Not to mention the role of bone in blood cell synthesis! The bone is a fascinating organ and sadly misunderstood by most people. They are, essentially, living

Not to mention the role of bone in blood cell synthesis! The bone is a fascinating organ and sadly misunderstood by most people. They are, essentially, living rocks. Our bones are innervated, full of living cells, and constantly engaging in a chemical equilibrium with the body on CaPO4 as you point out. Furthermore, the bone needs to grow along with the creature. It has to be soft enough to accommodate childbirth but allow growth and hardening as the organism matures. The process by which this occurs in natural bone is complex and remarkable. Metal bones can't do any of this stuff.