Hair Extensions: Anatomic, Biological, and Clinical Implications in Alopecia

Background: Hair extensions are widely used for cosmetic and medical reasons, but durable attachment systems (thermopolymer, adhesive, mechanical, and temporary) can impose mechanical, chemical, and thermal stresses that disrupt scalp biology and precipitate hair loss. Objective: To synthesize anatomic, biological, and clinical evidence on hair-extension practices with emphasis on alopecia risk, diagnosis, prevention, and management. Methods: Narrative review (1990–2025) across dermatology and trichology literature covering biomechanics, barrier/microbiome, thermal injury, phenotypes, diagnostic imaging, prevention, and therapies. Results: Sustained traction remodels the perifollicular matrix, compromises microcirculation, shortens anagen, and promotes miniaturization with frontal/temporal predilection. Adhesives and removers can disrupt the stratum corneum and trigger irritant or allergic contact dermatitis. Occlusion elevates humidity/temperature, shifts pH/TEWL, and fosters dysbiosis. Early load reduction with anti-inflammatory and barrier-repair care reverses most early disease; advanced cases show perifollicular fibrosis with limited reversibility. Standardized wear cycles, anchor-load distribution, patch testing, hygiene/ventilation, and stylist–clinician collaboration reduce risk. Regenerative adjuncts (PRP and low-level laser) have supportive evidence in nonscarring alopecias; exosomes remain investigational. Conclusions: Extensions confer psychosocial benefit yet impose nontrivial biological costs. A prevention-first, medical model—integrating risk stratification, standardized technique, early surveillance, and tiered therapy—can minimize iatrogenic alopecia and maintain scalp health.