PATENT ABSTRACT
A surgical severing and stapling instrument clamps tissue in a staple applying assembly comprising a lower jaw of an elongate channel holding a staple cartridge and a pivotally opposed upper jaw (anvil). A firing handle drives a rack in a handle connected to a firing rod and bar that pass through an elongate shaft and into the staple applying assembly to sever and staple tissue. Upon release of the firing handle, a retraction spring assists in withdrawing the rack and other firing components. To advantageously assist in firing, a spring biased plunger is cocked prior to firing to impart assistance upon depression of the firing trigger. Alternatively, a torsion coil spring ratchet mechanism is preloaded to act through a gear into the rack to assist. As a further alternative, a retraction spring is preloaded and disengaged until the rack is fully fired.

PATENT DESCRIPTION
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to surgical stapler instruments that are capable of applying lines of staples to tissue while cutting the tissue between those staple lines and, more particularly, to improvements relating to stapler instruments and improvements in processes for forming various components of such stapler instruments.  
       BACKGROUND OF THE INVENTION  
       [0002]     Endoscopic and laparoscopic surgical instruments are often preferred over traditional open surgical devices since a smaller incision tends to reduce the post-operative recovery time and complications. The use of laparoscopic and endoscopic surgical procedures has been relatively popular and has provided additional incentive to develop the procedures further. In laparoscopic procedures, surgery is performed in the interior of the abdomen through a small incision. Similarly, in endoscopic procedures, surgery is performed in any hollow viscus of the body through narrow endoscopic tubes inserted through small entrance wounds in the skin.  
         [0003]     Laparoscopic and endoscopic procedures generally require that the surgical region be insufflated. Accordingly, any instrumentation inserted into the body must be sealed to ensure that gases do not enter or exit the body through the incision. Moreover, laparoscopic and endoscopic procedures often require the surgeon to act on organs, tissues and/or vessels far removed from the incision. Thus, instruments used in such procedures are typically long and narrow while being functionally controllable from a proximal end of the instrument.  
         [0004]     Significant development has gone into a range of endoscopic surgical instruments that are suitable for precise placement of a distal end effector at a desired surgical site through a cannula of a trocar. These distal end effectors engage the tissue in a number of ways to achieve a diagnostic or therapeutic effect (e.g., endocutter, grasper, cutter, staplers, clip applier, access device, drug/gene therapy delivery device, and energy device using ultrasound, RF, laser, etc.).  
         [0005]     Known surgical staplers include an end effector that simultaneously makes a longitudinal incision in tissue and applies lines of staples on opposing sides of the incision. The end effector includes a pair of cooperating jaw members that, if the instrument is intended for endoscopic or laparoscopic applications, are capable of passing through a cannula passageway. One of the jaw members receives a staple cartridge having at least two laterally spaced rows of staples. The other jaw member defines an anvil having staple-forming pockets aligned with the rows of staples in the cartridge. The instrument includes a plurality of reciprocating wedges which, when driven distally, pass through openings in the staple cartridge and engage drivers supporting the staples to effect the firing of the staples toward the anvil.  
         [0006]     An example of a surgical stapler suitable for endoscopic applications is described in U.S. Pat. Appl. Pub. No. US 2004/0232196 A1, the disclosure of which is hereby incorporated by reference in its entirety. The disclosed surgical stapling and severing instrument enables a clinician to close the jaw members upon tissue to position the tissue prior to firing. Once the clinician has determined that the jaw members are properly gripping tissue, the clinician can then fire the surgical stapler to sever and staple the tissue. The simultaneous severing and stapling avoids complications that may arise when performing such actions sequentially with different surgical tools that respectively only sever or staple. In addition, a retraction spring assists in retracting firing components so that the jaws may be unclamped to release the stapled and severed tissue portions.  
         [0007]     Thus, while such surgical staplers have provided a significant advancement in surgical procedures, further features would be desirable, such as reducing the force to fire. The requirement of a higher force to operate the trigger may be inconvenient or difficult for a surgeon with limited hand strength. One successful approach to lowering the force to fire is utilizing multiple firing strokes, as described in U.S. patent application Ser. No. 11/052,632, entitled “MULTI-STROKE MECHANISM WITH AUTOMATED END OF STROKE RETRACTION”, to Swayze et al., filed 07 Feb. 2005, the disclosure of which is hereby incorporated by reference in its entirety. However, it may be desirable in some instances to retain the simplicity of a single firing stroke, or to reduce force to fire during each stroke of multiple firing strokes.  
         [0008]     Consequently, a significant need exists for an improved surgical stapling and severing instrument that effectively severs and staples tissue, but with a reduced amount of force required to pull a firing trigger to cause the severing and stapling.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The invention overcomes the above-noted and other deficiencies of the prior art by providing a surgical instrument that performs a surgical procedure by firing a firing member through an elongate implement portion. This firing motion occurs by the action of a firing actuator moved by the surgeon. The surgeon is assisted by preloading an assistance mechanism in a handle of the surgical instrument, perhaps by a surgical nurse during the preparation of the surgical procedure. This preloaded bias, which is either toward firing or toward retraction, is coupled to the firing member when the firing member is to move in a firing direction or a retracting direction, respectively.  
         [0010]     In one aspect of the invention, a surgical instrument has an assistance mechanism with a follower member proximate and aligned for movement between an unfired and fired positions of a proximal portion of a firing drive train in a handle. A biasing member urges the follower member in a selected direction between unfired and fired positions after being preloaded by the movement of the shuttle in the opposite direction. An engagement mechanism releases the follower member, which is preloaded by the biasing member and engaged to the proximal portion of the firing mechanism. The release is in response to the proximal portion of the firing mechanism being moved in the selected direction.  
         [0011]     In another aspect of the invention, a surgical instrument includes a proximal portion of a firing mechanism that is held in place by a brake of an engagement mechanism when a follower member is moved from a fired position to an unfired position, which preloads a biasing member in the opposite direction, and engages the biased follower member to the firing mechanism. Firing of the firing mechanism releases the brake to enable assisted firing. Thereby, an effective firing of the implement portion is achieved even if high firing forces are required and the surgeon has limited strength to move the firing mechanism to achieve the required firing forces.  
         [0012]     In yet another aspect of the invention, a surgical instrument incorporates preloading of a retraction assistance mechanism that is disengaged during firing to reduce opposition to firing, yet readily engages after firing to retract the firing member through the elongate implement portion.  
         [0013]     These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 
     
    
     DESCRIPTION OF THE FIGURES  
       [0014]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.  
         [0015]      FIG. 1  depicts a right side view in elevation of a surgical stapling and severing instrument in an open position with an open end effector (staple applying assembly), partially cut away elongate shaft and a firing assistance mechanism.  
         [0016]      FIG. 2  depicts a right side view of the open end effector taken in longitudinal cross-section along the lines  2 - 2  of FIG. I of the surgical stapling and severing instrument.  
         [0017]      FIG. 3  is a front left isometric view of the surgical stapling and severing instrument of  FIG. 1  with a staple cartridge removed.  
         [0018]      FIG. 4  is a left isometric view of a handle of the surgical stapling and severing instrument of  FIG. 1  with a left handle housing shell and firing trigger retraction spring removed to expose a tension-spring firing assistance mechanism in a retracted (preloaded) state.  
