A fall, followed by a crack – many people are no stranger to this. Broken bones are painful, but the majority heal very well. The secret lies in stem cells and bone’s natural ability to renew itself.

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Bone heals by making cartilage to temporarily plug the hole created by the break. This is then replaced by new bone.

Many people think of bones as being solid, rigid, and structural. Bone is, of course, key to keeping our bodies upright, but it is also a highly dynamic and active organ.

Old bone is constantly being replaced by new bone in a finely tuned interplay of the cells present. This mechanism of daily maintenance comes in handy when we are faced with a broken bone.

It allows stem cells to first produce cartilage and then create new bone to heal the break, all of which is facilitated by a finely tuned sequence of events.

Each year, around 15 million fractures, which is the technical term for broken bones, occur in the United States.

The immediate response to a fracture is bleeding from the blood vessels dotted throughout our bones.

The clotted blood collects around the bone fracture. This is called a hematoma, and it contains a meshwork of proteins that provide a temporary plug to fill the gap created by the break.

The immune system now springs into action to orchestrate inflammation, which is an essential part of healing.

Stem cells from the surrounding tissues, bone marrow, and blood respond to the immune system’s call, and they migrate to the fracture. These cells start off two different pathways that allow bone to heal: bone formation and cartilage formation.

New bone starts to form mostly at the edges of the fracture. This happens in much the same way that bone is made during normal, everyday maintenance.

To fill the void space between the broken ends, cells produce soft cartilage. This may sound surprising, but it is very similar to what happens during embryonic development and when children’s bones grow.

Cartilage, or soft callus, formation peaks around 8 days after injury. However, it is not a permanent solution because cartilage is not strong enough to withstand the pressures that bones experience in our daily lives.

The soft callus is replaced first with a hard, bone-like callus. This is pretty strong, but it is still not as strong as bone. Around 3 to 4 weeks after the injury, the formation of new mature bone starts. This can take a long time – several years, in fact, depending on the size and site of the fracture.

However, there are cases wherein bone healing is not successful, and these cause significant health problems.

Fractures that take an abnormally long time to heal, or those that do not join back together at all, occur at a rate of around 10 percent.

However, a study found that the rate of such non-healing fractures was much higher in people who smoke and people who used to smoke. Scientists believe that this may be due to the fact that blood vessel growth in the healing bone is delayed in smokers.

Non-healing fractures are particularly problematic in areas that carry a lot of load, such as the shinbone. An operation to fix the gap that will not heal is often necessary in such cases.

Orthopedic surgeons can use either bone from elsewhere in the body, bone taken from a donor, or man-made materials such as 3-D-printed bone to fill the hole.

But in the majority of cases, bone makes use of its remarkable ability to regenerate. This means that the new bone that fills the fracture closely resembles the bone before the injury, without a trace of a scar.