preferred embodiments of the surgical instrument and method disclosed herein will be described in terms of a minimally invasive direct coronary artery bypass ( midcab ) procedure wherein a vascular anastomosis is created by joining a section of a harvested vessel , e . g ., the internal memory artery ( ima ) to bypass an occlusion in a coronary artery , e . g ., the left anterior descending artery ( lad ). however , the presently disclosed surgical anastomosis instrument may also be utilized in performing anastomosis of other tubular luminal body structures . for example , the presently disclosed surgical anastomosis instrument may also be utilized to perform an a - v fistula to facilitate hemodialysis . this procedure consists of an end - to - side anastomosis joining an artery and a vein in the forearm or near the elbow . the a - v fistula allows a single puncture at the dialysis unit for blood cleansing . referring now in detail to the drawing figures in which like reference numerals identify similar or identical elements , one embodiment of the present disclosure is illustrated generally in fig1 and is designated therein as surgical instrument 100 . briefly , surgical instrument 100 includes a handle / actuator assembly 110 having a disposable loading unit 112 which is removably and rotatably attached to the handle / actuator assembly 110 by way of intermediate section 114 which extends from a distal end of housing / actuator assembly 110 . referring now to fig2 - 8 , disposable loading unit 112 includes a two - part fastener support member made up of split half - sections 116 a and 116 b which form a hinge and are connected to each other by hinge pin 118 . split half - section 116 a includes an actuator barrel 120 which forms a bore that opens at a proximal end . actuator barrel 120 includes opposed longitudinal slots 122 and 124 to facilitate operation of an actuator member 126 which is slidably mounted in actuator barrel 120 . actuation member 126 is spring biased to a proximal - most orientation by a coil spring 128 . actuator 126 is further retained within actuator barrel 120 by a lock ring 130 which includes an annular groove 132 defined by proximal and distal flanges 134 and 136 , respectively . each of the proximal and distal flanges 134 and 136 include a pair of opposed slotted portions which are aligned to facilitate locking of ring 130 anterior portion of a mounting hub 138 . for example , by way of passing lock ring 130 over opposed extended tabs 140 and 142 , as best seen in fig8 at the rotating lock ring 130 one quarter turn such that the pairs of opposed slots formed in proximal flange and distal flange 134 and 136 , respectively , are rotated 900 with respect to tabs 140 and 142 . lock ring 130 is biased against the distal facing surfaces of extended tabs 140 and 142 by the force of compressed coil spring 128 which is disposed in the bore formed longitudinally within actuator 126 . in particular , a shoulder portion 144 of actuator 126 is biased against an annular lip 146 ( fig4 ) which is formed on the distal facing surface of lock ring 130 . actuator member 126 further includes a pair of opposed transversely extending slots 148 and 150 formed on the outer surface thereof which , as will be described in further detail herein , provide an engagement surface for a fastener pusher member . split half - sections 116 a and 116 b of the fastener support member are provided with fastener retaining sections 152 and 154 . each of fastener retaining sections 152 and 154 are provided with a plurality of longitudinal channels 156 which are configured and dimensioned to support a “ c ”- shaped surgical fastener or clip 158 therein by , for example , friction or partial compression of clips 158 . fastener retaining sections 152 and 154 are preferably arranged such that longitudinal channels 156 form an eccentric array radially about the outer surface of fastener support member 116 . this arrangement facilitates the efficacious formation of an angled connection or fistula between the vessels to be joined . such a connection facilitates better fluid flow through the anastomotic site . a fastener pusher member 160 is provided to facilitate deformation of clips 158 during firing of the instrument . fastener pusher member 160 is made up of split half - sections 160 a and 160 b which are secured to split half - sections 116 a and 116 b , respectively , by way of mounting plates 162 and 164 and screws 166 which are threaded into threaded holes formed in receiving posts 168 , 170 ; and 172 , 174 disposed on split half - sections 116 a and 116 b , respectively . mounting plates 162 and 164 are attached to split half - sections 116 a and 116 b in a manner to permit longitudinal sliding movement of split half - sections 160 a and 160 b of fastener pusher member 160 relative to fastener support member 116 . this sliding movement is facilitated by longitudinally aligned slots 176 , 178 ; and 180 , 182 which are provided on split half - sections 160 a and 160 