Patent Publication Number: US-8523844-B2

Title: Surgical instrument with tendon preload-and-locking device

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a surgical apparatus having an elongated shaft for laparoscopic and endoscopic procedures. In particular, the disclosure relates to a surgical apparatus having a tensile member extending through the elongated shaft and a preload mechanism for adjusting a tensile force in tensile member. 
     2. Background of Related Art 
     Typically in a laparoscopic, an endoscopic, or other minimally invasive surgical procedure, a small incision or puncture is made in a patient&#39;s body. A cannula is then inserted into a body cavity through the incision, which provides a passageway for inserting various surgical devices such as scissors, dissectors, retractors, or similar instruments. To facilitate operability through the cannula, instruments adapted for laparoscopic surgery typically include a relatively narrow shaft supporting an end effector at its distal end and a handle at its proximal end. Arranging the shaft of such an instrument through the cannula allows a surgeon to manipulate the handle from outside the body to cause the end effector to carry out a surgical procedure at a remote internal surgical site. This type of laparoscopic procedure has proven beneficial over traditional open surgery due to reduced trauma, improved healing and other attendant advantages. 
     A steerable laparoscopic or endoscopic instrument may provide a surgeon with a range of operability suitable for a particular surgical procedure. For example, the instrument may be configured such that the end effector may be aligned with a longitudinal axis of the instrument to facilitate insertion through a cannula, and thereafter, the end effector may be caused to articulate or move off-axis as necessary to appropriately position the end effector to engage tissue. When the end effector of a steerable, articulating instrument comprises a pair of jaw members for grasping tissue, the jaw members may need to be oriented relative to the tissue once properly positioned. 
     Often a steerable endoscopic instrument will include a tensile member extending through the elongated shaft to provide the surgeon with control of the end effector. A tensile member may be coupled to the end effector or a distal portion of the elongated shaft in such a manner that longitudinal motion of the tensile member effects motion in the end effector. For example, a surgeon may manipulate an actuator on the handle to induce longitudinal motion of the tensile to articulate the end effector or to move the pair of jaw members between open and closed configurations. In some instances, providing a surgeon with control over the amount of tension in the tensile member may facilitate operation of the instrument. The instrument may be adjusted to provide a responsiveness or degree of tactile feedback suitable for a particular purpose or preference of the surgeon. 
     SUMMARY 
     The present disclosure describes a surgical instrument including a handle having an elongated shaft extending distally from the handle. An end effector extends distally from the elongated shaft, and at least one tensile member extends at least partially through the elongated shaft. A preload mechanism is provided for adjusting a tensile force in the tensile member. The preload mechanism includes first and second guides in contact with the tensile member to restrain the tensile member at respective first and second lateral distances from a longitudinal axis. A third guide is disposed longitudinally between the first and second guides and is in contact with the tensile member to restrain the tensile member at a third lateral distance from the longitudinal axis. The third guide is movable to vary the third lateral distance and a corresponding length of the tensile member disposed longitudinally between the first and second guides to vary the tensile force in the tensile member. 
     The first, second and third guides may be generally ring-shaped, and each of the first, second and third ring-shaped guides may be centered about a longitudinal axis defined by the elongated shaft. The tensile member may be in contact with each of the first and second ring-shaped guides on an interior surface, and a radius of the first ring-shaped guide may be less than a radius of the second ring-shaped guide such that the first lateral distance is less than the second lateral distance. The third ring-shaped guide may be movable in the longitudinal direction to vary the third lateral distance, and the tensile member may be in contact with the third ring-shaped guide on an interior surface such that the movement of the third ring-shaped guide in the longitudinal direction toward the second ring-shaped guide has a tendency to increase the tensile force in the tensile member. 
     Alternatively the third ring-shaped guide may be movable in a rotational direction about the longitudinal axis to vary the third lateral distance. For example, the tensile member may be in contact with a cam surface defined on an interior surface of the third ring-shaped guide. 
     The tensile member may be in contact with the third ring-shaped guide on an exterior surface such that the movement of the third ring-shaped guide in the longitudinal direction toward the first ring-shaped guide has a tendency to increase the tensile force in the tensile member. 
     The elongated shaft may include an articulating portion wherein the tensile member is coupled to a distal end of the articulating portion, and wherein longitudinal motion in the tensile member induces pivotal motion of the articulating portion with respect to the longitudinal axis. Alternatively, the end effector may include a pair of opposable jaw members movable between an open configuration for receiving tissue and a closed configuration for maintaining a closure pressure on the tissue, and the tensile member may be coupled to the end effector such that longitudinal motion in the tensile member induces movement of the jaw members between the open and closed configurations. At least one of the jaw members may be coupled to a source of electrosurgical energy. 
     According to another aspect of the disclosure, a surgical instrument includes a handle adapted for manipulation by a user to control the surgical instrument, an elongated shaft extending distally from the handle and including a proximal portion coupled to the handle and a distal portion pivotally coupled to the proximal portion. An end effector is coupled to the distal portion of the elongated shaft such that the end effector articulates relative to the longitudinal axis as the distal portion of the elongated shaft pivots relative to the proximal portion of the elongated shaft. An elongated tensile member defines a tensile axis and is coupled to the distal portion of the elongated shaft such that axial motion in the tensile member induces articulation of the end effector. A preload mechanism is provided for adjusting a tensile force in the tensile member. The preload mechanism includes a guide configured to impart a force on the tensile member in a lateral direction with respect to the tensile axis to bend the tensile member in the lateral direction. An increase in the lateral movement of the tensile member corresponds to an increase in the tensile force in the tensile member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the detailed description of the embodiments given below, serve to explain the principles of the disclosure. 
