Fracture resistance and failure modes of post and core systems in endodontically treated teeth with severely enlarged root canals: an in vitro study

Background This in vitro study compared fracture resistance and failure mode distribution among endodontically treated maxillary second premolars with experimentally enlarged canals restored with custom CAD/CAM zirconia, cast nickel–chromium (Ni-Cr), cast gold-colored copper-based dental alloy (NPG) post-and-core systems, or a prefabricated glass fiber post with a composite core. Methods Forty extracted maxillary second premolars underwent root canal treatment and coronal canal enlargement, leaving an approximately 1.5 mm coronal dentin wall at four measured sites. Specimens were randomly allocated to four groups ( n = 10), restored with standardized full metal crowns, thermocycled, and subjected to cyclic mechanical loading. Fracture resistance and failure modes were evaluated. Data were analyzed using Kruskal–Wallis and Dunn’s tests with Bonferroni correction; failure modes were analyzed using Fisher’s exact test (α = 0.05). Results Median fracture resistance was 492.27 N (IQR, 345.90–688.75) for the custom CAD/CAM zirconia post-and-core group, 369.44 N (IQR, 245.75–547.16) for the cast Ni-Cr post-and-core group, 173.66 N (IQR, 150.21–320.40) for the cast NPG post-and-core group, and 127.96 N (IQR, 100.13–143.03) for the glass fiber post group. Fracture resistance distributions differed significantly (H = 17.918, df = 3, P <.001). The glass fiber post group had lower fracture resistance than the custom CAD/CAM zirconia (adjusted P <.001) and cast Ni-Cr post-and-core groups (adjusted P =.012). The custom CAD/CAM zirconia post-and-core group had the highest observed median but did not differ significantly from the cast Ni-Cr (adjusted P = 1.000) or cast NPG post-and-core group (adjusted P =.077). Failure mode distribution also differed significantly ( P =.010); favorable failures occurred in 70% of fiber post specimens, 30% of NPG post-and-core specimens, and 10% of both zirconia and Ni-Cr post-and-core specimens. Conclusions The tested systems differed in fracture resistance and failure mode distribution. The custom CAD/CAM zirconia post-and-core group had the highest observed median fracture resistance; however, it was not statistically different from either the cast Ni-Cr or cast NPG post-and-core group. The prefabricated glass fiber post group had significantly lower fracture resistance than the custom CAD/CAM zirconia and cast Ni-Cr post-and-core groups but showed the greatest proportion of favorable failures.