Abstract: Objective: To discuss the effects of conventionally divergent preparation and minimally invasive preparation of undercut on inlay through three-dimensional finite element method. Methods: CBCT scanning data were used to establish the model of mandibular first molar restored by inlay with different undercuts diameters (1 mm, 2 mm, 3 mm). Two preparation methods were performed. Group A was minimally invasive preparation of undercut. Group B was conventionally divergent preparation of undercut. The size and distribution of stress were studied through finite element methods. Results: When filling material and diameter of the concave were the same, the equivalent stress of enamel was the same in group A and group B under vertical loading. The equivalent stress of dentin in group A was slightly larger than that in group B, and the equivalent stress of prosthesis in group B was significantly larger than that in group A. Under oblique loading, the stress of enamel, dentin, and restoration of group B were larger than those of group A. Different undercut filling materials had little effect on the equivalent stress value of enamel, dentin, and restoration. The stress of fluid resin was relatively small and uniform. Conclusion: From the view of biomechanics, the minimally invasive preparation of undercut is beneficial to protect the enamel and improve the stress of prosthesis, especially in the case of lateral loading But attention should be paid to protect the dentin during tooth preparation and root canal treatment. The filling material in the undercut area has little effect on the whole. In general, fluid resin is a better choice.

Key words: mandibular first molar, undercut, tooth preparation, inlay