A bone resector tool (100) comprises an ultrasonic transducer (8), typically generating longitudinal mode vibrations at around 40 kHz, and having an elongate blade portion (2) mounted thereto. The transducer (8) and blade portion (2) are mounted to a rotatably- drivable converter element (3). Rotation of the converter element (3) produces reciprocal longitudinal motion of the transducer (8) and blade portion (2). A counterweight (5B) is also mounted to the converter element (3), moving exactly out of phase with the transducer (8) and blade portion (2), such that a centre of mass of the whole system is stationary, reducing vibration of the tool (100) in a user"s hand. The peak velocity due to the ultrasonic vibrations of a distal tip (6A) of the blade portion (2) is up to seven times greater than its peak velocity due to the reciprocal longitudinal motion. This permits rapid, low-effort cutting of bone with easy removal of cut debris and minimal consequent necrosis.