b , respectively . split half - sections 160 a and 160 b are further provided with molded surfaces such as protrusions 184 and depressions 186 as shown on split half - section 160 a which interfit with corresponding protrusions and depressions formed on split half - section 160 a so as to prevent relative longitudinal movement of split half - sections 160 a and 160 b . stepped camming or pusher surfaces 188 are formed on the distal ends of each of split half - sections 160 a and 160 b and serve to cam a proximally oriented portion of clips 158 towards a distal oriented portion of clips 158 . this motion effects deformation of the clips upon firing of surgical instrument 100 . finally , a hinge lock 190 is mounted between mounting plate 162 and split half - section 160 b . hinge lock 190 is provided with latch portions 192 and 194 which extend transversely across one side of disposable loading unit 112 to latch onto raised correspondingly shaped surfaces 196 and 198 , respectively , formed on the outer surface of split half - section 160 a . hinge lock 190 is further provided with a pair of longitudinally oriented slots 200 and 202 which are radially aligned with slots 180 and 182 of split half - section 160 b to facilitate relative longitudinal movement of hinge lock 190 with respect to both split half - section 160 b and split half - section 116 a . a flexible return lockout tab 204 extends proximally from the distal end of slot 202 and is cantilevered outwardly away from a plane defined by slots 202 and 204 such that it is positioned in a tab receiving recess 206 formed on the inner surface of mounting plate 162 when the dlu 112 is in a pre - fired condition . a lockout slot 208 is formed longitudinally aligned with tab receiving recess 206 and spaced distally therefrom such that upon distal movement of hinge lock 190 ( when split half - section 160 b is moved distally ), return lockout tab will be deflected to a flattened condition and will extend into lockout slot 208 in order to prevent subsequent proximal movement of hinge lock upon proximal movement of split half - section 160 b after firing of surgical instrument 100 . in this manner , after firing of surgical instrument 100 , latch portions 192 and 194 of hinge lock 190 will remain distal relative to raised surfaces 196 and 198 of split half - section of 160 a by permitting the split half - sectioned dlu 112 to be opened by pivoting split half - sections 116 b and 160 a away from split half - sections 116 a and 160 b to facilitate removal of the vessel segment from within the vessel pathway formed through the distal end of dlu 112 and out through a lateral opening 210 formed by aligned molded recesses formed in each of split half - sections 116 a , 116 b ; and 160 a and 160 b of fastener support member 116 and fastener pusher member 160 , respectively . referring now to fig9 - 12 , the handle / actuator assembly 110 of surgical instrument 100 will now be described in detail . handle / actuator assembly 110 includes a housing formed of half - sections 212 a and 212 b which are preferably molded to have recessed inner surfaces and contours formed therein to house the various components which are contained within handle / actuator assembly 110 . in particular , handle / actuator assembly 110 includes a dlu mounting assembly which facilitates detachable mounting of a dlu 112 to the distal end of handle / actuator assembly 110 . the dlu mounting assembly includes an elongated holding tube 214 which is held longitudinally and rotationally fixed relative to housing half - sections 212 a and 212 b . this mounting arrangement may be accomplished for example , by way of an annular flange 216 formed adjacent the proximal end of holding tube 214 being held within an annular groove defined by partition wall segments 218 and 220 formed in each of housing half - sections 212 a and 212 b . holding tube 214 is further prevented from rotational movement by way of flattened sidewall portions 222 formed on opposing sides of holding tube 214 being retained within and abutting flat surfaces 224 and 226 formed long partition wall segments 218 and 220 , respectively , in each of housing half - sections 212 a and 212 b . dlu locking tube 215 is secured within a collar 228 formed on slide 217 . tube 215 may be secured in slide 217 by any suitable means , for example , friction fitting bonding , adhesives , or the like . coil spring 219 is interposed in housing half - sections 212 a and 212 b between partitioned segments 218 and the proximal end surface of collar 228 . in this manner , locking tube 215 is biased in a distal - most position which corresponds to a locked position to retain dlu 112 on the distal end of surgical instrument 100 . the distal end of dlu holding tube 214 is provided with a semi - annular groove 230 which is dimensioned to receive mounting hub 138 formed at the proximal end of dlu 112 . semi - annular groove 230 is dimensioned to permit dlu mounting hub 138 to rotate within the groove . handle and