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Wound Healing Physiology and Pathologic Scarring

Overview

Wound healing is a dynamic, coordinated process that involves cellular, humoral, and molecular mechanisms working in concert to restore tissue integrity after injury. For the surgeon, understanding the biology of normal wound healing is not merely academic -- it directly informs decisions about wound management, timing of interventions, and the prevention and treatment of complications including chronic wounds and pathologic scarring.

Phases of Wound Healing

Hemostasis (Minutes)

The moment a tissue is injured, the body's first priority is stopping the bleeding. Vascular damage exposes subendothelial collagen, which triggers platelet adhesion and aggregation at the wound site. As platelets degranulate, they release a cocktail of growth factors -- PDGF, TGF-beta, EGF, and thromboxane A2 -- that will orchestrate subsequent healing phases. Simultaneously, the coagulation cascade generates a fibrin clot that forms a provisional wound matrix. This scaffold is not just a physical barrier; it serves as the structural framework that allows inflammatory cells to infiltrate and begin the repair process.

Inflammatory Phase (Days 1-5)

Neutrophils are the first responders, arriving within 24 hours and predominating for approximately 48 hours. Their primary role is phagocytosis and bacterial killing through reactive oxygen species. An important nuance, however, is that neutrophils are not essential for wound healing in the absence of infection -- they are expendable if the wound is clean.

The truly indispensable cell arrives next. Monocytes and macrophages infiltrate the wound by 48 to 72 hours and are widely regarded as the single most critical cell type in the entire healing process. Macrophages undergo a phenotypic transition from M1 (pro-inflammatory) to M2 (reparative), and as they do so they secrete the growth factors -- TGF-beta, FGF, VEGF, and PDGF -- that drive the transition into the proliferative phase. They also coordinate extracellular matrix deposition and angiogenesis. Lymphocytes arrive later in this phase and help modulate the shift from inflammation to proliferation.

Proliferative Phase (Days 5-21)

This phase is defined by three concurrent processes. First, angiogenesis creates new capillary networks driven by VEGF and FGF. Second, fibroplasia brings fibroblasts into the wound, where they proliferate and begin depositing collagen -- initially type III collagen, which will later be replaced by the stronger type I collagen during remodeling. Some fibroblasts differentiate into myofibroblasts (identifiable by their expression of alpha-smooth muscle actin), which are responsible for wound contraction. Third, epithelialization occurs as keratinocytes migrate from wound edges and skin appendages across the moist wound surface beneath the scab. When the migrating epithelial edges meet, contact inhibition halts further migration. Together, these processes produce granulation tissue -- the characteristic combination of new capillaries, fibroblasts, macrophages, and loose connective tissue that fills a healing wound.

Remodeling Phase (Day 21 to 1+ Year)

The remodeling phase is the longest and most subtle. Type III collagen is progressively replaced by type I collagen through crosslinking and reorganization, governed by a balance between collagen synthesis and degradation by matrix metalloproteinases (MMPs). Cellularity and vascularity decrease as the wound matures. A critical fact for surgeons is that maximum wound tensile strength reaches only about 80% of unwounded skin -- full strength is never recovered. The timeline of strength recovery is clinically relevant: at one week, a wound has roughly 3% of its final strength; at three weeks, about 20%; and peak tensile strength is reached at approximately eight weeks.

Types of Wound Closure

Primary Intention (First Intention)

In primary intention healing, wound edges are directly approximated, as in a clean surgical incision. This results in minimal granulation tissue formation, the fastest healing, and the least scarring.

Secondary Intention (Second Intention)

When a wound is left open -- typically because it is contaminated or there is significant tissue loss -- it heals by granulation, contraction, and epithelialization. This approach results in greater scarring and a longer healing time, but it is the appropriate choice when primary closure would trap bacteria or create tension.

Tertiary Intention (Delayed Primary Closure)

In this hybrid approach, a wound is initially left open to allow contamination to clear over three to five days, then closed. This is commonly used for contaminated traumatic wounds and combines the benefits of initial drainage with the more favorable cosmetic and functional outcomes of primary closure.

