Patent Publication Number: US-2021177469-A1

Title: Unilateral implant holders and related methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/843,618, filed on Dec. 15, 2017, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Unilateral implant holders and related methods are disclosed herein. 
     BACKGROUND 
     Fixation systems can be used in orthopedic surgery or neurosurgery to maintain a desired spatial relationship between multiple bones or bone fragments. For example, various conditions of the spine, such as fractures, deformities, and degenerative disorders, can be treated by attaching a spinal fixation system to one or more vertebrae. Such systems may include a spinal fixation element, such as a rod, that is coupled to the vertebrae by one or more bone anchors, such as screws or hooks. The fixation system can also include various other implants, such as connectors for attaching multiple rods to one another. Once installed, the fixation system holds the vertebrae in a desired position until healing or spinal fusion can occur, or for some other period of time. 
     There may be various difficulties in manipulating and handling implants at a surgical site, particularly in the case of minimally-invasive procedures or procedures that involve areas with narrow anatomical constraints, such as the cervical spine. Existing surgical pick-up tools, such as forceps and tweezers, may fail to provide sufficient clamping force to resist the multi-directional forces exerted on the implant as it is manipulated within the surgical site, making it difficult to position the implant and increasing the risk of dropping the implant. Insertion instruments that rigidly attach to the implant may have considerable bulk and can limit the degree or manner in which the implant can be manipulated, impede insertion of a rod or other component into the implant, or cause other challenges. 
     SUMMARY 
     Various embodiments of unilateral implant holders and related methods are disclosed herein. An exemplary unilateral implant holder can include a surgical instrument that includes a unilateral locking mechanism arranged at a distal end of the instrument for rigidly holding an implant, such as a rod-to-rod connector or bone anchor. The locking mechanism can be configured to lock onto one end of an implant (i.e., a unilateral portion). For example, the locking mechanism can be configured to lock onto a unilateral portion of the implant such that access to an open recess or slot (e.g., for receiving a rod or a set screw) is not blocked. The locking mechanism can be configured to lock onto the unilateral portion of the implant by engaging a counterpart locking interface defined therein. By engaging the instrument&#39;s locking mechanism with the implant&#39;s counterpart locking interface, sufficient clamping force can be applied by the locking mechanism to resist multi-directional forces exerted on the implant during a surgical procedure. In some embodiments, the surgical instrument can be a stand-alone unilateral implant holder. In some embodiments, the surgical instrument can be configured to perform a surgical task while concurrently holding the implant in place using a unilateral locking mechanism. For example, the surgical instrument can include or be used with a rod reducer, a set screw inserter, or the like. 
     In some embodiments, a surgical instrument configured to unilaterally hold an implant can include a handle and a unilateral locking mechanism. The unilateral locking mechanism can include an elongated body having a proximal end coupled to the handle and a distal end defining multiple locking elements configured to engage a unilateral portion of the implant. The locking elements can include a proximal facing bearing surface, a distal-facing bearing surface, and a lateral-facing bearing surface configured to engage counterpart surfaces of the unilateral portion of the implant. The proximal-facing bearing surface can be formed on a clasp movable upwards to engage the implant. The locking elements of the unilateral locking mechanism can be configured to engage the unilateral portion of the implant such that the locking elements are laterally offset from a proximal-distal axis of an open recess of the implant. The body of the unilateral locking mechanism can define a pair of spaced apart arms forming an implant-receiving pocket therebetween. 
     The distal-facing bearing surface of the unilateral locking mechanism can extend transversely between the pair of spaced apart arms. The distal-facing bearing surface can be configured to contact a counterpart proximal-facing bearing surface of the unilateral portion of the implant and to thereby constrain longitudinal movement of the implant in a proximal direction. The distal-facing bearing surface can be formed on a stop beam. 
     The lateral-facing bearing surface of the unilateral locking mechanism can protrude longitudinally along at least one of opposing faces of the pair of spaced apart arms at or adjacent to a front of the pocket. The lateral-facing bearing surface can be configured to mate and slide along a lateral-facing counterpart groove formed in the unilateral portion of the implant and to thereby constrain lateral movements of the implant. The lateral-facing bearing surface can be formed on an insertion tab. 
     The proximal-facing bearing surface of the clasp can be disposed between opposing faces of the pair of spaced apart arms at or adjacent to a back of the pocket. The proximal-facing bearing surface can be configured to interlock with a counterpart distal-facing bearing surface of the unilateral portion of the implant and to thereby constrain longitudinal movement of the implant in a distal direction. The proximal-facing bearing surface can be formed on a lateral protrusion of the clasp. 
     The surgical instrument can include a control shaft having a proximal end moveably coupled to the handle and a distal end coupled to the clasp. The control shaft can be configured to move longitudinally in a proximal direction and to thereby move the clasp into a locked configuration in which the proximal-facing bearing surface of the lateral protrusion engages the counterpart distal-facing bearing surface of the unilateral portion of the implant. The control shaft can be configured to move longitudinally in a distal direction and to thereby move the clasp into an unlocked configuration in which the proximal-facing bearing surface of the lateral protrusion releases the counterpart distal-facing bearing surface of the unilateral portion of the implant. The clasp can be movable upward and inward to lock the surgical instrument onto the unilateral portion of the implant and can be movable downward and outward to unlock the surgical instrument from the unilateral portion of the implant. The control shaft can be configured to move longitudinally in a proximal direction and thereby move the clasp upward to engage the unilateral portion of the implant and lock the surgical instrument onto the implant. 
     The surgical instrument can include a pin and slot interface defined between the control shaft and the body of the unilateral locking mechanism to guide movements of the clasp between the locked configuration and unlocked configuration. A spring element can be defined in a surface of the handle portion and configured to exert a force against the second control shaft so that the pin slides along an edge of the slot interface. The elongated body of the unilateral locking mechanism can include one or more body segments angled to offset a longitudinal axis of the handle from a proximal-distal axis of an open recess defined in a body of the implant. 
     In some embodiments, the surgical instrument can include a clasp guide. The clasp guide can include at least a pair of clasp guide structures protruding between opposing faces of the pair of spaced apart arms at or adjacent to a back of the pocket. The pair of clasp guide structures can have proximal-facing ramped bearing surfaces configured to urge the clasp upward and inward towards the locked configuration in response to proximal movements of the control shaft. The pair of clasp guide structures can have distal-facing ramped bearing surfaces configured to guide the clasp downward and outward away from the locked configuration in response to distal movements of the control shaft. 
     The clasp can have a narrow body region formed between a proximal portion and a distal portion of the clasp and configured to pass through a spatial region formed between opposing faces of the pair of clasp guide structures. The clasp can include at least a pair of counterpart distal-facing ramped bearing surfaces extending laterally at a proximal end of the narrow body region. The counterpart distal-facing ramped bearing surfaces of the clasp can be configured to slide against the proximal-facing ramped bearing surfaces of the clasp guide structures. The clasp can include at least a pair of counterpart proximal-facing ramped bearing surfaces extending laterally at a distal end of the narrow body region. The counterpart proximal-facing ramped bearing surfaces of the clasp can be configured to slide against the distal-facing ramped bearing surfaces of the clasp guide structures. 
     In some embodiments, the surgical instrument can include a clasp guide. The clasp guide can include a proximal-facing ramped bearing surface, at least a pair of distal-facing ramped bearing surfaces, and a lateral-facing vertical bearing surface for guiding lateral movements and longitudinal movements of the clasp between the locked configuration and the unlocked configuration. 
     The proximal-facing ramped bearing surface can be formed on a lateral beam that extends transversely between the distal ends of the pair of spaced apart arms. The proximal-facing ramped bearing surface can be obliquely angled to engage a counterpart distal-facing ramped bearing surface of the clasp in response to distal movements of the control shaft and thereby guide lateral movements of the clasp outward away from the locked configuration. 
