In studies on a functionally isolated avian-bone preparation to which external loads could be applied in vivo, we determined the following information.
1. Removal of load-bearing resulted in substantial remodeling endosteally, intracortically, and, to a lesser extent, periosteally. Since the balance of this remodeling was negative, bone mass declined. It therefore appears that functional load-bearing prevents a remodeling process that would otherwise lead to disuse osteoporosis.
2. Four consecutive cycles a day of an externally applied loading regimen that engendered physiological strain magnitudes but an altered strain distribution prevented remodeling and was thus associated with no change in bone mass. A small exposure to, or the first effect of, a suitable dynamic strain regimen appears to be sufficient to prevent the negatively balanced remodel ing that is responsible for disuse osteoporosis.
3. Thirty-six 0.5-hertz cycles per day of the same load regimen also prevented intracortical resorption but was associated with substantial periosteal and endosteal new-bone formation, Over a six-week period, bone-mineral content increased to between 133 and 143 per cent of the original value. Physiological levels of strain imposed with an abnormal strain distribution can produce an osteogenic stimulus that is capable of increasing bone mass. Neither the size nor the character of the bone changes that we observed were affected by any additional increase in the number of load cycles from thirty-six to 1800.
CLINICAL RELEVANCE: The results of this experiment must be considered in relation to the type and duration of the non-physiological loads that were imposed. The sensitivity of bone-remodeling in this model to prevailing mechanical circumstances is evident. Functional levels of bone mass in patients may only be maintained under the effects of continued load-bearing. The osteogenic effect of an unusual strain distribution suggests that a diverse exercise regimen may engender a greater hypertrophic response than an exercise program that is restricted. A substantial osteogenic response may be achieved after remarkably few cycles of loading.