Bones heal differently and this discussion will be an overview with some generalizations. Typically long bones have a shaft with cortical bone which is more compact. The same long bones have a more dense type of bone at the ends (base and head) as well as short bones can be more dense with bone called cancellous bone. Both cortical and cancellous bones heal differently and at different rates. Bones that are fixated with fixation such as screws and plates also heal differently than those left to heal “on their own.”
A typical fracture interrupts the blood vessels to some extent depending on the energy the fracture occured at and the amount of displacement of fragments. A hematoma then forms (blood clot) which provides an early scaffold for new cells. Osteoblasts are cells that produce osteoid, this osteoid when it becomes mineralized is very disorganized at early stages and forms a primary callous. This is seen on x-ray like a cloudy area at the fracture site. This callous at each end of the fracture fragments will eventually bridge the gap. This external callous forms a very stable fracture site. Once the site is stable the gap between the fragments will reorganize with a uniting callous of bone or fibrocartilage. X-rays do not show cartilage so at this stage the bone gap may appear widened, which will narrow over time as the cartilage is replaced with lamellar bone. The external callous then undergoes a modeling and remodeling phase over many years. Healing time greatly varies but generally the bone is stable in 6-8 weeks for most bones. Age, nutrition, smoking, type of bone, and location of fracture are all important factors to consider when determining fracture healing time.