Purpose: This in vitro study aimed to evaluate and compare the fracture resistance
of zirconia mandibular molar crowns with the traditional trapezoidal (TT) shaped access
and three novel conservative access designs. The designs tested were the TT, conservative
trapezoidal (CT), ultraconservative circular (UC), and two conservative single accesses
(TC). Materials and Methods: A mandibular right first molar typodont tooth was prepared
for a full coverage zirconia crown. The preparation was scanned, and a total of 56
monolithic zirconia crowns were milled. Resin dies were printed for each restoration.
Crowns were cemented with dual curing resin cement to the printed dies. The specimens
were artificially aged with 10,000 thermal cycles between 5 degrees C and 55 degrees
C with a dwell time of 30 s and were loaded to failure using a universal testing machine.
Furthermore, scanning electron microscope (SEM) images were taken to evaluate the
fracture patterns of the specimens. The fracture load was analyzed using the Shapiro-Wilks
normality test, followed by a one-way ANOVA with a post hoc Tukey test. Results: The
fracture resistance of mandibular molar zirconia crowns significantly varied according
to the endodontic access cavity design. Crowns with UC access had the highest fracture
resistance (2722 +/- 113 N, p < 0.001), followed by the CT (2469 +/- 126 N) and the
TT access (1915 +/- 71 N). The two-conservative single access design had the lowest
value (1064 + 129 N). Conclusions: All endodontic access designs for mandibular molar
zirconia crowns appeared to withstand the average occlusal forces. Crowns with UC
access demonstrated the highest fracture resistance among different endodontic access
designs. Two conservative single accesses in the same crowns significantly diminished
the fracture resistance.
