## 2 Materials and Methods  
S Sixty extracted bovine mandibular incisives were stored in physiological saline solution at a temperature between 5 \({}^{\circ}\)C and 10 \({}^{\circ}\)C [23]. They were embedded in autopolymerizable methacrylate resin block of dimensions 35 \(\times\) 50 \(\times\) 14 mm (ProBase Cold, Ivoclar Vivadent, Bologna, Italy), with the cement-enamel junction above 1 mm and orthogonal the resin base. After horizontal preparation of 1-1.5 mm using a diamond chamfer bur on lingual surface near the cementum-enamel junction of each tooth, air-polishing was performed (S2, EMS) with sodium bicarbonate-based powders (EMS) on 60 teeth for 30 s at distance of about 3 centimeters on total surface of each tooth (Figure 1). All teeth were imported in virtual environment by laboratory scanner (Smart Big Open Technology, Rezzato (BS), Italy). Using a Cad software (Exocad DentaCad, Darmstadt, Germany), one master model of RBFDP with a cantilevered single-retainer was designed (Figure 2). Figure 1: Air-polishing used to clean incisive embedded in resin block. The RBFDP presented the same parameter setting, shape, and size of the cantilever tooth, independently from the anatomical morphology of the bovine mandibular incisives. The retainer wings were fabricated with a uniform thickness of 0.7 mm. The connectors were designed with a height of 3 mm and a width of 1.5 mm [24,25]. Twenty RBFDPs with a cantilevered single-retainer design for the groups were fabricated with 3 different materials: zirconia (Katana ML, Kuraray, Milano, Italy), lithium disilicate (IPS e.max Press LT A2, Ivoclar Vivadent, Bologna, Italy), and PMMA-based material with ceramic fillers (HIPC, Bredent GmbH, Senden Germany). Each RBFDP has been created by the same CAD design with different procedures based on the type of material. The twenty RBFDPs in HIPC were milled by a computer numerical control machine (Roland DWX-50, Roland DG Corporation, Osaka, Japan). The excessive material was removed with tungsten bur (Komet Dental; H250 NEX). All of the samples were polished using a polisher machine (Sirio Dental, Meldola (FC), Italy). Twenty zirconia RBFDPs were milled by a CNC machine (Roland DWX-50, Roland DG Corporation, Osaka, Japan), polished using diamond burs, and sintered (Nabertherm LHT 01/17 D, Nabertherm, Lilienthal, Germany) for 12 h at 1450 \({}^{\circ}\)C following the manufacturing instructions. The 20 RBFDPs in lithium disilicate were fabricated with wax lost technique. A wax block (CeraWax, Co.N.Ce.P.T, Busseto (PR), Italy) was used to mill the RBFDPs. A spruce was waxed onto the cantilever tooth of all RBFDPs. They were then embedded in a universal investment material (IPS PressVest Premium investment, Ivoclar Vivadent, Bologna, Italy), with 2 RBFDPs (Figure 3). Figure 3: Cantilever RBFDPs milled in wax. Figure 2: Design of master model of cantilever BFDP. The wax was removed in a heated furnace (Sirio SR 750-In Fire, Sirio Dental S.r.l., Meldola (FC), Italy) and the RBFDPs were pressed with lithium disilicate glass-ceramic (IPS e-max Press LT A2, Ivoclar Vivadent, Bologna, Italy) in the machine-calibrated furnace (Luxor Press-SR 862, Sirio Dental S.r.l., Meldola (FC), Italy) following manufacturer recommendations (Figure 4). The sprue was removed using a metallic disk and all RBFDPs in lithium disilicate were polish with diamond burs. We obtained 60 comparable RBFDPs with a cantilever design without preparation of the abutment in three different materials. The thickness of the wings of all RBFDPs were measured using a caliper. All of the abutment teeth were treated with the same procedure: 15 s of etching (Scotchbond Universal Etching, 3M, Milano, Italy), 15 s of rinsing with water, 15 s of drying, and the bonding (Scotchbond Universal Adhesive, 3M, Milano, Italy) were applied for 20 s and polymerized for 60 s using a lamp (Valo, Ultradent, South Jordan, UT, USA).