In vitro study on nanosecond-pulsed Q-switched Er:YAG laser-induced selective removal for caries dentin

Abstract

Since 2000, Federation Dentaire Internationale has advocated for “minimal intervention” dental treatments. The dental field has demanded less-invasive caries treatment methods. However, recent caries treatments using a microsecond-pulsed free-running Er:YAG laser with a pulse duration of 200–300 μs have issues with non-selectivity for caries due to the thermal effect caused by the laser. To improve the selectivity by reducing the thermal effect, the ablation characteristics of a nanosecond-pulsed Er:YAG laser are investigated. A Q-switched Er:YAG laser with a rotating mirror Q-switching system attached to a flash lamp-pumped Er:YAG laser and a free-running Er:YAG laser were used. The pulse duration of the Q-switched Er:YAG laser was 80–130 ns, while that of the free-running Er:YAG laser was around 200 μs. The pulse repetition rate of the free-running Er:YAG laser was constant at 10 Hz, but that of the Q-switched Er:YAG laser fluctuated between 10 and 23 Hz. The fluence was 6, 10, or 14 J/cm2. The samples were bovine sound dentin slices and caries models. Caries models were prepared by immersing sound dentin slice samples into a 0.1-M lactic acid solution for 24 h at 37 °C. The samples were irradiated with the two lasers horizontally without water spray for 1, 2, or 4 s. Laser irradiation experiments indicate that the selectivity towards caries occurs at the fluence of 6 J/cm2 without water spray. The ablation depth of a demineralized dentin slice by the Q-switched Er:YAG laser is about three times that by the free-running Er:YAG laser at the fluence of 10 J/cm2. In addition, the Q-switched Er:YAG laser suppresses the thermal effect reducing the range of the thermal melting area. Caries dentin may be selectively removed without water spray using the Q-switched Er:YAG laser with suppressing dental pulp necrosis because the temperature increase is below 5 °C. The ablation depths by the Q-switched Er:YAG are about three times larger than those using the free-running Er:YAG laser.