A surgical instrument (20) adapted for stapling and cutting tissue in a highly controlled manner, comprising:a frame having a proximal end and a distal end, with a handle (21) positioned at the proximal end and an end effector (80) positioned at the distal endthe end effector includes a U-shaped supporting structure (81) shaped and dimensioned for selectively receiving a cartridge module (120) containing a plurality of surgical fasteners and a knife (126)a firing mechanism associated with the end effector and the cartridge module for selective actuation of the surgical fasteners and knife for treatment of an individualthe cartridge module includes a cartridge housing (121) in which the surgical fasteners and knife are housed and an anvil (122) shaped and dimensioned for engagement with the surgical fasteners and knife to facilitate cutting and stapling, the cartridge housing and anvil being relatively movable between a first spaced apart position and a second position in close approximation with one another anda retainer (160) selectively positioned between the anvil and the cartridge housing to prevent undesired movement of the cartridge housing toward the anvil.