         FIG. 1  is a perspective view of a surgical instrument in accordance with the present disclosure including an end effector aligned with a longitudinal axis; 
         FIG. 2A  is a partial, perspective view of a distal end of the instrument of  FIG. 1  depicting the end effector in an articulated position with respect to the longitudinal axis; 
         FIG. 2B  is a partial, perspective view of the distal end of the instrument of  FIG. 2A  depicting the end effector in a rotated position with respect to a shoulder axis; 
         FIG. 2C  is a partial, perspective view of the distal end of the instrument of  FIG. 2B  depicting the end effector in a rotated position with respect to a wrist axis; 
         FIG. 2D  is a partial perspective view of the distal end of the instrument of  FIG. 2C  depicting the end effector in a closed configuration; 
         FIG. 3  is a partial side view of the instrument of  FIG. 1  having a housing component removed and depicting a coaxial drive mechanism; 
         FIG. 4  is a cross-sectional perspective view of the drive mechanism of  FIG. 3  in a first or “home” configuration for maintaining the end effector in alignment with the longitudinal axis; 
         FIG. 5  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a second configuration for articulating the end effector with respect to the longitudinal axis; 
         FIG. 6A  is a schematic view of a tendon preload and locking device; 
         FIG. 6B  is a schematic view of an alternate embodiment of a tendon preload and locking device depicting a movable guide having a cam surface; 
         FIG. 6C  is a schematic view of an alternate embodiment of a tendon preload and locking device depicting a movable guide for exerting an outwardly directed force on a tendon; 
         FIG. 7  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a third configuration for effecting a shoulder roll of the distal end of the instrument; 
         FIG. 8  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a fourth configuration for locking the distal end of the instrument in the shoulder roll configuration; 
         FIG. 9  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a fifth configuration for effecting a wrist roll of end effector; 
         FIG. 10A  is schematic view of a wrist roll joint at the distal end of the instrument; 
         FIG. 10B  is a schematic view of an alternate embodiment of a roll joint; 
         FIG. 11  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a sixth configuration for closing a pair of jaw members at the end effector; 
         FIG. 12  is an enlarged, cross-sectional, partial perspective view of the drive mechanism in a seventh configuration for advancing a knife through the pair of jaw members; 
         FIG. 13  is a partial side view of the instrument depicting a knife lock mechanism to permit advancement of the knife only after the pair of jaw members has been closed; and 
         FIGS. 14A through 14D  are perspective views depicting various individual components of the drive mechanism including an articulation sphere ( 14 A), an articulation spool ( 14 B), a shoulder roll knob ( 14 C), and a locking collar ( 14 D). 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , a steerable endoscopic instrument is depicted generally as instrument  10 . Instrument  10  includes a handle  12  near a proximal end, an end effector  16  near a distal end and an elongated shaft  18  therebetween. Elongated shaft  18  includes a proximal portion  20  extending from the handle  12  and an articulating distal portion  22  supporting the end effector  16 . The proximal portion  20  defines a longitudinal axis A-A, and is sufficiently long to position the end effector  16  through a cannula (not shown). At least one joint  28  is established between the proximal and distal portions  20 ,  22  of the elongated shaft  18  permitting the distal portion  22  and the end effector  16  to articulate or pivot relative to the longitudinal axis A-A as described in greater detail below (see  FIG. 2A ). The end effector  16  defines an end effector axis B-B, which is aligned with the longitudinal axis A-A when the articulating distal portion  22  of the elongated shaft  18  is in a “home” configuration. 
     The end effector  16  includes a pair of opposing jaw members  30  and  32 . The jaw members  30 ,  32  are operable from the handle portion  12  to move between an open configuration to receive tissue, and a closed configuration (see  FIG. 2D ) to clamp the tissue and impart an appropriate clamping force thereto. When the end effector  16  is in the open configuration, a distal portion of each of the jaw members  30 ,  32  is spaced from the distal portion of the other of the jaw members  30 ,  32 . When the end effector  16  is in the closed configuration, the distal portions of the jaw members  30 ,  32  are closer together. The end effector  16  is configured for bilateral movement wherein both jaw members  30  and  32  move relative to the end effector axis B-B as the end effector  16  is moved between the open and closed configurations. However, unilateral motion is also contemplated wherein one of the jaw members, e.g.,  32  remains stationary relative to the end effector axis B-B and the other of the jaw members, e.g.,  30  is moveable relative to the axis B-B. 
     Handle  12  is manipulatable by the surgeon from outside a body cavity to control the positioning, orientation and operation of the end effector  16  when the end effector  16  is positioned inside the body at a tissue site. To provide this operability, the handle  12  supports various actuators that are operable to induce movement in the end effector  16  through various modes. These actuators include an articulation trigger  40 , which is operable to articulate the distal portion  22  of the elongated shaft  18  with respect to the longitudinal axis A-A (see  FIG. 2A ), a shoulder roll knob  42 , which is operable to rotate the articulating distal portion  22  about the longitudinal axis A-A (see  FIG. 2B ), and a wrist roll knob  44 , which is operable to rotate the end effector  16  about the end effector axis B-B (see  FIG. 2C ). The articulation trigger  40 , wrist roll knob  44  and shoulder roll knob  42  cooperate to permit the end effector  16  to be appropriately positioned and oriented in a three dimensional environment to effectively engage tissue. Once the end effector  16  is positioned and oriented, the surgeon may approximate a pivoting handle  46  relative to a stationary handle  48  to move the jaw members  30 ,  32  to the closed position (see  FIG. 2D ). 
     The surgeon may also manipulate a finger trigger  50  to lock the pivoting handle  46  in an approximated position with respect to the stationary handle  48 , and thus maintain the jaw members  30 ,  32  in the closed configuration. When the jaw members  30 ,  32  are in the closed configuration, the surgeon may initiate the delivery of electrosurgical energy to the jaw members  30 ,  32  by manipulating a push button  50   a  provided on the handle  12 . In alternate embodiments, the delivery of electrosurgical energy may be initiated with the finger trigger  50 . 
