Patent Publication Number: US-11653953-B2

Title: Implant receivers and connectors with grip grooves for rod fixation

Description:
FIELD 
     Implant receivers and connectors with grip grooves for improved rod fixation are disclosed herein. 
     BACKGROUND 
     Fixation systems can be used in orthopedic surgery 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 typically include a spinal fixation element, such as a rigid or flexible rod or plate that is coupled to the vertebrae by attaching the element to various anchoring devices, such as screws, hooks, or wires. 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. 
     In screw and rod spinal fixation constructs, stability of the implanted construct is crucial in allowing the body to accomplish bony fusion through the operative levels. Instability of, or motion within an implanted construct can result in a pseudoarthrosis, a “non-union” where new bone formation fails to take over loads experienced by the screw and rod construct. Where there is too much motion allowed between instrumented vertebrae, growth activity is hindered; new bone cannot fuse between two bodies that are constantly moving relative to one another. To solve this problem and provide stable fixation, polyaxial screw-heads, rod to rod, and screw-head to rod connectors all need to hold securely to the rod. These implants must resist motion relative to the longitudinal rods in the construct; this includes the rod sliding, rotating, or pulling away from the connector or bone screw. 
     There are many instances in which it may be desirable to connect multiple implants to each other. For example, some revision surgeries involve extending a previously-installed construct to additional vertebral levels by coupling a newly-installed spinal rod to a previously-installed rod. In addition, with vertebral implants or constructs fixated in the cervical and thoracic regions of the spine, a rod-to-rod connector can be used to bridge the transition between the constructs or implants in the cervical and thoracic regions. In this example, and in other transition regions, torsional slip between the implants on the rod or rods connecting them to each other is a serious risk, which can be caused by routine and repetitive movements, for example the patient twisting their head. By way of further example, aspects of the patient&#39;s anatomy, the surgical technique used, or the desired correction may require that multiple spinal rods be connected to one another. As yet another example, coupling multiple rods to one another can improve the overall strength and stability of an implanted construct. 
     There can be various difficulties associated with connecting multiple implants to each other. The available space for the implanted construct can often be very limited, particularly in the cervical area of the spine. Also, manipulating and handling these relatively small implants in the surgical wound may be challenging or cumbersome for the surgeon. There is a continual need for improved implant connectors and related methods. 
     SUMMARY 
     Certain examples of the present disclosure include an implant with a body having a rod-receiving recess, where the body has first and second sides defining openings to the rod-receiving recess and the rod-receiving recess defines a central longitudinal rod axis extending between the openings of the first and second sides. At least a portion of the rod-receiving recess can be formed by an inner surface of the implant, with the inner surface defining two grip grooves extending parallel to each other and the central longitudinal rod axis. Each grip groove defines two edges where the grip groove intersects the inner surface, the four edges of the two grip grooves together defining a circular radius about the central longitudinal rod axis. The implant also includes a retaining member configured to move with respect to the body, exert a force against a rod in the rod-receiving recess that can be perpendicular to the central longitudinal rod axis, and engage the rod against the four edges of the two grip grooves. Additionally, engagement of the four edges of the grip grooves against the rod can restrain rotational movement of the rod about the central longitudinal rod axis. 
     In some examples, the rod-receiving recess defines a gap between the two grip grooves sized and positioned to allow the force against the rod in the rod-receiving recess to permit deflection of one or both of the edges and the rod where the edges engage the rod, the deflection causing movement of the rod into the gap. The inner surface of the rod-receiving recess between the two grip grooves can be positioned a distance away from the central longitudinal rod axis that is larger than a radius of the rod. 
     In some examples, the implant includes a compression member disposed in a cavity formed in the body, with the compression member having an inner surface defining at least a portion of the rod-receiving recess, with an inner surface of the compression member having the two grip grooves formed therein. In some examples, the grip grooves extend along an entire length of the inner surface of the rod-receiving recess in the direction of the central longitudinal rod axis. In some examples, the grip grooves are positioned opposite the retaining member with respect to the central longitudinal rod axis. 
     The rod-receiving recess can define an open end sized to accept the rod and a closed end sized to contact the rod, where the grip grooves are arranged symmetrically about an axis extending from the open end to the closed end. The body of the implant can define the inner surface forming the rod-receiving recess. In some examples, the intersection between the grip grooves and the inner surface defines sharp edges. 
     The inner surface can define a groove intersecting at least one grip groove, the intersection of the groove segmenting the edges of the at least one grip groove and defining four corners for resisting translation of the rod along the central longitudinal rod axis when the rod is engaged with the edges. In some examples, the groove intersecting at least one grip groove is oriented perpendicular to the grip grooves. 
     The grip grooves can be formed by protrusions extending from the inner surface. In some examples, at least one grip groove defines an inner surface having formed therein one or more protrusions, the one or more protrusions extending to edges arranged to contact the rod when the rod is engaged with the edges of the grip grooves. 
     In some examples, the implant includes a connector and the rod-receiving recess is a first rod-receiving recces, the body defining a second rod-receiving recesses, with one or both of the first and second rod-receiving recesses having the two grip grooves. The body has proximal and distal ends that define a proximal-distal axis extending therebetween, with the retaining member slidably disposed within a tunnel formed in the body and configured to translate with respect to the body along a rod pusher axis. 
     In some examples, the second rod-receiving recess is defined by a pair of spaced apart arms of the body. The first rod-receiving recess can be open in a distal direction and the second rod-receiving recess can be open in a proximal direction. The rod pusher axis can be substantially perpendicular to the proximal-distal axis. In some examples, the implant further includes a set screw threadably received in the body to lock a first rod within the first rod-receiving recess and to lock a second rod within the second rod-receiving recess. 
     The implant can include a bone anchor assembly, with the body having a receiver member of the bone anchor assembly and the retaining member having a set screw or locking element. 
     Another example of the present disclosure is an implant having a body having a rod-receiving recess, a grip insert configured to be positioned in an open end of the receiving recess, and a retaining member configured to move with respect to the body. The body has first and second sides defining openings to the rod-receiving recess and the rod-receiving recess defines a central longitudinal rod axis extending between the openings of the first and second sides. The grip insert has an inner surface for contacting a rod disposed in the rod-receiving recess, with the inner surface defining two grip grooves extending parallel to each other and the central longitudinal rod axis, where each grip groove defines two edges where the grip groove intersects the inner surface and the four edges of the two grip grooves together defining a circular radius about the central longitudinal rod axis. The retaining member can be configured to exert a force against a rod in the rod-receiving recess and engage the four edges of the grip against the rod, where the engagement of the four edges of the grip grooves against the rod serves to restrain rotational movement of the rod about the central longitudinal rod axis. 
     Any of the features or variations described above can be applied to any particular example of the present disclosure in a number of different combinations. The absence of explicit recitation of any particular combination is due solely to the avoidance of repetition in this summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a perspective view of a prior art bone anchor assembly; 
         FIG.  1 B  is an exploded view of the bone anchor assembly of  FIG.  1 A ; 
         FIG.  1 C  is a top view of the bone anchor assembly of  FIG.  1 A ; 
         FIG.  1 D  is a cross-sectional view of the bone anchor assembly of  FIG.  1 A ; 
         FIG.  2 A  is a perspective view of a prior art connector; 
         FIG.  2 B  is an exploded perspective view of the connector of  FIG.  2 A  shown with first and second spinal rods; 
         FIG.  2 C  is a sectional side view of the connector of  FIG.  2 A  in a first configuration; 
         FIG.  2 D  is a sectional top view of the connector of  FIG.  2 A  in the first configuration; 
         FIG.  2 E  is a sectional side view of the connector of  FIG.  2 A  in a second configuration; 
         FIG.  2 F  is a sectional top view of the connector of  FIG.  2 A  in the second configuration; 
         FIG.  2 G  is a sectional side view of the connector of  FIG.  2 A  in a third configuration; 
         FIG.  2 H  is a sectional top view of the connector of  FIG.  2 A  in the third configuration; 
         FIG.  2 I  is a side view of the connector of  FIG.  2 A  coupled to first and second spinal rods; 
         FIG.  2 J  is a perspective view of the connector of  FIG.  2 A  shown with a saddle; 
         FIG.  2 K  is an exploded perspective view of the connector and saddle of  FIG.  2 J  shown with first and second spinal rods; 
         FIG.  2 L  is a sectional side view of the connector and saddle of  FIG.  2 J  coupled to first and second spinal rods; 
         FIG.  3 A  is a cross-sectional view of one embodiment of a connector with a rod recess having two grip grooves; 
         FIG.  3 B  is a cross-sectional view of the rod-receiving recess of the connector of  FIG.  3 A ; 
         FIG.  3 C  is a cross-sectional view of the grip grooves of the rod-receiving recess of the connector of  FIG.  3 A ; 
         FIG.  4 A  is a cross-sectional view of a prior art rod-receiving recess; 
         FIG.  4 B  is a cross-sectional view of one embodiment of a rod-receiving recess having v-shaped grip grooves; 
         FIG.  4 C  is a cross-sectional view of the rod-receiving recess of  FIG.  4 B  showing a comparison of the position of a rod with and without the grip grooves; 
         FIG.  4 D  is a perspective view of a rod showing lines of contact made by the edges of the two grip grooves of  FIG.  4 B  when the rod is positioned in the rod-receiving recess of  FIG.  4 B ; 
         FIG.  4 E  is a perspective view of a surface of one embodiment of a rod-receiving recess with a segmented grip groove; 
         FIG.  4 F  is a top-down view of the segmented grip groove of  FIG.  4 E ; 
         FIG.  4 G  is an schematic illustration of the contact points on a rod engaged with the segmented grip groove of  FIG.  4 E ; 
         FIG.  4 H  is a perspective of a surface of a rod showing lines of contact made by the edges of the two segmented grip grooves of  FIG.  4 E  when the rod is positioned in the rod-receiving recess of  FIG.  4 E ; 
         FIG.  5 A  is a cross-sectional view of one embodiment of a rod-receiving recess having grip grooves of an alternative shape; 
         FIG.  5 B  is a cross-sectional view of one embodiment of a rod-receiving recess having grip grooves formed by protrusions; 
         FIG.  6 A  is a cross-sectional view of one embodiment of a rod-receiving recess and a rod pusher having grip grooves; 
         FIG.  6 B  is a cross-sectional view of an alternate embodiment of a rod-receiving recess and with a rod pusher having grip grooves; 
         FIG.  6 C  is a cross-sectional view of one embodiment of a rod-receiving recess and rod pusher both having grip grooves; 
         FIG.  7 A  is a cross-sectional view of one embodiment of a single grip groove having an internal protrusion arranged to contact the rod in the grip groove perpendicular to the edges of the grip groove; 
         FIG.  7 B  is a top-down view of the grip groove of  FIG.  7 A  showing the perpendicular edges of the internal protrusion; 
         FIG.  7 C  is a cross-sectional view of the grip groove of  FIG.  7 A  showing the edges of the internal protrusions contacting the rod when the rod is engaged with the grip groove; 
         FIG.  7 D  is a perspective view of a rod showing lines of contact made by the edges of the two grip grooves of  FIG.  7 A  with internal protrusions; 
         FIGS.  7 E and  7 F  are cross-sectional views of embodiments of a single grip groove having an internal protrusion with two different configurations; 
         FIG.  8 A  is a cross-sectional view of a prior art rod-receiving recess; 
         FIG.  8 B  is a cross-sectional view of one embodiment of a rod-receiving recess with two grip grooves; 
         FIG.  8 C  is a cross-sectional view of one embodiment of a rod-receiving recess and a rod engagement element with two grip grooves; 
         FIG.  8 D  is a cross-sectional view of an alternative embodiment of a rod-receiving recess and a rod engagement element with two grip grooves; 
         FIG.  9    is a perspective view of one embodiment of a receiving member having a rod-receiving recess with two grip grooves; 
         FIG.  10    is a perspective view of one embodiment of a receiving member having a rod-receiving recess with two circumferential grooves; 
         FIGS.  11 A and  11 B  are perspective views of one embodiment of a receiving member having a rod-receiving recess with two segmented grip grooves formed by the intersection of two grip grooves and two circumferential grooves; 
         FIG.  12    is a perspective view of one embodiment of a receiving member having a rod-receiving recess with two grip grooves having multiple internal protrusions; 
         FIG.  13    is cross-sectional view of one embodiment of a connector with a rod-receiving recess having two grip grooves; 
         FIG.  14    is cross-sectional view of one embodiment of a bone anchor assembly with a compression member forming a rod-receiving recess and a rod engagement element with two grip grooves; and 
         FIG.  15    is a perspective view of a human spine with a fixation system attached thereto. 
