Low Bone Mass May Not Be the Only Cause of Skeletal Fragility in Osteoporosis

Skeletal fragility in postmenopausal osteoporosis is not due solely to reduction in bone mass. This fact explains some of the overlap in bone mineral measurements observed between patients who are fracturing and age- and sex-matched normals who are not. Changes in skeletal architecture and bone remodeling occur with age which can account for some of the fragility. These changes are exaggerated in patients with postmenopausal osteoporosis who are suffering spine fractures. Three abnormalities have been described by histomorphometric methods which can account for skeletal fragility out of proportion to the degree of bone loss. They are: (i) loss of trabecular connectivity such that vertical weight-bearing bars lose their cross-attachments with each other, thus becoming susceptible to buckling; (ii) inefficient and prolonged microdamage repair due to periods of pause in the formation phase of remodeling; and (iii) accumulation of unrepaired microdamage in unremodeled bone tissue in the central part of trabeculae due to reduced osteon wall thickness coupled with maintenance of trabecular thickness. Recognition of these abnormalities should broaden our approach to the study of skeletal fragility in the syndrome of postmenopausal osteoporosis.