Patent Publication Number: US-8540724-B2

Title: Anterior distractor-inserter with linear countersink adjustment

Description:
BACKGROUND 
     A distractor-inserter is a device that can be used to distract (e.g., separate) two elements and insert another element between the two separated elements. For example, in the field of spinal medicine, a distractor-inserter is commonly used to separate two vertebrae and insert a spinal implant therebetween. The implant facilitates bone growth between the two vertebrae to thereby reduce back pain caused by a degenerated disc or other condition. 
     Distraction and insertion typically can be through an anterior approach (i.e., through the front of the patient) or a posterior approach (i.e., through a back side of the patient). When a surgeon uses a distractor-inserter, the surgeon inserts blades of the distractor-inserter into a cavity in the patient&#39;s body, positioning the blades between the two vertebrae to be distracted. The blades are then separated to distract or separate the vertebrae apart to create room for the implant to be inserted. After distracting the two vertebrae, the distractor-inserter is manipulated to insert the implant between the distracted vertebrae. 
     Typically, and particularly in the case of anterior distraction-insertion, there is very little room for a doctor to work within a patient&#39;s body during the distraction-insertion process. During the distraction-insertion process it is preferable that the distractor-inserter has minimal contact and interference with the patient&#39;s internal organs and vasculature to minimize trauma to the patient. 
     BRIEF SUMMARY 
     Embodiments presently disclosed generally relate to a distractor-inserter. More specifically, embodiments relate to an anterior distractor-inserter providing linear countersink depth adjustment. The countersink depth can be set to a value within a range of values, which may include more than two values. Adjustment may be analog, where any value within the range can be selected, or discrete, where whole values within the range can be selected. Embodiments may further include constant height retraction members. Further still, embodiments may include a ratchet mechanism that allows for automatic forward motion, but prevents backward motion unless the ratchet is manually disengaged. 
     An embodiment of a distractor-inserter includes a handle forming a passage disposed along a longitudinal axis, a drive shaft disposed along the longitudinal axis through the passage and moveable along the longitudinal axis, and a drive shaft grip coupled to a proximate end of the drive shaft and configured for use in imparting motion to the drive shaft. The distractor-inserter further includes first and second blades having respective first ends connected to the handle and respective second ends that can be opened, the first and second blades being curved away from the longitudinal. 
     Further still, an embodiment of the distractor-inserter includes a head block assembly coupled to a distal end of the drive shaft and disposed within the space between the first and second blades. The head block assembly is moveable along the longitudinal axis in response to movement of the drive shaft, wherein movement toward the second ends of the blades causes the second ends to open. The head block assembly includes first and second retraction members projecting in opposite directions away from the longitudinal axis and disposed within respective first and second elongate channels of the first and second blades. 
     Still further, an embodiment of the head block assembly includes a countersink adjustment mechanism including a tip at a distal end. The countersink adjustment mechanism is configured to linearly adjust a distance between the tip and the first and second retraction members to thereby linearly adjust a countersink depth of an implantable element pushed by the tip to a position between two distractable elements separated by the second ends of the first and second blades when the second ends are opened. The countersink adjustment mechanism may be configured to linearly adjust the countersink depth within a range including more than two depths. Still further, the countersink adjustment mechanism may provide analog adjustment of the countersink depth. Further yet, the countersink adjustment mechanism may provide stepwise adjustment of the countersink depth. 
     Still further, the range of countersink depths may comprise a lower limit of zero millimeters, and wherein a countersink depth of zero millimeters corresponds to an outer face of the implantable element being substantially flush with one or both outer faces of the two distractable elements. The first and second retraction members may be slidable along the respective first and second elongate channels of the first and second blades, and remain at a generally constant height relative to the respective first and second blades, regardless of their locations along lengths of the first and second blades. 
     An embodiment of a handle of a distractor-inserter includes a ratchet mechanism that includes an arm mounted on a pivot pin within the handle. Catch members are disposed on a first end of the arm and facing the drive shaft to engage threads of the drive shaft. The catch members allow for forward linear movement of the drive shaft but prevent backward linear movement of the drive shaft. The catch members may further allow forward and backward rotational movement of the drive shall. The ratchet mechanism may further include a push button on a second end of the arm and exposed on the handle. The push button is depressable to cause the arm to pivot about the pivot pin to disengage the catch members from the threads of the drive shaft. Disengagement of the catch members allows for backward linear movement of the drive shaft. 
     An embodiment of a countersink adjustment mechanism includes a countersink member carriage disposed along the longitudinal axis, a countersink member slidably coupled to the countersink member carriage, the countersink member having a distal end including the tip, and a countersink adjustment shall disposed through the passage in the countersink member carriage and coupled to the countersink member. At least a portion of the countersink adjustment shaft is threaded. The countersink adjustment mechanism further includes a countersink adjustment interface configured to engage the threaded portion of the countersink adjustment shaft to thereby cause longitudinal movement of the countersink adjustment shaft to impart corresponding longitudinal movement on the countersink member. The distractor countersink member may form a sleeve around the countersink member carriage. The countersink adjustment interface may include a thumbwheel. 
