| | Noninvasive skin measurements after CO2 and erbium laser resurfacing☆☆☆Accepted 2 October 2002. Abstract Background: Improvement in skin after laser resurfacing depends on treatment to a precise level. The target treatment level will depend on skin type, anatomic location, and severity of the presenting problem. The development of noninvasive skin measurements that correlate with histologic changes would provide a useful intraoperative guide for treatment. Objective: The relationship among different types of lasers and levels of applied energy; objective real-time noninvasive measures of the skin; and histologic condition were studied in an attempt to gain information that could aid in achieving safer and more predictable laser skin resurfacing. Methods: The paraspinal skin of Yorkshire hybrid pigs was used for the study. In pig 1, study sites were treated with the Ultrapulse CO2 laser (Coherent, Inc, Santa Clara, CA) with the CP-G at 300 mJ for 0 to 5 passes. The Sciton Contour erbium:YAG laser (Contour, Sciton, Palo Alto, CA) set at 200 μm of pure ablation was used to treat a separate but adjacent area for 0 to 5 passes. Treated sites and control sites were evaluated by measurements of biomechanical properties, digital imaging, and barrier integrity. In pig 2, the study was repeated with the Sciton Contour erbium:YAG laser at 10 different energy levels. Full-thickness punch biopsy specimens were obtained immediately after treatment and correlated with the biomechanical measures. Results: After the third pass with the CO2 laser, there was no additional tissue ablation or change in surface biophysical measurements. There was an apparent increase in inflammation in the lower dermis with passes 4 and 5 seen on day 2 biopsies that suggested deeper thermal effects. The erbium laser continued to ablate with increasing applied energy. The change in viscoelastic creep correlated with the laser energy applied and inversely correlated with the remaining dermal depth of the skin after laser treatment. Conclusions: The measurement of immediate changes in viscoelastic creep should be further studied as a means of guiding intraoperative erbium laser resurfacing. Histologic evaluation suggests that the erbium: YAG laser may be a more effective tool for dermal ablation than the CO2 laser. (Aesthetic Surg J 2003;23:20-27.)
Laser resurfacing is a significant tool for improving skin conditions such as severe sun damage and acne scarring. The treatment remains empiric, and we are only now beginning to understand the reparative process of the skin. Fundamental to treatment strategy is that depth of injury is critical to the outcome. Treatment deeper than necessary risks increased scar formation, whereas treatment superficial to the desired level results in less-than-optimal improvement. The optimal mix of thermal and ablative effects needed to maximize improvement in skin condition in each patient is still unclear.
In general, intraoperative judgments during CO2 laser treatment are based on changes in the color and texture of the skin resulting from applied laser energy. With erbium laser resurfacing, the character of dermal bleeding, along with the level of applied energy, have guided treatment. We believe that the safety, outcome, and predictability of laser resurfacing could be improved by quantifying the biophysical properties of the skin before and during laser resurfacing and correlating this with treatment depth, histologic findings, and outcome.
We sought to develop a noninvasive method of quantitatively characterizing skin changes during laser resurfacing procedures as a function of laser power input and to correlate data from noninvasive measurements with the changes in the epidermis/dermis revealed on histologic examination. The changes in epidermal/dermal architecture (ie, in the collagen and elastin) that occur during laser ablation would be reflected as changes in biomechanical properties, including elasticity, viscoelasticity, and elastic deformation. Results from animal studies with 2 commonly used lasers, a Coherent Ultrapulse CO2 laser (Coherent, Inc, Santa Clara, CA) and a Sciton Contour erbium:YAG laser (Contour, Sciton, Palo Alto, CA), were studied, and their histologic effects were compared. We hope that the results will facilitate the implementation of safer and more predictable laser skin resurfacing procedures in human subjects.