     
    
    
     DETAILED DESCRIPTION 
     Implants with grip grooves and related methods are disclosed herein. The implants can include connectors and receiver members of bone anchor assemblies. In some examples, a connector can include a low-profile portion to facilitate use of the connector in surgical applications where space is limited. In some embodiments, a connector can include a biased rod-pusher to allow the connector to “snap” onto a rod and/or to “drag” against the rod, e.g., for provisional positioning of the connector prior to locking. 
     Certain aspects of the present disclosure provide for increased torsional gripping capacity of an implant on a rod. One example presented is a rod-to-rod connector, but the feature may be applied to various spinal implants such as screw heads of bone anchor assemblies. Aspects of the present disclosure include single or multiple longitudinal grooves cut in the rod slot of an implant that add extra lines of contact between the connector and the rod. In operation, when locked, a very small portion of the cross-sectional rod perimeter is wedged into the groove, causing both edges where the groove cut begins to press into the rod. Even if the rod tangency is not perfectly located relative to this groove, the groove provides an edge, rather than a flat face, to grind against the rod and prevent further rotation should rotation begin to occur from forces exerted upon the rod or connector. This micro-shearing of the material is the principle of increasing torsional gripping capacity. 
     Alternatively, these grip grooves can be on the surface of the rod instead of the connector, screw head, or receiving implant. In addition, the semi-circular grip grooves can be other geometries, such as rectangular, right angle or other angles, trapezoidal, etc. 
     A variation of the grip groove to increase the axial slip (longitudinal rod sliding) gripping capacity is to have the grooves in a circumferential orientation relative to the rod. The longitudinal and the circumferential grip grooves can also be combined to increase torsional and axial resistance. The resulting features may resemble pegs or corners. 
     Certain exemplary embodiments will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. 
       FIGS.  1 A- 1 D  illustrate a prior art bone anchor assembly  10  including a bone anchor  12 , a receiver member  14  for receiving a spinal fixation element, such as a spinal rod  22 , to be coupled to the bone anchor  12 , and a closure mechanism  16  to capture a spinal fixation element within the receiver member  14  and fix the spinal fixation element with respect to the receiver member  14 . The bone anchor  12  includes a proximal head  18  and a distal shaft  20  configured to engage bone. The receiver member  14  has a proximal end  26  having a pair of spaced apart arms  28 A,  28 B defining a recess  30  therebetween and a distal end  32  having an inner surface  35  for polyaxially seating the proximal head  18  of the bone anchor  12  and distal end surface  34  defining an opening through which at least a portion of the bone anchor  12  extends. The closure mechanism  16  can be positionable between and can engage the arms  28 A,  28 B to capture a spinal fixation element, e.g., a spinal rod  22 , within the receiver member  14  and fix the spinal fixation element with respect to the receiver member  14 . 
     The proximal head  18  of the bone anchor  12  is generally in the shape of a truncated sphere having a planar proximal surface  36  and an approximately spherically-shaped distal surface  38 . The illustrated bone anchor assembly is a polyaxial bone anchor designed for posterior implantation in the pedicle or lateral mass of a vertebra. The proximal head  18  of the bone anchor  12  engages the distal end  32  of the receiver member  14  in a ball and socket like arrangement in which the proximal head  18  the distal shaft  20  can pivot relative to the receiver member  14 . The distal surface  38  of the proximal head  18  of the bone anchor  12  and a mating surface within the distal end  32  of the receiver member  14  can have any shape that facilitates this arrangement, including, for example, spherical (as illustrated), toroidal, conical, frustoconical, and any combinations of these shapes. 
     The distal shaft  20  of the bone anchor  12  can be configured to engage bone and, in the illustrated embodiment, includes an external bone engaging thread  40 . The thread form for the distal shaft  20 , including the number of threads, the pitch, the major and minor diameters, and the thread shape, can be selected to facilitate connection with bone. Exemplary thread forms are disclosed in U.S. Patent Application Publication No. 2011/0288599, filed on May 18, 2011, and in U.S. Provisional Patent Application Ser. No. 61/527,389, filed Aug. 25, 2011, both of which are incorporated herein by reference. The distal shaft  20  can also include other structures for engaging bone, including a hook. The distal shaft  20  of the bone anchor  12  can be cannulated, having a central passage or cannula extending the length of the bone anchor to facilitate delivery of the bone anchor over a guide wire in, for example, minimally-invasive procedures. Other components of the bone anchor assembly, including, for example, the closure member  16 , the receiver member  14 , and the compression member  60  (discussed below) can be cannulated or otherwise have an opening to permit delivery over a guide wire or to permit the insertion of a driver instrument to manipulate the bone anchor. The distal shaft  20  can also include one or more sidewall openings or fenestrations that communicate with the cannula to permit bone in-growth or to permit the dispensing of bone cement or other materials through the bone anchor  12 . The sidewall openings can extend radially from the cannula through the sidewall of the distal shaft  20 . Exemplary systems for delivering bone cement to the bone anchor assembly  10  and alternative bone anchor configurations for facilitating cement delivery are described in U.S. Patent Application Publication No. 2010/0114174, filed on Oct. 29, 2009, which is hereby incorporated herein by reference. The distal shaft  20  of the bone anchor  12  can also be coated with materials to permit bone growth, such as, for example, hydroxyl apatite, and the bone anchor assembly  10  can be coated partially or entirely with anti-infective materials, such as, for example, triclosan. 
     The proximal end  26  of the receiver member  14  includes a pair of spaced apart arms  28 A,  28 B defining a U-shaped recess  30  therebetween for receiving a spinal fixation element, e.g., a spinal rod  22 . Each of the arms  28 A,  28 B can extend from the distal end  32  of the receiver member  14  to a free end. The outer surfaces of each of the arms  28 A,  28 B can include a feature, such as a recess, dimple, notch, projection, or the like, to facilitate connection of the receiver member  14  to instruments. For example, the outer surface of each arm  28 A,  28 B can include an arcuate groove at the respective free end of the arms. Such grooves are described in more detail in U.S. Pat. No. 7,179,261, issued on Feb. 20, 2007, which is hereby incorporated herein by reference. At least a portion of the proximal end surface  48  of the receiver member  12  defines a plane Y. The receiver member  14  has a central longitudinal axis L. 
     The distal end  32  of the receiver member  14  includes a distal end surface  34  which is generally annular in shape defining a circular opening through which at least a portion of the bone anchor  12  extends. For example, the distal shaft  20  of the bone anchor  12  can extend through the opening. At least a portion of the distal end surface  34  defines a plane X. 
