## Materials and Methods
### Fabrication of Endocrowns and Cementation  
Group LD endocrowns were fabricated with pressable Lithium disilicate glass-ceramic. The wax patterns were fabricated directly over the teeth samples by the single dental technician. The dimension from the central groove to the pulpal floor in the wax pattern (Geo classic opak, Renfert, Germany) was verified with a digital caliper (fino Pra Ceci caliper; FINO GmbH, Bad Bocklet, Germany). The occlusal anatomy of all samples was standardized with the help of a single silicone mold. The wax patterns were invested (Pressvest speed, Ivoclar Vivadent, Liechtenstein, Germany), burn out in a furnace, and heat pressed with an IPS Empress, Programat, EP 5000 furnace (Ivoclar Vivadent AG Schaan, Liechtenstein, Germany) at 920 \({}^{\circ}\)C. The pressed endocrowns were finished with a brief sandblasting with 100 Al\({}_{2}\)O\({}_{3}\) at 1 bar (15 psi) pressure and steam cleaned. In conclusion, the specimens were rinsed in an ultrasonic cleaner with distilled water for 10 min and were coated with a single layer of the neutral shade glaze by firing at 765 \({}^{\circ}\)C. The endocrowns fabrication from HTZ and PIC were fabricated utilizing the Amann Girrbach system (Amann Girrbach AG, Koblach, Austria). The digital impression of the endocrowns preparations was obtained by a laboratory scanner (Ceramil Map, Amann Girrbach AG, Koblach, Austria). The standardized endocrowns was designed (Ceramil mind, Amann Girrbach AG, Koblach, Austria) at a 2.5% lesser volume for uniform luting cement space during cementation. The milling of endocrowns was accomplished with a computer-aided milling machine (ceramill motion 2, Amann Girrbach AG, Koblach, Austria).  
Intaglio surface of LD ceramic and PIC endocrowns were prepared by etching with 5% hydrofluoric acid (IPS ceramic etching gel) for 20 s and 60 s, respectively. Subsequently washed and ultrasonically cleaned for 10 min. Finally, mono-component silane (Monobond Plus, Ivoclar Vivadent AG, Schaan Liechtenstein) was applied and excess was air-dried. HTZ zirconia endocrowns surface was subjected to airborne-particle abrasion with 50 \(\upmu\)m alumina oxide particles at 2 bar pressure for 1 min. Washed  
Figure 1: Endocrowns designs for different groups.  
with water and ultrasonically cleaned for 10 min. Enamel surfaces were selectively etched with 37.5% phosphoric acid (Ultra-Etch, Ultradent Products, South Jordan, UT, USA), washed with water and gentle air-dried. All the endocrowns samples were cemented with self-adhesive luting cement (RelyX Unicem2 Automix, 3M ESPE, Seefeld, Germany), light-cured for 20 s for all the surfaces.  
All samples were thermo-cycled (1100; SD Mechatronik, Feldkirchen-Westerham, Germany) for 5000 cycles between 5\({}^{\circ}\) and 55 \({}^{\circ}\)C with the dwell time of 30 s. Static load at a crosshead speed of 1 mm/min was exerted with a universal testing machine (Instron, Norwood, MA, USA). The Load was applied at 45-degree angulation over buccal cusp until the fracture (Figure 2). The maximum load at the fracture is expressed in MPa. The fractured samples were evaluated with a stereomicroscope \(\times\)10 (Olympus/DeTrey, Germany) to determine the failure mode. The fractures were classified [26] as unfavorable if it involved the vertical fracture or fracture below assumed alveolar bone level requiring tooth extraction in the clinical situation (Figure 3). The failures were categorized as favorable if it included the debonding, cohesive fracture or crack in the crown. Obtained data were statistically analyzed using SPSS 19 software (IBM Corporation, Armonk, NY, USA). The data were assessed by one-way ANOVA and Tukey HSD (honestly significant difference) tests. Significance was set to \(p<0.05\).