## 4 Discussion
Furthermore, few studies focused on plasma treatment of ceramic surfaces, and to the authors' knowledge there are no studies on plasma surface treatment on LIC and RMC. The few studies that have focused on plasma surface treatment investigated its effect especially on zirconia surfaces [30; 31; 34; 35; 36; 37; 38; 39; 47; 51; 52; 53; 54; 55], and it is thus not possible to make comparisons of their results with those obtained in this study; therefore, comparisons will be made with other materials. [PERSON] et al. [56] studied the effect of atmospheric plasma treatment on lithium disilicate ceramics. The gas used was a combination of an 85% hexamethyldisiloxane (HMDSO) monomer and 15% argon for 30 min. The roughness obtained on the lithium disilicate was lower than those obtained in the LIC group. The authors observed a higher surface energy in the plasma treatment than in traditional HF acid etching, with the monomer being the key of the results obtained. These data were also found by [PERSON] et al. [57], in a similar study with the same combination of HMDSO and argon gas, but instead of using atmospheric plasma they used low-pressure plasma. The data could not be compared because of the differences in the materials and methodologies employed. Zirconia is a high-strength ceramic introduced as an alternative to metal-ceramic restorations [6]. Its popularity has increased considerably; however, resin-bonding protocols for zirconia are still controversial [14; 34]. Zirconia is a polycrystalline ceramic with no glass phase, and it is more difficult to obtain a proper bonding. Surface treatment by HF acid cannot be used for zirconia ceramics, because it fails to achieve adequate surface roughness [58]. Airborne alumina particle abrasion has been recommended to improve the surface roughness on the zirconia surface [59; 60; 61]. It also improves the surface energy and wettability [61]. However, it has been reported that particle abrasion by alumina can create mechanical damage on the surface of the ceramic [51]. To avoid this problem, alternative surface treatments have been proposed and, recently, surface treatment by plasma has been introduced [25; 47; 51]. The few studies found concluded that plasma treatment of zirconia surface decreased the contact angle, and therefore improve the surface energy [30; 31; 34; 35; 36; 47; 48; 53; 54; 55]. However, the surface roughness was not affected [35; 52; 53; 55]. The results were consistent to those obtained in the present study, although the ceramic materials analyzed were different. Although the most commonly used gases are argon and oxygen, to date it is not clear which of the two to use, or whether to mix both, and how long the exposure to gas should be. [PERSON] et al. [34] tested different plasma treatments on zirconia: atmospheric air, 100% oxygen, 100% argon, 10% argon, a 90% oxygen combination, a 20% argon, and an 80% oxygen combination. The lower contact angles values were those achieved by the group with 20% argon and the 80% oxygen combination. Therefore, there is no standardization regarding which gas is the most appropriate, the exposure times, the type of plasma, or the way in which to apply it. The use of laser technology for surface treatment has been also introduced. [PERSON] et al. [38] reported that a 1070 nm fiber laser can be considered as a good device to increase the adhesion of lithium disilicate ceramics. Several studies investigated the effect of laser on zirconia surface treatment and the results are inconclusive. [PERSON] et al. [62] reported that surface treatment with a CO\({}_{2}\) laser increased the shear bond strength between the resin cement and the zirconia ceramic. [PERSON] et al. [63] found that Nd:YAG laser irradiation produced significantly higher alterations in the surface roughness of zirconia than Er:YAG. Recently, it has been reported that repeating CO\({}_{2}\) laser treatment methods could be considered reliable approaches for zirconia surface treatment [61].