Expansion of the medullary cavity at the expense of cortex in postmenopausal osteoporosis.

Transilial biopsies from postmenopausal osteoporotic patients showed that static features and remodeling activity tended to form a pattern, with the area midway between the cortices having less bone and lower remodeling activity than the area closer to the inner boundary of the cortex. The appearance, enlargement, and coalescence of the resorption cavities within the subendosteal area of the cortex lead to negative bone balance and a progressive trabeculation of the cortex, resulting in the formation of a transitional zone. Therefore, in most cases we can distinguish two different zones, transitional and trabecular zones, within the area usually known as the trabecular bone area. The transitional zone undergoes more active remodeling than the trabecular zone and has an important role in evaluation of the traditional trabecular bone volume and remodeling dynamics depending on the field selection for this evaluation, i.e., the proportion between transitional and trabecular zones within the selected fields. We postulate that the appearance of resorption cavities in the subendosteal area of the cortex depends on excessive osteoclast work, and bone loss in the resulting transitional zone depends on a combination of excessive osteoclast work and defective osteoblast work, with the osteoclast excessive work predominating.