Wound Healing and Repair

Phase 1: Hemostasis (Immediate - Minutes)

• Platelet aggregation and fibrin clot formation establish primary hemostasis; thrombin activates coagulation cascade and converts fibrinogen to fibrin
• Platelets become trapped in fibrin mesh, forming provisional matrix that serves as scaffold for subsequent cellular infiltration
• Platelet activation releases tissue factor (TF) and phosphatidylserine, amplifying coagulation cascade
• Platelets degranulate, releasing platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-β), and von Willebrand factor (vWF)
• Thrombin activates protease-activated receptors (PARs) on fibroblasts and endothelial cells, initiating inflammatory cascade

Phase 2: Inflammation (0-3 Days, Overlaps with Hemostasis)

• Neutrophil infiltration begins within hours; mediated by C5a complement fragments, TNF-α, and IL-8 chemotaxis
• Neutrophils perform critical functions: bacterial killing via NADPH oxidase (respiratory burst), debridement of necrotic tissue, release of proteases (collagenase, elastase) for ECM remodeling
• By day 2-3, macrophages become dominant cell type; derived from recruited monocytes via CCL2/MCP-1 and macrophage colony-stimulating factor (M-CSF)
• Macrophages shift from pro-inflammatory (M1) to pro-healing (M2) phenotype, secreting IL-10, TGF-β, VEGF, FGF—critical for angiogenesis and fibroblast recruitment
• Endothelial cells upregulate adhesion molecules (ICAM-1, VCAM-1); vascular permeability increases via bradykinin, histamine, leukotrienes
• Lymphocytes (particularly T-helper cells) appear by day 3, producing IFN-γ and cytokines supporting fibroblast activation

Phase 3: Proliferation (3-21 Days)

• Fibroblast recruitment and activation driven by TGF-β1 (most potent), PDGF, FGF-2, and CTGF (connective tissue growth factor)
• Fibroblasts exhibit phenotypic switch to myofibroblasts (expressing α-SMA/alpha-smooth muscle actin), characterized by enhanced contractility via stress fiber formation and increased matrix deposition
• Angiogenesis occurs via VEGF-A, FGF-2, Ang-1/Ang-2 signaling; involves endothelial sprouting, migration, and tube formation
• Collagen deposition peaks by day 5-7; Type III collagen (immature, loose-knit) predominates initially, gradually replaced by Type I (mature, cross-linked)
• Epithelialization proceeds from wound margins via keratinocyte migration (loss of E-cadherin, upregulation of N-cadherin and integrins), driven by EGF, HGF, KGF
• Matrix metalloproteinases (MMPs) (especially MMP-2, MMP-9) facilitate ECM degradation and remodeling; balanced by tissue inhibitors of metalloproteinases (TIMPs)
• Neovascularization creates "red granulation tissue" characteristic of healthy proliferating wounds—abundant blood vessels, myofibroblasts, fibroblasts, macrophages

Phase 4: Remodeling/Maturation (21 Days - 1-2 Years)

• Collagen remodeling involves cross-linking via lysyl oxidase-catalyzed reactions, increasing tensile strength
• Myofibroblast apoptosis mediated by TGF-β withdrawal and FasL signaling, allowing wound contraction (normal process) while preventing excessive scarring
• Angiogenesis reversal: excess capillaries regress; scar loses red color, becoming pale and flat
• ECM reorganization: collagen fibers align along tension lines via mechanotransduction
• Tensile strength improves: ~70% of original strength by 3 weeks, ~80% by 3 months; never reaches 100% (maximum ~85% due to Type III collagen persistence)
• Apoptosis of endothelial cells and fibroblasts; protease-antiprotease balance shifts toward inhibition

Intrinsic Factors Affecting Wound Healing

• Advanced age: diminished inflammatory response, reduced angiogenic capacity, delayed fibroblast function, slower epithelialization
• Malnutrition: deficiencies in protein (impaired collagen synthesis), Vitamin C (defective hydroxylation of proline/lysine), Vitamin A (reduced epithelialization, macrophage function), zinc (protease cofactor), iron (collagen cross-linking)
• Diabetes mellitus: impaired neutrophil chemotaxis and bacterial killing, reduced angiogenesis via AGE-induced endothelial dysfunction, microvascular disease
• Immunocompromise: HIV/AIDS, chemotherapy, corticosteroid use (suppresses inflammatory phase)
• Chronic diseases: hepatic dysfunction (reduced albumin/clotting factors), renal failure (uremia), malignancy
• Genetic conditions: Ehlers-Danlos syndrome (collagen defects), osteogenesis imperfecta (Type I collagen mutations)
• Smoking: causes vasoconstriction, reduces oxygen delivery, impairs angiogenesis via reduced VEGF

