## 4 Discussion  
The purpose of this study was to evaluate the effect of femtosecond laser treatment on the surface roughness and flexural strength of translucent zirconia. Compared with the no treatment group, the surface roughness of translucent zirconia was significantly increased in the femtosecond laser group, while the flexural strength measured via three-point bending of zirconia modified by the femtosecond laser was found not to be significantly different from that of samples in the no treatment group. Thus, the null hypothesis of the study was partly rejected. The bending strength and fatigue resistance of translucent zirconia are higher than those of glass ceramics [22]. Thus, translucent zirconia is a material that offers both mechanical strength and aesthetic performance, and its clinical applications are becoming more and more extensive [22]. However, translucent zirconia may be adversely affected by the use of high-pressure airborne abrasion, as microcracks and defects are generated, and flexural strength is reduced [23]. To date, limited studies have evaluated the effect of FSL modification on Y-TZP surfaces and, to the best of our knowledge, no existing study has evaluated the effect of FSL for treating the surface of translucent zirconia. Studies have reported that the application of FSL forms groove- or pit-like structures on the surface of zirconia specimens, thereby increasing their surface roughness; however, this was accompanied by a reduction in flexural strength [24]. This approach may lead to sub-surface damage to the zirconia due to 50 \(\upmu\)m alumina blasting, which may cause microcracks that limit the life of zirconia restorations [25]. In this study, we explored a specific parameter of FSL in the surface modification of translucent zirconia, in order to improve surface roughness without producing significant grooves or pits. Even though no statistical significance was found, the results in this study revealed that surface roughness was improved. As studies incorporating the FSL method continue to improve, FSL may become more suitable as a surface treatment for zirconia materials. Although the use of APA with Al\({}_{2}\)O\({}_{3}\) particles is popular for the treatment of zirconia surfaces in order to increase the bond strength, studies have shown that the increased roughness imparted by this technique is accompanied by an increased fracture risk, thereby weakening the structure through the introduction of microcracks [26; 27; 28]. The results of our study demonstrated that FSL-treated translucent zirconia showed significantly higher mean R\({}_{\text{a}}\) values than those in the APA and NT groups, and there was no significant difference in the R\({}_{\text{a}}\) value between the APA and NT groups. These findings are consistent with those of the study of [PERSON] et al. [29], where the surface roughness of highly translucent Y-PSZ modified using Al\({}_{2}\)O\({}_{3}\) APA was not significantly enhanced, except for that of specimens comprising KATANA UTML (Kuraray Noritake, Japan). In contrast, FSL ablation of zirconia ceramic significantly enhanced surface roughness and improved the zirconia ceramic-resin bond strength, due to the presence of groove-like structures [30]. In our study, the translucent zirconia surface obtained following the FSL treatment presented uniform irregular structures without groove-like structures or pits, as confirmed in the SEM image shown in Figure 2e. Surface topography can be modified by femtosecond laser surface treatment and surface roughness of the zirconia can be increased; thus, the bond strength can be improved [31]. The main height parameters for evaluating surface roughness are average roughness (R\({}_{\text{a}}\)), the root mean square of the height of each point of the contour (Rq), and the ten-point height of microscopic unevenness (R\({}_{\text{z}}\)). The Ra value can represent the arithmetic mean deviation of the surface roughness profile amplitude parameter.