A medical instrument assembly comprises a shaft, a tool carried by the distal end of the shaft for performing a medical procedure on a patient, an end effector, a threaded housing in which the threaded distal shaft end is configured for being screwed, and a first threaded piece disposed in the threaded housing and configured for being moved to actuate the end effector. The assembly further comprises an actuation element extending within the shaft. The actuation element includes a second threaded piece that distally extends from the threaded distal shaft end. The second threaded piece is configured for being screwed to the first threaded piece. A robotic system comprises the assembly, a user interface configured for generating command(s), a drive unit coupled to the actuating element, and an electric controller configured, in response to the command(s), for directing the drive unit to move the actuating element to actuate the tool.