Extrinsic Factors

• Infection: bacterial colonization (especially >10^5 organisms/gram tissue) triggers excessive inflammation, protease release, tissue destruction
• Hypoxia: inadequate oxygen limits collagen hydroxylation, angiogenesis, oxidative bacterial killing
• Poor perfusion: atherosclerosis, peripheral vascular disease, vasculitis
• Foreign body/debris: sutures, particles—persistent inflammatory stimulus preventing resolution
• Mechanical disruption: excessive motion, tension, poor immobilization
• Edema: impairs oxygen/nutrient diffusion
• Radiation therapy: endothelial damage, fibrosis, impaired healing
• Medications: NSAIDs (inhibit PGE2, reduce inflammation), anticoagulants (increase hematoma)

Primary Intention (Clean Surgical Wounds)

• Minimal inflammatory response; wound edges tightly approximated
• Rapid epithelialization (24-48 hours)
• Minimal scar formation
• Healing by primary intention involves minimal granulation tissue, epithelial continuity restored within days

Secondary Intention (Contaminated/Open Wounds)

• Significant inflammatory exudate; erythema, warmth, edema
• Granulation tissue formation prominent (red, beefy, granular appearance)
• Slower epithelialization (weeks to months); contraction reduces wound size by 0.6-0.75 mm/day
• Increased scarring and potential deformity

Cardinal Symptoms of Normal Healing

• Inflammation: erythema, warmth, edema, exudation (peaks day 2-3, gradually resolves)
• Pain: peaks around day 2-3 as inflammatory mediators accumulate; improves with re-epithelialization
• Sensation of tightness/tension: reflects myofibroblast contraction and collagen deposition

Physical Exam Findings

• Wound margin blanching (good perfusion) vs. cyanosis/pallor (poor perfusion)
• Granulation tissue: friable, red, bleeding easily (normal finding in secondary wounds)
• Epithelialization: epithelial "bud" appearing at wound margins (pink, raised)
• Tissue turgor and elasticity: reflects hydration and collagen content

Delayed/Impaired Healing Signs

• Increased serous/purulent drainage beyond day 3-5 (suggests infection)
• Necrotic eschar/black tissue (indicates non-viable tissue)
• Pale, dull granulation tissue (suggests hypoxia or infection)
• Undermining/sinus tract formation (indicates fluid collection, inadequate drainage)

Clinical Assessment

• Wound inspection: color (red healthy, yellow slough, black necrotic), drainage (serous vs. purulent), odor, epithelialization progress
• Measurement: document length × width × depth; use photography for objective tracking
• Surrounding skin: assess for cellulitis (erythema, warmth, induration), edema, maceration (softening from moisture)
• Pain assessment: localized vs. widespread; distinguishes normal inflammation from infection

Histopathology

• Early inflammatory phase (0-3 days): fibrin clot, abundant neutrophils with intact nuclei, minimal fibroblast infiltration
• Mid-proliferative phase (3-7 days): granulation tissue characterized by:
• Capillary sprouts (new blood vessels with plump endothelial cells)
• Fibroblasts and myofibroblasts (spindle-shaped cells with eosinophilic cytoplasm)
• Abundant mucopolysaccharide matrix (loose, basophilic)
• Chronic inflammatory infiltrate (macrophages, lymphocytes)
• Minimal collagen initially
• Late proliferative/early remodeling (2-3 weeks): increased collagen deposition (pink, wavy appearance), organization of granulation tissue, reduced vascularity
• Remodeling phase (>3 weeks): dense collagen (Types I and III), aligned collagen bundles, minimal cellularity, few vessels

Microbiology

• Wound culture: quantitative culture >10^5 organisms/gram tissue indicates significant colonization
• Gram stain: identifies bacteria morphology
• Anaerobic culture: for deep/foul-smelling wounds

Labs

• Albumin <2.5 g/dL: indicates severe malnutrition, predicts poor healing
• Prealbumin: more sensitive marker of acute nutritional status
• Hemoglobin <7 g/dL: impairs oxygen-carrying capacity
• Blood glucose: glycemic control critical in diabetics
• Wound fluid analysis (rarely): cytology may show excessive inflammatory cells in chronic wounds

