An internal mandibular distractor comprising: a housing elongated in a longitudinal direction; at least one distal attachment plate attached to the housing, wherein the distal attachment plate is configured to be attached to a first bone surface; a drive rod arranged inside the housing and elongated in the longitudinal direction; at least one proximal attachment plate that is configured to be driven by the drive rod to move in the longitudinal direction, the at least one proximal attachment plate configured to be attached to a second bone surface, wherein the drive rod comprises a proximal end portion that is configured to be rotated to drive the at least one proximal attachment plate in the longitudinal direction; at least one housing engaging portion provided to the housing; and at least one drive rod engaging portion provided to the drive rod; characterized in that the at least one distal attachment plate is not movable with respect to the housing and the at least one housing engaging portion and the at least one drive rod engaging portion are configured to: be engaged in a first relative position of the housing and the drive rod so as to prevent relative rotation in both a first and a second direction of rotation between the drive rod and the housing caused by a force acting on the drive rod or the housing that is less than a predetermined force, and be moved out of engagement by driving the drive rod in one of a first and a second direction of rotation of the drive rod with a force equal to or greater than the predetermined force to a second relative position. a housing elongated in a longitudinal direction; at least one distal attachment plate attached to the housing, wherein the distal attachment plate is configured to be attached to a first bone surface; a drive rod arranged inside the housing and elongated in the longitudinal direction; at least one proximal attachment plate that is configured to be driven by the drive rod to move in the longitudinal direction, the at least one proximal attachment plate configured to be attached to a second bone surface, wherein the drive rod comprises a proximal end portion that is configured to be rotated to drive the at least one proximal attachment plate in the longitudinal direction; at least one housing engaging portion provided to the housing; and at least one drive rod engaging portion provided to the drive rod; characterized in that the at least one distal attachment plate is not movable with respect to the housing and the at least one housing engaging portion and the at least one drive rod engaging portion are configured to: be engaged in a first relative position of the housing and the drive rod so as to prevent relative rotation in both a first and a second direction of rotation between the drive rod and the housing caused by a force acting on the drive rod or the housing that is less than a predetermined force, and be moved out of engagement by driving the drive rod in one of a first and a second direction of rotation of the drive rod with a force equal to or greater than the predetermined force to a second relative position. be engaged in a first relative position of the housing and the drive rod so as to prevent relative rotation in both a first and a second direction of rotation between the drive rod and the housing caused by a force acting on the drive rod or the housing that is less than a predetermined force, and be moved out of engagement by driving the drive rod in one of a first and a second direction of rotation of the drive rod with a force equal to or greater than the predetermined force to a second relative position.