     The pair of distal-facing ramped bearing surfaces can be formed on distal ends of laterally opposing guide rails that extend longitudinally along the pair of spaced apart arms. The pair of distal-facing ramped bearing surfaces can be obliquely angled to engage a pair of counterpart proximal-facing ramped bearing surfaces of the clasp in response to proximal movements of the control shaft and thereby guide lateral movements of the clasp inward towards the locked configuration. 
     The lateral-facing vertical bearing surface can extend transversely between laterally opposing guide rails that extend longitudinally along the pair of spaced apart arms. The vertical bearing surface can be configured to engage a counterpart vertical bearing surface of the clasp and thereby guide longitudinal movements of the clasp between the locked and unlocked configurations in response to corresponding movements of the control shaft. 
     In some embodiments, the distal-facing bearing surface and the lateral-facing bearing surface can be formed on one or more prongs extending from the distal end of the elongated body of the unilateral locking mechanism. The lateral-facing bearing surface of each prong can slide along a lateral-facing counterpart groove formed in the unilateral portion of the implant to constrain lateral movements of the implant. The distal-facing bearing surface of each prong can contact a distal edge of the lateral-facing counterpart groove of the implant to constrain longitudinal movements of the implant in a proximal direction. 
     The surgical instrument can include a pocket formed in the distal end of the elongated body of the unilateral locking mechanism. The pocket can have a size and shape that accommodates the unilateral portion of the implant. The proximal-facing bearing surface of the clasp can be disposed through an opening at the back of the pocket. The proximal-facing bearing surface of the clasp can move upwards to interlock with a counterpart distal-facing bearing surface of the unilateral portion of the implant to constrain longitudinal movement of the implant in a distal direction. The size and the shape of the pocket can be configured to further accommodate a reduction tab proximally extending from the unilateral portion of the implant. The size and the shape of the pocket can be configured to further accommodate a distal head of an auxiliary instrument. The distal head of the auxiliary instrument can be a bell-shaped head of a nut driver. The surgical instrument can include a counter-torque lever extending substantially perpendicular to the handle. 
     In some embodiments, a surgical instrument configured to unilaterally hold an implant can include a handle and a unilateral locking mechanism. The unilateral locking mechanism can include an elongated body having a proximal end coupled to the handle and a distal end defining multiple locking elements configured to engage a unilateral portion of the implant. The unilateral locking mechanism can include a tubular locking shaft, a partial tubular locking shaft segment, a clasp, and a locking sleeve. The partial tubular locking shaft segment can be formed at a distal end of the tubular locking shaft. A window can be formed in the partial tubular locking shaft segment exposing an implant-receiving pocket. The clasp can have a proximal clasp portion fixedly attached to an outer surface of the partial tubular shaft segment and a free distal clasp portion aligned with the window. The locking sleeve can be configured to slide longitudinally over the tubular locking shaft and the partial tubular shaft segment. 
     The free distal clasp portion can enter the window towards the implant-receiving pocket in response to the locking sleeve sliding distally over the free distal clasp portion. The free distal clasp portion can exit the window away from the implant-receiving pocket in response to the locking sleeve sliding proximally away from the free distal clasp portion. The free distal clasp portion can define a surface protrusion configured to engage a counterpart groove formed in the unilateral portion of the implant. The surface protrusion of the free distal clasp portion can be configured to engage the counterpart groove of the implant in response to entering the window towards the implant-receiving pocket. The surface protrusion can be configured to disengage the counterpart groove of the implant in response to the free distal clasp portion exiting the window away from the implant-receiving pocket. 
     The surgical instrument can include an auxiliary instrument configured to pass through the tubular locking shaft in alignment with a proximal-distal axis of the open recess formed in the implant. The auxiliary instrument can be at least one of a rod reducer and a set screw reducer. 
     In some embodiments, an implant can include an implant body defining a locking interface in a unilateral portion of the implant body. A proximal-distal axis of the locking interface can be laterally offset from a proximal-distal axis of an open recess formed in the implant. The locking interface can include a top surface of the unilateral portion of the implant, at least two spaced apart grooves formed in an outer surface of the unilateral portion of the implant, and a surface protrusion extending from the outer surface of the unilateral portion of the implant between the at least two spaced apart grooves. The implant can be a bone anchor or a rod-to-rod connector. 
     In some embodiments, a method of a securing an implant to a surgical instrument can include aligning a unilateral locking mechanism of the surgical instrument with a unilateral portion of the implant, inserting the unilateral locking mechanism onto the unilateral portion of the implant until a distal-facing bearing surface of the unilateral locking mechanism engages a counterpart proximal-facing bearing surface of the unilateral portion of the implant, and controlling movement of a clasp of the unilateral locking mechanism such that a bearing surface of the clasp engages a counterpart bearing surface of the unilateral portion of the implant. The method can further include inserting a rod into the implant while the implant is secured to the instrument. 
     Inserting the unilateral locking mechanism onto the unilateral portion of the implant can include sliding at least a pair of lateral-facing bearing surfaces along at least a pair of lateral-facing grooves defined along the unilateral portion of the implant. Where the distal-facing bearing surface of the unilateral locking mechanism includes a distal-facing bearing surface formed on one or more prongs extending from a distal end of the unilateral locking mechanism, inserting the unilateral locking mechanism onto the unilateral portion of the implant can include sliding the one or more prongs along one or more lateral-facing grooves defined along the unilateral portion of the implant until the distal-facing surface of the one or more prongs contacts a distal edge of the one or more lateral-facing grooves. 
     Controlling the movement of the clasp can include actuating a control shaft coupled to the clasp such that movements of the control shaft in a first direction cause a proximal-facing bearing surface of the clasp to engage a counterpart distal-facing bearing surface of the unilateral portion of the implant. Controlling the movement of the clasp can include sliding a locking sleeve over an outer portion of the clasp such that a surface protrusion formed on an inner portion of the clasp engages a counterpart groove formed in the unilateral portion of the implant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments, and together with the general description given above and the detailed description given below, serve to explain the features of the various embodiments. 
         FIGS. 1A and 1B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a first embodiment. 
         FIG. 2  is a schematic diagram illustrating various counterpart locking elements of a unilateral locking interface defined in a unilateral portion of an implant according to an embodiment. 
         FIGS. 3A and 3B  are schematic diagrams illustrating various locking elements of a unilateral locking mechanism according to the first embodiment. 
         FIGS. 4A through 4D  are schematic diagrams illustrating a locking operation of the unilateral locking mechanism according to the first embodiment. 
         FIGS. 5A and 5B  are schematic diagrams illustrating the operation of a control shaft  108  according to the first embodiment. 
         FIGS. 6A and 6B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a second embodiment. 
         FIG. 7  is a schematic diagram illustrating the clasp and clasp guide of the locking mechanism according to the second embodiment. 
         FIGS. 8A and 8B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a third embodiment. 
         FIG. 9  is a schematic diagram illustrating the clasp and clasp guide of the locking mechanism according to the third embodiment. 
         FIGS. 10A through 10D  are schematic diagrams illustrating an operation of the clasp and clasp guide of the locking mechanism according to the third embodiment. 
         FIG. 11A and 11B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument having a unilateral locking mechanism according to a fourth embodiment. 
         FIGS. 12A and 12B  are schematic diagrams illustrating components of the partial tubular locking shaft segment according to the fourth embodiment. 
         FIGS. 13A and 13B  are schematic diagrams illustrating a locking operation of the unilateral locking mechanism according to the fourth embodiment. 
         FIGS. 14A and 14B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a fifth embodiment. 
         FIG. 15  is a schematic diagram illustrating various counterpart locking elements of a unilateral locking interface defined in a unilateral portion of an implant according to an embodiment. 
