Apparatus for aligning misaligned vertebrae including a partible screw which can be threadedly coupled to misaligned vertebrae and decoupled from the vertebrae after being aligned. The screw, which is rotatable about an axis, includes axially shiftable screw halves having complementally formed partial screw thread sections which can be aligned in any one of a plurality of axially spaced apart positions to define a helical screw thread. Apparatus is provided for detachably holding the screw halves together in any one of a plurality of axially spaced apart positions with the partial screw threads on each of the halves in alignment including an alignment collar and a locking collar which can be detachably threadedly received on, and locked to, the screw thread sections in locking positions when the screw is being threadedly coupled to and uncoupled from the vertebrae. The alignment collar and locking collar can be individually unthreaded and moved to a remote position removed from the screw threads to allow the screw halves to be axially relatively shifted relative to each other.