A fracture in a hollow bone is supported temporarily by a rigid plastics implant formed within a balloon within the bone. To assist implant removal, an access port, comprising a hollow cylinder, is inserted through an aperture made in an end of the bone and surrounds a proximal end of the implant. The access port is connected to a source of torsional-mode ultrasonic vibrations to soften the plastic implant and improve engagement between access port and implant. Removal of the ultrasound source allows introduction of a cutting tool through the access port, which guides it to hollow out the implant. The implant then collapses within the balloon and is withdrawn through the access port. One suitable cutting tool has a head with four helical fins separated by part-circular grooves and a concave distal face. Ultrasonically activated, the fin tips soften and cut the plastic, which flows away along the grooves.