Ablative erbium: Yag and ablative CO 2 laser resurfacing systems create a contiguous zone of controlled depth, cutaneous ablation and an associated thermal injury zone which is followed by a healing response that produces the desired improvement. In order to ablate skin without scarring, the laser must remove the epidermis and some papillary dermis while minimizing thermal injury. Since the ‘chromophore’ for the 10.6μm CO 2 and 2.94μm erbium: Yag laser is water, ablation occurs when the laser pulse is sufficient to almost instantaneously raise the intracellular epidermal fluid to its boiling point, at which time a steam plume is ejected out of the wound, quickly carrying away with it the generated heat before significant collateral tissue damage can occur. Subsequent passes with the CO 2 laser must induce thermal, conformational, and other changes in the dermal collagen to induce shrinkage and wrinkle reduction ( Figs 14.1 & 14.2 ). Because these thermal effects are thought to prolong healing times, the erbium: Yag lasers are marketed as an alternative ‘cool’ method of epidermal ablation and wrinkle improvement.
Fractionated CO 2 laser systems deeply treat about 20% of the skin surface area during each treatment. These small areas of treatment are known as micro thermal zones (MZT). The remaining untreated area promotes rapid healing in 2–3 days. Improvement continues for an additional few months after the procedure. Fractionated systems in other wavelengths (1440nm, 1540nm, 1550nm, 2940nm) are also available.
Comparison of CO 2 Ablation, Erbium Ablation, and Fractionated CO 2
Although both CO 2 and erbium wavelengths ablate tissues, with CO 2 more tissue is ablated per pulse and the zone of associated thermal injury is greater. The difference between erbium and CO 2 effects is largely related to the longer pulse duration of CO 2 (approx. 1msec) and its greater tissue penetration ( Fig. 14.3 ). The greater thermal injury and ablation associated with CO 2 are advantageous insofar as the patient has a greater improvement in wrinkling and one can resurface further into the vascular dermis without the encumbrance of significant bleeding ( Fig. 14.4 ). On the other hand, with erbium one can create a finer degree of epidermal ablation and the patients encounter proportionally quicker recovery times ( Fig. 14.5 ). Since erbium wavelength is absorbed more highly by water than is CO 2 , there is a corresponding difference in the minimal fluences necessary for ablation: for CO 2 laser, ablation occurs at or above 4–5J/cm 2 , whereas for erbium laser ablation occurs at >1.6J/cm 2 or higher ( Table 14.1 ).
|Laser||CO 2 ablative||CO 2 fractional||Er:YAG|
|Wavelength||10.6 μm||10.6 μm||2.94 μm|
|Usual pulse||950usec||Continuous or pulsed||250msec|
|Thermal damage||75-150 μm||10-50 μm|
|Vaporization||20-70 μm||Variable, upto 1500 μm or greater depth column of ablated tissue||2-400 μm|
|Tissue penetration||30 μm||See above||1 μm|
With fractionated CO 2 , only 20% of the skin surface is treated, but the CO 2 creates cylinders of vaporized tissue that are 70–100μm diameter and penetrate the skin up to a depth of 1.5mm or more. This results in rapid healing depending upon the depth of treatment, sometimes within 3–6 days. Patients generally require three treatments 1 month apart.
The first ablative CO 2 laser pass vaporizes the epidermis, leaving a residual desiccated debris which is wiped away. By contrast, several passes with the erbium: Yag laser are required to reach equivalent depth. No more resurfacing passes are necessary when the goals of treatment are limited to the epidermis. Subsequent laser passes may be applied to the papillary dermis in order to create deeper ablation. Because of the thermal effects associated with CO 2 , there is also an immediately noticeable thermal shrinkage and stimulation of a long-term healing response characterized by deposition of new subepidermal collagen (Grenz zone) and elastin fibers. This tightening of the dermis creates the wrinkle reduction ( Figs 14-6–14.10 ).
Fractional CO 2 is characterized by an immediate fine microarray of treatment spots which develop microcrusting, as well as immediate erythema and swelling.
Healing after ablative resurfacing requires the re-establishment of the barrier function of the epidermis and appropriate return of normal skin pigmentation. The new epidermis results from the differentiation and surface migration of a reservoir of basal-like keratinocytes residing deep to the epidermis in the pilar complexes. Hence, re-epithelialization may be impaired when the pilar complexes have been inactivated by isoretinoin (Accutane®) use, heavy X-ray irradiation, or destroyed (i.e. very deep resurfacing into the dermis). Hence, caution is prudent, especially when resurfacing the very thin eyelid skin or when attempting to erase very deep wrinkles whose bases may lie at the same depth as the reticular dermis of the adjacent tissue shoulder ( Fig. 14.11 ; Table 14.2 ). Likewise, hypopigmentation may result from deep resurfacing. Scarring may also be more likely in keloid-prone patients or over the edges of facelift flaps. Complete epithelialization is characterized by the cessation of weeping and oozing and a smooth, pink, dry skin surface. In general, re-epithelialization is completed within 7 days for erbium: Yag and 10–14 days for CO 2 resurfacing. Erythema typically takes a few weeks to resolve after erbium: Yag, but lasts around 12 weeks for CO 2 laser.
|Root of nose||144||324||223||691|
|Lobule of nose||111||918||735||1764|