Material and methods  The Yorkshire hybrid pig model was chosen because this was used by Fitzpatrick in early experimental work with the CO2 resurfacing laser.1 Quantitative measures of skin properties were made with the Cutometer SEM 575 (Courage and Khazaka, Koln, Germany) to establish the technical relationship for later evaluation of human skin responsiveness.2, 3, 4 This device measures the biomechanical properties of skin pulled into a small aperture and subjected to negative pressure. The measurements were taken during operation with 2 different laser resurfacing technologies. CO2 laser resurfacing (pig 1) Twelve treatment sites, each measuring 3 × 3 cm, were tattooed on the paraspinal area of a 14-week-old female Yorkshire hybrid mini-pig after removal of hair with electric clippers. The sites were arranged in a 2 × 6 grid along the paraspinal area to provide replicate treatments. Five pairs of sites were treated with an Ultrapulse CO2 laser with the CP-G at 300 mJ/cm2 for 1, 2, 3, 4, and 5 passes, respectively. Between successive passes, the laser saline solution-soaked gauze was used to remove any debris. The 2 remaining sites served as untreated controls. Skin biomechanical properties were measured at each site following each pass. One of the 2 treatment sites was used for collection of 4-mm full-thickness punch biopsy specimens immediately after surgery. Erbium:YAG laser resurfacing (pig 1) On postsurgical day 35, a third set of 6 sites was prepared. The sites were approximately 3 × 3 cm and were located on the sides of the animal (3 sites per side) to avoid the sites on the back that had been treated during the first surgery. The sites were treated with the Sciton Contour erbium:YAG laser at 250 μm in pure ablation made. Sites received 1, 2, 3, 4, or 5 passes of laser energy to achieve nominal depths of 250, 500, 750, 1000 and 1250 μm, respectively. One site served as an untreated control. Skin biomechanical properties were recorded after each pass. Full-thickness 4-mm punch biopsy specimens were obtained immediately after the treatments. Erbium:YAG laser resurfacing (pig 2) To further study the relationship between erbium:YAG laser-applied energy, noninvasive measures and histologic study, the paraspinal area of a second 14-week-old female Yorkshire hybrid mini-pig was prepared by removing the hair with electric clippers. Thirty 2.2 × 2.2-cm sites were marked on the skin with a surgical pen. The sites were smaller than those used for pig 1 to accommodate the larger number of skin sites for which measurements were made. They were arranged in a grid with 5 sites across the back and 6 sites along the length of the back. The sites were treated with the dual-mode erbium:YAG laser in the ablative mode with varying levels of power (ablative energy). Three replicates were treated with each of 10 depths of ablation: 20, 25, 50, 100, 200, 300, 400, 500, 600, and 700 μm. The treatments were performed in randomly assigned locations in the 5 × 6 grid to account for regional differences on the back. Three untreated skin regions along the paraspinal region and between the treated sites served as controls. Within each set of replicates, noninvasive measurements of skin condition were made on 2 of the sites treated at each level of power immediately after treatment, and full-thickness, 4-mm punch biopsy specimens were collected on the third site for histologic evaluation. Histologic study The biopsy specimens from the CO2 laser treatments for pig 1 and the erbium:YAG laser treatments for pig 2 were fixed in formalin overnight at 4°C. The tissues were subsequently dehydrated and embedded in paraffin by routine procedure. The samples were sectioned and stained with hematoxylin and eosin. The tissue samples from the erbium treatment of pig 1 were fixed in formaldehyde/glutaraldehyde for processing. The histologic samples were evaluated for qualitative effects on the epidermis and dermis by an experienced microscopist (R. E. B.) with standard light microscopic techniques. Depth of tissue ablation was determined semiquantitatively by measuring multiple regions of the remaining dermis with a micrometer. Statistical analyses The changes in biomechanical properties were evaluated with the paired t test statistical procedure with SigmaStat Software (Jandel Scientific, Chicago, IL). Analysis of variance and paired t test procedures were used to assess the effects as a function of laser power/ablation level. Correlations of biomechanical measurements with laser power and depth setting were determined with the Pearson and Spearman techniques. Results CO2 laser resurfacing (pig 1) One pass with the CO2 laser resulted in ablation of the stratum corneum and epidermis. The second pass resulted in additional ablation and some thermal damage. After the third pass, there was no further ablation, but rather an annealing together of the structural fibers, as shown in Figure 2.
Increased inflammatory reactions were observed in the lower dermis after passes 4 and 5. The response of pig skin to CO 2 laser energy input did not follow a dose-dependent pattern. The hair follicles were intact, indicating that tissue was available for epidermal regeneration. The findings also confirmed reports by other investigators. Burkhardt et al 5 concluded that injury depth did not increase after multiple passes and that the removal of water from the upper dermis prevented the injury. Fitzpatrick 6 reported that a plateau in ablation occurred after 3 passes and thereby provided a mechanism for controlling thermal damage. Changes were observed in the biomechanical properties of the skin after CO2 laser treatment. The elastic recovery, Ur, increased with the first 3 passes and then decreased with passes 4 and 5, as shown in Figure 3, A.
In addition, the first pass resulted in a decrease in the elastic deformation because of stress (Ue). This finding suggests that the stratum corneum and epidermis provide a significant amount of mechanical strength to the tissue. After 3 passes, however, the elastic deformation component increased, indicating a stiffening or lack of flexibility in the tissue. This finding is consistent with compaction of the fibrous tissue, possibly as a result of dehydration or annealing of collagen. Erbium:YAG laser resurfacing (pig 1) For pig 1, multiple passes were applied to the test sites. The erbium:YAG laser continued to ablate the tissues with each pass of applied energy. Examination of the histologic sections showed that the depth of ablation was proportional to the total applied energy (number of passes). The first pass removed the entire epidermis, as shown in Figure 4.
