Final answer:
Bone repair after a fracture involves the formation of a fracture hematoma, an inflammatory response which sets the stage for new bone formation, a fibrocartilaginous callus transformed into a bony callus, and finally, remodeling of the bony callus back into the shape of the original bone.
Step-by-step explanation:
The Four Steps of Bone Formation After Fracture
When a bone breaks, a multi-step bone repair process commences. Here's an outline of the four critical stages in bone formation after a fracture:
The first step involves the formation of a fracture hematoma. After a fracture, blood vessels within the bone tear and hemorrhage, leading to a collection of clotted blood at the site. The clotting also serves to seal the torn vessels, but unfortunately results in the death of bone cells that are deprived of nutrients.
Shortly following the injury, the body initiates an inflammatory response. Capillaries grow into the hematoma, and phagocytic cells clear away the dead bone cells, setting the stage for new bone formation. Subsequently, fibroblasts produce collagen, and osteoblasts begin to construct spongy bone. The material bridging the broken bone fragments is termed a fibrocartilaginous callus, comprising hyaline and fibrocartilage.
Over approximately two months, the fibrocartilaginous callus is replaced by a bony callus of spongy bone. This transformation is akin to endochondral ossification, where cartilage is ossified into bone, involving a matrix populated with osteoblasts and osteoclasts.
The concluding phase is remodeling. In this phase, osteoclasts and osteoblasts work together to hone the bony callus into the shape and structure of the original bone, a process which can extend over several months. The bone is sculpted by removing excess material on the outside and within the medullary cavity, while compact bone is laid down until the bone structure is akin to the original unbroken state.