Wound Healing by Tissue Type

Different tissues heal by different mechanisms and at different rates. Bone heals through callus formation that remodels into lamellar bone. Peripheral nerves undergo Wallerian degeneration distally, followed by axonal regeneration at roughly 1 mm per day. Tendons heal through both intrinsic (tenocyte-driven) and extrinsic (inflammatory cell-driven) pathways. Cartilage has very limited regenerative capacity because it is avascular. The liver is unique among solid organs in its remarkable regenerative capacity through hepatocyte proliferation.

Factors Impairing Wound Healing

Local Factors

The most significant local factor impairing wound healing is ischemia and hypoxia. Other local factors include infection (bacterial loads exceeding 10^5 organisms per gram of tissue overwhelm the wound's defenses), foreign bodies, radiation injury, venous insufficiency, and mechanical stress on the wound.

Systemic Factors

Diabetes mellitus impairs wound healing through multiple mechanisms including leukocyte dysfunction, microangiopathy, and neuropathy. Malnutrition is another major contributor: protein deficiency limits the raw material for repair, vitamin C deficiency (scurvy) impairs the hydroxylation of proline and lysine needed for collagen synthesis, and zinc deficiency disrupts cell proliferation. Immunosuppression from corticosteroids impairs both the inflammatory phase and collagen synthesis. Smoking causes vasoconstriction through nicotine and impairs oxygen delivery through carbon monoxide. Advanced age slows healing but does not qualitatively alter it. Obesity leads to relative hypoperfusion of subcutaneous tissue, and uremia impairs immune function.

Medications Affecting Healing

Corticosteroids impair all phases of healing, but their effects can be partially reversed by vitamin A supplementation (25,000 IU/day). Chemotherapeutic agents, particularly antimetabolites, interfere with cell division needed for repair. NSAIDs may impair the early inflammatory phase, though the clinical significance of this effect remains debated. Colchicine impairs microtubule function in fibroblasts. Bevacizumab (anti-VEGF) impairs angiogenesis and requires a six- to eight-week washout period before elective surgery.

Pathologic Scarring

Hypertrophic Scars

Hypertrophic scars are raised, erythematous scars that remain within the boundaries of the original wound. They typically develop within four to eight weeks of injury and are more common over joints and areas of mechanical tension. An important distinguishing feature is that hypertrophic scars often regress spontaneously over one to two years. Histologically, they show organized, parallel collagen bundles with nodular structures. Treatment options include pressure garments, silicone sheeting, intralesional corticosteroids, and laser therapy.

Keloids

Keloids extend beyond the original wound boundaries, which is the key distinction from hypertrophic scars. They are 15 to 20 times more common in darker-skinned individuals and tend to occur at characteristic sites: earlobes, the presternal area, deltoid, and upper back. Unlike hypertrophic scars, keloids do not regress spontaneously. Histologically, they show thick, disorganized, large collagen bundles known as "keloidal collagen." Excision alone carries a 50 to 80% recurrence rate, so treatment typically requires a combination approach: intralesional triamcinolone injection (first-line for small keloids), excision combined with adjuvant therapy (steroid injection, radiation, or pressure), silicone sheeting, cryotherapy, 5-fluorouracil injection, or radiation therapy (though radiation remains controversial due to the theoretical risk of malignancy).

Key Differences: Hypertrophic Scar vs. Keloid

Chronic Wounds

Chronic wounds are those that fail to proceed through the orderly healing phases within the expected timeframe, typically defined as longer than three months. The most common etiologies include pressure ulcers (sustained tissue compression over bony prominences, staged I through IV by the NPUAP), diabetic foot ulcers (driven by the triad of neuropathy, ischemia, and infection, classified by the Wagner system), venous stasis ulcers (the most common chronic lower extremity wound, characteristically in the gaiter distribution), and arterial insufficiency ulcers (painful, punched-out lesions in a distal location).