     The bone anchor  12  can be selectively fixed relative to the receiver member  14 . Prior to fixation, the bone anchor  12  is movable relative to the receiver member  14  within a cone of angulation generally defined by the geometry of the distal end  32  of the receiver member and the proximal head  18  of the bone anchor  12 . The illustrated bone anchor is a favored-angle polyaxial screw in which the cone of angulation is biased in one direction. In this manner, the bone anchor  12  is movable relative to the receiver member  14  in at least a first direction, indicated by arrow A in  FIG.  1 D , at a first angle C relative to the central longitudinal axis L of the receiver member  14 . The bone anchor  12  is also movable in at least a second direction, indicated by arrow B in  FIG.  1 D , at a second angle D relative to the longitudinal axis L. The first angle C is greater than the second angle D and, thus, the shaft  20  of the bone anchor  12  is movable more in the direction indicated by arrow A than in the direction indicated by arrow B. The distal shaft  20  of the bone anchor  12  defines a neutral axis  48  with respect to the receiver member  14 . The neutral axis  48  can be perpendicular to the plane X defined by the distal end surface  34  and intersects the center point of the opening in the distal end surface  34  through which the distal shaft  20  of the bone anchor  12  extends. The neutral axis  48  can be oriented at an angle to the central longitudinal axis L of the receiver member  14 . The plane Y defined by at least a portion of the proximal end surface  48  of the receiver member  14  intersects the plane X defined by at least a portion of the distal end surface  34  of the receiver member  12 . The proximal end  26  of the receiver member  14  can include a proximal first bore  50  coaxial with a first central longitudinal axis N (which is coincident with longitudinal axis L) and a distal second bore  52  coaxial with a second central longitudinal axis M (which is coincident with the neutral axis  48 ) and the first central longitudinal axis N and second central longitudinal axis M can intersect one another. The angle between the plane X and the plane Y and the angle between the axis L and the axis M can be selected to provide the desired degree of biased angulation. Examples of favored angled polyaxial screws are described in more detail in U.S. Pat. No. 6,974,460, issued on Dec. 13, 2005, and in U.S. Pat. No. 6,736,820, issued on May 18, 2004, both of which are hereby incorporated herein by reference. Alternatively, the bone anchor assembly can be a conventional (non-biased) polyaxial screw in which the bone anchor pivots in the same amount in every direction and has a neutral axis that is coincident with the central longitudinal axis L of the receiver member. 
     The spinal fixation element, e.g., the spinal rod  22 , can either directly contact the proximal head  18  of the bone anchor  12  or can contact an intermediate element, e.g., a compression member  60 . The compression member  60  can be positioned within the receiver member  14  and interposed between the spinal rod  22  and the proximal head  18  of the bone anchor  12  to compress the distal outer surface  38  of the proximal head  18  into direct, fixed engagement with the distal inner surface of the receiver member  14 . A proximal portion of the compression member  60  can include a pair of spaced apart arms  62 A and  62 B defining a U-shaped seat  64  for receiving the spinal rod  22 . A distal portion of the compression member  60  can include a sidewall having an inner cylindrical surface  67  that is connected to an outer cylindrical surface  68  by a distal-facing surface  66 , 
     At least a portion of the distal surface  66  of the compression member  60  can be shaped as a negative of the proximal portion  18  of the bone anchor  20 , against which the distal surface  66  abuts when the compression member  60  is fully inserted into the receiver member  14 . Thus, when the shaft  20  of the bone anchor  12  is oriented along the longitudinal axis L, the contact area between the distal surface  66  of the compression member  60  and the proximal head  18  is maximized. Where the angle of the shaft  20  with respect to the longitudinal axis L is not zero, however, the contact area between the distal surface  66  of the compression member  60  and the head  18  can be reduced, thus increasing a risk of slippage of the bone anchor  12  with respect to the receiver member  14 . 
     As best seen in  FIG.  1 B , the compression member  60  is configured to slide freely along the longitudinal axis L within the recess  30  of the receiver member  14 . To secure the compression member  60  within the receiver member  14 , the compression member  60  can be configured to mate with the receiver member, for example by mechanically deforming a portion of the compression member  60  against the receiver member  14 . In the illustrated embodiment, opposing bores formed in the arms  62 A,  62 B of the compression member  60  are aligned with bores formed in the arms  62 A,  62 B of the receiver member  14 , such that opposing pins can be inserted through the passageways defined by the bores to compress or “swage” the compression member  60  against the receiver member  14 . The swaging process can prevent subsequent removal of the compression member  60  from the receiver member  14 . 
     The proximal end  26  of the receiver member  14  can be configured to receive a closure mechanism  16  positionable between and engaging the arms  28 A,  28 B of the receiver member  14 . The closure mechanism  16  can be configured to capture a spinal fixation element, e.g., a spinal rod  22 , within the receiver member  14 , to fix the spinal rod  22  relative to the receiver member  14 , and to fix the bone anchor  12  relative to the receiver member  14 . The closure mechanism  16  can be a single set screw having an outer thread for engaging an inner thread  42  provided on the arms  28 A,  28 B of the receiver member  14 . In the illustrated embodiment, however, the closure mechanism  16  comprises an outer set screw  70  positionable between and engaging the arms  28 A,  28 B of the receiver member  14  and an inner set screw  72  positionable within the outer set screw  70 . The outer set screw  70  is operable to act on the compression member  60  to fix the bone anchor  12  relative to the receiver member  14 . The inner set screw  72  is operable to act on the spinal rod  22  to fix the spinal rod  22  relative to the receiver member  14 . In this manner, the closure mechanism  16  permits the bone anchor  12  to be fixed relative to the receiver member  14  independently of the spinal rod  22  being fixed to the receiver member  14 . In particular, the outer set screw  70  can engage the proximal end surfaces of the arms  62 A,  62 B of the compression member  60  to force the distal-facing surface  66  of the compression member  60  into contact with the proximal head  18  of bone anchor  12 , which in turn forces the distal surface  38  of the proximal head  18  into fixed engagement with the distal inner surface of the receiver member  14 . The inner set screw  72  can engage the spinal rod  22  to force the spinal rod  22  into fixed engagement with the rod seat  64  of the compression member  60 . 
     The outer set screw  70  includes a first outer thread  74  for engaging a complementary inner thread  42  on the arms  28 A,  28 B of the receiver member  14 . The outer set screw  74  includes a central passage  96  from a top surface  98  of the outer set screw  74  to a bottom surface  100  of the outer set screw  74  for receiving the inner set screw  72 . The central passage  96  can includes an inner thread  102  for engaging a complementary outer thread  104  on the inner set screw  72 . The thread form for the inner thread  102  and the outer thread  104 , including the number of threads, the pitch, major and minor diameter, and thread shape, can be selected to facilitate connection between the components and transfer of the desired axial tightening force. The top surface  98  of the outer set screw  74  can have one or more drive features to facilitate rotation and advancement of the outer set screw  74  relative to the receiver member  14 . The illustrated outer set screw  74  includes drive features in the form of a plurality of cut-outs  106  spaced-apart about the perimeter of the top surface  98 . The inner set screw  104  can include drive features for receiving an instrument to rotate and advance the inner set screw  72  relative to the outer set screw  74 . The illustrated inner set screw  104  includes drive features in the form of a central passage  108  having a plurality of spaced apart, longitudinally oriented cut-outs for engaging complementary features on an instrument. 
     The bone anchor assembly  10  can be used with a spinal fixation element such as rigid spinal rod  22 . The various components of the bone anchor assemblies disclosed herein, as well as the spinal rod  22 , can be constructed from various materials, including titanium, titanium alloys, stainless steel, cobalt chrome, PEEK, or other materials suitable for rigid fixation. In other embodiments, the spinal fixation element can be a dynamic stabilization member that allows controlled mobility between the instrumented vertebrae. 
     In use, bone can be prepared to receive the bone anchor assembly  10 , generally by drilling a hole in the bone which is sized appropriately to receive the bone anchor  12 . If not already completed, the bone anchor assembly  10  can be assembled, which can include assembling the bone anchor  12  and the receiver member  14 , so that the distal shaft  20  extends through the opening in the distal end  32  of the receiver member  14  and the proximal head  18  of the bone anchor  12  is received in the distal end  32  of the receiver member  14 . A driver tool can be fitted with the bone anchor  12  to drive the bone anchor  12  into the prepared hole in the bone. The compression member  60  can be positioned within the receiver member  14  such that the arms  62 A,  62 B of the compression member are aligned with the arms  28 A,  28 B of the receiver member  14  and the lower surface of the compression member  14  is in contact with the proximal head  18  of the bone anchor  12 . A spinal fixation element, e.g., the spinal rod  22 , can be located in the recess  30  of the receiver member  14 . The closure mechanism  16  can be engaged with the inner thread  42  provided on the arms  28 A,  28 B of the receiver member  14 . A torsional force can be applied to the outer set screw  70  to move it within the recess  30  using a tool which can engage the plurality of cut-outs  106  in the upper facing surface of the outer set screw  70 , so as to force the compression member  60  onto the proximal head  18  of the bone anchor  12 . Torsional forces can then be applied to the inner set screw  72  to move it relative to the outer set screw  70  so that it contacts the spinal rod  22  and can, for example, fix the spinal rod  22  relative to the receiver member  14  and the bone anchor  12 . 
     One or more embodiments of inventive bone anchor assemblies are described below. 
     Except as indicated below, the structure, operation, and use of these embodiments is similar or identical to that of the bone anchor assembly  10  described above. Accordingly, a detailed description of said structure, operation, and use is omitted here for the sake of brevity.  FIGS.  3 A- 12    show various embodiments of rod-receiving recesses similar to the recesses formed by the receiver member  14  and/or the compression member  60  shown in  FIG.  1 B  but with gripping recesses or features formed on the inner surface of the rod-receiving recess for gripping a cylindrical rod  22  with greater friction as compared with the receiver member  14  and/or the compression member  60  of the bone anchor shown in  FIG.  1 B . The rod-receiving recesses shown in  FIGS.  3 A- 12    can be used with the bone anchor assembly shown in  FIGS.  1 A- 1 D , or with various other bone anchor assemblies known in the art.  FIGS.  8 C and  8 D  show rod-receiving recesses with locking members arranged to compress an insert having gripping recesses or features formed therein against the cylindrical rod  22 . The inserts shown in  FIGS.  8 C and  8 D  can be used with the bone anchor assembly shown in  FIGS.  1 A- 1 D , or with various other bone anchor assemblies known in the art, and can be urged against the cylindrical rod  22  by a locking mechanism such as the outer set screw  70  or inner set screw  72  shown in  FIGS.  1 A- 1 D . 
