MELANIN ABSORPTIONSPECTRUMLasersfor hair reduction arebased on the theory of selectivephotothermolysis, which implies asite-specific, thermally mediated injuryof microscopic tissue targets by theselective absorption of pulses ofradiation by the chromophores1.The chromophore employed in laser hair reduction is melanin1. Although the absorption of melanin corresponds towavelengths from 400 to 1200 nm, the best lasers for hair removal have wavelengths between 600 and 1100 nm, arange where there is more selective absorption by melanin and the light penetrates relatively deeply into the skinwithout dissipation to surrounding tissues2.PRECISE IMPACT ON HAIR GROWTHPhases of active hair growth, or anagen, are separated by periods of quiescence, or telogen. Only anagen hairs areparticularly sensitive to physical insults3. The mechanism of action of laser hair removal is reflected in the immediatehistologic changes in the skin, as well as its effects on the hair growth cycle4.Anagen phaseTreatmentCatagenCLUB HAIRActive growth phase(3-6 years)Laser treatmentTransition phase (1-2 weeks)TelogenSECONDARYGERM CELLSDERMALPAPILLAReturn to AnagenHAIR MATRIX FORMINGNEW HAIRResting phase (1-2 weeks)MicroscopicallyHistologicallyTreated follicles display immediate changes ofMost follicles are in the telogen phase 1 month after treatment,keratinocyte swelling, scattered apoptotic and necroticwhereas fibrosis with a foreign body giant cell reactionkeratinocytes, and full-thickness necrosis of the folliclesreplaces others4. There is a period of alopecia lasting fromdepending on the amount of energy absorbed4.several weeks to a few months until a portion of hair folliclesrecover and commence another anagen cycle4.
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