     An embodiment of a method of performing a distraction-insertion procedure includes selecting a countersink depth setting with a linear countersink adjustment mechanism of a distractor-inserter. Setting the countersink depth may include linearly adjusting a longitudinal distance between retraction members of the distractor-inserter and a tip configured to push an implant between two vertebral bodies. The method further includes positioning distal ends of blades of the distractor-inserter between the two vertebral bodies to be separated and driving a head block assembly of the distractor-inserter forward until the retraction members of the distractor-inserter engage respective outer faces of the vertebral bodies, wherein driving the head block assembly forward causes the distal ends of the blades to open, thereby separating the vertebral bodies, and wherein driving the head block assembly forward further comprises pushing the tip to a countersink depth between the vertebral bodies corresponding to the countersink depth setting. The method may further include retracting the distal ends of the blades and the tip from between the vertebral bodies, wherein the vertebral bodies hold the disc implant between the vertebral bodies at the selected countersink depth. 
     In an embodiment of a method, selecting the countersink depth setting includes adjusting the countersink depth adjustment interface. Selecting the countersink depth setting may include selecting a countersink depth from among a linear range of countersink depths. The linear countersink adjustment mechanism may include markings indicating a linear range of available countersink depths. 
     An embodiment of an apparatus for distracting two distractable elements and inserting an implantable element therebetween includes an elongate handle, first and second blades having respective proximal ends connected to a distal end of the elongate handle, and respective distal ends configured to distract the two distractable elements. The apparatus further includes a longitudinally moveable drive shaft disposed through a passage formed by the elongate handle and a head block assembly mounted on a distal end of the drive shaft and disposed within a space formed between the first and second blades. The head block assembly includes a tip configured to push the implantable element to a selected depth between the distracted elements, first and second retraction members slidably disposed within respective first and second channels of the first and second blades, wherein the first and second retraction members are configured to engage respective outer faces of the distracted elements, and means for linearly adjusting a distance between the tip and the first and second retraction members to thereby linearly adjust a countersink depth of the implantable element between the distracted elements. 
     In an embodiment of the apparatus, the means for linearly adjusting the distance between the tip and the retraction members includes a countersink member carriage having a first passage disposed therethrough along a longitudinal axis, a countersink member moveably coupled to the countersink member carriage and having a distal end comprising the tip, a thumbwheel having a second passage disposed therethrough along the longitudinal axis, the second passage having a threaded surface, and a countersink adjustment shaft disposed along the longitudinal axis and extending through the first passage and the second passage. The countersink adjustment shaft has a distal end coupled to the countersink member and a proximal end that is threaded. The threaded surface of the second passage engage with threads of the countersink adjustment shaft, and rotation of the thumbwheel causes longitudinal movement of the countersink adjustment shaft. Longitudinal movement of the countersink adjustment shaft causes corresponding longitudinal movement of the countersink member. 
     Further still, the distal end of the drive shaft may be twistably coupled to the head block assembly. The handle may include a ratchet mechanism configured to engage the drive shaft to prevent backward linear movement of the drive shaft unless the ratchet mechanism is disengaged from the drive shaft. The ratchet mechanism may permit forward linear movement when engaged with the drive shaft. The first and second retraction members may remain at a constant height relative to the respective first and second blades as the first and second retraction members slide along a length of the respective first and second blades. The countersink member carriage may include markings indicating a range of selectable countersink depths. The countersink member may include a sleeve around the countersink member carriage. The sleeve may expose countersink depth markings on the countersink member carriage as the sleeve moves forward. In some embodiments, the distal end of at least one of the blades has a friction element adapted to engage a face of the distractable element. This friction element may be shaped to generally match at least a portion of the distractable element face. In a particular embodiment, the distractable element is a vertebra, and the friction element is shaped to generally match at least a portion of the end plate of the vertebra. 
     This summary provides only a general outline of some embodiments disclosed herein. Many other objects, features, advantages and other possible modifications to the disclosed embodiments will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an anterior distractor-inserter according to one embodiment of the disclosure. 
         FIG. 2  is an elevation view of the distractor-inserter of  FIG. 1 . 
         FIG. 3  is a plan view of a distal end of a blade of the distractor-inserter of  FIG. 1 . 
         FIG. 4  is a cross sectional elevation view of a head block assembly of the distractor-inserter of  FIG. 1 . 
         FIG. 5  is an exploded perspective view of the head block assembly including a countersink adjustment mechanism according to an embodiment. 
