Fracture strength of zirconia and lithium disilicate restorations following endodontic access

Objectives To compare the fracture load of zirconia and lithium disilicate crowns prepared with endodontic access with fine and coarse diamond instruments. Materials and methods 0.8 mm (3Y zirconia) or 1 mm (lithium disilicate) crowns were luted to resin composite dies with resin-modified glass ionomer (zirconia) or self-adhesive resin (lithium disilicate) cement. A 2.5 mm endodontic access hole was placed in each crown with fine (8369DF.31.025FOOTBALL) or coarse (6379 DC.31.023FOOTBALL) diamond instruments and restored with composite. A control group was prepared without access holes. Crowns were thermocycled for 10,000 cycles (5-55 degrees C) and tested in compression with a steel indenter until failure (n = 8/group). A one-way ANOVA and Dunnett 2-sided test (alpha = 0.05) compared differences in fracture load between groups. Results For zirconia, there was no statistical difference between the control group (2335 +/- 160 N) and coarse diamond group (2345 +/- 246 N); however, the fine diamond group (2077 +/- 216 N) was significantly lower. For lithium disilicate, there was no statistical difference between the control group (2113 +/- 183 N) and the fine (2049 +/- 105 N) or coarse (2240 +/- 118 N) groups. Conclusions 3Y zirconia crowns became weaker when accessed with a fine diamond instrument. There was no negative effect of the endodontic access with bonded lithium disilicate crowns. Clinical significance Conservative endodontic access openings in high-strength ceramic restorations do not have a negative effect on their static fracture load. The coarse zirconia-cutting diamond rotary instrument is more efficient and has a less detrimental effect on the strength of the crowns than a fine diamond rotary instrument.