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 [5] 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  [6].  Higher  temperatures  of up  to 800 8 C  are required  to  remove  the  more  tightly  bound  water  [6]. 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