Abstract
Osteoporosis is characterized by defective bone remodeling in the sense that bone is removed inappropriately. The evidence found in iliac biopsy specimens is reduced numbers of trabeculae and reduced cortical thickness. As seen in Table 1, however, abnormalities in bone histomorphometry are not striking. Nor are the distributions of the variables significantly different in osteoporotics compared with normals. The lack of findings may be because they are absent by the time a patient undergoes biopsy late in life, or because they are so subtle that the method does not possess sufficient statistical power to detect them. Another explanation may be that they are removed at the time the bone itself is removed. On the other hand, older literature on remodeling in cortical bone of the rib showed clear differences in patients with osteoporosis compared with normals. These data, which have been ignored to some extent, showed loss of bone from the inner cortex, decreased appositional rates, decreased bone formation rates, and prolonged bone remodeling periods in patients compared with normals. These would permit accumulation of unrepaired microdamage in the cortical skeleton, loss of adaptation to normal mechanical usage, and excessive fragility of cortical bone. Confirmation of sluggish remodeling would result in sluggish repair of microdamage and accumulation of excessive unrepaired microdamage which, in turn, would result in increased fragility cortical bone. This cannot be confirmed until we have satisfactory methods of studying microdamage in living humans. The serum and urine bone markers of global bone remodeling rates are not entirely concordant with the iliac or rib histomorphometric data in that the distributions are not as extreme. The differences between findings by histomorphometry and by bone markers may be due to heterogeneity of remodeling in different areas of the skeleton. The apparent discordance will only be resolved by continued research in the area.