         FIGS. 16A and 16B  are schematic diagrams illustrating front and perspective views of the various locking elements at the distal end of the unilateral locking mechanism according to the fifth embodiment. 
         FIGS. 17A and 17B  are schematic diagrams illustrating a locking operation between the unilateral locking mechanism according to the fifth embodiment. 
         FIGS. 18A and 18B  are schematic diagrams illustrating a surgical procedure that may be performed using the surgical instrument according to the fifth embodiment. 
         FIG. 19A through 19G  are schematic diagrams of various implants to which the various embodiments may be applied. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the claims. 
     Various embodiments are disclosed herein of a surgical instrument that includes a unilateral locking mechanism arranged at a distal end of the instrument for rigidly holding an implant, such as a rod-to-rod connector or bone anchor. The locking mechanism may be configured to lock onto one end of an implant (i.e., a unilateral portion). For example, the locking mechanism may be configured to lock onto a unilateral portion of the implant such that access to an open recess or slot (e.g., for receiving a rod or a set screw) is not blocked. The locking mechanism may be configured to lock onto the unilateral portion of the implant by engaging a counterpart locking interface defined therein. By engaging the instrument&#39;s locking mechanism with the implant&#39;s counterpart locking interface, sufficient clamping force may be applied by the locking mechanism to resist multi-directional forces exerted on the implant during a surgical procedure. In some embodiments, the surgical instrument may be a stand-alone unilateral implant holder. In some embodiments, the surgical instrument may be configured to perform a surgical task while concurrently holding the implant in place using a unilateral locking mechanism. For example, the surgical instrument may include or be used with a rod reducer, a set screw inserter, or the like. 
       FIGS. 1A and 1B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a first embodiment. As shown, the instrument  100  may include a proximal handle  102  and a distal unilateral locking mechanism  104 . The handle  102  may include at least one handle segment  106  configured to attach an auxiliary instrument (e.g., a rod reducer, a set screw reducer, or the like). The locking mechanism  104  may include an elongated body having a proximal end coupled to the handle  102  and a distal end defining multiple locking elements configured to engage a unilateral portion of an implant. The locking elements may be configured to engage the unilateral portion of the implant such that the locking elements are laterally offset from a proximal-distal axis of an open recess for receiving, e.g., a screw and/or a rod. At least one of the locking elements may include a clasp  110  extending from a control shaft  108 . The control shaft  108  may be moveably coupled to the handle  102  by a retaining clip  112  and to the locking mechanism  104  by a pin  114  and slot  116  interface. The control shaft  108  may be configured to move in response to rotation of a knob  118  or other type of actuation control, thereby causing the clasp  110  to lock or unlock the implant. The control shaft  108  may include a proximal shaft portion  120  threadably coupled to a distal shaft portion  122 . As described in more detail below, the locking elements may be configured to contact, mate, interlock, or otherwise engage counterpart locking elements defined in a unilateral locking interface of an implant. 
       FIG. 2  is a schematic diagram illustrating various counterpart locking elements of a unilateral locking interface defined in a unilateral portion of an implant according to an embodiment. Although the illustrated implant  200  is a rod-to-rod connector, the counterpart unilateral locking interface may be integrated into other types of connectors and bone anchors. As shown, the implant  200  may include one or more open recesses  202  formed in the body of the implant (e.g., for receiving a rod and/or a set screw). A unilateral portion of an implant may be a portion of the implant that is located towards one end of the implant. For example, the unilateral portion  205  may correspond to an end portion of the implant  200  opposing the open recesses  202 . By defining the locking interface towards one side of the open recesses  202 , the recesses may continue to be accessible (e.g., not blocked) after the instrument&#39;s locking mechanism  106  is engaged in a locked configuration. 
     As shown, the locking interface of the implant  200  may include a top or proximal-facing bearing surface  210 , laterally-facing grooves  220 , and a distal-facing bearing surface  230 . Each of these counterpart locking elements may be configured to contact, mate, interlock, or otherwise engage the locking elements of the instrument&#39;s locking mechanism  104 , thereby constraining movement of the implant in all directions. For example, as shown in  FIG. 2 , the top or proximal-facing bearing surface  210  may correspond to a top surface of the implant  200  which may partially surround the upper edge of an open recess  202 . The pair of laterally-facing grooves  220  may correspond to a pair of vertical grooves formed in a sidewall of the connector adjacent to the open recess  202 . The vertical grooves  220  may intersect with the recess  202 , as shown in  FIG. 2 , or may be spaced a distance apart from the recess. The distal-facing bearing surface  230  may be formed on a lateral locking protrusion  232  at the back surface of the implant and extend transversely between the pair of vertical grooves  220 . In some embodiments, the implant may also include a horizontal groove or notch (not shown) formed along a proximal end of the unilateral portion below the top or proximal-facing bearing surface. 
       FIGS. 3A and 3B  are schematic diagrams illustrating various locking elements of a unilateral locking mechanism according to the first embodiment. As shown, the locking mechanism  106  may include a pair of parallel arms  300   a  and  300   b  (collectively  300 ). The spacing and dimensions of the arms  300  may be configured to form an implant-receiving pocket  302  between opposing faces of the arms  300 . The pocket  302  may be configured to accommodate a width and a depth of the unilateral portion of the implant (e.g.,  205  of  FIG. 2 ). 
     The locking mechanism  106  may include a horizontal stop beam  306  that extends transversely between opposing faces of the arms  300 . A height of the stop beam  306  relative to the distal end of the arms  300  may be configured to accommodate, or at least partially accommodate, the height of the unilateral portion of the implant. The stop beam  306  may have a distal-facing bearing surface  308  configured to contact the top or proximal bearing surface of the implant&#39;s locking interface (e.g.,  210  of  FIG. 2 ), thereby constraining longitudinal movements of the implant in a proximal direction (e.g., upward movements). The stop beam  306  may have a shape that conforms to the shape of the top bearing surface of the implant&#39;s locking interface (e.g.,  210  of  FIG. 2 ). For example, where the implant&#39;s top bearing surface forms an outer edge of an open recess for receiving a rod or a set screw, the forward and distal faces of the stop beam  306  may be shaped such that the stop beam does not block or otherwise interfere with the open recess of the implant. 
     The locking mechanism  106  may include a pair of opposing insertion tabs  310   a  and  310   b  (collectively  310 ) that protrude longitudinally along opposing faces of the arms  300  at or adjacent to the front of the pocket  302 . The insertion tabs  310  may have lateral-facing bearing surfaces configured to mate and slide along lateral-facing counterpart grooves formed in the unilateral portion of the implant (e.g., grooves  220  of  FIG. 2 ), thereby constraining lateral movements of the implant (e.g., side-to-side and front-to-back movements). 
     The locking mechanism  106  may include a retractable clasp  314  disposed between the opposing faces of the arms  300  at or adjacent to the back of the pocket  302 . The clasp  314  may include a generally rectangular-shaped body forming a lateral locking protrusion  318  that extends along a width of the distal end of the clasp  314 . A proximal-facing bearing surface may be formed on the lateral locking protrusion  318  and configured to interlock with a counterpart distal-facing bearing surface formed in the unilateral portion of the implant (e.g.,  230  of  FIG. 2 ), thereby constraining longitudinal movements of the implant in a distal direction (e.g., downward movements). 
     The clasp  314  may be configured to move upward and inward towards a locked configuration in which the clasp&#39;s proximal-facing bearing surface  316  is forced against the distal-facing bearing surface of the implant&#39;s locking interface (e.g.,  230  of  FIG. 2 ). The clasp  314  may be configured to move downward and outward away from the locked configuration towards an unlocked configuration in which the clasp&#39;s proximal-facing bearing surface  316  is disengaged from the distal-facing bearing surface of the implant&#39;s locking interface. 
