A surgical instrument that may include a housing, a transducer engaged with the housing which can produce vibrations, and an end-effector engaged with the transducer. The surgical instrument can include an adjustable sheath extending from the housing where the sheath is movable relative to the distal tip of the end-effector and where the distance between the distal tip of the sheath and the distal tip of the end-effector can enable the sheath to act as a depth stop. The sheath can be adjusted such that, when the distal tip of the sheath contacts the tissue or bone being incised, the surgeon can determine that the appropriate depth of the incision has been reached. In other embodiments, the end-effector can be moved with respect to the sheath to adjust the distance between the distal tip of the end-effector and the distal tip of the sheath.