This post is to discuss a research letter by Dr. MICHAEL JOHN MURPHY entitled “Q-switched 532nm laser energy causes significant vascular damage in the capillary plexus – how does this affect laser tattoo removal?” to b published in the British Journal of Dermatology. This research letter discusses the using of pressing technique to minimize the side effects of using the laser for tattoo removal.
Tattoos can be effectively removed using Q-switched and picosecond lasers at four wavelengths – 1064, 755, 694 and 532nm. This mainly depends on the photoacoustic properties of the extremely short pulse width. The intensive fibration will break the ink particle into small particles, and the body macrophage will start cleaning them. However, there are two particular problems with the 532nm line. Firstly, it is well absorbed by the melanin in the epidermis, due to its relatively high absorption coefficient5, (μa_mel = 56 cm-1 for typical Caucasian skin). Secondly, 532nm is also strongly absorbed in the hemoglobin located in the capillary plexus5 (μa_HbO = 260 cm-1).
In this small study, Dr. Murphy compared the effects of Q-switched pulses using all four of the above wavelengths on the non-tattooed skin. In particular, the effects of absorption in the blood layer were studied. The results indicate that treatments with 532nm may be slightly more complicated than first thought.
A Lynton Q+ Ruby/Nd: YAG laser generated the 532nm, 694nm, and 1064nm wavelengths while a Candela Trivantage Q-switched alexandrite laser was used to generate the 755nm wavelength.
The author subjected himself to these tests (Caucasian, Fitzpatrick type 2). Both dorsal forearms were irradiated – the right forearm was treated with all four wavelengths at 10 J/cm2 in 3 mm diameter spots, except for the 532nm wavelength which was set at its maximum output of 5.5 J/cm2. The left forearm was treated with 532nm and 1064nm at 5.5 J/cm2 in 3 mm spots, to compare them directly at the same radiant exposure (fluence).
The ‘glass slide technique’ was discussed by the author in 20146. This technique comprises the compression of a tattooed skin site by a standard microscope glass slide through which the laser energy is delivered.
Half of the irradiated areas were compressed with a glass slide, while the other half were treated directly.
The difference between the sites irradiated with 1064, 755 and 694nm wavelengths and the 532nm wavelength was marked. The 532nm sites all instantly displayed the tell-tale ‘whitening’ or ‘frosting’ often seen during laser tattoo removal treatments, much more so than with the other wavelengths. Note that there was no tattoo ink present in any of the above sites and no blood appeared on the skin surface. It is likely that the whitening appearance is mostly due to absorption of the 532nm laser energy by the melanin in the epidermis
Twelve minutes after irradiation the initial whitening had faded significantly. However, three hours after irradiation there was a clear difference between the compressed spots and the non-compressed spots. The compressed areas showed significantly less erythema and edema than the uncompressed set.
Figure 1 shows the extent of this erythema with a marked rise in the blood-filled spots 48 hours after irradiation. Clearly, there is a significant difference between the two sets of spots.
At 48 hours there appeared to be only a very marginal difference between the uncompressed and compressed sets of irradiated laser spots in 1064, 755 and 694nm wavelengths. Only the 532nm spots show any obvious difference, with significantly more vascular damage occurring in the uncompressed skin regions. This is due to the relatively low absorption coefficients for 1064, 755 and 694nm compared with 532nm in blood5.
This indicates that much of the incident 532nm laser energy is absorbed by the blood layer leaving significantly less energy available to deeper levels, where large amounts of tattoo ink may be located.
The use of 532nm is commonplace in laser tattoo removal. The cumulative absorption of this wavelength in both melanin and blood reduces the total amount of energy which can reach the reticular dermis, while also mechanically damaging melanin granules and capillary vessels.
Compressing the skin with glass slides appears to be sufficient to occlude many of the vessels in the superficial capillary plexus. By doing so, the 532nm laser energy absorption in blood is reduced significantly as can be seen in the ‘compressed’ irradiated spots, leaving more energy available for deeper targets. Anecdotal evidence from the patients also appeared to indicate enhanced healing rates following treatment.
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