Biomechanical Evaluation of Ultra-Short Posterior Implants as Adjuncts to Two-Implant Mandibular Overdentures in the Atrophic Edentulous Mandible: A Three-Dimensional Finite Element Analysis Study

Progressive alveolar resorption complicates prosthetic rehabilitation of the atrophic edentulous mandible, and the conventional two-implant mandibular overdenture concentrates occlusal forces on anterior fixtures through extended posterior cantilevers. This study evaluated and compared the biomechanical performance of a conventional two-implant-supported mandibular overdenture with configurations incorporating additional ultra-short (4 mm) posterior implants at varying positions using three-dimensional finite element analysis. A 3D atrophic edentulous mandible model was generated from Visible Human Project computed tomography data. Six configurations were analyzed: Model 1 (control; two 4.1 × 12 mm Ti-15Zr implants at 33 and 43) and Models 2–6, which supplemented the control with two 4.1 × 4 mm ultra-short Ti-15Zr implants at 35–45, 36–46, 35–46, 35–47, and 36–47, respectively. All models were subjected to bilateral oblique (100 N at 45°) and vertical (200 N, 300 N) loading. Von Mises stress, maximum principal stress (P1), and minimum principal stress (P3) were evaluated on all implants. All experimental configurations reduced anterior implant stress compared with the control. Models 5 (35–47) and 4 (35–46) achieved the greatest anterior stress reduction, while Model 6 (36–47) exhibited the most favorable overall stress distribution, with the lowest posterior peak von Mises stress (14.49 MPa). Oblique loading produced higher anterior stress than vertical loading across all models despite its lower magnitude. All stresses remained below the yield strength of Ti-15Zr. Ultra-short posterior implants reduced anterior implant stress and improved load distribution. Asymmetric configurations with a wider anteroposterior spread demonstrated the most favorable biomechanical profiles, supporting their use as minimally invasive adjuncts to standard overdenture protocols.