## 2 Materials and Methods  
Thirty implants with an internal conical tapered configuration with a 4 mm diameter and 11.5 mm length (Cyroth--AoN Implants) were embedded in acrylic resin (Palapress; Heraeus Kulzer) to provide slight movability of the tooth, simulating natural tooth mobility during chewing simulation and fracture resistance testing [1]. The resin supports were drilled by burs supplied by the manufacturer and then tapped to allow the insertion of the implant using a wrench limited to 60 Ncm torque, at the end of the screwing only. Each implant received a standard titanium abutment (Moncone per Zirconia--AoN Implant), fastened with 25 Ncm torque applied to the abutment screw, and then sandblasted with aluminum oxide particles with an average diameter of 30 \(\upmu\)m (Cojet sand, 3M Espe) (Figure 1). A right maxillary second pre-molar was designed in a CAD environment (Lava Design 7 3M Espe) with two different configurations (see Figure 2). One type was designed for fitting to a customized zirconia abutment whereas the second type was designed for fitting to a standard titanium abutment, with the size and shape of the tooth remaining unchanged. From this CAD design, 30 crowns were milled (Trentotech--3M Espe authorized milling center, TN Italy) in a new resin composite material (Lava Ultimate 3M Espe), 15 for each type of design crown fitting configuration (Figure 3). Figure 1: Preparation of samples: (**a**) implant insertion into the resin supports, (**b**) sandblasting of abutments, and (**c**) evidence of the sandblasting process. Figure 2: Design sketches of the samples: (**a**) group 1 and (**b**) group 2. In this way, two experimental groups were created. In group 1, 15 Ultimate Lava nano ceramic resin crowns were cemented directly to a titanium abutment that was previously screwed into the fixture and sandblasted with aluminum dioxide powder (Figure 4). Each crown and titanium abutment were coated with Scotchbond Universal adhesive (3M Espe) and then RelyX Ultimate (3M Espe) was placed directly on the crown for cementation. After removing the excess cement, photopolymerization was achieved using an LED curing light for 40 s (Valo Led Ultradent), maintaining a manual pressure on the parts. Group 2 consisted of 15 Lava Ultimate nano ceramic resin crowns, cemented on zirconia abutments and screwed to the fixture, and embedded in the resin supports as described above. For this group, a master custom abutment was made by simulating the natural emergence profile of a right maxillary pre-molar. The master abutment was optically scanned and then milled in zirconia dioxide ceramic (Lava(tm) Zirconia--3M Espe--Trentotech, 3M Espe authorized milling center, TN, Italy) (see Figure 4). The internal surfaces of the Lava Ultimate crowns were sandblasted with aluminum oxide (Cojet sand, 3M Espe) and the same treatment, both inside and outside, was used for the zirconia abutments that had two distinct adhesive interfaces. First, the zirconia and titanium abutments were cemented together, carefully following the manufacturer's instructions. Before cementation, a specific primer (Relix Ceramic Primer, 3M Espe) was applied on both bonding surfaces and then the two abutments were cemented (Relyx Unicem, 3M Espe). After removing the excess cement, and maintaining pressure, they were light cured for 40 s around the finishing line (Valo Led-Ultradent). The second step cemented the Lava Ultimate crowns to the zirconia abutments. After brushing a thin coat of Scotchbond Universal adhesive (3M Espe) on both adhesive interfaces, without curing the layer, the nano ceramic resin crowns were cemented to the zirconia abutments using Relix Ultimate (3M Espe). During the cementation, pressure was maintained on the structure and the samples were light cured for 40 s (Valo Led-Ultradent). Excess cement resin was removed before photopolymerization by means of a micro-brush. Figure 5 shows the result of the procedure, in which the thin layer of Relix Ultimate material is visible between the crown and the zirconia abutment.