## 1 Introduction  
The concept of biomaterials, which is widely used nowadays, could include everything from metal to ceramic, plastic, and any other material that is well tolerated by the human body. In the field of fixed orthodontics, brackets are used very often during the treatment. These appliances are mostly made of metal alloys, polycarbonate, ceramic, zirconia [1, 2], different kind of wires, and various bonding materials. Although the materials from which the brackets were made have significantly evolved, the metallic ones are still widely used due to their mechanical and chemical properties [3]. Intense research over the years on these materials has led to a great diversity of ferroalloys used for brackets, wires, bands, and other orthodontic accessories, such as cobalt-chromium alloy (Cr-Co), stainless steel, and titanium alloys, all of which have nickel (Ni) as one of the main components [4].  
C ceramic brackets began to be used in the late 1980s, and nowadays, they are commonly used for orthodontic fixed therapy [5]. They were designed in an attempt to satisfy the esthetic requirements of the patients with fixed orthodontic appliances. These translucent brackets are made from aluminum oxide (alumina) particles and are available in the monocrystalline and polycrystalline form [5]. Aluminum oxide is an inert crystalline powder, insoluble in water and odorless [6].  
Aluminum oxide has a chemical formula Al\({}_{2}\)O\({}_{3}\). It is amphoteric in nature and has been used in various chemical, industrial, and commercial applications. It is considered an indirect additive used in food contact substances by the FDA [6]. The crystal structure of aluminum oxide is illustrative of the principles involved with ceramics having a substantial ionic bonding character. The crystal structure [7] consists of a nearly hcp arrangement of the larger oxygen anions (O\({}^{2-}\)), with the smaller aluminum cations (Al\({}^{3+}\)) located in two-thirds of the octahedral interstitial sites in the hcp structure. The layers in the structure provide the maximum separation of the Al\({}^{3+}\) ions. These octahedral sites have sixfold coordination, i.e., each aluminum ion is surrounded by six oxygen ions. The crystal structure is determined by the ratio of the radii of the aluminum and oxygen ions, and the requirement of an electrically neutral unit cell.  
Both physical and chemical characteristics of aluminum oxide products vary depending on the processing method that leads to changes in the crystal structure. The variety formed at very high temperatures is quite inert chemically [5; 6].  
Concern about enamel damage occurs especially when ceramic brackets are used. The first generations of ceramic brackets resulted in a major alteration of the enamel. Thus far, they have undergone significant improvements. The purpose of debracketing is to remove brackets and adhesive material from the tooth, giving it the initial appearance and contour, and minimizing the loss of iatrogenic hard tissue during this maneuver [8]. The manufacturers of the new generations of ceramic brackets assure that their use in orthodontic treatment is safe, due to the design of the base. Technological evolution allows researchers to quantitatively measure the surface of the enamel, which can be reduced after debracketing. The use of metal brackets registers the presence of the remaining adhesive material and leads to a loss of hard tooth substance of 20-50 \(\upmu\)m after their removal [6; 8].  
The objective of this in vitro study was to compare the changes in the enamel surface following the removal of ceramic and metal brackets, namely, the appearance of new cracks or the deepening of the existing ones. Quantitative analysis was performed using dispersive X-ray spectroscopy in the base of the brackets, and the ARI\({}_{\text{tooth}}\) score [9] is retained.