Bone Microarchitecture and Stiffness in Premenopausal Women with Idiopathic Osteoporosis

Context: Idiopathic osteoporosis (IOP) is an uncommon disorder in which low areal bone mineral density (aBMD) and/or fractures occur in otherwise healthy premenopausal women. Objectives: Our objectives were to characterize bone mass, microarchitecture, and trabecular bone stiffness in premenopausal IOP and to determine whether women with low aBMD who have never fractured have abnormal microarchitecture and stiffness. Design, Setting, and Patients: We conducted a prospective cohort study of 27 normal controls and 31 women with IOP defined by low trauma fracture (n = 21) or low BMD (Z score <==-2.0; n = 10). Main Outcome Measures: We assessed aBMD by dual-energy x-ray absorptiometry; volumetric BMD and cortical and trabecular microarchitecture of the radius and tibia by high-resolution (82 mu m) peripheral quantitative computed tomography; and trabecular bone stiffness (elastic moduli), estimated by micro-finite element analysis. Results: Fracture subjects did not differ from controls by age or body mass index, which was lower in low-BMD subjects than controls. Fracture subjects also had lower aBMD than controls at all sites (P < 0.05-0.0001). Bone size was similar in controls and fracture subjects but 10.6% smaller in low-BMD subjects (P < 0.05). Every trabecular parameter in both fracture and low-BMD groups was markedly worse than controls (P < 0.01-0.0001). Cortical thickness was significantly lower in both fracture and low-BMD groups at the tibia but not radius. Bone stiffness estimated by micro-finite element analysis was comparably reduced in low-BMD and fracture groups. Conclusion: Premenopausal women with IOP had marked trabecular microarchitectural deterioration at the radius and tibia. Cortical parameters were affected only at the tibia. Although they had not fractured, microarchitectural deterioration was similar in IOP women with low BMD and those with fractures. (J Clin Endocrinol Metab 94: 4351-4360, 2009)