Effect of occlusal loading and crown material on stress and strain distribution in bone around 4-mm ultra-short implants: a 3D-FEA

Ultra-short (4-mm) dental implants represent a minimally invasive solution for the rehabilitation of posterior atrophic regions; however, concerns remain regarding stress and strain concentration in bone around implants under functional loading. This study aimed to evaluate the influence of occlusal loading conditions, prosthetic configuration, and crown material on stress and strain distribution using three-dimensional finite element analysis (3D-FEA). Three-dimensional finite element models of single-crown restorations and the splinted two-crown configuration supported by 4-mm ultra-short implants were developed. Zirconia and composite resin crowns were evaluated under vertical and 30° oblique loading, with and without a simulated food bolus. The bolus was assigned a Young’s modulus of 10 MPa and a Poisson’s ratio of 0.30. Primary outcomes were volume-weighted mean von Mises stress and equivalent elastic strain within a fixed 0.50-mm peri-implant volumetric region of interest (ROI). Mesh convergence was assessed using three nominal local refinement levels and a 5% acceptance threshold. In single-crown models without bolus, oblique loading increased cortical ROI stress from approximately 10.01 to 43.02 MPa and trabecular ROI stress from approximately 4.95 to 12.35 MPa. With bolus interposition, corresponding oblique values decreased to approximately 19.57 and 5.63 MPa. In models of the splinted two-crown configuration, cortical ROI stress under oblique loading ranged from 8.63 to 10.43 MPa. Zirconia and composite resin produced nearly identical ROI outcomes in the single-crown configurations, whereas a modest configuration-specific difference was observed under oblique loading in the splinted two-crown configuration. All primary outcomes met the 5% convergence criterion. Within the assumptions of this finite element model, loading direction and prosthetic configuration had greater effects on peri-implant stress and strain than crown material. Bolus interposition reduced the ROI outcomes under oblique loading. Under the modeled oblique conditions, the splinted two-crown configuration was associated with lower peri-implant ROI outcomes than the single-crown configuration; however, the different total loads prevent isolation of splinting as an independent causal factor. These comparative findings do not establish clinical superiority or biomechanical reliability and require experimental and clinical validation.