Bone Repair

What happens when a bone is fractured? Fractures are classified by the type and location of the break. See the image below:

In this illustration, each type of fracture is shown on the right femur from an anterior view. In the closed fracture, the femur is broken in the middle of the shaft with the upper and lower halves of the bone completely separated. However, the two halves of the bones are still aligned in that the broken edges are still facing each other. In an open fracture, the femur is broken in the middle of the shaft with the upper and lower halves of the bone completely separated. Unlike the closed fracture, in the open fracture, the two bone halves are misaligned. The lower half is turned laterally and it has protruded through the skin of the thigh. The broken ends no longer line up with each other. In a transverse fracture, the bone has a crack entirely through its width, however, the broken ends are not separated. The crack is perpendicular to the long axis of the bone. Arrows indicate that this is usually caused by compression of the bone in a superior-inferior direction. A spiral fracture travels diagonally through the diameter of the bone. In a comminuted fracture, the bone has several connecting cracks at its middle. It is possible that the bone could splinter into several small pieces at the site of the comminuted fracture. In an impacted fracture, the crack zig zags throughout the width of the bone like a lightning bolt. An arrow indicates that these are usually caused by an impact that pushes the femur up into the body. A greenstick fracture is a small crack that does not extend through the entire width of the bone. The oblique fracture shown here is travelling diagonally through the shaft of the femur at about a thirty degree angle.

Types of Fractures. By OpenStax College [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons. Link: https://upload.wikimedia.org/wikipedia/commons/3/35/612_Types_of_Fractures.jpg

Repair of fractures occurs in four major stages, assuming that the bones have been somewhat realigned. Realigning of the bone is known as reducing (setting) the fracture. https://upload.wikimedia.org/wikipedia/commons/1/12/613_Stages_of_Fracture_Repair.jpg

Stages of Bone Repair. By OpenStax College License: CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons Link: https://upload.wikimedia.org/wikipedia/commons/1/12/613_Stages_of_Fracture_Repair.jpg

  1. Hematoma Forms: Stage 1 involves the formation of a hematoma, a mass of clotted blood that forms within a few hours after injury. This is caused by rupture of the associated blood vessels in the bone and in the tissues surrounding the bone, recall that bone is highly vascular. The hematoma is fragile and can be easily damaged, this is why broken bones are immobilized by casts or boots while they are healing.
  2. Bone Generation: External Callus and Internal Callus Form: In the next stage of repair, over several days, blood vessels grow into the hematoma bringing macrophages, and stem cells that can differentiate into the various bone and cartilage cells. During the next few weeks, an external callus forms around the fracture to encircle the bone like a collar to stabilize the fracture. Then, an internal callus is formed as a network of spongy bone to unite the inner surfaces of the bone. The macrophages clean up the cellular debris as osteoclasts break down the fragments of dead bone.
  3. Boney Callous: Fibroblasts and chondroblasts then form a fibrocartilage callus that can be used as a model for formation of new bone. Once the fibrocartilage callus is formed the osteoblasts begin to convert the fibrocartilage callus to bone. Over a period of about 8 weeks, the third stage of repair is complete and results in the formation of a bony callus.
  4. Bone Remodeling: The newly formed bone is woven bone, which is remodeled into lamellar bone by the osteoclasts and osteoblasts in the final stage of repair. The remodeling phase takes several months to a few years to complete. This remodeling also hollows out the medullary cavity and returns the bone to its original shape.

The healing process described above is known as indirect healing. Another type of healing can occur if the broken ends of the bone are precisely and rigidly held in place. This type of healing (direct healing) typically takes place only in the bones that have been surgically realigned and held tightly in place, usually with metal plates and screws. With this type of healing, new, lamellar bone forms directly, without the development of the callus.

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