The present invention relates in certain embodiments to medical devices for treating osteoplasty procedures such as vertebral compression fractures. More particularly, embodiments of the invention relate to instruments and methods for controllably restoring vertebral body height by controlling the geometry of fill material introduced into cancellous bone. An exemplary system utilizes Rf energy in combination a conductive bone fill material for polymerizing the surface of the inflow plume to control the geometry of the fill material and the application of force caused by inflows of fill material. In another embodiment, method of treating bone includes injecting a volume of fill material into a bone and selectively modifying a viscosity of a selected portion of the bone filler to control the direction of flow of the fill material within the bone. A system for treating bone using this method includes an introducer for delivering fill material into the bone and an energy source selectively coupleable to the fill material to alter the viscosity of the fill material as it flows out of the introducer.