       FIGS. 4A through 4D  are schematic diagrams illustrating a locking operation between the unilateral locking mechanism according to the first embodiment. As shown in  FIG. 4A , the locking mechanism  104  may be initially positioned over a proximal end of an implant  200  such that the implant-receiving pocket  302  is aligned with the unilateral portion  205  of the implant. As the locking mechanism  104  is inserted distally towards the implant  200 , the insertion tabs  310  of the locking mechanism may slide longitudinally along the lateral-facing grooves  220  of the locking interface, thereby guiding the unilateral portion  205  of the implant  200  proximally into the pocket  302 . As shown in  FIG. 4B , the insertion tabs  310  may continue to slide along the grooves until the distal-facing bearing surface  308  of the stop beam  306  contacts or abuts the top or proximal bearing surface  210  of the implant&#39;s locking interface. 
     Once the implant&#39;s top bearing surface  210  contacts or abuts the stop beam  306 , the clasp  314  may be engaged to lock the implant in place. As shown in  FIGS. 4C and 4D , a pin  114  and slot  116  interface may be used to control the movement of the clasp  314  from an unlocked configuration to a locked configuration relative to the implant. A pin  114  may be configured to protrude from at least one of the opposing faces of the arms  300  through a slot  116  formed in the control shaft  108 . The slot  116  may include a ramped portion that extends obliquely with respect to the longitudinal axis of the clasp  314 . Accordingly, movement of the pin  114  along the slot  116  can be effective to move or flex the clasp  314  radially-inward or radially-outward relative to the arms  300 . Other configurations of the pin and slot interface may be employed to control the movement of the clasp  314 . 
     As shown in  FIG. 4C , the clasp  314  may start from an unlocked configuration in which the clasp&#39;s proximal-facing bearing surface  316  is disengaged from the implant  200  at a position located down and away relative to the implant in the pocket  302 . As the control shaft  108  moves in a proximal direction, the clasp  314  may traverse a proximal path guided by the geometry of the slot interface  116 . 
     As shown in  FIG. 4D , the clasp  314  may be pulled upward (e.g., proximally along the y-axis) and inward (e.g., laterally along the x-axis) towards a locked configuration, thereby forcing the clasp&#39;s proximal-facing bearing surface  316  against a distal-facing bearing surface  230  of the implant  200 . As previously discussed, the distal-facing bearing surface  230  may be formed on a lateral locking protrusion (e.g.,  232  of  FIG. 2 ). In this locked configuration, the implant  200  may be captured within the pocket  302  at one end (e.g., the unilateral portion  205  of  FIG. 2 ) and constrained from longitudinal movement (e.g., upward and downward movement along a y-axis) and lateral movements (e.g., side-to-side and forward-to-back movements in a x-z plane). To release the implant from the locking mechanism  104 , the clasp  314  may traverse a distal path guided by the geometry of the slot interface  116  towards the unlocked configuration in response to the control shaft  108  moving in a distal direction. 
       FIGS. 5A and 5B  are schematic diagrams illustrating the operation of a control shaft  108  according to the first embodiment. As shown, the control shaft may be configured to have a proximal shaft portion  120  disposed in the handle  102  and a distal shaft portion  122  disposed in the elongated body of the unilateral locking mechanism  104 . The elongated body of the unilateral locking mechanism  104  may be shaped to include one or more angular body segments  502   a  and  502   b  (collectively angular segments  502 ) such that the body segments  502  are angled to offset a longitudinal axis A 1  of the handle  102  from a proximal-distal axis A 2  of an open recess (e.g.,  202  of  FIG. 2 ) defined in the body of the implant (e.g.,  200  of  FIG. 2 ). 
     The proximal shaft portion  120  may have a knob  118  coupled to a proximal end and a threaded portion  502  at a distal end. The proximal shaft portion  120  may disposed within a hollow interior of the handle portion  102  by a retaining clip  112  and configured to rotate about the longitudinal axis of the handle portion in response to turning of the knob  118 . A threaded portion  504  of the proximal shaft portion  120  may be threadably coupled to a proximal nut  508  of the distal shaft portion  122 . 
     By threadably coupling the proximal and distal shaft portions, rotations of the proximal shaft portion  120  may induce translational movements of the distal shaft portion  122 . For example, rotations of the proximal shaft portion  120  in one direction may cause the distal shaft portion  122  to move proximally away from the locking mechanism  104 , such that the clasp  314  moves towards a locked configuration to engage the implant  200 . Conversely, rotations of the proximal shaft portion  120  in an opposite direction may cause the distal shaft portion  122  to move distally towards the locking mechanism  104 , such that the clasp  314  moves towards an unlocked configuration to disengage the implant  200 . 
     A pin  114  and slot  116  interface may be configured to guide the translational movements of the distal shaft portion  122  between the locked and unlocked configurations. The pin  114  may be biased against an edge of the slot  116  using a spring-loaded biasing element  510  formed in the handle portion  104  of the elongated body. The spring-loaded biasing element  510  may be configured to exert a force on a surface protrusion  512  of the distal shaft portion  122  (e.g., a tooth), such that the exerted force urges the pin  114  to slide along an edge of the slot  116  during translation of the distal shaft portion  122 . 
       FIGS. 6A and 6B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a second embodiment. As shown, the instrument  600  may include a proximal handle  102  and a distal unilateral locking mechanism  602 . The handle  102  may include at least one handle segment  106  configured to attach an auxiliary instrument (e.g., a rod reducer, a set screw reducer, or the like). The locking mechanism  602  may include an elongated body having a proximal end coupled to the handle  102  and a distal end defining multiple locking elements configured to engage a unilateral portion of an implant. 
     With reference to  FIGS. 1A through 6B , the locking elements of the locking mechanism  602  may have a structure and operation similar to the structure and operation of the instrument  100 . However, as discussed in more detail below, the locking mechanism  602  may include a clasp  610  and a clasp guide  620  configured with various ramped bearing surfaces. The ramped bearing surfaces of the clasp  610  and the clasp guide  620  may be employed to alleviate the need for a pin and slot interface (e.g.,  114  and  116 ) to control the movements of the clasp between the locked and unlocked configurations. The clasp  610  may extend from a control shaft  108  and the control shaft may be moveably coupled to the handle  102  by a retaining clip  112 . The control shaft  108  may be configured to move in response to rotation of a knob  118  or other type of actuation control, thereby causing the clasp  610  to lock or unlock the implant. The control shaft  108  may include a proximal shaft portion  120  threadably coupled to a distal shaft portion  122 . 
       FIG. 7  is a schematic diagram illustrating the clasp and clasp guide of the locking mechanism according to the second embodiment. As shown, the clasp guide  620  may include a pair of clasp guide structures  702   a  and  702   b  (collectively  702 ) protruding between the opposing faces of the arms  300  at or adjacent to the back of the pocket  302 . A narrow spatial region  704  may be formed between the opposing faces of the guide structures. The clasp guide structures  702  may be configured to have a cross-sectional shape of a rhombus or other parallelogram and oriented to form a pair of proximal-facing ramped bearing surfaces  708   a  and  708   b  (collectively  708 ) and a pair of distal-facing ramped bearing surfaces  710   a  and  710   b  (collectively  710 ). The distal-facing ramped bearing surfaces  710  may be obliquely angled to guide the clasp  610  upward and inward towards the locked configuration in response to proximal movements of the control shaft. The proximal-facing ramped bearing surfaces  708  may be obliquely angled to guide the clasp  610  downward and outward away from the locked configuration. 
