From mechanostat theory to development of the "Functional Muscle-Bone-Unit"

Bone densitometric data are often difficult to interpret in children and adolescents because of large inter- and intraindividual variations in bone size. Here, we propose a functional approach to bone densitometry that addresses two questions: is bone strength normally adapted to the largest physiological loads, that is, muscle force? Is muscle force adequate for body size? The theoretical background for this approach is provided by the mechanostat theory, which proposes that bones adapt their strength to keep the strain caused by physiological loads close to a set point. Because the largest physiological loads are caused by muscle contractions, there should be a close relationship between bone strength and muscle force or size. The proposed two-step diagnostic algorithm requires a measure of muscle force or size and a measure of bone mineral content (BMC) at a corresponding location. The results can be combined into four diagnostic groups. In the first situation, muscle force or size is adequate for height. If the skeleton is adapted normally to the muscle system, the result is interpreted as "normal". If it is lower than expected for muscle force or size, a "primary bone defect" is diagnosed. In the second situation, muscle force or size is too low for height. Even if the skeleton is adapted adequately to the decreased mechanical challenge, this means that bone mass and presumably strength are still too low for body height. Therefore, a "secondary bone defect" is diagnosed. It is hoped that the more detailed insights thus gained could help to devise targeted strategies for the prevention and treatment of pediatric bone diseases.