         [0019]      FIG. 5  is a front left isometric view of a disassembled surgical stapling and severing instrument of  FIG. 1  with the end effector omitted.  
         [0020]      FIG. 6  depicts a left side elevational view of the handle portion of the surgical stapling and severing instrument of  FIG. 1  with a left handle housing shell and assistance mechanism removed to expose interior closure and firing components in an unclamped, unfired (“start”) position.  
         [0021]      FIG. 7  depicts a left side elevational view of the handle of the surgical stapling and severing instrument of  FIG. 7  with the left handle housing shell and assistance mechanism removed to expose interior closure and firing components in a “clamped” position.  
         [0022]      FIG. 8  depicts a left side elevational view of the handle of the surgical stapling and severing instrument of  FIG. 8  with the left handle housing shell and assistance mechanism removed to expose interior closure and firing components in the stapled and severed (“fired”) position.  
         [0023]      FIG. 9  depicts an isometric view of the end effector at the distal end of the surgical stapling and severing instrument of  FIG. 1  with the anvil in the up or open position exposing the staple cartridge and cutting edge of a firing bar.  
         [0024]      FIG. 10  depicts an isometric, exploded view of the implement portion of the surgical stapling and severing instrument of  FIG. 1 .  
         [0025]      FIG. 11  depicts an isometric view of the end effector at the distal end of the surgical stapling and severing instrument of  FIG. 1  with the anvil in the up or open position with the staple cartridge largely removed exposing a single staple driver and a double staple driver as exemplary and a wedge sled in a start position against a middle pin of the firing bar.  
         [0026]      FIG. 12  depicts an isometric view of the distal end of the surgical stapling and severing instrument of  FIG. 1  with the anvil in the up or open position with the staple cartridge completely removed and a portion of an elongate channel removed to expose a lowermost pin of the firing bar.  
         [0027]      FIG. 13  depicts a side elevation view in section showing a mechanical relationship between the anvil, elongate channel, and staple cartridge in the closed position of the surgical stapling and severing instrument of  FIG. 1 , the section generally taken along lines  13 - 13  of  FIG. 9  to expose the wedge sled, staple drivers and staples but also depicting the firing bar along the longitudinal centerline.  
         [0028]      FIG. 14  depicts a section view of the end effector of the surgical stapling and severing instrument with the cartridge and firing bar in the start position taken along line  14 - 14  of  FIG. 9 .  
         [0029]      FIG. 15  depicts a section view taken along line  15 - 15  of  FIG. 14  showing the cross-sectional relationship between the firing bar, elongate channel, wedge sled, staple drivers, staples and staple cartridge.  
         [0030]      FIG. 16  is a top view of the tension spring firing assistance mechanism of  FIG. 4  in an initial, start state with a small rack (rectangular plate of a firing drive train) retracted and an unloaded shuttle.  
         [0031]      FIG. 17  is a top view of the tension spring firing assistance mechanism of  FIG. 16  in an armed state (preloaded) with the small rack retracted by the shuttle causing an extended tension spring.  
         [0032]      FIG. 18  is a top view of the tension spring firing assistance mechanism of  FIG. 17  in a triggered state (firing) with the small rack being distally biased by the extended tension spring via the engaged shuttle while being simultaneously distally advanced by a firing trigger.  
         [0033]      FIG. 19  is a top view of the tension spring firing assistance mechanism of  FIG. 18  in a unloaded, fired state with the small rack being proximally retracted to return to the initial state after disengagement from the distally-positioned shuttle.  
         [0034]      FIG. 20  is a left side view in elevation of a top aft portion of the handle with the left handle housing shell removed from the surgical stapling and severing instrument of  FIG. 1  exposing a firing handle being fired to release a small rack brake of the firing assistance mechanism of  FIG. 4 .  
         [0035]      FIG. 21  is a front left isometric view of the small rack and brake assembly of  FIG. 20 .  
         [0036]      FIG. 22  is a left side view in elevation of a top aft portion of a right handle housing shell having a laterally elongate firing axle slot and an axle bias leaf spring of the firing assistance mechanism of  FIG. 4 .  
         [0037]      FIG. 23  is a left side detail view of the laterally elongate firing axle slot of the right handle housing shell of  FIG. 22 .  
         [0038]      FIG. 24  is a diagrammatic aft view in vertical cross section of the small rack and brake assembly of the firing assistance mechanism of  FIG. 4 .  
         [0039]      FIG. 25  is a left side detail view of a portion of the small rack, brake pad and brake linkages of the firing assistance mechanism of  FIG. 4 .  
         [0040]      FIG. 26  is a left side detail view of the small rack and the small rack brake of  FIG. 24  in a locked state;  
         [0041]      FIG. 27  is a left side detail view of the small rack and the small rack brake of  FIG. 27  moved to an unlocked state.  
         [0042]      FIG. 28  is a left side view in elevation of a top aft portion of the handle with the left handle housing shell removed from the surgical stapling and severing instrument of  FIG. 1  exposing a firing handle being fired to release an alternative small rack brake for the firing assistance mechanism of  FIG. 4 .  
         [0043]      FIG. 29  is a left side detail view of the alternative small rack brake of  FIG. 28 .  
         [0044]      FIG. 30  is a top view of an alternative compression spring firing assistance mechanism in an initial, start state with a small rack retracted and an unloaded shuttle for the surgical stapling and severing instrument of  FIG. 1 .  
         [0045]      FIG. 31  is a top view of the compression spring firing assistance mechanism of  FIG. 30  in an armed state (preloaded) with the small rack retracted and engaged to a retracted shuttle with a compressed compression spring.  
         [0046]      FIG. 32  is a top view of the tension spring firing assistance mechanism of  FIG. 31  in a triggered state (firing) with the small rack being distally biased by the compression spring via the engaged shuttle while simultaneously being distally advanced by the firing trigger.  
         [0047]      FIG. 33  is a top view of the tension spring firing assistance mechanism of  FIG. 32  in a retraction state after firing with the small rack being proximally retracted (shown in phantom) to return to the initial state after disengagement from the distally-positioned plunger.  
         [0048]      FIG. 34  is a front right perspective view of a handle of a surgical stapling and severing instrument with a wind-up firing assistance mechanism.  
         [0049]      FIG. 35  is a bottom view of the handle of the surgical stapling and severing instrument of  FIG. 34  with a left handle housing shell omitted and a bottom portion of a right handle housing shell cut away to expose the wind-up firing assistance mechanism.  
         [0050]      FIG. 36  is a bottom detail view of the wind up firing assistance mechanism of  FIG. 35 .  
         [0051]      FIG. 37  is a front left isometric view of a winding knob, portion of housing, ratchet gear and an assisted spur gear of the wind up firing assistance mechanism of  FIG. 34 .  
         [0052]      FIG. 38  is a front left isometric view of the winding knob, ratchet gear and an assisted spur gear multiplier gear of  FIG. 37  with a small outer spur gear removed to expose a torsion coil spring of the ratchet gear.  
         [0053]      FIG. 39  is a side view of the torsion coil spring of  FIG. 38  in a relaxed, loose coil.  
         [0054]      FIG. 40  is a side view of the torsion coil spring of  FIG. 39  in an actuated, tight coil.  
         [0055]      FIG. 41  is a front left perspective view of a handle of a surgical stapling and severing instrument including a retraction disengagement firing assistance mechanism.  
