 . The grip  96  can further include a biasing member such as a torsion spring  124  that biases the lower grip member to pivot in a forward direction about the pivot location  113  toward the nose  86  from the disengaged position of the grip  96  to the engaged position of the grip  96 . The grip surface  118  can extend eccentrically about the pivot location  113  such that the grip surface  118  moves upward toward the upper grip member  114  as the lower grip member  112  pivots forward to the engaged position. 
     The lower grip member  112  defines a first or forward stop surface  126  and the nose  86  defines a complementary second or rear stop surface  127 . When the traveler rod  111  is in a forward position, the grip  96  is in the disengaged position whereby the torsion spring  124  biases the forward stop surface  126  against the rear stop surface  127 . When the grip is in the disengaged position, the gap  116  is sized greater than the thickness of the free end  35  of the strap  32  as illustrated in  FIG. 7A . Accordingly, the free end of the strap  35  can be received in the gap  116  between the first and second grip surfaces  118  and  115  as illustrated in  FIG. 8A . As the traveler rod  111  moves rearward, the lower grip member  112  pivots forward about the pivot location  113  such that the grip surface  118  moves toward the grip surface  115  of the upper grip member  114  as illustrated in  FIG. 7B , thereby reducing the gap  116  until the grip  95  captures the free end  35  of the strap  32  that is received in the gap  116  between the first and second grip surfaces  118  and  115  under the force of the torsion spring  124  ( FIG. 8B ). It should thus be appreciated that as the traveler rod  111  further moves rearward, the free end  35  of the strap  32  will move rearward along with the grip  95  and the traveler rod  111 . 
     Referring now to  FIGS. 4 and 6 , the traveler rod  111  defines a second rear end  117   b  opposite the front end  117   a . The rear end  117   b  of the traveler rod  111  is connected to the tension limiter  102 . The tension limiter  102  includes a spring member  128  such as a helical compression spring that defines a first or front end  130   a  and a second or rear end  130   b . The spring member  128  is fixed with respect to movement relative to the traveler rod  111  at its rear end  130   b , and movable with respect to the traveler rod  111  at its front end  130   a . The spring member  128  is further coupled to the trigger  84  at its front end. Thus, the spring member  128  is coupled to the trigger  84  at its end that is also movable with respect to the traveler rod  111  so as to generate a force to the traveler rod  111  that biases the traveler rod rearward away from the locking mechanism  38  of the bone fixation member  24 . 
     In accordance with the illustrated embodiment, the tension limiter  102  includes a stationary end cap  129  that can be threaded onto or otherwise secured to the traveler rod  111 , for instance to the rear end  117   b  of the traveler rod  111 . The rear end  130   b  of the spring member  128  is attached to the end cap  129  so as to secure the spring member  128  to the traveler rod  111  at its rear end  130   b  such that the rear end  130   b  is fixed with respect to movement relative to the rod  111 . Thus, as the rear end  130   b  of the spring member  128  moves rearward, the traveler rod moves rearward along with the rear end  130   b  of the spring member  128 . The tension limiter  102  can further include a movable spring seat  132  that is fixed to the movable front end  130   a  of the spring member  128 . In accordance with the illustrated embodiment, the traveler rod  111  extends through the spring seat  132 , such that the spring seat  132  is movable in the longitudinal direction L along the traveler rod  111 . The traveler rod  111  can define a longitudinally elongate slot  134  extending therethrough that can receive a projection of the spring seat  132 . Thus, the slot  134  defines a length that allows the movable spring seat  132  to translate from its forward-most position to its rearward-most position. 
     The tension assembly  70  further includes the force transfer member  104  illustrated as a force transfer arm  136  that is pivotally connected between the movable spring seat  132  and the trigger  84 . Otherwise stated, the movable front end  130   a  of the spring member  128  is coupled to the trigger  84  via the force transfer arm  136 . In accordance with the illustrated embodiment, the force transfer arm  136  defines a front end  138   a  and an opposed rear end  138   b . The force transfer arm  136  is pivotally coupled, for instance at its rear end  138   b  to the spring seat  132  at a pivot location  139  that defines a lateral pivot axis. The force transfer arm  136  is further pivotally coupled, for instance at its front end  138   a  to the trigger  84  at a pivot location  140  that defines a lateral pivot axis. 