Imaging

• X-ray: identifies foreign body, gas (suggesting anaerobic infection), bone involvement
• Ultrasound: assesses fluid collections, sinus tract depth
• MRI: superior for soft tissue detail, abscess identification in deep wounds

Gross Pathology Appearance

• Healthy healing wound: red/pink granulation tissue, minimal purulent drainage, clean wound base, epithelialization at margins
• Infected wound: yellow/green exudate, friable tissue, foul odor, surrounding cellulitis
• Chronic wound: pale/dull granulation, may show slough (dead tissue), eschar (thick necrotic crust)

Diagnostic Criteria

• Clinical infection: ≥2 of following: erythema, warmth, edema, purulent drainage, pain disproportionate to wound appearance
• Colonization vs. infection: colonization is bacterial presence without clinical signs; infection causes clinical inflammation
• Critical colonization: high bacterial burden (10^5-10^6) without overt signs—early stage of infection

Principles of Wound Care

First-Line Management (Supportive Care)

• Wound cleansing: gentle saline irrigation removes debris, bacteria, exudate; avoid cytotoxic antiseptics (povidone-iodine, chlorhexidine > certain concentrations inhibit fibroblasts)
• Debridement: removal of non-viable tissue essential for healing progression
• Mechanical: gauze with saline (non-selective, can damage healthy tissue)
• Enzymatic: collagenase, papain (selective for necrotic tissue)
• Autolytic: moist dressings promote endogenous protease activity
• Surgical: sharp debridement for large necrotic areas or urgent infection
• Moist wound environment: maintains hydration, promotes epithelialization, facilitates autolytic debridement; use transparent films, hydrogels, hydrocolloids based on drainage
• Infection control:
• Systemic antibiotics for spreading cellulitis (cover S. aureus, Streptococcus initially; adjust based on culture)
• Topical antibiotics: bacitracin, mupirocin for prevention; limited value in heavily colonized wounds
• Antiseptic dressings: silver, iodine-containing dressings for heavily contaminated wounds
• Dressing selection:
• Low-exudate wounds: petroleum gauze, transparent film, hydrogel
• Moderate exudate: foam dressing, alginate
• High exudate: calcium alginate, absorptive foam
• Malodorous wounds: charcoal dressings, anaerobic antimicrobial agents

Nutritional Optimization

• Protein supplementation: 1.5-2.0 g/kg daily (normally 0.8 g/kg); essential for collagen synthesis
• Vitamin C: 500-1000 mg daily (cofactor for prolyl hydroxylase); particularly important if deficient
• Vitamin A: 10,000-25,000 IU daily (enhances macrophage function, epithelialization); especially beneficial if on steroids
• Zinc: 15-30 mg daily (protease cofactor); deficiency impairs healing
• Iron: ensure adequate stores for collagen cross-linking
• Glucose control: target HbA1c <7% in diabetics; tight glycemic control improves leukocyte function, reduces infection risk

Management of Specific Complications

Infection

• Systemic antibiotics for signs of spreading infection (cellulitis, fever, lymphangitis)
• Empiric coverage: MRSA-coverage agent (vancomycin, linezolid) + beta-lactam (ceftriaxone) pending culture
• Adjust based on culture/susceptibilities
• Duration typically 7-14 days for localized infection; longer for deep/osteomyelitis
• Increased dressing frequency (daily or more)
• Surgical drainage of abscess
• Necrotizing fasciitis: emergent surgical debridement + broad-spectrum antibiotics (life-threatening)

Hypoxia/Poor Perfusion

• Vascular assessment: ankle-brachial index (ABI), doppler ultrasound
• Vascular intervention: angioplasty, bypass grafting for critical ischemia
• Hyperbaric oxygen: 90-120 minutes at 2.4-3.0 atm; enhances angiogenesis, fibroblast function (FDA approved for diabetic foot ulcers)
• Topical oxygen therapy: limited evidence, may benefit select wounds

Excessive Inflammation/Chronic Wounds

• Steroid reduction: minimize corticosteroid use if possible (impairs wound healing)
• Immunosuppressive optimization: avoid excessive immunosuppression
• Anti-inflammatory agents: topical calcineurin inhibitors (tacrolimus) may benefit chronic wounds
• Advanced wound therapies: see below

