A hand prosthesis (15) including a hand chassis (15), a thumb member (22) mounted on the hand chassis for rotation of the thumb member in relation to the hand chassis about an axis extending generally along the length of the thumb member, a motor (64) disposed on one of the hand chassis and the thumb member, the motor being operable to drive a worm (56) and a worm gear wheel (54) disposed on the other of the hand chassis and the thumb member, the worm being in engagement with the worm gear wheel such that, upon operation of the motor, the thumb member rotates in relation to the hand chassis.