actuator assembly 110 further includes a firing assembly which facilitates movement of actuator 126 housed within the actuator barrel 120 . in the embodiment illustrated in fig9 - 15 , the firing assembly is a linkage mechanism which imparts reciprocating longitudinal movement in an actuator rod by way of movement of an actuator lever . however , it is within the scope of the present disclosure that the actuator rod may be reciprocatingly moved by any suitable known methods . the firing assembly basically includes an actuator rod 232 which is connected to an actuator lever 234 by way of links 236 pivotally attached at either end to actuator rod 232 and actuator lever 234 , respectively . actuator rod 232 is slidably disposed within a longitudinal bore formed through dlu holding tube 214 . actuator rod 232 is biased in a proximal - most position by way of a torsion spring 238 being mounted on a post 240 formed near the proximal end of housing halfsection 212 b . extended legs 238 a and 238 b are provided on torsion spring 238 to bias actuator lever 234 upwardly away from housing half - sections 212 a and 212 b thereby pulling actuator rod 232 toward its proximal - most position as determined by the abutment of annular shoulder 242 against partition 244 formed in each of housing half - sections 212 a and 212 b . to facilitate comfort and ease of operation for the user , a cover 246 is secured over actuator lever 234 and is independently pivotally mounted to housing half - sections 212 a and 212 b . cover 246 is provided with an ergonomic surface 248 which is contoured and configured to be comfortably actuated by the thumb of a user when handle and actuator assembly 210 is held in the palm of the user &# 39 ; s hand . a firing safety mechanism is also provided to prevent premature firing of surgical instrument 100 . in the illustrated embodiment , the firing safety assembly includes an inverted leaf spring 250 having a cut - out portion 252 formed therein which biases against a shoulder 254 formed on actuator rod 232 . spring 250 is further provided with a hook portion 256 formed at the proximal end thereof which latches onto a protruding surface 258 formed in housing half - sections 212 a and 212 b . in this manner , spring 250 prevents distal movement of actuator rod 232 when the safety mechanism is armed . a safety release slide 260 which includes an upwardly extending contact surface which is exposed from the upper surface of housing half - sections 212 a and 212 b to permit the user to operate the slide and forms a clevis on a lower portion thereof which receives a flattened section 266 of actuator rod 332 . safety release slide 260 is further provided with a camming surface 268 along the lower edge thereof which cams the spring 250 out of contact with shoulder 254 upon proximal movement of safety release slide 260 . in use , as shown in fig1 - 31 , surgical instrument 100 facilitates the performance of a vascular anastomosis without the need for manual suturing of the vessels . the method and usage described herein will be addressed in terms of minimally invasive vascular anastomosis performed on a beating heart such as in a midcab procedure . however , the presently disclosed surgical instrument may also be used in performing anastomoses of other tubular or luminal body structures without departing from the scope of the present disclosure . for example , surgical instrument 100 may be used in conventional open cabg procedures using a median stemotomy or other large incision without stopping the heart . alternatively , the thoracic “ window ” procedure may be used to achieve access to the heart . the “ window ” approach involves a smaller incision and less displacement of the ribs , and therefore is less traumatic to the patient . for this approach , conventional surgical techniques are used to determine the location of the incision to access the chest cavity . in particular , a surgical retractor assembly may be used to separate the ribs at the site of the incision as shown in fig1 . specifically , a base 270 is placed on the chest of the patient with the central opening defined by the base being positioned over the operative site . retractor assemblies 272 are mounted to the base at various locations . each of retractor assemblies 272 includes a blade having a hook to engage either a rib or the sternum therewith . the retractor assemblies are mounted and used to retract ribs until a sufficiently large opening in the chest cavity is defined to provide direct access to the heart . for example , the sternum and the fourth and fifth ribs can be split apart to create a window . other configurations of spreading the ribs and / or selectively cutting individual ribs away from the sternum may also be utilized for a particular procedure . once the desired access to the heart is achieved , the graft vessel , e . g ., the internal memory artery ( ima ) is dissected from the surrounding cartilage and muscle , in a free end of the vessel is exposed . the occluded