Methods
CAD, fracture
CAD
Twenty Ø4.1-mm implant laboratory analogs (Straumann Bone Level, Straumann Co, Switzerland) were embedded into Ø2-cm epoxy resin blocks [
Abutments tightened to implant analogs embedded into resin blocks. In the preparable abutment group, abutments were tightened onto the analogs and their screw access channels blocked with a barrier material (Liquidam; Discuss Dental, LLC). In the ti-base group, ti-bases were tightened to the implant analogs and scan bodies (Sirona scanbody; Dentsply Sirona, Switzerland) were snapped over the abutments. A laboratory scanner (InEos X5 lab scanner, Dentsply Sirona) was used to scan all specimens.A CAD software program (CEREC software; Dentsply Sirona) was used to design the crowns for both groups standardizing the crown parameters. All crowns were milled (CEREC MC X5; Dentsply Sirona) from lithium disilicate CAD blocks having A2 shade (IPS e.max CAD; Ivoclar AG, Liechtenstein). Specially designed CAD blocks with prefabricated screw access channel were used for the ti-base group (IPS e.max CAD CER/INLAB; Ivoclar AG, Liechtenstein ) while solid CAD blocks were used for the preparable abutment group and screw access channels were prepared later after milling of the crowns and before crystallization (Fig. 
Lithium disilicate CAD blocks used for crown fabrication. Prior to cementation, a spectrophotometer (Easy shade1, Vita, Germany) was used to measure the shade of the crown specimen from 4 different aspects (buccal, lingual, mesial, distal) evaluating the degree of perceptible color based on 3 coordinates; (l*, a* b*) (Fig. 
A spectrophotometer was used to measure the shade of the specimens. Cementation was done by using a translucent shade dual-polymerizing self-adhesive resin cement (Panavia SA Cement universal; Kuraray Co, Japan) following the manufacturer’s instructions. The technique of crowns cementation to their corresponding abutments mentioned by Khamis MM et al. [The spectrophotometer was used again after cementation to measure the shade of the specimens (Fig. The vertical distance between the abutment shoulder and the most apical part of the crown represents the marginal gap [
Measurement of the marginal gap by using stereomicroscope at X50 magnificationThe specimens were then aged using a thermocycling machine made specifically for the purpose (Dental biomaterials Department, Alexandria University, Egypt) for 2000 cycles, or 3 months of clinical service, in water baths between 5 and 55 degrees Celsius with dwell times of 1 min in each bath and relaxation periods of 30 s in air between the 2 baths [After aging, stereomicroscopy was used to evaluate the effect of thermocycling and cyclic loading on the marginal adaptation of the specimens. The samples were then transferred and mounted on a universal testing machine (Department of dental biomaterials, Alexandria University, Egypt) set at 0.5 mm per minute cross head speed and the test for fracture resistance was performed. A holder was constructed with 4 retentive screws to hold the epoxy resin blocks with opposing holes in the epoxy resin blocks to accommodate the retentive screws. A specially designed ball head attachment with dimensions equal to the occlusal diameter of the crowns was fabricated and mounted on the universal testing machine resting on all the cusps and deflecting them when the test starts (Fig. 
Specimen mounted on a universal testing machine to measure the tensile load required to fracture the crownData were collected and statistically analyzed with a statistical software program (IBM SPSS Statistics for Windows, v23.0; IBM Corp). Normality was checked for all variables by using the Shapiro–Wilk test [