Subsequent passes removed tissue in the papillary and reticular dermis. The entire dermis was removed with 5 passes. Changes in biomechanical properties of the tissue were observed as ablation progressed. The elastic deformation, Ue, increased for first 2 passes and leveled off after the third pass. With additional ablation in passes 4 and 5, Ue again increased. After pass 5, the dermis was completely ablated and the observed elastic deformation was attributed to blood vessels and subcutaneous fat. Correlations among changes in mechanical properties relative to baseline (before treatment) values and laser power were determined. The elastic recovery parameter, Ur, was significantly correlated with the laser power (correlation coefficient = 0.87, P = .05), as shown in Figure 3, B. The relationship between elastic recovery and erbium:YAG laser power input indicates that the difference between elastic recovery and the baseline, untreated skin condition increases as the laser power increases. This finding suggests that elastic recovery is generally related to the applied power and therefore to the depth of penetration into the dermis. Erbium:YAG laser resurfacing (pig 2) The objectives of the second pig study were to determine the relationship between the amount of ablation and changes in the biomechanical properties at the lower end of the erbium:YAG power range, beginning with very low levels (20 μm) to simulate very superficial skin treatments. Evaluation of the histologic sections indicated that the laser settings of 20, 25, 50 and 100 μm produced minimal changes in the epidermis. For the 20-, 25-, and 50-μm treatments, there appeared to be no correlation between epidermal changes and power level. In each sample, the stratum corneum was present and intact. There was epidermal condensation of the keratinocyte nuclei and cytoplasmic vacuolization. At 100 μm, keratinocyte aberrations were evident and the stratum corneum appeared to be abraded, with a small amount of loss. Some viable epidermis remained in the 200-μm sample and there was extensive vacuolization. Substantially more of the stratum corneum and stratum granulosum had been removed. The epidermis was completely removed at power levels greater than 300 μm. There was evidence of some cellular condensation in the papillary dermis of these samples. In the tissues treated with 500, 600, and 700 μm, the extent of dermal fragmentation was increased relative to samples at lower power settings. Figure 5 illustrates the histologic features observed at various power levels.
Changes in biomechanical properties of pig skin relative to the baseline untreated skin condition were observed with increasing erbium:YAG laser power in the second pig as well. Total recovery (Ua), viscoelastic creep (Uv), total extensibility (Uf), and elastic recovery (Ur) were positively correlated with the laser power setting in micrometers. The correlation coefficients are shown in Table 2.
| | |  | Change in biomechanical property | Correlation coefficient | P value | |
|---|
| Total recovery, Ua | 0.85 | .002 | |
| Viscoelastic creep, Uv | 0.84 | .002 | |
| Total extensibility, Uf | 0.85 | .002 | |
| Elastic recovery, Ur | 0.88 | .001 | | | | |
Changes in biomechanical properties were evaluated for the combined erbium:YAG resurfacing data from pigs 1 and 2 to determine the relationships across the entire power range. As shown in Table 3, significant positive correlations were found for Ua, Uv, Uf, and Ur.
| | | | Change in biomechanical property | Correlation coefficient | P value | |
|---|
| Total recovery, Ua | 0.67 | .009 | |
| Viscoelastic creep, Uv | 0.82 | .0004 | |
| Total extensibility, Uf | 0.70 | .005 | |
| Elastic recovery, Ur | 0.76 | <.001 | | | | |
The relationship between the change in viscoelastic creep (v) relative to the baseline skin condition and laser power in microns for the combined data sets is shown in Figure 6.
