Lasers were first developed by physicists during the 1950s as a tool to study molecules, leading to the invention of the first machine in 1960 by Theodore Maiman. They are now widely used for a range of applications, with the most common use being within CD and DVD players.
Dermatologists and ophthalmologists (1961) were the first to start using the devices for medical treatments. Dermatologists already had a long history of using light treatments for skin disorders, with one of the earliest being the Finsen lamp in 1899. Leon Goldman first started using the devices for skin conditions in 1961 and is honoured as the ‘father of lasers in medicine’. His work initially started with pigmented lesions, and then progressed to vascular lesions.1Geiges ML. History of lasers in dermatology. Curr Probl Dermatol. 2011;42:1-6. doi:10.1159/000328225 Subsequent research and development of lasers has led to refined techniques to improve their clinical use.
Today, lasers are widely used around the world for numerous skin conditions, including vascular lesions, pigmentation, scars, and rejuvenation.
How does laser therapy work?
Wavelengths, chromophores, and depth
Common light sources (such as sunlight) are made up of multiple different colours (wavelengths) and are termed polychromatic (many colours). In contrast, lasers emit a single, coherent wavelength, or monochromatic light. Laser is an acronym, which succinctly describes it: light amplification by stimulated emission of radiation.
They are made up of:
- An energy or pump source that is usually electrical.
- A resonant chamber that contains the medium and reflective mirrors.
- A medium is the source of the atoms producing the laser light. The medium is usually gas, crystals, dyes, or semiconductors. They are typically named for their medium, such as:
- Gases: argon (Ar, 488–514 nm), carbon dioxide (CO2, 10,600 nm), and excimer (excited dimers), which are a combination of two gases.
- Solid-state lasers, typically crystals such as sapphire or ruby sometimes combined with other substances to alter their properties such as neodymium in Nd:YAG (1064 nm) lasers (neodymium-doped: yttrium aluminium garnet).
- Semiconductors (typically diodes) have wavelengths of 800-900 nm.
- Dyes in a liquid solution like pulse dye lasers have wavelengths of 585 nm and 595 nm.
In dermatology, the laser wavelength will be absorbed by a specific chromophore or structure in the skin. This means that the energy will primarily be absorbed by the target chromophore while surrounding structures will absorb a minimal amount of this energy. Heating of the target structure results in its destruction.
The other main relevance of wavelength is that it influences the depth of penetration the beam can reach. Longer wavelengths penetrate deeper into the tissues. This means that different devices may be used for the same chromophore (e.g., haemoglobin) depending on how deep the lesion extends into the skin.
In a real-world example:
- Pulse-dyed laser (PDL) has a wavelength of 585 or 595 nm and has a yellow colour. It is absorbed by the oxyhaemoglobin (blood) chromophore making it a very good option for the removal of vascular structures.
Various wavelengths are available.
There are numerous different types of lasers in use, all of which have different applications, pros, and cons.
Laser beam characteristics
The effect of spot size of laser on penetration depth.
While the wavelength is the primary feature that determines treatment, the emitted beam can be modified in numerous ways to alter the effect on tissues and the treatment outcome.
Common parameters:
- Continuous wave lasers produce a continuous beam.
- Pulsing of the laser beam over short durations. The ideal pulse duration is influenced by the target structure’s thermal relaxation time. Typically, the pulse duration is less than the thermal relaxation time allowing the target tissues to cool between pulses. When the pulse duration is longer than the thermal relaxation time, heat will spread and damage surrounding structures.
- Spot size is defined as the diameter of the beam.
- The amount of energy delivered is measured in joules and fluence is a measure of energy density (J/cm2). Typically, larger structures need a higher fluence. Power is measured in joules per second, or watts and power density in watts per cm (watts/cm2)
The safety of lasers
Lasers typically have much higher power than normal light sources, so special consideration needs to be given for safe use. The biggest concern regarding the use of these devices is inadvertent exposure to the eye. Damage to the retina can cause blindness within nanoseconds of exposure with some devices.
Correct eye protection is of paramount importance when using a laser to avoid irreversible damage.
Pregnancy
The basic physics principals of commonly used lasers determines that there should be no exposure to the foetus. Simply put, these devices generally do not penetrate any deeper than the skin. Lasers introduce heat into the skin and do not transfer chemicals or other substances into the body. A review of the available evidence supports this expectation.2Wilkerson EC, Van Acker MM, Bloom BS, Goldberg DJ. Utilization of Laser Therapy During Pregnancy: A Systematic Review of the Maternal and Fetal Effects Reported From 1960 to 2017. Dermatol Surg. 2019;45(6):818-828. doi:10.1097/DSS.0000000000001912
The main concern with laser procedures is the use of topical anaesthetics, which can be absorbed into the bloodstream. Some ingredients in commonly used creams have been known to have impacts on the foetus and are best avoided. Thankfully, some modern lasers and energy devices are starting to negate the need for topical anaesthetic creams due to the more efficient use of energy, resulting in less pain.
