Does Bone Mass Equate with Bone Health? An Argument for the Negative

Abstract This article argues that diagnosing osteopenias and osteoporoses by bone mass Z-scores, and viewing disorders detected in that way as diseases, fostered a belief that those scores evaluate whole-bone strength and bone health equally well (here, mass has its meaning in absorptiometry, not in physics). However, load-bearing bones normally adapt their strength to the past and present voluntary loads on them in ways that make strong muscles make strong bones, and make persistently weak muscles make weak bones. The resulting adaptations also make bones strong enough to keep voluntary mechanical usage from fracturing them. Such observations suggest a new criterion for a bone's health that would depend on the relationship among its strength, the sizes of the voluntary loads it carries, and any spontaneous fractures caused by those loads. Such a relationship could indicate a bone's health independently of its mass. For example, mouse and horse femurs differ in strength more than 1000-fold, but they could be equally healthy if they were to satisfy that criterion in the animals they were born from. That idea would distinguish abnormal in a strictly statistical sense (which T- and Z-scores would evaluate) from unhealthy in the sense of both abnormal and diseased (which the above relationship would evaluate). Thus, a bone with a negative Z-score could still be healthy if voluntary loads do not fracture it. These ideas and some of their implications question some long-held views. This article presents the ideas, evidence they stand on, and some of their implications.