       FIGS.  2 A- 2 L  illustrate a prior art connector  200  with a traditional configuration for securing a rod to the connector  200 . As shown, the connector  200  can include a body  202  that defines first and second rod-receiving recesses  204 ,  206 , a rod pusher  208 , a bias element or spring wire  212 , and a locking element or set screw  216 . The rod pusher  208  can be configured to translate laterally within the body  202 , and can be biased by the spring wire  212  in a direction that urges the rod pusher into a first rod R 1  disposed in the first rod-receiving recess  204 . The set screw  216  can be tightened to lock the connector  200  to both the first rod R 1  and to a second rod R 2  disposed in the second rod-receiving recess  206 . The illustrated connector  200  can thus allow for one-step locking of first and second rods R 1 , R 2  to the connector. The connector  200  can include one or more low-profile portions to facilitate use in tight spaces. For example, the first rod-receiving recess  204  can be formed in a portion of the connector body  202  having a reduced-profile, e.g., to fit between bone anchors implanted in adjacent levels of the cervical spine. 
     The body  202  can include proximal and distal ends  202   p ,  202   d  that define a proximal-distal axis A 1 . The proximal end  202   p  of the body  202  can include a pair of spaced apart arms  218 ,  220  that define the second rod-receiving recess  206  therebetween. A rod R 2  disposed in the second rod-receiving recess  206  can have a central longitudinal rod axis A 2 . The second rod-receiving recess  206  can be open in a proximal direction, such that a rod R 2  can be inserted into the recess by moving the rod distally with respect to the connector  200 . Each of the arms  218 ,  220  can extend from the distal portion  202   d  of the body  202  to a free end. The outer surfaces of each of the arms  218 ,  220  can include a feature (not shown), such as a recess, dimple, notch, projection, or the like, to facilitate coupling of the connector  200  to various instruments. For example, the outer surface of each arm  218 ,  220  can include an arcuate groove at the respective free end of the arms for attaching the connector  200  to an extension tower or retractor. The arms  218 ,  220  can include or can be coupled to extension or reduction tabs (not shown) that extend proximally from the body  202  to functionally extend the length of the arms  218 ,  220 . The extension tabs can facilitate insertion and reduction of a rod or other implant, as well as insertion and locking of the set screw  216 . The extension tabs can be configured to break away or otherwise be separated from the arms  218 ,  220 . The inner surfaces of each of the arms  218 ,  220  can be configured to mate with the set screw  216 . For example, the inner surfaces of the arms  218 ,  220  can include threads that correspond to external threads formed on the set screw  216 . Accordingly, rotation of the set screw  216  with respect to the body  202  about the axis A 1  can be effective to translate the set screw with respect to the body axially along the axis A 1 . 
     The distal end  202   d  of the body  202  can define a tunnel  228  in which the rod pusher  208  can be disposed. The tunnel  228  can extend along a rod pusher axis A 3  between the second rod-receiving recess  206  and the first rod-receiving recess  204 . The rod pusher  208  can be configured to translate within the tunnel  228  along the axis A 3 . The axis A 3  can be perpendicular or substantially perpendicular to the axis A 1 . The axis A 3  can also be perpendicular or substantially perpendicular to the axis A 2 . The tunnel  228  can have a shape that is substantially a negative of the exterior shape of the rod pusher  208 . A through-bore  224  can be formed in the body  202  such that the through-bore intersects with the tunnel  228 . The through-bore  224  can extend perpendicular or substantially perpendicular to the axis A 3 . The through-bore  224  can be sized to receive the spring wire  212  therein, as described further below. The through-bore  224  can be open at both ends or one or both ends can be closed. 
     The body  202  can include a cantilevered wing portion  230  that defines the first rod-receiving recess  204 . A rod R 1  disposed in the first rod-receiving recess  204  can have a central longitudinal rod axis A 4 . The axis A 4  can be parallel to the axis A 2  as shown, or can be perpendicular or obliquely angled with respect to the axis A 2 . The wing portion  230  can extend radially-outward from the second arm  220  of the body  202 . The wing portion  230  can have a width  230 W and a height  230 H. A ratio of the width  230 W to the diameter of the first rod-receiving recess  204  (or of a rod R 1  disposed therein) can be less than about 1.5:1, less than about 2:1, and/or less than about 3:1. A ratio of the height  230 H to the diameter of the first rod-receiving recess  204  (or of a rod R 1  disposed therein) can be less than about 0.5:1, less than about 1:1, and/or less than about 2:1. The height  230 H can be less than about 5 mm, less than about 4 mm, and/or less than about 3 mm. The first rod-receiving recess  204  can be open in a distal direction such that a rod R 1  can be inserted into the recess by moving the connector  200  distally with respect to the rod. The first rod-receiving recess  204  can be open in a proximal direction, e.g., by flipping the wing portion  230  and forming it such that it extends from a distal portion of the body  202 , or in a lateral direction. 
     As noted above, the rod pusher  208  can be slidably disposed within the tunnel  228  of the body  202  and can be configured to translate with respect to the body along the axis A 3 . The rod pusher  208  can include a first bearing surface  244 A configured to contact and bear against a first rod R 1  disposed in the first rod-receiving recess  204 . The bearing surface  244 A can extend at an oblique angle with respect to a longitudinal axis of the rod pusher  208  such that the bearing surface is ramped. The bearing surface  244 A can be planar as shown, or can be convex, concave, pointed, sharpened, etc. For example, the bearing surface  244 A can be concave and can define a section of a cylinder, such that the bearing surface matches or approximates the contour of a cylindrical rod R 1  disposed in the first rod-receiving recess  204 . The rod pusher  208  can include a second bearing surface  244 B configured to contact and bear against a second rod R 2  disposed in the second rod-receiving recess  206 . The bearing surface  244 B can extend at an oblique angle with respect to a longitudinal axis of the rod pusher  208  such that the bearing surface is ramped. The bearing surface  244 B can be planar as shown, or can be convex, concave, pointed, sharpened, etc. For example, the bearing surface  244 B can be concave and can define a section of a cylinder, such that the bearing surface matches or approximates the contour of a cylindrical rod R 2  disposed in the second rod-receiving recess  206 . 
     The rod pusher  208  can include a through bore  226 . The through-bore  226  can extend perpendicular or substantially perpendicular to the axis A 3 . The through-bore  226  can be sized to receive the spring wire  212  therein. In at least some positions of the rod pusher  208  with respect to the body  202 , the through-bore  226  of the rod pusher can be aligned with the through-bore  224  of the body, such that the spring wire  212  extends through both through-bores  224 ,  226 . As best shown in  FIGS.  2 D,  2 F, and  2 H , the through-bore  226  can include a middle portion and opposed end portions. The middle portion of the through-bore  226  can approximate the dimensions of the spring wire  212 . For example, the middle portion can be cylindrical and can have a diameter that is substantially equal to the diameter of the spring wire  212 . The end portions of the through-bore  226  can be elongated or can otherwise have a dimension greater than the diameter of the spring wire  212  to allow the rod pusher  208  to translate along the axis A 3  and to accommodate the bend radius of the spring wire  212  during such translation. 
     The bias element can be configured to bias the rod pusher  208  towards the first rod-receiving recess  204 . In the illustrated view, the bias element is a cylindrical spring wire  212 . The spring wire  212  can be formed from a resilient material such that, when deformed from a straight line, the spring wire tends to flex back towards its straight resting configuration. Accordingly, when deformed by movement of the rod pusher  208 , the spring wire  212  can exert a force against the interior of the through-bore  226  to urge the rod pusher  208  towards the first rod-receiving recess  204 . While a straight, cylindrical spring wire  212  is shown, various other bias elements can be used instead or in addition, such as non-straight or non-cylindrical wires, leaf springs, spring clips, wave springs, coil springs, and the like. The bias element can be omitted. For example, the rod pusher  208  can be free to float within the tunnel  228 , or can be retained by a pin or other retention feature without being biased towards the first rod-receiving recess  204 . 
     The set screw  216  can include an exterior thread configured to mate with the interior threads formed on the arms  218 ,  220  of the body  202  to allow the set screw to be advanced or retracted along the axis A 1  with respect to the body by rotating the set screw about the axis A 1 . The set screw  216  can include a driving interface  248  configured to receive a driver for applying a rotational force to the set screw about the axis A 1 . The distal surface of the set screw  216  can be configured to contact and bear against a rod R 2  disposed in the second rod-receiving  206  recess to lock the rod to the connector  200 . When tightened against the rod R 2 , the set screw  216  can prevent the rod from translating relative to the connector  200  along the axis A 2  and/or from rotating with respect to the connector about the axis A 2 . While a set screw  216  is shown, it will be appreciated that other locking elements can be used instead or addition, such as a closure cap that advances and locks by quarter-turn rotation, a closure cap that slides in laterally without rotating, a nut that threads onto an exterior of the connector  200 , and so forth. 
     Operation of the connector  200  is illustrated schematically in  FIGS.  2 C- 2 H . 
     As shown in  FIGS.  2 C and  2 D , the connector  200  can have a resting configuration in which no rod is disposed in the first or second rod-receiving recesses  204 ,  206 . In this configuration, the biasing force of the spring wire  212  can cause the rod pusher  208  to slide towards the first rod-receiving recess  204 . 
     In the resting configuration, the wing portion  230  of the body  202  and the free end of the rod pusher  208  can define an aperture  250  that is smaller than the diameter of a first rod R 1  to which the connector  200  is to be coupled. Accordingly, as shown in  FIGS.  2 E and  2 F , as the rod R 1  is inserted into the first rod-receiving recess  204 , the rod bears against the rod pusher  208  to move the connector  200  out of the resting configuration. Insertion of the rod R 1  can move the rod pusher  208  along the axis A 3 , thereby deforming the spring wire  212  from its resting state. As the largest cross-sectional portion of the rod R 1  is positioned in the aperture  250 , the rod pusher  208  can be displaced to its furthest distance from the first rod-receiving recess  204 . 
     As shown in  FIGS.  2 G and  2 H , once the largest cross-sectional portion of the rod R 1  clears the aperture  250  as the rod is seated in the first rod-receiving recess  204 , the biasing force of the spring wire  212  can cause the rod pusher  208  to move back along the axis A 3  towards the first rod-receiving recess. This movement can at least partially close the aperture  250  around the rod R 1  to capture the rod in the first rod-receiving recess  204 . The biasing force of the spring wire  212  can resist retrograde movement of the rod pusher  208  and thus resist disconnection of the connector  200  from the first rod R 1 . The geometry of the connector  200  can be selected such that, when the rod R 1  is fully seated in the first rod-receiving recess  204 , the spring wire  212  is deformed from its resting state. The spring wire  212  can thus press the rod pusher  208  against the rod R 1  to provide a friction or drag effect, before the set screw  216  is tightened and/or before a second rod R 2  is positioned in the connector  200 . 
