Bone fracture healing is a biological process that allows the skeletal system to repair itself after an injury. This natural capacity ensures bones regain their original strength and function by initiating a complex sequence of events.
Fracture Hematoma Formation
The immediate response to a bone fracture involves the disruption of blood vessels within the bone and surrounding tissues. This rupture leads to bleeding, and a blood clot, known as a fracture hematoma, rapidly forms around the injury site. The hematoma consists of blood cells from peripheral and intramedullary sources, and its formation is a first step in the healing cascade.
An acute inflammatory response follows, peaking within 24 hours and subsiding within seven days. This inflammatory reaction releases various chemical mediators, including cytokines and growth factors, which attract inflammatory cells like neutrophils and macrophages to the fracture site. These cells work to clear debris and initiate tissue repair, while the hematoma provides a temporary scaffold for subsequent cellular activity.
Soft Callus Formation
Following the initial inflammatory phase, the fracture hematoma begins to organize, transitioning into a more stable structure. Within two weeks, mesenchymal stem cells and other progenitor cells are recruited to the fracture site. These cells differentiate into chondroblasts and fibroblasts, which start to produce collagen and fibrocartilage.
This newly formed tissue is called granulation tissue. It gradually forms a soft bridge, known as the soft callus, across the fracture gap. This soft callus provides initial stability to the fractured bone, although it remains flexible and is not yet rigid enough to bear significant weight.
Hard Callus Formation
The soft callus then undergoes a transformation into a rigid hard callus. Osteoblasts, bone-forming cells, invade the soft callus and begin depositing woven, immature bone, leading to the mineralization of the cartilaginous framework.
This process involves both endochondral ossification, where cartilage is converted into bone, and intramembranous ossification, where bone tissue is directly formed. The resulting hard callus provides mechanical stability to the fracture, and this stage can last for several months, depending on the injury’s severity and location.
Bone Remodeling
The final stage of fracture healing is bone remodeling, a long-term process. During this phase, the woven bone of the hard callus is gradually replaced by stronger lamellar bone. This transformation restores the bone’s original form and mechanical strength.
Osteoclasts, bone-resorbing cells, resorb the woven bone of the hard callus. Simultaneously, osteoblasts lay down new lamellar bone, aligning it in response to mechanical stresses. This coordinated activity reshapes the bone, recreating its original structure.