Objective     To determine the effect of different root morphology of first mandibular molar on its immediate loading after immediate implant based on the three dimensional (3-D) finite element method. Methods           Two groups of first mandibular molars with different root morphology were used to establish the model after immediate implant

and immediate loading of the first mandibular molars were built. The 2 groups of tooth were further divided according to their different interradicular angle (IA) and root trunk height (RT). Load of 100 N was applied to the buccal cusps in 4 different directions

including axial

oblique 45°

lingual

and mesial directions. The displacements of implant and the Von Mises stresses of bone were analyzed with finite element analysis software ANSYS. Results       The 3-D finite element model of immediate loading of first mandibular molar after immediate implant was established successfully. The results of the analysis indicated that with the increase of root separation and the shorter root trunk height

the maximum displacement of the implant and maximum Von Mises stresses of the implant bone interface were in a downward trend. The maximum displacement of the implant reached 0.461 mm when the mesial loading at RT of 5.3 mm

and the minimum displacement was only 0.008 mm when the axial loading at IA of 42°. The stresses concentrated upon the cortical bone of the neck of implant and the weak cancellous bone beside thread. The maximum Von Mises stresses of the implant bone interface was observed when the mesial loading at IA of 10°

while the minimum was when the axial loading at IA of 18°. Conclusion      Different root anatomical morphology of mandibular first molar exert certain effects on the formation of osseointegration in the implant. The decrease of available bone in interradicular septa enhances the risk of failure in the immediate loading of mandibular first molar after immediate implant.