     A second rod R 2  can be positioned in the second rod-receiving recess  206 , and the set screw  216  can be tightened to lock the connector  200  to the first and second rods R 1 , R 2 . As the set screw  216  is tightened, the second rod R 2  can press against the second bearing surface  244 B of the rod pusher  208 , urging the rod pusher towards the first rod-receiving recess  204  and firmly into contact with the rod R 1 . When the set screw  216  is tightened, the connector  200  can be locked to the first and second rods R 1 , R 2  to resist or prevent translation of the rods R 1 , R 2  with respect to the connector along the axes A 2 , A 4  and to resist or prevent rotation of the rods R 1 , R 2  with respect to the connector about the axes A 2 , A 4 . 
     As shown in  FIG.  2 I , the second rod-receiving recess  206  can be shaped to encourage contact between the second rod R 2  and the second bearing surface  244 B of the rod pusher  208 . In other words, the recess  206  can be shaped to reduce or eliminate the risk that the second rod R 2  will only bear against the floor of the recess  206  when the set screw  216  is tightened, without applying sufficient force to the bearing surface  244 B. As shown, the recess  206  can include a relief disposed in alignment with the end of the tunnel  228  such that the rod pusher  208  protrudes into the recess. The recess  206  can thus be asymmetrical about the axis A 1 , and can deviate from a symmetrical U-shape. When the rod R 2  is bottomed out in the recess  206 , the central longitudinal axis A 2  of the rod can be offset from the axis A 1 . The central longitudinal axis of the rod R 2  when the rod is fully seated is shown in  FIG.  2 I  as axis A 5 . The recess  206  can be configured such that, as the rod R 2  is seated within the recess  206 , it translates distally along the axis A 1  and laterally along the axis A 3 . 
     As shown in  FIGS.  2 J- 2 L , the connector  200  can include a saddle  210 . The saddle  210  can be included in addition to the asymmetrical recess  206  or as an alternative thereto. The saddle  210  can be positioned within a cavity  222  formed in the body  202 . The saddle  210  can be generally cylindrical with first and second arms  232 ,  234  extending in a proximal direction to respective free ends of the arms. The first and second arms  232 ,  234  can be aligned with the first and second arms  218 ,  220  of the body  202  such that a recess defined therebetween is aligned with the second rod-receiving recess  206 . Accordingly, the second rod R 2  can be simultaneously cradled between the arms  232 ,  234  of the saddle  210  and the arms  218 ,  220  of the body  202  when the rod is disposed in the second rod-receiving recess  206 . The saddle  210  can include a ramped bearing surface  240  configured to contact and bear against the second bearing surface  244 B of the rod pusher  208 . The bearing surface  240  can extend at an oblique angle with respect to the axis A 1 . The bearing surface  240  can be planar as shown, or can be convex, concave, pointed, sharpened, etc. In operation, a force applied to the saddle  210  along the direction A 1 , e.g., by tightening the set screw  216  down onto the saddle or down onto a rod R 2  disposed in the saddle, can cause the saddle  210  to translate distally with respect to the body  202  and cause the bearing surface  240  to ramp along the bearing surface  244 B of the rod pusher  208 , urging the rod pusher towards the first rod-receiving recess  204  along the axis A 3 . Accordingly, tightening the set screw  216  can be effective to simultaneously lock both rods R 1 , R 2  to the connector  200 . The saddle  210  can allow for locking of rods having different diameters in the second rod-receiving recess  206 , while still ensuring that, regardless of the diameter of the second rod R 2 , sufficient force is applied to the rod pusher  208  to lock the first rod R 1 . 
     Sometimes, the arms  232 ,  234  can extend proximally past the maximum dimension of the rod R 2  and the set screw  216  can include an outer screw configured to bear against a proximal-facing surface of the arms. An inner set screw can be threadably mounted within the outer set screw. Accordingly, the outer set screw can be tightened first to press down on the saddle  210  and lock the first rod R 1  in the first rod-receiving recess  204 . Then, the inner set screw can be tightened to press down on the second rod R 2  and lock the second rod in the second rod-receiving recess  206 . The dual set screw can thus facilitate independent locking of the first and second rods R 1 , R 2  to the connector  200 . While not shown in  FIGS.  2 J- 2 L , connectors  200  that include a saddle  210  can also include a bias element as described above for biasing the rod pusher  208  towards the first rod-receiving recess  204 . 
     The connector  200  can thus be used to connect a first spinal rod R 1  to a second spinal rod R 2 . While use of the connector  200  with first and second spinal rods is generally described herein, it will be appreciated that the connector can instead be configured for use with other types of orthopedic hardware, whether implanted or external. For example, one or both halves of the connector  200  can be modified to couple other various components to each other (e.g., to couple a rod to a plate, to couple a plate to a plate, to couple a rod to cable, to couple a cable to a cable, and so forth). 
     The connector  200  can provide various benefits for the user and/or patient. For example, the biased rod pusher  208  can provide tactile feedback when the connector  200  is “snapped” onto the first rod R 1 , giving the user confidence that the rod has been attached successfully before tightening the connector. The biased rod pusher  208  can also apply friction or “drag” to the rod R 1  prior to locking the set screw  216 , helping to keep the connector in place and prevent “flopping” while still allowing free movement when intended by the user. By way of further example, the low-profile geometry of the wing portion  230  of the connector  200  can allow the connector to be used in surgical areas where space is limited (e.g., in the cervical area of the spine). In an exemplary method, the wing portion  230  of the connector  200  can be hooked onto a first rod R 1  at a location between two bone anchors to which the rod is coupled, the two bone anchors being implanted in adjacent vertebral levels of the cervical spine. As yet another example, the connector  200  can facilitate simultaneous and/or single-step locking of the first and second rods R 1 , R 2 . This can allow the connector  200  to be locked to both rods R 1 , R 2  with minimal steps. In other instances, the connector  200  can facilitate independent locking of the rods R 1 , R 2 , e.g., with use of a saddle  210  and dual set screw. 
     Exemplary connector and implants are disclosed in U.S. Patent Application Publication No. 2017/0333088, filed on Oct. 4, 2016, and in U.S. Patent Application Publication No. 2017/0333087, filed May 18, 2016, both of which are incorporated herein by reference. 
       FIG.  3 A  is a cross-sectional view of a connector with a rod recess having two grip grooves. As shown, the connector  300  can include a body  202  that defines first and second rod-receiving recesses  304 ,  306  and a rod pusher  308 . The rod pusher  308  can be configured to translate laterally within the body  302 , and can be biased in a direction that urges the rod pusher  308  into a first rod R 1  disposed in the first rod-receiving recess  304 . A set screw  216  can be tightened to lock the connector  300  to both the first rod R 1  and to a second rod R 2  disposed in the second rod-receiving recess  306 . The illustrated connector  300  can thus allow for one-step locking of first and second rods R 1 , R 2  to the connector (as shown in  FIGS.  2 A- 2 L ). The connector  300  can include one or more low-profile portions to facilitate use in tight spaces. For example, the first rod-receiving recess  304  can be formed in a portion of the connector body  202  having a reduced-profile, e.g., to fit between bone anchors implanted in adjacent levels of the cervical spine. 
     The body  302  can include proximal and distal ends that define a proximal-distal axis A 1 , as shown in  FIG.  2 B . The proximal end of the body  202  can include a pair of spaced apart arms  318 ,  320  that define the second rod-receiving recess  306  therebetween. A rod R 2  disposed in the second rod-receiving recess  306  can have a central longitudinal rod axis A 2 , as shown in  FIG.  2 B . The second rod-receiving recess  306  can be open in a proximal direction, such that a rod R 2  can be inserted into the recess by moving the rod distally with respect to the connector  200 . Each of the arms  318 ,  320  can extend from the distal portion of the body  302  to a free end. The outer surfaces of each of the arms  318 ,  320  can include a feature (not shown), such as a recess, dimple, notch, projection, or the like, to facilitate coupling of the connector  300  to various instruments. For example, the outer surface of each arm  318 ,  320  can include an arcuate groove at the respective free end of the arms for attaching the connector  300  to an extension tower or retractor. The arms  318 ,  320  can include or can be coupled to extension or reduction tabs (not shown) that extend proximally from the body  302  to functionally extend the length of the arms  218 ,  220 . The extension tabs can facilitate insertion and reduction of a rod or other implant, as well as insertion and locking of the set screw  216 , as shown in  FIG.  2 B . The extension tabs can be configured to break away or otherwise be separated from the arms  218 ,  220 . The inner surfaces of each of the arms  318 ,  320  can be configured to mate with the set screw  216 . For example, the inner surfaces of the arms  318 ,  320  can include threads that correspond to external threads formed on the set screw  316 . Accordingly, rotation of the set screw  316  with respect to the body  302  about the axis A 1  can be effective to translate the set screw with respect to the body axially along the axis A 1 . 
     The body  302  can include a cantilevered wing portion  330  that defines the first rod-receiving recess  304 . As shown in  FIG.  2 B , a rod R 1  disposed in the first rod-receiving recess  304  can have a central longitudinal rod axis A 4 . The axis A 4  can be parallel to the axis A 2  as shown, or can be perpendicular or obliquely angled with respect to the axis A 2 . The wing portion  330  can extend radially-outward from the second arm  320  of the body  302 . The first rod-receiving recess  304  can be open in a distal direction such that a rod R 1  can be inserted into the recess by moving the connector  300  distally with respect to the rod. The first rod-receiving recess  304  can be open in a proximal direction, e.g., by flipping the wing portion  330  and forming it such that it extends from a distal portion of the body  302 , or in a lateral direction. 
