AbstrAct
Topical  fluoride  treatment  prevents  dental  caries.  However,    the    resulting    calcium-fluoride-like    deposits  are  soft  and  have  poor  wear  resistance;  therefore,  frequent  treatment  is  required.  Lasers  quickly  heat  surfaces  and  can  be  made  portable  and  suitable  for  oral  remedies.  We  examined  the  morphology,    nanohardness,    elastic    modulus,    nanowear,  and  fluoride  uptake  of  fluoride-treated  enamel followed by CO2 laser irradiation for 5 and 10 sec, respectively. We found that laser treatments significantly  increased  the  mechanical  properties  of  the  calcium-fluoride-like  deposits.  The  wear  resistance  of  the  calcium-fluoride-like  deposits  improved  about  34%  after  laser  irradiation  for  5  sec  and  about  40%  following  irradiation  for  10  sec. We also found that laser treatments increased fluoride  uptake  by  at  least  23%.  Overall,  laser  treatment significantly improved fluoride incorpo-ration into dental tissue and the wear resistance of the protective calcium-fluoride layer.

KEY WORDS:laser therapy, dental enamel, flu-orides,   mechanical   phenomena,   nanomedicine,   scanning electron microscopy.

Introduction
Many  studies  have  reported  that  fluoridation  provides  a  cost-effective  means  of  preventing  dental  decay  (Ripa,  1990).  Fluoridation  can  be  achieved via a dentifrice, water fluoridation, or topical fluoride treatment. The effectiveness  of  a  dentifrice  requires  good  dental  hygiene  habits,  and  water  fluoridation  risks  fluorosis  (Szpunar  and  Burt,  1988;  Marinho  et  al.,  2003;  Wong et al., 2010). Therefore, topical fluoride treatment is preferred by many dental practitioners. Children, as well as adults undergoing orthodontic treat-ment, especially need topical fluoride treatment. However, the calcium-fluo-ride (CaF2)-like deposits generated on the enamel surface during treatment are generally eroded away within days or weeks as a natural consequence of eat-ing and chewing (Øgaard, 2001). Jeng et al. (2008, 2011) discovered that the hardness  and  elastic  modulus  of  the  CaF2-like  deposits  were  approximately  0.77 and 53 GPa, respectively, while the wear depth was about 5 times higher than  that  of  native  enamel.  Therefore,  methods  are  required  to  increase  the  wear  resistance  of  these  deposits  to  improve  the  long-term  effectiveness  of  the treatment.

A laser was first used in dentistry, in the 1960s, by Stern et al. (1966), who showed  that  using  a  ruby  laser  to  irradiate  dental  enamel  significantly  improved its resistance to acid. Since then, many other forms of laser irradia-tion have been shown not only to improve the acid resistance of enamel, but also to increase fluoride uptake on the enamel surface (Yamamoto and Sato, 1980; Nelson et al., 1986; Featherstone et al., 1998; Hsu et al., 2004). Of the various  forms  of  laser  systems  available,  the  pulsed  CO2  laser  is  one  of  the  most appropriate for dental health care applications (Rodrigues et al., 2004). Because the wavelengths of visible and near-infrared laser systems (e.g., Ar and  Nd-YAG  lasers)  are  only  weakly  absorbed  by  enamel  (Gonzalez  et  al., 1996), high irradiation intensity is required to achieve the desired effect, and thus  the  risk  of  damaging  the  underlying  pulp  increases.  In  contrast,  CO2lasers  have  an  infrared  wavelength  that  is  readily  absorbed  by  the  enamel  surface. Consequently, the scattered energy is very low, and the laser irradia-tion energy is confined to the enamel surface. Furthermore, the use of a pulsed irradiation technique minimizes the accumulated energy within the  enamel  surface,  which  further  reduces  the  risk  of  thermal  pulp damage (Kantorowitz et al., 1998).dental laser

results
The    Table    summarizes    the    results    obtained for the surface roughness, nano-hardness,   elastic   modulus,   and   mean   wear  depth  of  the  enamel  surfaces  sub-ject  to  laser  exposure  with  or  without  topical fluoride treatments.

discussion
Previously,  we  reported  that  clinical  fluoride  treatment  gener-ates  fluoride-containing  deposits  only  hundreds  of  nanometers  thick  (Jeng  et  al.,  2008,  2011).  However,  clinical  evidence  (Lowndes et al., 1996; Ishikawa et al., 2002) has confirmed that even a layer this thin is significantly protective, because it con-tributes  to  the  enamel’s  ability  to  resist  cariogenic  challenges.  The  nanometer  coating  significantly  affects  various  physical,  chemical,  and  even  biological  properties  of  the  bulk  materials,  which suggests that the potential contribution of nanocoating to biological  systems  should  not  be  overlooked.  Studies  of  the  effects  of  nanotribological  properties  on  overall  materials  per-formance  showed  a  34%  increase  in  the  wear  resistance,  and  CO2  lasing  is  anticipated  to  improve  that  percentage  (Bhushan  et al., 1995; Bhushan, 2007; Szlufarska et al., 2008).We  report  here  the  first  nanotribological  mapping  of  fluoride  treatment  deposits  and  the  enamel  from  the  surface  to  the  dentin-enamel junction (DEJ) with integrated in situ chemical composition analysis,  which  provides  comprehensive  biomechanical-chemical  insight  into  the  clinically  observed  protective  effect  of  tradi-tional fluoride treatment and into its potential limitations (Jeng et  al.,  2008,  2011).  Lasers  are  among  the  most  effective  and  clinically  acceptable  approaches  for  the  treatment  of  various  conditions (Miller and Truhe, 1993; Gupta and Kumar, 2011). In this study, we combined lasers with topical fluoride treatments to improve this nanomechanical clinical practice.In  addition,  we  found  that  the  enamel  sheaths  on  the  tooth  surface  shrank  after  they  had  been  irradiated  and  were  shorter  than  the  central  enamel  rods  (Appendix  Fig.  2).  This  was  expected  because  the  sheaths  have  a  relatively  higher  protein  content than rods (Ge et al., 2005) and protein decomposes rap-idly  because  of  the  intense  heat  generated  by  laser  irradiation.