Ĭonventional over-the-counter (OTC) whitening approaches include physical removal of stains with abrasives and detergents and chemical degradation of chromophores with peroxide-based agents. According to the American Academy of Cosmetic Dentistry, tooth whitening has become one of the most popular and common procedures in cosmetic dentistry. It was reported that 28% of adults in the United Kingdom, 34% of adults in the United States, and 56% of adults in China are dissatisfied with the appearance or color of their teeth. Intrinsic discoloration, such as tetracycline teeth or dental fluorosis, is caused by the incorporation of chromophores into enamel and dentin, while extrinsic discoloration refers to superficial stains on the tooth surface, which are usually caused by the consumption of tobacco, coffee and tea. In addition to this physiological change, tooth color is also affected by a combination of intrinsic and extrinsic discoloration. For example, an age-related change in tooth color can occur due to a reduction in enamel thickness, which makes the teeth appear darker since the dentin shines through more. Any process that changes the tooth characteristics, such as gloss, curvature and texture, opacity and translucency, can affect the light reflection and thereby influence the tooth appearance. When the incident light falls on the tooth surface, it undergoes a process of specular and diffuse reflection at the surface and absorption and scattering within the dental tissue. The color of the tooth is a complex optical phenomenon. The whitening effect seemed to be concentration-dependent. In conclusion, nano-HAP toothpaste has a satisfying postbrushing whitening effect and good resistance to mechanical forces. For both nano-HAP groups, HAP single crystallites and agglomerates were identified, and their sizes grew with nano-HAP reapplication. The △ E and △ L values were slightly reduced after HSF ( P < 0.05). After three applications, the △ E mean value of the 10 wt% nano-HAP group was 4.47. The 10 wt% nano-HAP group showed significantly higher △ E values than the 1 wt% nano-HAP group ( P < 0.05). We found that nano-HAP-treated enamel exhibited higher L* values and lower a* and b* values than the control groups ( P < 0.05). Two-way mixed ANOVA was performed to evaluate the influence of the concentration and repeated application on the tooth-whitening effect of nano-HAP. Tooth color ( L*, a* and b* values) was measured by a spectrophotometer, and color changes ( △ E, △ L, △ a and △ b values ) were calculated. The teeth surfaces were observed by SEM after each application. Each tooth was treated with the toothpaste three times and hydrodynamic shear force (HSF) once. Forty bovine incisors were randomly assigned into four groups: 10 wt% nano-HAP, 1 wt% nano-HAP, toothpaste without nano-HAP as a negative control and water as a blank control. Two concentrations of nano-HAP (10 wt% and 1 wt%) were incorporated in nonabrasive toothpastes. The impact of the concentration on the whitening performance of nano-HAP toothpaste was also investigated. The aim of this study was to evaluate the postbrushing tooth-whitening effect of toothpaste containing hydroxyapatite nanoparticles (nano-HAPs).
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