Fracture load and mode of failure of ceramic veneers with different preparations

Statement of problem. Fracture is a clinical failure modality for ceramic v eneers. Whether design of tooth preparation can affect the strength of cera mic veneers remains controversial. Purpose. This in vitro study evaluated fracture load and mode of failure of ceramic veneers, with 4 tooth preparation designs, that were bonded on ext racted human maxillary central incisors. Identical parameters were also mea sured on unrestored intact teeth for comparison. Material and methods. Fifty maxillary central incisors were randomly divide d into 5 equal groups. Each group was assigned a different tooth preparatio n design: (1) no incisal reduction, (2) 2 mm incisal reduction without pala tal chamfer (butt joint), (3) 1 mm incisal reduction and 1 mm height palata l chamfer, (4) 4 mm incisal reduction and 1 mm height palatal chamfer, and (5) unrestored (control). Forty teeth were prepared to accommodate ceramic veneers of equal thickness and incisocervical length. Stone dies were fabri cated and veneers made from IPS Empress ceramic. Ceramic veneers were bonde d and all teeth mounted in phenolic rings with epoxy resin. Fracture loads were recorded with a mechanical testing machine. Results. Mean fracture loads (SD) in kgf were as follows: group 1, 23.7 (6. 11); group 2, 27.4 (9.63); group 3, 16.4 (3.44); group 4, 19.2 (6.18); and group 5, 31.0 (10.38). Modes of failure were also analyzed for both ceramic veneers and teeth. One-way ANOVA with multiple comparisons revealed 3 sign ificant subsets: groups 1-2-5, groups 4-1, and groups 3-4 (P<.05). Groups 1 and 2 had no ceramic veneer fractures; group 3 had 3 ceramic veneer fractu res, and group 4 had 6 ceramic veneer fractures. Conclusion. Groups 1 and 2 recorded the greatest fracture loads that were c omparable to an unrestored control.