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Phases of Wound Healing

The body uses a systematic approach for healing a break in the skin, the largest organ of the body. It is the first line of defense against environmental organisms. Once broken, our bodies are at risk for fluid loss and invasion of opportunistic invaders.

4 Phases of Wound Healing

There are four phases to wound healing:

Phase 1- Hemostasis

This phase starts as soon as the wound occurs. Blood vessels constrict and platelets (clotting factor in the blood) are activated and sent to the injury site to stop blood loss. Once platelets are activated, they release growth factors and cytokines (control wound healing). A fibrin structure is created by the platelets which help cells migrate during this phase. This phase lasts from 1-2 days.

Wound Healing

Phase 2- Inflammation

The cytokines and growth factors released in phase 1 start the inflammatory phase. The fibrin matrix has been created and the bleeding has stopped.

The goal of this phase is to clean any foreign material and dead tissue from the wound and control bacteria that may have entered the wound. White blood cells (WBC) - specifically, neutrophils and monocytes - are sent to the wound. The neutrophils destroy bacteria. Once destroyed, macrophages - another component of the WBC - are created by the monocytes. These macrophages eliminate dead bacteria and cellular debris. Macrophages release additional cytokines that help transition into the third phase of wound healing, proliferation.

The inflammatory phase overlaps with the next, proliferation phase. Macrophages play a key role in the early phases of repair. Impaired macrophage activity will delay wound healing, often keeping the wound in the inflammatory state.

In normal wound healing, the inflammatory phase typically lasts from day 1 to day 10.

 

Phase 3 – Proliferation

This phase is also known as the regeneration phase. The cytokines in the inflammatory phase along with growth factors signal fibroblasts to enter the area of injury and generate connective tissue proteins for building granulation tissue. Granulation tissue is also known as the extracellular matrix (ECM).

Fibroblast can normally be found in the undamaged dermal layer of the skin. The largest amount of connective tissue protein in the body is collagen. This builds the scaffolding for new blood vessels and types III collagen. Endothelial progenitor cells in the bone marrow are called to the site to help with blood vessel formation. This is called angiogenesis. The angiogenesis is quite active during this phase and the thickness of the blood vessels is greater than in normal tissue.

Type III collagen is not strong and will convert to collagen type I once the scarring is complete. This will increase the tensile strength of the scar. Full-thickness wounds that heal by secondary intention (from the bottom up) will contract. Special fibroblast cells called myofibroblasts have components of smooth muscle. These cells create a stronger pull, contracting the edges of the wound together, closing the defect in the skin. The color of the scar formation is red.

This phase lasts between 8 and 30 days of the wound.

Phase 4 – Maturation/Remodeling

As the collagen is replaced from type III to type I, the tensile strength of the tissue at 21 days is low. It will increase to 60% about 2-3 months after wound closure. The maximum strength that can be achieved by the end of 1 year is 80%. The thick blood vessels decrease, and most will disappear. The red scar will eventually become pale pink in color as the blood vessels diminish. Contraction of the wound continues to decrease the size of the defect.

Sometimes the body continues to produce new collagen and fails to break down type III collagen, causing a hypertrophic or keloid scar. This is a scar that is raised and overgrown and is reddish to deep pink in color.

Remodeling begins about day 17 and can last up to 2 years.

The time frame for each phase of healing is dependent on normal progression without complication. Wound healing can be delayed based on the body’s failure to manage each phase. Factors that delay wound healing include:

If these factors delay healing, the underlying condition must be resolved for the wound to progress.

The body uses three mechanisms for healing. Granulation tissue formation is when the connective tissue is deposited into the wound bed to fill the defect. Contraction pulls the granulation together to close the defect. Epithelialization is the growth of epithelial cells (skin) over the defect.

 

Healing by Primary Intention

Surgical wounds that have been sutured or stapled closed heal by primary intention. Epithelial cells close the small gaps in the skin. Granulation will still occur below the surface. No contraction is needed because the surgeon reconnects all layers of the wound.

Healing by Secondary Intention

Full-thickness wounds such as a pressure ulcer or dehisced (opened) surgical wound require granulation, contraction, and epithelialization to occur. These wounds will have a larger scar than those that heal by primary intention and epithelialization requires a larger surface area.

Healing by Tertiary Intention

This type of healing combines secondary and primary intentions. It is reserved for wounds that are infected or have been contaminated with large amounts of debris. The wound is initially left open to heal by secondary intention, allowing for antibiotic therapy to be completed, decreasing infection and bacteria within the wound. Once infection or bacteria load has improved, the wound can then be closed by primary intention.

 

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