A device and method for performing interspinous fixation or spinal fusion is provided. The device may generally include a first plate and a second plate, each having an adjustable length, and a mechanical actuator for increasing or reducing the length of the plates and, correspondingly, the device. Operation of the mechanical actuator causes first and second inner expansion portions of each plate to simultaneously slide on an outer fixed portion of the plate, to increase or decrease (depending on the direction of operation of the mechanical actuator) the separation between the first and second inner expansion portions, and thus, lengthen or shorten the plates and device.