         [0056]      FIG. 42  is a front left isometric detail view of a small rack and shuttle of the retraction disengagement firing assistance mechanism of  FIG. 41 .  
         [0057]      FIG. 43  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 41  in an initial state, start state with a small rack and shuttle retracted.  
         [0058]      FIG. 44  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 43  being armed by distally advancing the shuttle with the small rack remaining retracted.  
         [0059]      FIG. 45  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 44  being armed by further distally advancing the shuttle with the small rack remaining retracted.  
         [0060]      FIG. 46  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 45  armed (pre-loaded) by a dual locking cam assembly being pivoted within the shuttle into engagement with the handle housing while the small rack remains retracted.  
         [0061]      FIG. 47  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 46  remaining armed (pre-loaded) as the small rack distally advances during firing of the surgical stapling and severing instrument.  
         [0062]      FIG. 48  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 47  with the small rack distally advanced to approximately full travel into contact with the dual locking cam assembly to effect disengagement from the handle housing.  
         [0063]      FIG. 49  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 48  with the small rack fully distally advanced to rotate the dual locking cam assembly fully out of engagement with the handle housing and into engagement with the small rack.  
         [0064]      FIG. 50  is a top view of the retraction disengagement firing assistance mechanism of  FIG. 49  with the small rack retracting under the retraction bias from the shuttle.  
         [0065]      FIG. 51  is an isometric view of the dual locking cam assembly of  FIG. 43 .  
         [0066]      FIG. 52  is an isometric disassembled view of the dual locking cam assembly of  FIG. 44 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0067]     Turning to the Drawings, wherein like numerals denote like components throughout the several views, in  FIG. 1 , a surgical stapling and severing instrument  10  is depicted that is capable of practicing the unique benefits of the present invention. The surgical stapling and severing instrument  10  incorporates an end effector  12 , which in the illustrative version is a staple applying assembly having an E-beam firing member (“firing bar”)  14  ( FIG. 2 ) that controls the spacing of the end effector  12 . In particular, an elongate channel  16  and a pivotally translatable anvil  18  are maintained at a spacing that assures effective stapling and severing. Firing this firing bar  14  requires an amount of force to sever tissue, form staples, and to overcome mechanical resistance in the surgical stapling and severing instrument  10 . Consistent with aspects of the present invention, an assistance mechanism  19  is incorporated into a handle portion  20  to reduce the amount of force necessarily imparted by the surgeon at the time of firing to achieve this amount of firing force imparted to the firing bar  14 . A preloading actuator, depicted as a firing assistance plunger  21  accessible on a left, aft surface of the handle portion  20 , allows a surgeon or surgical nurse to prepare the surgical stapling and severing instrument  10  by preloading an amount of mechanical potential energy that subsequently assists in firing.  
         [0068]     It will be appreciated that the terms “proximal” and “distal” are used herein with reference to a clinician gripping the handle portion  20  of the surgical stapling instrument  10 . Thus, the end effector  12  is distal with respect to the more proximal handle portion  20 . The end effector  12  is viewed from the front and the handle portion  20  is viewed from aft. It will be further appreciated that for convenience and clarity, spatial terms such as “vertical” and “horizontal” are used herein with respect to the drawings. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be limiting and absolute.  
         [0069]     The surgical and stapling and severing instrument  10  includes an implement portion  22  comprising a shaft  23  proximally attached to the handle portion  20  and distally terminating in the end effector  12 . The handle portion  20  includes a pistol grip  24  toward which a closure trigger  26  is pivotally drawn by the clinician to cause clamping, or closing, of the anvil  18  toward the elongate channel  16  of the end effector  12 . A firing trigger  28  is farther outboard of the closure trigger  26  and is pivotally drawn by the clinician to cause the stapling and severing of clamped tissue in the end effector  12 .  
         [0070]     In preparation for use, a surgical nurse may draw the firing assistance plunger  21  proximally (aft) until locked into place ( FIG. 4 ). An adjustment knob  29  ( FIG. 1 ) on a right side of the handle portion  20  may be turned to vary that amount of initial force required on the firing trigger  28  to activate the assistance mechanism  19 . A desired type of staple cartridge  37  would also be inserted into the end effector  12 . The surgeon then depresses the closure trigger  26  to close the end effector  12  and then may position the implement portion  22  through a cannula of a trocar (not shown) to a desired surgical site within the patient&#39;s body. Closure trigger  26  is actuated first. Once the clinician is satisfied with the positioning of the end effector  12 , the clinician may draw back the closure trigger  26  to its fully closed, clamped position proximate to the pistol grip  24 . Then, the firing trigger  28  is actuated. The firing trigger  28  springedly returns when the clinician removes pressure. A release button  30 , when depressed on the proximal end of the handle portion  20 , releases the clampled closure trigger  26 .  
         [0071]     The closure motion and firing motion are transferred down the elongate shaft  23 . In particular, a closure sleeve  32  encloses a frame  34 , which in turn encloses a firing drive rod  36  that is positioned by the firing trigger  28 . The frame  34  connects the handle portion  20  to the end effector  12 . With the closure sleeve  32  withdrawn proximally by the closure trigger  26  as depicted, the anvil  18  springedly opens, pivoting away from the elongate channel  16  and translating proximally with the closure sleeve  32 . The elongate channel  16  receives a staple cartridge  37 .  
         [0072]     In  FIG. 2 , the firing bar  14  includes three vertically spaced pins that control the spacing of the end effector  12  during firing. In particular, an upper pin  38  is staged to enter an anvil pocket  40  near the pivot between the anvil  18  and elongate channel  16 . When fired with the anvil  18  closed ( FIG. 13 ), the upper pin  38  advances distally within a longitudinal anvil slot  42  extending distally through anvil  18 . Any minor upward deflection in the anvil  18  is overcome by a downward force imparted by the upper pin  38 . Similarly, if insufficient tissue is clamped, the upper pin  38  may hold up the anvil  18  for proper staple formation. Firing bar  14  also includes a lower most pin, or firing bar cap,  44  that upwardly engages a channel slot  45  in the elongate channel  16 , thereby cooperating with the upper pin  38  to draw the anvil  18  and the elongate channel  16  slightly closer together in the event of excess tissue clamped there between or to space part in the event of insufficient tissue. The firing bar  14  includes a middle pin  46  that passes through a firing drive slot  47  formed in a lower surface of the cartridge  37  proximate to an upward surface of the elongate channel  16 , thereby driving the staples therein as described below. The middle pin  46 , by sliding against a lower surface of the staple cartridge  37  that rests upon the elongate channel  16 , resists any tendency for the end effector  12  to be pinched shut at its distal end. A distally presented cutting edge  48  between the upper and middle pins  38 ,  46  on the firing bar  14  traverses through a proximally presented, vertical slot  49  in the staple cartridge  37  to sever clamped tissue. The affirmative positioning of the firing bar  14  with regard to the elongate channel  16  and anvil  18  assure that an effective cut is performed.  
