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Full-field devices
Full-field devices
Many different wavelengths of light are produced by commercially available devices for the treatment of photoaging. Lasers emit monochromatic, coherent light at a single specific wavelength, and this is typically used to target a single chromophore. Conversely, IPL is structurally composed of a flashlamp which emits polychromatic, noncoherent light composed of many wavelengths, commonly from 500 to 1, 200 nm. While lasers are more selective for a given chromophore, IPL is advantageous because it produces light of various wavelengths and thus can target several different chromophores at once. Current devices are coupled with various filters in order to make
the output slightly more selective, eliminating all light with a wavelength shorter than the filter threshold. This versatility makes IPL a very useful device for those seeking to use a single device that can treat multiple conditions. On the other hand, the flexibility of IPL devices can also be a handicap if used incorrectly, as the nonspecific output can make outcomes less predictable and therefore result in complications. Consequently, more experience with this device is warranted for more predictable results to be obtained.
Light-based devices generate significant heat during use, which can cause unwanted dyspigmentation, blistering, and scarring via nonspecific bulk heating. To avoid this complication, devices are coupled with a cooling mechanism that either functions by direct contact cooling or a cryogen spray mechanism. In addition to the device cooling, many providers use cold gel or forced air in order to protect the epidermis and prevent unwanted burns or pigmentary changes. In addition to providing epidermal protection, the use of cold gel improves light penetration by reducing the refractive index between the air and the skin.59 The duration of cooling is proportional to the depth of the targeted structure, with less cooling used for epidermal targets and longer cooling duration useful when treating deeper dermal structures.60
One of the most commonly observed manifestations of photoaging are solar lentigines. These noticeable lesions are the result of chronic ultraviolet light exposure and are often cosmetically unacceptable to patients. The melanin within lentigines has a broad absorption spectrum peaking at 335 nm and steadily decreasing to just beyond 700 nm. As there are no specific peaks, it can be targeted with multiple wavelengths, and most surgeons select a laser with output in the 500 nm range such as KTP (532 nm) or an IPL with filters inclusive of the low 500 nm. The benefit of IPL in the treatment of these benign but unwanted pigmentary lesions is significant and has been well studied.61 When using appropriate treatment parameters, benign lentigines may be very responsive to therapy, and atypical or resistant lesions should be considered for biopsy to exclude melanocytic lesions such as a nevi or even lentigo maligna.
Benign solar lentigines are composed of melanosomes which accumulate at the base of the epidermis along the rete ridges. These melanosomes serve a protective function in chronically photodamaged skin by aggregating intracellularly above the keratinocyte nucleus. This shields the nuclear DNA from ultraviolet radiation, and is thought to prevent carcinogenesis. During treatment, these melanosomes preferentially absorb photons, are degraded during light conversion to thermal energy, and subsequently aggregate within the epidermis as โmicrocrusts.โ62 These crusts are then extruded from the epidermis over the subsequent 7 to 10 days, clinically observed as a coffee-ground like material on the surface of the skin.
The telangiectasias of chronically photodamaged skin can be similarly treated by understanding the absorption spectrum of the hemoglobin within them. Hemoglobin has
high relative absorption at 542 nm and 577 nm which can be targeted with either KTP, PDL, or IPL with appropriate filters inclusive of those peaks. According to the theory of selective photothermolysis, the energy pulse duration should be slightly shorter than the thermal relaxation time (TRT) of the target chromophore, which is related to the square of the chromophore size. The relatively small melanosomes of solar lentigines are roughly 0.5 ฮผm, and thus have a TRT of approximately 70 to 250 nanoseconds. These short TRTs are the reason that nanosecond spectrum Q-switched lasers are commonly used for pigmented lesions. Larger telangiectasias often measure 30 to 100 ฮผm, and thus are effectively treated with a pulse duration in the 1 to 20 ms range.59 Understanding this difference in chromophore size and TRT is important because it explains the cause of some common complications. When using relatively longer pulse durations in the 10 to 25 ms range typical for targeting vessels, pigmentary complications are more common as this exceeds the TRT of melanosomes and may lead to bulk tissue heating.
In addition to the treatment of vascular and pigmented lesions, over time it has become apparent that there is an unrelated photorejuvenation effect from light-based devices, particularly when using the longer wavelengths emitted in the near infrared spectrum with IPL (800โ1,200 nm). In this spectrum, the emitted light targets water as a chromophore, and has been shown to have proliferative effects on tissue fibroblasts. Photodamaged skin is composed of fragmented, thickened collagen fibers, as well as an increased ratio of collagen 3 to collagen 1. Tissue fibroblasts treated with IPL demonstrate increased production of collagen 1, as well as a decrease in matrix metalloproteinase enzymes responsible for collagen degradation.63
With all light-based treatments, patient selection is essential to success. Selective photothermolysis requires that the tissue target has a unique absorption spectrum relative to the surrounding structures in order to be selective. For example, a Fitzpatrick type I patient with dark solar lentigines would be a relatively good candidate because of the absorption spectrum contrast between the low melanin skin and melanin-laden lentigines: the melanin within the lentigines would readily absorb the photoenergy while leaving the surrounding melanin-poor skin unaffected. Conversely, a tan Fitzpatrick type III patient with the same lentigines would be much more difficult to treat safely because of the increase in baseline epidermal melanin content throughout the skin. In the second scenario, the chromophore is more uniformly distributed throughout the skin, and treatment may lead to unwanted hypopigmentation or burns.
In addition to selecting the appropriate patient for treatment, it is also imperative to evaluate the site of treatment.