Bone has two forms: dense cortical bone in the outer shaft wall, and porous trabecular (spongy) bone forming a strut lattice inside. Both are continuously remodeled: osteoclasts resorb a small volume of old bone, then osteoblasts fill it back in â a cycle lasting weeks in real bone, sped up here for visualization.
- Wolff's law: bone remodels to match mechanical load â higher loading (exercise) raises osteoblast formation efficiency above the resorbed amount, net density rises; disuse (bed rest, spaceflight) does the opposite.
- Osteoporosis: aging and hormone loss (e.g. post-menopause estrogen drop) raise the resorption rate and cut formation efficiency, so each cycle replaces less than was removed â struts thin, some disappear, and the lattice becomes fragile.