Abstract: BACKGROUND: Three-dimensional finite element method was used to analyze the stress of implants in different cortical bone thicknesses, and the results showed that cortical bone thickness affected the displacement value of the implant-bone interface, the equivalent stress value, and the equivalent stress value of each component of the implant. There are still uncertainties in the clinical application of ultrashort implants in different mandibular bones. 
OBJECTIVE: To analyze the stress distribution of ultrashort implant in different bone and bone tissue of mandible by three-dimensional finite element method.
METHODS: Cone-beam CT images of a patient with missing mandibular first molar were selected. Mimics software was used to create the model of the first molar region of the mandible. The required implant and upper prosthesis models were drawn according to the model of TS IIIφ5 mm×5 mm in the Orthodontic system. Geomagic Studio bias command was used to obtain the models of four different types of mandibular bone (classes I, II, III, and IV). All the models were integrated and cross-combined to give different directions of loading so as to analyze the Fann equivalent stress distribution of each model after being stressed.
RESULTS AND CONCLUSION: (1) Under vertical loading, the maximum stress of cortical bone in classes I, II, and III bone tended to be more stable than that in class IV bone, and the force gradually concentrated from around implant to the junction between implant neck and cortical bone when it was distributed in class IV bone. The peak value of equivalent stress in class I bone was in the screw region. In classes II, III, and IV bone, the maximum stress peak was at the junction between the implant neck and the abutment. (2) Under lateral loading, the maximum stress of cortical bone increased gradually with the weakening of bone condition. The maximum stress peaks in I-IV bone were concentrated at the junction of implant and abutment. In classes I-III bone, the stress of implant itself increased gradually with the weakening of bone, while in class IV bone, the stress decreased. This rule was also applicable to the stress of prosthesis abutment and cancellous bone. (3) The maximum stress of all kinds of bone under oblique load was larger than that under vertical load.

Key words: ultrashort implant, three-dimensional finite element, different bones, stress distribution, bone tissue