     Referring again to  FIG. 4 , tension assembly  70  further includes the trigger  84  that includes a lower grip portion  85  extending down from the housing  88  at a location spaced forward of the handle  82  and an upper securement portion  89  pivotally connected to the force transfer arm  136  and further pivotally connected to the housing  88 . The trigger  84  defines an aperture  91  that extends longitudinally through the upper securement portion  89  and receives the traveler rod that extends through the aperture, and thus the upper securement portion  89 . The securement portion  89  of the trigger  84  is pivotally connected to the force transfer arm  136  at the pivot location  140 , which defines a first lower pivot location of the trigger  84 . The securement portion  89  of the trigger  84  is further pivotally coupled to the housing  88  at a second upper pivot location  142  that defines a lateral pivot axis. The user can grasp the handle  82  and the grip  85  of the trigger  84  with one hand, and squeeze the trigger  84 , which causes the trigger  84  to pivot rearward about the second upper pivot location  142 . Because the lower pivot location  140  is disposed between the upper pivot location  142  and the grip  85 , the lower pivot location  140  moves rearward when the trigger  144  is moved rearward by the user. The tension assembly  70  further includes a spring member  146  that is connected between the handle  142  and the trigger  144  that biases the trigger  144  forward to its initial position. The fixation instrument  22  includes a stop member illustrated as a dowel  148  that extends laterally between the housing members  90   a  and  90   b . The front edge of the securement portion  89  of the trigger  84  abuts the dowel  148  so as to locate the trigger  144  in its first initial position under the force of the spring member  146 . 
     Operation of the fixation instrument will now be described with further reference to  FIGS. 8A-E . For instance, as illustrated in  FIGS. 7A and 8A , the trigger  84 , and thus the tension assembly  70 , is in a first initial position whereby the front end  117   a  of the traveler rod  111  is disposed adjacent and proximate to the nose  86 , such that the forward stop surface  126  of the lower grip member  112  abuts the complementary rear stop surface  127  of the nose  86 , thereby causing the gap  116  to define a transverse thickness greater than that of the free end  35  of the strap  32 . The free end  35  is inserted into the gap  116  and the nose  86  is positioned so as to abut the locking mechanism  38 . The nose  86  thus provides a brace member that prevents the locking mechanism  38  from traveling rearward when the fixation instrument  22  applies a rearward force to the free end  35 . 
     When the tension assembly  70  is in the first initial position, the spring member  128  can be in an initial position that can be a neutral position, or the movable spring seat  132  can be disposed at a forward location with respect to the neutral position such that the initial position of the spring member  128  can be flexed, such as slightly extended, from its neutral position. For instance, the force of the spring member  146  that biases the trigger  84  forward can be greater than the force of the spring member  128  that biases the trigger  84  rearward when the movable spring seat  132  is displaced forward from its neutral position. When the spring member  128  is extended from its neutral position, the spring member can apply a rearward compressive force onto the end cap  129 , and thus the traveler rod  111 , which biases the lower grip member  112  forward against the nose  86 . It is appreciated that the torsion spring  124  that biases the lower grip member  112  forward against the nose  86  creates an equal and opposite force against the traveler rod  111  that biases the traveler rod rearwards. Thus, the forward force of the spring member  128  applied to the traveler rod  111  when the spring member  128  is slightly extended is greater than the rearward biasing force of the torsion spring  124  against the traveler rod  111 . Because the forward biasing force of the spring member  128  is greater than the rearward biasing force of the torsion spring  124  when the spring member  128  is slightly extended, the spring member  128  can actively maintaining the grip  96  in its disengaged position. 
     Alternatively, the movable spring seat  132 , and thus, the spring member  128 , can be in a neutral position when the tension assembly  70  is in the first initial position, such that the spring member  128  does not apply a forward biasing force or a rearward biasing force to the traveler rod  111 . As described above, the torsion spring  124  can apply a force to the traveler rod  111  that biases the traveler rod rearwards  111 . However, rearward movement of the traveler rod  111  would cause the rear end  130   b  of the spring member  128  to move away from the front end  130   a  of the spring member  128 , thereby causing the spring member  128  extend from its neutral position. The spring member  128  can have an spring constant that is greater than the spring constant of the torsion spring  124  such that the spring member  128  resists extension under the forces of the torsion spring  124 , and thereby passively maintains the lower grip member  112  in its disengaged position whereby the forward stop surface  126  rests against the complementary rear stop surface  127  of the nose  86  and the gap  116  is thus thicker in the transverse direction T than the free end  35  of the strap  32  that is received in the gap  116 . 
