The Short Answer: Through a process called bone remodelling, osteoclasts break down old bone while osteoblasts build new bone. This continuous cycle means your skeleton is completely replaced approximately every 7 to 10 years.

We tend to think of bones as fixed and unchanging, like the girders in a building. But bone is one of the most metabolically active tissues in the human body. At any given moment, roughly 10% of your skeleton is being actively remodelled — old bone dissolved, new bone laid down in its place.

How Bone Remodelling Works

The process relies on two types of cell working in tandem. Osteoclasts are large cells that attach to the bone surface and dissolve old or damaged tissue using acid and enzymes. They create small pits in the bone surface. Behind them come the osteoblasts, which fill those pits with fresh collagen matrix that then mineralises into new bone.

This cycle — called the bone remodelling unit — takes approximately 3 to 6 months to complete at any single site. Across the entire skeleton, the combined effect of millions of these units means that virtually all your bone tissue is replaced over a period of about 10 years.

Why It Matters in Medicine

Bone remodelling is why fractures heal. When a bone breaks, the body accelerates local remodelling to bridge the gap with new tissue. It is also why weight-bearing exercise strengthens bones — mechanical stress signals osteoblasts to build more bone in areas under load.

When the balance between osteoclasts and osteoblasts tips toward breakdown, conditions like osteoporosis develop. Understanding this cycle has led to drugs like bisphosphonates, which slow osteoclast activity, and newer treatments that stimulate osteoblast bone formation.

Sources

• Clarke B, Normal Bone Anatomy and Physiology, Clinical Journal of the American Society of Nephrology, 2008.
• NHS, Osteoporosis Overview, 2023.