Surgical fastener applying apparatus including fluid-activated firing mechanism

1. A surgical fastener applying apparatus, comprising:

an end effector assembly including an anvil assembly and a cartridge assembly, at least one of the anvil assembly and the cartridge assembly movable relative to the other between an open position and a closed position for grasping tissue therebetween, the cartridge assembly configured to retain a plurality of surgical fasteners therein;

a fluid-actuated drive assembly including a distal piston member configured to oscillate between a proximal position and a distal position, the distal piston member including a base and a rack extending distally from the base, the rack slidably disposed within the cartridge assembly;

a drive beam configured for translation through the cartridge assembly to eject the plurality of surgical fasteners from the cartridge assembly; and

a pawl assembly coupling the drive beam to the rack such that the drive beam is incrementally advanced along the rack and through the cartridge assembly as the distal piston member oscillates between the proximal position and the distal position.

2. The surgical fastener applying apparatus according to claim 1 , wherein the fluid-actuated drive assembly includes a distal piston assembly including a piston housing, the distal piston member disposed within the piston housing, wherein fluid pressure within the piston housing urges the distal piston member to move from the proximal position to the distal position.

3. The surgical fastener applying apparatus according to claim 2 , wherein the distal piston member is biased towards the proximal position such that, in the absence of fluid pressure within the piston housing, the distal piston member is returned from the distal position to the proximal position.

4. The surgical fastener applying apparatus according to claim 1 , wherein the drive assembly further includes a proximal piston assembly and an elongated tube coupling the proximal piston assembly and the distal piston member, the elongated tube substantially filled with a fluid such that oscillation of the proximal piston assembly between first and second positions effects oscillation of distal piston member between the proximal and distal positions.

5. The surgical fastener applying apparatus according to claim 4 , further comprising a motor assembly coupled to the proximal piston assembly and configured to drive the proximal piston assembly to oscillate between the first and second positions.

6. The surgical fastener applying apparatus according to claim 5 , wherein the motor assembly includes an axle and a drive member eccentrically mounted about the axle, the motor configured to rotate the axle such that the drive member is rotated by the axle to drive the proximal piston assembly to oscillate between the first and second positions.

7. The surgical fastener applying apparatus according to claim 1 , wherein the drive beam is further configured to engage the anvil assembly and the cartridge assembly upon distal translation of the drive beam to move the end effector assembly from the open position to the closed position.

8. The surgical fastener applying apparatus according to claim 1 , wherein the pawl assembly is configured to engage the drive beam to the rack upon movement of the distal piston member from the proximal position to the distal position and wherein the pawl assembly is configured to disengage the drive beam from the rack upon movement of the distal piston member from the distal position back to the proximal position.

9. The surgical fastener applying apparatus according to claim 8 , wherein the pawl assembly includes a pawl pivotably coupled to the drive beam, the pawl including a free end configured to engage a tooth of the rack upon movement of the distal piston member from the proximal position to the distal position and configured to slid about teeth of the rack upon movement of the distal piston member from the distal position back to the proximal position.

10. The surgical fastener applying apparatus according to claim 1 , further comprising a retraction assembly coupled to the drive beam and the pawl assembly, the retraction assembly selectively actuatable for translating the drive beam proximally through the cartridge assembly.

11. The surgical fastener applying apparatus according to claim 10 , wherein the retraction assembly include a retraction cord coupled to the drive beam and the pawl assembly at a first end thereof and having a retraction knob disposed at a second end thereof.