         FIGS. 6A-6B  are perspective views of the head block assembly including the countersink adjustment mechanism set to different countersink depth settings. 
         FIGS. 7A-7C  are elevation views of the head block assembly between the blades, including the countersink adjustment mechanism set to different countersink depth settings. 
         FIG. 8  is an exploded perspective view of a drive shaft coupling to the head block assembly according to one embodiment. 
         FIG. 9  is a cross sectional elevation view of a ratchet system of the distractor-inserter of  FIG. 1  according to one embodiment. 
         FIG. 10  is an elevation view of blades, head block assembly and countersink depth adjustment mechanism inserting an implant between adjacent vertebral bodies at a selected countersink depth. 
         FIG. 11  is a flow diagram illustrating steps in a process of using a distractor-inserter in a distraction-insertion procedure according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments presently disclosed generally relate to a distractor-inserter. More specifically, embodiments relate to an anterior distractor-inserter providing linear countersink depth adjustment. The countersink depth can be set to a value within a range of values, which may include more than two values. Adjustment may be analog, where any value within the range can be selected, or discrete, where whole values within the range can be selected stepwise. Embodiments may further include constant height retraction members. Further still, embodiments may include a ratchet mechanism that allows for automatic axial forward motion, but prevents axial backward motion unless the ratchet is manually disengaged. 
     A distractor-inserter is a device that can be used to distract (i.e., separate) two elements and insert another element therebetween. For example, in spinal surgery, a distractor-inserter may be used to separate two adjacent vertebral bodies and insert a spinal implant between the two separated vertebral bodies. The implant may be selected from a variety of implants, including without limitation, cages or spacers used to promote fusion, dynamic stabilization devices or artificial discs, and the like. The implants may be metal (e.g., titanium), or plastic (e.g., PEEK), or other biocompatible materials including other metals and/or plastics. Once inserted between two vertebral bodies, some spinal implants may ease pain by facilitating bone growth and fusion between the two vertebral bodies. 
     An embodiment of a distractor-inserter includes a head block assembly coupled to a distal end of a drive shaft, which can move the head block assembly forward and backward along a longitudinal axis of the distractor-inserter. The head block assembly is slidably disposed between two blades that can open at a distal end to separate two distractable elements, such as vertebral bodies. When the distal ends of the blades are positioned between the two distractable elements, forward movement (i.e., toward the distal ends) of the head block assembly cause the blades to open and thereby separate the two distractable elements. 
     According to various embodiments, the head block assembly includes a tip configured to push an implantable element toward the distal ends of the blades. After the distal ends of the blades have opened to separate the distractable elements, continued forward motion of the head block assembly causes the tip to push the implantable element beyond the open distal ends of the blades and into a space created between the two separated elements. When the tip is retracted from the space, the two separated elements engage the implantable element, thereby holding the implantable element in place between the separated elements. 
     According to one or more embodiments, the head block assembly includes retraction members above and below the tip. When the head block assembly is moved forward to its furthest extent, the retraction members engage with outer faces of at least one, and often both of the separated elements. Further forward force applied to the head block assembly causes the retraction members to push against the outer faces of the separated elements. The retraction members cause the tip to retract from between the separated elements when the retraction members push against outer faces of the separated elements. 
     In various embodiments, the location of the head block assembly tip is adjustable relative to the rest of the head block assembly to provide for adjustability of a countersink depth of the implantable element between the separated elements. More specifically, the distance between the tip and the retraction members is adjustable, and corresponds to the distance between the outer faces of the separated elements and the implantable element when the tip is fully extended into the space between the separated elements. The adjustable distance between the retraction elements and the tip therefore corresponds to the countersink depth of the implantable element. 
     According to at least one embodiment, the countersink depth can be adjusted linearly within a range of depths. In one embodiment, the range is zero millimeters to eight millimeters; however, other ranges are possible. In some embodiments selection of the countersink depth is discrete, wherein only whole values (e.g., nonfractional) can be selected. In other embodiments, selection of the countersink depth is analog, wherein any value (e.g., fractions of millimeters) within the range can be selected. Setting of the countersink depth to zero millimeters causes the implantable element to be positioned such that an outer face of the implantable element is substantially flush with at least one or both outer faces of the separated elements on either side of the implantable element. 
     Some embodiments include retraction members having constant height relative to outer faces of blades of the distractor-inserter. In these and other embodiments, the retraction members may be arms having faces that engage with respective faces of separated elements, such as vertebral bodies. The arms are anchored at a base portion of the head block assembly. The arms are disposed within, and guided by, channels in the blades. When the head block assembly moves forward and backward, the height of the arm faces remains generally constant, regardless of where the arms are along the length of the blades. 
