Alpha hydroxy acids (AHAs) are natural fruit, carboxylic acids that are nontoxic. Due to their relatively high safety profile, they have become the most popular technique, not only of plastic surgeons and dermatologists but also of the cosmetic therapists and aestheticians. Glycolic acid or 2-hydroxyethanoic acid is the most commonly used AHA. Glycolic acid is derived from sugar cane and is typically used in concentrations of 20% to 70%. Other commonly used AHAs are lactic acid (2-hydroxypropanoic acid), found in sour milk and tomato juice, and citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid), found in citrus fruit. Salicylic acid, in concentrations of 20% to 30%, are also used for superficial chemoexfoliation. AHAs affect both the epidermis and most superficial layer of the dermis by creating a loss of cohesion of keratinocytes of the stratum granulosum. This allows sloughing of the abnormal cells and affords thinning of a thickened stratum corneum. These benefits can last for 2 to 3 weeks.

The Jessner’s solution, composed of 14 grams (g) of salicylic acid, 14 g of lactic acid, and 14 g of resorcinol in 100 mL of ethanol, can be used safely as a superficial peeling agent. Its depth is coat dependent and applying one to three coats, exfoliation, or removal of the stratum corneum, can be achieved. However, by applying 5 to 10 coats, the depth of penetration extends to the basal cell layer. Similarly, a simple 50% resorcinol solution, applied for up to an hour, can result in a superficial peel.

A false sense of safety should be avoided with AHAs. For example, without neutralization, glycolic acid can penetrate deeply. Its time-dependent action should be neutralized with water. By leaving glycolic acid unneutralized, dermal penetration will ensue and healing problems, crusting, and scarring can result. The desired endpoint of an AHA peel should be erythema and light peeling of the epidermis.

Medium-depth peeling agents are those that create injury through the depth of papillary dermis and create some inflammation in the upper reticular dermis. The recent gold standard of the medium-depth peel has been trichloroacetic acid (TCA). This agent has no systemic toxicities, and in its crystalline form, it lacks light sensitivity and does not require refrigeration for stabilization. However, it has a narrow margin of error, and the risk of scarring is much higher than with phenol-based solutions. TCA concentration is based upon a strict weight by volume standard (g/100 mL distilled water). There have been anecdotal accounts of miscommunication between the physician and the pharmacist in which the pharmacist instead used a weight-to-weight concentration. The results ended with patients being exposed to concentrations greater than 50% TCA with resultant scarring. For clarification, scarring has also been reported when using TCA concentrations of 50%. No emulsifiers, additives, or surfactants have been found to lessen this risk.

Combining TCA 35% solution with another less potent agent allows practitioners to achieve the same effects of a 50% TCA without the aforementioned risks. The additional agent is applied first as an epidermolytic to allow the TCA solution to penetrate more deeply into the dermis.

Monheit has written extensively regarding the combination of the Jessner’s peel solution and 35% TCA. The Jessner’s peel destroys the epidermis and is followed by an immediate application of a 35% TCA solution. This allows for even application of the TCA solution and affords a deeper penetration. In contrast to the phenol peels, the frost does not form immediately and an adequate time of 3 to 4 minutes must be allowed for the full frost to appear. Additional applications can be made to the desired depth. This combination has shown to produce an increase in type I collagen deposition, increased fibroblast activation, and decreased elastic fibers relative to 35% TCA alone. Application of multiple layers of the 35% TCA solution has an additive effect, which can cause a deeper peel leading to hypopigmentation and scarring.

Similarly, the combination of glycolic acid with 35% TCA increases the depth of the peel. Histologic biopsies have proven that this is a slightly deeper level peel than Monheit’s Jessner/35% TCA combination peel and was found to cause more neoelastogenesis, neovascularization, and papillary dermal fibrosis. The mentioned combination peels have a risk of scarring of less than 1%, which is similar to that of CO₂ laser resurfacing and phenol-croton oil-based peels.

When medium-depth peels do not afford enough penetration to treat the deepest of rhytids, specifically the “crow’s feet” and perioral region, a deeper treatment is considered. The deep peel relies on injury to the reticular dermis to affect change to these areas. The Baker-Gordon peel (Table 30.1) provides considerable improvement of deeper rhytids; however, the irreversible hypopigmentation risk and widely discussed phenol cardiac and renal toxicity shifted resurfacing practices away from this once popular formulation. While the papillary dermis heals through reorganization, the reticular dermis is thought to heal through the process of scarring. Further, with additional healing time due to the depth of treatment, additional risks and complications rise. Despite these well-known risks, in selected patients and situations, the Baker-Gordon formulation provides an effective treatment option for those patients with Fitzpatrick type I and II skin who present with deepened rhytids in discrete anatomic subunits. It is then combined with medium-strength peel solutions throughout the rest of the face. Conservative and even application with awareness of frost development will further help to avoid potential complications.

Croton oil is pressed from the seeds of Croton tiglium, a small shrub found in India and Ceylon. The oil consists mainly of oleic, linoleic, myristic, and arachadonic acids. Less than 5% of the oil is made up of a resin, which has been known since 1895 in scientific literature to possess irritant and toxic properties. When applied to the skin, this toxic resin creates severe vesiculations of the skin and a resultant wound taking almost 3 weeks to
heal. In order to better understand the role of croton oil, Hetter performed multiple chemical peels of different concentrations of phenol and croton oil. He found that phenol penetrated more deeply with increased concentrations, that higher concentrations of phenol (88%) without Septisol penetrated more deeply than lower concentrations (50% and 35%), and that increasing the concentration of croton oil added to peel formula resulted in a more profound dermal effect and deeper peel. It should be noted that multiple coats will increase the depth of injury. Finally, 1% croton oil solutions were noted as the upper threshold for safe use to avoid serious risk of hypopigmentation.

Today, it is optimal to have a more standardized means of measuring the croton oil concentrations, instead of relying on droppers, which are inherently inconsistent. Drops are now converted to cubic centimeters by having 25 drops equal one cubic centimeter. Using this conversion, a stock solution of 0.04 mL of croton oil per 1 mL of phenol, from which one can make varying croton oil concentrations of 0.4%, 0.8%, 1.2%, and 1.6% in a constant phenol concentration, needing only Septisol, phenol, and water. Using these formulations, the practitioner can decide between a phenol concentration of 35% or 48.5%.

Due to the differences in skin throughout the face, it is common practice to apply different depths of peeling for individual subunits of the face. This can also be translated into the use of varying concentrations of croton oil in different regions of the face. While the lower nose can tolerate croton oil concentrations up to 1.2%, the cheeks and forehead only tolerate concentrations up to 0.8%, and the upper nose, temple, and lateral brow can only withstand concentrations up to 0.4% before the risk of complications arise.

While the depth of penetration is dictated not only by the components of the peeling solution but also by the concentrations of these components, the application of these varying concentrations must be controlled to create desired results. For example, Stone’s work revealed that varying the concentration of phenol and croton oil produced equivalent levels of fibrosis when applied with 50 rubs. However, when fewer strokes were used, thus increasing the concentration of phenol, a greater depth of peel was observed. This was again confirmed with the use of a lower phenol concentration and found that increasing concentrations of croton oil lowers the threshold number of applications needed to achieve the same depth of peel. This work emphasizes that the technique and application of the peel formula to the skin is as important as the formula used. This is where we feel the experience of the peeler and the “art” of peeling become critical to achieving a successful outcome.