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 .