Fracture resistance and failure modes of maxillary premolars restored with different endocrown designs and materials after artificial aging

Endocrowns have been used successfully for molars. However, they are still questionable for premolars due to their limited surface area for bonding. This study aimed to evaluate the effect of different designs and materials on the fracture resistance of maxillary premolar endocrowns. Three different endocrown designs and two ceramic materials were studied. Conventional post-and-core crowns served as control.

Fifty-six maxillary premolars were endodontically treated and shortened to a level of 2 mm from the cervical line. They were randomly allocated into six endocrowns groups of two ceramic materials and three preparation designs: flat occlusal table with no ferrule design, 1.5 mm circumferential ferrule, and 1.5 mm buccal ferrule only. A post-and-core crown control group was designed with 1.5 mm ferrule, zirconia posts, and composite resin cores and lithium disilicate crowns. All restorations were bonded using Panavia V5 and subjected to a thermo-mechanical fatigue test with a 10 kg dynamic load for 1,200,000 cycles and thermocycling between 5 and 55 °C.

The fracture resistance ranges from a minimum of 661 (± 143) N to a maximum of 1440 (± 316) N. Within the same material group, the designs showed a highly significant effect (p≤ 0.001), while the effect of a material variable within the same design was dependent on the design used.  The ferrule design improved the fracture resistance of the specimens and were comparable to the post-and-core crown control group (p>0.05). The buccal ferrule groups fracture strength values were significantly lower than those of the control group (p≤0.001). The zirconia endocrowns with a flat occlusal table showed significantly higher fracture resistance than lithium disilicate endocrowns (p=0.001). Only two post-and-core samples revealed a favorable failure, all other specimens showed catastrophic failure modes.

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