By way of review, the surface of bone is completely covered with metabolically active cells. These cells respond to specific calciotrophic hormones and mineral concentrations to adjust serum calcium and bone remodeling activity. Many local factors, such as interleukin 6, platelet derived growth factor, fibroblast growth factor, transforming growth factor beta, and other bone morphogenetic proteins also regulate surface cells. Since trabecular bone has a much greater surface area than cortical bone, it is more metabolically active. In the typical skeleton, 80% of bone surfaces are devoted to ion exchange and 20% are undergoing remodeling.