Abstract: Expandable intervertebral spacer system having a top plate and a bottom plate which are separated and held apart a changeable distance forming a cage. The plates are held apart by four locking mechanisms in the cage at or near the corners or the periphery of the plates, leaving the cavity of the cage substantially unobstructed. Each locking mechanism is made of two saw-toothed posts that cooperate to lock the top plate a desired distance from the bottom plate. The saw-tooth posts are biased against each other with a spring. Once the spacer is implanted in a patient with a removable insertion tool, the top and bottom plates are forced apart incrementally, one saw tooth at a time, in a ratchet-like motion, using a removable expansion mechanism. Optionally, each stanchion is surrounded by a sheath to prevent bone particles and other debris from interfering with the mating of the saw teeth.