Effect of Shoulder Design on Failure Load of PTCercon Crowns

Objective: To evaluate the effect of shoulder design on the failure load of Press-to-Cercon (PTC) crowns. Method: Two groups were studied: Overpressed crowns with a zirconia free PTC shoulder (CS) and overpressed crowns with zirconia up to the margin (CC). The zirconia-free shoulder extended 0.8 mm over the finishing line of the coping. Eight zirconia copings per group of first maxillary anteriors were fabricated with CerconBrain CAM system (DeguDent). The thickness was 0.6 mm standard. After milling, the copings were sintered at 1350°C to final density. After sintering the coping was waxed-up to a standard contour, sprued and invested in CarraraUniversalDustless Investment (Elephant). The PTC Ceramic was pressed at 940°C over the zirconia coping. After devesting and separation from the sprues the crowns were veneered with two layers CerconCeramS porcelain (DeguDent). The crowns were cemented on a CoCr die with zinc phosphate cement and held under constant load of 5 kg during setting. The crowns were inspected using SEM for surface fracture analysis. Failure loads were measured using vertical compression loading at 0.5 mm/min. Results: Failure loads [kN (SD)]: Group CS: 4228(515) and group CC: 5408(806).Conclusion: A significant (p<0.05) decrease of 22 percent in breaking strength was observed with the overhanging shoulder as compared to fully supported PTC Crowns. Surface fracture analysis revealed the crack initiation site was typically located on the inside of the coping at the glass-zirconia interface.