coronary artery , e . g ., the left anterior descending artery ( lad ), is then prepared for receiving the ima graft . the heart is positioned in the desired orientation either by traction sutures passing through the pericardium or by manipulation with heart manipulation instruments which are held by the surgical personnel or clamped in a fixed orientation to a base such as the retractor assembly base . one such heart manipulating instrument is available from united states surgical corporation of norwalk , conn . blood flow through the lad can be restricted by cardiopulmonary bypass and pericardial cooling . alternatively , a damping instrument may be applied directly on the lad to restrict blood flow and reduce movement of the heart near the lad . such a heart stabilizing instrument is also available from united states surgical corporation of norwalk , conn . referring to fig1 , the free end of the ima is inserted through lateral opening 210 of dlu 112 and passed out the distal end opening of the dlu . the free end of the ima is then everted over the distal end of the dlu such that the end of the ima is retained by the distal end oriented portions of clips 158 as best shown in fig1 and fig1 . everting of the tissue may be achieved by any suitable known techniques such as y using graspers . with the ima loaded in dlu 112 , the dlu is manipulated preferably detached from the handle and actuator assembly 110 in approximation to an incision “ i ” formed in the lad , as shown in fig1 . referring to fig1 and 19 , the dlu with the everted ima is inserted into the incision “ i ” of the lad such that the walls of the lad surrounding the incision are retained between the everted end of the ima and the proximal ends of clips 158 , as shown in fig1 . as previously noted , the distal end of dlu 112 is configured with an angle relative to a transverse plane of the dlu in order to optimize the anastomosis and to facilitate optimal blood flow across the graft site from the ima to the lad . this junction creates “ heel ” and “ toe ” portions in which an acute or obtuse angle between the vessels is defined . once dlu 112 with the everted ima has been successfully inserted through the incision of the lad , the surgeon may then attach the handle / actuator assembly 110 to dlu 112 as shown in fig2 and 21 . in particular , the dlu lock slide 217 is moved proximally as indicated by arrow “ a ” in order to retract locking tube 215 and thereby expose the distal end of dlu holding tube 214 , and in particular , semi - annular groove 230 . dlu 112 is inserted on the distal end of the handle and actuator assembly 110 by placing mounting hub 138 within semi - annular groove 230 and releasing the force applied on dlu lock slide 217 as indicated by arrow “ b ” in fig2 , to permit coil spring 219 to return lock slide 217 and locking tube 215 to their distal - most orientations thereby securing the dlu in place . referring to fig2 and 23 , when the surgeon is ready to complete the anastomosis , the safety release slide 260 is moved proximally as indicated by arrow “ c ” in fig2 thereby causing camming surface 268 to cam spring 250 downwardly away from shoulder 254 as indicated by arrow “ d ”. thereafter , the surgeon may depress cover 246 towards handle half - sections 212 a and 212 b , as indicated by arrow “ e ” in fig2 , causing actuator rod 232 to drive actuator 126 of dlu 112 distally as indicated by arrow “ f ”, thereby moving pusher member split half - sections 160 and 160 b distally to deform clips 158 as shown in fig2 and 26 . as best illustrated in fig2 , one particularly advantageous feature of the presently disclosed surgical instrument is that upon actuation of handle / actuator assembly 110 , safety release slide 260 is urged back into its initial locked orientation by way of annular flange 216 of actuator rod 232 pushing the lower portion of safety release slide 260 during the distal movement of actuator rod 232 . in this manner , upon release of cover handle 246 , the potential energy created by the compression of torsion spring 238 will cause actuator rod 232 to be pulled back proximally thereby engaging shoulder 254 with cutout 252 of spring 250 . this will serve to prevent accidental re - firing action of surgical instrument 100 . referring to fig2 - 30 , a further uniquely advantageous feature of surgical instrument 100 is hinge lock 190 and its operation . upon firing of surgical instrument 100 , return lockout tab 204 of hinge lock 190 , which extended into lockout slot 208 during distal movement of hinge lock 190 with split half - sections 160 and 160 b , serves to retain hinge lock 190 distal of its original seated location in tab receiving recess 206 . by retaining hinge lock 190 at this position , latch portions 192 and 194 are released from raised surfaces 196 and 198 of split half - section 160 a of the pusher member . in this manner , split half - section 116 b and 160 a may be pivoted away from split half - sections 116 a and 160 b as shown in fig3 to