Introduction

While skin rejuvenation dates back to the Egyptian era, laser facial rejuvenation is a recent innovation. It is said that Cleopatra used soured milk (lactic acid) to rejuvenate her skin. Thus chemical peels as well as a variety of mechanical exfoliations have been practiced for thousands of years. The laser, however, is a recent addition to the skin rejuvenation armamentarium. In 1960, Theodore Maiman produced the first laser energy, utilizing a Ruby crystal.¹ New laser development was primarily a product of the communications industry, and in fact much of the early work was done in Bell Laboratories. Many of the lasers used in medicine today were developed for industrial use and later adapted to medical applications. Patel published his discovery of the carbon dioxide laser in 1964.²

Laser application in medicine gained momentum in the late 1960s and early 1970s. Dr Leon Goldman commonly considered ‘the father of laser medicine’ and many other pioneers recognized the value of the laser in treating a variety of medical conditions.³ Ophthalmology was one of the early adopters of laser energy for treating intraocular neovascularization. The technology was soon applied to vascular birthmarks, such as port-wine stains. These early lasers, however, were very crude devices with poor control of the laser parameters, thus leading to mixed results. Commonly a port-wine stain was exchanged for a scarred, hypopigmented area of skin. The satisfactory treatment of many skin conditions awaited the development of new technology and understanding.

The concept of selective photothermolysis (SPTL) was introduced by Anderson and Parrish in 1983 and proved to be a turning point in the laser treatment of skin conditions.⁴ This theory recognized that the optimal treatment of a variety of skin disorders was dependent on optimization of several factors related to the target tissue and the laser parameters. The target of vascular disorders for instance, was the hemoglobin inside the vessels. The laser wavelength that was preferentially absorbed in hemoglobin was that wavelength which was heavily absorbed by hemoglobin and minimally absorbed by other competing pigments in the tissue. Absorption curves which plot percent absorption versus wavelength for a single tissue are not linear but have many peaks and troughs. When the absorption curves for the various absorbers, termed chromophores, in the target area are overlaid it is possible to choose optimal wavelengths that are heavily absorbed by the target (e.g. hemoglobin) and minimally absorbed by other chromophores such as water, xanthophyll, and melanin that would compete with the hemoglobin to absorb the laser energy. Since nearly all lasertissue interaction is thermally induced, the controlled heating of the target tissue is created by the absorption of the laser energy which is induced by photons impacting the molecules of the tissue. Vibrational energies disrupt the molecules and the cells of the tissue target.

As part of the SPTL theory, the concept of thermal relaxation time was introduced to explain the selective heating of the various tissue components. In essence, this concept states that the confinement of energy (heat) within a structure can be controlled by limiting the exposure time. This thermal relaxation time is proportional to the size of the structure, meaning that extremely small tissue components such as melanosomes have an extremely short thermal relaxation time, and thus should be treated with extremely short pulse duration. Typically these nanometer structures would be treated with a nanosecond pulse duration. By limiting the pulse duration the energy is thus inhibited from spreading to surrounding structures causing unwanted damage.

Energy loading or heating of target tissue to achieve a specific goal such as disruption of the cells tends to be an all-or-none phenomenon. Thus, like the high jumper who has to jump 6 feet to clear the bar, the ablation threshold has to be reached in order to achieve cell disruption. Below the ablation threshold nonselective heating of the tissue can occur, which will lead to untoward results. This ablation threshold is also related to the size of the target structure. The concept of selective photothermolysis (SPTL) opened the door to many successful laser treatments.

The result of the SPTL theory led to the development of the first laser which was specifically designed and produced to treat a medical condition. This was the pulsed dye laser used for treating port-wine stains. The pulsed dye laser was almost instantly accepted in treating port-wine stains in very young patients which had hitherto been very problematic.⁵ It was not long before new lasers were introduced, having been designed according to the theory of SPTL for treating a variety of ailments such as lentigines, tattoos, and skin aging.^5,6

Unfortunately the theory of selective photothermolysis is not ideal in every situation. While it has led to a greater understanding and thus more effective treatment in most lesions, there are exceptions to this generalization. For instance, the original flash lamp pulsed dye laser was designed to deliver energy at 577 nm, a peak in the absorption curve of hemoglobin. Because of the considerable variation in size, color, and depth of vessels within a port-wine stain, the newer lasers have emitted wavelengths at 585 to 595 nm. In addition the pulse duration of the original pulsed dye laser, 450 ns has been extended to the millisecond range for the same reason. This compromise from the SPTL theory ideal has led to a much improved outcome while minimizing undesirable side effects.

