Purpose. The purpose of this finite element study was to compare stresses, strains, and displacements of double versus single implant, in immediate loading for replacing mandibular molar. Materials and Methods. Two 3D FEM models were made to simulate implant designs. The first model used 6？mm wide-diameter implant to support a single molar crown. The second model used 3.75-3.75 double implant design. Each model was analyzed with a single force magnitude of 70？N in oblique axis in three locations. Results. This FEM study suggested that micromotion can be well controlled by both double implants and 6？mm single wide-diameter implant. The Von Mises stress for double implant had 31%–43% stress reduction compared to the 6？mm implant. Conclusion. Within the limitations of the paper, when the mesiodistal space for artificial tooth is more than 12.5？mm, under immediate loading, the double implant support should be considered. 1. Introduction Threaded root form osseointegrated implants were designed originally to support complete arched fixed implant supported restorations for completely edentulous patients [1]. Now, this type of implant is used to support single-implant supported crowns and fixed partial dentures in partially edentulous areas [2]. Many in vitro and animal studies attempted to predict the biomechanical and clinical behavior of dental material and technique associated with implant-supported prosthesis [3–5]. In vitro methods include conventional in vitro model analyses [6], photo elastic analyses [4, 7, 8], and finite element analyses [9]. In vitro studies are less complicated and less expensive than clinical trials and produce results relatively quickly compared to randomized controlled trials [10]. FEM analysis has been used to provide analytical solutions to problems involving complex geometric forms [9]. Posterior teeth have two or three roots, having from 450 to 533？mm2 as a total anchorage area in good quality bone [11], whereas surface area of 3.75？mm implant varies from 72 to 256？mm2 depending on its length. The molar has a crown surface area of approximately 100？mm2, whereas 3.75？mm implant has cross section area 10.9？mm2. Therefore, the tooth can dissipate occlusal forces efficiently, whereas masticatory forces are exerted at angle mesiodistally and buccolingually creating bending and torquing vectors [12] on the implant. The cross-sectional area for 2 (3.75？mm) implants is 21.9？mm2, whereas for single (5？mm) implant 19.6？mm2 and for 2 (4？mm) implant 19.6？mm2 [13]. Greater dimension of bone buccolingually is required for placement of
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