Adaptation of bone geometry. (a) Cross-section of long bone with marrow cavity (purple) and cortical bone (yellow), bordered by endosteal surface to the marrow and periosteal envelope at the outside. (b) Endosteal resorption (dotted area) leads to porosity of cortical bone and eventually to the expansion of the marrow cross-sectional area (CSA). (c) Compensatory periosteal bone apposition (area between dashed and solid line) results in an increase of total bone CSA. Solid lines, present bone envelopes; dashed lines, past bone envelopes.