Recent innovations in the major facial rejuvenation have involved other wave lengths and other sources of radiant energy. Examples of these would include the intense pulsed light (IPL) as well as radiant energy sources such as microwave and radiofrequency excited sources. The intense pulsed light is a noncoherent light source consisting of a broad spectrum emission of relatively modest energy. The function of the microwave and radiofrequency sources is to stimulate collagen tightening and deposition.

The carbon dioxide (CO₂) laser with a wave length of 10,600 nm in the far infrared portion of the spectrum is capable of coagulation and ablation of tissue. Thus it is used to remove lesions by evaporation as well as to thermally stimulate the tissue. The normal response can therefore be quite varied, depending on a large number of factors such as tissue hydration as well as the laser parameters such as pulse duration and energy density. Although the traditional carbon dioxide laser does not meet any of the parameters of the theory of selective photothermolysis, nevertheless this laser can be designed to achieve satisfactory ablation or cutting while minimizing thermal damage.⁸

As an ablative instrument the carbon dioxide laser has been used to remove a variety of benign and malignant lesions.⁹ These include verrucae, syringomas, seborrheic keratoses, and other benign lesions. Actinic keratoses as well as superficial basal cell carcinoma can be removed with these lasers, however, due to a lack of histologic confirmation one can never be certain that the lesion is totally removed by the laser.¹⁰

Two conditions which respond extremely well to the carbon dioxide laser treatment are rhinophyma and actinic cheilitis.^11,12 A generic treatment regimen for these types of lesion would include the use of a 1 mm handpiece with a focused and defocused beam, usually pulsed mode with sufficient energy to evaporate the tissue. Coagulated tissue may require removal by wiping between passes. Commonly some minor scarring and hypopigmentation can be seen at the site of treatment.

Methods

As mentioned above, a variety of chemical and mechanical processes have been used in the past to rejuvenate the skin. Most of these required removal of a large portion of the epidermis as well as portions of the dermis, anticipating that the healing process would rejuvenate the epidermal elements including pigment, and contraction of the dermal collagen elements with additional collagen being deposited during the healing process. It was quickly recognized that the laser could produce a similar effect. In particular the carbon dioxide laser was used for treating photoaged skin in the 1980s. Early attempts with conventional carbon dioxide lasers however led to many unsatisfactory results, including scarring.¹³ With the advent of high peak power, rapidly pulsed or scanned carbon dioxide lasers, this difficulty was overcome in accordance with the theory of SPTL, mentioned above.

There are generally two methods of achieving the necessary energy in a sufficiently short time frame in order to achieve evaporation/ coagulation with minimal thermal damage. These would be the extremely short pulsed, high energy lasers as well as the rapidly scanned continuous-wave laser. The super pulsed carbon dioxide laser was a step in the direction of satisfactory laser rejuvenation. The UltraPulse laser by Coherent Inc. was capable of delivering 5 j/cm² in a sufficiently short period of time in order to allow vaporization of 20 to 30 μ of tissue and residual thermal damage of 40 to 120 μ depth after two or three passes.¹⁴ Most of the laser resurfacing is accomplished with a robotic scanner which Coherent termed the computerized pattern generator (CPG). Histologically much of the epithelium is removed. Although elastin may persist, type I collagen is denatured between 60 ° and 70 °C, causing it to shorten.¹⁵ Epithelialization was complete in seven days, but total healing as indicated by an abating of the erythema required several weeks. In fact, collagen deposition may be seen for many months following the laser resurfacing. Comparison of laser resurfacing with chemical peel and dermabrasion showed similar healing and results, with the exception of the phenol peel which required considerably longer to heal. No scarring was observed in this treatment study.¹⁶

A competing technology for laser resurfacing is the rapidly scanned carbon dioxide laser produced by Sharplan called the Silk Touch. This continuous wave CO₂ laser is rapidly scanned across the skin surface, thus reducing the swell time and therefore the effective pulse duration.

Indications and Contraindications