Bone healing, or fracture healing, is a proliferative

Cell growth

The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...

 physiological process in which the body facilitates the repair of a bone fracture

Bone fracture

A bone fracture is a medical condition in which there is a break in the continuity of the bone...

.

Generally bone fracture treatment consists of a doctor pushing dislocated bones back into place via relocation with or without anaesthetic, stabilizing their position, and then waiting for the bone's natural healing process to occur.

Physiology and process of healing

In the process of fracture healing, several phases of recovery facilitate the proliferation and protection of the areas surrounding fractures and dislocations. The length of the process depends on the extent of the injury, and usual margins of two to three weeks are given for the reparation of most upper bodily fractures; anywhere above four weeks given for lower bodily injury.

The process of the entire regeneration of the bone can depend on the angle of dislocation or fracture. While the bone formation usually spans the entire duration of the healing process, in some instances, bone marrow within the fracture has healed two or fewer weeks before the final remodeling phase.

While immobilization and surgery may facilitate healing, a fracture ultimately heals through physiological processes. The healing process is mainly determined by the periosteum

Periosteum

Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

 (the connective tissue

Connective tissue

"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 membrane covering the bone). The periosteum is one source of precursor cells which develop into chondroblasts and osteoblasts that are essential to the healing

Healing

Physiological healing is the restoration of damaged living tissue, organs and biological system to normal function. It is the process by which the cells in the body regenerate and repair to reduce the size of a damaged or necrotic area....

 of bone. The bone marrow

Bone marrow

Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

 (when present), endosteum

Endosteum

In anatomy the endosteum is a thin layer of connective tissue that lines the surface of the bony tissue that forms the medullary cavity of long bones. This endosteal surface is usually resorbed during long periods of malnutrition, resulting in less cortical thickness...

, small blood vessels, and fibroblasts are other sources of precursor cells.

Phases of fracture healing

There are three major phases of fracture healing, two of which can be further sub-divided to make a total of five phases;

• 1. Reactive Phase
  • i. Fracture and inflammatory phase
  • ii. Granulation tissue formation
• 2. Reparative Phase
  • iii. Cartilage Callus formation
  • iv. Lamellar bone deposition
• 3. Remodeling Phase
  • v. Remodeling to original bone contour

Reactive

After fracture

Bone fracture

A bone fracture is a medical condition in which there is a break in the continuity of the bone...

, the first change seen by light and electron microscopy is the presence of blood cells within the tissues which are adjacent to the injury site. Soon after fracture, the blood vessels constrict, stopping any further bleeding. Within a few hours after fracture, the extravascular blood cells form a blood clot, known as a hematoma

Hematoma

A hematoma, or haematoma, is a localized collection of blood outside the blood vessels, usually in liquid form within the tissue. This distinguishes it from an ecchymosis, which is the spread of blood under the skin in a thin layer, commonly called a bruise...

. All of the cells within the blood clot degenerate and die. Some of the cells outside of the blood clot, but adjacent to the injury site, also degenerate and die. Within this same area, the fibroblast

Fibroblast

A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...

s survive and replicate. They form a loose aggregate of cells, interspersed with small blood vessels, known as granulation tissue

Granulation tissue

Granulation tissue is the perfused, fibrous connective tissue that replaces a fibrin clot in healing wounds. Granulation tissue typically grows from the base of a wound and is able to fill wounds of almost any size it heals...

.

Reparative

Days after fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal to the fracture gap develop into chondroblast

Chondroblast

A chondroblast is a cell which originates from a mesenchymal stem cell and forms chondrocytes, commonly known as cartilage cells. Chondroblasts that become embedded in the matrix are called chondrocytes. They lie in the space or lacunae present in the groups of two or more. The groups are formed by...

s which form hyaline cartilage

Hyaline cartilage

Hyaline cartilage consists of a slimy mass, pearly bluish in colour with firm consistency and considerable collagen. It contains no nerves or blood vessels, and its structure is relatively simple....

. The periosteal cells distal to the fracture gap develop into osteoblast

Osteoblast

Osteoblasts are mononucleate cells that are responsible for bone formation; in essence, osteoblasts are specialized fibroblasts that in addition to fibroblastic products, express bone sialoprotein and osteocalcin.Osteoblasts produce a matrix of osteoid, which is composed mainly of Type I collagen...

s which form woven bone. The fibroblasts within the granulation tissue develop into chondroblasts which also form hyaline cartilage. These two new tissues grow in size until they unite with their counterparts from other parts of the fracture. These processes culminate in a new mass of heterogenous tissue which is known as the fracture callus. Eventually, the fracture gap is bridged by the hyaline cartilage and woven bone, restoring some of its original strength.

The next phase is the replacement of the hyaline cartilage and woven bone with lamellar bone. The replacement process is known as endochondral ossification

Endochondral ossification

Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral...

with respect to the hyaline cartilage and bony substitution with respect to the woven bone. Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone. The lamellar bone begins forming soon after the collagen matrix of either tissue becomes mineralized. At this point, the mineralized matrix is penetrated by channels, each containing a microvessel

Microvessel

Microvessel or microvasculature can refer to:* Arteriole, a small diameter blood vessel that extends and branches out from an artery and leads to capillaries* Capillary, the smallest blood vessels...

 and numerous osteoblasts. The osteoblasts form new lamellar bone upon the recently exposed surface of the mineralized matrix. This new lamellar bone is in the form of trabecular bone. Eventually, all of the woven bone and cartilage of the original fracture callus is replaced by trabecular bone, restoring most of the bone's original strength.

Remodeling

The remodeling process substitutes the trabecular bone with compact bone. The trabecular bone is first resorbed by osteoclast

Osteoclast

An osteoclast is a type of bone cell that removes bone tissue by removing its mineralized matrix and breaking up the organic bone . This process is known as bone resorption. Osteoclasts were discovered by Kolliker in 1873...

s, creating a shallow resorption pit known as a "Howship's lacuna". Then osteoblasts deposit compact bone within the resorption pit. Eventually, the fracture callus is remodelled into a new shape which closely duplicates the bone's original shape and strength. The remodeling phase takes 3 to 5 years depending on factors such as age or general condition.