     The clasp  610  may have a substantially rectangular body coupled to a control shaft  108  at a proximal end and a lateral locking protrusion  714  having a proximal-facing bearing surface  716  configured to engage the implant formed along the width of the distal end. The clasp  610  may further include a counterpart guide structure formed between the proximal and distal ends of the clasp. As shown, the counterpart guide structure may include a narrow body region  718  formed in the body of the clasp  610  between a proximal portion and a distal portion of the clasp. The narrow body region  718  may be configured to have a reduced width less than respective widths of the proximal portion and the distal portion of the clasp. The reduced width of the narrow body region  718  may be configured to pass through the width of the narrow spatial region  704  formed between the opposing faces of the guide structures  702 . The counterpart guide structure may further include a pair of counterpart distal-facing ramped bearing surfaces  720   a  and  720   b  (collectively  720 ) extending laterally at a proximal end of the narrow body region  718  and a pair of counterpart proximal-facing ramped bearing surfaces  722   a  and  722   b  (collectively  722 ) extending laterally at a distal end of the narrow body region  718 . 
     In operation, the clasp  610  may start from an unlocked configuration in which the clasp&#39;s proximal-facing bearing surface  716  is disengaged from the implant at a position located down and away relative to the implant in the pocket. As the control shaft  108  is moved in a proximal direction, the clasp  610  may be pulled upward such that the clasp&#39;s counterpart proximal-facing ramped bearing surfaces  722  begin to slide against the distal-facing ramped bearing surfaces  710  of the guide structures  702 . The clasp  610  may continue to slide along the guide structures  702  until reaching the locked configuration at which the clasp&#39;s proximal-facing bearing surface  716  of the lateral locking protrusion  714  is forced against a distal-facing bearing surface of the implant (e.g.,  230  of  FIG. 2 ). The distal-facing ramped bearing surfaces  710  of the guide structures may be configured at an oblique angle, such that the clasp  610  may be pulled upward (e.g., proximally along the y-axis) and inward (e.g., laterally along the x-axis) towards the locked configuration. 
     As the control shaft  108  is moved a distal direction, the clasp  610  may be pushed downward, such that the clasp&#39;s counterpart distal-facing ramped bearing surfaces  720  begins to slide against the proximal-facing ramped bearing surfaces  708  of the guide structures  702 . The clasp  610  may continue to slide along the guide structures  702  until reaching the unlocked configuration at which the clasp&#39;s proximal-facing bearing surface  716  of the lateral locking protrusion  714  is released from a distal-facing bearing surface of the implant (e.g.,  230  of  FIG. 2 ). The proximal-facing ramped bearing surfaces  708  of the guide structures may be configured at an oblique angle, such that the clasp  610  may be slid downward (e.g., distally along the y-axis) and outward (e.g., laterally along the x-axis) away from the locked configuration. 
       FIGS. 8A and 8B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument that includes a unilateral locking mechanism according to a third embodiment. As shown, the instrument  800  may include a proximal handle  102  and a distal unilateral locking mechanism  802 . The locking mechanism  802  may include an elongated body having a proximal end coupled to the handle  102  and a distal end defining multiple locking elements configured to engage a unilateral portion of an implant. 
     With reference to  FIGS. 1A through 8B , the locking elements of the locking mechanism  802  may have a structure and operation similar to the structure and operation of the instrument  100 . However, as discussed in more detail below, the locking mechanism  802  may include a modified clasp  810  and a clasp guide  820  configured with various ramped and vertical bearing surfaces. The ramped and vertical bearing surfaces of the clasp  810  and the clasp guide  820  may be employed to alleviate the need for a pin and slot interface (e.g.,  114  and  116 ) to control the movements of the clasp between the locked and unlocked configurations. The clasp  810  may extend from a control shaft  108  and the control shaft may be moveably coupled to the handle  102 . The control shaft  108  may be configured to move in response to rotation of a knob  118  or other type of actuation control, thereby causing the clasp  810  to lock or unlock the implant. The control shaft  108  may include a proximal shaft portion  120  threadably coupled to a distal shaft portion  122 . 
       FIG. 9  is a schematic diagram illustrating the clasp  810  and clasp guide  820  of the locking mechanism  802  according to the third embodiment. As shown, the clasp guide  820  may include a proximal-facing ramped bearing surface  910 , at least a pair of distal-facing ramped bearing surfaces  920   a  and  920   b  (collectively  920 ), and a lateral-facing vertical bearing structure  930 . The guide&#39;s proximal-facing ramped bearing surface  910  may be formed on a lateral beam  912  that extends transversely between the distal ends of the arms  300 . The guide&#39;s proximal-facing ramped bearing surface  910  may be obliquely angled to guide lateral movement of the clasp  810  away from a locked configuration towards an unlocked configuration (e.g., outward along the x-axis) in response to distal movements of the control shaft  108 . 
     Proximal to the lateral beam  912 , the guide&#39;s distal-facing ramped bearing surfaces  920  may be formed on the distal ends of laterally opposing guide rails  922   a  and  922   b  (collectively  922 ) that extend longitudinally along the arms  300  of the locking mechanism towards a proximal end. The guide&#39;s distal-facing ramped bearing surfaces  922  may be obliquely angled to guide the lateral movement of the clasp  810  away from an unlocked configuration towards the locked configuration (e.g., inward along the x-axis) in response to proximal movements of the control shaft  108 . The guide&#39;s vertical bearing surface  930  may extend transversely between the opposing guide rails  922  forming an elongated recess. The guide&#39;s distal-facing vertical bearing surface  930  may be configured to guide longitudinal movements of the clasp  810  between the locked and unlocked configurations (e.g., along the y-axis) in response to proximal or distal movements of the control shaft  108 . 
     The clasp  810  may have an elongated body with a proximal end coupled to a control shaft  108  and a distal end forming a lateral locking protrusion  950 . The locking protrusion  950  of the clasp may have a proximally-facing bearing surface  952  formed thereon for engaging an implant (e.g.,  200  of  FIG. 2 ) in a locked configuration. The clasp  810  may be configured to have a counterpart guide structure that includes at least a pair of proximal-facing ramped bearing surfaces  954   a  and  954   b  (collectively  954 ), a distal-facing ramped bearing surface  956 , and a lateral-facing vertical bearing surface  958 . 
     The lateral-facing vertical bearing surface  958  of the clasp&#39;s guide structure may be formed on a substantially rectangular portion  960  that extends longitudinally between the proximal and distal ends of the clasp  810 . The proximal-facing ramped bearing surfaces  954  of the clasp&#39;s guide structure may be formed on a proximal end of the lateral locking protrusion  950  on opposite sides of the vertical bearing surface  958 . The distal-facing ramped bearing surface  956  of the clasp&#39;s guide structure may be formed along the width of the distal end of the lateral locking protrusion  950 . 
       FIGS. 10A and 10D  are schematic diagrams illustrating an operation of the clasp and clasp guide of the locking mechanism according to the third embodiment. The clasp  810  may initially start from an unlocked configuration in which the proximal-facing bearing surface  952  along the clasp&#39;s lateral locking protrusion  950  is disengaged from the implant  200  at a position located down and away relative to the implant  200 . As the control shaft  108  is moved in a proximal direction, the clasp  810  may be pulled upward such that the clasp&#39;s proximal-facing ramped bearing surfaces  954  may begin to slide against the guide&#39;s distal-facing ramped bearing surfaces  920  formed on at the distal ends of the guide rails  922 . The distal-facing ramped bearing surfaces  920  of the guide structures may be configured at an oblique angle, such that the clasp  810  may be pulled upward (e.g., proximally along the y-axis) and inward (e.g., laterally along the x-axis) towards the locked configuration. The clasp  810  may continue to slide along the guide&#39;s distal-facing ramped bearing surfaces  920  until the end of the clasp&#39;s proximal-facing ramped bearing surfaces  954  is reached. When the end of the clasp&#39;s proximal-facing ramped surface  954  is reached, the clasp&#39;s vertical bearing surface  958  may be flush or at least in contact with the guide&#39;s vertical bearing surface  930 . As the control shaft continues to move in the proximal direction, the clasp&#39;s vertical bearing surface  958  slides proximally along the guide&#39;s vertical bearing surface  930  until reaching the locked configuration at which the clasp&#39;s proximal-facing bearing surface  952  is forced against a distal-facing bearing surface of the implant (e.g.,  230  of  FIG. 2 ). In the locked configuration, the guide rails  922  can abut an outer back surface of the lateral locking protrusion  950  to prevent the protrusion  950  from moving laterally-outward away from the implant. 
