Bonding and Thickness Effect on Fracture Load of CAD/CAM Crowns

Objective: To measure the fracture load of CAD/CAM lithium disilicate glass-ceramic crowns (e.maxCAD, Ivoclar Vivadent, Inc.) milled with two thicknesses and cemented either conventionally (Vivaglass CEM, Ivoclar Vivadent, Inc.) or adhesively (Multilink Automix, Ivoclar Vivadent, Inc.).

Methods: Four groups were used for this study: (1) Standard thickness and conventional cementation (2) Reduced thickness and conventional cementation (3) Standard thickness and adhesive cementation and (4) Reduced thickness and adhesive cementation. Fifteen specimens were produced for each group. A standardized molar crown was scanned using the Cerec InLab (Sirona). Thirty crowns were designed with thicknesses exceeding the minimal dimensions for e.maxCAD material (Standard thickness). Thirty more crowns were milled with the thickness reduced by 0.6 mm. The occlusal morphology was the same for all the crowns. The specimens were then fired and glazed in one firing according to the manufacturer's instructions in a P500 Programat oven (Ivoclar Vivadent, Inc.). The crowns were cemented to composite (Heliomolar, Ivoclar Vivadent) preparations which were formed from the internal surface of each crown using either a conventional cement (VivaglassCEM) or an adhesive cement (Multilink Automix) and stored in water for 24 h at 37°C. The crowns were loaded using a 15 mm diameter steel ball at a crosshead speed of 0.5 mm/min with an Instron Universal Testing machine until a crack was produced.

Results: The failure load values (N) of the groups were Group 1: 1595.3±228.3, Group 2: 651.1±68.0, Group 3: 2730.4±473.9 and Group 4: 1554.4±246.2. Using ANOVA (p≤0.05)) a statistically difference was found. Group 3 was greater than all the other groups while Group 2 was lower than all the other groups. The most interesting finding was that Groups 1 and 4 were not statistically different.

Conclusion: Within this study, increased thickness and adhesive cementation greatly increased the failure load of CAD/CAM lithium disilicate crowns.