Fracture Resistance of Various Thickness e.max CAD Crowns Cemented on Different Supporting Substrates

Purpose: To investigate the influence of abutment material properties on the fracture resistance and failure mode of lithium disilicate (IPS e.max) CAD/CAM (computer-aided design/manufacturing) crowns on traditionally and minimally prepared simulated tooth substrates. Materials and Methods: Sixty lithium disilicate (IPS e.max) CAD/CAM crowns were divided into three groups (n=20): TD: traditional thickness crowns cemented on Paradigm MZ100 abutments; MD: minimal thickness crowns cemented on Paradigm MZ100 abutments; ME: minimal thickness crowns cemented on e.max abutments. The 3Shape system was used to scan, design and mill all abutments and crowns with a die space set to 40 micrometers. Traditional thickness crowns were designed based on manufacturer guidelines with 1.5mm occlusal thickness and 1.0mm margins. Minimal thickness crowns were designed with 0.7mm occlusal thickness and 0.5mm margins. MZ100 composite and e.max abutments were selected to simulate dentin and enamel substrates respectively based on their elastic-modulus. Variolink Esthetic was used to cement all samples following manufacturer’s instructions. Half the specimens in each group were stored in water for 7 days before fracture load was tested. The other half were stored in water for 120 days with 37,500 thermocycles from 5ºC and 55ºC. A universal testing machine was used to load all specimens to fracture with a 3mm radius stainless steel hemispherical tip at a crosshead speed 0.5mm/minute along the longitudinal axis of the abutment with a 1mm thermoplastic film placed between the loading tip and crown surface. Data was analyzed using ANOVA and Bonferroni post hoc assessment. Fractographic analysis was performed with scanning electron microscopy (SEM). Results: The mean fracture load (standard deviation) was 1600 (350) N for TD; 1595 (479) N for MD; and 1571 (295) N for ME. When comparing the means of all data for each group (non-aged and aged), no statistically significant difference between groups was observed (p=0.970, F=0.030). However, some of the 120-day storage groups were significantly higher in value than the 7-day storage groups (p= <0.001, F= 7.561). SEM illustrated larger micro-fracture dimensions in Group MD than Group ME. Conclusion: Minimal thickness e.max crowns did not demonstrate statistical difference in fracture resistance from traditional thickness crowns. Fracture mechanisms of minimal thickness e.max crowns may be affected by the e-modulus of the substrate. Minimal thickness e.max crowns may be a viable restorative option when supported by high e-modulus materials.