A Three-dimensional Finite Element Analysis On Stress Distribution In Edentulous Mandibular With All-on-4 Implant Restoration

OBJECTIVE: To study the effect of different lengths and tilt angles of the distal implants、different lengths of the cantilevers 、and different loading positions on the stress value and distribution of the All-on-4 implant denture and cortical bone around the implants by three-dimensional finite element method(FEM),and to provide biomechanical reference for its clinical application.Methods: Three-dimensional solid model of edentulous mandible was established using MSC / PATRAN(2010)software and the obtained three-dimensional data of edentulous mandible by CT scanning.On the basis of the obtained solid model,the three-dimensional finite element models of edentulous mandible All-On-4 implant-supported fixed dentures were built under different experimental conditions,and the stress value and distribution were calculated and analyzed by FEM.The experiment design is as follows:Group 1:To study the effect of different tilt angles of the distal implants on the stress value and distribution of the All-on-4 implant denture and cortical bone around the implants.The finite element models of All-on-4 implant dentures with the angles of the distal implants 0°~45°tilted were established and the tilt angles effect on the stress was analyzed;Group 2:To study the effect of different lengths of the distal implants on the stress value and distribution of the All-on-4 implant denture and cortical bone around the implants.The finite element models of All-on-4 implant dentures with the angles of the distal implants 30° tilted and lengths of the distal implants 13 mm,14mm,15 mm respectively were established and the distal implant length effect on the stress was analyzed;Group 3:To study the effect of different lengths of the cantileves on the stress value and distribution of the All-on-4 implant denture and cortical bone around the implants.The finite element models of All-on-4 implant dentures with the angles of the distal implants 30° tilted and lengths of the cantileves 0mm,5mm,10 mm respectively were established and the cantileve length effect on the stress was analyzed when the loading positions remained unchanged;Group 4:To study the effect of loading position on the stress value and distribution of the All-on-4 implant denture and cortical bone around the implants.The finite element models of All-on-4 implant dentures with the angles of the distal implants 30° tilted and the cantileves 10 mm long ere established and the effect of different loading positions on the stress was analyzed;Results : 1.Load on the bilateral posterier teeth,the maximum stress of the distal implants was located on the distal buccal necks,and that of the surrounding cortical bone appeared close to the distal buccal necks of the distal implants correspondingly.2.The stress of the implants and the surrounding bone tissue decreased as the tilt angles of the distal implants were increased.3.The stress of the implants and the surrounding bone tissue decreased as the lengths of the distal implants were increased.4.When the lengths of the cantilever beams were increased and the load and other conditions remained unchanged,the stress of the distal implant and the surrounding bone was unchanged.5.The stress of the implants and the surrounding bone tissue was affected by the loading position greatly,as the loading positions moved backwards,the stress value of implants and surrounding bone increased apparently.Conclusions: 1.When the mandibular edentulous All-on-4 implant denture adopted,the distal buccal necks of the distal implants and the surrounding cortical bone are areas of stress concentration with the load on bilateral posterior teeth,where are prone to implant fatigue 、fracture or cortical bone resorption;2.Other conditions unchanged,the stress of the implants and the surrounding bone tissue decreases with the increase of the tilt angle of the distal implants,which proves the design of the distal tilt implant to be helpful for the protection of the implants and the surrounding bone tissue.3.The length of the distal tilt implants can reduce the stress of the implants and the surrounding cortical bone,but the effect of the length on the stress is smaller than that of the tilt angle.4.The loading position is one of the main factors that affect the stress value and distribution of All-on-4 implant denture implants and its surrounding bone.When the loading position remains unchanged and the lengths of cantilevers are increased,the stress peaks of the implants and the cortical bone stay the same,while the loading positions are moved backwards,the stress significantly increases,suggesting that shortened dental arch design should be adopted for mandibular All-on-4 implant denture to avoid the distal loading force with too long arms.