Trabecular bone is believed to heal mainly through inter-trabecular (otherwise known as intermembranous) bone formation. This process is characterized by direct bone formation occurring freely in the metaphyseal marrow. In some cases, mechanical instability may lead to cartilage formation as well as an external callus, which otherwise is rather unspecific to healing of trabecular bone tissue. The fractured fragments need to be anatomically correct to each other and remain within a very close distance . The inter-trabecular healing process starts with a hematoma consisting of blood and marrow cells (Figure ). Blood vessels directly neighboring the fracture site undergo rupture and release blood into the fracture, allowing immune cells to access the site. It is followed by inflammation, which causes the hematoma to coagulate . Next, the mesenchymal stem cells (MSCs) proliferate at three days from the injury 2216 . If the bone is mechanically stable, the mesenchymal cells proliferate directly into osteoblasts lining the trabeculae directly to the fracture and osteocytes, forming bone in an instant manner . Simultaneously, macrophages known as "osteomacs" play a crucial role in supporting the osteoblasts, substantially enhancing direct ossification event . In cases of mechanical instability of the fracture a cartilage and an external callus may appear, which are otherwise unusual .