     Referring now to  FIGS. 4 and 8B , when the user applies a rearward directed force F to grip portion  85  of the trigger  84 , the trigger  85  and thus also the tension assembly  70  moves from the first initial position to the second grip position that causes the grip  96  to iterate from the disengaged position to the engaged position. In accordance with the illustrated embodiment, the rearward force applied to the grip portion  85  of the trigger  84  causes the trigger  84  to pivot rearward about the upper pivot location  142 , which causes the lower pivot location  140  to move rearward. It should be appreciated that the lower pivot location  140  moves rearward about an arc-shaped travel path about the upper pivot location  142 . Rearward movement of the lower pivot location  140  causes the force transfer arm  136  to likewise moves rearward, thereby transferring the force from the trigger to the movable spring seat  132  and the front end  130   a  of the spring member  128 . It should be appreciated that the force transfer arm  136  pivots relative to the trigger  84  about the lower pivot location  140  as the force transfer arm  136  moves rearward. 
     As the force transfer arm  136  moves rearward, the force transfer arm  136  further causes the movable spring seat  132  and the front end  130   a  of the spring member  128  to translate rearward along the travel rod  111  in the longitudinal direction L toward the rear end  130   b  of the spring member  128 , thereby flexing the spring member  128  from the first initial position. It should be appreciated that the force transfer arm  136  pivots relative to the movable spring seat about the pivot location  139 . As the front end  130   a  of the spring member  128  moves rearward toward the rear end  130   b  of the spring member  128 , the spring member  128  flexes from its neutral position so as to apply a rearward biasing force to the traveler rod  111 . In accordance with the illustrated embodiment, the spring member  128  compresses when the end  130   a  of the spring member  128  moves rearward, and thus biases the end  130   b , the end cap  129 , and also therefore the traveler rod  111  to move rearward. Alternatively, the movement of the end  130   a  of the spring member  128  can be sufficient so as to reduce the force of the spring  128  that biases the traveler rod  111  to a level that is less than the force of the torsion spring  124  that biases the traveler rod  111  rearward. The fixation instrument  22  can define an aperture  119  that extends through the rear end  78   b  of the body  76 . The aperture  119  is sized to receive the end cap  129  such that the end cap  129  can extend beyond the housing  88  due to rearward travel of the rear end  130   b  of the spring member  128 . 
     Accordingly, referring also to  FIGS. 7A-B , the traveler rod  111  is biased, under a biasing force applied by the torsion spring  124 , the spring member  128 , or both to move rearward away from the nose  86 . As the rod  111  moves away from the nose  86 , the torsion force of the torsion spring  124  causes the lower grip member  112  to pivot forward about the pivot location  113  along the direction of Arrow B so as to reduce the gap  116  until the grip surfaces  118  and  115  capture the free end  35  of the strap  32  in the gap  116 , thereby securing the free end  35  to the fixation instrument  22  while the nose  86  abuts the loop  55 , and in particular the housing  50  of the locking mechanism  38 . Thus, it can be said that moving the trigger  84  from the first neutral position to the second neutral position causes the grip  96  to iterate from its disengaged position to its engaged position 
     Referring now to  FIGS. 4, 6, and 8C , continuous application of the force F to the grip portion  85  of the trigger  84  causes the trigger  84  to further pivot about the upper pivot location  142 , thereby causing the trigger  84 , and thus the tension assembly  70 , to move from the second grip position to a third tension position that causes the traveler  98  to move rearward, thereby applying tension to the free end  35  of the strap  32  when the tension in the strap  32  is less than the desired tension, such as a maximum tension as determined by the tension limiter  102 . Because engagement of the nose  86  of the fixation instrument  22  with the loop  55 , and in particular the housing  50  of the locking mechanism  38 , provides a brace that prevents the loop  55  from moving rearward along with the free end  35 , when the tension assembly  70  places the free end  35  in tension, the strap  32  and in particular the locking region  42  is further pulled through the locking mechanism  38 . As described above with reference to  FIG. 2B , the locking teeth  58  and  48  engage so as to allow the locking region  42  to be pulled through the locking mechanism so as to reduce the size of the loop  55  or to place the loop  55  in further tension about the bone segments that are disposed in the loop  55 . Whether the size of the loop  55  is reduced or whether the loop  55  is further tightened about the bone segments disposed in the loop, it can be said that the fixation instrument places the bone fixation member  24  in tension when the tension assembly moves from the second grip position to the third tension position. 