     As the control shaft  108  is moved in a distal direction, the clasp  810  may be pushed downward, such that the clasp&#39;s counterpart distal-facing ramped bearing surface  956  begins to slide against the proximal-facing ramped bearing surface  910  of the lateral beam  912 . The clasp  810  may continue to slide along the lateral beam  912  until reaching the unlocked configuration at which the clasp&#39;s proximal-facing bearing surface  952  of the lateral locking protrusion  950  is released from a distal-facing bearing surface of the implant (e.g.,  230  of  FIG. 2 ). 
       FIG. 11A and 11B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument having a unilateral locking mechanism according to a fourth embodiment. As shown in the illustrated embodiment, a unilateral locking mechanism may integrated into an instrument that is configured to perform another surgical task, such that the surgical task may be performed concurrently while holding the implant in place. Although the instrument as shown is a rod reducer, the unilateral locking mechanism may be integrated into other types of surgical instruments, such as a screw driver, inserter, or the like. 
     The instrument  1100  may include a handle portion  1110 , a rod reducing element  1120 , and a unilateral locking mechanism  1130 . The unilateral locking mechanism  1130  may include a tubular locking shaft  1140 , a partial tubular locking shaft segment  1145 , a locking sleeve  1150 , a locking pin  1155 , a clasp  1160 , and a coupling mechanism  1170 . The handle portion  1110  may be threadably coupled to a proximal end of the rod reducing element  1120 , such that rotations of the handle portion  1110  may cause the rod reducing element  1120  to translate along a longitudinal axis defined therebetween. The handle portion  1110  may be further coupled to a proximal end of the tubular locking shaft  1140 , such that the rod reducing element  1120  may extend longitudinally through the hollow interior of the tubular locking shaft  1140 . A coupling mechanism  1170  of any type may be used to fix the handle portion  1110  to the proximal end of the tubular locking shaft  1140 . The tubular locking shaft  1140  may define a cutaway portion at a distal end to form the partial tubular locking shaft  1145 . The partial tubular locking shaft segment  1145  may include multiple locking elements, including the clasp  1160 . The multiple locking elements may be configured to contact, mate, interlock, or otherwise engage counterpart locking elements defined in a unilateral locking interface of an implant. For example, as discussed in more detail below, the locking sleeve  1150  may be configured to slide longitudinally over the outer surface of the tubular locking shaft  1140  to engage or disengage the clasp  1160  relative to a unilateral portion of an implant. A locking pin  1155  may be inserted at a distal end of the locking sleeve  1150  perpendicular to a longitudinal axis of the sleeve. Once inserted, the locking pin  1155  may extend transversely across an inner portion of the sleeve, such that the pin may exert a force against the clasp  1160  as the sleeve slides over it. 
       FIGS. 12A and 12B  are schematic diagrams illustrating components of the partial tubular locking shaft segment according to the fourth embodiment. As shown in the illustrated embodiment, the partial tubular locking shaft segment  1145  may have a structure and operation similar to the structure and operation of the instrument  100 . For example, the partial tubular locking shaft segment  1145  may define or otherwise have formed thereon a pair of arms  300  configured to form an implant-receiving pocket  302 , a stop beam  306  configured to prevent proximal movements of the implant (e.g., upward movements), and a pair of insertion tabs  310  configured to preventing lateral movements of the implant (e.g., side-to-side and front-to-back movements) as previously shown and described with reference to  FIGS. 3A, 3B, 4A and 4B . 
     The partial tubular locking shaft segment  1145  may further include a clasp  1160  having a proximal end portion  1202  and a distal end portion. The proximal end portion  1202  of the clasp  1160  may be fixedly attached to an outer surface of the partial tubular locking shaft segment  1145 . The distal end portion may be unattached to the locking shaft segment  1145 , forming a free clasp head  1204 . The free clasp head  1204  may be aligned with a window  1210  defined in the body of the locking shaft segment  1145  to expose the unilateral portion of an implant situated in the implant-receiving pocket (e.g., the pocket  302  of  FIG. 3A and 3B ). The free clasp head  1204  may have dimensions sufficient to fit through the window  1210 . The face  1206  of the free clasp head  1204  may define a lateral-facing surface protrusion  1208  configured to engage a counterpart locking element formed in the unilateral portion of the implant, such as a groove or notch. As discussed in more detail below, the free clasp head  1204  may be configured to enter the window  1210  towards the back of the implant-receiving pocket (e.g.,  302 ) in response to the locking sleeve  1150  sliding distally over the free clasp head  1204 , such that the locking pin  1155  may urge against the free clasp head  1204 . Conversely, the free clasp head  1204  may be configured to exit the window away from the implant-receiving pocket (e.g.,  302 ) in response to the locking sleeve sliding proximally away from the free clasp head  1204 . 
       FIGS. 13A and 13B  are schematic diagrams illustrating a locking operation of the unilateral locking mechanism according to the fourth embodiment. The locking sleeve  1150  may have a substantially tubular body having an inner diameter that is larger than an outer diameter of the tubular locking shaft  1140 , thereby enabling the  1150  sleeve to slide longitudinally along the outer surface of the shaft  1140 . As shown in  FIG. 13A , the locking sleeve  1150  may be initially positioned in an unlocked or released configuration towards a proximal end of the locking shaft  1140 . With the locking sleeve  1150  away from the free clasp head  1204 , the free clasp head  1204  may be laterally biased to a resting position aligned outside the window  1210 . In this unlocked configuration, the unilateral portion of an implant (e.g.,  205 ) may be received into the pocket  302  formed in the partial tubular locking shaft  1145  as previously shown and described with reference to  FIGS. 3A, 3B, 4A and 4B . 
     As shown in  FIG. 13B , once the implant is positioned within the pocket, the locking sleeve  1150  may be moved distally to engage the free clasp head with a counterpart locking element formed in the unilateral portion of the implant  205 , such as a groove or notch. For example, as the locking sleeve  1150  slides distally along the outer surface of the tubular locking shaft  1140 , the inner surface  1203  and locking pin  1155  may slide along and exert a force against the proximal clasp portion  1202  towards the free clasp head  1204 . As the sleeve  1150  reaches the free clasp head  1204 , the force exerted on the free clasp head  1204  may urge the head into the window  1210 , thereby enabling the lateral-facing surface protrusion  1208  on its face to engage a groove or notch formed in the unilateral portion of the implant (e.g.,  205 ) 
       FIGS. 14A and 14B  are schematic diagrams illustrating perspective and exploded views of a surgical instrument  1400  that includes a unilateral locking mechanism  1404  according to a fifth embodiment. As shown, the surgical instrument  1400  may include a proximal handle  1402  and a distal unilateral locking mechanism  1404 . The handle  1402  may include a fixed attachment bar or shaft  1406  extending substantially perpendicular or obliquely to the handle  1402  and configured for attaching to a counter-torque lever (not shown). The locking mechanism  1404  may include an elongated body having a proximal end coupled to the handle  1402  and a distal end defining multiple locking elements configured to engage a unilateral portion of an implant. 
