The first of these questions must be answered against the backdrop of the state of the art at that time (i.e., when laser-assisted skin resurfacing first emerged). Chemical skin exfoliation (chemical peels) developed almost in parallel with mechanical skin exfoliation and both became established modalities during the mid- to late 1960s.1–5 Chemical peeling is a process whereby a chemical cauterant is applied to the skin to induce exfoliation. A variety of agents can be used that produce a variety of effects, ranging from a light peel, in which the stratum corneum is affected, to a deep peel, in which necrosis can extend all the way to the reticular dermis. These include salicylic acid, trichloroacetic acid (TCA), the α-hydroxy group of acids, Jessner’s solution (i.e., salicylic acid, resorcinol, lactic acid, and ethanol), and phenol.^6–10 By contrast mechanical skin exfoliation refers to dermabrasion usually accomplished with a power-driven rotary dermabrader. A direct comparison of these two modalities revealed consistent changes.¹¹ In general, the papillary dermis enlarged and the depth of the injury correlated well with the benefit derived from treatment to a point. Hayes et al.¹² showed that the upper reticular dermis heals by regeneration from residual adnexial structures, whereas the deeper reticular dermis heals with scar tissue formation. An injury extending down to the reticular dermis is therefore likely to leave irreversible scarring. A more superficial injury is likely to heal by regeneration. This is the fundamental basis of aesthetic and therapeutic skin exfoliation. Kligman et al.¹⁰ studied the long-term histologic changes associated with chemical peels and noted a newly formed wide band of thin compact collagen bundles, parallel to the overlying skin. They also noted elastin fibers within the neocollagen and, once again, parallel to the surface of the overlying skin. The clinical correlation was apparent as a smoothing of the skin. Obliteration and subsequent regeneration of the epidermis from residual adnexiae resulted in elimination of dyschromias, keratoses, and the reestablishment of the normal vertical polarity of skin. The benefits of limited skin exfoliation were thus obvious.

Despite some excellent results, several limitations and some devastating complications became evident.¹³ The major limitations related to a lack of precision. With regard to chemical peeling, it became evident that there was variability in the success rate among patients and that this was probably due to an inherent lack in the control of the depth of exfoliation. So many variables determined the depth of exfoliation that any classification of peeling agents was virtually meaningless.¹⁴ For example, the application of 25% TCA with a cotton-tipped swab to the face of a patient with thick oily skin that has not been primed before treatment will result in superficial intradermal sloughing of the skin. If a piece of gauze, saturated with the same concentration of TCA (25%), is rubbed repeatedly on the face of a thin-skinned woman whose skin was primed before treatment with 0.1% retinoic acid, the extent of injury will be much deeper and will probably extend to the midpapillary dermis. The variables that determine the depth of the peel appear to be related to the type of agent used and to the technique with which it is applied:

Peeling agent and its concentration
Number of coats applied
Technique used to apply the agent
Method of skin cleansing prior to the application
Whether the skin was pretreated in the weeks just before the peel
Skin type
Anatomic location of the peel
Duration of contact with skin

Not only were there complications relating to the actual peel, such as scarring, hypopigmentation, and hyperpigmentation, but complications resulting from an adverse effect of the specific agent used became evident. Resorcinol and salicylic acid can cause systemic toxicity, and even death in rare instances, several of the other agents can provoke an allergic reaction.^{13, 15, 16}

As with chemical peeling, during dermabrasion, an appreciation of the depth of the treatment at any point in time is difficult and only comes with experience. Once again, this lack of precision led to variability in results.^{4, 17} Profuse bleeding not only obscures the endpoint, but aerosolization of blood and tissue by the high-speed burrs, with the attendant risks of airborne infection, should discourage against using this technique.

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