Objective To investigate stress distribution in mandibular first premolar restored by composite resin with root screw of different length in different occlusion. Methods An three dimensional finite element model of mandibular first premolar restored by composite resin with root screw was constructed by using spiral CT image reconstruction technique, Mimics, Pro/Engineer, Pro/Mechanica software. Based on this model，stress distribution in dentin, composite resin and root screw was analyzed in the situation of different screw length and different occlusion. Results When the occlusion was tripod contact and cusp tip- ovoid fossa contact, the stress of dentin, composite resin and root screw were all much less than their compressive strength. And also in this situation, the stress of bond surface between the composite resin and dentin was less than the bond strength of composite resin. The screw was longer but no more than 8 mm, the dentin stress distributes better. Conclusion In the restoration of large area mandibular first premolar crown defection, tripod contact and cusp tip- ovoid fossa contact with 8 mm root screw is recommended.
