BONE MINERAL DENSITY AT DISTAL TIBIA USING DUAL-ENERGY X-RAY ABSORPTIOMETRY IN NORMAL WOMEN AND IN PATIENTS WITH VERTEBRAL OSTEOPOROSIS OR PRIMARY HYPERPARATHYROIDISM

To assess the effect of age and disease on mineral distribution at the distal third of the tibia, bone mineral content (BMC) and bone minera l density (BMD) were measured at lumbar spine (spine), femoral neck (n eck), and diaphysis (Dial and distal epiphysis (Epi) of the tibia in 8 9 healthy control women of different age groups (20-29, n = 12; 30-39, n = 11; 40-44, n = 12; 45-49, n = 12; 50-54, n = 12; 55-59, n = 10; 6 0-69, n = 11; 70-79, n = 9), in 25 women with untreated vertebral oste oporosis (VOP), and in 19 women with primary hyperparathyroidism (PHPT ) using dual-energy x-ray absorptiometry (DXA; Hologic QDR 1000 and st andard spine software). A soft tissue simulator was used to compensate for heterogeneity of soft tissue thickness around the leg. Tibia was scanned over a length of 130 mm from the ankle joint, fibula being exc luded from analysis. For BMC and BMD, 10 sections 13 mm each were anal yzed separately and then pooled to define the epiphysis (Epi 13-52 mm) and diaphysis area (Dia 91-130 mm). Precision after repositioning was 1.9 and 2.1% for Epi and Dia, respectively. In the control group, at any site there was no significant difference between age groups 20-29 and 30-39, which thus were pooled to define the peak bone mass (PBM). Mean decrease in BMD from PBM (mean +/- SEM) was significant after age 49 at Epi (-11.8 +/- 12.7%), after age 54 at spine (-14.2 +/- 5.9%) a nd neck (-11.2 +/- 8.6%), and after age 59 at Dia (-10.9 +/- 6.9%). Ep iphysis and diaphysis BMC but not BMD were correlated with body weight (r = 0.22, p < 0.04 for both sites). In patients with PHPT mean tibia l BMD expressed as Z scores was significantly lower than in controls, from 39 to 130 mm but was similar to that in controls from 0 to 39 mm as well as at spine and neck. In patients with untreated VOP, mean BMD (Z scores) was lower than in controls at spine and neck as well as ti bia from 0 to 52 mm (p < 0.01) but not between 52 and 130 mm. We concl ude that DXA measurement at tibia is a suitable tool to assess bone lo ss and distribution of bone mass between Dia (cortical) and Epi (with a substantial amount of trabecular bone) in a single weight-bearing bo ne in health and bone diseases.