A medical instrument converts linear movement of a shaft at a proximal end of the instrument to a rotational movement of an end effector of the instrument. According to one embodiment, the medical instrument includes an end effector formed of a snare loop connected to a shaft that is located within a sleeve. Once the sleeve is located near a desired location within a patient, the snare loop may be rotationally positioned with linear movement of the shaft by the operator. The shaft is formed with a threaded portion that matches threads formed in a torque transmitter element fixed within the sleeve. Linear movement of the shaft through the torque transmitter element causes the shaft to rotate. The medical instrument may also include a pivot connector for reducing torque build-up of the shaft and/or a clutch assembly for restricting controlled rotation of the snare loop to only one direction.