An adjustable spinal implant (10) is provided for use in connecting elongate members (12) as well as vertebral spacers such as corpectomy devices, intervertebral fusion devices, and other prostheses. The implant (10) may have fittings (80) on either end comprising fixed (100) or articulating (200) jaws, endplates, or other engagement structures. The implant (10) comprises a housing (40) with an internal rotor (60); an extending shaft (20); and a locking collar (70). The extending shaft (20) has an external helical groove (23) that meshes with an internal helical groove (63) on rotor (20). Length adjustment occurs by transforming axial movement of the extending shaft (20) into a rotary movement of the rotor (60) via helical engagement. The locking collar (70) comprises protrusion (73) engaging grooves (63) of rotor (60), thus providing a simple, positive locking mechanism without requiring the surgeon to apply excessive force to lock the length.