     Additionally, handle  12  supports a locking lever  52  that is operable to prevent unintended actuation of the articulation trigger  40  and shoulder roll knob  42 . Thus, the end effector  16  may be maintained in a stable position. Also, a knife spool  54  is supported at a proximal end of the handle  12 . The knife spool  54  is operable to advance a knife blade  54   a  (see  FIG. 2B ) through the jaw members  30 ,  32 . With the exception of the finger trigger  50  and the push button  50   a , each of the actuators described above transmits mechanical motion to the end effector  16  through a drive assembly  100  (see  FIG. 3 ). The operation of the drive assembly  100  is discussed in greater detail below. 
     Push button  50   a  is in electrical communication with a source of electrosurgical energy such as electrosurgical generator  50   b . The electrosurgical generator  50   b  serves to produce electrosurgical energy and also to control and monitor the delivery of the electrosurgical energy. Various types of electrosurgical generators  50   b , such as those generators provided by Covidien-Energy-based Devices, of Boulder, Colo., may be suitable for this purpose. Electrosurgical generator  50   b  may be housed within the stationary handle  48  as depicted schematically in  FIG. 1 , or may alternatively be electrically and mechanically coupled to the instrument  10  by a cable (not shown). The electrosurgical generator  50   b  is in electrical communication with at least one of the jaw members  30 ,  32 . 
     Referring now to  FIG. 2A , the end effector  16  is moved in an articulation mode in the direction of arrow “A 0 ,” or generally to the right from a perspective of a user. In this articulated position, the end effector axis B-B is oriented at an articulation angle “α” with respect to the longitudinal axis A-A. Any angle “α” may be achieved by bending distal portion  22  of the elongated shaft  18  to an appropriate degree. The distal portion  22  includes a plurality of segments  56  that are nested with one another such that each segment  56  is pivotally arranged with respect to a neighboring segment  56 . Each segment  56  is constructed in a similar manner and includes a pair of steering bores  58  extending therethrough. The steering bores  58  are laterally arranged near an outer circumference of the link  56  and have a radial spacing of about 180 degrees. Thus, the steering bores  58  define a plane of articulation in which the distal  22  may bend. Although the distal portion  22  of the elongated shaft  18  may articulate in a single plane of articulation, other embodiments are also envisioned in which additional steering bores (not shown) permit articulation in multiple planes of articulation. 
     The steering bores  58  permit passage of a pair of steering tendons  110  (see also  FIG. 4 ). The steering tendons  110  are coupled to a leading segment  56   a  such that a tension imparted to one of the steering tendons  110  is transferred to the leading segment  56   a . Thus, the distal portion  22  of the elongated shaft  18  may be articulated. For example, a tendon  110  disposed through the bores  58  on the right side of the distal portion  22  may be pulled proximally in the direction of arrow “A 4 ” to draw the right side of the leading segment  56   a  proximally, and to curve the distal portion  22  to the right. Similarly, the distal portion  22  may be curved to the left by drawing the opposite tendon  110  proximally. 
     Each of the segments  56  includes a central bore (not shown) extending therethrough. The central bore permits passage of various components through the distal portion  22  to the end effector  16 . For example, an electrosurgical conductor  178  ( FIG. 10A ) may pass through the distal portion  22  to provide electrosurgical energy to the end effector  16 . Control cables, flexible rods, and torsion members may also be passed through the central bores to transfer mechanical motion to the end effector  16  as discussed in greater detail below. Although the distal portion  22  of the elongated shaft  18  has been described as including a plurality of pivoting segments  56 , other embodiments are contemplated in which a single pivot joint is provided to orient the end effector axis B-B at angle “α” with respect to the longitudinal axis A-A. 
     Referring now to  FIG. 2B , the end effector  16  is moved through a second mode of motion herein referred to as a “shoulder roll.” In the shoulder roll mode, the end effector  16 , the distal portion  22  of the elongated shaft  18  and an outer tubular member  60  of the elongated shaft are all concurrently rotated about the longitudinal axis A-A as indicated by arrows “S 0 .” The end effector  16  is maintained at the angle “α,” and is swept through a three dimensional arc “β” such that the position of the end effector  16  is moved from the right generally to the left from the perspective of a user. The shoulder roll may be continued until any desired angle “β” is achieved. Since any angle “α” and any angle “β” is achievable, the end effector  16  may be positioned at any appropriate position in a three dimensional environment. 
     A knife blade  54   a  (depicted in phantom) is advanceable through the jaw members  30 ,  32  in the direction of arrow “K 0 ” in a cutting mode as described in greater detail below with reference to  FIG. 12 . The embodiment of instrument  10  described herein includes a knife lock mechanism (see  FIG. 13 ) to prevent the knife blade  54   a  from advancing to the position depicted in  FIG. 2B  when the end effector  16  is in the open configuration. Other embodiments are envisioned, however, which permit the knife blade  54   a  to be advanced when the end effector  16  is in the open configuration. 
     Referring now to  FIG. 2C , the end effector  16  is moved through a third mode of motion herein referred to as a “wrist roll.” The wrist roll may be initiated by rotating the wrist roll knob  44  ( FIG. 1 ) in either direction. The end effector  16  is rotated about the end effector axis B-B through any wrist roll angle “δ” until an orientation of the end effector  16  is achieved that is appropriate for contacting tissue. Once positioned and oriented, the end effector  16  may be moved through a fourth mode of motion herein referred to as the “clamping mode.” The clamping mode may be initiated to move the jaw members  30 ,  32  in the direction of arrows “C 0 ” to the closed configuration depicted in  FIG. 2D  to capture tissue therebetween. In this closed configuration, an appropriate clamping force along with electrosurgical energy may be delivered to the tissue to seal the tissue. Thereafter, the surgeon may elect to sever the tissue captured between the jaws  30 ,  32  by advancing the knife blade  54   a  ( FIG. 2B ) through the tissue in a fifth mode of motion herein referred to as the “cutting mode.” 