         [0073]     In  FIG. 4 , the handle portion  20  is partially disassembled from the depiction of  FIG. 3  to expose the assistance mechanism  19  that is integrated onto a left side of internal components of the handle portion  20 , the latter as described in the U.S. Pat. Appl. Pub. No. US 2004/0232196 A1 cross referenced above. In  FIG. 5 , the handle portion  20  is fully disassembled. In particular, the handle portion  20  is comprised of left and right handle housing shells  50  and  52 , which are molded from a polymeric material such as a glass-filled polycarbonate. The left handle housing shell  50  is provided with a plurality of cylindrical-shaped pins  54 . The right handle housing shell  52  includes a plurality of extending members  56 , each having a hexagonal-shaped opening  58 . The cylindrical-shaped pins  54  are received within the hexagonal-shaped openings  58  and are frictionally held therein for maintaining the left and right handle housing shells  50 ,  52  in assembly.  
         [0074]     With particular reference to  FIG. 5 , a rotating knob  60  has a bore  62  extending completely through it for engaging and rotating the implement portion  22  about its longitudinal axis. The rotating knob  60  includes an inwardly protruding boss  64  extending along at least a portion of the bore  62 . The protruding boss  64  is received within a longitudinal slot  66  formed at a proximal portion of the closure sleeve  32  such that rotation of the rotating knob  60  effects rotation of the closure sleeve  32 . It will be appreciated that the boss  64  further extends into a proximal slot  67  in the frame  34  to effect rotation of the elongate shaft  23 . Thus, the end effector  12  ( FIGS. 1-3 ) rotates with the rotating knob  60 .  
         [0075]     A proximal end  68  of the frame  34  passes proximally through the rotating knob  60  and is provided with a circumferential notch  70  that is engaged by opposing channel securement members  72  extending respectively from the left and right handle housing shells  50 ,  52 . Only the channel securement member  72  of the right handle housing shell  52  is shown. The channel securement members  72  extending from the left and right handle housing shells  50 ,  52  serve to secure the frame  34  to the handle portion  20  such that the frame  34  does not move longitudinally relative to the handle portion  20 , but is allowed to rotate about its longitudinal axis.  
         [0076]     The closure trigger  26  has a handle section  74 , a gear segment section  76 , and an intermediate section  78 . A bore  80  extends through the intermediate section  78 . A cylindrical support member  82  extending from the right handle housing shell  52  passes through the bore  80  for pivotably mounting the closure trigger  26  on the handle portion  20 .  
         [0077]     In the afore-referenced U.S. Pat. Appl. Pub. No. US 2004/0232196 A1, the firing trigger  28  was pivotally supported upon a second, smaller cylindrical support member that coaxially extends to the left and passes through a bore  81  of firing trigger  28  and into engagement with the left handle housing shell  50  of the handle portion  20  (not shown). Instead, the assistance mechanism  19  advantageously includes a longitudinally movable firing trigger axle  83  that passes though the bore  81  of the firing trigger  28  and its lateral ends slide within longitudinally elongate oval guides  84  formed into both the left and right handle housing shells  50 ,  52 .  
         [0078]     A closure yoke  86  is housed within the handle portion  20  for reciprocating movement therein and serves to transfer motion from the closure trigger  26  to the closure sleeve  32 . Support members  88 , extending from the right handle housing shell  52  and securement member  72 , which extends through a recess  89  in the yoke  86 , support the yoke  86  within the handle portion  20 .  
         [0079]     A proximal end  90  of the closure sleeve  32  is provided with a flange  92  that is snap-fitted into a receiving recess  94  formed in a distal end  96  of the yoke  86 . A proximal end  98  of the yoke  86  has a gear rack  100  that is engaged by the gear segment section  76  of the closure trigger  26 . When the closure trigger  26  is moved toward the pistol grip  24  of the handle portion  20 , the yoke  86  and, hence, the closure sleeve  32  move distally, compressing a spring  102  positioned between a distal narrowed ring  103  defined in the left and right handle housing shells  50 ,  52  and the distal end  96  of the closure yoke  86 , that biases the yoke  86  proximally. Distal movement of the closure sleeve  32  effects pivotal translation movement of the anvil  18  distally and toward the elongate channel  16  of the end effector  12  and proximal movement effects closing, as discussed below.  
         [0080]     The closure trigger  26  is forward biased to an open position by a front surface  130  interacting with an engaging surface  128  of the firing trigger  28 . Clamp first hook  104  that pivots top to rear in the handle portion  20  about a pin  106  restrains movement of the firing trigger  28  toward the pistol grip  24  until the closure trigger  26  is clamped to its closed position. Hook  104  restrains firing trigger  28  motion by engaging a lockout pin  107  in firing trigger  28 . The hook  104  is also in contact with the closure trigger  26 . In particular, a forward projection  108  of the hook  104  engages a member  110  on the intermediate section  78  of the closure trigger  26 , the member  110  being outward of the bore  80  toward the handle section  74 . Hook  104  is biased toward contact with member  110  of the closure trigger  26  and engagement with lockout pin  107  in firing trigger  28  by a release spring  112 . As the closure trigger  26  is depressed, the hook  104  is moved top to rear, compressing the release spring  112  that is captured between a rearward projection  114  on the hook  104  and a forward projection  116  on the release button  30 .  
         [0081]     As the yoke  86  moves distally in response to proximal movement of the closure trigger  26 , an upper latch arm  118  of the release button  30  moves along a proximal, upper surface  120  on the yoke  86  until dropping into an upwardly presented recess  122  in a proximal, lower portion of the yoke  86 . The release spring  112  urges the release button  30  outward, which pivots the upper latch arm  118  downwardly into engagement with the upwardly presented recess  122 , thereby locking the closure trigger  26  in a tissue clamping position, such as depicted in  FIG. 7 .  
         [0082]     Returning to  FIG. 5 , the latch arm  118  can be moved out of the recess  122  to release the anvil  18  by pushing the release button  30  inward. Specifically, the upper latch arm  118  pivots upward about pin  123  of the right handle housing shell  52 . The yoke  86  is then permitted to move proximally in response to return movement of the closure trigger  26 .  
         [0083]     A firing trigger return spring  124  is located within the handle portion  20  with one end attached to pin  106  of the right handle housing shell  52  and the other end attached to a pin  126  on the firing trigger  28 . The firing return spring  124  applies a return force to the pin  126  for biasing the firing trigger  28  in a direction away from the pistol grip  24  of the handle portion  20 . The closure trigger  26  is also biased away from pistol grip  24  by engaging surface  128  of firing trigger  28  biasing front surface  130  of closure trigger  26 .  
         [0084]     As the closure trigger  26  is moved toward the pistol grip  24 , its front surface  130  engages with the engaging surface  128  on the firing trigger  28  causing the firing trigger  28  to move to its “firing” position. When in its firing position, the firing trigger  28  is located at an angle of approximately  45 ° to the pistol grip  24 . After staple firing, the spring  124  causes the firing trigger  28  to return to its initial position. During the return movement of the firing trigger  28 , its engaging surface  128  pushes against the front surface  130  of the closure trigger  26  causing the closure trigger  26  to return to its initial position. A stop member  132  extends from the right handle housing shell  52  to prevent the closure trigger  26  from rotating beyond its initial position.  