Histologic changes are also indicated. These data demonstrate consistent results between the 2 experiments. Discussion Skin resurfacing techniques promote the repair of damage (eg, ultraviolet ray exposure and photoaging) to the dermal structures that provide the elastic properties to the skin. Tsukahara et al7 demonstrated that treatment with the CO2 laser restored the skin mechanical properties in the skin of Sprague-Dawley rats in which UVB rays had been used to induce wrinkles. Histologic evaluations conducted 6 weeks after treatment indicated recovery of the 3-dimensional architecture of the elastic structures. Fitzpatrick8 demonstrated an improvement in wrinkling in human beings after treatment with the CO2 laser and indicated that the mechanism involved shrinkage of collagen from the heat. Smith et al9 used the pig model to investigate the effects of thermal damage with the CO2 laser. They found that treatment of the skin with 2 passes (energy of 300 mJ, duration of 60 s resulted in denaturing of the collagen and viable cell death in the upper papillary dermis. An additional pass affected the tissues deeper in the papillary dermis and that collagen hyalinization was related to the extent of thermal damage. Stuzin et al10 investigated the effects of CO2 laser resurfacing on facial skin in subjects with photodamage before facial rhytidectomy. Their histologic evaluations indicated the generation of neocollagen in the superficial and mid-dermis, as well as new collagen in the dermis. New elastin fibers were evident in the dermis, and depth of effect in the dermis was dose-dependent. Several reports have compared the effects on skin of various types of lasers and resurfacing treatments.11, 12, 13, 14, 15 de Noronha et al11 compared the effects of the CO2 laser with those of an erbium:YAG system. For both lasers, 2 passes caused more ablation and thermal damage than 1 pass. When tissues were treated with 1 pass of the CO2 laser and then 1 pass of the erbium laser, they exhibited the same amount of ablation found after 1 CO2 laser pass, but significantly less thermal damage. Greene et al12 compared the histologic effects of the CO2 laser, the erbium:YAG laser, and blended lasers on the periauricular skin of patients 4 to 6 months post-treatment and found that CO2 laser treatment produced the greatest amount of collagen, whereas erbium:YAG laser treatment produced the least. Erbium:YAG laser treatment resulted in the greatest rate of decrease in erythema and inflammation. Combination treatments with both lasers produced effects that were between the two individual systems. The authors indicated that the histologic findings correlated positively with the clinical changes. Trelles et al14 compared 2 commercial brands of CO2 lasers with a 90-day split-face facial resurfacing protocol in 22 subjects. The histologic analyses indicated that the lasers differed in the degree of inflammation, collagen proliferation, compaction, and elastin levels. Utley et al15 compared the histologic effects of the treatment with the CO2 laser, the erbium:YAG laser, and combinations with both lasers on periauricular skin over 7 days after treatment. The authors concluded that collagen injury and thermal damage could be minimized and dermal fibrous material could be increased by use of fewer CO2 laser passes followed by an erbium:YAG laser treatment. The relationship between the mechanical properties of the skin (eg, elasticity, viscoelastic creep, deformation) and the histologic features of the dermis has been established.7, 16, 17 Nishimori et al16 measured the biomechanical properties of photodamaged human skin and ultraviolet-irradiated hairless mouse skin with the Cutometer and found significant correlations between the reduction in total extensibility (Uf), elastic deformation (Ue), and viscoelastic creep (Uv) and degeneration of the dermal collagen fiber bundles. Skin laxity and wrinkling have been attributed to deficiencies in dermal collagen and elastin in aging and photoaging.17, 18, 19 Because dermal collagen and elastin are the targets for laser resurfacing methods, we used the noninvasive measures of skin elasticity to determine the changes in the dermis in relation to the laser power input. In our series, we observed that the relationship between applied erbium laser energy and ablation was not linear, but that there was continued ablation with applied energy for the erbium laser, as shown in Fig. 3, Fig. 5. With CO2 laser resurfacing, there was no additional ablation after the third pass. There was an increase in the inflammatory response after passes 4 and 5 that may represent an increased thermal effect (Figure 2). In addition, the inflammatory changes seen on posttreatment histologic sections in the remaining dermis were less for the erbium laser than for the CO2 laser. Conclusion Our findings indicate that noninvasive measurement of biomechanical properties, specifically viscoelastic creep after deformation of the skin, before and during resurfacing procedures, should be further studied in human facial skin and correlated with histologic changes and to long-term clinical outcomes. If this relationship is confirmed in human skin, it may be developed into a useful guide for cosmetic laser resurfacing procedures. Our results confirm the experience of others that the dual-mode erbium laser may be a more effective tool for dermal ablation than the CO2 laser. The basic biologic features of skin restoration need to be further studied to improve techniques and strategies for correction of intrinsic aging skin changes. Supplementary Files
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Dermatol Clin. 1986;4:433–446. MEDLINE Dr. Kitzmiller and Dr. Stanton are from the Division of Plastic, Reconstructive, and Hand Surgery, University of Cincinnati Medical Center; Dr. Wickett and Mr. Tolia are from the College of Pharmacy, University of Cincinnati Medical Center; Dr. Boissy is from the Department of Dermatology, University of Cincinnati Medical Center; and Dr. Visscher is from the Skin Sciences Institute, The Children's Hospital Medical Center, Cincinnati, OH. ☆ Reprint requests: W. John Kitzmiller, MD, Division of Plastic, Reconstructive & Hand Surgery, University of Cincinnati Medical Center, 231 Albert Sabin Way, Cincinnati, OH 42567-0558. ☆☆ 1090-820X/2003/$30.00 + 0 PII: S1090-820X(03)90019-7 doi:10.1016/S1090-820X(03)90019-7 © 2003 American Society for Aesthetic Plastic Surgery, Inc. Published by Elsevier Inc. All rights reserved. | |
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