Endochondral ossification is a process where bone replaces cartilage. It occurs during fetal development and throughout childhood as the bones of the body grow. When people experience fractures, endochondral ossification is part of the healing process, with the body first forming cartilage known as a callus and later replacing it with bone. This method for bone formation relies on replacing a model made from cartilage with fully ossified bone.
The process starts in the middle of the bone, with a small cluster of bone cells. These cells will proliferate, allowing for the production of more bone and establishing a mineralized matrix. The cartilage starts to weaken, and the cells form a bone collar on the outside of the bone to support it while the endochondral ossification takes place. Blood vessels grow from the outside in, extending through the bone collar to supply fresh blood to the developing bone.
In fetal development, the skeleton first forms in cartilage, and then endochondral ossification starts to replace it. The bones do not form completely as they still need room to grow. Children have deposits of cartilage at the ends of the long bones to allow them to fully develop, and over time, these growth plates, as they are known, start to ossify. This may not occur completely until people reach their 20s and their bodies are fully grown.
People at their mature height can still experience endochondral ossification in fracture healing. If a doctor sets a fracture correctly, the body will start to produce cartilage at the site before slowly replacing it with bone. A doctor can use x-rays to monitor the progress of healing and make sure the bone is developing properly. Failure to form a cartilage callus is a cause for concern, as the bone cannot develop if it doesn't have a framework to start growing on.
The body needs lots of oxygen and nutrients for this process. Proper nutrition is key to prevent issues like weak bones, which may become a problem later in life. In the case of people healing from fractures, they may use physical therapy to keep the muscles and ligaments in the area healthy so they can support the bone after it heals. There will be some residual weakness while the body slowly works to heal the site of the fracture, and the history of the fracture will always be visible in the bone.