         [0085]     The surgical stapling and severing instrument  10  additionally includes a firing reciprocating assembly (“firing drive train”)  134  that transfers firing motions from the firing trigger  28  to the end effector  12  for severing and stapling tissue. Although a separate closure approach is incorporated in the illustrative version, applications consistent with the present invention may use a first portion of the distal travel of the firing drive train  134  to close the end effector  12  with the later portions of the distal travel used to sever and staple. A proximal portion of the firing drive train  134  is received within the handle portion  20  and a distal portion of the firing drive train  134  is received within the implement portion  22 . The proximal portion of the firing drive train  134  begins with a longitudinally elongate rectangular plate  135  having a proximal gear rack segment  136  that engaged to a gear segment  137  upwardly presented on the firing trigger  28  above the pivot axis defined by the bore  81  and firing trigger axle  83 . A distal gear rack segment  138  on the rectangular plate  135  engages a leftward small spur gear (pinion)  139  of a coaxial dual spur gear (multiplier gear)  140 , which freewheels on a laterally aligned gear axle  143  and whose large spur gear (large pinion)  141  engages a rod gear rack  142  formed on a proximal end of the firing drive rod  36 . The firing rod  36  and firing bar  14  thus form the distal portion of the firing drive train  134  that transfer this firing motion through the implement portion  22 .  
         [0086]     A first notch  144  is provided on the rectangular plate  135  intermediate the proximal and distal gear rack segments  136 ,  138 . During return movement of the firing trigger  28 , a tooth  146  on the firing trigger  28  engages with the first notch  144  on the rectangular plate  135  for returning the rectangular plate  135  to its initial position after staple firing. A second notch  148  is located at a proximal end of the firing drive rod  36  for locking the firing drive rod  36  to the upper latch arm  118  of the release button  30  in its unfired position.  
         [0087]     Before describing the components and operation of the assistance mechanism  19 , in  FIGS. 6-8 , the handle portion  20  is depicted with the left handle housing shell  50  and the assistance mechanism  19  removed for clarity in describing the closure and firing components. In  FIG. 6 , the handle portion  20  is in the start position (open and unfired). In  FIG. 7 , the handle portion  20  is in a clamped position (closed and unfired). In  FIG. 8 , the handle portion is in a fired position. In order to prevent staple firing before tissue clamping has occurred, the upper latch arm  118  on the release button  30  is engaged with the second notch  148  on the rectangular plate  135  such that the firing drive rod  36  is locked in its proximal-most position, as depicted in  FIG. 6 . When the upper latch arm  118  falls into the recess  122 , the upper latch arm  118  disengages with the second notch  148  to permit distal movement of the firing drive rod  36 , as depicted in  FIG. 8 .  
         [0088]     Because the distal gear rack segment  138  on the rectangular plate  135  and the rod gear rack  142  on the firing drive rod  36  are engaged with the multiplier gear  140 , movement of the firing trigger  28  causes the firing drive rod  36  to reciprocate between a first reciprocating position, shown in  FIG. 7 , and a second reciprocating position, shown in  FIG. 8 . Since the diameter of the large pinion gear  141  is greater than the diameter of the small pinion gear  139 , the multiplier gear  140  moves the distal portion of the firing drive train  134  (i.e., firing drive rod  36  and firing bar  14 ) a greater distance than the rectangular plate  135  is moved by the firing trigger  28 . The relative diameters of the larger and small pinion gears  139 ,  141  may be selected to permit the length of the stroke of the firing trigger  28  and the force required to move it to be varied.  
         [0089]     The end effector  12  of the surgical stapling and severing instrument  10  is depicted in further detail in  FIGS. 9-15 . As described above, the handle portion  20  produces separate and distinct closing and firing motions that actuate the end effector  12 . The end effector  12  maintains the clinical flexibility of this separate and distinct closing and firing (i.e., stapling and severing). In addition, the end effector  12  introduces the aforementioned ability to affirmatively maintain the closed spacing during firing after the clinician positions and clamps the tissue. Both features procedurally and structurally enhance the ability of the surgical stapling and severing instrument  10  by ensuring adequate spacing for instances where an otherwise inadequate amount of tissue is clamped and to enhance the clamping in instances where an otherwise excessive amount of tissue has been clamped.  
         [0090]     In  FIG. 9 , the end effector  12  is in an open position by a retracted closure sleeve  32  and includes a staple cartridge  37  installed in the elongate channel  16 . On a lower surface  200  of the anvil  18 , a plurality of stapling forming pockets  202  are arrayed to correspond to a plurality of stapler apertures  204  in an upper surface  206  of the staple cartridge  37 . The firing bar  14  is at its proximal position, with the upper pin  38  aligned in a noninterfering fashion with the anvil pocket  40 . The anvil pocket  40  is shown as communicating with the longitudinal anvil slot  42  in the anvil  18 . The distally presented cutting edge  48  of the firing bar  14  is aligned with and proximally from removed from the vertical slot  49  in the staple cartridge  37 , thereby allowing removal of a spent cartridge and insertion of an unfired cartridge, which is snapfit into the elongate channel  16 . Specifically, extension features  208 ,  210  of the staple cartridge  37  engage recesses  212 ,  214  (shown in  FIG. 11 ) of the elongate channel  16 .  
         [0091]     In  FIG. 10 , the implement portion  22  of the surgical stapling and severing instrument  10  is in disassembled form. The staple cartridge  37  is shown as being comprised of a cartridge body  216 , a wedge sled  218 , single and double drivers  220 ,  221  staples  222  ( FIGS. 13, 15 ), and a cartridge tray  224 . When assembled, the cartridge tray  224  holds the wedge sled  218 , single and double drivers  220 ,  221  and staples  222  inside the cartridge body  216 .  
         [0092]     Having a wedge sled  218  integral to the staple cartridge  37  enables a number of flexible design options as compared to incorporating camming surfaces onto a firing bar itself. For instance, a number of different staple cartridges  37  may be selected for use in the surgical stapling and severing instrument  10  with each staple cartridge  37  having a different configuration of rows of staples  222 , each thus having a unique wedge sled  218  configured to contact the middle pin  46  of the firing bar  14  while causing the driving of the staples  222 . As another example, the integral wedge sled  218  provides an opportunity for a number of lockout features.  
         [0093]     The elongate channel  16  has a proximally placed attachment cavity  226  that receives a channel anchoring member  228  on the distal end of the frame  34  for attaching the end effector  12  to the handle portion  20 . The elongate channel  16  also has an anvil cam slot  230  that pivotally receives an anvil pivot  232  of the anvil  18 . The closure sleeve  32  that encompasses the frame  34  includes a distally presented horseshoe shaped aperture  234  that engages an anvil feature  236  proximate but distal to the anvil pivot  232  on the anvil  18  to thereby effect opening and closing of the anvil  18 . The firing drive rod  36  is shown as being assembled to the firing bar  14  by a firing connector  238  by pins  240 , which in turn is rotatingly and proximally attached to the firing drive rod  36 . The firing bar  14  is guided at a distal end of the frame  34  by a slotted guide  239  inserted therein.  
         [0094]     With particular reference to  FIG. 11 , a portion of the staple cartridge  37  is removed to expose portions of the elongate channel  16 , such as recesses  212 ,  214  and to expose some components of the staple cartridge  37  in their unfired position. In particular, the cartridge body  216  (shown in  FIG. 10 ) has been removed. The wedge sled  218  is shown at its proximal, unfired position with a pusher block  242  alligned to contact the middle pin  46  (hidden in  FIG. 11 ) of the firing bar  14 . The wedge sled  218  is in longitudinal sliding contact upon the cartridge tray  224  and includes wedges  228  that force upward the single and double drivers  220 ,  221  as the wedge sled  218  moves distally. Staples  222  (not shown in  FIG. 11 ) resting upon the drivers  220 ,  221  are thus also forced upward into contact with the anvil forming pockets  202  on the anvil  18  to form closed staples  222 . Also depicted is the channel slot  45  in the elongate channel  16  that is aligned with the vertical slot  49  in the staple cartridge  37 .  
