## 2 Materials and Methods  
The study was carried out after obtaining the ethical approval of its design from the research ethics committee of the Faculty of Dentistry at King Abdulaziz University (IRB protocol #362-12-21). Twenty extracted mandibular third molars free of caries with approximately similar root lengths and crown dimensions were collected from local clinics; teeth were extracted for routine clinical indications. Extracted teeth with cracks in the crown or roots of teeth identified after extraction were excluded from the study. First, teeth were cleaned using an ultrasonic scaler. Then, teeth were sectioned parallel to the occlusal plane 2 mm above the cementoenamel junction (CEJ) using a low-speed diamond saw (Allied TechCut 4 Low-Speed Diamond Saw, Rancho Dominguez, CA, USA) under water coolant. Next, pulp chambers and canals were accessed and cleaned from the remaining soft tissue. All specimens were ultrasonically cleaned (PowerSonic 405, Hwashin, Seoul, Republic of Korea). Finally, the pulp chamber walls were smoothed for the path of insertion. To achieve a symmetrical pulp chamber with an average depth of 3 mm, chambers were filled with flowable resin composite (Tetric N-Flow Bulk Fill, Ivoclar Vivadent, Amherst, NY, USA) and light cured (E-Morlit.Apoza, NewTaipei, Taiwan). Then, teeth were randomly divided into two groups (n = 10). The control group has all the tooth structure walls at the same level. Test group: box extension on the proximal walls was 2 mm apical to the level of the buccal and lingual walls. The remaining tooth structure was measured using a digital caliper. For the conventional endocrown design, the ranges of thicknesses were as follows: buccal wall width ranged 2-3.6 mm, lingual wall width ranged 1.7-3.3 mm, mesial wall thickness ranged 2.3-3.3 mm, distal wall thickness ranged 2-3.2 mm. For the endocrown with mesial and distal extension boxes, the ranges of thicknesses were as follows: buccal wall width ranged 1.8-3.1 mm, lingual wall width ranged 1.7-2.8 mm, mesial box depth ranged 2-2.5 mm, distal box depth ranged 2-2.5 mm, mesial box width range 2.5-4 mm, distal box width range 2-5 mm. The remaining wall thicknesses for both conventional endocrown and boxed designs were homogeneous among the specimens for each wall thickness, which were analyzed using the normality test Shapiro-Wilks. The root portion of the teeth was covered with a thin layer (0.35 mm) of light polyvinyl siloxane material (Virtual extra light body, Ivoclar Vivadent, Schaan, Liechtenstein) to simulate the periodontal ligament [42]. Then, the teeth were embedded in auto-polymerizing acrylic denture base resin (Probase Cold, Ivoclar Vivadent, Schaan, Liechtenstein) at 2 mm apical to the cementoenamel junction (CEJ) to simulate alveolar bone level [43]. Mean chamber depth and wall thickness were measured. Specimens were scanned using an intraoral scanner (Cerec AC Bluecam; Sirona Dental Systems GmbH, Bensheim, Germany), and acquired data were transferred to software for restoration design (Cerec SW 4.0, Sirona Dental Systems GmbH, Bensheim, Germany). Endocrowns were designed with approximately 4 mm height established for all specimens. The crown material was lithium dislicate (IPS e.max CAD, Ivoclar Vivadent, Schaan, Liechtenstein) and was milled using (Cerec MC XL; Sirona Dental Systems GmbH, Bensheim, Germany). The post-milling firing was accomplished following the manufacturer's protocol in ceramic furnaces (Programat S, Ivoclar Vivadent, Schaan, Liechtenstein). Abutment preparations for conventional endocrown design and endocrown design with mesial and distal extension boxes are shown in Figures 1 and 2, respectively. Silicon indicating paste (Fit Checker Advanced Blue cartridge pack, GC America, St Alsip, IL, USA) was applied to check the seating, and adjustment was made if needed. The intaglio surface of endocrowns was etched with 9% hydrofluoric acid for 20 s. Then, each surface was rinsed with water and dried.