     Referring now to  FIG. 3 , a coaxial drive assembly  100  is disposed generally within the handle  12 . The coaxial drive assembly  100  receives mechanical motion from the actuators (e.g., articulation trigger  40 , shoulder roll knob  42 , wrist roll knob  44 , pivoting handle  46 , locking lever  52  and knife spool  54 ), and transmits a corresponding motion through the elongated shaft  18  to induce the various modes of motion in the end effector  16  discussed above. The mechanical motion transmitted through the elongated shaft  18  is either longitudinal motion directed in a general direction parallel to the longitudinal axis A-A, or rotational motion about the longitudinal axis A-A. Accordingly, the drive assembly  100  may be referred to as “coaxial.” 
     Referring now to  FIG. 4 , the coaxial drive assembly  100  is depicted in a first “home” configuration corresponding to the “home” configuration of the elongated shaft  18  depicted in  FIG. 1 . The drive assembly  100  includes two main subsystems  102  and  104 . A first positioning subsystem  102  drives the articulation and shoulder roll modes, and provides a lock to maintain the end effector  16  at particular angles “α” and “β” The second subsystem  104  drives the wrist roll, clamping and cutting modes. The two subsystems  102 ,  104  operate independently of one another in that the operation one subsystem, e.g.  102 , does not transfer motion to the other subsystem, e.g.  104 . 
     The first positioning subsystem  102  includes an articulation sphere  106  coupled to tendons  110  by a pair of tendon pins  112 . The articulation sphere  106  (depicted in greater detail in  FIG. 14A ) is generally spherical in shape and includes a pair of coupling channels  114  ( FIG. 14A ) therein for receiving the tendon pins  112 . The tendon pins  112  are captured within the connection channels  114 , and the tendons  110  are secured to the tendon pins  112 . Thus, the tendons  110  are coupled to the articulation sphere  106 . Tendon relief slots  116  ( FIG. 14A ) extend longitudinally from the connection channels  114  and provide strain relief and permit rotational movement of the articulation sphere  106  without interfering with the tendons  110 . 
     A pair of pivot axles  118  ( FIG. 14A ) extend laterally from the articulation sphere  106  in a diametrically opposed fashion. The pivot axles  118  define a pivot axis C-C ( FIG. 14A ) about which the articulation sphere  106  may be induced to pivot. A drive slot  120  is defined in the articulation sphere  106  laterally spaced from the pivot axis C-C such that longitudinal motion imparted to the drive slots  120  induces pivotal motion in the articulation sphere  106  about the pivot axis C-C. A second drive slot  120 ′ is formed opposite drive slot  120 . The second drive slot  120 ′ provides additional symmetry to the articulation sphere  106  to ease manufacturing and assembly. However, second drive slot  120 ′ is inactive in the embodiment of drive assembly  100  discussed herein with reference to  FIG. 4 , i.e., the second drive slot  120 ′ is not engaged to impart pivotal motion to the articulation sphere  106 . A central opening  122  is formed longitudinally through the articulation sphere  106 . The central opening  122  permits passage of various components through the articulation sphere  106  without interference with the motion of the articulation sphere  106 . 
     An articulation spool  124  (depicted in greater detail in  FIG. 14B ) is generally cylindrical in shape and radially surrounds the articulation sphere  106 . A drive post  126  extends into an interior of the articulation spool  124  such that the drive post  126  may engage the drive slot  120  of the articulation sphere  106 . A spool feature  128  at the proximal end of the articulation spool  124  provides a double-flange interface for engaging the articulation trigger  40  (see  FIG. 3 ). Clearance slots  130  ( FIG. 14B ) are formed in the walls of the articulation spool  124  to provide clearance for the axles  118  of the articulation sphere  106 . 
     Referring now to  FIG. 5 , the coaxial drive assembly is moved to a second configuration to initiate motion of the end effector  16  in the articulation mode. The articulation spool  124  may be driven longitudinally to induce rotational movement in the articulation sphere  106 . For example, a surgeon may drive articulation trigger  40  distally in the direction of arrow “A 1 ” ( FIG. 3 ), which in turn drives the articulation spool  124  distally in the direction of arrows “A 2 .” The drive post  126  drives the drive slot  120  distally, and since the axles  118  ( FIG. 14A ) of the articulation sphere  106  are longitudinally restrained (as discussed below), the articulation sphere  106  is induced to rotate in the direction of arrow “A 3 ” about the pivot axis C-C. This rotation induces differential longitudinal tension and motion in the tendons  110  as indicated by arrows “A 4 .” As discussed above with reference to  FIG. 2A , differential longitudinal motion in the tendons  110  imparts movement in the end effector  116  in a first direction in the articulation mode. A surgeon may similarly induce articulation of the end effector  116  in an opposite direction by drawing the articulation trigger  40  proximally. 
     To induce motion of the end effector  16  in the articulation mode, the drive assembly  100  employs tension in the tendons  110  and a system spring rate. The tendons  110  are preloaded with a tensile force to manage the amount of slack or play in the articulation mode. In some instances, an adjustment to the amount of tension in the tendons may facilitate use of the coaxial drive assembly  100 . For instance, over time, fatigue or creep tension losses may occur in the tendons  110 . This loss in tension may be associated with a decline in responsiveness of the device  10 , which may frustrate the intent of a surgeon. The ability to increase the tension in the tendons  110  may improve the survivability of the drive assembly  100  and facilitate operation of the instrument  10 . In other instances, the general tension in the tendons  110  may be increased to prohibit any inadvertent movement of the end effector  16  in the articulation mode. In this manner, an operator may lock the end effector  16  at a particular angle “α” ( FIG. 2A ) with respect to the longitudinal axis A-A. 