     Accordingly, once a trigger stroke has been completed whereby the trigger  84  has been moved to its rearward-most position as illustrated in  FIG. 8C , the user can release the trigger  84  which causes the spring member  146  to bias the trigger  84  to its forward position shown in  FIG. 8D . Once the trigger  84  has moved to its forward position, the trigger  84  and thus the tension assembly  70  are in the first initial position, such that the lower grip member  112  is biased against the nose  86  thereby placing the grip  96  in its disengaged position as described above. Accordingly, while the free end  35  of the strap  32  is no longer secured to the fixation instrument  22 , the engagement of the locking teeth  58  and  48  prevent the increased tension induced in the loop  55  from allowing the free end  35  to move through the housing in a direction opposite Arrow A that would reduce the tension in the loop  55 . 
     It should be appreciated that the motion of the trigger  84  from the second grip position to the third tension position can be a continuation of the movement from the first initial position to the second grip position. Thus, the trigger  84  can move from the first initial position to the second grip position to the third tension position in a single fluid motion. 
     The nose  86  can again be placed in contact with the locking mechanism  38  and the free end  35  can be received in the gap  116  of the grip  96 , and the tension assembly  70  can be iterated from the first initial position to the second grip position to the third tension position as many times as desired, for instance until the tension induced in the loop  55  reaches a predetermined maximum tension. When the tension in the loop  55  reaches the maximum tension, the tension limiter  102  prevents the traveler  98  from moving rearward when the actuator  100  is moved to the tension position. 
     For instance, referring now to  FIGS. 4, 6, and 8E , it should be appreciated that the spring member  128  has a spring constant that is configured to apply a predetermined biasing force to the traveler rod  111  when the front end  130   a  of the spring member  128  has translated rearward a distance corresponding to full rearward actuation of the trigger  84 . Accordingly, so long as the tension in the loop  55  is less than the desired force as defined by the spring member  128  and length of travel of the front end  130   a  of the spring member  128 , compression of the spring member  128  will cause the force applied by the spring member  128  to move the traveler rod  111  rearwards. However, once the tension induced in the loop  55  reaches the desired force, compression of the spring member  128  due to rearward movement of the front end  130   a  will cause the spring member  128  to apply a rearward biasing force against the traveler rod  111  that is insufficient to overcome the force necessary to further tighten the loop  55 . 
     It should appreciated that the spring member  128  can be configured to apply a rearward biasing force against the traveler rod  111  that is greater than the tension induced in the loop  50  about the target bone  28 . For instance, the spring member  128  can apply a force that is sufficient to overcome both the tension of the loop  50  and the additional force that causes one or both of the locking teeth  48  and  52  to deflect as the teeth ride over each other when tightening the loop  55 . Accordingly, the tension limiter  102  can be configured apply a force that is greater than, but corresponds to, the tension in the loop  55  about the target bone. Thus, the maximum force applied by the tension limiter  102  can correspond to the maximum desired tension in the loop  55 , it being appreciated that once the teeth  48  and  52  ride over and past each other, the tension in the loop  55  can decrease somewhat as the teeth  48  and  52  interlock. In accordance with one embodiment, the tension limiter  102  can apply a maximum force as desired, for instance up to approximately 430 Newtons or any other force as desired that corresponds to a desired maximum tension in the loop  55  about the target bone  28 . As a result, once the desired maximum tension in the loop  55  has been induced about the target bone  28 , the force applied by the tension limiter  102  when the trigger  84  is fully actuated is insufficient to cause the teeth  48  and  58  to ride past each other and further tighten the loop  55 . Accordingly, once the maximum tension in the loop  55  has been induced about the target bone  28 , the force applied by the tension limiter  102  will be insufficient to cause the traveler rod  111  to translate rearward a sufficient distance that further tightens the loop  55  about the target bone  28 . 
     It should be appreciated that the spring member  128  can alternatively be configured as an extension spring. For example, in accordance with an alternative embodiment, the front end  130   a  can be connected to the traveler rod  111  and fixed with respect to the traveler rod  111 , and the rear end  130   b  can be movable relative to the traveler rod  111 . For instance, the rear end  130   b  can be coupled to the trigger  84  via the force transfer member  104  in the manner described above. The tensile force applied to the rear end  130   b  and corresponding rearward displacement of the rear end  130   b  can thus cause the spring force to bias the traveler rod  111  to move rearward, thereby inducing tension in the strap  32  in the manner described above. Thus, the initial position of the spring member  128  initial position that can be a neutral position as described above, or the initial position of the spring member  128  can be flexed, such as slightly compressed, from its neutral position. 
