Nd:YA¥laser promises improved action at 1.34 µm
Roland Rou
A report comparing the merits of various dental and surgical lasers concludes that output from a neodymium-doped yttrium aluminum perovskite (YAP) laser emitting at 1.34 µm is absorbed 20 times more than the output of a Nd:YAG laser, but less than that of erbium:YAG (Er:YAG) and carbon dioxide (CO2) lasers. These results indicate that a Nd:YA¥laser should provide a better compromise between tissue cutting and coagulation than has been previously possible.
The report was published by Lokki SA (Vienne, France), which specializes in development and commercialization of medical-laser systems. The results are based on investigations carried out together with the Université d`odontologie (Lyon, France) and several French dentists and surgeons. The lasers were assessed based on the results of laser/tissue interaction.
In surgical applications, laser energy is absorbed into the tissues, leading to a temperature increase in the cells constituting these tissues. According to Lokki, the therapeutic effects of the laser are directly related to the temperature achieved. Only tissue heating occurs at temperatures between 37°C and 60°C; between 60°C and 65°C cellular protein denaturation occurs; and from 65°C to 90°C coagulation and necrosis take place. Above 100°C, cellular carbonization occurs if the temperature increases progressively or cellular volatilization occurs if the temperature rises rapidly.
The actual temperature achieved depends on the tissue concerned, the laser used, and its wavelength. The primary lasers used in medical applications are argon-ion lasers emitting at 488 and 544 nm, helium-neon lasers at 633 nm, Nd:YAG lasers at 1.06 µm, Nd:YA¥lasers at 1.34 µm, Er:YAG lasers at 2.94 µm, and CO2 lasers at 10.6 µm.
Beam propagation in the tissue depends on the type of tissue and laser wavelength. This relationshi¥is illustrated by the absorption curve of water, which is the main constituent of biological tissue (see Fig. 1). From the curve, one can see that the 10.6-µm output of a CO2 laser is highly absorbed, and its action in biological tissues is, therefore, only superficial, leading to easy cutting but with low hemostatic effect. The Er:YAG laser output at 2.9 µm also coincides with a very high absorption peak, and the laser`s action is consequently superficial with a low hemostatic effect. The Nd:YAG-laser output at 1.06 µm, however, is minimally absorbed, and its tissue action is deeper and hemostatic but causes necrosis and is inefficient for cutting.
The output of a Nd:YA¥laser emitting at 1.34 µm, however, is more highly absorbed than that of the Nd:YAG laser but less than that of Er:YAG and CO2 lasers, which leads to better compromise between cutting and coagulation (see Fig. 2).
Other factors relevant to evaluating the lasers include beam delivery. Silica fiberoptics have a propagation limit of 2.5 µm, which means that beam delivery for Er:YAG and CO2 lasers involves an articulated arm with beam-steering mirrors or use of new metallic-halide fibers.
Another factor is the operating mode. Several lasers such as CO2 lasers are continuous-wave or can be adjusted to deliver pulses from 0.1 to several seconds--in this case, however, the continuous heat supply creates a thermal gradient that can cause fissures in hard tissues such as enamel and dentine. Other lasers such as Nd:YA¥deliver relatively short pulses (100 to 200 µs), leading to higher peak power than the average power.
Given these considerations and the results of the report, Lokki has developed a Nd:YAP-laser-based dental system capable of delivering high peak power. Pulse energies range from 100 to 400 mJ with a pulsewidth of 150 µs to produce 1- to 2.6-kW peak power and 1- to 10-W average power with a repetition rate between 5 and 30 Hz.
The high peak power causes a very fast temperature increase and consequently vaporizes the superficial tissue layer, avoiding the carbonization phase that appears with traditional CO2 laser use. The rapid temperature increase on the dentine surface also sterilizes the cavity bottom or canal walls. The short pulse duration and the low repetition rate limit the thermal diffusion reducing the risk of necrosis as well as the pain perceived by the patient.