Q-switched Nd:YAG lasers have been used to target topically applied carbon particles that have previously been applied to the hair follicle. This method was one of the first available laser hair removal techniques. This short term hair removal technique has also been used without the prior application of carbon.
Immediately after Q-switched 1064-nm laser irradiation of carbon coated hairs, the carbon is heated to its vaporization temperature of about 3,700 °C. Vaporization leads to a huge volume expansion with resultant supersonic proliferation of high pressure waves. These shock waves, in turn, produce mechanical damage, as well as the development of heat. It is not clear how much mechanical and/or heat energy produced by this mechanism is required for destruction of a hair follicle. However, histo-logic evidence of follicular damage is seen after such a laser exposure. This results in a clinical delay of hair growth.
Depending on the position and amount of the topically applied chromophore, as well as the energy administered, it may be possible to occasionally irreversibly damage a hair follicle even with a Q-switched laser.
Histologic studies have documented the presence of carbon in the follicle after low flu-ence Nd:YAG lasing. This carbon appears to penetrate superficially in a large number of follicles with and without a hair shaft in place; deep follicular penetration is rare. The disadvantages of this technique, therefore, appear to relate to the fact that that the carbon granules may not consistently reach the requisite hair bulge or bulb.
Different studies have compared the effectiveness of Q-switched Nd:YAG laser hair removal with ruby and alexandrite laser treatments. Millisecond pulse ruby and alexandrite lasers showed greater hair reduction than was seen with Q-switched Nd:YAG lasers. Relatively weak absorption by the innate target chro-mophore melanin of Q-switched Nd:YAG laser energy translates into less energy available to damage the follicle. Therefore, a lesser hair removal effect is seen.
Several studies have examined the 1064-nm Q-switched Nd:YAG laser with and without a topically applied chromophore. However, in one controlled study (Nanni et al. 1997), using a single Q-switched Nd:YAG laser treatment, 100% hair regrowth was observed at 6 months irrespective of the treatment. Although capable of inducing delayed regrowth, Q-switched Nd:YAG laser treatment appears to be ineffective at producing long-term hair removal.
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