A staple cartridge is disclosed. The staple cartridge can comprise a cartridge body comprising a deck and a bottom surface opposite the deck. The staple cartridge can comprise a plurality of staple cavities, wherein each staple cavity extends into the cartridge body from the deck to the bottom surface. Additionally, a plurality of wells can be defined into the staple cartridge from the deck to a lowermost surface of the well. A plurality of staples can be removably positioned in the staple cavities. The staple cartridge can comprise a tissue thickness compensator releasably secured to the cartridge body, wherein the tissue thickness compensator comprises a compensator body and a plurality of extensions extending from the compensator body into the wells, wherein at least one extension is compressed within one of the wells. Each well can surround at least one staple cavity and/or can extend between at least two staple cavities.