During the third step, bony callus (spongy bone) forms. Osteoclasts (bone destroying cells) and osteoblasts (bone forming cells) move into the area and increase in number. As this happens, bony callus replaces the fibrocartilage callus.
The final step is bone remodeling. This process can take weeks or months depending on the severity of the fracture. As the bone remodels, the callus gets much stronger and acts as a glue that holds the bone together.
And finally, after nearly 5 months of wearing casts, Homer's leg is finally healed!
Bone remodeling is important outside of bone fractures, too. It allows for bones to stay healthy and grow as the body grows. Since the bones are living entities, they need to be managed so that they stay healthy. Remodeling occurs in response to the amount of calcium in the blood and how much the muscles and gravity are pulling on the bones.
When the amount of calcium is below a certain level, the parathyroid glands release parathyroid hormone (PTH) into the blood. The PTH activates osteoclasts. These cells break down the bone matrix and calcium is brought back into the blood.
If there is too much calcium, some is put into the bone matrix to form salts of hard calcium.
Bones become thicker and have growths to account for the pressure of the attached muscles. Osteoblasts place new matrix and get trapped to add layers to the existing bone to add strength.