An expandable interbody implant is expandable from a contracted configuration to an expanded configuration by moving opposing first and second vertebral-engaging surfaces apart from one another. The implant includes a locking system for restraining contraction of the implant. The locking system may have a locked configuration, in which the first and second surfaces are prevented from moving back towards the contracted configuration, and the locking system may have an unlocked configuration, in which the first and second surfaces are permitted to move back towards the contracted configuration. The locking system may be controlled by rotation of one or more pinions. The pinions may, in turn, be controlled by linear movement of a rack. The rack may be configured so as to bias the locking system towards the locked configuration. The implant may also include a stop for constraining the maximum expansion of the implant.