permit the removal of the ima from within the pathway of the dlu thereby completing the vascular anastomosis as shown in fig3 . referring to fig3 a , a portion of the vascular anatomy of an arm which may be an alternative utilized anastomosis site as illustrated wherein instead of joining the ima to the lad in a bypass procedure , an a - v fistula is performed utilizing surgical instrument 100 to join the radial artery end - to - side with the cephalic vein ( sometimes called the radial vein ). other a - v fistulas which may also be achieved utilizing surgical instrument 100 include joining the ulnar artery end - to - side with the basilic vein ( sometimes called the ulnar vein ). such a - v fistulas are performed to facilitate hemodialysis for end stage kidney disease to allow a single puncture at the dialysis unit for blood cleansing . the fistula allows a greater flow rate through the dialyzer ( not shown ). referring to fig3 - 39 , an alternative embodiment of the firing safety mechanism illustrated in fig2 - 24 is illustrated which also prevents the unintended firing of surgical instrument 100 . the firing safety mechanism includes a lock spring 350 and a safety release slide 360 . referring to fig3 and 33a , safety release slide 360 includes a pair of camming surfaces 360 a and 360 b which interact with cam follower surfaces 350 d and 350 e . lock spring 350 , as best illustrated in fig3 , includes a body portion 350 a that defines a yoke , a pair a leaf spring portions 350 b and 350 c , and a pair of cam follower surfaces 350 d and 350 e formed along a top portion thereof . an actuator rod 332 , fig3 , is configured and dimensioned to pass through yoke portion 350 a of lock spring 350 . in the initial ( pre - fired ) configuration , as shown in fig3 , safety release slide 360 is disposed above lock spring 350 such that camming surfaces 360 a and 360 b contact cam following surfaces 350 e and 350 e , respectively , when safety release slide 360 is in the distal - most position . in this orientation , leaf spring portions 350 b and 350 c serve to bias lock spring 350 upwardly within the instrument housing so that a shoulder portion 332 a of actuator rod 332 abuts against a proximal facing lower surface 350 f , as shown in fig3 , to prevent firing of the instrument . when the surgeon desires to fire the instrument , safety release slide 360 is moved proximally as indicated by arrow “ g ” in fig3 . in this manner , camming surfaces 360 a and 360 b travel over cam follower surfaces 350 e and 350 d , respectively , thereby urging lock spring 350 downwardly in the direction of arrows “ h ” to overcome the spring force of leaf spring portions 350 b and 350 c . this movement displaces proximal surface 350 f below shoulder portion 332 a thereby permitting actuator rod 332 to move distally . upon distal movement of actuator rod 332 as indicated by arrow “ i ” in fig3 , a second shoulder portion 332 b formed proximal of shoulder portion 332 a on actuator rod 332 contacts a rear wall portion 360 c of safety release slide 360 thereby urging safety release slide distally in a direction of arrow “ i ” as well . upon release of actuator handle cover 246 , actuator rod 332 moves proximally as indicated by arrow “ j ” in fig3 thereby permitting lock spring 350 to move upwardly as indicated by arrow “ k ” in fig3 to reset the firing safety mechanism . it will be understood that various modifications may be made to the embodiment shown herein . for example , the instruments may be sized to perform an anastomosis for other vessels and luminal tissue . therefore , the above description should not be construed as limiting , but merely as exemplications of preferred embodiments . those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto .
a surgical instrument for performing an anastomosis includes a housing having a proximal end and a distal end ; a shaft extending from the housing distal end ; and a disposable loading unit configured and dimensioned to retain a plurality of surgical fasteners , the disposable loading unit extending from the shaft and forming a juncture with the shaft , the disposable loading unit including : i ) opposed split - sections pivotably connected to facilitate movement of the opposed split - sections between open and closed configurations , the opposed split sections being movable through a firing stroke from a first position to a second position and back to the first position to form a plurality of surgical fasteners ; and ii ) a pivot lockout member including a latch portion which extends between the opposed split - sections , the latch portion including a contact surface which interacts with a corresponding surface formed on at least one of the opposed split - sections to retain the opposed split - sections in the closed configuration , wherein upon return of the fastener pusher member to the first position from the second position , the pivot lockout member is displaced relative to the opposed split sections to an unlocked configuration .