[Effect of cementation on fracture toughness of chair-side CAD/CAM ceramic]

Chair-side CAD/CAM ceramic restorations are esthetic with a time-saving procedure. However, ceramic restorations often failed due to fracture. This study was to investigate the effect of cement thickness on fracture toughness and hardness of CAD/CAM ceramic.

Machinable ceramic blocks for CAD/CAM (Sirona CEREC Blocs) were cut into 9 slices (12 mm×14 mm×2 mm) using a diamond saw (Leica SP 1600) and polished carefully with sandpaper / diamond pastes to yield a mirror-like surface. LIGHT-CORE(TM) was used to prepare resin slices with dimension of 12 mm×14 mm×2 mm. The polished ceramic slices were then randomly divided into 3 groups and cemented to resin slices using Multilink Automix: Group 1, no cement; Group 2, the thickness of cement was 50 μm; Group 3, the thickness of cement was 100 μm. Hardness (H) was determined by Vickers indentation technique using micro-hardness tester (HMV-2T Shimadzu) with a load of 19.6 N for 15 s. The fracture toughness (K(IC)) was calculated by K(IC) = 0.016 (E/H)(0.5)(P/c(1.5)), where P=applied load, c=crack length, E=elastic modulus, H= (0.47 P/a(2)), a=half diagonal of the indentation.

The fracture toughness for Group 1 (no cement) was (1.02±0.11) MN/m(3/2), for Group 2 and Group 3 was (0.99±0.10) MN/m(3/2) and (0.97±0.14) MN/m(3/2). Group 3 showed lower fracture toughness than group 1 (P<0.05). The Weibull parameter m of group 2 was the highest and K(0) of three groups were 1.08 MN/m(3/2), 1.04 MN/m(3/2) and 1.03 MN/m(3/2) respectively. The hardness for Group 1 was (6.68±0.73) GPa, group 2 [(6.85±0.64) GPa], group 3 [(6.81±0.98) GPa]. There was no significant deference in hardness among the 3 groups (P>0.05).

Thickness of cementation may affect fracture toughness of machinable ceramic, therefore, a thinner film of cementation is recommended for Chair-side CAD/CAM ceramic restorations.