[Analysis of the effect of mesial implant position on surrounding bone stress of mandibular edentulous jaw under dynamic loads].

Objective: To evaluate the effect of different placement of mesial implants in edentulous jaws on the stress of the implant and the surrounding bone tissue by three-dimensional (3D)finite element analysis. Methods: Cone-beam CT data of mandibular edentulous patients was transferred into Mimics 17.0 and UG NX8.5 software, and three groups of 3D solid model were established: two mesial implants were implanted in the anterior region of the mandible (bilateral central incisor, lateral incisor, canine), and two distal oblique implant with 30° were implanted in the mandibular second premolar area(5 mm near the mental foramen). Set mandible 3D model with 4 implant by using UG NX8.5 software, produced 3 groups (mandible Ⅰ-Ⅲ). We took dynamic loading on models with help of software Abaqus 6.12, working side posterior teeth loading was set to 150 N and the simulation cycle was 0.875 s. The first stage: 0.000 s to 0.130 s, the lower jaw moves outward (moving toward the side of the work), with no contact between the upper and lower teeth; the second stage: 0.130 s to 0.150 s, mandibular upward, the same tooth tip of the working side were relative, the loading position were the posterior buccal tip, tongue tip; the third stage: 0.150 s to 0.260 s, the buccal slopes of buccal tips of mandibular posterior teeth glide along the lingual slopes of buccal tips of maxillary posterior teeth, the loading force was from the buccal side to the lingual side, the long axis of the tooth was 45°, loaded on the buccal slopes of buccal tips of posterior teeth; the fourth stage: 0.260 s to 0.300 s, the lingual slopes of buccal tips of mandibular posterior teeth glide along the buccal slopes of the tongue tips of maxillary posterior teeth, separate from the tip of the tooth at half the length, the loading force was from the lingual side to the buccal side, the long axis of the tooth was 45°, loaded on the lingual slopes of buccal tips of posterior teeth; the fifth stage: 0.300 s to 0.875 s, at the unloading stage, mandibular posterior teeth were separated from the maxillary teeth and returned to the intercuspal position. The loading position varied according to the mastication cycle. The stress distribution of implant and surrounding bone tissue at different stages of each model were observed. Results: From the early stage to chew occlusal contact to the end of the mastication cycle, three groups of models were displayed: the stress of distal implants was greater than that of mesial implants and the neck stress reached the maximum and gradually decreased to the root tip. The stress of distal implant bone was greater than that of mesial implant bone and the stress of distal bone of distal implant was greater than that of mesial bone of distal implant. All the stress peak showed a gradual increase, and the stress reach the maximum at the fourth stage. In the 3 models, the bone stress around the distal implant of model of the anterior implant located in the lateral incisor region was the lowest. The peak stress of cortical bone of the distal position of implant was 58.7 MPa. The bone stress around the distal implant of model of the anterior implant located in the canine region reached the maximum, and the peak stress of cortical bone of the distal position of implant was 135.6 MPa. Conclusions: When mesial implants of edentulous jaws located in the lateral incisor region, it is good for stress dispersion.