A method for stabilizing at least one spinal motion segment includes an expandable spacer in a collapsed state into the interspinous space between superior and inferior vertebrae such that the spacer extends through the sagittal plane defined by the spinous processes of the superior and inferior vertebrae. The spacer has a flexible membrane defining an interior cavity. The method includes expanding the spacer to an expanded state by forcing a fill material in a fluid state into the cavity with the spacer disposed in the interspinous space, and thereafter solidifying the fill material to a solid state in the cavity. The method may be performed through a percutaneous penetration in the patient's skin and may cause the first and second vertebrae to be distracted.