Two important factors in determining how bad a burn is include how hot the skin gets and how long the burn lasts. The location is also important because skin varies in thickness, water and oil content, the amount of subcutaneous fat, and the number of blood vessels from one location in the body to another.




The seriousness of a burn injury depends on how deep the injury is and how much of the body has been burned. It is common for a person with a large burn injury to have burns of different depths. The deepest injury is usually at the center of a burned area. There are a number of tests used to determine the depth of a burn injury, but an experienced burn specialist’s examination continues to be the most reliable way of evaluating the depth of burns.


A potentially dangerous condition that some burn victims experience is damage to the pulmonary system so they can’t breath. Inhalation injury is the most common cause of death in burn victims. Flash burns often harm the face but rarely involve the airway, unlike severe burns from prolonged heat exposure associated with smoke inhalation.




The skin has several layers that include the Epidermis, the Dermis and the Subcutaneous layer. The deeper the burn injury is, the greater the number of layers that are damaged. Sweat glands and the roots of hair follicles are in the deeper layers and will be destroyed with a deep burn. Deep injuries heal more slowly, are more difficult to treat, and are more likely to have complications than superficial injuries.




The names given to burn injuries of various depths have changed in recent decades. Injury to the top layer of skin (the epidermis) is now called a superficial burn (formerly called a first-degree burn). Injury to the second layer of skin (the dermis) is now called a partial thickness or dermal injury (formerly called a second-degree burn). An injury that extends down to the third layer (the subcutaneous tissue, which includes fat) is called a full thickness injury (formerly called a third-degree burn). Burns that damage muscles underneath the subcutaneous skin layer are described as full thickness burns with injury to the underlying muscle (sometimes formerly called fourth degree burns).


Superficial burns normally heal within 5 to 7 days. A common type of superficial burn is a sunburn. Because the top layer of skin (the epidermis) is thin (about the thickness of a piece of paper), it is easily replaced. Even when the skin is not injured, the skin completely replaces the epidermis every 45 to 75 days. Healing from a superficial burn usually occurs without scarring, although there may be some permanent discoloration.


The dermis is 15 – 40 times thicker than the epidermis. As a result, the seriousness of a partial thickness (or dermal) burn depends on how much of the dermis has been injured. A deep and large partial thickness burn will usually be treated with skin grafting. Partial thickness burns usually leave scars.


A full thickness burn destroys all three layers of skin, resulting in the loss of not only the skin but also the hair follicles, sweat glands, and the region where new skin cells are formed. For these reasons, full thickness burns require skin grafts. Fourth-degree burns extend through the skin into underlying tissues – ligaments and muscle – and are often life-threatening.




Different systems have been developed to estimate the percentage of total skin (total body surface area (TBSA) that has been burned. One of the older systems is known as the Rule of Nines, which is based on the rough approximation that each arm has 9% of the body’s total skin, the head 9%, each leg 18% (two 9’s), the front of the torso 18%, the back of the torso 18%, and the neck 1%. (see image above)


When partial thickness or full thickness burns exceed thirty percent of an adult’s total body surface area, it is usually necessary to perform grafts in stages because the patient does not have enough healthy skin to graft the burned area in a single operation.


Around a burn is the necrosis zone, the stasis zone, and the hyperemia zone. Inadequate blood flow causes the zone of stasis to become necrotic; adequate fluid resuscitation and proper treatment are needed to enable the zone of stasis to heal.


The etiology of burn injuries are different in different countries; people in less developed countries experience burns more often and about half of all burn injuries worldwide occur in Southeast Asia. Women are more apt to be burned in cooking accidents while men are more prone to industrial accidents.




The ultimate success of a skin graft, or its “take,” depends on nutrient uptake and vascular ingrowth from the recipient bed, which occurs in 3 phases.


The first phase takes place during the first 24-48 hours. The graft is initially bound to the recipient site through the formation of a fibrin layer and undergoes diffusion of nutrients by capillary action from the recipient bed by a process called plasmatic imbibition.


The second phase involves the process of inosculation, in which the donor and recipient end capillaries are aligned and establish a vascular network.


Revascularization of the graft is accomplished through those capillaries as well as by ingrowth of new vessels through neovascularization in the third and final phase, which is generally complete within 4-7 days.


Reinnervation of skin grafts begins approximately 2-4 weeks after grafting and occurs by ingrowth of nerve fibers from the recipient bed and surrounding tissue. Sensory return is greater in full-thickness grafts because they contain a higher content of neurilemmal sheaths. Similarly, hair follicles may be transferred with a full-thickness graft, which allows the graft to demonstrate the hair growth of the donor site. Split-thickness grafts are ultimately hairless.


The amount of dermis present in the graft determines the degree of contraction immediately after harvest from the donor site and following placement and revascularization in the recipient bed. Freshly harvested grafts undergo immediate recoil as a result of elastin in the dermis in a phenomenon termed primary contraction. Therefore, a full-thickness skin graft contracts more initially following harvest as it contains the dermis in its entirety. Secondary contraction is likely due to myofibroblast activity and is defined as the contraction of a healed graft. The degree of secondary contraction is inversely related to the thickness of the skin graft.


Accordingly, split-thickness skin grafts contract more than full-thickness grafts following placement in the recipient bed. For that reason, full-thickness grafts are preferably used in areas that would be significantly impacted functionally or aesthetically by scarring or scar contracture, such as the head and neck, hands, genitals, or breast.