Implants have been successfully used to replace missing teeth for many years. Notwithstanding the high success rates, complications such as screw loosening and/or fracture, prosthesis fracture, and even implant fracture are inevitable. The reasons of the complication may be related to decreased proprioception and low tactile sensitivity which makes implant-supported crowns more susceptible to occlusal overload than tooth-supported crowns.

Metal-based superstructures are frequently used for implant-supported restorations. Since the metal grayish shade and opaque structure prevent light transmission, they demonstrate limited esthetic results that cannot meet esthetic demands. Besides this esthetic disadvantage, the more important problem is the biological one, the potential corrosion process between the titanium and metal-based superstructure, which can cause implant failures. Therefore, natural-looking appearance, biocompatibility, and improved mechanical properties have led to the use of all ceramics in implant-supported restorations, as in fixed partial dentures. In nowadays, continual marketing of new all-ceramic systems and advances in CAD/CAM (computer-aided design/computer-aided manufacturing) technology make the use of all ceramics a more preferable alternative that can be fabricated monolithic or manually veneered.

Lithium disilicate (LDS) which is the most popular glass ceramic has small needle-shaped crystals and glass matrix. It can be manufactured by CAD/CAM (IPS e.max CAD, Ivoclar Vivadent) and can be used for substructure or full-contour restorations (monolithic restorations). Recently, a new glass ceramic, zirconia-reinforced lithium silicate (ZLS; Vita Suprinity, Vita Zahnfabrik, Celtra Duo, Dentsply DeTrey) has been marketed (Table 1).

This material has a fine-grained, homogenous microstructure and flexural strength value comparable to lithium disilicate. It is reinforced with approximately 10% zirconia which strengthens the ceramic structure by avoiding crack propagation and demonstrates enhanced mechanical and esthetic features due to the combination of the positive material properties of zirconia and glass ceramic. In a clinical study of Zimmermann et al., it was concluded that ZLS ceramic restorations had a high clinical success rate (96.7%) after 12 months.

In the literature, a limited number of studies have demonstrated the outcomes of all ceramic implant-supported restorations. Therefore, the aim of this study was to compare the failure load of CAD/CAM-manufactured implant-supported crowns with different restoration techniques and stress distribution on prosthesis-implant-bone complex. The null hypotheses were that (1) there were no differences in the failure load of two different monolithic glass ceramic crowns; (2) veneering technique did not affect the failure load of the different glass ceramics; and (3) veneered crowns have similar failure load values.