AbstractThe purpose of this study was to determine the fracture toughness (KIC) of human dentin and to test the null hypothesis that KIC is not affected by the orientation of dentinal tubules relative to the plane of crack propagation. Triangular prisms (4 × 4 × 4 × 8 mm) were obtained from human molars and tested using the notchless triangular prism (NTP) specimen KIC test. Dentin prisms were prepared so that the plane of crack propagation would have three different orientations relative to the orientation of dentinal tubules: perpendicular (PE), parallel aligned (PAA), and parallel transverse (PAT). The prepared specimens were secured in the specimen holder and loaded in tension until fracture or crack arrest. The maximum load recorded was used to calculate KIC. There was no significant difference between the KIC of PAA specimens (1.97 ± 0.17 MPa/m1/2) and PAT (2.02 ± 0.18 MPa/m1/2). The KIC for the PE specimens (1.13 ± 0.36 MPa/m1/2) was significantly lower. The SEM images of the fractured surfaces showed distinct differences that were correlated with the determined KIC values. The fractured surfaces of PAA and PAT specimens were rougher compared to PE specimens. Both the hyper mineralized peritubular dentin and the orientation of collagen fibrils surrounding the tubules could be responsible for the significant differences in KIC. The results of this study identified a significant anisotropy of dentin with respect to its KIC. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 66A: 507–512, 2003