Alendronate increases bone strength by increasing the mean degree of mineralization of bone tissue in osteoporotic women

The mean degree of mineralization of bone (MDMB) was measured by quantitati ve microradiography on transiliac bone biopsies taken from 53 postmenopausa l osteoporotic women who had been treated with alendronate (ALN; 10 mg/day) during 2 (9 patients) or 3 years (16 patients) or with placebo (PLA; 15 an d 13 patients, respectively). In the same patients, bone mineral density (B MD) values were obtained by dual-energy X-ray absorptiometry of the lumbar spine and femoral neck at the beginning and end of treatment. Histomorphome tric parameters and activation frequency of new remodeling units were also measured on the iliac biopsies. After 2 years of ALN, MDMB in compact bone was 9.3% (p = 0.0035) and in cancellous bone was 7.3% (p = 0.0009) higher, respectively, than PLA, After 3 years of ALN, MDMB in compact bone was 11.6 % (p = 0.0002) and in cancellous bone was 11.4% (p = 0.0001) higher, respec tively, than PLA, After 2 and 3 years of ALN, and compared with the corresp onding PLA, the distribution of the degree of mineralization in compact and cancellous bone showed a clear shift toward the highest mineralization val ues and a decrease in the number of bone structure units having low values of mineralization, The between-group differences in MDMB were similar to th ose of BMD at the lumbar spine BMD (+8.7% after 2 years and +9.6% after 3 y ears, respectively), suggesting that MDMB augmentation probably accounted f or the majority of the increase in BMD seen with ALN, The data support the hypothesis that the reduction in activation frequency caused by the antires orptive effect of ALN is followed by a prolonged secondary mineralization t hat increases the percentage of bone structure units having reached a maxim um degree of secondary mineralization and, through this mechanism, MDMB, Th at these effects contribute to improved bone strength is demonstrated by th e reduction in fracture incidence previously demonstrated in these patients . (C) 2000 by Elsevier Science Inc. All rights reserved.