     Referring now to  FIGS. 4-5B , the fixation device  22  includes a cutter assembly  72  that includes a cutter arm  106  having a front end  107   a  and a rear end  107   b , and a movable cutter blade  108  carried by the cutter arm  106 , for instance at the front end  107   a . The cutter arm  106  can be sized and shaped as desired, and is constructed in accordance with the illustrated embodiment such that the front and rear ends  107   a  and  107   b  extend substantially longitudinally. The cutter arm  106  can also include an offset segment  107   c  that extends laterally outward along a forward direction from the rear end  107   b  toward the front  107   a . Thus, the front end  107   a  can be offset laterally outward with respect to the rear end  107   b . For instance, the rear end  107   b  can be disposed between the opposed sides  80  and the front end  107   a  can be coupled to one of the side  80  at a location laterally outward of the side  80 . The cutter arm  106  can further include an intermediate longitudinal segment  107   d  that extends forward from the offset segment  107   c  and is thus extends substantially parallel to the front and rear ends  107   a  and  107   b . The cutter arm can also include a transversely offset segment  107   e  that extends between the intermediate longitudinal segment  107   d  and the front end  107   a , and extends transversely down along a forward direction. Accordingly, the front end  107   a  can be transversely offset with respect to the rear end  107   b . The cutter arm  106  can further include a grip  150  at disposed at the rear end  107   b.    
     Referring also to  FIG. 6 , the cutter assembly  72  can include a seat  158  that receives the cutter arm  106  at an upper end of the body  76  when the cutter arm  106  is in its disengaged position. In accordance with the illustrated embodiment, the seat  158  is sized and shaped so as to receive the cutter arm  160 . Accordingly, when the cutter arm  106  is seated in the seat  158 , the cutter arm  106  can be said to be in a seated disengaged position. The seat  158  can further include at least one retainer member  162 , such as a pair of laterally opposed retainer members  162  that are spaced apart a distance slightly less than the lateral thickness of the cutter arm  106 . The retainer members  162  can be spring loaded to the position spaced apart a distance slightly less than the lateral thickness of the cutter arm  106 . For instance, the cutter arm  106  can define a recess  164  that is sized to receive the retainer members  162  when the cutter arm  106  is in its seated disengaged position. Accordingly, when moving the cutter arm  106  to and from its seated disengaged position, an engagement force along the direction of Arrow C can be applied to the cutter arm  106  that is sufficient to overcome the retention force of the retainer members  162 , which causes the retainer members  162  to pop out of the recess  164  and free the cutter arm  106  from the seat  158 . 
     Referring also to  FIGS. 7A-B , the cutter arm  106  can be pivotally attached to the body  76 , for instance at one of the sides  80 . In accordance with the illustrated embodiment, the cutter arm  106  is pivotally attached at its front end  107   a  to the nose  86  at a pivot location  152  that defines a lateral pivot axis. For instance, the cutter assembly  72  can include a blade support body  154  that is substantially cylindrical and extends through a side wall of the nose  86  and rotatable about its central axis so as to allow the cutter arm  106  to pivot between its engaged position and its disengaged position. When the cutter arm  106  is in its disengaged position, the cutter blade  108  is spaced above a complementary and substantially stationary cutter blade  87  of the nose  86  so as to define a gap  156  that has a transverse dimension greater than that of the free end  35  of the strap  32  so that the free end  35  is loosely received in the gap  156 , which is disposed forward with respect to the grip  96 , and thus the gap  116 . 
     Referring again to  FIGS. 5A-B , the cutter arm  106  can be pivoted up and forward about the pivot location  152  along the direction of Arrow C to its engaged position, which causes the cutter blade  108  to travel down along an arc-shaped path toward the complementary cutter blade  87  so as to reduce the size of the gap  156 . The cutter blade  108  is recessed rearward with respect to the complementary cutter blade  87  such that the cutter blade  108  slides past the cutter blade  87 . Accordingly, the cutter blades  108  and  87  cooperate so as to cut the free end  35  of the strap  102  that is received in the gap  156  as the cutter blade  108  slides past the cutter blade  87  (see  FIG. 9 ). Accordingly, when the nose  86  abuts the locking mechanism  38  of the bone fixation member  24  such that the gap  156  is aligned with the outlet end of the strap-receiving slot  52  of the housing  50  of the locking mechanism (see  FIGS. 2B and 3C ) and the arm  106  is moved to its engaged position, the cutter assembly  72  can cut the free end  35  at a location that is spaced from the housing  50  a distance substantially equal to the distance between the front end of the nose  86  and the cutter blade  108 . Therefore, once the fixation instrument  22  has applied the desired amount of tension in the strap  32 , the free end  35  can be cut in the manner described above to substantially remove the free end  35  from the loop  55 . 
