Cooling technique puts a freeze on fractional resurfacing efficacy

Key Points

Grapevine, Texas - Skin cooling performed during fractional photothermolysis impacts the size of epidermal zones of microthermal damage, and may, therefore, impact treatment efficacy, according to a new study.

"There are many different devices now that work by creating fractional thermal damage patterns," says Dieter Manstein, M.D., the study's principal investigator and an instructor in dermatology at Harvard Medical School and Massachusetts General Hospital.

In a study involving the first of these devices to reach the market (Fraxel SR 750, Reliant Technologies), he says, "We have demonstrated that the diameter of the fractional lesions depends on the skin cooling itself - the cooler the skin, the smaller the lesions."

To arrive at this conclusion, researchers performed fractional photothermolysis with the 1,550 nm fiber laser on full-thickness, previously frozen cadaver skin. Before treatment, researchers placed the skin on wet gauze on a thermal plate (Cole-Parmer) that allowed them to maintain the samples at preset five-degree temperature intervals between zero and 45 degrees Celsius.

Before laser exposure, investigators also applied Optiguide Blue (Reliant) and a thin layer of 30 percent lidocaine ointment to better approximate clinical conditions.

Each treatment included two passes at 10 mJ per pulse or microthermal zone (MTZ) and a density of 250 MTZ/cm² per pass. Before and after each exposure, researchers measured skin temperature to verify that it remained consistent.

After each exposure, investigators took 6 mm punch biopsies at the center of the exposed area and sent samples for histological assessment, which included nitroblue tetrazolium chloride (NBTC) staining. This method allowed for monitoring of tissue necrosis caused by the irreversible thermal alteration of essential mitochondrial enzymes, Dr. Manstein tells Dermatology Times. In short, it stains viable cells blue and leaves thermally damaged cells unstained, he explains.

Next, a blinded investigator examined the epidermal samples under a light microscope and assessed the diameter of 48 randomly selected (non-overlapping) MTZs for each temperature under investigation. Investigators then calculated the mean MTZ diameter and its standard deviation, and analyzed the thermally damaged surface area per MTZ.

Study results

They found that as temperatures rose from zero to 45 degrees Celsius, MTZ diameter grew from 93 to 147 millimicrons (58 percent). As for MTZ area, it likewise increased from 6,870 to 17,050 millimicrons (148 percent).

"Skin temperature has a direct positive linear correlation with diameter of the MTZ," and with MTZ area for the range of temperatures studied, Dr. Manstein says.

For example, researchers found that when one uses simultaneous skin cooling to decrease skin temperature to 20 degrees Celsius, MTZ area decreases approximately 40 percent versus a similar treatment performed at standard body temperature.

One could hypothetically increase the amount of energy delivered per MTZ to overcome the decrease in MTZ diameter associated with lower skin surface temperature, he says. However, the relationship between this change in thermally damaged surface area and clinical outcomes remains unclear, Dr. Manstein notes. Therefore, researchers recommend further study in this regard.

For now, Dr. Manstein says that in order to accurately compare fractional resurfacing devices, "We believe it's mandatory to talk not only about energy and exposure parameters, but also to document the amount of cooling used."

"When I treat a patient with a fractional resurfacing device," says Henry H. Chan, M.D., "I make sure my nurse documents both the laser parameters and cooling parameters," keeping both as consistent as possible.

"The cooling is a separate variable that could determine the outcome," says Dr. Chan, who is a study co-author and honorary clinical associate professor of dermatology, University of Hong Kong.

Disclosure: Dr. Chan owns stock in Reliant.

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