Bone is constantly changing. Old bone breaks down and new bone is formed in a continuous process called remodeling. The bone breakdown cells, osteoclasts are linked with the bone forming cells, osteoblasts in a cycle of bone breakdown and formation. Remodeling occurs in sequence initiated by microdamage to start the repair process.
In remodeling, bone on the surface of trabecular bone or in the interior of the cortical bone is removed and then replaced at the same site. This process is vital for bone health to keep in good repair and not accumulate too much old bone.
Ten to 15% of the skeleton is broken done and renewed each year. Therefore, most of the adult skeleton is replaced about every 10 years. The different types of bone turnover at different rates. Normal balance is a about 10% of cortical bone and 30% to 40% of trabecular bone remodeled per year.
Information about bone remodeling can be obtained through measuring bone turnover markers in blood or urine. With aging, menopause, or other secondary causes, bone remodeling increases with a deficit of new bone leading to thinning of bone and ultimately, increased fracture risk.