A two-part tissue thickness compensator assembly can include a first tissue thickness compensator configured to be positioned relative to an anvil of a surgical stapler, a second tissue thickness compensator configured to be positioned relative to a staple cartridge of the surgical stapler, and a hinge connecting the first tissue thickness compensator to the second tissue thickness compensator. The first and/or second tissue thickness compensators may include additional engagement features, such as a raised ridge that engages a slot in the anvil and/or the staple cartridge. In certain embodiments, the first and/or second tissue thickness compensators may include an encasement that contains a suitable biologic agent. An end effector assembly may be provided for attachment to a surgical instrument that includes, for example, a staple cartridge, an anvil, a first tissue thickness compensator positioned on the anvil, and a second tissue thickness compensator positioned on the staple cartridge.