Bone healing

Primary healing (also known as direct healing) requires a correct anatomical reduction which is stable, without any gap formation. Such healing requires only the remodeling of lamellar bone, the Haversian canals and the blood vessels without callus formation. This process may take a few months to a few years. Contact healing When the gap between the bone ends is less than 0.01 mm, and interfragmentary strain is less than 2%, contact healing can occur. In this case, cutting cones, which consists of osteoclasts, form across the fracture lines, generating cavities at a rate of 50–100 μm/day. Osteoblasts fill up the cavities with the Haversian system. This causes the formation of lamellar bone that orients longitudinally along the long axis of the bone. Blood vessels form that penetrate the Haversian system. Remodelling of lamellar bone results in healing without callus formation. Gap healing If the fracture gap is less than 800 μm to 1 mm, the fracture is filled by osteoblasts and then by lamellar bone oriented perpendicular to the axis of the bone. This initial process takes three to eight weeks. Perpendicular orientation of lamellar bone is weak, thus a secondary osteonal reconstruction is required to re-orient the lamellar bone longitudinally. == Secondary healing ==

Secondary healing (also known as indirect fracture healing) is the most common form of bone healing. It usually consists of only endochondral ossification. Sometimes, intramembranous ossification occurs together with endochondral ossification. Intramembranous ossification, mediated by the periosteal layer of bone, occurs with the formation of callus. For endochondral ossification, deposition of bone only occurs after the mineralised cartilage. This process of healing occurs when the fracture is treated conservatively using orthopaedic cast or immobilisation, external fixation, or internal fixation. Reaction After bone fracture, blood cells accumulate adjacent to the injury site. Soon after fracture, blood vessels constrict, stopping further bleeding. Within a few hours, the extravascular blood cells form a clot called a hematoma Seven to nine days after fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal to (on the near side of) the fracture gap develop into chondroblasts, which form hyaline cartilage. The periosteal cells distal to (at the far end of) the fracture gap develop into osteoblasts, which form woven bone through bone resorption of calcified cartilage and recruitment of bone cells and osteoclasts. • Non-union: no progression of healing within six months of a fracture occurring. The fracture pieces remain separated and can be caused by infection and/or lack of blood supply (Ischaemia) to the bone. There are two types of non-union, atrophic and hypertrophic. Hypertrophic involves the formation of excess callus leading to bone ends appearing sclerotic causing a radiological "Elephants Foot" appearance • Delayed union: healing times vary depending on the location of a fracture and the age of a patient. Delayed union is characterised by 'persistence of the fracture line and a scarcity or absence of callus formation' on x-ray. Healing is still occurring but at a much slower rate than normal. Gallery File:Woven bone matrix.jpg|Collagen fibers of woven bone File:Osteoclast.jpg|Osteoclast displaying many nuclei within its "foamy" cytoplasm. File:Active osteoblasts.jpg|Light micrograph of decalcified cancellous bone displaying osteoblasts forming new bone tissue, containing two osteocytes, within a resorption pit. Radiologic timeline in young children On medical imaging, secondary bone healing displays the following features over time in young children: == Footnotes ==