     The rod pusher  308  can be slidably disposed within the tunnel  328  of the body  302  and can be configured to translate with respect to the body along the axis A 3 , as shown in  FIG.  2 B . The rod pusher  308  can include a first bearing surface  344 A configured to contact and bear against a first rod R 1  disposed in the first rod-receiving recess  304 . The bearing surface  344 A can extend at an oblique angle with respect to a longitudinal axis of the rod pusher  308  such that the bearing surface is ramped. The bearing surface  344 A can be planar as shown, or can be convex, concave, pointed, sharpened, etc. For example, the bearing surface  344 A can be concave and can define a section of a cylinder, such that the bearing surface matches or approximates the contour of a cylindrical rod R 1  disposed in the first rod-receiving recess  204 . The rod pusher  308  can include a second bearing surface  344 B configured to contact and bear against a second rod R 2  disposed in the second rod-receiving recess  306 . The bearing surface  344 B can extend at an oblique angle with respect to a longitudinal axis of the rod pusher  208  such that the bearing surface is ramped. The bearing surface  344 B can be planar as shown, or can be convex, concave, pointed, sharpened, etc. For example, the bearing surface  344 B can be concave and can define a section of a cylinder, such that the bearing surface matches or approximates the contour of a cylindrical rod R 2  disposed in the second rod-receiving recess  306 . 
       FIG.  3 A  shows the first rod-receiving recess  304  including two grip grooves  301  opposite to the bearing surface  344 A. The grip grooves  301  are formed as recesses in the inner surface of the first rod-receiving recess  304 , as shown more clearly in  FIG.  3 B . 
       FIG.  3 B  is a cross-sectional view of the rod-receiving recess  304  of the connector of  FIG.  3 A .  FIG.  3 B  shows that the grip grooves  301  each define two edges  312 ,  313 ,  322 ,  323  that are configured to contact the surface of the cylindrical rod R 1  disposed in the first rod-receiving recess  304 . The grip grooves  301  extend along the axis A 4  and the first rod-receiving recess  304  is sized and shaped such that the side of the cylindrical rod R 1  that faces the grip grooves  301  is urged into contact against the edges  312 ,  313 ,  322 ,  323  by the rod pusher  308 , such that additional force applied to the cylindrical rod R 1  by the rod pusher  308  further engages the cylindrical rod R 1  against the edges  312 ,  313 ,  322 ,  323 . This can be accomplished by, for example, having the inner surface of the rod-receiving recess  304  and the location of the grip grooves  301  sized and shaped such that the edges  312 ,  313 ,  322 ,  323  define a radius about the axis A 4  that approximately matches the radius of the cylindrical rod R 1  and, when the cylindrical rod R 1  is urged into the rod-receiving recess  304 , the half of the cylindrical rod R 1  opposite the first bearing surface  344 A of the rod pusher contacts the rod-receiving recess  304  only at engages the edges  312 ,  313 , as shown in more detail in  FIG.  3 C . 
       FIG.  3 C  is a cross-sectional view of the grip grooves  301  of the rod-receiving recess  304  of the connector  300  of  FIG.  3 A .  FIG.  3 C  shows the cylindrical rod R 1  (solid line) disposed in the rod-receiving recess  304  and in contact with the edges  312 ,  313 , such that a small portion of the outer surface of the cylindrical rod R 1  is received inside each grip groove  301 . This illustrated configuration shows that the cylindrical rod R 1  makes contact with the rod-receiving recess  304  along 4 points of contact, in contrast with the prior art rod-receiving recess  204  without grip grooves. This prior art configuration is shown in  FIG.  3 C  as the position of a cylindrical rod R 1 ′ is overlaid the cylindrical rod R 1 . The position of a cylindrical rod R 1 ′ (dotted line) corresponds to position the cylindrical rod R 1 ′ in the rod-receiving recess  304  without grip grooves  301  as shown more clearly in  FIG.  4 A .  FIG.  3 C  shows the radius  398  of the cylindrical rod R 1  and the distance  399  of each edge  312 ,  313  from the central longitudinal axis A 4  of the rod-receiving recess  304  are approximately the same. 
       FIG.  4 A  is a cross-sectional view of a prior art rod-receiving recess.  FIG.  4 A  shows a cylindrical rod R 1  disposed in a prior art rod-receiving recess  204  without grip grooves. In this configuration, the cylindrical rod R 1  contacts the inner surface of the rod-receiving recess  204  at two points P 1 , P 2 . In operation, these two points P 1 , P 2  can define parallel lines of contact along the axis A 4  (e.g., along the surface of the cylindrical rod R 1  that contacts the inner surface of the rod-receiving recess  204 ). The particular interface of the cylindrical rod R 1  and the rod-receiving recess  204  at these points P 1 , P 2  is a curved surface (e.g., the cylindrical rod R 1 ) against a flat or angled surface (e.g., the rod-receiving recess  204 ). With this engagement, both the rotation of the cylindrical rod R 1  about axis A 4  and translation of the cylindrical rod R 1  in the A 4  direction are restrained by the force applied by surface of the rod-receiving recess  204  against the cylindrical rod R 1 , which depends on the force applied by the rod pusher  208  and the friction forces between the cylindrical rod R 1  and the surface of the rod-receiving recess  204 . The radius  498  of the cylindrical rod R 1  is shown with respect to the central axis R of the cylindrical rod R 1 . 
     As illustrated in  FIG.  4 B , aspects of the present disclosure provide improved retaining of the cylindrical rod R 1  in a rod-receiving recess  304 .  FIG.  4 B  is a cross-sectional view of a rod-receiving recess  308  having v-shaped grip grooves  401 . The-shaped grip grooves  401  define edges  412 ,  413  that are positioned to be contacted by the surface of the cylindrical rod R 1  at points E 1 -E 4 . That is, the v-shaped grip grooves  401  are formed in the surface of the rod-receiving recess  304  such that the edges  412 ,  413  define four points on a circle with a radius approximately the same as the cylindrical rod R 1 . In operation, the cylindrical rod R 1  rests against the four edges  412 ,  413  and rod pusher  308  applies a force  488  to the cylindrical rod R 1  that urges the surface of the cylindrical rod R 1  against the edges  412 ,  413 . In contrast to the prior art surface-to-surface engagement of  FIG.  4 A , the curved surface of the cylindrical rod R 1  is pressed against sharp edges  412 ,  413  (e.g., corners or apexes creating an edge of a sufficiently small radius or thickness to be able to be deformed by the cylindrical rod R 1  or to deform the surface of the cylindrical rod R 1  to allow the cylindrical rod R 1  to be urged into the grip grooves  401 ), such that rotation of the cylindrical rod R 1  while pressed against the four edges  412 ,  413  requires micro shearing of the edges  412 ,  413  at the points of contact E 1 -E 4 . Accordingly, the grip grooves  401  constrain the rotation of the cylindrical rod R 1  in the rod-receiving recess  304  in a manner that requires material property failure to allow movement between the edges  412 ,  413  and the cylindrical rod R 1 . As a result, for an equal force applied to the cylindrical rod R 1  by the rod pusher  308 , the grip grooves  401  are able to withstand higher torque on the cylindrical rod R 1  about axis A 4  before movement occurs. 
     In some instances, in order to ensure the engagement between the cylindrical rod R 1  and the edges  412 ,  413 , a pocket  450  is formed in the rod receiving recess  304 , such that the movement of the cylindrical rod R 1  against the edges  412 ,  413  as driven by the rod pusher  308  is not interrupted by the cylindrical rod R 1  contacting the inner surface of the rod receiving recess  308 . This configuration can allow the initial urging of the cylindrical rod R 1  against the edges  412 ,  413  to create some deformation at the points of contact E 1 -E 4  (e.g., deformation of either of the material of the cylindrical rod R 1 , the edges  412 ,  413 , or both), such that any initial rotation of the cylindrical rod R 1  induces further material deformation at the points of contact E 1 -E 4 . 
     While  FIG.  4 B  shows the body  302  having two grip grooves  401 , in some instances, the body  302  has only one grip groove  401 , for example, with the single grip groove  401  located opposite the clamping force vector, such that the force vector  488  and the center of the grip groove  401  intersects the central axis A 4  of the rod R 1 . 
       FIG.  4 B  also shows the radius  498  of the cylindrical rod R 1  and the distance  499  of each edge  412 ,  413  from the central longitudinal axis A 4  of the rod-receiving recess  304  can be approximately the same to allow the surface of the cylindrical rod R 1  to evenly contact the edges  412 ,  413 , in which case the central axis R of the cylindrical rod R 1  is concentric with the central longitudinal axis A 4  of the rod-receiving recess  304 . 
       FIG.  4 C  is a cross-sectional view of the rod-receiving recess  304  of  FIG.  4 B  showing a comparison of the position of a cylindrical rod R 1  with and without the grip grooves  401 .  FIG.  4 C  shows both the position of cylindrical rod R 1  engaged with the grip grooves  401  as shown in  FIG.  4 B , as well as the same cylindrical rod R 1  positioned in the rod-receiving recess  204  without the grip grooves  401  (as shown by dotted line R 1 ′). Compared with the position of the cylindrical rod R 1 ′ against the rod-receiving recess  204 , the cylindrical rod R 1  in the rod-receiving recess  304  having the grip grooves  401  is positioned farther into the pocket  450  because a small portion of the surface of the cylindrical rod R 1  is received into the grip grooves  401  to allow the edges  412 ,  413  to all contact the surface of the cylindrical rod R 1 . 
       FIG.  4 D  is a perspective view of a cylindrical rod R 1  showing lines of contact E 1 -E 4  made by the edges  412 ,  413  of the two grip grooves  401  when the cylindrical rod R 1  is positioned in a rod-receiving recess  304  (as shown in  FIG.  4 B ).  FIG.  4 D  shows the resultant contact between a rod-receiving recess  304  with grip grooves  401  that span the length of the cylindrical rod R 1 . This allows the cylindrical rod R 1  to be pressed into the edges  412 ,  413  along the lines of contact E 1 -E 4  such that the force applied to the cylindrical rod R 1  is not disrupted by contact between the cylindrical rod R 1  and the rod-receiving recess  304  except for along the lines of contact E 1 -E 4 . In some instances, and as explained in more detail below, additional points and lines of contact between the cylindrical rod R 1  and the rod-receiving recess  304  are within the scope of this disclosure, however, such additional contacts still permit a sufficient portion of the force applied to cylindrical rod R 1  to be directed against the edges  412 ,  413  along the lines of contact E 1 -E 4  in order to create the condition whereby rotation of the cylindrical rod R 1  is resisted by a requirement for material deformation of one or both of the cylindrical rod R 1  and the edges  412 ,  413  along the lines of contact E 1 -E 4 .  FIG.  4 D  shows that the lines of contact E 1 -E 4  are oriented to resist rotation of the cylindrical rod R 1  in both the clockwise CW and counterclockwise CCW directions. 