         [0095]     In  FIG. 12 , the end effector  12  of  FIG. 11  is depicted with all of the staple cartridge  37  removed to show the middle pin  46  of the firing bar  14  as well as portion of the elongate channel  16  removed adjacent to the channel slot  45  to expose the firing bar cap  44 . In addition, portions of the shaft  23  are removed to expose a proximal portion of the firing bar  14 . Projecting downward from the anvil  18  near the pivot, a pair of opposing tissue stops  244  prevent tissue from being positioned too far up into the end effector  12  during clamping.  
         [0096]     In  FIG. 13 , the end effector  12  is closed in a tissue clamping position with the firing bar  14  unfired. The upper pin  38  is in the anvil pocket  40 , vertically aligned with the anvil slot  42  for distal longitudinal movement of the firing bar  14  during firing. The middle pin  46  is positioned to push the wedge sled  218  distally so that wedge  228  sequentially contacts and lifts double drivers  221  and the respective staples  222  into forming contact with staple forming pockets  202  in the lower surface  200  of the anvil  18 .  
         [0097]     In  FIG. 14 , the upper surface  206  of the staple cartridge  37  is depicted with the firing bar  14  in its unfired, proximal position. The stapler apertures  204  are arrayed on each side of the vertical slot  49  in the staple cartridge  37 .  
         [0098]     In  FIG. 15 , the portion of the elongate channel  16  of the end effector  12  near the pivot has opposing ramp portions  246  to thereby cooperate with the tissue stops  244  of the anvil  18  ( FIG. 12 ) to prevent tissue from jamming the end effector  12 . Also depicted in greater detail are the double drivers  221  and their relation to the staples  222 .  
         [0099]     Returning to  FIGS. 4-5 , the assistance mechanism  19  complements the operation of the afore-described firing components of the handle portion  20  having a spring biased follower member, which in the illustrative version is a linearly moving shuttle  300 , that selectively urges a proximal portion of the firing drive train  134 , which in the illustrative version is the rectangular plate  135 . The shuttle  300  has a lateral through slot  302  in which a cam finger  304  is centrally engaged for horizontal rotation by a vertical pin  306  residing in a pin hole  308 . A proximal end of the shuttle  300  is attached to the firing assistance plunger  21  whose cylindrical shaft  310  slides through a plunger hole  312  ( FIG. 4 ) formed in the left handle housing shell  50  ( FIG. 5 ). A stop surface of the plunger  21  is provided by an increased diameter cylindrical finger grip  314  at an aft end of the cylindrical shaft  310 . A distal end of the shuttle  300  is attached to a proximal hook end  315  of a tension spring  316  residing with a distally open lateral slot  317  in the shuttle  300  by a pin  318 . A distal hook end  320  of the tension spring  316  is attached to a distal portion of the left handle housing shell  50  such that retraction of the plunger  21 , as in  FIG. 4 , stretches the tension spring  316  storing mechanical potential energy therein. A shuttle tray structure  323  formed in the left handle housing shell  50  guides the tension spring  316  and shuttle  300  and limits the distal movement of the shuttle  300  ( FIG. 4 ).  
         [0100]     In  FIGS. 16-19 , interaction is depicted between the shuttle  300  of the assistance mechanism  19  and the rectangular plate  135  of the firing drive train  134 . The cam finger  304  is generally an elongate rectangular plate with tapered in long sides presenting a front right corner that rotates clockwise, when viewed from above into a corresponding notch  322  about midway along a top left edge of the rectangular plate  135 . Thus, the cam finger  304  may engage the rectangular plate  135  if inserted into the notch  322  with the shuttle  300  biased distally. In  FIG. 16 , tension spring firing assistance mechanism  19  of  FIG. 4  in an initial, start state with the rectangular plate  135  retracted and with the plunger distally positioned and disengaged. An aft left surface of the cam finger  304  rests upon a kick-out cam step  324  attached to an internal surface of the left handle housing shell  50 . It should be appreciated that a torsion coil spring and/or the lateral spacing around the cam finger  304  may bias the front, right surface of the cam finger  304  into contact with the rectangular plate  135  unless rotated counterclockwise, when viewed from above, by contact with the kick-out cam step  324 . In  FIG. 17 , the tension spring firing assistance mechanism  19  of  FIG. 16  has been armed by retracting the plunger  21  to an armed state (preloaded). The cam finger  304  is engaged into the notch  322  of the rectangular plate  135  and the tension spring  316  has been longitudinally stretched to store mechanical potential energy. In  FIG. 18 , the tension spring firing assistance mechanism  19  of  FIG. 17  is being fired distally, the rectangular plate  135  being urged by the firing trigger  28  (not shown in  FIGS. 16-19 ) and assisted by the spring-biased shuttle  300 . The cam finger  304  has approached its most distal position as a beveled proximal surface of the kick-out cam step  324  contacts the aft left surface of the cam finger  304 . In  FIG. 19 , tension spring firing assistance mechanism  19  of  FIG. 18  has reached its unloaded, fired state with the cam finger  304  rotated counterclockwise as viewed from above out of engagement with the notch  322  in the rectangular plate  135 , allowing a retraction bias and/or reverse movement of the firing trigger to retract the rectangular plate  135  (as shown in phantom)..  
         [0101]     In  FIGS. 5 and 20 - 27 , a selective engagement mechanism of the firing assistance mechanism  19  that couples a firing assistance force during firing includes both the selective engagement of the shuttle  300  to the rectangular plate  135  but also the selective release of the combination of the rectangular plate  135  and the shuttle  300  from a held proximal position relative to the handle portion  20 . To that end, a small rack brake assembly  340  of the assistance mechanism  19  holds the rectangular plate  135 , and thus the firing drive train  134 , in a proximal position until firing commences. The firing trigger axle  83  passes through a distal end hole  342  of a horizontal brake link  344 . A proximal abutment surface  346  of the horizontal brake link  344  is urged distally by a distally bowed leaf spring  348  having an upper looped end  350  engaged to a lateral pivot pin  352  extending from the left handle housing shell  50 . A lower end  354  of the distally bowed leaf spring  348  is adjustably positioned longitudinally between the left and right handle housing shells  50 ,  52  by a cylindrical cam  356  having an off-center lateral through hole  358  that receives an adjustment axle  360  rotationally fixed by a pin  362  and turned by the adjustment knob  29 . The cylindrical cam  356  is rightward biased from contact with the left handle housing shell  50  by a spring  363  ( FIG. 5 ). A locking finger  364  passing transversely through the adjustment axle  360  frictionally engages at both ends a frictional ring surface  366  ( FIG. 22 ) formed on an inner surface of the right handle housing shell  52  to hold the adjusted position. Adjustment allows the user to vary the amount of spring force distally biasing the firing trigger axle  83  via horizontal brake link  344 .  
