NONABLATIVE LASER RESURFACING

Selective cooling provides an added safety measure of protection to the epidermis during laser treatment of the skin. The novel 1320-nm Nd:YAG laser system combines cryogen spray cooling with selective photocoagulation in the upper dermis. This article reviews the clinical results from applications of this laser system to treat facial rhytides.

NONABLATIVE LASER SYSTEMS
There are an increasing number of devices available for nonablative photorejuvenation, or skin renewal. They can be divided into two classes: visible light devices for treating rosacea and pigmentary changes caused by photodamage, and midinfrared devices for promoting di­rect collagen remodeling. Visible light devices such as the dye laser are predominantly used on lighter skin types and can have some ef­fect on dermal collagen,although the light is absorbed primarily in chromophores such as pigment or hemoglobin. Midinfrared light is absorbed primarily in water and collagen and can be used to heat tissue uniformly indepen­dent of skin pigmentation.

The first such midinfrared device is the Nd:YAG laser operating at 1320 nm (CoolTouch, Roseville, CA). Energy at this wavelength can penetrate to the papillary and upper reticular dermis, with minimal effect to the epidermis. This device is used in a pulsed mode to confine the photothermal response to a uniform, thin subepidermal layer of skin. A pulse width of about 20 msec to 50 msec confines the thermally mediated area to a band approximately 200-u thick. At 1320 nm there is a significant amount of backscattering. The scattering causes photons to lose their energy and dissipate before penetrating into the lower dermis. Also, many photons near the epidermal surface scatter out of the tissue without depositing energy in the epidermis. The result is a temperature profile in skin that peaks just below the surface. The tem­perature in skin can be set to a desired value by carefully adjusting the laser fluence at the skin surface. The treatment temperature in the epidermis is reduced by the use of a pulsed cooling device synchronized with the laser pulse.

With the 1320-nm laser, pulsed cooling can be operated in either a dynamic or nondynamic precooling mode or in a postcooling thermal quenching mode. In the precooling mode, a cryogen spray is administered to the tissue surface before the laser pulse. A cooling front propagates through skin tissue from the sur­face. A subsurface layer of tissue drops in tem­perature as heat diffuses out toward the skin surface. The layer then warms again as body heat from deeper layers diffuses upward. The subsurface layer of tissue effectively senses a cooling pulse whereby its temperature drops, reaches a minimum, and then subsequently warms again. If the tissue is exposed to the laser immediately subsequent to the cooling pulse, then the surface is at its lowest tem­perature, and adjacent subsurface layers of tis­sue will continue to drop their temperatures. The resulting cooling front advances into tissue while the laser energy simultaneously heats tis­sue up, causing a dynamic temperature profile and providing epidermal protection. This pro­cess can be referred to as dynamic cooling. Laser treatment occurs during the dynamic phase of cooling.

In contrast, the laser can be fired after a dwell time of several milliseconds after the end of the cooling pulse when a particu­lar subsurface layer of tissue is at minimal or nondynamic temperature. This timing of laser discharge allows the cooling front to dif­fuse fully through the epidermis and reach a static equilibrium just before the laser exposure takes place, resulting in a nondynamic,tem­perature profile. The depth of the subsurface layer determines the required dwell time. For the papillary dermis, dwell times of 10 msec to 30 msec are used. Nondynamic precooling allows for full-thickness cooling of skin tissue.

Other midinfrared lasers with wavelengths at 1540 nm and 1450 nm have been used with dynamic precooling for photorejuvenation. Energy at these wavelengths has much higher absorption in water and has been used in an attempt to create temperature profiles in skin with peak temperatures closer to the skin surface in combination with dynamic cool­ing techniques. Most of the energy at these wavelengths is absorbed in the epider­mis. The temperature profile is overwhelm­ingly influenced by the cooling technique, however, so that the resulting temperature pro­files for 1320 nm, 1450 nm, and 1540 nm lasers with precooling are similar. Cooling the epi­dermis effectively cools the papillary dermis. Using a laser with greater absorption in water requires increased epidermal cooling, resulting in more cooling of the papillary dermis and effectively negating any advantage.

A recent development, using thermal quenching (postcooling) in combination with the 1320-nm laser, is a unique modality that shows promise of creating a peak temperature closer to the surface. In this process, the cooling is applied immediately after the laser exposure. Because the papillary dermis is not cooled, much lower fluences are required, and the temperature profile peaks closer to surface. Thermal quenching is the subject of ongoing investigations.

