A device and method for fixation of bone fractures has a bone screw comprising a shank with a threaded end portion, on the outer surface. The screw has a through bore with two bore portions differing in diameter. A step in the diameter is formed between these bore portions and is located within the end of the screw having the thread. This step in diameter can support a metal insert which in turn supports a polymer pin when the latter if pressurized with a sonotrode in the bone screw. Together with an applied ultrasonic vibration the pressure fluidizes the polymer pin and presses the material through holes configured in the wall of the bone screw and into surrounding bone.