How Bones Grow, Break and Heal

A bone that breaks today will, weeks later, be filled in with new tissue at exactly the point where it cracked. Bone is not a fixed structure that simply holds still until damaged. It is living tissue that is constantly being broken down and rebuilt, even when nothing is wrong.

Bone looks solid and unchanging, but it is one of the most metabolically active tissues in the body. At any given moment, small patches of old bone are being removed while new bone is being laid down elsewhere, a continuous process called remodelling. Two cell types in constant balance Remodelling depends on two opposing cell types. Osteoclasts break down old or damaged bone tissue, releasing the minerals stored inside it into the bloodstream. Osteoblasts then build new bone tissue in its place, laying down a protein framework and depositing fresh calcium and phosphate into it. In healthy bone, these two processes stay roughly balanced, so bone strength is maintained over time. Why bone remodels itself at all Remodelling serves several purposes at once. It repairs the microscopic damage that ordinary daily activity produces, long before that damage could add up into a fracture. It also allows bone to respond to the load placed on it, becoming denser where it is stressed regularly and thinner where it is not. On top of this, remodelling lets bone act as a mineral reserve. It releases calcium into the blood when the body needs it elsewhere, such as for normal muscle and nerve function. What happens when a bone actually breaks A fracture does not heal by scarring over, the way skin does. Instead, it heals through the same remodelling machinery that maintains bone every day, only concentrated at the fracture site. Blood immediately clots between the broken ends, and within days a soft bridge of tissue begins to form across the gap. Osteoblasts gradually convert this soft bridge into a harder, temporary structure, which is denser than normal bone but not yet properly organised. Over weeks and months, osteoclasts and osteoblasts remodel this temporary structure into bone that closely resembles the original, restoring most of its strength. Why healing time depends on more than the break itself The pace of this process depends on factors such as age, blood supply to the area, and how well the broken ends are held in position. This is part of why a doctor immobilises a fracture with a cast or splint. Movement at the break disrupts the soft bridge before it can harden. This is why an unstable fracture heals slowly, or heals in a poor position, if it is not stabilised. Why bone density changes across a lifetime Bone density is not fixed. It rises through childhood and early adulthood as new bone formation outpaces removal. It typically peaks in a person's late twenties or early thirties. It then gradually declines as removal starts to outpace formation with age. Hormonal changes, activity levels, and diet all influence this balance, which is why bone density becomes a specific medical concern later in life. What a doctor checks A doctor assessing bone health or a suspected fracture is checking both the structure of the bone itself and the balance of the remodelling process behind it. Imaging. An X ray shows whether a bone is broken, how the pieces are positioned, and, over time, whether healing is progressing as expected. Bone density scanning. A specific scan can measure how dense a bone is compared with an expected healthy range, which is used to assess the long term balance between bone formation and removal. Blood tests. Blood tests measuring calcium, phosphate, and hormones involved in bone metabolism can reveal whether an imbalance in the remodelling process is affecting bone strength. When this system is affected When bone removal consistently outpaces bone formation over a long period, bones can become fragile. They may fracture from a fall or bump that would not normally cause a break. This is explored further in the lesson "Joint Pain and Swelling", which covers how a doctor investigates pain and injury affecting the skeleton more broadly.

Bone tissue is continuously broken down and rebuilt by two opposing cell types working in balance, which is also why a fracture heals through the same rebuilding process rather than through scarring.