Early work at 1320 nm showed that at con­stant treatment fluence, results varied from patient to patient. Other investigators have found that consistent results could be obtained by adjusting the fluence so that the result­ing peak surface temperature of the treated skin was 42°C to 48°C, correlating to an optimal higher peak temperature in the up­per reticular dermis. Surface temperature is monitored by a noncontact radiometer inte­grated into the laser delivery device. The el­evated temperature initiates an inflammatory response, leading to long-term collagen remod­eling and resulting in improvement of facial rhytides.

The standard protocol calls for adjusting the treatment fluence to achieve a peak surface temperature reading of 40°C to 45°C, with non­dynamic pulse precooling with a 10-msec to 20-msec delay between pulsed cooling and laser treatment. Laser energy is delivered into adjacent 5-mm spots, fully covering the desired treatment area. Two passes are used to achieve uniform transient erythema, which lasts up to 2 hours.

NONABLATIVE LASER TREATMENT FOR RHYTIDES

Clinical and Histologic Evaluations

In 1999, Kelly et al described the use of nonablative Nd:YAG (wavelength 1320 nm) laser in treating facial rhytides (Figs. 1 and 2). In this study, statistically significant improve­ments were noted at 12 weeks. At 24 weeks, however, statistically significant improvement could be demonstrated only in the severe rhytides group. Goldberg reported clinical im­provements in 8 of the 10 patients in his study. Histologic changes were also found to be sig­nificant in this study (Fig. 3). Menaker et al found an increase in homogenization of col­lagen in their study. These changes were re­ported as not statistically significant, however. Menaker et al also reported hyperpigmentation in four of the ten patients. Hyperpigmentation also was found not to be statistically significant. Golberg pointed out that in Menaker's study a previous version of the prototype 1320-nm Nd:YAG laser was used for clinical and histologic evaluations. In Menaker's study, the laser used did not have the thermal sensor that was used in Goldberg's study. The different models of this laser might have accounted for the differences in clinical and histologic results found in Menaker's and Goldberg's studies.

NONABLATIVE LASER TREATMENT FOR RHYTIDES IN ASIAN PATIENTS

Background

A thorough literature search revealed no pre­vious study examining the efficacy and safety of the Nd:YAG laser system in Asian patients. The following data are the first known report of nonablalive laser treatment in pigmented skin in the literature.

Materials and Methods
Twelve patients, three men and nine women, with class I to III rhytides were enrolled in the prospective study. All patients were Asian, with Fitzpatrick skin types III to V. Their ages ranged from 49 to 75 years. All patients presented with rhytides in the peri­orbital area. The rhytides were treated with the Cool Touch Thermescent Skin Treatment Nd:YAG laser system (Cool Touch, Roseville, CA). The fluence was set between 30 J/cm2 and 39 J/cm2. The peak measured therapeutic tem­perature ranged from 38°C to 48°C, and one to two passes were performed. The cryogenic spray was set at 20-msec duration prior to the laser discharge. Four patients underwent se­rial treatments at intervals of 4 weeks, for a total of two treatments. A total of 6 months follow-up was implemented for all twelve pa­tients. No patients were lost to follow-up study. Clinical improvements were assessed by an in­dependent clinician. Biopsies of skin removed in the periorbital areas for histologic evalua­tions were performed before and at 2 weeks after treatment in six patients from the single-treatment group. The Wilcoxon signed-rank test was used to evaluate results from the sin­gle treatment group, and the Friedman test was used for the serial-treatment group.

Results
For the group that received a single treat­ment, only two patients noted improvements of rhytides at 6 months. These improvements were not statistically significant. All four pa­tients who received serial treatments noted im­provements of rhytides at 6 months (Fig. 4). The improvements in these four patients were found to be statistically significant by the Friedman test. Transient erythema was noted in all patients but subsided approximately 3 hours after treatment. One patient was found to have swelling 1 day after the procedure. Two patients reported pain that lasted no more than 2 hours after treatment. No hyperpigmentation, milia, hypersensitivity, or pruri­tus were noted. With the exception of tran­sient erythema, the unwanted side effects observed in this study were not statistically significant. Histologic evaluations performed preoperatively and 2 weeks after treatment re­vealed no evidence of increased melanin in the dermis.

