ОПТИКА И СПЕКТРОСКОПИЯ, 2013, том 115, № 1, с. 168-176
ЛАЗЕРЫ И ИХ ПРИМЕНЕНИЕ
YM 621.373.8;615.47
VALIDITY OF RECIPROCITY RULE ON MOUSE SKIN THERMAL DAMAGE
DUE TO CO2 LASER IRRADIATION
© 2013 n P. Parvin*, H. R. Dehghanpour**, M. Shojaei Moghadam***, and V. Daneshafrooz*
* Physics Department, Amirkabir University of Technology, Tehran, Iran ** Physics Group, Tafresh University, Tafresh, Iran *** RasoulAkram Hospital, Tehran Medical University, Tehran, Iran E-mail: Parvin@aut.ac.ir, h.dehghanpour@aut.ac.ir Received September 24, 2012
CO2 laser (10.6 p.m) is a well-known infrared coherent light source as a tool in surgery. At this wavelength there is a high absorbance coefficient (860 cm-1), because of vibration mode resonance of H2O molecules. Therefore, the majority of the irradiation energy is absorbed in the tissue and the temperature of the tissue rises as a function of power density and laser exposure duration. In this work, the tissue damage caused by CO2 laser (1-10 W, ~40-400 Wcm-2, 0.1-6 s) was measured using 30 mouse skin samples. Skin damage assessment was based on measurements of the depth of cut, mean diameter of the crater and the carbonized layer. The results show that tissue damage as assessed above parameters increased with laser fluence and saturated at 1000 J cm-2. Moreover, the damage effect due to high power density at short duration was not equivalent to that with low power density at longer irradiation time even though the energy delivered was identical. These results indicate the lack of validity of reciprocity (Bunsen-Roscoe) rule for the thermal damage.
DOI: 10.7868/S0030403413070131
INTRODUCTION
The CO2 laser emits far infrared light at 10.6 ^m. Its beam is strongly absorbed by light scattering tissues with high water content, and because the influence of laser is restricted to shallowest layers of the skin, the extent of damage is controllable. The diversity of clinical applications of CO2 laser in surgery is still growing. The search for simpler techniques for skin resurfacing has led many groups to investigate the usefulness of lasers as precision tools for photo-dermabrasion. A modality for superficial skin rejuvenation using a dye-assisted low level energy CO2 lasers was used [1]. This technique has been performed to smooth out peri-oral or periorbital fine and deep rhytids in the treated patients. All subjects were female between the age of 50 and 75 years with light to medium skin complexion. All patients were treated only once and were followed in clinic every month. The entire treatment with dye-assisted dermabrasion is performed setting under topical/local anesthesia and lasts only 20-30 min. The pulsed mode CO2 laser was used in all patients at low power output 2-4 W and was defocused (hand-piece 3-5 cm from skin) using a painting or sir-brush technique. Superficial reepithelialization was observed within the first two weeks post-therapy and full healing was achieved in 2 months. No scarring or permanent hyper/hypopigmentation was observed; although some laser abraded skin had a pinkish hue that gradu-
ally blended with the surrounded skin color. Significant improvement in the skin texture, appearance, and elasticity was observed in all patients with more uniform, smoother surface. Near complete regression of superficial rhytids (80% subjective improvement) and significant ablation of deep rhytids with at least 50% improvement was observed. The physician's learning curve with CO2 lasers is shorter and easier than with chemical or mechanical dermabrasion such that the results appear to be more uniform and predictable. Assessing the effects of laser therapy and its possible dose dependency on the healing of CO2 laser surgical wounds was studied [2]. Laser-treated groups showed a healing process characterized by more prominent fi-brolastic proliferation. While young fibrolasts actively produced collagen, no myofibroblasts were found. By using that methodology, CO2 laser therapy offers a positive effect in wound healing and the dose exhibits no influence on the treatment. Experience gained in the management of oral mucosal lesions by CO2 laser in an outpatient clinical treatment was carried out [3]. Lasers have indications for use in dentistry for incision, excision, and coagulation of intraoral soft tissue. Advances in laser technology have provided the specific delivery systems for the laser energy with short interaction items on tissue to be ablated. Laser excision was well tolerated by patients with no intraoperative adverse effects. All patients healed postsurgrically with