     Referring now to  FIG. 6A , a tendon preload-and-locking mechanism  130   a  may be incorporated into the device  10  at any convenient longitudinal location, and may be employed to adjust the tension in a tendon  110 . The mechanism  130   a  includes first and second stationary tendon guides  132  and  134 , respectively. Each of the stationary tendon guides  132 ,  134  is generally ring shaped and centered about the longitudinal axis A-A. The stationary tendon guides  132 ,  134  have a fixed longitudinal position with respect to the stationary handle  48  ( FIG. 1 ), but in some embodiments may be free to rotate about the longitudinal axis A-A. Tendon  110  defines a tensile axis and is situated to extend through the rings and to contact an inner surface of each of the guides  132 ,  134 . The first fixed tendon guide  132  has a smaller inner diameter than the second fixed tendon guide  134 , and thus the tendon  110  extends obliquely with respect to the longitudinal axis A-A. The first fixed tendon guide  132  restrains the tendon  110  at a first lateral distance from the longitudinal axis A-A that is less than a second lateral distance from the longitudinal axis A-A at which the second fixed tendon guide  134  restrains the tendon  110 . A straight-line distance between the points of contact between the tendon  110  and the stationary guides  132 ,  134  is represented by dimension “L 0 .” Arranging the first tendon guide  132  distally with respect to the second fixed tendon guide  134  may direct the tendon  110  toward the longitudinal axis A-A for passage through the elongated shaft  18 . 
     Arranged longitudinally between the stationary tendon guides  132 ,  134  is a movable guide  136   a . Similar to the stationary tendon guides  132 ,  134 , the movable guide  136   a  is ring shaped and centered about the longitudinal axis A-A. The movable guide  136   a  has an intermediate diameter, i.e., greater than the first stationary tendon guide  132  and less than the second stationary tendon guide  134  to restrain the tendon  110  at a third lateral distance from the longitudinal axis. The movable guide  136   a  is movable in the direction of arrow “G 0 ” along the longitudinal axis A-A by an actuator (not shown), which is accessible to an operator at least during maintenance of the instrument  10 . The movable guide  136   a  may be arranged such that an inner surface of the movable guide  136  contacts the tendon  110  as depicted in  FIG. 6A , and exerts a force on the tendon  110  in the direction toward the longitudinal axis A-A. The tendon  110 , thus assumes an angular configuration defining angle “Ω,” with an apex at the movable guide  136   a  and forming straight-line segments between the movable guide  136   a  and each of the stationary guides  132 ,  134 . These segments have lengths represented by the dimensions “L 1 ” and “L 2 .” 
     In this angular configuration, the total length (L 1 +L 2 ) of the tendon  110  between the stationary guides  132 ,  134  is greater than the straight-line distance (L 0 ) between the stationary guides  132 ,  134 . Moving the movable guide  136   a  longitudinally in the direction of arrow “G 0 ” toward the second stationary guide  134  increases the angle “Ω” and the total length (L 1 +L 2 ) of the tendon  110  between the guides. This increase in length is associated with an increase in tension in the tendon  110  due to structural elastic deformation, compensating spring load, or elongation. 
     The longitudinal position of the movable guide  136   a  may be fixed at a location wherein an appropriate tension is maintained in the tendon  110  to suit a particular purpose. For example, the movable guide  136   a  may be moved sufficiently close to the second stationary guide  134  that the frictional force between the tendon  110  and the guides  132 ,  134 ,  136   a  is sufficient to effectively lock the position of the tendon  110 . In this way, the tendon preload-and-locking mechanism  130   a  may be used to prevent inadvertent articulation of the end effector  16 . 
     Referring now to  FIG. 6B , an alternate embodiment of a preload-and-locking mechanism is depicted generally as  130   b . The mechanism  130   b  includes tendon  110  situated through stationary guides  132 ,  134  as described above with reference to  FIG. 6A . A movable guide  136   b  includes an irregular inner profile defining a cam surface  138 . The movable guide  136   b  may be rotated about the axis A-A in the direction of arrow “G 1 ” to vary the distance from the axis A-A in which the tendon  110  contacts the cam surface  138 . Thus, the path of tendon  110  between the stationary guides  132 ,  134  is varied along with the contact force encountered by the tendon  110  and the tension in tendon  110 . 
       FIG. 6C  depicts another embodiment of a preload-and-locking mechanism  130   c  in which a movable tendon guide  136   c  contacts tendon  110  in an opposite direction to bend the tendon  110  generally away from the longitudinal axis A-A. A cam surface  140  is provided on an outer surface of the movable tendon guide  136   c  to vary the distance from the longitudinal axis A-A in which the tendon contacts the tendon guide  136   c . Thus, the tension in tendon  110  may be adjusted by rotating the movable tendon guide  136   c  about the longitudinal axis in the direction of arrow “G 2 .” Alternatively, the tension may be adjusted by movement of the moveable tendon guide  136   c  in the longitudinal direction of arrow “G 3 ” toward the first tendon  132  to increase the tension in the tendon. 
     Referring now to  FIG. 7 , the coaxial drive assembly  100  is moved to a third configuration inducing the end effector  16  to move in the shoulder roll mode. A surgeon may engage external bosses  142  on the shoulder roll knob  42  to rotate the shoulder roll knob  42  about the longitudinal axis A-A in the direction of arrows “S 1 ” The shoulder roll knob  42  is fixedly coupled to the outer tubular member  60  of the elongated shaft  18 , and thus the outer tubular member  60  rotates in the direction of arrow “S 2 ” along with the shoulder roll knob  42 . A distal end of the outer tubular member  60  is fixedly coupled to a trailing segment  56   h  ( FIG. 2A ) of the distal portion  22  of the elongated shaft  18 . Thus, the entire distal portion  22  of the elongated shaft  18  rotates along with the shoulder roll knob  42  to achieve the motion in the shoulder roll mode. 
     Rotation of the shoulder roll knob  42  induces a corresponding rotation in each element of the positioning subsystem  102  about the longitudinal axis A-A. An interior surface  148   a  (FIG.  14 C) of the shoulder roll knob  42  may be splined to include ridges (not shown) extending longitudinally therein to correspond with longitudinal channels (not shown) on an exterior surface  148   b  ( FIG. 14B ) of the articulation spool  124 . The ridges and channels allow the articulation spool  124  to move longitudinally within the interior of the shoulder roll knob  42  while providing a means for transmitting torque from the shoulder roll knob  42  to the articulation spool  124 . This type of arrangement is hereinafter denoted a “splined slip joint.” 