       FIG. 1  is a perspective view of an example distractor-inserter  100  according to one embodiment of the disclosure.  FIG. 2  is an elevation view of the distractor-inserter  100 .  FIG. 3  is a plan view of a distal end of blades of the distractor-inserter  100 . For ease of discussion,  FIGS. 1-3  are referenced together in the following description. 
     The distractor-inserter  100  includes a handle  102  which enables a user to hold the distractor-inserter  100  during use. Handle  102  has a proximate end  104  and a distal end  106 . The handle  102  forms a passage  108  (seen through a cut-out portion of the handle  102 ) extending through the length of the handle  102 . In the illustrated embodiment, distractor-inserter  100  includes a drive shaft  110  extending through the passage  108  formed by the handle  102 . Accordingly, the passage  108  may be a substantially cylindrical shape, or some other shape that is capable of receiving a cylindrically shaped member, such as drive shaft  110 . 
     Handle  102  and drive shaft  110  are disposed along a longitudinal axis  112  that extends the length of the distractor-inserter  100 . A grip  114 , such as the illustrated “T” grip, is connected to a proximate end of drive shaft  110 . The drive shaft  110  is slidably disposed through passage  108  of handle  102 , such that the shaft  110  can be driven in a forward motion and a backward motion along the longitudinal axis  112 , using the grip  114 . 
     Connected to the distal end  106  of handle  102  are first and second blades  116 . In the illustrated embodiment, first and second blades  116  are opposing. Opposing blades  116  have respective proximate ends  118  and distal ends  120 . Opposing blades  116  have respective outer faces  122 , facing away from the longitudinal axis  112 , and an inner faces  124 , facing the longitudinal axis  112 . In the illustrated embodiment, the blades  116  are substantially concave. For example, middle sections of the length of the blades  116 , between the distal ends  120  and the proximate ends  118 , are curved away from the longitudinal axis  112 . 
     The blades  116  are constructed of a material that renders them relatively flexible, such that the blades  116  can be disposed in a separated position or an unseparated position. In a separated position, opposing inner faces  124  of the blades  116  are open (e.g., not in contact with each other) at the distal ends  120 . In the unseparated position, the inner faces  124  of the blades  116  are typically closed (e.g., in contact with each other) at the distal ends  120 . 
     First and second blades  116  include stop members  126  which are mounted on outer faces  122  of the blades  116  near the distal ends  120  of the blades  116  (see, e.g.,  FIG. 3 ). Each stop member  126  has an engagement face  128  that faces the distal end  120  of the blade  116 . Each engagement face  128  is substantially perpendicular to the respective outer face  122  of the blade  116  that the stop member  126  is mounted on. 
     In the illustrated embodiment, the first and second blades  116  include respective elongate channels  130  extending at least a portion of the length of the blades  116 . In one embodiment, a stop member  126  is mounted on each side of the elongate channel  130  of the respective blade  116 . Stop members  126  may be arranged on the blades  116  in different arrangements than those shown, depending on the particular implementation. 
     During operation, each engagement face  128  of the stop members  126  engages (e.g., abuts) a face of a distractable element, such as a vertebral body, which is to be separated. Stop members  126  may engage the face of one or both distractable elements depending on, for example, the shape of the distractable element faces, the position of the distractable elements, the surrounding tissue, as well as other factors. The engagement faces  128  thereby allow only a distal portion of each blade  116  to extend between the distractable elements (e.g., vertebral bodies). Specifically, the portions of the blades  116  allowed to extend between the separated elements extend from the furthest distal ends  120  of the blades  116  to the engagement faces  128  of the stop members  126 . The stop members  126  may be any of numerous shapes and sizes, depending on the particular implementation, preferably to facilitate minimal contact with bodily structures (e.g., organs or vasculature) around the vertebral bodies (e.g., superior and inferior vertebrae). 
     With further regard to the blades  116 , distal ends  120  further include friction elements  132 . Friction elements  132  engage interior faces of distractable elements, such as the end plates of vertebral bodies. The friction elements  132  help stabilize the distal ends  120  of the blades  116  between the vertebral bodies during operation, by creating a friction force between the blades  116  and the vertebral bodies. In the illustrated embodiment, the friction elements  132  include a number of raised elements, such as bumps. Bumps are merely one example of types of friction elements that can be employed. Other embodiments may employ other friction elements such as serrations. 
     The bumps  132  (or other types of friction elements) may be beneficially oriented on outer faces  122  of the blades  116  in ways that correspond to the general shape or curvature of typical vertebral bodies (or other distractable elements). Such orientations of friction elements  132  may facilitate stabilization of the blades  116  between vertebral bodies better than orientations that do not correspond to the shapes or curvatures of the vertebral bodies. For example, in the illustrated embodiment, bumps  132  are disposed substantially diagonally from near the channel  130  distally outward toward the outer edges of the blades  116 . In some embodiments, bumps  132  are in a curved orientation that may generally match a portion of the vertebral body. For example, in a particular embodiment bumps  132  are positioned to generally align with the apophyseal ring of a vertebral body. Although three bumps  132  are shown on each side of the channel  130  (see, e.g.,  FIG. 3 ), it is to be understood that any number of bumps  132  (or other types of friction elements) can be employed to achieve different results. 
