An expandable intervertebral spacer system having a cage formed by a top plate and a bottom plate which are held apart a changeable distance. An expansion mechanism within the cavity is configured to force the top plate apart from the bottom plate a desired distance. One expansion mechanism is an expansion spring that is held in a compressed state with a removable trigger wire that temporarily secures the top and bottom plates together. Once the spacer is implanted in a patient, the trigger wire is removed permitting the expansion spring to relax, in a single motion, to the farthest extent possible in an intervertebral space. The top plate and the bottom plate are locked apart using a locking mechanism 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.