       FIG.  4 E  is a perspective view of a surface  410  of a rod-receiving recess  304  with a segmented grip groove  403 . The segmented grip groove  403  is formed when a grip groove  401  is intersected by one or more recesses or grooves  408 .  FIG.  4 E  shows the segmented grip groove  403  formed by a grip groove  401  that is intersected by two recesses  408  oriented at right angles to the grip groove  401 . In some embodiments, however, other angles are possible such that the one or more recesses or grooves  408  are transverse to the grip groove  401  (e.g., oblique to, etc.). The intersection of the grip groove  401  by the grooves  408  creates breaks in the grip groove edges  412 ,  413  along the length of the cylindrical rod R 1 , as shown in  FIG.  4 F , which are now referred to as segmented edges  412 ,  413 . 
       FIG.  4 F  is a top-down view of the segmented grip groove of  FIG.  4 E . In operation, the segmented edges  412 ,  413  serve to constrain movement of the cylindrical rod R 1  along the length of the segmented grip groove  403  (i.e., translation along axis A 4 ) by creating point edges  480  (e.g., small corners as the rod is urged into the groove and each point edge  480  forms a small corner of contact with the rod R 1 ), as shown in  FIG.  4 G  that engage the surface of the cylindrical rod R 1  and enable the segmented edges  412 ,  413  to apply a force on the cylindrical rod R 1  in the A 4  direction.  FIG.  4 G  is a schematic illustration of the contact points F 1 -F 4  on the cylindrical rod R 1  engaged with the segmented grip groove  403  of  FIG.  4 E .  FIG.  4 G  shows that when the cylindrical rod R 1  is pressed into engagement with the segmented grip groove  403 , the resulting contact lines F 1 , F 2  of the segmented edges  412 ,  413  are segmented as well. With the cylindrical rod R 1  engaged with the segmented edges  412 ,  413 , the point edges or small corners  480  define the transition between a point on the cylindrical rod R 1  where the segmented edges  412 ,  413  are and are not contacting the cylindrical rod R 1 . If, for example, the material properties of the cylindrical rod R 1  and the segmented edges  412 ,  413  are chosen such that the surface of the cylindrical rod R 1  is deformed when in contact with the segmented edges  412 ,  413 , then the point edges  480  together resist translation of the rod along the segmented grip groove  403  (i.e., translation along axis A 4 ) by virtue of this movement resulting in two of the four illustrated point edges  480  being driven into an un-deformed region of the surface of the cylindrical rod R 1 . Therefore, the segmented grip groove  403  can resist both rotation and translation of the cylindrical rod R 1  in the rod-receiving recess  403 , as shown in  FIG.  4 F . 
       FIG.  4 H  is a perspective of a surface of a cylindrical rod R 1  showing segmented lines of contact F 1 -F 4  made by the edges of two segmented grip grooves  403  when the cylindrical rod R 1  is positioned in the rod-receiving recess  304 . Clockwise CW and counterclockwise CCW rotation of the cylindrical rod R 1  is resisted by the segmented grip grooves  403  in the same manner as described above with respect to non-segmented grip grooves  401 , and translation of the cylindrical rod R 1  is resisted by the point edges  480  created between the segmented edges  412 ,  413   
     While  FIG.  3 A  shows grip grooves  301  having a cylindrical shape and  FIG.  4 B  shows grip grooves  401  having a V-shape, other shapes are possible. For example,  FIG.  5 A  is a cross-sectional view of a rod-receiving recess  403  having grip grooves  501  of a rectangular shape, where the edges  512 ,  513  are right angles.  FIGS.  3 A- 5 A  show the grip grooves  301 ,  401 ,  501  as recesses formed on the inner surface of the rod-receiving recess  304 , but the grip grooves can also be formed by protrusions  540  extending from the inner surface of the rod-receiving recess  304 , as shown in  FIG.  5 B . 
       FIGS.  3 A- 5 B  show the grip grooves  301 ,  401 ,  501  formed on wing portion  330  of the body  302  with the rod pusher  308  arranged to apply a force to the cylindrical rod R 1  to drive the cylindrical rod R 1  against the grip grooves  301 ,  401 ,  501 , but other configurations are possible. For example,  FIG.  6 A  is a cross-sectional view of a rod-receiving recess  304  with the rod pusher  308  having grip grooves  601 . As shown, the grip grooves  601  define edges  612 ,  613  that contact the cylindrical rod R 1  at four contact points E 1 -E 4  that define a circle of approximately the same radius as the cylindrical rod R 1 . In  FIG.  6 A , the surface of the rod-receiving recess opposite the rod pusher  308  defines a curved surface  651  that contacts the cylindrical rod R 1  to apply a force that opposes the force applied to the cylindrical rod R 1  by the edges  612 ,  613  of the grip grooves  601  of rod pusher  308 . In operation, the grip grooves  601  of the rod pusher  308  function in the same manner as the grip grooves  301 ,  401 ,  501  described above.  FIG.  6 B  is a cross-sectional view of a rod-receiving recess  304  with a pocket  450  and a rod pusher  308  having grip grooves  601 .  FIG.  6 C  shows both the rod-receiving recess  304 , grip grooves  501 , and the rod pusher  308  having grip grooves  601 . In this configuration, the position of the cylindrical rod R 1  is highly constrained by the grip grooves  501 ,  601  as the eight points of contact E 1 -E 8  all engage with the surface of the cylindrical rod R 1 . However, in some instances, the tolerances in the positions of the eight points of contact E 1 -E 8  is reduced upon initial contact with the cylindrical rod R 1  such that the cylindrical rod R 1  is highly constrained once sufficient force is applied to sufficiently engage the cylindrical rod R 1  about the eight points of contact E 1 -E 8 . 
       FIG.  7 A  is a cross-sectional view of a single grip groove  501  having an internal protrusion  710  arranged to contact the surface of the cylindrical rod R 1  between the edges  512 ,  513  of the grip groove  501 . In order to increase the restraint of the cylindrical rod R 1  in the translational direction (i.e., along axis A 4 ), additional grip features can be included inside the grip groove  501  in order to provide translational restraint with a minimal to negligible effect on the rotational restraint provided by the edges  512 ,  513 . In  FIG.  7 A  the grip groove  501  includes one or more protrusions  701  along the length of the grip groove  501 . The internal protrusions  710  extend towards the cylindrical rod R 1  and include a feature, such as end edge  711  that is positioned to contact the cylindrical rod R 1  when the cylindrical rod R 1  is engaged with the edges  512 ,  513  of the grip groove  501 .  FIG.  7 A  shows an internal protrusion  710  with an edge  711  contacting the cylindrical rod R 1  along a line of contact L 1  that is perpendicular to the lines of contact E 1 , E 1  created by the edges  512 ,  513 . In operation, the position and shape of the edge  711  in the grip groove  501  can be configured to vary the strength of the contact line L 1  between the cylindrical rod R 1  and the edge  711  when the cylindrical rod R 1  is in contact with the edges  512 ,  513 , as shown in more detail in  FIGS.  7 D and  7 E . 
       FIG.  7 B  is a top-down view of the grip groove  501  of  FIG.  7 A  showing the perpendicular edges  711  of two internal protrusions  710  creating two parallel lines of contact L 1 , L 2  on the cylindrical rod R 1 .  FIG.  7 C  is a cross-sectional view of the grip groove  501  of  FIG.  7 A  showing the edges  711  of the internal protrusions  710  contacting the cylindrical rod R 1  when the rod is engaged with the grip groove  501 . 
       FIG.  7 D  is a perspective view of a cylindrical rod R 1  showing lines of contact E 1 -E 4  made by the edges  512 ,  513  of two grip grooves  501  and the lines of contact L 1 -L 4  made by two internal protrusions  710  in each grip groove  501 . In operation, the lines of contact E 1 -E 4  made by the edges  512 ,  513  resist rotation of the cylindrical rod R 1  in the clockwise CW and counterclockwise CCW directions, and the lines of contact L 1 -L 4  made by two internal protrusions  710  resist translation of the cylindrical rod R 1  in the A 4  direction. 
       FIGS.  7 E and  7 F  are cross-sectional views of a single grip groove  501  having an internal protrusion  710  with two different edge configurations. In a first configuration, shown in  FIG.  7 E , the internal protrusion  710  extends to a flat edge  711  that will define a line of contact L 1  with the cylindrical rod R 1  that lengthens as the cylindrical rod R 1  is urged against the edges  512 ,  513 . Because the initial size of the line of contact L 1  is small, the resistance to movement of the cylindrical rod R 1  into the grip groove  510  is minimal, ensuring that a positive engagement between the cylindrical rod R 1  and the internal protrusion  710  is established. In a second configuration, shown in  FIG.  7 F , the internal protrusion  710  has a curved edge  712  that is shaped to initially contact the cylindrical rod R 1  with a longer line of contact L 1 , in order to increase the translational restraint of the cylindrical rod R 1  upon initial contact with the edges  512 ,  513 . The resultant strength of the restraint of the two different edge shapes  711 ,  712  can depend on the material properties of the cylindrical rod R 1 , edges  512 ,  513 , and internal protrusion  710  and the location of the edge  711 ,  712  with respect to the edges  512 ,  513 . For example, if the cylindrical rod R 1  is made from a metal that is softer than the metal of the internal protrusion  710 , then the edge  711 ,  712  can extend closer to the radius of the edges  512 ,  513  (i.e., the expected location of the cylindrical rod R 1 ), such that the cylindrical rod R 1  first contacts the edge  711 ,  712  and the edge  711 ,  712  is driven into the surface of the cylindrical rod R 1  as the cylindrical rod R 1  interfaces with the edges  512 ,  513 . 