     Many of the other components of the coaxial drive assembly  100  may define a splined slip joint with radially adjacent components such that the radially adjacent components rotate together while maintaining independent longitudinal motion capabilities. For example, a splined slip joint is also defined between the articulation spool  124  and a locking collar  154 . Thus, the locking collar  154  rotates along with the articulation spool  124  and the shoulder roll knob  42 . The locking collar  154  is fixedly coupled to a flange  154   a  at a proximal end thereof. The flange  154   a  includes a series of radially oriented teeth and detents thereon, which may be engaged by a flexible rib (not shown) projecting from the stationary handle  48 . Since the splined slip joints permit the flange  154   a  to rotate concurrently with the shoulder roll knob  42 , the teeth on the surface of the flange  154   a  may be employed to index the shoulder roll mode and to provide a variable resistance lock for shoulder roll rotation. This indexed rotation of the flange  154   a  provides tactile feedback to the surgeon while employing the shoulder roll mode, and stabilizes the shoulder roll mode once an appropriate shoulder roll angle “β” ( FIG. 2B ) has been established. The splined slip joints also permit the locking collar  154  to move in a longitudinal direction to provide an additional lock to the articulation and shoulder roll modes as described below with reference to  FIGS. 3 and 8 . 
     The splined slip joints defined in the positioning subsystem  102  permit the entire positioning subsystem  102  to rotate concurrently to ensure that the tendons  110  do not become entangled in the shoulder roll mode. The articulation mode is thus independent of the shoulder roll mode in that the articulation mode may be initiated irrespective of the rotational position (e.g. the angle “β,”  FIG. 2B ), of the end effector. Likewise, the shoulder roll mode may be initiated irrespective of the articulation angle (e.g. the angle “α,”  FIG. 2A ). 
     The interior surface  148   a  ( FIG. 14C ) of the shoulder roll knob  42  includes clearance channels  150  therein. The clearance channels  150  are diametrically opposed to receive the axles  118  ( FIG. 14A ) of the articulation sphere  106 . The axles  118  are longitudinally restrained in the clearance channels  150 . The shoulder roll knob  42  also includes a tapered interior surface  152   a  extending between the interior surface  148  and a ring surface  152   b . The tapered surface  152   a  may function as a conical volume feeder cone to guide the tendons  110  from the articulation sphere  106  to the ring surface  152   b . The ring surface  152   b  guides the tendons  110  through the elongated shaft  18 , and may serve as the first stationary tendon guide  132  as described above with reference to  FIGS. 6A through 6C . 
     Referring now to  FIGS. 3 and 8 , the coaxial drive assembly  100  may be moved to a fourth configuration to prohibit motion of the end effector  16  in the articulation and shoulder roll modes simultaneously. When satisfactory articulation and shoulder roll positions have been achieved, a surgeon may manipulate locking lever  52  as depicted in  FIG. 3  to induce appropriate movement of the locking collar  154  as depicted in  FIG. 8 . The locking lever  52  ( FIG. 3 ) is supported to rotate about the longitudinal axis A-A in the direction of arrows “F 1 ” and includes a cam surface on a proximal end thereof. The cam surface on the locking lever  52  engages a corresponding surface on a locking slider  156 . The locking slider  156  is restrained from rotating by a splined slip joint with the handle portion  12  or a similar feature, and thus, rotation of the locking lever  52  in the direction of arrow “F 1 ” induces longitudinal motion in the locking slider  156  in the direction of arrow “F 2 .” The locking slider  156  engages the flange  154   a  that is coupled to locking collar  154  such that the longitudinal motion in the locking slider  156  is transmitted to the locking collar  154  as depicted in  FIG. 8 . The engagement of the locking slider  156  with the flange  154   a  permits the indexed rotational motion of the flange  154   a  in the shoulder roll mode as described above with reference to  FIG. 7  while transmitting the longitudinal motion in the locking slider  156  to the locking collar  154 . 
     As depicted in  FIG. 8 , the locking collar  154  is induced to move in the direction of arrow “F 0 ” in response to the longitudinal motion of the locking slider  156 . The locking collar  154  is moved proximally in the direction of arrow “F 0 ” until an interior concave surface  156  ( FIG. 14D ) of the locking collar  154  contacts the articulation sphere  106 . The articulation sphere  106  encounters a frictional resistance to motion since the tendons  110  exhibit a fixed length, and thus do not permit the articulation sphere  106  to move proximally. The frictional resistance to motion prohibits the articulation sphere  106  from pivoting about the pivot axis C-C ( FIG. 14A ) and thus prohibits motion of the end effector  16  in the articulation mode. 
     The frictional resistance generated between the locking collar  154  and the articulation sphere  106  also supplements the resistance to rotation provided by the indexing feature on the flange  154   a  as discussed above with reference to  FIG. 7 . The frictional resistance between the articulation sphere  106  and the locking collar  154  discourages rotation of the locking collar  154  about the longitudinal axis A-A. As discussed above, the locking collar  154  may include longitudinal ridges and channels on an outer surface thereof to define a splined slip joint with corresponding ridges and channels on an interior of the articulation spool  124 . The locking collar  154  includes longitudinal slots  160  ( FIG. 14D ) therein to permit passage of the axles  118  of the articulation sphere  106  and the drive post  126  of the articulation spool  124 . The frictional resistance to rotation in the locking collar  154  may thus be transmitted to the articulation spool  124 , and further to the shoulder roll knob  42  through the splined slip joint defined between articulation spool  124  and the shoulder roll knob  42 . Thus, the locking collar  154  provides a more positive stop to the shoulder roll mode than the indexing feature on the flange  154   a  alone. 