The pathophysiology of chronicity involves a self-perpetuating cycle: elevated matrix metalloproteinases (particularly MMP-1, MMP-8, and MMP-9) degrade growth factors and extracellular matrix, while a persistent inflammatory state populated by senescent fibroblasts prevents the transition to proliferative healing. Bacterial biofilm formation further impairs healing without necessarily reaching the threshold of clinical infection. The net result is reduced growth factor activity and stalled repair.

Wound Healing Adjuncts

Several adjunctive therapies can help wounds that are not progressing through normal healing. Negative pressure wound therapy (NPWT/VAC) removes exudate, reduces edema, increases tissue perfusion, and promotes granulation tissue formation. Hyperbaric oxygen therapy increases tissue oxygen tension, with the best evidence supporting its use for diabetic foot ulcers, compromised flaps, and chronic refractory osteomyelitis. Becaplermin, a recombinant PDGF-BB growth factor, is approved for diabetic foot ulcers. Skin substitutes, including acellular dermal matrices and bioengineered skin equivalents, provide scaffold and biological signals for healing. Platelet-rich plasma (PRP) has emerging but not yet definitive evidence supporting its use.

<image>Detailed medical illustration showing the four overlapping phases of wound healing (hemostasis, inflammation, proliferation, remodeling) depicted as a timeline diagram with key cellular players at each phase. Show platelets and fibrin clot in hemostasis, neutrophils and macrophages in inflammation, fibroblasts and new blood vessels in proliferation, and organized collagen in remodeling. Include a graph showing wound tensile strength over time reaching 80% of normal.</image>

<image>Cross-sectional histological comparison of normal scar, hypertrophic scar, and keloid. Normal scar showing organized parallel collagen fibers; hypertrophic scar showing nodular structures with whorled collagen bundles confined to wound borders; keloid showing thick, disorganized, large collagen bundles extending beyond the original wound margins into surrounding dermis. Label the epidermis, dermis, and collagen architecture.</image>

<image>Anatomical diagram of a chronic wound bed showing the pathologic microenvironment: elevated matrix metalloproteinases degrading growth factors, senescent fibroblasts, bacterial biofilm on wound surface, persistent inflammatory cells, and impaired angiogenesis. Contrast with an adjacent panel showing a normally healing wound bed with active granulation tissue, organized neovascularization, and migrating keratinocytes.</image>

Clinical Pearls

The macrophage is the single most important cell in wound healing. Wounds can heal without neutrophils in a clean wound, but they cannot heal without macrophages. Maximum wound tensile strength reaches only 80% of unwounded tissue, and this is never fully recovered. Vitamin A at 25,000 IU per day can partially reverse the wound healing impairment caused by corticosteroids, making it a practical clinical tool. Vitamin C deficiency (scurvy) impairs collagen crosslinking by inhibiting prolyl and lysyl hydroxylase. Zinc, a cofactor for RNA and DNA polymerase, is essential for cell proliferation, and its deficiency impairs epithelialization. Wounds in the face heal faster than wounds on the extremities due to the superior blood supply of the head and neck. Keloids virtually never occur on the eyelids, palms, soles, or genitalia. A serum albumin below 3.0 g/dL is associated with significantly impaired wound healing. Smoking cessation for at least four weeks before surgery significantly reduces wound complications. In mature scar, type I collagen constitutes approximately 80% of dermal collagen, while type III predominates early in healing.

References

• Gurtner GC, Werner S, Barrandon Y, Longaker MT. Wound repair and regeneration. Nature. 2008;453:314-321.
• Broughton G, Janis JE, Attinger CE. The basic science of wound healing. Plast Reconstr Surg. 2006;117(7S):12S-34S.
• Gauglitz GG, Korting HC, Pavicic T, et al. Hypertrophic scarring and keloids: pathomechanisms and current and emerging treatment strategies. Mol Med. 2011;17:113-125.
• Sen CK, Gordillo GM, Roy S, et al. Human skin wounds: a major and snowballing threat to public health and the economy. Wound Repair Regen. 2009;17:763-771.

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