no loss of function. CO2 laser is a successful surgical option when performing excision of benign intraoral lesions. Advantages of laser therapy include minimal postoperative pain, conservative site-specific minimally invasive surgeries and elimination of need for sutures. The CO2 laser gives an effective treatment for vascular malformations of the skin of a nevus flam-meus as well as treatment of polypoid hemangioma [4]. Layer-by-layer char-free facial skin resurfacing was done at very low CO2 laser power levels with a miniature "silk touch" microprocessor-controlled optomechanical flashscanner [5]. That device provides an excellent ablation depth control with minimal thermal damage to the dermis. Indications for laser therapy was studied in aesthetic surgery include perioral, lips and perioorbital wrinkles. The clinical usefulness of laser surgery in pathologic diagnosis following exci-sional biology of human oral mucosa by CO2 laser and electrotome was investigated [6]. Despite the differences between CO2 laser and electrotome methods, similar thermal denaturation such as carbonization, vacuolar degeneration, and elongation of nuclei were obsereved at the excisional margins for both methods. In pathological diagnosis, the use of CO2 laser, particularly in pulse mode, significantly reduce the amount of thermal denaturation compared to the electrotome. Thermal damages due to CW—CO2 laser exposure at high fluences were also studied [7, 8]. The damage-zone thickness is approximately constant around periphery of the cut, consistent with the existence of liquid layer which stores heat and leads to tissue damage accordingly. On the other hand, thermal damages after CW—CO2 laser exposure at various durations were studied. The damage adjacent to incisions created in dorsal mouse skin by CO2 laser shots having 250 ^s duration was investigated histologically respect to the injuries induced by long-term heating. In fact, the multiple laser shots usually create larger thermal damage. It was demonstrated how the lateral thermal damage near the laser cuts arises from the heat conduction. In fact, it is essential to introduce a tissue layer as a heat reservoir to be liquefied during the cutting process. The mechanical properties of tissue after CO2 laser ablation were reported as well [8—10]. The effect of CO2 laser pulse repetition rate on the ablation process and consequent thermal damage was also investigated. The ablation rate and thermal damage in skin produced by a superpulsed CO2 laser operating at pulse repetition rates of 1—900 Hz was studied [11]. When delivering a fixed number of pulses ~60% increase in the tissue ablation was experienced up to 200 Hz. At pulse repetition rates greater than 200 Hz, no further increase was seen. Under identical conditions, an 80% increase in the thermal damage zone was observed when the pulse repetition rate rises up from 1 to 60 Hz. The evidence of large increase in tissue ablation and corresponding
damage indicates the existence of mixed-phase (i.e. liquid — vapor) layer or metastable liquid which deposits significant amounts of thermal energy between successive pulses. The data suggests that CO2 laser operation at relatively low repetition rates assure optimal performance.
Numerous reports have been published on skin rejuvenation by the so called fractional laser devices that deliver a laser beam in a dot form over a grid pattern. The effects of a fractional CO2 laser on atrophic acne scars at the clinical levels were characterized [12]. A fractional CO2 laser device was used with irradiation parameters set i.e. the output power 10 W, pulse width 600 ^s, dot spacing 800 ^m with 0.91 J cm-2 irradiation output power. Histologically, outgrowths of many degenerated elastic fibers were observed as irregular rod-shaped masses in the superficial dermis prior to the treatment in the region of the acne scars. Several weeks after the multiple weekly irradiations, the degenerated elastic fibers were no longer seen. It was concluded that the fractional CO2 laser was considered to be very effective for treating atrophic acne scars.
CO2 laser ablative fractional resurfacing produces skin damage, with removal of the epidermis and variable portions of the dermis as well as associated residual heating, resulting in new collagen formation and skin tightening. The non-resurfaced epidermis helps tissue to heal rapidly, with short term postoperative erythema. The CO2 laser for fractional treatment (2 Hz, 60 W 60-120 mJ) was used in super pulsed mode [13]. Laser pulsing can operate repeatedly on the same spot or to be moved randomly over the skin, using several passes to achieve a desired residual thermal effect. Postoperatively, resurfaced areas were treated with an ointment of gentamycin, retinol palmitate and DL-methionine. Once epithelialization was achieved, anti-pigmentation and sun protection agents were recommended. Immediately after treatment, vaporization was produced by stacked pulses, with clear ablation and collateral heat coagulation. An increased number of random pulses removed more epidermis and with denser pulse per area, a thermal deposit was noted histologically. After 2 months, a thicker, multi-cellular epidermis and an evident increase in collagen were observed. Fractional CO2 laser permits a variety of resurfacing setting that obtain safe, effective skin
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