Abstract: An apparatus for stabilizing a plurality of bone segments, such as vertebrae of a spinal column, comprises one or more fixation devices. Each fixation device comprises an elongated rod having a longitudinal slot through its upper and lower surfaces and having threaded lateral surfaces. Slotted or axially threaded block members are slideably or rotatably movable along each rod into predetermined positions corresponding to locations selected for securing the apparatus to the bone. Posts are provided having have threaded distal ends, such that the distal end of each such post is inserted into the bone. The proximal end of each post is passed through the longitudinal slot in the rod and also through a vertical bore in the block member. The locations at which corrective forces are applied by the posts may be changed relative to the rods in three planes.