       FIG.  8 A  is a cross-sectional view of a prior art rod-receiving recess  206  of, for example, a receiver member  14  of a bone anchor assembly  10  or a body  202  of a connector  200 . A cylindrical rod R 1  is disposed in the rod-receiving recess  206  and contacts the inner surface of the rod-receiving recess  206  at two points P 1 , P 2  along the axis of the rod-receiving recess  206 . A locking element or set screw  104 ,  216  contacts the cylindrical rod R 1  at point P 3  and applies force to urge the cylindrical rod R 1  into the rod-receiving recess  206  and against the two points P 1 , P 2 . 
       FIG.  8 B  is a cross-sectional view of a rod-receiving recess  306  with two grip grooves  801  formed as parallel recesses in the inner surface of the rod-receiving recess  306 .  FIG.  8 B  shows a cylindrical rod R 1  disposed in the rod-receiving recess  306  and engaged with the edges  812 ,  813  of the grip grooves  801 . A threaded locking element or set screw  104 ,  216  is arranged to secure the cylindrical rod R 1  in the rod-receiving recess  306  by applying a force to the cylindrical rod R 1  to urge the cylindrical rod R 1  against the edges  812 ,  813  of the grip grooves  801 .  FIG.  8 B  also illustrates the position of the cylindrical rod R 1  would be in the rod-receiving recess  306  if the grip grooves  801  were missing, which is shown as R 1 ′. The difference between R 1 ′ and the position of the cylindrical rod R 1  with the grip grooves  801  is due to the grip grooves  801  defining edges  812 ,  813  along the inner surface of the rod-receiving recess  306  at four locations with approximately the same radius from an central axis of the rod-receiving recess  306  as the cylindrical rod R 1 , which thereby permits the central axis of the cylindrical rod R 1  to be concentric with the axis defined by the equal radius location of the edges  812 ,  813 . Said otherwise, the grip grooves  801  are positioned to allow the cylindrical rod R 1  to rest against all 4 edges  812 ,  813  when urged into the rod-receiving recess  306  by the locking element or set screw  104 ,  216 . In some instances, the rod-receiving recess  306  defines a pocket or gap  850  below the designed position of the cylindrical rod R 1  (as determined by the grip grooves  801 ), in order to allow additional urging of the cylindrical rod R 1  into the rod-receiving recess  306  by the set screw  104 ,  216  to result in increased pressure on the edges  812 ,  813 . 
       FIGS.  8 C and  8 D  illustrate embodiments where the grip grooves  801  are positioned on an insert  816  that is driven against the cylindrical rod R 1  in the rod-receiving recess  306 . The insert  816  is driven by set screw  104 ,  216  against the cylindrical rod R 1 , but does not rotate with the set screw  104 ,  216  because the insert  816  is shaped to extend the grip grooves  801  along a length of the cylindrical rod R 1 . The insert  816  can include a peg  817  to be received by the set screw  104 ,  216  in order to couple the grooves  801  with the body  302  (into which the set screw  104 ,  216  is threaded, not shown) to allow the body  302  to oppose rotation of the cylindrical rod R 1  via the cylindrical rod&#39;s R 1  engagement with the grip grooves  801 .  FIG.  8 C  shows the rod-receiving recess  306  having a circular section to provide surface contact with the cylindrical rod R 1  to oppose the force of the insert  816 , and  FIG.  8 D  shows the rod-receiving recess  306  having a tapered closed end with a gap  850  providing two lines of contact P 1 , P 2  with the cylindrical rod R 1  to oppose the force of the insert  816 . 
       FIG.  9    is a perspective view of a receiving member  902  having a rod-receiving recess  904  with two grip grooves  801  formed therein.  FIG.  10    is a perspective view of a receiving member  902  having a rod-receiving recess  904  with two circumferential grooves  1020  cut into the rod-receiving recess  904  perpendicular to the central longitudinal axis of the receiving recess  904 . 
       FIGS.  11 A and  11 B  are perspective views of a receiving member  902  having a rod-receiving recess  904  with two segmented grip grooves  403  formed therein by the intersection of two grip grooves  801  and two circumferential grooves  1020 .  FIG.  11 B  shows the point edges  480  created in by the segmented grip grooves  403 .  FIG.  12    is a perspective view of a receiving member  902  having a rod-receiving recess  906  with a grip groove  801  having multiple internal protrusions  710  formed therein. The receiving member  902  and rod-receiving recess  904  of  FIGS.  9 - 11 B  could be a body  302  of a connector  300 , for example, or a receiver member  14  of a bone anchor assembly  10 , or any other implant configured to be secured to a rod, cable, or other spinal fixation element. 
       FIG.  13    is an illustration of a connector  300  with a saddle  301  that defines a rod-receiving recess with grip grooves  1301 . In the connector  300  of  FIG.  13   , the cylindrical rod R 1  is secured to the receiving member  14  by being engaged with a rod-receiving recess formed by an inner surface the saddle  301 , and the inner surface of the saddle  60  includes two grip grooves  1301  configured to grip the cylindrical rod R 1  when the locking screw  216  applies a force on the cylindrical rod R 1  to urge the cylindrical rod R 1  into the grip grooves  1301 . 
       FIG.  14    is an illustration of a bone anchor assembly  10  with a compression member  60  that defines a rod-receiving recess with grip grooves  1401 . In the bone anchor assembly of  FIG.  14   , the cylindrical rod R 1  is secured to the receiver member  14  by way of contact with a compression member  60  disposed in the receiver member, where an inner surface of the compression member  60  that forms the rod-receiving recess includes the grip grooves  1401  that contact the cylindrical R 1 . In operation, the inner set screw  102  applies a force to the cylindrical rod R 1  to urge that cylindrical rod R 1  into the grip grooves  1401 . 
       FIG.  15    is a perspective view of a human spine with a fixation system attached thereto. 
     An exemplary method of using the bone anchors and connectors disclosed herein is described below. In some instances, the bone anchors and connectors are each secured to one or two rods in order to, for example, bridge between constructs in the cervical and thoracic regions of a patient&#39;s spine. The bone anchors and connectors can be secured to the rods using rod-receiving recesses with grip grooves in order to increase resistance to movement of the rods, connectors, ad bone anchors, relative to one another. 
     The procedure can begin by forming an open or percutaneous incision in the patient to access a target site. The target site can be one or more vertebrae, a long bone or multiple portions of a long bone, or any other bone or non-bone structure of the patient. As shown in  FIG.  15   , the target site can be multiple vertebrae in the patient&#39;s cervical and thoracic spine. 
     Bone anchors can be driven into one or more of the vertebrae and spinal rods can be attached thereto using known techniques. In the illustrated example, bilateral spinal rods R 1 , R 2  are coupled to four adjacent vertebrae V 1 -V 4  using eight bone anchors S 1 -S 8 . In addition, bilateral rods R 3 , R 4  are coupled to the next two adjacent vertebrae V 5 -V 6  using four bone anchors S 9 -S 12 . The rods R 1 , R 2  can be connected to the rods R 3 , R 4 , respectively, using four connectors C 1 -C 4  of the type described herein (e.g., connector  300 ) and the bone anchors S 1 -S 8  can be connected to the rods R 1 -R 4  using receivers of the type described herein (e.g., receiver  902 ). 
     As shown, the low-profile nature of the connectors C 1 -C 4  can allow them to be installed at adjacent vertebral levels on the same rod (e.g., between V 2 /V 3  and between V 3 /V 4 ). As also shown, the connectors C 1 -C 4  can connect to the rods R 1 , R 2  between bone anchors installed in adjacent vertebral levels. 
     The connectors C 1 -C 4  can receive the rods in respective rod-receiving recesses, with the rod-receiving recesses having grip grooves to secure the connector to the rods R 1 , R 2 , thereby providing improved rotational and, in some instances, axial restraint of motion of the rods R 1 , R 2  relative to the connectors. 
     The connectors C 1 -C 4  can include independent locking features such that they can be locked to the rods R 1 , R 2  prior to being locked to the rods R 3 , R 4  or vice versa. 
     The connectors C 1 -C 4  can include single-step locking features such that they can be simultaneously locked to their respective rods. For example, connector C 1  can be simultaneously locked to rods R 1  and R 3 . 
     All of the rods R 1 -R 4 , the connectors C 1 -C 4 , and the bone anchors S 1 -S 12  can be installed in a single procedure. 
     Alternatively, the rods R 1 , R 2  and the bone anchors S 1 -S 8  may have been installed in a previous procedure, and the current procedure can be a revision procedure in which the rods R 3 , R 4 , the connectors C 1 -C 4 , and the bone anchors S 9 -S 12  are installed to extend the previously-installed construct to additional levels. 
     The connectors C 1 -C 4  can be attached to position the rods R 1 -R 4  such that they are substantially parallel to one another and substantially lie in a common coronal plane as shown. The connectors C 1 -C 4  can also be rotated 90 degrees from the orientation shown to position the rod pairs R 1 , R 3  and R 2 , R 4  such that they substantially lie in respective common sagittal planes. 
     The above steps can be repeated to install additional rods and/or connectors at the same or at different vertebral levels. Final tightening or other adjustment of the construct can be performed and the procedure can be completed using known techniques and the incision closed. 
     It should be noted that any ordering of method steps expressed or implied in the description above or in the accompanying drawings is not to be construed as limiting the disclosed methods to performing the steps in that order. Rather, the various steps of each of the methods disclosed herein can be performed in any of a variety of sequences. In addition, as the described methods are merely exemplary embodiments, various other methods that include additional steps or include fewer steps are also within the scope of the present disclosure. 
     While the methods illustrated and described herein generally involve attaching spinal rods to multiple vertebrae, it will be appreciated that the connectors 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 connectors 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. 
     Although specific embodiments are described above, it should be understood that numerous changes may be made within the spirit and scope of the concepts described.