Fracture Strength of All-Ceramic Restorations After Fatigue Loading

Objectives: To measure and compare fracture strength of monolithic and bilayered LAVA and e.max crowns after load cycling.

Methods: To compare fracture strengths of bilayered and monolithic LAVA zirconia/3M ESPE and e.max lithium disilicate/Ivoclar Vivadent crowns prepared with varying manufacturing techniques and thicknesses (table).  A full crown preparation was made on typodont mandibular first mandibular (Kilgore201), scanned/LAVA COS/3M ESPE and sent to 3M ESPE to have Z100 composite resin dies prepared. Crowns were fabricated with full contour Lava Design 5.0 Software. For DVS system, DVS fusion porcelain was applied to the occlusal surface of the copings and fused by sintering. Veneering was done by experienced technicians for hand-veneered groups.

Experimental Groups	Dimensions	Fracture strength(N) Mean±SD
LAVA-DVS	t=1.2mm/(0.6mm zirconia core+0.6mm DVS)	2624.76±289.86
LAVA-Hand veneered	t=1.2mm/(0.6mm zirconia core+0.6mm feldspathic porcelain hand layered)	2655.25±590.04
LAVA-Monolithic	t=0.6mm/(zirconia core)	1668.48±311.06
IPS e. max CAD	t=1.2mm/(monolithic lithium disilicate)	1465.20±329.89
	t=1.5mm/(monolithic lithium disilicate)	2026.83±364.47
	t=1.5mm/( bilayered lithium disilicate core+feldspathic porcelain hand layered)	1731.90±315.26

Crowns were cemented (RelyX Luting Plus RMGI cement/3M ESPE), stored (37°C/24hours), load cycled (200,000cycles/25N/40 cycles-minute). At 50,000 cycles crowns were examined using digital microscope (Keyence VHX-600) at 40 and 100X for chipping/cracks/fracture by staining the crown surface with 10% methylene blue. At the end of 200,000 cycles they were loaded to failure (1mm/min) using a universal testing machine (Instron/Model 5565).

Results: Microscopic evaluation of the surface did not reveal cracks but minor wear facets were observed. LAVA bilayered groups showed two-step failure. Crowns from other groups fractured as single event (Keyence high speed camera).

Conclusions: Zirconia bilayered crowns showed the highest fracture loads, lithium disilicate monolithic crowns showed the lowest. Monolithic zirconia crowns of 0.6mm thickness resulted in relatively high magnitude fracture loads. Supported in part by a grant from 3M ESPE.