A telescopically adjustable surgical instrument for a distraction apparatus used in performing an elongation osteotomy comprises a rigid support rail having a guide chamber or guide recess and a rigid extension rail axially slidable in said recess. The extension rail has hinged bone connecting plates by means of which the extension rail may be attached to the distal part of a bone. An extension drive mechanism may be releasably connected to the two rails. The instrument is completely implantable. After the distraction, the position of the extension rail is fixed and the drive mechanism may be easily removed.