The present invention is directed to an interspinous spacer assembly for implantation and/or affixation between spinous processes of adjacent superior and inferior vertebrae and associated tools and methods for stabilizing the human spine. The interspinous spacer assembly includes a spacer body, a first lateral plate, a second lateral plate and a locking mechanism extending through a channel provided in the spacer body and coupling the first and second lateral plates to the spacer body.
