# Surface Morphology Changes of Bleached Dental Ceramics
## 1 Introduction  
An aesthetically pleasing smile is the aspiration of many patients and dentists, and this is determined by the colour, shape, size, texture, and position of natural or restored teeth. The most common aesthetic and restorative dental procedures are teeth whitening, composite bonding, and restoring teeth with veneers and inlays, overlays, and crowns. Silicon dioxide (silica) containing (feldspathic porcelains, leucite-reinforced ceramics, and lithium disilicate ceramics) and alumina and zirconia core-based ceramics were mostly favoured restorative materials in dentistry during the last few decades due to their well-controlled manufacturing process, great aesthetics, high fracture strength and optimal biocompatibility [1]. The teeth whitening effect is achieved using different bleaching agents, such as hydrogen peroxide (HP) or carbamide peroxide (CP). The bleaching effect on restorative ceramics in the mouth depends on restorative material's properties and type of whitening agent, its concentration, pH, and exposure time [2; 3; 4; 5]. Although the teeth whitening procedure has been shown to be safe and effective for hard tooth tissues [6], increased surface roughness of the tooth with and without restoration is often observed as a side effect of bleaching. Rough surfaces can cause discolorations, secondary caries, and soft tissue inflammation due to increased oral microorganisms' adhesion to the restoration [7]. The threshold for clinically acceptable surface roughness value is 0.2 \(\upmu\)m [8]. In some cases, the tooth whitening procedure may lead to erosion of restorative materials [9]. On the other hand, the surface roughness is responsible for diffusescattering of light incident on the tooth and inversely correlates with the brightness of the tooth enamel [10]. The concentration of bleaching materials can range from 5% to 40% [11] and can be used with additional light source such as light emitting diode (LED), LED plus lasers, lasers, ultraviolet lamp, halogen curing lights, and non-thermal atmospheric pressure plasmas (NAPP) [12; 13; 14; 15]. Hydrogen peroxide (HP) reacts directly with discolorations and releases active carbon molecules to break the double carbon bonds between chromophore molecules, leaving smaller and colourless fragments that are converted into water and carbon dioxide [11; 16; 17]. Though the concentration of HP varies, the highest concentration allowed for treatment of patients in Europe is 6%. HP concentration > 15% does not improve whitening efficiency but may lead to significant changes in tooth surface morphology [18]. Contacting with tooth surface carbamide peroxide (CP), also known as urea hydrogen peroxide [19], slowly breaks down into HP and urea [20] and then follows the same reaction route as HP. Due to slow decomposition, in order to achieve the same bleaching effect CP should interact with the surface significantly longer than HP [21]. The authors of this article concluded that bleaching for 126 h with 10% and 16% carbamide peroxide had no effect on the surface roughness of fluorapatite glass-ceramic and feldspathic ceramics. In a study by [PERSON] et al. [9], bleaching polished polymer-infiltrated ceramic network and polished resin nano-ceramics with 35% and 40% hydrogen peroxide, 10% and 16% carbamide peroxide showed no changes in surface roughness and topography. However, these bleaching materials had a remarkable effect on the optical properties and the colour of the investigated ceramics [22]. The opposite results were observed in investigation performed by [PERSON] et al. [23], where it was shown that bleaching feldspathic porcelain with 10%, 20%, and 35% carbamide peroxide, the strongest effect on surface roughness was achieved with 20% and 35% bleaching agents. The study conducted by [PERSON] et al. [7] revealed that bleaching of glazed lithium disilicate and leuctite-reinforced glass-ceramics samples with 16% carbamide peroxide for 6 h per day for seven days increased surface roughness of both types of ceramics.