Mechanism of Water Augmentation During IR Laser Ablation of Dental Enamel
Background and Objectives: The mechanism of water augmentation during IR laser ablation of dental hard tissues is controversial and poorly understood. The in- fluence of an optically thick applied water layer on the laser ablation of enamel was investigated at wavelengths in which water is a primary absorber and the magnitude of absorption varies markedly. Study Design/Materials and Methods: Q-switched and free running Er: YSGG (2.79 m m) and Er:YAG (2.94 m m), free running Ho:YAG and 9.6 m m TEA CO 2 laser systems were used to produce linear incisions in dental enamel with and without water. Synchrotron-radiation IR spectro- microscopy with the Advanced Light Source at Lawrence Berkeley National Laboratory was used to determine the chemical changes across the laser ablation profiles with a spatial resolution of 10- mm.
Results: The addition of water increased the rate of abla- tion and produced a more desirable surface morphology during enamel ablation with all the erbium systems. More- over, ablation was markedly more efficient for Q-switched (0.15 microsecond) versus free-running (150 microsecond) erbium laser pulses with the added water layer. Although the addition of a thick water layer reduced the rate of ablation during CO 2 laser ablation, the addition of the water removed undesirable deposits of non-apatite mine- ral phases from the crater surface. IR spectromicroscopy indicates that the chemical composition of the crater walls deviates markedly from that of hydroxyapatite after Er:YAG and CO 2 laser irradiation without added water. New mineral phases were resolved that have not been previously observed using conventional IR spectroscopy. There was extensive peripheral damage after irradiation with the Ho:YAG laser with and without added water without effective ablation of enamel.
Conclusions: We postulate that condensed mineral phases from the plume are deposited along the crater walls after repetitive laser pulses and such non-apatitic phases interfere with subsequent laser pulses during IR laser irradiation reducing the rate and efficiency of abla- tion. The ablative recoil associated with the displacement and vaporization of the applied water layer removes such loosely adherent phases maintaining efficient abla- tion during multiple pulse irradiation. Lasers Surg. Med.
Key words: erbium laser; CO 2 laser; dental laser tips
; laser ablation; infrared spectromicroscop.
INTRODUCTION It has been well established that extensive water ap- plication is necessary for the efficient ablation of dental hard tissues with Er:YAG and Er:YSGG laser irradiation. The mechanism of interaction between the water-layer, the laser radiation, and the hard tissues is not clearly understood and is somewhat controversial. Early mechani- stic studies focused on tissue dehydration [1– 4]. However, water absorption and diffusion studies in enamel indicate that only approximately half of the water is actually diffusible  and the rate for water diffusion is quite slow, on the order of several hours to days. Thermal analysis studies indicate that the tissue has to be heated to tem- peratures exceeding 200– 300 8 C before the diffusible water is removed . Higher temperatures of up to 800 8 C are required to remove the more tightly bound water . Therefore, it is unlikely that simple dehydration by dif- fusion has a significant effect during laser irradiation. Other more novel hypotheses include cavitation bubbles, accelerated water droplets, and apatite crystalline frag- ments. One proposed mechanism dubbed the ‘‘hydrokinetic effect’’ suggests that water droplets are rapidly accelerated into the enamel by absorption in the laser beam [7,8]. Altshluler et al. [9,10] have proposed that solid particles of ablated material are accelerated against the walls of the crater resulting in a polishing effect that removes debris and any protruding sharp edges. In the same study, the authors found that the ablation of enamel could be effectively enhanced by a factor of four via ablation with a quartz or sapphire contact-mode fiber during the initial few laser pulses. Solid particles were reflected by the fiber tip and particles originated from failure of the fiber.http://www.dentlasertip.com