How bones grow and heal

Bones stop growing longer when the growth plates turn to bone, usually in the early twenties.

Three kinds of bone cell

Bone is built, maintained and reshaped by three types of cell. Osteoblasts are bone-forming cells that lay down new bone material. When an osteoblast becomes surrounded by the material it made, it is called an osteocyte, a mature bone cell. Osteoclasts do the opposite job: they break down and reabsorb bone. Growth, repair and everyday reshaping all come from the balance between building and removing.

From cartilage to bone

The skeleton begins to form in the first weeks after conception. By the end of the eighth week its pattern is laid out in cartilage and in sheets of connective tissue, and bone starts to replace them. Some flat bones of the skull form directly from the connective tissue sheets. Most other bones form from a cartilage model that is gradually replaced by bone, starting in the middle of the shaft and spreading toward the ends. Later, usually after birth, new centres of bone form in the ends as well.

Growth plates

Between each end and the shaft of a growing long bone there remains a disc of cartilage called the epiphyseal plate, or growth plate. On one side of the plate the cartilage keeps multiplying, and on the other side it is replaced by bone, so the bone gets longer. This continues through childhood and the teenage years. Growth is driven by growth hormone from the pituitary gland and by sex hormones. When the cartilage stops growing, usually in the early twenties, the plate turns completely into bone. Only a thin line is left, and the bone can no longer grow in length.

Bones can still grow thicker throughout life in response to stress from muscle activity or weight. Osteoblasts add compact bone on the outside while osteoclasts remove bone on the inside around the marrow cavity, so the bone gets wider without becoming too heavy. This constant rebuilding is called remodeling, and it is one reason the skeleton responds to how you use it.

How a broken bone mends

Most fractures heal in four overlapping stages, according to a review in the U.S. National Library of Medicine. First, blood from torn vessels forms a clot at the break, called a hematoma, and inflammation brings in cells that clear away damaged tissue; this stage lasts about 5 days. Second, a soft callus of cartilage and fibrous tissue bridges the gap, starting around day 5. Third, the soft callus is replaced by woven bone, forming a hard callus, which usually lasts up to about 4 weeks after the injury. Finally, in remodeling, osteoclasts and osteoblasts reshape the new bone into compact, strong bone, a stage that can take months to years.

Why does a cast or splint help? Healing bone needs the broken ends to stay lined up and fairly still while the callus forms, so that the bridge of new tissue grows across the gap in the right place. How long a bone needs support depends on the break.