2.4. Tooth Preparation
dentine, tooth
CAVITY, GROUP B
Orthodontic resin (Ortho-Resin, DeguDent GmbH, Hanau, Germany) was used to cover the roots of the teeth up to 5 mm below the CEJ. The preparation of samples was performed by one investigator to maintain uniformity. The proximal box preparations (mesial or/and distal) were conducted using a using a high-speed contra-angle handpiece Ti-Max Z900L (NSK, Nakanishi Inc., Tochigi, Japan) at a speed range of 320,000–400,000 rpm with a round-end tapered diamond (Bur # TR-14, ISO 198/022, Mani Inc., Tokyo, Japan), which was replaced by a new one for each tooth. The final tooth preparations had the following dimensions: 3 mm wide isthmus; 2 mm wide box at the gingival third and 4 mm occlusal width. The apical depth of the boxes in Group B was prepared 2 mm above the level of CEJ, and for Groups C and D, it was prepared 2 mm and 4 mm below the CEJ, respectively. For etching the dentin, 37% phosphoric acid gel (Prime-Dent, Chicago, IL, USA) was used for 15 s and rinsed with water. The excess water was then carefully removed by a brief burst of air, leaving the dentine and enamel surface slightly moist with a shiny surface. A fully saturated brush was used to apply the bonding agent (Bonding resin, Prime-Dent, Chicago, IL, USA) on the etched surface in two consecutive coats; it was air dried for 10 s and light-cured using an LED curing light (Smart lite max, Dentsply Caulk, Milford, DE, USA) for another 10 s on both the occlusal and proximal surfaces. The rest of the cavity in all samples was filled incrementally using a microhybrid composite, Filtek Z250 (3M ESPE, St. Paul, MN, USA), followed by light curing for 20 s.Based on current principles of tooth preparation, for all-ceramic zirconia crowns, the occlusal surfaces were reduced by 2.0 mm, with a function-cusp bevel. A circumferential reduction by 1.0 mm was conducted, with the margins terminating at the CEJ. The final preparation had an axial wall taper of 6 to 8 degrees. The preparations were carried out using a flat-end long tapered diamond (Bur # TF-14, ISO 172/023, Mani Inc., Japan) with a highspeed contra-angle handpiece Ti-Max Z900L (NSK, Nakanishi Inc., Tochigi, Japan).