The thermal effects of Nd:YAG, argon, and C02 laser beams are observed on enamel, dentin, and dental pulp by means of computerized infrared thermography and thermocouple. This study shows that the Nd:YAG laser beam deeply diffuses through the enamel and dentin to the pulp. The argon laser effects are inconsistent depending on whether the enamel surface is cleaned, but after cleaning, the superficial and deep temperatures are low. With the C02 laser, the enamel and dentin surfaces reach very high temperatures, but only low temperatures are measured in the pulp chamber.
Key words: dentistry, CO2 laser, dental laser, Nd:YAG laser, dental enamel, dentin
Although lasers are now widely used in medical practice [Brunetaud et al., 1980; Brunetaud and Des- comps, 1982; Brunetaud and Mordon, 19841, the dental applications of this technology are not yet clearly de- fined. Since 1965 [Kinersly et al., 19661 it has been known that a laser is able not only to cut soft tissues but also to serve as a substitute for traditional rotary instru- ments in drilling hard dental tissues. Various mechanisms have been sought to find pos- sible dental uses for lasers, most of them with a prophy- lactic purpose. Literature reviews reveal that most medical lasers have been tried on dental tissues, but the effects of lasers on dental pulp have not been studied. Therefore, fuller information about the quantitative anal- ysis of laser thermal effects on dental tissues is needed. To date some quantitative analyses of these thermal effects are lacking in precision. Different authors have reported use of thermistors [Goldman et al., 19651 or thermocouples [Lobene and Fine, 19661. After laser treatment, measurements of the surface temperature with an infrared thermometer and in the pulp cavity with thermocouples have been reported, but the results were not quantified [Nagasawa, 19831. The purpose of this report is to quantify thermal effects of laser treatment on the dental pulp; knowledge of the surface temperature may influence research on modifying the phy sicai or chemical properties of dental hard tissues.
MATERIALS AND METHODS
Lasers Three types of laser were used for this experimental study: 1. A COz laser, Ercelas 40 (Biophysic Medical, clermont-Ferrand, France) : wavelength 10.6 pm; power 0.1-30 W; continuous pulse or 0.1, 0.2, 0.5, or 1-sec pulses; focal length lens 125 mm; spot size 0.6-mm diameter. 2. A Nd:YAG laser, Y.M. 100 (Cilas, Marcous- sis, France): wavelength 1.06 pm, power 0-120 W; con- tinuous pulse or 0.2-0.7-sec pulses; spot size 2-mm diameter. 3. An argon laser Cooper 770 (Lasersonics, Palo Alto, CA): wavelength 487-544 nm; power 0.5-10 W; 0.1-20-sec pulses; spot size 1-mm diameter. Eighty-eight human, single-rooted, freshly ex- tracted teeth, individually stored in saline solution, were used in this study. Surface Temperature Measurements To measure the surface temperature of dental hard tissues after laser treatment, we used infrared thermog- raphy with the AGA 720 thermovision system (Agema, Danderyd, Sweden). The lens aperture of the infrared.
Computerized Data Processing
To quantify thermal images, computerized data processing has been developed in our laboratory [Mor- don et al., 19831. Thus the infrared tissue emission is recorded on a 3/4-inch U-Matic video recorder (Sony, Tokyo, Japan) during laser irradiation. For data process- ing, the video recorder is connected to a computer. Two types of data may be obtained: the temperature profile giving the spatial distribution of the thermal energy during laser irradiation, dental laser tips
and the temperature evolution of a predetermined point of the laser spot for periods of 12 sec.
Internal Temperature Measurements The temperature rise in the pulp cavity during irra- diation of the surface hard tissues by the laser is mea- sured with a HYP-0 thermocouple (Omega, Stamford, CT). It is a cooper-constantan thermocouple, with a temperature range of -270" to 400°C. This thermocou- ple is a 2.5-cm long and has a 0.2-mm diameter steriliz- able needle. The small diameter permits a very short response time: 7. lOP3sec. The thermocouple is con- nected to an OMNI-AMP I1 unit (Omega) and then to a UIV CM4 501 Schlumberger recorder. To obtain maximum accuracy, laser shots are fo- cused on the middle of the buccal or lingual areas of the teeth, and the thermocouple position inside the pulp cav- ity is controlled by radiography. Experimental Procedure Thermal measurements during single laser shot on enamel surface. All three types of laser were tested with the same experimental procedure. The thermocou- ple is inserted inside the tooth, and the infrared camera and laser are focused. The external and internal temper- ature measurements are carried out during laser shot. Thermal measurements during single laser shot on dentinal surface. Side (3 mm) and deep (1.5 mm) cavities are drilled with a high-speed burn in the cervical one-half of buccal face of single-rooted teeth. The pulpal face is carefully flattened, and the teeth are well dried before the laser shot. After our previous results on enamel, and because of other reports [Melcer et al., 1982; 1983; 1984a,b; 19851, only the C02 laser has been tested on dentin. RESULTS Single Laser Shot on Enamel Surface Nd:YAG laser. Ten shots were done with the Nd:YAG laser. The power varied from 3 W to 35 W, energy from 6 to 70 J, and energy densities from 190 to 2,228 J/cm2. The macroscopic aspect of the enamel surface was never altered. Thermal results showed a very light absorption of the Nd:YAG beam by enamel and dentin. Energy is transmitted to the pulp, and the temperature rises into the pulp cavity is in direct ratio to the applied power. External and internal temperature rise and lack of surface alteration show Nd:YAG beam diffusion through the hard tissues to the pulp. The external temperature rise is consistent with absorption of a small amount of the incident energy. Argon laser. First, 20 shots were applied with the power varying from 5 to 8 W, energy from 2.5 to 80.8 J, and energy densities from 318 to 10,287 J/cm2. The results were inconsistent. The temperatures ranged between 200 and 1,34O"C, and only eight values were detectable. In these cases, the temperature rose a maximum of 900"C, and a blackish crater was formed in the enamel. In cross-section, this cone-shaped alteration may extend to the pulp. For the 12 other cases, the temperatures never reached 200"C, and no macroscopic tissue alterations were seen. Another 20 argon laser shots were applied after cleaning the enamel surface with an ultrasonic device and polishing with silicon cups. The parameters reached were 6 W, 3-60 J, and 381-7,632 J/cm2. In this range, the temperatures never reached 20O0C, and the enamel surface was not altered. The internal temperature increased in proportion to the energy used and the shooting time, but the results were not consistent. CO2 laser. The results are given in Figures 1, 2, and 3. Figure 1 shows the surface effect of temperature on the function of fluence on the immature tooth enamel. The temperature increase is very important, and 1,340"C is obtained for fluences between 200 J/cm2 and 1,OOO J/cm2. Figure 2 shows the effect of surface temperature on the function of fluence on mature tooth enamel. In Figure 2, the temperature increase is lower than that in Figure 1, and the dispersion of data is greater. Figure 3 shows the effect of surface temperature on the function of fluence on dentin. There is only slight dispersion of the data. The temperature increase is lower than that obtained on enamel.