A loading unit for use with a surgical stapling apparatus is provided and includes: a tool assembly having a cartridge assembly and an anvil assembly that are movable in relation to one another； a surgical buttress 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； and a drive assembly slidably translatable through the tool assembly between proximal and distal positions. The anvil assembly has pairs of side recesses, at least one of which is a keyhole-shaped, or has an enlarged head portion.
