How  do Laser and Intense Pulsed Light (IPL) treatments work in skin rejuvenation?

Introduction

Lasers and Intense Pulsed Light (IPL) devices are pivotal tools in dermatology, leveraging the power of light to treat various skin conditions. The term “laser” stands for Light Amplification by Stimulated Emission of Radiation, characterized by its monochromatic, coherent, and collimated high-intensity light beams. IPL, on the other hand, emits polychromatic, non-coherent light across a broad spectrum, making it less intense but versatile. The development of the theory of selective photothermolysis in the 1980s revolutionized their use by enabling precise targeting of skin chromophores, minimizing damage to surrounding tissues, and enhancing safety and efficacy.

Skin Optics

Understanding how light interacts with biological tissues is essential for selecting appropriate laser or IPL settings and ensuring safe, effective treatments. Light can interact with skin in the following ways:

1. Absorption: The Grotthuss-Draper law states that light must be absorbed to exert an effect. Chromophores such as melanin, oxyhemoglobin, and water in the skin absorb light, converting it to thermal energy and leading to the desired therapeutic effects.
2. Scattering: Light scattering, primarily caused by dermal collagen, can reduce the beam’s power density as it penetrates the skin. Longer wavelengths and larger spot sizes decrease scattering, enhancing light penetration.
3. Reflection: A portion of light is reflected off the skin surface, primarily by the stratum corneum, and does not contribute to clinical effects.
4. Transmission: Light that passes through the skin without absorption, scattering, or reflection does not produce a clinical effect.

Effective treatment requires optimizing the balance between absorption and scattering to ensure sufficient light reaches the target chromophore. Longer wavelengths penetrate deeper into the skin due to reduced scattering and fewer superficial chromophores that absorb these wavelengths.

Selective Photothermolysis

Selective photothermolysis is the principle guiding the use of lasers and IPL in dermatology. It involves using light to selectively destroy skin targets based on the following parameters:

• Wavelength: The chosen wavelength should be preferentially absorbed by the target chromophore and penetrate the skin to the required depth.
• Pulse Duration: Light must be delivered in a time frame short enough to prevent excessive heat transfer to surrounding tissues.
• Fluence: The energy delivered per unit area must be sufficient to achieve the desired effect while minimizing collateral damage.

These parameters help in tailoring treatments for specific conditions, such as targeting hemoglobin in vascular lesions with a pulsed dye laser at 585 nm.

Therapeutic Parameters

Several laser parameters are crucial for determining clinical outcomes:

1. Wavelength: Different chromophores absorb light at specific wavelengths (Figure 2). Longer wavelengths generally penetrate deeper, while shorter wavelengths are absorbed more superficially.
2. Pulse Duration: This should match the thermal relaxation time of the target to confine heat and avoid damage to surrounding tissues. Larger targets, like blood vessels, have longer thermal relaxation times than smaller structures like melanosomes.
3. Fluence: The amount of energy delivered per unit area. Higher fluence is required for larger structures or deeper targets.
4. Irradiance: The rate of energy delivery per unit area, impacting how quickly a target heats up. High irradiance can vaporize tissue, while low irradiance leads to coagulation.
5. Spot Size: The diameter of the laser beam. Larger spot sizes penetrate deeper due to reduced scattering, making them essential for targeting deep dermal structures.

Fractional Photothermolysis

Fractional photothermolysis creates microthermal zones (MTZs) of coagulated or ablated tissue, sparing surrounding skin. This technique, commonly used in skin resurfacing, promotes faster healing and remodeling.

Skin Cooling

Cooling techniques protect the skin from excessive heat, allowing higher energy delivery to target structures and reducing pain. Methods include cold gases, liquids, or solids applied before, during, or after treatment.

Classification of Devices

Lasers and IPL devices are categorized based on their operational modes:

1. Continuous Wave Lasers: Emit a continuous beam and include argon and CO2 lasers.
2. Pulsed Lasers: Emit light in short pulses with high peak power, suitable for targeting specific structures like blood vessels or tattoo pigments.
3. Fractionated Lasers: Use fractional photothermolysis to create MTZs, ideal for resurfacing and scar treatment.
4. Intense Pulsed Light (IPL): Emit non-coherent polychromatic light, versatile for treating both vascular and pigmented lesions, although requiring more sessions than lasers.

Other Laser/Light Tissue Interactions

Lasers and IPL devices can also induce non-thermal effects, such as:

• Excimer Laser: Delivers UVB light for conditions like psoriasis.
• Photodynamic Therapy: Uses a photosensitizer activated by light to treat conditions such as actinic keratoses and skin cancers.

Conclusion

Lasers and IPL devices are powerful tools in dermatology, offering precise and effective treatments for various skin conditions. Understanding their principles and therapeutic parameters is crucial for optimizing clinical outcomes and minimizing side effects.