Bone healing, or fracture healing.

 Bone healing is a proliferative physiological process in which the body facilitates the repair of a bone fracture.

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

2-Adequate nutrient intake has been found to significantly affect the integrity of the fracture repair. Age, Bone type, drug therapy and pre existing bone pathology are factors which affect healing. The role of bone healing is to produce new bone without a scar as seen in other tissues which would be a structural weakness or deformity.

3-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.

4-While immobilization and surgery may facilitate healing, a fracture ultimately heals through physiological processes. The healing process is mainly determined by the periosteum (the connective tissue membrane covering the bone). The periosteum is one source of precursor cells which develop into chondroblasts and osteoblasts that are essential to the healing of bone. The bone marrow (when present), endosteum, small blood vessels, and fibroblasts are other sources of precursor cells.

There are three major phases of fracture healing

1. Reactive phase

       Fracture and inflammatory phase

  Granulation tissue formation

2. Reparative phase

       Cartilage callus formation

      Lamellar bone deposition

3. Remodeling phase

    . Remodeling to original bone contour

Reactive

After fracture, the first change seen by light and electron microscopy is the presence of blood cells within the tissues adjacent to the injury site. Soon after fracture, the blood vessels constrict, stopping any further bleeding. Within a few hours after fracture, the extra vascular blood cells form a blood clot, known as a hematoma. These cells release cytokines and increase blood capillary permeability. 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 fibroblasts survive and replicate. They form a loose aggregate of cells, interspersed with small blood vessels, known as granulation tissue. This tissue reduces strain across the fracture site. Osteoclasts move in to reabsorb dead bone ends and other necrotic tissue are removed.

Reparative

Days after fracture, the cells of the periosteum replicate and transform. The periosteal cells proximal (closest) to the fracture gap develop into chondroblasts which form hyaline cartilage. The periosteal cells distal to (further from) the fracture gap develop into osteoblasts 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 heterogeneous 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 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 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.^[12] 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.

Remodelling

The remodeling process substitutes the trabecular bone with compact bone. The trabecular bone is first resorbed by osteoclasts, 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. This process can be enhanced by certain synthetic injectable biomaterials, such as cerament, which are osteoconductive and actively promote bone healing.

Obstructions to Bone Healing

1.   Poor blood supply which leads to the death of the osteocytes. Bone cell death is also dependent on degree of fracture and disruption to the Haversian system.

2.   Condition of the soft tissues. Soft tissue in between bone ends restrict healing.

3.   Nutrition and drug therapy. Poor general health reduces healing rate. Drugs that impair the inflammatory response impede healing also.

4.   Infection. Diverts the inflammatory response away from healing towards fighting of the infection.

5.   Age. Young bone unites more rapidly than adult bone.

6.   Pre existing Bone malignancy.

7. Fracture healing is determined by mechanical factors and obstructions to healing include the bone not aligned and too much or little movement. Excess mobility can disrupt the bridging callus interfering with union. Slight biomechanical motion is also seen to improve callus formation

The Stages of Wound Healing

1-    Haemostasis,

2-    Inflammation,

3-    Proliferation and

4-    Maturation.

Although the stages of wound healing are linear, wounds can progress backward or forward depending on internal and external patient conditions. The four stages of wound healing are:

Hemostasis Phase

Haemostasis is the process of the wound being closed by clotting.

Haemostasis starts when blood leaks out of the body. The first step of haemostasis is when blood vessels constrict to restrict the blood flow. Next, platelets stick together in order to seal the break in the wall of the blood vessel. Finally, coagulation occurs and reinforces the platelet plug with threads of fibrin which are like a molecular binding agent. The haemostasis stage of wound healing happens very quickly. The platelets adhere to the sub-endothelium surface within seconds of the rupture of a blood vessel's epithelial wall. After that, the first fibrin strands begin to adhere in about sixty seconds. As the fibrin mesh begins, the blood is transformed from liquid to gel through pro-coagulants and the release of prothrombin. The formation of a thrombus or clot keeps the platelets and blood cells trapped in the wound area. The thrombus is generally important in the stages of wound healing but becomes a problem if it detaches from the vessel wall and goes through the circulatory system, possibly causing a stroke, pulmonary embolism or heart attack.

Inflammatory Phase

Inflammation is the second stage of wound healing and begins right after the injury when the injured blood vessels leak transudate (made of water, salt, and protein) causing localized swelling. Inflammation both controls bleeding and prevents infection. The fluid engorgement allows healing and repair cells to move to the site of the wound. During the inflammatory phase, damaged cells, pathogens, and bacteria are removed from the wound area. These white blood cells, growth factors, nutrients and enzymes create the swelling, heat, pain and redness commonly seen during this stage of wound healing. Inflammation is a natural part of the wound healing process and only problematic if prolonged or excessive.

Proliferative Phase

The proliferative phase of wound healing is when the wound is rebuilt with new tissue made up of collagen and extracellular matrix. In the proliferative phase, the wound contracts as new tissues are built. In addition, a new network of blood vessels must be constructed so that the granulation tissue can be healthy and receive sufficient oxygen and nutrients. Myofibroblasts cause the wound to contract by gripping the wound edges and pulling them together using a mechanism similar to that of smooth muscle cells. In healthy stages of wound healing, granulation tissue is pink or red and uneven in texture. Moreover, healthy granulation tissue does not bleed easily. Dark granulation tissue can be a sign of infection, ischemia, or poor perfusion. In the final phase of the proliferative stage of wound healing, epithelial cells resurface the injury. It is important to remember that epithelialisation happens faster when wounds are kept moist and hydrated. Generally, when occlusive or semi occlusive dressings are applied within 48 hours after injury, they will maintain correct tissue humidity to optimize epithelialisation

Maturation Phase

Also called the remodelling stage of wound healing, the maturation phase is when collagen is remodelled from type III to type I and the wound fully closes. The cells that had been used to repair the wound but which are no longer needed are removed by apoptosis, or programmed cell death. When collagen is laid down during the proliferative phase, it is disorganized and the wound is thick. During the maturation phase, collagen is aligned along tension lines and water is reabsorbed so the collagen fibres can lie closer together and cross-link. Cross-linking of collagen reduces scar thickness and also makes the skin area of the wound stronger. Generally, remodeling begins about 21 days after an injury and can continue for a year or more. Even with cross-linking, healed wound areas continue to be weaker than uninjured skin, generally only having 80% of the tensile strength of unwounded skin.

The stages of wound healing are a complex and fragile process. Failure to progress in the stages of wound healing can lead to chronic wounds. Factors that lead up to chronic wounds are venous disease, infection, diabetes and metabolic deficiencies of the elderly. Careful wound care can speed up the stages of wound healing by keeping wounds moist, clean and protected from reinjury