     The locking elements may be configured to engage the unilateral portion of an implant such that the locking elements are laterally offset from a proximal-distal axis of an open recess for receiving, e.g., a screw, nut and/or a rod. At least one of the locking elements may include a clasp  1410  extending from a distal end of a control shaft  1408 . The control shaft  1408  may be configured to move in response to rotation of a knob  1418  or other type of actuation control, thereby causing the clasp  1410  to lock or unlock the implant. The control shaft  1408  may include a proximal shaft portion  1420  threadably coupled to a distal shaft portion  1422 . As described in more detail below, the locking elements may be configured to contact, mate, interlock, or otherwise engage counterpart locking elements defined in a unilateral locking interface of an implant. 
       FIG. 15  is a schematic diagram illustrating various counterpart locking elements of a unilateral locking interface defined in a unilateral portion of an implant  1500  according to an embodiment. Although the illustrated implant is a fixed U-U rod-to-rod connector  1500  having an extended reduction tab  1502 , the counterpart unilateral locking interface may be integrated into other types of connectors and bone anchors. As shown, the implant  1500  may include one or more open recesses  1504   a  and  1504   b  (collectively  1504 ) formed in the body of the implant (e.g., for receiving a rod, nut, and/or a set screw). A unilateral portion of an implant may be a portion of the implant that is located towards one end of the implant. For example, the unilateral portion  1510  may correspond to an end portion of the implant  1500  opposing the open recesses  1504 . As shown, the extended reduction tab  1502  may extend proximally from the unilateral portion  1510  of the implant. By defining the locking interface towards one side of the open recesses  1504 , the recesses may continue to be accessible (e.g., not blocked) after the instrument&#39;s locking mechanism is engaged in a locked configuration. 
     As shown, the locking interface of the rod-to-rod connector  1500  may include a vertical groove or slot  1520  having a lateral-facing bearing surface formed in one or more of the sidewalls of the unilateral portion of the implant  1510  below the extended reduction tab  1502 . The groove  1520  may also have a proximal-facing bearing surface formed on a distal edge  1525  of the groove or slot  1520 . The grooves  1520  may intersect with the recess  1504   a,  as shown in  FIG. 15 , or may be spaced a distance apart from the recess. The locking interface of the implant may also include a distal-facing bearing surface  1530  formed on a lateral locking protrusion  1532  at the back surface of the implant and extending transversely between the pair of vertical grooves  1520 . As discussed below, each of these counterpart locking elements may be configured to contact, mate, interlock, or otherwise engage the locking elements of the instrument&#39;s locking mechanism, thereby constraining movement of the implant in all directions. 
       FIGS. 16A and 16B  are schematic diagrams illustrating front and perspective views of the various locking elements at the distal end of the unilateral locking mechanism  1404  according to the fifth embodiment. As shown, the locking mechanism may include a pocket  1610  formed at the distal end of the elongated body  1600 . The pocket  1610  may be configured to have a size and shape that accommodates a width and depth of a unilateral portion of an implant (e.g.,  1510  of  FIG. 15 ). For example, the pocket  1610  may include an implant-receiving pocket  1612  having a concave or other suitable shape that extends about or encompasses a unilateral end portion of a rod-to-rod connector (e.g.,  1500 ) or other implant. The pocket  1610  may also include a tab-receiving pocket  1614  having a substantially rectangular or other suitable shape positioned above the implant-receiving pocket  1612  to accommodate a reduction tab extending proximally from the unilateral end portion of an implant. For example, the tab-receiving pocket  1614  may be a rectangular recess formed in the distal body  1600  above the implant-receiving pocket  1612  to accommodate the extended reduction tab  1502  of the fixed rod-to-rod connector  1500  of  FIG. 15 . The pocket  1610  may also include an instrument-receiving pocket  1616  having a size and shape configured to accommodate a distal head of an auxiliary instrument, such as a bell-shaped head of a nut driver. As shown, the pocket  1616  may have a substantially concave shape that expands distally towards the sidewalls of the body  1600  from a proximal tapered end of the pocket  1616 . The implant-receiving pocket  1612  and the extended tab-receiving pocket  1614  may be formed as recesses in the inner surface of the instrument-receiving pocket  1616 . 
     The locking mechanism may further include one or more prongs  1620   a  and  1620   b  (collectively or individually  1620 ) extending distally from the elongated body  1600 . As shown, a pair of prongs  1620  may extend distally from a pair of arms  1650   a  and  1650   b  (collectively or individually  1650 ). Each prong  1620  may have a lateral-facing bearing surface  1622  and a distal-facing bearing surface  1624 . The lateral-facing bearing surface  1622  of each prong  1620  may be configured to slide along a lateral-facing counterpart groove or slot formed in the sidewall of the unilateral portion of the implant (e.g.  1520  of  FIG. 15 ), thereby constraining lateral movements of the implant (e.g., side-to-side and front-to-back movements). The distal-facing bearing surface  1624  of each prong  1620  may be configured to contact a proximal-facing bearing surface of a distal edge of the groove or slot (e.g.,  1525  of  FIG. 15 ), thereby constraining longitudinal movements of the implant in a proximal direction (e.g., upward movements). Although the use of the prongs  1620  may avoid the need for a stop beam having a distal-facing bearing surface previously described with reference to  FIGS. 1A-13B  for constraining proximal movements of an implant, other embodiments may include both prongs and stop beams for constraining proximal movements of an implant. 
     The locking mechanism may further include a retractable clasp  1630  disposed between the opposing faces of the spaced apart arms  1650  and through an opening at the back of the implant-receiving pocket  1612 . The clasp  1630  may have a substantially rectangular body coupled at a proximal end to a control shaft (e.g.,  1408  of  FIG. 14B ). The clasp  1630  may include a lateral locking protrusion  1632  having a proximal-facing bearing surface  1634 . 
     The clasp  1630  may be configured to move upward and inward towards a locked configuration in which the clasp&#39;s proximal-facing bearing surface  1634  is forced against the distal-facing bearing surface of the implant&#39;s locking interface (e.g.,  1530  of  FIG. 15 ). The clasp  1630  may be further configured to move downward and outward away from the locked configuration towards an unlocked configuration in which the clasp&#39;s proximal-facing bearing surface  1634  is disengaged from the distal-facing bearing surface of the implant&#39;s locking interface. 
     As shown, the clasp  1630  may have a structure and operation similar to the clasp  610  shown and described with reference to  FIGS. 6A through 7 . For example, the clasp  1630  may include a narrow body region  1636  defined between a proximal end portion  1638  and a distal end portion  1640 . The height, width and/or shape of the narrow body region  1636  may be configured to facilitate movement of the clasp between a pair of clasp guide structures  1652   a  and  1652   b  (collectively or individually  1652 ) protruding between opposing faces of the spaced apart arms  1650 . Distal-facing bearing surfaces of the proximal end portion  1638  and proximal-facing bearing surfaces of the distal end portion  1640  may extend laterally from the narrow body region  1636  and be configured to slide against the clasp guide structures  1652  to guide the clasp  1630  in and out of a locked configuration with the implant. For example, the clasp guide structures  1652  may each have a cross-sectional shape of a rhombus or other parallelogram oriented to form distal-facing ramped bearing surfaces  1654  and proximal-facing ramped bearing surfaces  1656 . The distal-facing ramped bearing surfaces  1654  may be obliquely angled to guide the clasp  1630  upward and inward towards the locked configuration in response to proximal movements of the control shaft. The proximal-facing ramped bearing surfaces  1656  may be obliquely angled to guide the clasp  1630  downward and outward away from the locked configuration in response to distal movements of the control shaft. Alternatively, the clasp  1630  may be configured to have a structure and operation similar to the structure and operation of any of the embodiments shown and described with reference to  FIGS. 1A-13B . 
