Preparation of test groups

This study tested the current glass ceramic ZLS by comparing LDS with monolithic and conventional veneering techniques in implant-supported crowns: group L-M: lithium disilicate ceramic (monolithic), group L-V: lithium disilicate ceramic (conventional veneering), group ZL-M: zirconia-reinforced lithium silicate ceramic (monolithic), group ZL-V: zirconia-reinforced lithium silicate ceramic (conventional veneering) (Table 2).

Bone-level implants with a diameter of 4.1 mm and a length of 12 mm (Hager & Meisinger GmbH, Germany) and prefabricated cement-type abutments (5 mm in diameter, 6 mm in height, serial number A66756, Hager & Meisinger GmbH, Germany) were used for simulating the clinical situation of a missing mandibular first molar teeth.

Forty-eight implants (Hager & Meisinger GmbH, Germany) were embedded in a plastic ring using an auto polymerizing acrylic resin (Vertex-Dental BV, Zeist, Holland) vertically to the horizontal plane. The same number of prefabricated abutments (Hager & Meisinger GmbH, Germany) was fixed to them at 30 N.cm of torque according to the manufacturer’s instructions.

Manufacture of the crowns

Contrast spray (IPS Contrast Spray Labside; Ivoclar Vivadent, Schaan, Liechtenstein) was scattered on an implant-abutment complex for optical impression (InEos Blue, Sirona Dental Systems, GmbH, Bensheim, Germany). After scanning abutment, the crown design was performed with Cerec InLab V15.0 based on the mandibular first molar tooth morphology (Fig. 1) and was calibrated to 1.5 mm in fossa, 2 mm in cusps, and the entire crown. “Multilayer” was selected among different design modes. In this design technique, the fully anatomical shape can be splinted into a core and a covering crown. Therefore, the cores of the two veneering groups and monolithic groups were fabricated in a standardized way.

For monolithic restorations, the design mode “unsplit” was defined to obtain fully anatomical crowns. A total of 24 monolithic crowns (n = 12 for group L-M and n = 12 for group ZL-M) were fabricated by a milling machine (InLab MC XL, Sirona Dental Systems, GmbH, Bensheim, Germany).