         [0102]     A block-shaped brake pad  368  is moved into braking contact with a proximal undersurface of the rectangular plate  135  proximal to the proximal gear rack segment  136  when the horizontal brake link  344  is at its forward position with the firing trigger axle  83  at a distal end of the longitudinally elongate oval guides  84  laterally aligned in each handle housing shell  50 ,  52 . The block-shaped brake pad  368  is moved out of braking contact with the proximal undersurface of the rectangular plate  135  when the horizontal brake link  344  is at its aft position with the firing trigger axle  83  moved away from the distal end of the longitudinally elongate oval guides  84 . This aft movement occurs when the firing trigger  28  is depressed with some of the resistance reacted by the firing drive train  134  being transferred to the firing trigger axle  83  and ultimately into the distally bowed leaf spring  348 .  
         [0103]     Horizontal motion of the horizontal brake link  344  is converted to an aft and up rotational motion (when viewed from the left when engaging) at the brake pad  368  by an assembly that rotates as a plane laterally aligned by left and right lateral spacing pins  370 ,  372  that extend respectively into sliding contact with the left and right handle housing shells  50 ,  52 . In particular, a lower rod-shaped axle  374  passes through a proximal lateral through hole  376  in the horizontal brake link  344  and through a lower horizontal through hole  378 ,  380  respectively in left and right upper brake arms  382 ,  384  that flank the horizontal brake link  344 . The left and right lateral spacing pins  370 ,  372  extend respectively outward from the left and right upper brake arms  382 ,  384  from respective left and right holes  386 ,  388  above the respective lower horizontal through hole  378 ,  380 . An upper square axle  390  passes through left and right upper through holes  392 ,  394  in the respective upper brake arms  382 ,  384  and a generally horizontally elongate rectangular through hole  396  ( FIGS. 26-27 ) in the block-shaped brake  368 . Left and right rectangular prism ends (shoe)  398 ,  400  with a lateral square cross section are attached to the ends of the upper square axle  390  on the lateral outside respectively of the left and right upper brake arms  382 ,  384 . Each cubic end  398 ,  400  is received within a respective rectangular boss  402 ,  404  ( FIGS. 21-22 ,  24 - 27 ) formed on the inner surface respectively of the left and right handle housing shells  50 ,  52 .  
         [0104]     It should be appreciated that the firing drive train  134  is generally held in a retracted position by the firing trigger return spring  124  rotating the firing trigger  28  clockwise, when viewed from the left. When the small rack brake assembly  340  is in its default, engaged position, as depicted in  FIGS. 20-21 ,  25 - 26 , with the horizontal brake link  344  forward and the upper brake arms  382 ,  384  top aft, the rectangular plate  135  is allowed to retract, as depicted as occurring in  FIG. 19 . However, the brake pad  368  creates sufficient frictional force to offset any tendency of the firing drive train  134  to be distally advanced solely under the influence of the assistance mechanism  19  that might otherwise overcome this retraction force. With particular reference to  FIG. 25-26 , the small rack brake assembly  340  is urged into the default, engaged position by the forward movement of the horizontal brake link  344 . The left and right rectangular bosses  402 ,  404  are canted such that forward movement at the connection between the horizontal brake link  344  and upper brake arms  382 ,  384  causes the left and right shoes  398 ,  400  to move and aft and upward in the respective left and right bosses  402 ,  404 . The upper square axle  390  can slide within the generally horizontal rectangular through hole  396  in the brake pad  368  to allow for retraction of the rectangular plate  135  and thus the firing drive train  134 . In particular, the generally horizontal rectangular through hole  396  is slightly canted upward at its distal end. Thus frictional contact of the brake pad  368  to the under surface of the rectangular plate  135  during retraction slides the brake pad  368  proximally with the upper square axle  390  positioned in a slightly higher distal portion of the generally horizontal through hole  396 , allowing a slight gap to occur between the surfaces ( FIG. 27 ). Conversely, a distal bias on the firing drive train  134 , such as when the assistance mechanism  19  is preloaded and engaged to the firing drive train  134 , causes a slight distal movement of the brake pad  368 , moving the upper square axle  390  to a slightly lower proximal portion of the generally horizontal through hole  396  in the brake pad  368 , urging the brake pad  368  upwardly into braking contact with the rectangular plate  135 .  
         [0105]     In  FIGS. 28-29 , an alternate small rack braking assembly  440  includes the adjustable distally bowed leaf spring  348  that forward biases a horizontal brake link  442  and thus the firing trigger axle  83  as described above. An upper brake arm  444 , however, is perpendicularly and rigidly attached at a midpoint of the horizontal brake link  442  to selectively rotate an inverted J-shaped brake arm  446 . A resilient guide  448 , formed in the left handle housing shell  52 , positions a top portion  450  of the inverted J-shaped brake arm  446  so that a downwardly curved proximal end  452  is rotated downward into binding contact with a recessed top surface  454  formed in the proximal end of the rectangular plate  135 . A distal downwardly hanging portion  456  of the inverted J-shaped brake arm  446  may be rotated proximally by a proximally moving upper brake arm  442  causing the downwardly curved proximal end  452  to lift off of the recessed top surface  454  allowing the firing drive train  134  to fire.  
         [0106]     In  FIGS. 30-33 , an alternative firing assistance mechanism  500  includes the shuttle  300  that engages the rectangular plate  135  of the firing drive train  134  as previously described but urged with-a compression spring  502  between a raised distal ring  504  formed on the cylindrical portion  310  of the plunger  21  and the plunger hole  312 . In  FIG. 30 , the rectangular plate  135  and firing driving train  134  are retracted with the alternative firing assistance mechanism  500  in a distal, fired position with the compression spring  502  unloaded. In  FIG. 31 , the plunger  21  has been drawn aft, engaging the cam finger  304  and thus the shuttle  300  to the rectangular plate  135  as the compression spring  502  is compressed, storing mechanical potential energy. In  FIG. 32 , the assistance mechanism  500  has been triggered, the firing drive train  134  and shuttle  300  moving in unison as the cam finger  304  begins to contact the kick-out cam step  324 . In  FIG. 32 , with further distal movement, the kick-out cam step  324  has rotated the cam finger  304  counterclockwise as viewed from above out of engagement with the notch  322  in the rectangular plate  135 , which would be followed by retraction of the firing drive train  134 . The alternative firing assistance mechanism  500  may utilize either brake assembly  340 ,  440 .  
         [0107]     In  FIGS. 34-40 , another alternative wind-up firing assistance mechanism  600  incorporated into the surgical stapling and severing instrument  10  includes a wind-up knob  602  that stores mechanical potential energy in a torsion coil spring  604  in a ratchet gear  606  that is in gear engagement to a third spur gear  605  coaxially attached to the right side of the coaxial dual spur gear (multiplier gear)  140  ( FIGS. 35, 36 ). A shaft  608  connects to the wind-up knob  602 , passes through the right handle housing shell  52  and is attached to a circular flange  610  that pivotally holds a downwardly biased ratchet pawl  612  that engages a rotatingly received ratchet gear  614  with teeth angled such that counterclockwise rotation of the ratchet gear  614 , when viewed from the right, is prevented the torsion coil spring  604  has an outer end  616  that is engaged to the ratchet gear  614  and an inner end  618  that is engaged to the shaft  608 . A spur gear  620  is attached for rotation with the ratchet gear  614  and is in gear engagement with the third spur gear  605  added to the coaxial dual spur gear (multiplier gear)  140 . The third spur gear  605  that is coaxially attached to the right side of the dual spur gear  140  that is in gear engagement with the spur gear  620  of the ratchet gear  606  may be sized to provide a desired gear ratio between the assistance mechanism  500  and the firing drive train  134 . The alternative firing assistance mechanism  600  may utilize either brake assembly  340 ,  440 .  
