Abstract: Objective: To evaluate the effect of restoration design with different preparation depths and with or without ferrule on the failure load of maxillary short coronal molar restored with endocrown. Methods: Thirty permanent molars with complete morphology from the neck to the apical were collected. Three kinds of pulp cavity preparation depth, i.e. 3 mm, 4 mm, and 5 mm and two kinds of margin design were randomly divided into six groups, which were restored with zirconia endocrowns by CAD-CAM technique. The flexural strength test was recorded with the fracture load value and the fracture mode. Results: No matter the butt margin group or the shoulder group, the maximum loading value of the six groups was 3mm<4mm<5mm, however, there was no significant difference between two groups. The maximum load value of the shoulder group was greater than that of the butt margin group, and there was a statistically significant difference between the 3mm and 4mm groups. In the fracture mode of six groups of samples, irremediable fracture was the main one, and all samples in the 5mm group presented irremediable fracture. Conclution: When maxillary short coronal molars were restored with zirconia endocrowns by CAD-CAM technology, the preparation depth of pulp cavity at 3mm and 4mm could meet the requirements of clinical flexural strength, however, no more than 4mm. In addition, the introduction of 1mm dentin shoulder collar could be used as a clinical preparation method to effectively increase the flexural strength of teeth and prostheses.

Key words: endocrown, preparation depth, ferrule, fracture resistance