Ablative Laser Skin Resurfacing, in the simplest sense, describes the process of removing the epidermal and superficial dermal layers of the skin in order to reduce cutaneous signs of photoaging.  Other indications for ablative laser skin resurfacing include patients who exhibit scarring, actinic keratoses, seborrheic keratoses, and facial wrinkles.

The use of lasers for ablating and resurfacing involves the concept of selective thermolysis of the epidermal and dermal layers of the skin through the delivery of light energy [1] [2]. Light energy emitted by the laser is absorbed by the skin's two main chromophores, melanin and water, which then emit thermal energy causing destruction to the surrounding skin tissues. 

The process of laser resurfacing has undergone a number of breakthroughs in the last few decades, with the first application of continuous carbon dioxide (CO2) laser beginning in the 1980s [3]. The implementation of pulsed CO2 lasers (versus continuous CO2 lasers) and the subsequent development of the erbium:yttrium aluminum garnet (Er:YAG) laser, which gained popularity in the late 1990s, further improved the precision and depth of cutaneous ablation and reduced the incidence of adverse effects [4].

Further refinement in skin resurfacing occurred in the early 2000s with the advent of fractional lasers, which are defined as lasers that use narrow, microscopic columns of laser light to treat a defined portion of the skin. This less destructive modality further reduced the incidence of adverse events and provided a greater degree of therapeutic control whilst still seemingly providing comparable results to non-fractional modalities [5].

Depending on the indication, the technician may choose to employ a specific ablative laser (e.g., CO2, Er:YAG) with a multitude of different settings, including fractional versus non-fractional, to achieve the desired result and, more importantly, minimize laser-associated complications such as scarring, persistent erythema, and dyspigmentation.

All in all, ablative lasers represent a safe and effective tool for skin resurfacing, some nuances of which will be discussed herein.

SKIN BASICS

An understanding of the function and anatomy of the skin and its appendages are important in order to thoroughly appreciate the applications of ablative laser therapy. As the body’s largest organ, the skin can be divided into several distinct layers including the epidermis, dermis, and hypodermis: 

1. The epidermis is an epithelial layer derived from the ectodermal germ layer of the embryo. The layers of the epidermis, from deep to superficial, include:
   1. Stratum basale is a single layer of columnar cells, which are the only layer to undergo mitosis in the epidermis. Cells will migrate from the basal layer until they are ultimately shed from the skin surface.
   2. Stratum spinosum is eight to ten layers of irregularly shaped cells with prominent intercellular bridges known as desmosomes given it a spiny appearance under a microscope.
   3. Stratum granulosum is where the process of surface keratin formation where cells are arranged in a sheet of two to four layers filled with granules known as keratohyalin and small bodies of glycophospholipids
   4. Stratum lucidum is flat keratinocytes that are closely packed and clear with typically absent nuclei and is often absent in thin skin (i.e. around the eyelids)
   5. Stratum corneum is the most superficial layer of the epidermis and consists of a very thin squamous layer mostly consisting of skin cells that are dead and shed. This layer is replenished continually by the deeper layer of the epidermis, filled with keratin, and moved to the surface of the skin through keratinization. This layer functions as a barrier to water, physical trauma, and other environmental threats.
2. The dermis is a relatively dense and vascular connective tissue layer that may average more than 4mm in thickness. It is divided into a thin papillary and a thicker reticular layer.
   1. Papillary layer: The presence of dermal papillae composed of loose connective tissue and thin collagen and elastic fibers is evident in the papillary layer of the dermis. These are responsible for the formation of frictional ridges in the skin (important to the formation of fingerprints).
   2. Reticular layer: This deeper layer has a much thicker network of fibers of mostly collagen and elastin. It serves as a layer of attachment for numerous skeletal and smooth muscle fibers and contains many somatic sensory receptors as well as hair follicles and other skin glands.
3. The hypodermis is the subcutaneous layer, which is not directly part of the skin but lies deep to it. It contains mostly loose fibrous and adipose tissue. It is responsible for carrying the major blood supply and innervation to the skin.

LASER BASICS

Within the ablative laser therapies, the two major laser types are CO2 lasers and Er:YAG lasers, with therapeutic options being further subdivided into non-fractionated and fractionated laser ablation. Non-fractionated lasers will provide treatment over the entire targeted area of skin, versus fractionated lasers, which will target therapy to specific fractions or columns of the targeted area, thereby minimizing the risk of adverse effects on the skin and hastening recovery. Ablative lasers vaporize tissue and are generally considered more aggressive therapies and produce the most dramatic outcomes compared with nonablative lasers, which produce less dramatic improvements but leave the epidermis otherwise intact. Non-ablative lasers are beyond the scope of this review but can be useful to minimize the appearance of less prominent rhytides and dyspigmentation without a significant recovery period.

