Fracture Resistance of Lithium Disilicate Restorations Following Endodontic Access Preparation; An In Vitro Study
The purpose of this in vitro study was to determine the effect of endodontic access preparation on the fracture resistance and microstructural integrity of monolithic pressed and monolithic milled lithium disilicate complete coverage restorations. Twenty monolithic milled (IPS e.max Press) and twenty monolithic milled (IPS e.max CAD) lithium disilicate restorations were fabricated and adhesively bonded to forty identical epoxy resin dies. Ten of the pressed and ten of the milled crowns were accessed for a simulated endodontic treatment and subsequently repaired using a porcelain repair system and composite resin. All specimens were cyclically loaded for 250,000 cycles (1.6Hz, 50-250N) and loaded to failure in a universal testing machine. Force data were recorded and analyzed using a statistical analysis software. An analysis of variance (ANOVA) followed by a post hoc test (Sidak?s correction) was used to indicate significant differences between the groups (p<0.05). A Weibull analysis was also performed in order to calculate the Weibull parameters (modulus and characteristic failure load) for each group, and therefore compare their mechanical reliability. Eight (four pressed and four milled) additional restorations were also fabricated, in order to complete an SEM analysis and evaluate the surface damage created by the endodontic access preparation.
A statistically significant difference (p=0.019) was noted between the pressed intact and pressed repaired restorations, as well as between the pressed intact and milled repaired restorations (p=0.002). There was no statistically significant difference noted between the milled intact and milled repaired restorations. The milled repaired restorations had the lowest mean failure load, whereas the pressed intact restorations had the highest. Specimens that were examined under SEM showed edge chipping that involved primarily the glaze (IPS e.max Ceram) layer around the access openings.
Endodontic access preparation of lithium disilicate restorations resulted in a significant decrease in the fracture resistance of the pressed (IPS e.max Press) specimens, but not in the milled (IPS e.max CAD) restorations.