A vertebral spacer includes an expandable member having a self sealing percutaneous fluid access port and engaged to an anchor member that is secured the vertebrae. The anchor member may be an arcuate channel mechanically secured to adjacent spinous processes by bone screws through slots in the legs of the channel. The port allows for adding or removing fluid from the expandable member in order to adjust the volume thereof and thus the interspinous process spacing. Subsequent to initial expansion, the volume of the expandable member may be increased or decreased by using a needle to add or remove fluid from the expandable member via the port. The fluid may be a liquid, gel, or a viscous polymer and may remain in a liquid state or harden to a viscoelastic state.