A loading unit (16) for use with a surgical stapling apparatus (10) is provided and includes: a tool assembly (17) having a cartridge assembly (18) and an anvil assembly (20) that are movable in relation to one another a surgical buttress (B1) releasably secured to a tissue contacting surface of the anvil assembly and/or the cartridge assembly, wherein the surgical buttress secured to the anvil assembly is secured by at least one anchor (S) and a drive assembly (50) slidably translatable through the tool assembly between proximal and distal positions. The anvil assembly has pairs of side recesses (570d,570e), at least one of which is a keyhole-shaped, or has an enlarged head portion (570d3,570e3).
