Determinants of bone loss during immobilization

Abstract

Bone losses occur in clinical cases of immobilization, such as stroke and spinal cord injury. In those cases, the rate of bone loss is initially high and levels off after 2-5 years. Experimental bed rest, which is often used as a model of deconditioning effects, is also associated with bone losses from the legs, but not from the arms. Similar effects can be elicited by unilateral lower limb suspension, which disfavours the hypothesis that increased venous pressure can prevent immobilization-induced bone loss. Adequate resistive exercise, conversely, has the potential to act as a countermeasure against bone losses. Data suggest that resistive exercise may be more effective when combined with whole body vibration. There seems to be a wide variation in the time course of bone losses from immobilized limbs, but probably not in the amount of bone losses. The question arises as to what determines this biological variation. Recent data show that bone structure can partly explain different rates of bone loss. Moreover, there is a possibility that the kidney’s ability of calcium excretion is a limiting factor and might hamper immobilization-induced bone losses.