     Referring again to the blades  116 , the concavity of the blades  116  forms a space  134  between the inner faces  124  of the blades  116 . The distal end of the drive shaft  110  extends into the space  134 . A head block assembly  136  is mounted at the distal end of the drive shaft  110 . The head block assembly  136  provides for retraction of the distal ends  120  of the blades  116  from separated elements and countersink depth adjustment, and is discussed in further detail below with reference to  FIGS. 4-7 . 
     Turning to  FIG. 4 , a cross-sectional elevation view of head block assembly  136  is shown. The head block assembly  136  includes two retraction members, such as upper and lower elongate arms  202 . The head block assembly  136  is positioned between the blades  116  such that the arms  202  project laterally away from the longitudinal axis  112  and extend through respective elongate channels  130  of the blades  116 . The channels  130  form tracks that guide the respective arms  202  as the head block assembly  136  moves forward (toward the distal ends  120 ) and backward (toward the handle  102 ) on the drive shaft  110 . Arms  202  have height  204  that remains constant relative to the outer faces  122  of the respective blades  116 . By maintaining a generally constant height  204  of arms  202  relative to outer faces  122  of blades  116 , arms  202  engage tissue surrounding the disc space at a known orientation and position. Such a feature may be particularly useful when implanting into a small or short disc space, to help arms  202  avoid significant contact with soft tissue, vessels, and the like. 
     A tip  206  is disposed at a distal end of the head block assembly  136 . The tip  206  is configured for pushing an implantable element toward the distal ends  120  of the blades  116  as the head block assembly  136  is driven forward by the drive shaft  110 . When the distal ends  120  are opened, a space is created between two separated elements and the tip  206  pushes the implantable element between the two separated elements. A substantially longitudinal offset  208  between the tip  204  and the arms  202  corresponds to a countersink depth at which the implantable element is inserted. 
     The head block assembly  136  includes a countersink depth adjustment mechanism  210  for adjusting the offset  208  corresponding to the countersink depth of an implanted element. In one embodiment the countersink depth adjustment mechanism provides for linear adjustment of the countersink depth within a range of countersink depths. An embodiment of the countersink depth adjustment mechanism is shown in  FIGS. 5-7  and is discussed in further detail below. 
       FIG. 5  is an exploded perspective view of the head block assembly  136  according to one embodiment. In the view of  FIG. 5 , the countersink depth adjustment mechanism  210  is also exploded. With further regard to the arms  202 , each arm  202  includes a rider member  212 . Rider members  212  stabilize the arms  202  within the channels  130  of respective blades  116  and facilitate sliding of the arms  202  longitudinally forward and backward along the lengths of the blades  116 . Rider members  212  facilitate maintaining constant height  204  of the arms  202  as the arms  202  slide along the blades  116 . 
     The head block assembly  136  further includes a buttress  214  configured for anchoring the arms  202  and the drive shaft  110  within the distractor-inserter  100 . Proximate ends of the arms  202  include coupling members  216  for coupling to a coupling point  218  of the buttress  214 . The buttress  214  further includes upper and lower arced slots  220  into which bottom portions of respective elongate arms  202  fit. The coupling members  216  align with the coupling point  218  when the arms  202  are in the respective arced slots  220 . When coupling members  216  and the coupling point  218  are aligned, fasteners, such as pins  222 , are inserted therethrough to anchor the arms  202  to the buttress  214 . 
     The buttress  214  further includes a core member  224  to which the distal end of the drive shaft  110  can be coupled. Coupling of the drive shaft  110  to the core member  224  is discussed in further detail below with reference to  FIG. 8 . In the illustrated embodiment, the core member  224  has a longitudinally disposed passage  226  formed therethrough. Core member passage  226  is discussed in further detail below with reference to the countersink depth adjustment mechanism  210 . 
     In the illustrated embodiment the countersink depth adjustment mechanism  210  generally includes a countersink adjustment member  228 , a countersink adjustment member carriage  230 , a countersink adjustment interface member  232  and a countersink adjustment shaft  234 . The countersink adjustment mechanism  210 , and components thereof, are generally disposed along the longitudinal axis  112  of the distractor-inserter  100 . 