     Referring now to  FIG. 9 , the coaxial drive assembly  100  is moved to a fifth configuration to induce the end effector  16  to move in the wrist roll mode. The wrist roll mode is driven by subsystem  104 , which operates independently of the positioning subsystem  102 . Thus, the wrist roll mode, and the other modes of motion driven by the subsystem  104 , may be initiated irrespective of the configuration of the positioning subsystem  102 . To initiate the wrist roll mode, a surgeon may engage external bosses  162  on the wrist roll knob  44  to rotate the wrist roll knob  44  about the longitudinal axis A-A in the direction of arrows “W 1 .” The wrist roll knob  44  is coupled to a jaw spool  164  through a splined slip joint such that the jaw spool  164  rotates in the direction of arrow “W 2 ” along with the wrist roll knob  44 . The jaw spool  164  is fixedly coupled to an intermediate tubular member  166  of the elongated shaft  18 , and thus the intermediate tubular member  166  rotates in the direction of arrow “W 3 ” along with the wrist roll knob  44 . The intermediate tubular member  166  extends distally through the positioning subsystem  102  and through the outer tubular member  60 . A distal end of the intermediate tubular member  166  is coupled to a torsion cable  166   a  ( FIG. 10A ), which transmits torque from the intermediate tubular member  166  to a roll joint  168   a  ( FIG. 10A ). The roll joint  168  supports the end effector  16 , and thus the torque transmitted to the roll joint  168   a  may induce the end effector  16  to rotate in the wrist roll mode. The torsion cable  166   a  is flexible, and extends through the articulating distal portion  22  of the elongated shaft  18  such that torque may be transmitted through the distal portion  22  regardless of the articulation angle “α” achieved. 
     Referring now to  FIG. 10A , the torsion cable  166   a  extends to a roll joint  168   a . The roll joint  168   a  includes a first tubular structure  170  and a second tubular structure  172 . The first tubular structure  170  extends distally from the elongated shaft  18  where a shoulder  170   a  is provided to rigidly couple the first tubular structure  170  to leading segment  56   a . The second tubular structure  172  is rigidly coupled to an end effector housing  16   a  (see also  FIG. 2C ) that supports the end effector  16 . The first and second tubular structures  170 ,  172  are coupled to one another by a bearing set  174 , which permits the second tubular structure  172  to rotate relative to the first tubular structure  170  about end effector axis B-B. The bearing set  174  may be provided as a duplex pair, e.g. a pre-manufactured pair of bearings with a precisely controlled distance between races to provide a particular pre-load to the bearings, and may be fixedly coupled to the first and second tubular structures  170 ,  172  by welding or a similar process. The torsion cable  166   a  is coupled to the second tubular structure  172  to transmit torque thereto. When the intermediate tubular member  166  ( FIG. 9 ) is induced to rotate in the direction of arrow “W 3 ” ( FIG. 9 ) by the rotation of the wrist roll knob  44 , the torsion cable  166   a  induces the second tubular structure  172  to rotate in the direction of arrows “W 4 .” Since the second tubular structure  172  is rigidly coupled to the end effector housing  16   a , the end effector  16  may be moved through the wrist roll mode in the direction of arrows “W 0 ” ( FIG. 2C ) about the end effector axis B-B. 
     The first tubular structure  170  includes a cable wrap volume  176  therein to provide sufficient space for electrosurgical conductor  178  to be coiled about the end effector axis B-B. The cable wrap volume  176  extends between a bulkhead  176   a  and the bearing set  174 . The electrosurgical conductor  178  is configured to conduct electrosurgical energy to the end effector  16  in response to an appropriate actuation of finger trigger  50  and/or push button  50   a  ( FIG. 1 ). Conductor  178  may exhibit a round, flat or other geometry to facilitate winding or unwinding of the conductor  178  within the cable wrap volume  176 . Winding the conductor  178   a  permits the end effector  16  to rotate in the wrist roll mode without unduly straining or tangling the conductor  178   a . Sufficient slack is provided in the conductor  178   a  to permit rotation of the end effector  16  in either direction. The number of wrist roll rotations permitted may be limited by the geometric construction and winding configuration of the conductor  178   a . To relieve any undue strain on the conductor  178  that may occur as a result of reaching the limit of wrist-rolls, a structural stop  180  may be provided. The structural stop  180  is positioned such that the second tubular structure  172  engages the stop  180  to prohibit further rotational motion before the slack in the conductor  178   a  is taken up. First and second tubular structures  170 ,  172  are include a longitudinal bore extending therethrough to permit passage of various implements to effect motion in the end effector  16 . For example, a reciprocating member  188  is longitudinally movable in the clamping mode to open and close the jaws, and compression member  194  is longitudinally movable in the cutting mode to advance the knife blade. 
     Referring now to  FIG. 10B , an alternate embodiment of a roll joint is depicted generally as  168   b . The roll joint  168   b  includes a bearing set  174  coupled between first and second tubular structures  170 ,  172  as described above with reference to  FIG. 10A . An electrosurgical conductor  178   b  extends to an electrically conductive slip ring  182  defined on the interior of first tubular member  170 . An electrically conductive receptor  184  is coupled to the second tubular structure  172  such that the receptor  184  maintains electrical contact with the slip ring  182  regardless of the wrist roll angle “γ” ( FIG. 2C ) achieved. The receptor  184  is in electrical communication with the end effector  16  through electrosurgical conductor  178   c , and thus the end effector  16  may be provided with an electrosurgical current. The slip ring arrangement of roll joint  168   b  permits an unlimited number of wrist roll rotations of the end effector  16 . 
