Biomechanical Analysis of Prosthetic Designs Retained by Two or Four Implants in the Edentulous Mandible: A Three-Dimensional Finite Element Analysis.

Background: Edentulism remains a prevalent condition worldwide, significantly compromising masticatory efficiency, quality of life, and oral health. Various implant-retained and implant-supported prosthetic options have been developed to address mandibular edentulism, yet their biomechanical performance under functional loading remains incompletely understood.

Aim: To compare stress distribution among seven prosthetic designs for the edentulous mandible: two-implant bar and locator overdentures; four-implant bar and locator overdentures; an anterior fixed-posterior removable prosthesis retained with clasps; its precision-attachment variant; and an All-on-Four fixed prosthesis.

Materials and methods: A 3D finite element model of a D2-quality mandible (2-mm cortical, 1-mm mucosa, remaining cancellous bone) was created. Standard implants (4.3×11 mm) and distal-tilted (30°) long implants (4.3×14 mm) were simulated. A 100-N load was applied on the mandibular first molar vertically and obliquely. Literature-based material properties were assigned. Outcomes were implant von Mises stress and cortical bone maximum and minimum principal stresses.

Results: The lowest implant and bone stresses occurred in the anterior fixed-posterior clasp-retained design. Non-rigid connections (bar, locator, clasp) reduced implant stresses compared with precision attachments and fully implant-supported designs. Four-implant overdentures showed higher crestal stress than two-implant counterparts, likely due to greater framework rigidity and distal implant proximity to the load. Precision-attachment hybrid and All-on-Four configurations generated higher stresses at the implant neck and crestal cortical bone. Oblique loading produced higher stresses than vertical across all models.

Conclusion: Within the limits of an idealized, fully osseointegrated FEA, the anterior fixed-posterior clasp-retained prosthesis provided the most favorable stress distribution, supporting consideration of cost-effective, tissue-supported, non-rigid designs for selected mandibular edentulous cases. Increasing implant number did not uniformly reduce stresses; design rigidity and implant position were more influential than implant count. Clinical studies are warranted to validate these simulations.