Emergencies — act before imaging

• Necrotizing soft tissue infection: bacterial spread along fascial planes with thrombosis of perforating vessels produces ischemic overlying skin; signaled by pain out of proportion, rapidly advancing erythema, bullae, crepitus, "dishwater" gray discharge, and systemic toxicity. The IDSA skin and soft tissue infection guideline calls for emergent surgical exploration and debridement plus broad-spectrum antibiotics, with clindamycin added to suppress streptococcal/staphylococcal exotoxin production. Imaging must never delay the operating room.
• Fascial dehiscence with evisceration: failure of the fascial closure while the wound still has low tensile strength (classically postoperative days 5–10, the trough between inflammation and collagen cross-linking). Heralded by sudden salmon-colored serosanguinous drainage. Next step: cover bowel with saline-moistened sterile gauze, keep patient NPO, emergent operative repair.
• Hemorrhage: erosion of granulation tissue into an underlying vessel or graft, a recognized hazard of negative-pressure wound therapy placed over exposed vessels or anastomoses.

Impaired-healing complications

• Surgical site infection: bacterial burden overwhelms neutrophil clearance, proteases degrade new matrix; purulence, spreading erythema, or fever after day 3–5. CDC surgical site infection prevention guidance emphasizes normothermia, glycemic control, and appropriate prophylaxis timing.
• Chronic wound / sinus tract / enterocutaneous fistula: persistent stimulus (foreign body, ischemia, radiation) locks the wound in inflammation with high MMP and low TIMP activity; undermining edges and pale granulation are the tell.
• Incisional hernia and intra-abdominal adhesions: late scar failure or fibrous bands; adhesions are the leading cause of small bowel obstruction after laparotomy.

Excessive-healing complications

• Keloid, hypertrophic scar, and contracture: unchecked TGF-β signaling with failed myofibroblast apoptosis; contractures across joints or in burn patients cause functional loss.
• Marjolin ulcer: squamous cell carcinoma arising in a chronic wound or burn scar; a heaped-up, non-healing nodule at the margin demands biopsy, not another dressing change.

Treatment-related

• Antibiotic toxicity: vancomycin nephrotoxicity (dose to 24-hour AUC/MIC 400–600 per the 2020 IDSA/ASHP consensus) and Clostridioides difficile colitis.
• Hyperbaric oxygen: middle-ear barotrauma, pneumothorax, CNS oxygen-toxicity seizure.
• Debridement of pyoderma gangrenosum: pathergy enlarges the ulcer — the classic iatrogenic trap.

• The macrophage is the master cell of repair: it dominates by day 3, switches M1→M2, and supplies TGF-β and VEGF. A stem describing failed transition out of the inflammatory phase is pointing at macrophage dysfunction, not neutrophil deficiency.
• Vitamin C is the cofactor for prolyl and lysyl hydroxylase: deficiency blocks collagen triple-helix stabilization → perifollicular hemorrhage, corkscrew hairs, bleeding gums, and wounds that reopen. Copper is the cofactor for lysyl oxidase (cross-linking) — think Menkes disease. Zinc is a metalloproteinase cofactor; deficiency gives delayed healing with perioral/acral dermatitis (acrodermatitis enteropathica).
• Keloid vs hypertrophic scar is the single most tested association: a keloid extends beyond the original wound borders, favors earlobes, sternum, and shoulders, is more common in darker-skinned patients, and recurs after excision — first-line management is intralesional corticosteroid rather than simple excision. A hypertrophic scar stays within the wound margins and often regresses.
• Best next step for a postoperative wound with sudden serosanguinous drainage: inspect the fascia and take the patient to the OR if it has separated. Do not order a CT first if bowel is visible.
• Steroids impair healing by suppressing the inflammatory phase and fibroblast activity; vitamin A is the classic agent said to partially reverse this effect. NSAIDs blunt healing too — a frequent "which medication delays healing" answer.
• Tensile strength never returns to baseline; a sutured wound is weakest in the first week or two, which is why sutures are removed only after epithelial closure and why early strenuous activity causes dehiscence.
• Common distractor — granulation tissue is not granulomatous inflammation. Granulation tissue is capillary sprouts, fibroblasts/myofibroblasts, and loose matrix; granulomas are epithelioid macrophage collections. Examiners deliberately swap these words.
• Chronic wound that will not close: think ischemia (check ankle-brachial index), unrecognized osteomyelitis, retained foreign body, or malignant transformation (Marjolin ulcer) before escalating dressings. ADA Standards of Care support glycemic optimization and structured foot care in diabetic ulceration.