A method and apparatus for producing a change in the spacing between a pair of selected, spaced skeletal bones in a patient utilizing a single-surgery, elongate selectively adjustable, length-change bone-spacing device The device includes (a) an elongate, pre-stressed, length-change, spring-force component having a relaxed condition toward which it is self-biased, and in which relaxed condition it is either one of longer or shorter than when it is in its pre-stressed condition, (b) a pair of spaced, relatively moveable bone drivers coupleable to selected bones, and drivingly connected, one each, to opposite ends of the spring-force component, and (c) a two-state, RF-activated, stress-control constraining medium embedding at least a portion of the length of the spring-force component, and having a hardened state wherein it immobilizes that length portion against any length change, and a softened, fluid state to which it converts reversibly under the influence of received radio-frequency (RF) energy of a defined character which is specific to the constraining medium, and in which softened state, the constraining medium permits length change of the spring-force component's embedded length portion.