What Causes Keloids: Definition, Mechanisms, and Risk Factors

Defining Keloids as a Distinct Clinical Entity

A keloid is a benign fibrous tumor that arises from an exaggerated healing response following skin injury. Unlike normal scars that remain within the boundaries of the original wound, keloids extend beyond the initial damage margins, invading surrounding healthy tissue. They present as raised, firm, rubbery nodules or plaques that range in color from flesh-toned to erythematous or hyperpigmented depending on skin phototype and lesion maturity.

The defining histopathological feature is the accumulation of abnormally thick, hyalinized collagen bundles arranged in a haphazard, whorled pattern. These bundles consist predominantly of type I and type III collagen deposited by hyperactive fibroblasts. The extracellular matrix also shows elevated levels of proteoglycans and glycosaminoglycans, contributing to the lesion's firm consistency and resistance to regression.

Clinically, keloids rarely regress spontaneously and tend to enlarge progressively over months to years. They may cause pruritus, tenderness, or a burning sensation, though many remain asymptomatic beyond their cosmetic impact. The diagnosis is primarily clinical, supported by the characteristic history of progressive expansion beyond the original wound margins and the typical histological findings on biopsy when confirmation is needed.

Cellular and Molecular Mechanisms Driving Formation

Keloid pathogenesis centers on a fundamental dysregulation of the wound healing cascade. In normal healing, the proliferative phase transitions to remodeling with balanced collagen synthesis and degradation. In keloid-prone individuals, this balance fails: fibroblasts exhibit prolonged activation, excessive proliferation, and resistance to apoptosis. These cells produce collagen at rates several times higher than normal dermal fibroblasts, while simultaneously downregulating matrix metalloproteinases that would normally degrade excess matrix.

Multiple signaling pathways contribute to this fibroblast phenotype. Transforming growth factor-beta (TGF-β) signaling, particularly the TGF-β1 and TGF-β2 isoforms, is markedly upregulated in keloid tissue. This cytokine drives fibroblast-to-myofibroblast differentiation, enhances collagen gene transcription, and suppresses collagenase expression. Concurrently, the insulin-like growth factor (IGF) pathway, platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF) are all overexpressed, creating a self-sustaining loop of cellular activation and angiogenesis.

Epigenetic modifications further entrench the abnormal phenotype. DNA methylation patterns and histone modifications in keloid fibroblasts silence tumor suppressor genes and activate profibrotic genes. MicroRNA dysregulation — particularly downregulation of miR-29 family members that normally target collagen genes — removes a critical brake on matrix production. These molecular alterations persist even when keloid fibroblasts are cultured in vitro, confirming an intrinsic cellular memory rather than solely microenvironmental influence.

Genetic Architecture and Hereditary Predisposition

A strong genetic component underlies keloid susceptibility. First-degree relatives of affected individuals have a significantly elevated risk, with concordance rates in twin studies supporting a polygenic inheritance pattern. Genome-wide association studies have identified susceptibility loci on chromosomes 2q23, 7p11, and 15q21, among others. These regions contain genes involved in immune regulation, extracellular matrix organization, and growth factor signaling.

Specific gene variants have been implicated across populations. Polymorphisms in the TGF-β1 gene promoter, particularly the -509 C/T variant, correlate with increased TGF-β1 production and keloid risk in multiple ethnic groups. Variants in the NEDD4 ubiquitin ligase gene, which regulates TGF-β receptor degradation, also associate with susceptibility. In African and African-descended populations, where keloid prevalence reaches 6-16%, distinct haplotype blocks around the SMAD3 and SMAD7 genes — intracellular mediators of TGF-β signaling — show strong association signals.

Despite these discoveries, no single gene determines keloid formation. The trait exhibits incomplete penetrance and variable expressivity: family members sharing risk alleles may develop keloids after minor trauma, remain unaffected, or form lesions only at specific anatomical sites. Environmental triggers, hormonal status, and epigenetic modifications interact with the genetic background to determine whether the pathogenic cascade initiates and progresses.

Inciting Trauma and Environmental Triggers

Virtually any breach of skin integrity can initiate keloid formation in susceptible individuals. Surgical incisions, burn injuries, acne vulgaris, chickenpox lesions, ear piercing, vaccination sites, and even minor abrasions or insect bites have all been documented as antecedent events. The severity of the initial trauma does not predict keloid development; minimal injuries frequently produce extensive lesions, while major surgeries may heal without abnormal scarring in the same patient.

Inflammation plays a pivotal permissive role. Prolonged or intense inflammatory responses — whether from infection, foreign material, or repeated mechanical irritation — amplify the profibrotic signaling milieu. Wounds that heal by secondary intention, those crossing joint lines subject to tension, and sites of chronic irritation such as acne-prone areas show disproportionately high keloid rates. The duration of inflammation correlates with risk; wounds taking longer than three weeks to epithelialize carry substantially greater likelihood of keloid transformation.

