AGING CHANGES MECHANICAL LOADING THRESHOLDS FOR BONE-FORMATION IN RATS

The effect of aging on the mechanical loading thresholds for osteogene sis was investigated in rats. We applied mechanical loads varying from 30 to 63 N to the tibiae of 43 19-month-old rats using a four-point b ending apparatus, Bone formation rates were measured on the periosteal and endocortical surfaces of the tibial midshaft using double-label h istomorphometry, Bone formation rates from the old rats were compared with results from adult (9-month-old) rats that we reported earlier.(( 4)) Bone formation on the periosteal surface of the old rats was predo minantly woven-fibered. Periosteal bone formation was observed in a lo wer percentage of the old rats compared with the younger adult rats fo r applied loads of 40 N and greater (59% old, 100% adult), However, in the old rats that formed woven bone there were no significant differe nces in woven bone area (p = 0.1) or surface (p = 0.24) compared with younger adult rats. Therefore, the periosteum of old rats had a higher threshold for activation by mechanical loading, but after activation occurred, the cells had the same capacity to form woven bone as younge r adult animals, On the endocortical surface, relative bone formation rates in old rats showed a marginal (p = 0.06) increase in response to an applied load of 64 N but was not increased at lower loads. The rel ative bone formation rate in the old rats was over 16-fold less than t hat reported for the younger adult rats at an applied load of 64 N and the relative bone forming surface in old rats in this study was 5-fol d less than it was in younger rats under similar loading conditions, I n the younger adult rats, a mechanical threshold for Lamellar bone for mation of 1050 mu strain was calculated for the endocortical bone surf ace, The old rats required over 1700 mu strain on the endocortical sur face before bone formation was increased, The data suggest that both t he periosteal and endocortical surfaces of the tibiae of older rats ar e less responsive to mechanical stimuli.