     Referring now to  FIGS. 4 and 6 , the cutter assembly  72  further includes a safety mechanism  110  that moves from a disengaged position, whereby the cutter arm  106  can move from the seated disengaged position toward the engaged position, and an engaged position that prevents the cutter arm  106  from moving from the seated disengaged position toward the engaged position. In accordance with the illustrated embodiment, the cutter assembly  72  includes a first engagement member  166  that extends down from the cutter arm  106  and a second engagement member  168  that extends up from the movable spring seat  132 . When the tension assembly  70  is in the first initial position, the first engagement member  166  is spaced rearward from the movable spring seat  132 . Otherwise stated, the first engagement member  166  is spaced from the movable spring seat  132  in the direction that the spring seat  132  moves when the tension assembly  70  is moved from the first initial position toward the third tension position. Accordingly, second engagement member  168  can move toward the first engagement member as the movable spring seat  132  moves rearward. Alternatively, the second engagement member  168  can extend from the traveler rod  111  and movable along with the traveler rod  111  so as to interlock with the engagement member  166 . 
     The first and second engagement members  166  and  168  are configured so as to interlock when they longitudinally overlap. For instance, the second engagement member  168  can be configured as a housing  170  having a slot  172  that receives the first engagement member  166 . The first engagement member  166  can define a flange  174  that overlaps the housing  170  in the transverse direction, such that the flange  174  abuts the housing  170  when an engagement force is applied to the cutter arm  106  along the direction of Arrow C so as to move the cutter arm  106  from its seated disengaged position toward its engaged position. In accordance with the illustrated embodiment, the first and second engagement members  166  and  168  interfere so as to prevent the cutter arm  106  from being unseated from the seat  158 . It should be appreciated that the first engagement members  166  and  168  can alternatively be sized and shaped as desired so as to selectively interfere with each other in the manner described herein. 
     Referring also to  FIG. 8A , when the tension assembly  70  is in the first initial position, the first and second engagement members  166  and  168  are longitudinally spaced apart such that the cutter arm  106  can be moved from the seated disengaged position to the engaged position along the direction of Arrow C. When the tension assembly  70  moves to the second grip position, the movable spring seat  32  translates longitudinally rearward, thereby also translating the second engagement member  168  longitudinally rearward toward the first engagement member  166 . In accordance with the illustrated embodiment illustrated in  FIG. 8B , the engagement members  166  and  168  remain longitudinally spaced from each other when the tension assembly  70  is in the second grip position, thereby allowing the cutter arm  106  to become unseated and move from the disengaged position toward the engaged position. Accordingly, referring to  FIG. 9 , after the fixation instrument  22  has induced a desired amount of tension in the loop  55 , the tension assembly  70  can be moved to the grip position so as to capture the free end  35  in the gap  116  prior to cutting the free end  35  in the manner described above. Thus, as illustrated in  FIG. 3A , the plurality of bone fixation members  24  can be tightened about the target bone  28  by actuating the tension assembly  70  as described herein, and can be subsequently cut by actuating the cutter assembly  72  as described herein. 
     In accordance with the illustrated embodiment shown in  FIG. 8C , the engagement members  166  and  168  are positioned so as to overlap when the tension assembly  70  has moved from the second grip position toward the third tension position. Thus, once the tension assembly  70  has advanced beyond the second grip position, the engagement members  166  and  168  interfere when the engagement force is applied to the cutter arm  106  along the direction of Arrow C. Therefore, the safety mechanism  110  prevents the fixation instrument  22  from cutting the free end  35  while the tension assembly  70  is applying tension to the strap  32 . Alternatively, the engagement members  166  and  168  can be positioned so as to at least partially overlap and interlock when the tension assembly  70  is in the second grip position, thereby defining an interference that prevents the cutter arm  106  from becoming unseated and moving to the engagement position along the direction of Arrow C. 
     Although the disclosure has been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present disclosure is not intended to be limited to the particular embodiments described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, composition of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure.