If you have ever broken a bone, then you know the healing process takes time. So what exactly happens under that cast? It’s actually a pretty amazing and complex healing process.

Dr. Bharat Desai of the Panorama Orthopedics & Spine Center in metro Denver recently explained the four healing stages of a fracture at a free, educational seminar.

“Think of bone as a living structure, it’s not static,” he said.. “That understanding is critical in understanding why this healing occurs.”

Dr. Desai compared the process to what happens after a disaster. Troops are sent in to evaluate the extent of the damage, a highway is built to make the site accessible, a cleaning crew is sent in and then reconstruction begins.

Getting the troops in is the inflammation stage. Inflammation occurs when the blood vessels inside the bone are torn, and it sends the message to the body that reinforcements are needed. The body brings in blood vessels from the surrounding area, creating a route, to carry away dead cells and tissue and to lay down new fibers.

In the second phase of healing, the body begins to build scar tissue, soft callus (healing bone) and potentially cartilage tissue. This ties the bone fragments together.

By the third stage of healing, which begins at around six weeks, the body is beginning to rebuild the bone. Finally, between three months and 18 months after the injury, the body enters the consolidation phase where it further refines, or remodels, the new bone.

“It’s reforming its original organization,” Dr. Desai said. “As you start walking, it remodels. It takes away bone, adds bone. If you start walking twice as much, then it adjusts it again.”

As you can see, even while you are resting a broken leg or arm, the healing process keeps that body part hard at work.