       FIGS. 17A and 17B  are schematic diagrams illustrating a locking operation between the unilateral locking mechanism according to the fifth embodiment. As shown in  FIG. 17A , the distal end of the locking mechanism  1600  may be initially positioned over a proximal end of an implant  1500  such that the implant-receiving pocket  1612  may be aligned with the unilateral portion of the implant  1510 . As the locking mechanism  1600  is inserted distally towards the implant  1500 , the extended reduction tab  1502  of the implant  1500  may be received in the tab-receiving pocket  1614 . As insertion of the locking mechanism  1600  continues, the prongs  1620  of the locking mechanism may slide longitudinally along the lateral-facing grooves or slots  1520  of the implant&#39;s locking interface, thereby guiding the unilateral portion  1510  of the implant  1500  proximally into the implant-receiving pocket  1612 . As shown in  FIG. 17B , the prongs  1620  may continue to slide along the grooves until the distal-facing bearing surface  1624  of the prongs  1620  contacts or abuts the proximal-facing bearing surface at the distal edge  1525  of the groove  1520 . In response to the prongs  1620  of the locking mechanism contacting the distal edge  1525  of the implant&#39;s lateral-facing groove or slot  1520 , further movement of the implant in the lateral and proximal directions is prevented. The clasp  1630  may be engaged to lock the implant in place as previously described with reference to  FIG. 7  in order to prevent movement of the implant in the distal direction. 
       FIGS. 18A and 18B  are schematic diagrams illustrating a surgical procedure that may be performed using the surgical instrument according to the fifth embodiment. In particular, the surgical instrument  1400  may be used to rigidly hold a rod-to-rod connector  1500  in place while concurrently performing a surgical procedure using an auxiliary instrument, such as a driver instrument  1800 . For example, the driver instrument  1800  may be used to tighten a locking nut or screw to the connector  1500 . To enable the driver instrument  1800  with direct access to the connector  1500 , the elongated body of the locking mechanism  1404  may include one or more angled body segments configured to laterally offset the handle portion  1402  away from a proximal-distal axis of the connector  1500 . While holding onto the handle portion  1402  of the surgical instrument  1400 , a surgeon can position the distal head  1802  of a driver instrument  1800  directly into engagement with the connector  1500  or a locking nut or screw thereof. To tighten a nut and/or screw to the connector  1500 , the driver instrument  1800  applies a torqueing force. To maintain the position of the connector  1500  while applying the torqueing force to the nut and/or screw, a counter-torque lever (not shown) may be attached to a fixed attachment bar or shaft  1406  extending substantially perpendicular or obliquely to the handle  1402 . The fixed attachment bar or shaft  1406  may include various coupling structures for connecting the counter-torque lever to the handle  1402 . The counter-torque lever may be any type of lever or handle known to one of ordinary skill in the art that can enable the surgeon to manually apply a counter-torqueing force opposite and substantially equal to the torqueing force applied using the driver instrument  1800 . For example, when the driver instrument  1800  is used to apply a clockwise torqueing force to the nut and/or screw, the surgeon may use the counter-torque lever attached to the fixed attachment bar or shaft  1406  to apply a counter-clockwise torqueing force to prevent the surgical instrument  1400 , and thus the connector  1500 , from twisting. 
       FIG. 18B  further illustrates the distal end of the locking mechanism  1600  having an instrument-receiving pocket  1616  that accommodates the size and shape of the distal head  1802  of the driver instrument  1800 , such that the distal head may freely rotate within the pocket  1616  while applying a torque to drive the nut and/or screw through the connector  1500 . As shown, the pocket  1616  may have a substantially concave shape that expands distally towards the sidewalls of the body  1600  from a proximal tapered end of the pocket  1616 . The implant-receiving pocket  1612  and the extended tab-receiving pocket  1614  may be formed as recesses in the inner surface of the instrument-receiving pocket  1616 . 
       FIG. 19A through 19G  are schematic diagrams of various implants to which the various embodiments may be applied. For example, the various embodiments of the instruments configured with a unilateral locking mechanism (e.g., instruments  100 ,  600 ,  800 ,  1100  and  1400 ) may be used to rigidly hold onto a unilateral portion of various types and configurations of bone anchors and/or connectors, such as without limitation a fixed rod-to-rod connector  1902 , an articulating rod-to-rod connector  1904 , a receiver head  1906  of a bone anchor, e.g., of a polyaxial bone screw, a rod-to-anchor connector  1908 , a side or bottom loading rod-to-rod connector  1910 , a bone or lamina hook  1912 , a multipoint implant  1914 , cross-connector adapters, open connectors, closed connectors, and so forth. Implants can include multiple counterpart locking elements or multiple sets of counterpart locking elements. For example, an implant having multiple arms, e.g., first and second opposed arms, can include a counterpart locking geometry (e.g., a set of counterpart locking elements) formed in any one or more of said arms. This can advantageously allow unilateral implant holders of the type described herein to be interchangeably attached to any one or more arms of the implant.  FIG. 19F  shows an exemplary implant having counterpart locking elements on each of first and second opposed arms of the implant. In some embodiments, the vertical groove locking elements of the implant can be open to or can intersect with a rod-slot of the implant, for example as shown in  FIGS. 19C, 19D, 19E, and 19F . In some embodiments, the vertical groove locking elements of the implant can be separate from and spaced a distance apart from a rod-slot of the implant, for example as shown in  FIGS. 19A and 19B . 
     Exemplary implants in which counterpart locking elements for use with the instruments described herein can be incorporated are disclosed in U.S. application Ser. No. 15/073,020 filed on Mar. 17, 2016 and entitled “MULTIPOINT FIXATION IMPLANTS” (now issued as U.S. Pat. No. 9,962,192); U.S. application Ser. No. 15/158,127 filed on May 18, 2016 and entitled “IMPLANT CONNECTORS AND RELATED METHODS” (now issued as U.S. Pat. No. 10,517,647); U.S. application Ser. No. 15/284,587 filed on Oct. 4, 2016 and entitled “IMPLANT CONNECTORS AND RELATED METHODS” (now issued as U.S. Pat. No. 10,321,939); U.S. application Ser. No. 15/377,449 filed on Dec. 13, 2016 and entitled “IMPLANT ADAPTERS AND RELATED METHODS” (now issued as U.S. Pat. No. 10,398,476); U.S. application Ser. No. 15/471,075 filed on Mar. 28, 2017 and entitled “ARTICULATING IMPLANT CONNECTORS AND RELATED METHODS” (now issued as U.S. Pat. No. 10,561,454); U.S. application Ser. No. 15/430,188 filed on Feb. 10, 2017 and entitled “TANDEM ROD CONNECTORS AND RELATED METHODS” (now issued as U.S. Pat. No. 10,238,432); and U.S. application Ser. No. 15/208,872 filed on Jul. 13, 2016 and entitled “BONE ANCHOR ASSEMBLIES AND RELATED INSTRUMENTATION” (now issued as U.S. Pat. No. 10,463,402): each of which is hereby incorporated by reference herein. 
     While the instruments and methods illustrated and described herein generally involve attaching to spinal implant hardware, it will be appreciated that the instruments and methods herein can be used with various other types of fixation or stabilization hardware, in any bone, in non-bone tissue, or in non-living or non-tissue objects. The implants disclosed herein can be fully implanted, or can be used as part of an external fixation or stabilization system. The devices and methods disclosed herein can be used in minimally-invasive surgery and/or open surgery. 
     The devices disclosed herein and the various component parts thereof can be constructed from any of a variety of known materials. Exemplary materials include those which are suitable for use in surgical applications, including metals such as stainless steel, titanium, or alloys thereof, polymers such as PEEK, ceramics, carbon fiber, and so forth. The various components of the devices disclosed herein can be rigid or flexible. One or more components or portions of the device can be formed from a radiopaque material to facilitate visualization under fluoroscopy and other imaging techniques, or from a radiolucent material so as not to interfere with visualization of other structures. Exemplary radiolucent materials include carbon fiber and high-strength polymers. 
     The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use what is described. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the scope of the present disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and features disclosed herein.