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Multicenter RCT 6-Year Survival of Zirconia-Reinforced Lithium Silicate Endocrowns: 90° Shoulder Versus Butt-Joint
Ping Xiao1, Ziting Zheng2, Yuting Zeng1
1Department of Stomatology, Nanfang Hospital, Southern Medical University, Guangzhou, Guangdong, China.
Objectives:
Margin design significantly influences endocrown biomechanical performance. Nevertheless, long-term clinical data comparing margin designs within a singular zirconia-reinforced lithium silicate (ZLS) system remain scarce. This study reports the 6-year follow-up results of a multicenter, prospective, randomized, assessor-blinded superiority trial. The trial compares CAD/CAM ZLS endocrowns with 90° shoulder margins and butt-joint margins in endodontically treated molars (ETMs).
Methods:
Two hundred ETMs in 200 patients meeting the inclusion criteria were randomly restored with chairside CAD/CAM ZLS endocrowns, which were then assigned to either the 90° shoulder margin group or the butt-joint margin group. All restorations were fabricated following a standardized digital workflow and adhesively luted in line with predefined cementation protocols. Subsequently, clinical and radiographic evaluations were performed at baseline and annually for up to 6 years using modified United States Public Health Service (USPHS) criteria. Cumulative tooth survival and clinical success rates were estimated using Kaplan-Meier analysis, and group comparisons were conducted using log-rank tests (α = 0.05).
Results:
Over 6 years, 35 restorations failed, and 32 were censored, leaving 133 endocrowns (79 butt-joint, 54 shoulder) available for clinical examination (6-year recall rate 66.5%). The 6-year cumulative tooth survival rate was 93.1% (91.7% butt-joint vs. 94.8% shoulder). The 6-year cumulative clinical success rate was 80.5% (81.4% butt-joint vs. 79.4% shoulder). Restoration fractures were the most prevalent failure type, with most affected teeth remaining restorable. No statistically significant difference was found between the two margin-design groups in 6-year tooth survival or clinical success.
Conclusions:
Within the limitations of this study, both margin designs demonstrated favourable long-term clinical outcomes for CAD/CAM ZLS endocrowns in ETMs when standardized protocols were followed. The available evidence does not support the superiority of the 90° shoulder margin over the butt-joint margin. Both designs are clinically acceptable for the restoration of ETMs with CAD/CAM ZLS endocrowns.