Bone Building

Bone formation and bone resorption are at constant equilbrium until either a disease presents itself or, in women, menaupause occurs.  When the equilibrium is disrupted the bone resorption (bone loss) side accelerates cause more bone to disappear and because bone formation continues at the same, then the net effect is more bone loss because bone formation can not keep up with the increased bone loss.

Most therapeutics slow down bone loss to make it equal to boen formation. However, if the bone has endured to much bone loss then the structure of the bone is weak. Think of a sponge and increased  bone loss removes the inside structure of the sponge, and no amount of slowing bone loss will repair the loss that has already occured. Thus it is important to address increases of bone loss quickly.

         Furthermore,  until recently there was only one therapeutic acting on the bone formation side (PTH).  Today we have additional therapeutics on the bone formation side acting thru the sclerostin mechanism, both therapeutics are biologics and only one is FDA approved (Amgen's Evenity).

Within the class of bone formation therapeutics the PTH mechanism does not uncouple bone formation from bone resorption, thus the difference between the two shows the anabolic window of the net bone building effect. The sclerostin mechanism however does act as a switch to uncouple bone formation from bone resorption. This leads to a larger net effect of bone formation.