A medical drill assembly includes a probe connectable at a proximal end to an electromechanical transducer for generating mechanical vibration of an ultrasonic frequency. The probe has a shaft with a central lumen or channel, the shaft being formed at a distal end with a head having a tapered distal side. A source of pressurized liquid communicates with the lumen or channel of the probe, and a translatory or linear drive is operatively connected to the probe for applying a distally directed force to the probe. A controller is operatively connected to the translatory or linear drive for periodically at least reducing the magnitude of the distally directed force.
