The reason for selecting YSGG over CO2 is only strengthened when comparing YSGG to low power CO2 lasers. Low Power CO2 lasers (typically 30 watts and below) exaggerate the problems of CO2 and have several independent problems as well. Histology obtained from data sheets for Deka’s SmartXide DOT and Alma’s Pixel CO2 are included to illustrate the problems with devices that do not have sufficient power and energy density in each spot.
Many people believe wavelength determines depth. For ablative lasers, this is not true. Wavelength determines the thermal characteristics of the laser, but depth of ablation is determined by power and energy density. With the right amount of power, all ablative wavelengths can ablate equally deep. Their thermal damage zones, however, vary significantly across devices.
A first typical problem of lower power CO2 lasers is ablation depth. To ablate deep, lasers must have high energy densities delivered in short time periods. This requires high power. Depth of ablation is directly proportional to treatment results up to about 400-800 µm deep. Beyond this depth, there are two schools of thought. One thought is that greater depth is better, the other is that greater density is better. There is likely some truth to both. Either way, however, device should be able to treat at least 400-800 microns deep.
In the histology image below, the image on the left shows a lesion from a single laser pulse. The ablation depth in this image is ~200µm (based on epidermal thickness of ~70µm). The histology image on the right shows a deeper treatment, but as can be seen, this required stacking 3 pulses (i.e. firing three pulses in the same location). Stacking pulses is challenging as even minor movements by the patient or practitioner can misalign the treatment holes as each spot is only several hundred microns in diameter. Further, it takes 3X longer to perform a treatment if stacked pulses are required. This means the patient experiences 3X as many shots per pass, which can reduce the tolerability of the treatment.
The second problem with low powered CO2 devices is pulse duration. The primary problem with any CO2 laser is excessive thermal damage. This thermal damage leads to increased potential for pigmentary complications and slows wound healing time. This is the reason all low power CO2 units were long ago abandoned for full resurfacing. This thermal damage is also a limiting factor or fractional CO2 devices and can lead to the same complications as observed with full resurfacing CO2 lasers if greater depth or density treatments are attempted. To properly perform a resurfacing procedure with a CO2 laser total exposure time should be less than ~600µs to confine thermal damage.
· This is documented in the text “Cutaneous Laser Surgery” Chapter 6, Carbon Dioxide Laser Surgery. In this text, it states that the thermal relaxation time for the volume of ablated tissue during CO2 resurfacing is about 695 us and pulse durations of less than 950 us (0.95 ms) are sufficient to prevent clinically significant thermal damage
Because the laser is low powered, long pulse durations must be used to deliver sufficient energy to ablate tissue. In fact, many pulse durations as much as 100X longer than was experimentally and clinically proven optimal for CO2 lasers.
The long pulse durations used with low power CO2 lasers can be seen histologically in the two images above. In both cases, the coagulation / thermally damaged tissue zone is ~250-300 microns wide and deep. This means the treatment diameter of the laser is not just the spot size, but the spot size plus ~250 µm on each side of thermally damaged tissue, meaning the spot size is ~500µm wider than stated – leading to greater downtimes if treatments are performed to equivalent depths for equivalent treatment results.
The end result of minimized ablation depth, excessive thermal damage, and large effective spot sizes is that treatments are predominately thermal (not ablative), and are not very deep. Fractional non-ablative devices were completely thermal, and deeper, but required 4-6 treatments. Low powered CO2 lasers do have maximized ablation (the only difference between ablative and non-ablative fractional lasers) meaning these treatments and results are closer to fractional non-ablative devices. Further, it is often necessary to perform multiple treatments to provide equivalent end results as highly ablative fractional devices (like Pearl Fractional). Many before and after pictures actually show results pre and post 3 treatments each spaced roughly 1 month apart.
Pearl Fractional was designed to reproducibly deliver deep fractional ablation with variable density to provide single-treatment results with maximum results and consistency. Accomplishing this and limiting downtime and complication risks required a wavelength capable of providing controlled thermal coagulation in a pulse-duration that is thermally confined. Pearl fractional was designed with sufficient power to ablate to necessary depths (typically 600-800 microns, which is ½ of maximum power). See comparison histology images below – image sizes are scaled to maintain approximately equal magnification. Histology images for the SmartXide DOT and the Alma Pixel CO2 were obtained from product brochures from each company.