Discussion
The new nonablative laser treatment pro­vides distinct advantages over ablative laser treatment. In previous studies, hyperpigmentation was noted to be prevalent in Asian patients after laser resurfacing. In this study, considerable shortening of recovery time was achieved. No hyperpigmentation was observed within 1 month of surgery and at 6-month follow up. The erythema lasted for only a few hours, and the swelling lasted no more than 24 hours. These side effects were not statistically significant. Menaker noted that 4 of the 10 patients developed hyperpigmentation of the periorbital area 1 month after treatment. Hyperpigmentation lasted for 3 months in 3 of the 4 patients. Even though the change in hyperpigmentation was not statistically significant, the fact that none of the patients in this study experienced hyperpigmentation, despite their darker skin, showed that thermal injury could play a significant role in the production of pigment in the postoperative period. Hyperpigmentation was well-documented in ablative laser resurfacing studies in which no thermal feedback technology was yet available. It should be noted that in the author's study of Asian patients, the peak measured therapeutic temperatures ranged between 38°C and 48°C. These temperatures were comparable with those in Goldberg's study. The lack of evidence of hyperpigmentation noted in the author's study indicated that it is probably safe to target peak measured therapeutic temperatures in treatments of skin types III to V between 38°C and 48°C. The one patient who experienced swelling lasting until the next day had a peak measured therapeutic temperature of 48°C.

Erythema was noted in Menaker's study to last from several hours up to 3 days after the procedure. Goldberg also found erythema in the immediate postoperative period. The duration of erythema, however, was not specified in Goldberg's study. Also in Goldberg's study, no blistering, pigmentary changes, or scarring were observed. In this study erythema was found to last no more than 3 hours after the procedure. No blistering or pitted scarring was seen in this study. Kelly et al reported blistering and subsequent hyperpigmentation in these blistered areas. Menaker et al observed pitted scars in several treated areas; two of these areas developed hyperpigmentation. Of note was the relatively low fluence used in Menaker's study. The treatment fluence in that study was set at 32 J/cm2, whereas those in this study ranged between 30 J/cm2 and 39 J/cm2 and those in Goldberg's study between 28 J/cm2 and 38 J/cm2. Once again, despite the higher fluences, the less unwanted effects noted in this study and in Goldberg's appear to be related to the thermal control in the form of thermal sensor available with the newer laser model.

At 6 months after the procedure, clinical improvements were noted in all 4 patients who had serial treatments, in contrast to only 2 of the group who had had a single treatment. The absence of postoperative unwanted effects allowed patients to return to daily activities almost immediate after the procedure. This advantage allowed physicians to treat patients several times without affecting the patients' work schedules. Serial treatments with nonablative laser resurfacing could provide a safe alternative to single ablative treatments with erbium:YAG or CO2 lasers.

No evidence of dermal or epidermal increase of melanin was noted in the author's study. Two weeks after the procedure, however, no change in collagen production or reorganization was discernible in this study. Both Menaker et al and Goldberg noted homogenization of collagen. The histologic evaluations in their studies, however, were done at 1, 3, and 6 months after treatment, in contrast with the 2-week evaluations performed in this author's study. The purpose of histologic evaluations in this study was to determine whether there was any increase in pigment changes in darker skin types. To detect homogenization of collagen, histologic evaluations must be performed at 1, 3, and 6 months after treatment. A period of 2 weeks is too brief to observe homogenization of collagen.

Conclusions
Patients with skin types IV and V responded well to serial nonablative treatment, with no lasting unwanted effects. Even though previous studies and the author's study have shown that the Q-switched Nd:YAG laser, when coupled with cryogen spray and thermal sensor, could be a safe and effective treatment modality for rhytides, long-term follow-up studies are necessary to determine the lasting effects of the treatment. Additional research focusing on thermal control during treatment could help to elucidate the role of thermal effect on collagen reorganization and pigment production. Long-term histologic evaluations of treatment effects in skin types III to V also are needed to provide objective documentation of laser efficacy.


SUMMARY
The accumulated data on nonablative laser treatments for rhytides showed that this new modality could be used safely in patients with skin types I to V, provided that epidermal temperatures were monitored carefully. The absence of hyperpigmentation noted in this author's study showed that thermal control could greatly reduce hyperpigmentation during the postoperative period. Other unwanted effects, such as pit scarring, also could be prevented with thermal feedback.

Clinical improvements in rhytides, although modest, were evidenced in all studies. Serial treatments appeared to provide greater improvements than a single treatment. The full effects of nonablative laser treatment cannot be appreciated until 3 to 6 months after the procedure, however. These effects are probably the results of collagen homogenization.

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