Purpose: This study tested the hypothesis that the tensile strength of enamel varies according to prismatic orientation. Materials and Methods: Eight extracted, caries-free human third molars were thoroughly cleaned with pumice and water. The entire enamel surface was conditioned with 37% phosphoric acid for 30 s, air dried, and bonded with Single Bond adhesive system. Several layers of composite (Z-100) were incrementally applied to the crown to build up a cube-like resin structure approximately 5 mm thick covering the entire crown of the teeth. The teeth were stored for 24 h in water at 37°C, and then the crown was serially, vertically sectioned in a mesio-distal direction to obtain several slices approximately 0.7 mm thick. The slices were sectioned into halves, and each half was gently trimmed from both sides with a diamond bur to reduce to cross-sectional area to a
eck located in enamel, either on the external slope or functional slope of the cusps. The specimens were trimmed to permit testing of enamel with its prisms either oriented parallel (PL) or perpendicular (PD) to the applied load. The trimmed specimens were glued to the fixtures of a Vitrodyne tester and stressed in tension at 0.5 mm/min. SEM images were taken from the fractured surfaces to examine the site of failure and confirm the prism orientation. Results: Mean tensile strength of enamel was 24.7 ± 9.6 MPa (n = 22) for PL and 11.4 ± 6.3 MPa (n = 22) for PD oriented enamel prisms (t value = 5.45, p<0.05). There was no significant difference between specimens originating from different slopes of the cusps (p>0.05). Conclusion: The results showed that tensile strength of enamel is dependent on the prismatic orientation.