Optical properties of root canal irrigants in the 300–3,000-nm wavelength region

Abstract

In root canal therapy, irrigating solutions are essential to assist in debridement and disinfection. Their spread and action is often restricted by canal anatomy, requiring some form of activation. Lasers have been shown to be promising tools for this purpose (laser-activated irrigation (LAI)). For LAI to be effective, high absorption of radiation in the irrigant is essential. Although the absorption spectrum of water is well established, little is known about the optical properties of other irrigating solutions. Therefore, root canal irrigants (sodium hypochlorite (NaOCl), citric acid (CA), chlorhexidine (CHX), ethylenediaminetetraacetate (EDTA), water) were subjected to UV/Vis spectrophotometry in the 300-3,000-nm region using synthetic quartz cells with an optical path length of 1 mm. Transmission data were used to plot the transmission spectrum and calculate the absorption coefficient (α) of each irrigant. The transmission spectra of the tested solutions proved to follow the spectrum of pure water to a large extent. All tested solutions displayed absorption peaks around 1,450 nm (α ≈ 14 cm(-1)), 1,950 nm (α > 30 cm(-1)), and above 2,500 nm (α > 30 cm(-1)). NaOCl showed higher absorption than water in the UV region. Slightly higher absorption than water was noted for CHX (Corsodyl) around 513 nm and for CA between 1,600 and 1,800 nm and around 2,200 nm. The absorption in all tested solutions for wavelengths greater than 2,500 nm is very high, meaning a great potential for laser-activated irrigation. Other wavelengths eligible for LAI are located around 1,450 and 2,000 nm but require further investigation.

Visualization of removal of trapped air from the apical region in simulated root canals by laser-activated irrigation using an Er,Cr:YSGG laser

“…Red dye was used to render the solution-air interface visible to the camera. EDTA was used as the irrigating solution in this study because it demonstrates no significant difference in optical properties compared with lowconcentration sodium hypochlorite solutions, pure water, and chlorhexidine solutions [20].…”

Rationale for using a double-wavelength (940 nm + 2780 nm) laser in endodontics: literature overview and proof-of-concept

“…whereas the 980-nm diode laser presents a slight absorption coefficient peak in water (μ a = 0.25 cm −1 ) [22]. Knowing that the root canal dentin presents high volume of water content within its molecular structure, such water peak absorption constrains 980-nm dentine penetration when compared with the other two NIR wavelengths (810 and 940 nm).…”

“…It cannot be also expected near-infrared lasers to activate irrigants such as NaOCl, citric acid or EDTA as their absorption coefficients in such solutions range only from μ a = 0.05-0.25 cm −1 [22] and an average penetration into intra-canal irrigants between 4 and 20 cm. Therefore, cavitation effects using these wavelengths should not be expected.…”

“…Knowing that the root canal dentin presents high volume of water content within its molecular structure, such water peak absorption constrains 980-nm dentine penetration when compared with the other two NIR wavelengths (810 and 940 nm). Consequently, having lower absorption coefficients in water, it should be expected that both 810 and 940 nm lasers could present better biophysical behaviour in terms of dentine penetration and deep disinfection in detriment of the 980 nm laser [22,32,33]. Based on the same biophysical properties-despite questionable conflicting findings in literature [34-37]-it cannot not be expected that any diode laser could effectively remove smear layer and expose dentinal tubules without exciding the dentin temperature threshold [36,[38][39][40][41].…”

“…Generally, mid-infrared erbium lasers operating at wavelengths of 2780 or 2940 nm demonstrate a high absorption coefficient for both water/aqueous solutions and hydroxyapatite whereas in the near infrared range diode lasers (810, 940, and 980 nm) are mostly absorbed by haemoglobin and melanin [21][22][23][24].…”