Posterior implant angulation without cantilever reduction in mandibular All-on-4: a controlled finite element analysis of stress transfer and implant macrodesign effects.

backgroundPosterior implant angulation is widely used in mandibular All-on-4 rehabilitation, but its proposed biomechanical benefit is often confounded by concurrent cantilever reduction and/or increased distal support. This finite element analysis isolated the effect of posterior angulation under standardized prosthetic geometry and assessed whether implant macrodesign modified stress transfer under healed, bonded-interface assumptions.

methodsFour three-dimensional finite element models of an edentulous mandible were created: axial posterior implants (VERT4) and 30° distally tilted posterior implants (ALL4). Each configuration was simulated with two implant macrodesigns (NobelActive and Straumann BL). A 12-unit screw-retained prosthesis extending to the first molars was modeled with identical emergence profile, distal extension, and cantilever length across all models, without angulation-dependent distal implant-length gain. A healed condition was simulated using bonded bone-implant interfaces, and linear static analyses were performed. Two unilateral vertical load cases (300 N) were applied via a rigid food bolus over the left canine or left first-molar region. Principal stresses were extracted for cortical and trabecular bone, and von Mises stresses for implants, multiunit abutments, and the framework and veneering complex. Ranking stability was tested by ± 10% load variation.

resultsWhen posterior angulation was applied without cantilever reduction or increased distal support, tilted models showed higher peak stresses than axial models. Under first-molar loading, ALL4-N produced the highest cortical stresses (Pmax 100.44 MPa; Pmin - 95.30 MPa) and posterior implant von Mises stress (925.82 MPa). Under canine loading, ALL4-N also showed the highest posterior implant stress (546.94 MPa). The highest anterior multiunit abutment stress occurred in ALL4-S under first-molar loading (287.27 MPa). Across matched configurations, NobelActive models showed higher implant and connection stresses than Straumann BL models. Relative rankings were unchanged in sensitivity analyses.

conclusionsWithin this linear-elastic, healed-interface model, posterior implant angulation did not provide a biomechanical advantage when used without cantilever reduction or increased distal support. It was associated with increased mechanical demand in peri-implant bone and implant-prosthetic components under unilateral loading. Findings should be interpreted as comparative trends within this model, not direct predictions of clinical failure or generalized inferiority of tilted implants in other scenarios. Implant macrodesign also influenced stress transfer.