     Referring now to  FIG. 11 , the coaxial drive assembly  100  is moved to a sixth configuration for moving the end effector  16  in the clamping mode. As discussed above with reference to  FIG. 1 , the clamping mode may be initiated to open and close the jaw members  30 ,  32 . To close the jaws  30 ,  32 , (see  FIG. 2D ) the jaw spool  164  is moved longitudinally in the proximal direction of arrow “C 2 ” by the approximation of the pivoting handle  46  with the stationary handle  48  in the direction of arrow “C 1 .” The pivoting handle  46  engages the jaw spool  164  between drive flanges  186  such that jaw spool  164  may be moved longitudinally in either direction by the approximation and separation of the pivoting handle  46  with respect to the stationary handle  48 . Since the jaw spool  164  is fixedly coupled to the intermediate tube member  166  as described above with reference to  FIG. 9 , the proximal longitudinal motion in the jaw spool  164  is transmitted to the intermediate tube member  166 . Thus, the intermediate tube member  166  moves in the proximal direction as indicated by arrow “C 3 .” 
     A distal end of the intermediate tubular member  166  is coupled to a narrow reciprocating member  188  ( FIG. 10A ). The reciprocating member  188  transmits the proximal motion of the intermediate tube member  166  through the articulating distal portion  22  of the elongated shaft  18 . The reciprocating member  188  may be a relatively thin wire or bar oriented such that the reciprocating member  188  is sufficiently flexible to bend in a single plane to accommodate any articulation angle “α” while remaining sufficiently rigid to transmit sufficient tensile and compressive forces to open and close jaw members  30 ,  32 . The jaw members  30 ,  32  may include any appropriate feature such that the jaw members  30 ,  32  are induced to close by the proximal longitudinal motion in the reciprocating member  188 . For example, the jaw members  30 ,  32  may exhibit cam features (not shown) thereon to transform the longitudinal motion of the reciprocating member  188  into pivotal motion of the jaw members  30 ,  32  as in the instrument described in U.S. Pat. No. 7,083,618 to Couture et al., entitled “VESSEL SEALER AND DIVIDER.” 
     When the jaw members  30 ,  32  are moved to the closed configuration to clamp tissue therebetween, an electrosurgical tissue seal may be created. To form an effective tissue seal, in one embodiment, a relatively high clamping force is typically generated to impart a closure pressure on the tissue in the range of from about 3 kg/cm 2  to about 16 kg/cm 2 . An appropriate gap distance of about 0.001 inches to about 0.006 inches may be maintained between the opposing jaw members  30 ,  32 , although other gap distances are contemplated. Since at least one of the jaw members  30 ,  32  is connected to a source of electrical energy  50   b  (see  FIG. 1 ), a surgeon may manipulate finger trigger  50  and/or push button  50   a  to initiate transmission of electrosurgical energy through tissue to effectuate a seal. 
     Referring now to  FIG. 12 , the coaxial drive assembly  100  is moved to a seventh configuration for advancing a knife blade  54   a  (see  FIG. 2B ) to induce end effector motion in the cutting mode. The knife blade may be advanced through the jaw members  30 ,  32  to sever or transect tissue once a tissue seal has been formed. The knife spool  56  forms a splined slip joint with the jaw spool  164  such that a surgeon may move the knife spool  56  longitudinally in the distal direction of arrow “K 1 ” against the bias of a spring  190  ( FIG. 13 ). The knife spool  56  is fixedly coupled to an inner tubular member  192  such that the distal longitudinal motion imparted to the knife spool  56  is transmitted to the inner tubular member  192 . Thus, the inner tubular member  192  moves distally as indicated by arrow “K 2 .” A distal end of the inner tubular member  192  is fixedly coupled to the bendable compression member  194  ( FIG. 10A ), which is configured similar to the reciprocating member  188  described above with reference to  FIGS. 11 and 10A . Thus, the compression member  194  may transmit longitudinal motion through the articulating distal end  22  of the elongated shaft  18  regardless of the articulation angle “α” achieved. A distal end of the compression member  194  is, in turn, fixedly coupled to the knife blade  54   a . Thus, the longitudinal motion imparted to the knife spool  56  may be transmitted ultimately to the knife blade  54   a  to drive the knife blade  54   a  through tissue. When the surgeon releases the knife spool  56 , the bias of spring  190  tends to retract the knife blade  54   a.    
     Referring now to  FIG. 13 , a knife lock mechanism is depicted generally as  200 . The knife lock mechanism  200  is configured to permit advancement of the knife blade  54   a  ( FIG. 2B ) in the cutting mode only when the jaw members  30 ,  32  have been moved to the closed configuration depicted in  FIG. 2D . The mechanism  200  includes a latch piston  202  mounted within the stationary handle  48 . The latch piston  202  is normally biased by compression spring  204  to a position in which the latch piston  202  prohibits distal translation of the knife spool  56 . A latch release piston  206  is biased by compression spring  208  in a distal direction such that a distal end of the latch release piston  206  abuts one of the flanges  186  of the jaw spool  164 . A proximal end of the latch release piston  206  includes a ramped cam surface  210  in engagement with the latch piston  202 . 
     A surgeon may approximate the pivoting handle  46  with the stationary handle  48  to induce longitudinal motion in the jaw spool  164  in the direction of arrow “C 2 ” and thereby move the jaw members  30 ,  32  to the closed configuration as described above with reference to  FIG. 11 . The jaw spool  164  bears on the release piston  206  such that the release piston  206  moves in the proximal direction indicated by arrow “C 4 ” against the bias of spring  208 . As the release piston  206  moves proximally, the ramped cam surface  210  engages the latch piston  202  such that the latch piston  202  is induced to move in the direction of arrow “C 5 ” against the bias of spring  204 . The latch piston  202  is thereby moved to a position that does not interfere with longitudinal motion of the knife spool  56  in the direction of arrow “K 1 ”. Thus, motion in the cutting mode may be initiated. When the surgeon separates the pivoting handle  46  from the stationary handle  48 , the bias of the springs  204  and  208  return the latch piston  202  to its normally biased position. In the normally biased position, the latch piston  202  extends into the path of the knife spool  56  prohibiting motion of the knife spool  56 . 
     Although the foregoing disclosure has been described in some detail by way of illustration and example, for purposes of clarity or understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.