     In one embodiment, the countersink adjustment member  228  is an elongate member having a distal end and a proximate end. The tip  206  is integrated on the distal end of the countersink adjustment member  228 . The proximate end of the countersink adjustment member  228  is configured to slidably couple to the countersink adjustment member carriage  230 . In one embodiment, the countersink adjustment member carriage  230  is integrated with the buttress  214 . Once coupled to the countersink adjustment member carriage  230 , the countersink adjustment member  228 , including the tip  206 , can move forward and backward relative to the countersink adjustment member carriage  230 . 
     In the illustrated embodiment, the countersink adjustment member  228  includes sleeves  236 . Sleeves  236  of the countersink adjustment member  228  fit over opposite sides  238  of the countersink adjustment member carriage  230  to engage the opposite sides  238  and provide sliding movement. As discussed in further detail below, the sleeves  236  expose countersink depth markings  240  on the countersink adjustment member carriage  230  as the sleeves  236  move forward, and cover the markings  240  as the sleeves  236  move backward. In this manner, the sleeves  236  and the markings  240  visibly indicate the current countersink depth setting. 
     In the illustrated embodiment, the countersink adjustment member carriage  230  has a passage  242  formed therethrough. The countersink adjustment interface  232  also has a passage  244  formed therethrough. The core member passage  226 , the carriage passage  242  and the interface member passage  244  are aligned with the longitudinal axis  112 . The countersink adjustment shaft  234  is generally an elongate member aligned with the core member passage  226 , the carriage passage  242  and the interface member passage  244 . When assembled, the countersink adjustment shaft  234  is positioned through the core member passage  226 , the carriage passage  242  and the interface member passage  244 . 
     In the illustrated embodiment, an interior surface  246  of the countersink adjustment interface member  232  that forms the passage  244  is threaded. A proximate end of the countersink adjustment shaft  234  is also threaded. When assembled, a portion of the threads of the countersink adjustment shaft  234  engage with the threads of the interior surface  246  of the interface passage  244 . Actuation of the countersink adjustment interface member  232  thereby engages and actuates the countersink adjustment shaft  234 . 
     In the illustrated embodiment the countersink adjustment interface member  232  is a thumbwheel that is rotatable about the countersink adjustment shaft  234 . The thumbwheel  232  may have tactile members to facilitate rotary use of the thumbwheel. When the thumbwheel  232  is rotated, the threads of the interior surface  246  engage with the threads of the countersink adjustment shaft  234  and tend to push or pull the countersink adjustment shaft  234  longitudinally forward or backward, depending on the direction of rotation. 
     A distal end of the countersink adjustment shaft  234  includes a coupling member  248  configured to couple with the countersink adjustment member  228 . In one embodiment, the coupling member  248  extends into a slot in the countersink adjustment member  228 . A fastener, such as a pin  250 , is disposed through the countersink adjustment member  228  and the coupling member  248  to thereby couple the countersink adjustment shaft  234  to the countersink adjustment member  228 . Once coupled, the countersink adjustment member  228  tends to move in corresponding longitudinal motion with the countersink adjustment shaft  234  as the thumb wheel  232  is rotated. 
     In the illustrated embodiment, the thumbwheel  232  provides for a level of discrete selection of the countersink depth. The thumbwheel  232  includes one or more detent mechanisms to hold the thumbwheel  232  and countersink adjustment member  228 , and thereby hold the countersink depth, at discrete values. For example, in some embodiments detent sockets  252  on the thumbwheel  232  face detent balls  254  of ball plungers  256 . As the thumbwheel  232  is rotated, the detent balls  254  engage respective detent sockets  252  when they are in alignment and disengage from the detent sockets  252  when they are out of alignment. When the detent balls  254  are engaged, more force is needed to turn the thumbwheel  232  in order to disengage the detent balls  254  from the sockets  252 . 
     In one embodiment, one or more detent sockets  252  are positioned radially at locations on the thumbwheel  232  that correspond to whole number unit offsets  208  (e.g., integer values of millimeters) between the tip  206  and faces of the arms. Detent sockets  254  may be located at other locations on the thumbwheel  232  to provide for fractional units of countersink depths (e.g., 1.5 mm, 1.8 mm, 2.2 mm, etc.). 
     In some embodiments, detent mechanisms are not included. In these embodiments, selection of the countersink depth is analog. The movement of the thumbwheel  232  in these embodiments is typically smooth and gradual. In these embodiments, the user can select virtually any depth within the range of depths. 
     Turning to  FIGS. 6A-6B , there are shown perspective views of the head block assembly  136 , including the countersink depth adjustment mechanism  210  adjusted to two different countersink depths. In  FIG. 6A , the countersink adjustment member  228  is in its furthest distal position. In this embodiment, the furthest distal position of the countersink adjustment member  228  corresponds to a countersink depth of eight millimeters, as indicated by the countersink depth markings  240 . In  FIG. 6B , the countersink adjustment member  228  is in its least distal position which corresponds to a countersink depth of zero millimeters, according to countersink depth markings  240 . In other embodiments, the maximum countersink depth can be more or less than eight millimeters, and may be for example, six millimeters (6 mm), eight millimeters (8 mm), ten millimeters (10 mm), twelve millimeters (12 mm) or the like. 
