In this video, we're going to examine the stages of bone repair after a fracture. So in stage one, because of the fracture, blood vessels within the bone have been torn, and this results in a bleeding out or a hemorrhage locally. The local hemorrhage, however, is sort of repaired through a clotting mechanism. So you form a hematoma. So you have this excess fluid which leads to a swelling, pain, inflammation, but in addition some of those bone cells, oops, those bone cells because of a lack of nutrition will die.
So the surrounding area that is not getting an adequate blood supply . will die off. So we transition then to stage two. Stage two we're going to call the fibrocartilaginous callus formation.
And so when you have that hematoma, eventually you're going to have phagocytic cells that move in. You're going to have new blood vessels that grow into the hematoma. So you have capillaries. So this is going to reconnect the periosteum and the endosteum to this region. And it is from the periosteum and endosteum that fibroblasts will invade this area.
You'll have some precursor cells become chondroblasts. You'll have osteoblasts kind of moving in, and each of these cells are going to do something different. The fibroblasts will begin to secrete collagen fibers that sort of temporarily splints the two fragments of bone together.
You'll have the chondroblasts start to secrete a cartilaginous matrix. And then the osteogenic cells will give rise to the osteoblasts, and these in turn will start to replace the cartilage that was deposited here with spongy bone. So the process mimics what we see with endocondrial ossification.
So we call this entire mishmash of material of collagen fibers, cartilage, and the beginnings of spongy bone formation the fibrocartilaginous callus. So we move then to stage three, the bony callus. So we begin to complete the ossification of the cartilage that was deposited that kind of temporarily splinted the two pieces of bone together.
So within the span of a week you have new trabiculae from the spongy bone and this becomes a much more robust structure, a harder structure, as opposed to the softer fibrocartilaginous callus. So within the span of about two months we've replaced the breakage with this spongy bone. So that leaves us with stage four where we need to remodel the bone.
So we need to re-establish the medullary cavity. We need to rebuild the compact bone that is on the perimeter. And then you have repair complete.
So this can take several months where material from the diaphysis can sort of be increased. So you have appositional growth based on stresses, and then the spongy bone that is in the medullary cavity can also be removed. So that ends stage four where bone remodeling occurred.