Laser for Burn Scar Treatment

History of Laser and Intense Pulse Light

Albert Einstein was the first to describe the theoretical physics of the laser in 1917. He described the interaction of atoms and molecules with electromagnetic energy in terms of the spontaneous absorption and emission of energy and concluded that stimulated emission of energy was also possible. In 1959, the first instrument was developed by Drs. Townes and Schawlow based on those concepts. It was known as the microwave amplification through the stimulated emission of radiation (MASER). Shortly thereafter, in 1960, Theodore Maimon developed the first laser light with use of a ruby crystal. Early clinical studies with the ruby laser began in 1964, with Dr. Leon Goldman, a dermatologist considered by many to be the father of laser medicine. The rapid development of additional lasers occurred with the helium-neon laser appearing in 1961, the argon laser in 1962, the carbon dioxide (CO ₂ ) laser in 1964, the Nd:YAG laser in 1964, and the erbium-YAG laser in the mid-1990s.

However, early lasers had limited control of energy parameters, leading to frequent thermal injury and scarring. The concept of selective photothermolysis (SPTL) introduced by Anderson led to the development of the first laser that was specifically designed to treat a medical condition; it was named the pulsed dye laser (PDL) and was almost instantly accepted in treating port wine stains in very young patients, a condition that had hitherto been very problematic.

Further developments over the ensuing decades included the introduction of pulsed energy resulting in decreased laser exposure time. Reliant technologies unveiled the ability to split the treated zone into specific area, resulting in multiple microtreatment zones within the target area; this was termed fractionated laser treatment. The advent of short-duration Er:YAG lasers in the mid-1990s offered an additional option for resurfacing, either alone or in combination with the CO ₂ laser. Around the same time, high-intensity flashlamp exposure was presented as a suitable tool for treating vascular lesions; this resulted in the release of intense pulsed light (IPL) as a medical device. In the following years, multiple technical modifications allowed easier handling, increased safety, and widened the spectrum of potential indications.

Physics of Laser

Lasers are devices that rely on the stimulated emission of radiation to produce a beam of light. The word LASER is an acronym for the term light amplification by stimulated emission of radiation . The device itself consists of an active medium, an energy source, and a resonating chamber.

In most lasers, there is a population of atoms known as an active medium . The active medium of a laser is a material of controlled purity, size, concentration, and shape. This material can be of any state: gas, liquid, solid, or plasma. The gas lasers consist of the argon, copper vapor, helium-neon, krypton, and CO ₂ devices. One of the most common liquid lasers, the PDL, contains a fluid with rhodamine dye. The solid lasers are represented by the ruby, neodymium:yttrium-aluminum-garnet (Nd:YAG), alexandrite, erbium, and diode lasers.

In the atom's resting state, electrons orbit around the nucleus at their ground state or lowest energy levels, in orbitals. When an electron absorbs energy from an energy source, electrons become excited and orbit into higher orbitals. The energy source used to excite the electrons can be a light source, an electrical field, or a chemical. When these excited electrons fall back into their resting orbitals, energy is released, generating photons (electromagnetic radiation). Photons have wavelengths specific to the atom excited. Since lasers have a consistent population of atoms, the photons emitted are all identical. These identical photons are considered monochromatic, meaning that all photons in a laser beam are of the same wavelength. By contrast, IPL consists of many different wavelengths.

The resonant chamber consists of two reflective mirrors between which photons reflect back and forth. If a photon collides with an excited electron, that electron falls back to its resting orbital, releasing another photon of the same wavelength. The two photons are “in phase” or “coherent,” meaning their wave patterns are synchronized and reinforce each other. By comparison, the photons in conventional light travel randomly. As these photons hit other excited electrons, more photons are released and the light energy increases. The resonant chamber allows for more and more atoms to become excited and then return to ground state, thereby amplifying the energy produced (amplification of stimulated emission) and allowing for photon coherence.

One of the mirrors is only partially reflective and partially transparent. Those photons traveling in a perfectly parallel direction will exit the transparent portion of the mirror, also known as the optical resonator, taking the form of the “laser beam.”

Light energy can be visible or invisible depending on its wavelength. The spectrum of electromagnetic radiation ranges from long radio waves (wavelength >10 cm) to extremely short γ rays (<10 ⁻¹¹ m). The entire spectrum includes radio, microwave, infrared, visible (400–700 nm), ultraviolet, X-ray, and γ rays.

The specific wavelength emitted by a laser will determine how the laser beam interacts with tissue. When the laser strikes tissue, a variety of desirable and undesirable effects result as the laser is reflected, scattered, transmitted, or absorbed.

Reflection is the proportion of laser that bounces off the surface and is redirected in a different direction. When laser is directed perpendicular to the skin, approximately 5% of the laser is reflected. Reflection of laser is one of the reasons why it is imperative to wear appropriate safety goggles at all times when performing laser treatments.

Scatter is the increase in spatial distribution of a laser beam as it passes farther through tissue, leading to irradiation over a larger area of tissue. The main scattering wavelengths are between 400 and 1200 nm (those where tissue water absorption is poor).

Absorption can be described as the conversion of the energy of the laser to heat when its photons strike a specific molecular target, known as a chromophore . The mechanism by which lasers are used to target specific tissue is called selective photothermolysis (photo = light and thermolysis = decomposition by heat).

Transmission occurs when a laser that has not been absorbed is transmitted into deeper tissue beyond the chromophore.

Multiple variables must be considered when selecting a type of therapeutic laser, in addition to the appropriate wavelength. The additional laser parameters that optimize the result are fluence , or power density (joules/cm ² ); pulse width or duration; mode of delivery; and spot size (increasing spot size increases penetration).

The theory of selective photothermolysis first described by Anderson and Parrish explains the principles behind the clinical application of photothermal lasers. The wavelength of laser light chosen must be selective and appropriate for the target tissue, which must be destroyed without damaging the surrounding tissues. The pulse width or duration of the laser pulse must be within the thermal relaxation time of the treated tissues. Thermal relaxation time is the amount of time it takes to transfer two-thirds of the resultant heat to the surrounding tissues.

The laser's effect on the epidermis can further be classified as ablative or nonablative depending on whether or not the epidermis is left intact. Fractional lasers treat only a portion (or fraction) of the tissue. Nonablative lasers will thermally injure the tissue, whereas ablative lasers will destroy entire columns of tissue, including epidermis.

Overview of Lasers in Hypertrophic Burn Scar

The ultimate goal in the treatment of hypertrophic scars is to make improvements both aesthetically and functionally, as well as reduce itching and pain related to the scars. Traditional and emerging laser- and light-based technologies offer new hope for patients with burn scars.