     As discussed, the countersink adjustment mechanism  210  allows for linear adjustment of the countersink depth among a range of depths, which may include more than two depths.  FIGS. 7A-7C  are elevation views of the head block assembly  136  with the countersink depth set to three different depths. In  FIG. 7A , the countersink depth is set to zero as indicated by depth marking  240 . At zero countersink depth, the longitudinal distance  208  from the tip  206  to each face  258  of the arms  202  is zero millimeters. Zero millimeters of countersink depth corresponds to a flush alignment of an implantable element  260  when it is inserted between two distracted elements. Depending in part on the shape or irregularity of the faces of the distracted elements, the angle at which the distractor-inserter is positioned through tissue, and other factors, element  260  may be flush with only one of the distracted elements. 
     In  FIG. 7B , the countersink depth is set to between two and three millimeters as indicated by markings  240 . In some embodiments, the countersink depth can be set in an analog manner including fractions of depth units. The depth indicated by marking  240  corresponds to the distance  208  between the tip  206  and faces  258  of the retraction members  202 , which in turn corresponds to the countersink depth of the implantable element  260 . In  FIG. 7C , the countersink depth is set to four millimeters as indicated by markings  240 . 
     Turning to  FIG. 8  there is shown a perspective view of an exploded coupling between the drive shaft  110  and the head block assembly  136 . A distal end of the drive shaft  110  extends into the space  134  between the blades  116 . The distal end of the drive shaft  110  includes a knob coupling member  302  including a knob  304  and recesses  306 . The core member  224  of the buttress  214  includes an opening (not shown) on its proximal face. 
     When assembled, the knob coupling member  302  is inserted into the core member  224 . Recesses  306  are aligned with coupling passages  308  of the core member  224 . Fasteners, such as pins  310  are inserted through the coupling passages  308  and through the width of the core member  224 . Within the core member  224 , the pins  310  pass through the recesses  306  to engage the knob  304 . In this configuration, the drive shaft  110  cannot be pulled out of the core member  224 , but the drive shaft  110  can be twisted about the longitudinal axis  112 . In alternative embodiments, coupling member  302  and core member  224  are connected together using other arrangements or mechanisms. For example, in one embodiment pins  310  may be replaced with a generally C-shaped clip. The C-shaped clip engages recesses  306  after knob  304  is place through a core member washer, such as that shown in  FIG. 8 . In this manner, drive shaft  110  and coupling member  302  are allowed to rotate relative to core member  224 , but are axially connected to core member  224  in a way which provides axial movement of core member  224  as drive shaft  110  is advanced or retracted. 
     Referring now to  FIG. 9 , there is shown a cross sectional elevation view of a ratchet system  402  of the distractor-inserter  100  according to one embodiment. In the illustrated embodiment, the shaft  110  is threaded. The threads  404  may comprise a leadscrew, such as a four leadscrew. The ratchet system  402  is configured to engage threads  404  of the shaft  110  as the shall  110  is pushed forward but prevent backward axial movement of the shaft  110  unless the ratchet is manually disengaged. 
     In one embodiment, the ratchet system  402  includes a ratchet arm  406  pivotally mounted on a pivot member, such as pivot pin  408 . Catch members  410  are integrated on a bottom side of the arm  406  facing threads  404  of the shaft  110 . Catch members  410  automatically insert between and catch threads  404 , holding the drive shaft  110  in place. A ratchet disengagement button  412  is integrated on a top side of the pivot arm  406  and exposed on the surface of the handle  102 . When the disengagement button  412  is depressed, the arm  406  pivots about the pivot pin  408 , thereby raising the catch members  410  to disengage the threads  404  of the drive shaft  110 . 
     In the illustrated embodiment, the drive shaft  110  cannot be pulled backward until the catches  410  are manually disengaged from the threads  404 . The ratchet mechanism  402  thereby prevents the shaft  110  from moving backward, unless the user manually disengages the catch members  410 . However, the coupling shown in  FIG. 8  allows a user to twist the drive shaft  110  to cause backward movement as the threads  404  can slide in the thread direction through the catch members  410 . 
     Referring now to  FIG. 10 , there is shown an example use scenario in which a distractor-inserter is used to distract two vertebral bodies (superior vertebral body  502 A and inferior vertebral body  502 B) and insert a spinal implant  260  therebetween. In the illustrated scenario, the countersink adjustment member  228  has been set to provide a countersink depth of between 2 mm and 3 mm, as indicated by countersink depth markings  240 . The distal ends  120  of the blades  116  are positioned between the superior vertebral body  502 A and the inferior vertebral body  502 B to points at which front faces  128  of the stop members  126  engage respective front faces  504  of the vertebral bodies  502 . In addition, friction elements  132  engage respective interior faces  506  of the vertebral bodies  502 . 