The laser utilized will emit light at a particular wavelength, CO2 at 10600nm and Er:YAG at 2940nm, which will be absorbed by the tissue's chromophores, namely water and melanin. The goal is to ensure adequate but safe depth of penetration as well as delivery of sufficient energy to the targeted tissues while ensuring no excessive heat transfer to adjacent structures [6].

Energy delivery to tissues is directly correlated to the power of a laser, measured in Watts. Power density or fluency is the energy delivered per unit area and must be sufficient to achieve the desired effect while minimizing tissue damage [7]. Excessive transfer of thermal energy particularly to the dermis can lead to adverse effects including scarring and dyspigmentation.

The depth of penetration of light energy is generally correlated with increasing light wavelengths. CO2 lasers generally produce a 20-60um depth of vaporization on the first pass compared with Er:YAG lasers that produce a 3-5um depth of vaporization on first pass [8]. The advantage of CO2 and Er:YAG lasers is that the energy they emit is highly absorbed by water in the epidermis, causing rapid vaporization of the epidermis, and ensuring that little light energy reaches the deeper layers of the skin. Nonetheless, there is residual thermal damage caused by penetration of energy which is believed to lead to collagen contraction, remodeling, and skin tightening. Thermal damage with traditional CO2 lasers and Er:YAG lasers is believed to reach depths of 100-150um and 10-40um respectively [9]. The choice of laser utilized often depends on a combination of clinician experience, patient factors, as well as laser availability.

CO2 Ablative Lasers

The initial uses of laser ablation for facial rejuvenation began with the use of a continuous CO2 laser [3]. The CO2 laser produces an invisible beam emits energy at a wavelength of 10600nm that is primarily absorbed by water. This makes this laser excellent for cutting, coagulation, and ablation of the skin. For instance, when it is pulsed at 5J/cm for <1ms, the light from the CO laser penetrates to a depth of approximately 20-30um [10]. The development of high-pulsed CO2 lasers improved the ability to control the depth of ablation.

CO2 lasers are thought to produce contraction of the skin immediately by denaturing collagen and subsequent stimulating the production of new collagen. Studies have demonstrated improvement in rhytides by up to 90% using CO2 laser therapy compared with surrounding untreated skin [11]. The properties of the CO2 laser system work best at alleviating fine wrinkles especially around the eyes or mouth [12].

The discomfort associated with CO2 laser therapy often requires local anesthetic with additional sedation, anxiolytics, and possible oral analgesia [9] compared with a topical anesthetic alone for Er:YAG lasers. Recovery times associated with CO2 laser therapy are approximately two weeks with persistent erythema for a period of several weeks to months. Re-epithelization of the epidermal layer following CO2 laser therapy occurs approximately eight days [13] [14]. Studies have demonstrated an improved clinical outcome and greater dermal collagen remodeling with CO2 lasers compared with Er:YAG laser on a per laser pass basis [15]. However, CO2 lasers are associated with a higher rate of dyspigmentation compared with Er:YAG laser [16] [17].  Photodamage is less severe in individuals with dark skin but laser-induced dyspigmentation is a major concern for darker skin individuals. For this reason, CO2 laser use is not recommended for skin phototypes IV or higher [14].

Fractional ablative laser therapy was first developed for CO2 lasers in 1998 with the Lumenis UltraPulse Encore [18]. Fractional laser therapy uses narrow columns of laser light on the skin to create microscopic thermal zones measuring less than 400um in diameter and up to 1300um in depth. The concept relied on smaller treatment areas with viable surrounding tissue allowing for rapid re-epithelization as opposed to traditional laser therapy, which requires migration of epidermal cells. Compared to traditional CO2 lasers, fractionated CO2 laser therapy possesses similar efficacy as traditional CO2 laser therapy based on a number of studies demonstrating up to 80% of patients reporting an acceptable reduction in rhytides [18]. Post-operative recovery is also more rapid than with traditional CO2 laser therapy, with a return to normal activity after 5-10 days depending on the intensity of treatment. In addition, post-operative topical hydroquinone cream, retinoids, or peeling agents may be prescribed to accelerate the resolution of erythema and edema [11][13].