         [0108]     In  FIGS. 41-52 , in addition to or as an alternative of direct assistance in moving a firing drive train  134  in a surgical stapling and severing instrument  10 , firing assistance may be incorporated by disengaging a retraction force on the firing drive train  134  during firing. Thereby, the surgeon is not required to exert as great a force on the firing trigger  28  yet retains the advantages of automatic retraction of the firing drive train  134  after firing. To that end, an assistance mechanism  700  is incorporated into the surgical stapling and severing instrument  10  with a shuttle  702  that is proximate to a left side and aligned for movement with the rectangular plate  135  having a reversed notch  704  near a distal end along a top left surface thereof. Rather than being connected directly to the firing trigger  28 , a retraction tension spring  706  whose fixed end (not shown) is attached in the pistol grip  24  has a moving end  708  that is connected to a cable  710  that turns on a pulley  712  and is attached to a proximal end of the shuttle  702 . The pulley  712  is aligned to the longitudinal travel of the shuttle  702  and is more proximal than a most proximal position of the shuttle  702 .  
         [0109]     A lateral through slot  714  near a distal end of the shuttle  702  receives a horizontally rotating dual engagement cam member  716  that selectively engages the left handle housing shell  50  or the firing drive train  134 . With particular reference to  FIGS. 51-52 , the dual engagement cam member  716  has a leftward canted distal end  718  that is distal to a vertical pivot through hole  720  and left and right proximal fingers  722 ,  724  that are proximal to the vertical pivot through hole  720 . A cam axle  726  has a cylindrical upper portion  728  that allows rotation of the cam member  716  with a square lower portion  730  that is fixedly received into the shuttle  702  below the lateral through slot  714 . A torsion coil spring  732  has an inner end  734  attached to the cylindrical upper portion  728 , is wrapped counterclockwise as viewed from above, and has an outer end  736  engaged to the cam member  716  such that the cam member  736  is biased clockwise into contact with the rectangular plate  135  of the firing drive train  134 .  
         [0110]     With particular reference to  FIG. 41 , a bent leaf spring  738  (shown in phantom) has a first end  740  inserted into the closure trigger  26  and a second end  742  inserted into the firing trigger  28  with a curved portion  744  there between biased to separate the two triggers  26 ,  28 . The bent leaf spring  738  compensates to a lesser degree for the disengagement of the retraction tension spring  706  from the firing trigger  28  by keeping the firing trigger  28  from sagging and interfering with use of the closure trigger  26 .  
         [0111]     In  FIGS. 41-42 , the assistance mechanism  700  is preloaded in preparation for use of the surgical stapling and severing instrument  10  by distally sliding a retraction assistance slide grip  746  that rides longitudinally along an actuator horizontal slot  748  ( FIG. 41 ) formed in the left handle housing shell  50 .  
         [0112]     In  FIGS. 43-50 , a small, distal ramped notch  750  and a large, proximal ramped notch  752  formed on an inner surface of the left handle housing shell  50  in combination with the aligned cam member  716  act as an engagement mechanism that selectively engage the shuttle  702  to the left handle housing shell  50 , the rectangular plate  135  of the firing drive train  134 , or to neither. In  FIG. 43 , the rectangular plate  135 , and thus the firing drive train  134 , are in a retracted, unfired position. The shuttle  702  is also retracted in an initial unloaded state. The cam member  716  is positioned to the right of the large, proximal ramped notch  752 . The clockwise bias of the torsion coil spring  732  on the cam member  716  tends to rotate the left proximal finger  722  into the large, proximal ramped notch  752  and the right proximal finger  724  out of the reversed notch  704  in the rectangular plate  135  to the extent allowed by the leftward canted distal end  718  as the shuttle  702  is distally advanced in  FIG. 44 , preloading the retraction tension spring  706  ( FIG. 41 ). In  FIG. 45 , the cam member  716  rotates counterclockwise as the left proximal finger  722  exits the large, proximal ramped notch  752  in preparation for engaging the right handle housing shell  50 , which occurs as the shuttle  702  reaches full distal travel in  FIG. 46 . The shallow nature of the small, distal ramped notch  750  allows sufficient clockwise rotation of the cam member  716  such that the left proximal finger  722  is in the small, distal ramped notch  750 , the right proximal finger  724  is approximately flush with a right edge of the shuttle  702 , and the leftward canted distal end  718  of the cam member  716  extends slightly into the space traversed by the rectangular plate  135  of the firing drive train  134 . In  FIG. 47 , the rectangular plate  135  of the firing drive train  134  is distally advancing as the firing trigger  28  ( FIG. 41 ) is depressed during firing. A distal left corner of the rectangular plate  135  is approaching the leftward canted distal end  718  of the cam member  716  as the reversed notch  704  moves along the right side of the right proximal finger  724  of the cam member  716 . In  FIG. 48 , the rectangular plate  135 , contacting the leftward canted distal end  718  of the cam member  716 , progressively rotates the cam member  716  counterclockwise, thus drawing the left cam finger  722  of the cam member  716  out of engagement with the small, distal ramped notch  750  in the left handle housing shell  50  and positions the right proximal finger  724  into the reversed notch  704  of the rectangular plate  135 . In  FIG. 49 , further distal advancement to nearly full firing travel of the rectangular plate  135  has disengaged the cam member  716  from the right handle housing shell  50  and engaged the cam member  716  to the rectangular plate  135  of the firing drive train  134 , thus coupling the extended retraction tension spring  706  ( FIG. 41 ) to create a retraction bias. In  FIG. 50 , as the firing trigger  28  ( FIG. 41 ) is released, the retraction bias retracts the shuttle  702  and thus the rectangular plate  135  (via the engaged cam member  716 ) back toward the initial state of  FIG. 43 .  
         [0113]     While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications may readily appear to those skilled in the art.  
         [0114]     For example, an assistance mechanism consistent with aspects of the invention may be coupled in other ways to the firing rod rather than interposing a multiplier gear between a primary rack that is attached to the firing rod and a small secondary rack.  
         [0115]     As another example, a retraction assistance mechanism, consistent with aspects of the invention, may store biasing energy in a compression spring or torsion coil spring or other resilient member (e.g., charged gas cylinder) rather than a tension spring, similar to the versions that biased the firing assistance mechanism.  
         [0116]     As yet another example, while a surgical stapling and severing instrument benefits from pre-loaded assistance in firing and/or retraction, other surgical instruments may benefit from aspects consistent with the present invention, to include but are not limited to a linear stapler, a circular stapler, an anastomosis instrument, etc.  
         [0117]     As yet an additional example, a surgical stapling and severing instrument may benefit from incorporating a dual firing and retraction assistance mechanism that includes preloading for both assistance during the firing stroke and during the retraction of a firing drive train.  
         [0118]     As yet a further example, while a single firing stroke has been described for clarity, a multiple firing stroke surgical instrument, such as described in the above-referenced U.S. patent application Ser. No. 11/052,632, may benefit from aspects consistent with the present invention.