STRAIN MAGNITUDE RELATED CHANGES IN WHOLE BONE ARCHITECTURE IN GROWING RATS

Short daily periods of controlled dynamic loading mere applied in vivo through the flexed carpus and olecranon to-the intact ulna of 240 g m ale Sprague-Dawley rats, This technique involved neither surgical prep aration, nor direct loading of the periosteum at a site close to the r egion of the bone in which adaptive modeling was subsequently assessed , The animals used their limbs normally between loading episodes, thus approximating to the natural situation, in which short periods of exe rcise are generally superimposed on longer periods of less strenuous a ctivity, The strain patterns associated with normal activities were es tablished for the rat ulna from strain gauges implanted in vivo, Typic al peak strain magnitudes during unrestricted locomotion varied betwee n -0.0007 and -0.0012, with peak strain rates between 0.023 and -0.038 (-1), Stride frequency was 1.5-4.2 Hz. The adaptive response to a sing le 10 min period of loading each day, causing peak dynamic strains of -0.002 (1200 cycles at 2 Hz, and a loading/unloading rate of +0.03 sec (-1)), involved modification of the normal growth related medial to la teral modeling drift, simultaneously reducing the rate of lateral peri osteal bone deposition and medial bone resorption, This change to the normal modeling pattern reduced the total amount of new bone formation as well as the midshaft curvature of the ulna, At higher peak strain amplitudes (-0.004), adaptive straightening was accompanied by an incr ease in bone mass, achieved by an increase in the mineral apposition r ate on the previously forming lateral face, and arrest of resorption o n the medial ulna surface, with reversal to formation, These experimen ts show that the growing rat ulna underwent adaptive changes in both b one mass and architecture when short daily periods of axial loading, p roducing strains within the physiological range and with near normal s train distribution, mere superimposed on the loading associated with n ormal activity, At moderate peak strain magnitude (-0.002), modificati on of drift produced a straighter bone, associated with a reduced peri osteal bone formation, At higher strain magnitude (-0.004), adaptive m odeling produced a straighter bone associated with increased periostea l bone formation. (C) 1997 by Elsevier Science Inc.