Er:YAG Ablative Lasers

The erbium-yttrium aluminum garnet (Er:YAG) laser was developed in the 1990s and produces laser light at 2940nm. The chromophore is water, similar to the CO2 laser, however energy absorption is 12-18 times greater. Re-epithelialization of the epidermal layer occurs more rapidly as compared with CO2 lasers, in approximately five days and with occurence of 3-4 weeks of erythema post-operatively [19].

Er:YAG has a similar mechanism of action to traditional CO2 lasers but typically has less of a skin tightening effect [13]. Er-YAG lasers additionally have a poorer coagulative effect than CO2 lasers which may limit utilization of multiple passes due to bleeding. Fluence levels of 5-15 J/cm are typically used for Er:YAG lasers, although microablative procedures with lower fluences and a single laser pass may also have some benefit for treatment of photoaging.

Topical anesthetics are generally sufficient for Er:YAG laser procedure with the possibility of local anesthesia if needed [13]. Shorter recovery periods are noted with Er:YAG laser compared to CO2 laser therapy with less postoperative edema and reduced adverse effects [14]. Traditional Er:YAG laser therapy requires more laser passes than traditional CO2 lasers to achieve a similar depth of ablation. Long pulsed Er:YAG lasers have been used for as a less efficacious treatment of deep rhytides as a substitute for pulsed CO2 laser [20]. However, outcomes are similar with some clinicians advocating superiority of Er:YAG compared to CO2 counterparts[20]. The precise skin ablation with Er:YAG can be useful for areas at great risk for scarring such as periorbital skin with some clinicians opting for Er:YAG for dyschromia and fine wrinkles as opposed to deep rhytides [13]. The more precise skin ablation with Er:YAG is sometimes desired but may also have reduced thermal damage to underlying collagen resulting in lesser skin tightening effect [14].

Fractional laser therapy for Er:YAG was developed and applied in a similar fashion to CO2 lasers. Postoperative and cosmetic outcomes were found to be similar to traditional Er:YAG [21]. Although adverse outcomes such as scarring are less frequent than with CO2 lasers, they still occur with Er:YAG laser, and appropriate patient selection is required to minimize the risk of these events. For patients at high risk for dyspigmentation and scarring (Fitzpatrick IV-VI), Er:YAG would be preferred versus CO2. Of note, the reactivation and local spread of HSV may occur following any ablative laser therapy [22]. 

Indications for ablative laser resurfacing include [23]:

• Photoaging
• Facial Wrinkles
• Acne Scars
• Scar Revisions
• Actinic Keratoses
• Seborrheic Keratoses
• Warts
• Moles and other nevi
• Xanthelasma
• Skin tags
• Rhinophyma
• Sebaceous hyperplasia
• Pyogenic granuloma
• Neurofibroma
• Angiofibroma
• Actinic cheilitis
• Tattoo Removal
• Cutaneous Vascular Lesions
• Keloids

Contraindications to ablative laser resurfacing include[23]:

• Fitzpatrick skin types IV-VI (increased risk for pigmentation issues)
• History of keloidal scarring
• Recent oral isotretinoin therapy (traditional teaching recommends cessation for 6 months prior to resurfacing procedures)
• Ectropion (particularly when considering infraorbital resurfacing)
• Morphea
• Scleroderma
• Prior Radiation Therapy
• Cutaneous Disorders (Vitiligo, Lichen Planus, Psoriasis – relative contraindication)
• Previous Facial Surgery (relative contraindication)
• active herpes outbreaks (should postpone surgery until the flare is resolved)
• ongoing ultraviolet exposure
• recent chemical peel (depending on the depth of the peel, may require 6 weeks to 6 months before considering ablative laser resurfacing)

Anesthetic

• Ablative laser procedures may be performed under local anesthesia, using a combination of topical and local anesthetic. This, of course, depends on the patient's demeanor and pain tolerance, as well as the technician's ability to employ effective sensory nerve blocks to the face if a full facial skin resurfacing is planned.
• Regarding topical anesthetic, a Eutectic Mixture of Local Anesthesia cream (EMLA) can be used by applying 2mg/cm topically under occlusion for 45-60 minutes, with removal of the occlusion just before the procedure is preformed.

Laser Selection

• CO2 or Er-YAG lasers
  • Fractional or Traditional

Laser-safe eye protection

• The patient's eyes should be protected using a wet gauze or an eye shield 
• personnel should use wavelength appropriate spectacles

Laser-safe instruments (i.e., non-flammable)

• Fire extinguisher
• Wet towels

Other Considerations

• Laser protocols, pre-op laser setting check
• Gloves, mask, and cap should be used by all personnel
• Povidone-iodine 5% solution for disinfection (alcohol should be avoided because it is inflammable)