# Laser Use in Creating Orthodontic Adhesion to Ceramic Surfaces
## 4 Discussion
\begin{tabular}{c} Alumina \\ Alumina and magnesium spinel \\ \end{tabular}  & \ \begin{tabular}{c} Anterior and posterior full crowns \\ \end{tabular}  & Y \\ \cline{2-4}  & Alumina and zirconia & Anterior and posterior full crowns & Y \\ \hline Polycrystalline & Alumina & \begin{tabular}{c} Veneer restoration, anterior and \\ posterior full crowns \\ \end{tabular}  & N \\ ceramics & Stabilised zirconiaGlass matrix ceramics consist of metallic and silica oxides within a glassy matrix, meaning they can be attacked by acids such as acidulated phosphate fluoride (APF) or hydrofluoric acid [24]. Among the glass matrix ceramics, some types are widely used even today (feldspathic, lithium disilicate, alumina), although the manufacturers are constantly working on improvements. In order to prevent the dispersion of cracks through the glass matrix, stronger polycrystals or crystalline structures are inserted into the material [24]. Contrary to glass matrix ceramics, polycrystalline ceramics (alumina, stabilised zirconia, zirconia-toughed alumina or alumina-toughened zirconia) do not contain any glass, and as a result, they are not etchable. All the atoms are included into a regular crystalline array through which it is much more difficult for cracks to disperse, compared to glass ceramics. Polycrystalline ceramics were not available in dental practice until the late 1980s when the computer-aided manufacturing was developed [24,26]. In dental practice, ceramics can be used as all-ceramic restorations or metal-ceramic. For metal-ceramic restorations, feldspathic ceramics are used to cover a metal coping. Clinical experience suggests the veneering ceramic should be about 1-2 mm thick for adequate strength and aesthetics. Whether a type of ceramic is etchable or not is important not only in prosthodontics but also for orthodontic treatments, as the adhesion between an orthodontic bracket and a prosthetic restoration is important for the treatment's success [24]. There are several surface conditioning techniques commonly used for bonding on zirconia restorations, such as silica coating [27] sandblasting [28,29], or laser irradiation with CO\({}_{2}\), Nd:YAG, or Er:YAG [30, 31, 32, 33]. CO\({}_{2}\) lasers have a gas active part which emits at a wavelength of 10.6 nm which is absorbed by water and hydroxyl apatite, but also by ceramic materials and it can create micro-cavities using superficial heat, enhancing the strength between resins and ceramics. Nd:YAG lasers determine melting and random re-crystallisation, determining surface roughness and increasing the strength of resin-ceramic bond [33]. However, there is still to be established an ideal surface-treatment that will provide sufficient bond strength to minimise bracket debonding from polycrystalline ceramic restorations. In this context, the aim of our research was to create a thorough image of laser use in ceramic preparation for bonding orthodontic attachments, as it is not always possible to use hydrofluoric acid for surface preparation of the ceramic restoration already placed intra-only. The original articles included in this review used a similar procedure for bonding brackets to ceramics: creating a ceramic surface of either feldspathic, zirconia or IPS Empress E-max type, bonding brackets using an orthodontic adhesive, mostly Transbond XT as the golden standard and debonding the bracket using an Instron machine that also measures shear bond strength. In some studies, thermocycling from 500 to 5000 cycles was used to create an environment that is similar to the oral cavity, as thermocycling influences adhesion. Some studies also searched the debonded ceramic samples under a scanning electron-microscope in order to find surface irregularities or cracks.