## 2 Materials and Methods
### Characterization of Resin Cements  
One adhesive resin cement (Panavia F2.0, Kuraray Medical Inc., Okayama, Japan) and a self-adhesive resin cement (RelyX U200, 3M ESPE, Neuss, Germany) were selected for this study (Table 1). As the MLC test was performed in a wet environment, the flexural properties and surface energy parameters of the two resin cements were studied. To evaluate the flexural properties of the resins, the mixed resin paste was syringed into a stainless steel split mold with a dimension of 2 mm \(\times\) 2 mm \(\times\) 25 mm and covered by a polyester film; a glass slide was used to expel extra resin material on top of the mold [16; 17]. The specimen was then light-cured for 40 s by placing the light guide tip of a dental light-curing unit (LCU, Bluephase\({}^{\text{\textregistered}}\) 20i, Ivoclar Vivadent, Schaan, Liechtenstein) against the slide in five overlapping sections [17]. All the resin specimens were stored in water for \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **Manufacturer** & **Component** & **Lot No.** \\ \hline \multirow{6}{*}{Panavia F2.0} & & A paste: dimethylates, & \\  & & 10-methacryloyloxydelyl & \\  & Kuraray Medical Inc., & dihydrogenphosphate, & A paste: 00599C; \\  & Okayama, Japan & camphorquinone, silica; B paste: & B paste: 00122A \\  & & dimethacrylates, barium glass, & sodium fluoride \\ \hline \multirow{6}{*}{RelyX U200} & & Substituted dimethacrylate, & \\  & & 1-benzyl-5-phenyl-barbic-acid, & \\  & & calcium salt, 1,12-dodecane & \\ \cline{1-1}  & & dimethacrylate, sodium \(p\)-toluene & \\ \cline{1-1}  & & sulfinate, glass, silica, calcium & \\ \cline{1-1}  & & hydroxide, titanium dioxide, & \\ \cline{1-1}  & & methacrylated aliphatic amine & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Two resin cements used. 24 h at 37 \({}^{\circ}\)C before testing. Dimensions of the specimens were determined by a digital caliper. The specimens (n = 5) were then subjected to a three-point bending test under a universal testing machine (UTS, Model 3343, Instron Inc., Canton, MA, USA; crosshead speed: 0.5 mm/min). The flexural strength (FS) in MPa was obtained with the equation [16; 17]: FS = (3 FI)/(2 bh\({}^{2}\)), where F is the load at fracture (N), l is the support span (20 mm), b is the specimen width (mm), and h is the specimen depth (mm). The flexural modulus (FM) in GPa was also calculated [17]: FS = [(F13)/(84 bh\({}^{3}\))] \(\times\) 10\({}^{-3}\), in which F/8 is the slope of the load-displacement curve (N/mm). To evaluate the surface energy of the resin cements, contact angle (CA) measurements were performed. A split mold was used to prepare disc-shaped resins (diameter: 8 mm; thickness: 1 mm). Five resin discs were made for each material in a similar way as for flexural testing. The CAs (\(\Theta\)) of a liquid triplet (water, glycerol, and methylene iodide) with known surface energy parameters were determined on the resin surfaces using a CA goniometer (OCA 15 plus, DataPhysics, Filderstadt, Germany) [3; 18; 19]. The calculation was based on the Young-Dupre equation combined with the Lifshitz-van der Waals/[PERSON] acid-base (LW/AB) theory: \(\gamma_{1}(1+\cos\Theta)=2[(\gamma_{\rm s}{}^{\rm LW}\gamma_{1}{}^{\rm LW})^{ 1/2}+(\gamma_{\rm s}{}^{+}\gamma_{1}{}^{-})^{1/2}+(\gamma_{\rm s}{}^{-}\gamma_ {1}{}^{+})^{1/2}]\), in which \(\gamma_{1}\) and \(\gamma_{\rm s}\) are the surface tensions of the liquid (l) and solid (s) surfaces, respectively; the superscripts \(+\) and \(-\) indicate the acid and base components, respectively. The total surface energy \(\gamma_{\rm s}\) was calculated from: \(\gamma_{\rm s}\) = \(\gamma_{\rm s}{}^{\rm LW}+2(\gamma_{\rm s}{}^{+}\gamma_{1}{}^{-})^{1/2}\).