     In the illustrated use scenario of  FIG. 5 , the shaft  110  has been driven forward (toward the distal end  120 ) to push the head block assembly  136  forward, thereby separating the distal ends  120  of the blades  116  and the vertebral bodies  502 . The head block assembly  136  is positioned at the furthest distal extent, where the faces  258  of the arms  202  engage respective faces  504  of associated vertebral bodies  502 . In the illustrated position where the arm faces  258  engage the vertebral body faces  504 , the implant  260  is between the vertebral bodies  502  at the selected countersink depth  208 . 
     In the position illustrated in  FIG. 5 , as the shaft  110  is continued to be driven forward, the arm  202  faces  258  press against outer faces  504  of the vertebral bodies  502 . The forward pressure of the arms  202  against the vertebral bodies  502  causes a backward, retractive force on the blades  116 . The backward force causes the distal ends  120  of the blades, including the stop members  126  and the friction elements  132 , to disengage from the vertebral bodies  502 . 
     As the blades  116  disengage and retract from the vertebral bodies  502 , the vertebral bodies  502  close together, applying compressive forces on upper and lower sides of the implant  260 . The compressive forces on the implant  260  hold the implant  260  at the desired location between the vertebral bodies  502  while the blades  116  are retracted completely from between the vertebral bodies  502 . Accordingly, the implant  260  becomes fixed between the vertebral bodies  502  as a result of the compressive force, and held at the selected countersink depth  208 . 
       FIG. 11  is a flow diagram illustrating steps in a distraction-insertion process  1100  using a distractor-inserter, such as the distractor-inserter of  FIG. 1 , according to an embodiment. Following the process  1100  of  FIG. 11 , a linear countersink depth adjustment mechanism is adjusted to provide for a desired countersink depth (block  1102 ). In one embodiment, countersink adjustment involves turning a thumb wheel of the distractor-inserter, which causes a tip portion that pushes an implantable element, to move along a longitudinal axis toward or away from a reference point (e.g., retraction member faces) in a linear fashion. 
     In some embodiments, the countersink adjustment mechanism enables the user to select a countersink depth within a range of multiple countersink depths, which may be more than two countersink depths. In one embodiment, the range of countersink depths is zero millimeters (mm) to eight mm, but other ranges may be employed. In some embodiments, the movement may be graduated, wherein movement from one countersink depth to another is analog. 
     Continuing with the process  1100  of  FIG. 11 , distal ends of blades of the distractor-inserter are positioned (block  1104 ) between two elements to be separated, such as adjacent vertebrae of a spinal column. In one embodiment, the blades of the distractor-inserter are inserted in an anterior fashion, for example through the abdomen of a patient. According to an embodiment, stop elements of the blades engage respective front faces of the adjacent vertebrae, at which point the blades are prevented from going any deeper between the adjacent vertebrae. 
     In one embodiment, when distal ends of the blades are positioned (block  1104 ) between adjacent vertebrae, one or more friction elements on outer faces of the blades engage interior faces of the vertebrae. In this embodiment, the one or more friction elements may be arranged on the outer faces of the blades to substantially correspond to curvature of the end plates of the vertebrae. 
     Continuing with the process  1100  of  FIG. 11 , a shaft of the distractor-inserter is driven (block  1106 ) forward toward the distal ends of the blades until retraction elements (e.g., arms  202 ,  FIG. 4 ) of a head block assembly engage respective outer faces of the vertebrae. Driving the head block assembly forward causes the distal ends of the blades to separate, thereby pushing the vertebral bodies apart. When the arms engage the outer faces of the vertebrae, the implantable element is positioned between the vertebrae at the selected (block  1102 ) countersink depth. 
     In the process  1100 , continuing (block  1108 ) to drive the head block assembly forward causes arms of the head block assembly to push against the outer faces of the vertebral bodies. When the arms push against the vertebral bodies, the distal ends of the blades disengage from the vertebral bodies and allow for retraction of the blades from the spinal region. As the blades retract, the vertebral bodies come together and the implantable element is held between them. The compression forces imparted on the implantable element between the vertebral bodies causes the implantable element to disengage from the tip of the distractor-inserter and remain fixed at the selected countersink (block  1102 ) depth between the vertebral bodies. 
     In conclusion, various systems, devices, methods and arrangements for distraction and insertion are disclosed. While detailed descriptions of one or more embodiments have been provided above, various alternatives, modifications, and equivalents are possible. Therefore, the above description should not be taken as limiting the scope of possible embodiments, which is defined by the appended claims.