Mechanical tension on healing tissue acts as a potent cofactor. Areas subject to repeated stretching — the presternal region with respiratory movement, the deltoid with arm motion, the earlobe with jewelry weight — demonstrate the highest incidence. This mechanotransduction effect likely operates through integrin-mediated signaling that converges on the same TGF-β and focal adhesion kinase pathways activated in keloid fibroblasts. Silicone sheeting and pressure therapy, standard preventive measures, function partly by modulating these mechanical forces during the critical remodeling window.

Anatomical Distribution and Site-Specific Vulnerability

Keloids exhibit a striking anatomical predilection. The presternal chest, deltoid region, upper back, mandibular border, and earlobes account for the vast majority of cases. These regions share characteristics: thick dermis, high sebaceous gland density, frequent exposure to mechanical stress, and rich vascular supply. Conversely, keloids rarely form on the eyelids, genitalia, palms, soles, or across the anterior tibial surface despite similar trauma exposure.

The earlobe presents a unique case study. Earlobe keloids almost exclusively follow piercing, yet only a minority of pierced individuals develop them. The combination of cartilage proximity, constant low-grade tension from jewelry, and a dense fibroblast population creates a permissive microenvironment. Presternal keloids frequently arise from acne, chickenpox, or minor trauma, while deltoid lesions commonly follow vaccination or intramuscular injection. Each site's characteristic triggers inform both prevention strategies and treatment planning.

Bilateral symmetry is occasionally observed, particularly in earlobe and presternal keloids, suggesting systemic factors modulate site-specific expression. Multiple simultaneous keloids at different anatomical locations indicate a generalized diathesis rather than isolated local response. This distribution pattern helps clinicians distinguish keloid-prone patients from those with single reactive hypertrophic scars, guiding surveillance and prophylactic counseling.

Demographic, Hormonal, and Physiological Modifiers

Age exerts a profound influence on keloid risk. The peak incidence occurs between ages 10 and 30, coinciding with puberty and early adulthood. Children under 10 rarely develop keloids despite frequent trauma, while new-onset keloids after age 50 are uncommon. This age curve parallels hormonal fluctuations, particularly sex steroid levels. Androgens and estrogens both modulate fibroblast proliferation and collagen synthesis in vitro, and keloid tissue expresses hormone receptors at higher density than normal dermis.

Skin phototype strongly correlates with prevalence. Individuals with Fitzpatrick skin types IV-VI (darker skin tones) develop keloids at rates 15-20 times higher than those with types I-II. This disparity reflects both genetic ancestry and melanocyte-fibroblast interactions. Melanocytes secrete paracrine factors including endothelin-1 and stem cell factor that stimulate fibroblast activity and collagen production. The evolutionary conservation of this pathway may relate to enhanced wound healing under high ultraviolet exposure, with keloids representing a maladaptive extreme.

Pregnancy represents a distinct high-risk state. Established keloids frequently enlarge during gestation, and new lesions may arise at sites of prior trivial trauma. The surge in estrogen, progesterone, and corticosteroid levels alters immune regulation and fibroblast responsiveness. Postpartum regression is variable; some lesions stabilize while others persist. Thyroid dysfunction, particularly hypothyroidism, has also been associated with increased keloid tendency, though the mechanistic link remains under investigation.

Differentiating Keloids Within the Spectrum of Fibrotic Scarring

Keloids occupy the severe end of a continuum of abnormal scarring that includes hypertrophic scars and widespread fibrosis syndromes. The critical diagnostic boundary lies in spatial behavior: keloids extend beyond the original wound margins, often with claw-like projections into adjacent skin, while hypertrophic scars remain confined within the injury boundary. Temporally, hypertrophic scars typically appear within weeks of injury, plateau, and often regress over 12-18 months; keloids may arise months to years after the inciting event and continue expanding indefinitely without intervention.

Histologically, both entities show increased collagen and myofibroblast density, but keloids demonstrate the characteristic broad, eosinophilic, hyalinized collagen bundles absent in hypertrophic scars. Keloid tissue also exhibits higher microvessel density, greater mast cell infiltration, and distinct expression profiles for decorin, versican, and tenascin-C — extracellular matrix proteins that modulate fibroblast behavior. These molecular signatures provide research tools for classification but have not replaced clinical assessment in routine practice.

Understanding this spectrum matters for prognosis and management. Hypertrophic scars often respond to conservative measures including silicone gel, pressure therapy, and intralesional corticosteroids. Keloids typically require multimodal approaches combining surgical excision with adjuvant radiation, cryotherapy, or immunomodulatory agents, and carry substantial recurrence risk. Accurate classification at initial presentation sets appropriate expectations and guides therapeutic intensity.