Monolithic CAD/CAM lithium disilicate versus veneered Y-TZP crowns: comparison of failure modes and reliability after fatigue.
PURPOSE The aim of this research was to evaluate the fatigue behavior and reliability of monolithic computer-aided design/computer-assisted manufacture (CAD/CAM) lithium disilicate and hand-layer-veneered zirconia all-ceramic crowns.
MATERIALS AND METHODS A CAD-based mandibular molar crown preparation, fabricated using rapid prototyping, served as the master die. Fully anatomically shaped monolithic lithium disilicate crowns (IPS e.max CAD, n = 19) and hand-layer-veneered zirconia-based crowns (IPS e.max ZirCAD/Ceram, n = 21) were designed and milled using a CAD/CAM system. Crowns were cemented on aged dentin-like composite dies with resin cement. Crowns were exposed to mouth-motion fatigue by sliding a WC-indenter (r = 3.18 mm) 0.7 mm lingually down the distobuccal cusp using three different step-stress profiles until failure occurred. Failure was designated as a large chip or fracture through the crown. If no failures occurred at high loads (> 900 N), the test method was changed to staircase r ratio fatigue. Stress level probability curves and reliability were calculated.
RESULTS Hand-layer-veneered zirconia crowns revealed veneer chipping and had a reliability of < 0.01 (0.03 to 0.00, two-sided 90% confidence bounds) for a mission of 100,000 cycles and a 200-N load. None of the fully anatomically shaped CAD/CAM-fabricated monolithic lithium disilicate crowns failed during step-stress mouth-motion fatigue (180,000 cycles, 900 N). CAD/CAM lithium disilicate crowns also survived r ratio fatigue (1,000,000 cycles, 100 to 1,000 N). There appears to be a threshold for damage/bulk fracture for the lithium disilicate ceramic in the range of 1,100 to 1,200 N.
CONCLUSION Based on present fatigue findings, the application of CAD/CAM lithium disilicate ceramic in a monolithic/fully anatomical configuration resulted in fatigue-resistant crowns, whereas hand-layer-veneered zirconia crowns revealed a high susceptibility to mouth-motion cyclic loading with early veneer failures.
There have been significant improvements in the ceramics that can be used for full-coverage crown and fixed partial denture restorations. This parallels improvements in software and milling devices for both in-lab and in-office CAD/CAM systems. This well designed research study comparing lithium disilicate and hand-layer-veneered zirconia all-ceramic crowns used a testing device that provides for cycled stress fatigue of the ceramic. The results indicate that it would be expected that lithium-disilicate CAD/CAM milled restorations would perform better than the porcelain-layered veneered zirconia crowns. This research testing protocol has also been extended to evaluate zirconia core designs to improve the durability of the porcelain-layered veneer. By modifying core design, the veneered porcelain demonstrated better durability with very minor porcelain chipping after fatigue stress. From this and other research, the reliability of these all-ceramic systems allows them to be used for crown restorations.
Commentary by Howard E. Strassler, DMD
Professor, Division of Operative Dentistry
Department of Endodontics, Prosthodontics and Operative Dentistry
University of Maryland Dental School