Abstract: A surgical instrument comprising a housing with a motor and source of power contained in the housing. A removable sheath is placed over the housing and a cutter assembly is removably mounted to the housing so that the housing is protected from external materials and contamination. The cutter assembly has a body, a tube projecting from the body, a blade positioned in the tube and a drive transfer mechanism mounted in the cutter body, connecting the blade to the motor. The tube defines an aperture and the blade in combination with the walls defining the aperture shears tissue entering the hole. A second passage is formed in the cutter body communicating with the tube to allow pressure differentials to be exerted to the tube.