Modulation of appositional and longitudinal bone growth in the rat ulna byapplied static and dynamic force

Appositional and longitudinal growth of long bones are influenced by mechan ical stimuli. Using the noninvasive rat ulna loading model, we tested the h ypothesis that brief-duration (10 min/day) static loads have an inhibitory effect on appositional bone formation in the middiaphysis of growing rat ul nae. Several reports have shown that ulnar loading, when applied to growing rats, results in suppressed longitudinal growth. We tested a second hypoth esis that load-induced longitudinal growth suppression in the growing rat u lna is proportional to time-averaged load, and that growth plate dimensions and chondrocyte populations are reduced in the loaded limbs. Growing male rats were divided into one of three groups receiving daily 10 min bouts of static loading at 17 N, static loading at 8.5 N, or dynamic loading at P N. Periosteal bone formation rates, measured 3 mm distal to the ulnar midshaf t, were suppressed significantly (by 28-41%) by the brief static loading se ssions despite normal (dynamic) limb use between the daily loading bouts. S tatic loading neither suppressed nor enhanced endocortical bone formation. Dynamic loading increased osteogenesis significantly on both surfaces. At t he end of the 2 week loading experiment, loaded ulnae were approximately 4% shorter than the contralateral controls in the 17 N static and dynamic gro ups, and approximately 2% shorter than the control side in the 8.5 N static group, suggesting that growth suppression was proportional to peak load ma gnitude, regardless of whether the load was static or dynamic. The suppress ed growth in loaded limbs was associated with thicker distal growth plates, particularly in the hypertrophic zone, and a concurrent retention of hyper trophic cell lacunae. Negligible effects, were observed in the proximal gro wth plate. The results demonstrate that, in growing animals, even short per iods of static loading can significantly suppress appositional growth; that dynamic loads trigger the adaptive response in bone; and that longitudinal growth suppression resulting from compressive end-loads is proportional to load magnitude and not average load. (C) 2001 by Elsevier Science Inc. All rights reserved.