This in vitro study aimed to evaluate fracture strength and failure modes of thin occlusal onlays fabricated
from direct resin composite, CAD/CAM polymer-infiltrated ceramic-network/hybrid ceramic, and CAD/CAM lithiumdisilicate
glass ceramic under compressive loading. Forty extracted maxillary molars were prepared, including occlusal
enamel and dentin removal, leaving two dentin slopes with peripheral enamel. Thirty teeth were restored with
0.6-mm-thick occlusal onlays using direct resin composite (Premise), CAD/CAM polymer-infiltrated ceramic-network/
hybrid ceramic (Vita Enamic), and CAD/CAM lithium-disilicate glass ceramic (IPS e.max CAD). Others were restored
with IPS e.max CAD crowns (n=10). The milled restorations were luted with adhesive resin cement. All restored
teeth were loaded vertically by means of a universal testing machine. Fracture loading data were recorded in Newtons
(N) and statistically analyzed. The failure modes were classified, and correlations between fracture strength
and failure mode were analyzed. The fracture strengths (mean±SD) were ranging from 1,949.59 to 2,870.44 N. The
IPS e.max CAD onlays showed significantly higher fracture strength than the IPS e.max CAD crowns (p<0.05). There
was no correlation between fracture strength and failure modes found within each material. In conclusion, the
fracture strength of the Vita Enamic occlusal onlay was comparable with that of the Premise occlusal onlay and
the IPS e.max CAD onlay. The IPS e.max CAD onlay provided higher fracture resistance than the IPS e.max CAD
crown. However, all restorations demonstrated higher fracture resistance than that required for the average force
of mastication.