Influence of different framework materials on stress distribution and micromovement in mandibular immediate loading All-on-4 treatment: a three-dimensional finite element analysis

To evaluate, using three-dimensional finite element analysis (FEA), the biomechanical effects of four prosthetic designs planned according to the All-on-4 concept and immediately loaded in a fully edentulous mandible. An edentulous mandibular model was created with two anterior vertical and two posterior 30° mesially tilted implants. Four prosthetic designs were modeled: a full acrylic prosthesis without a framework (M1), a glass fiber framework (M2), a carbon fiber-reinforced polyetheretherketone (CFR-PEEK with 30% carbon fiber content) framework (M3), and a polyetheretherketone (PEEK) framework (M4). Vertical and oblique 30° loads of 150 N were applied. Principal stresses in cortical bone, von Mises stresses in implants and frameworks, prosthesis displacements, and micromovements at the implant-bone interface were analyzed. Oblique loading produced higher stress and displacement than vertical loading. Cortical bone and implant stresses ranked as M1 > M4> M3 > M2 based on framework rigidity. The highest framework stress occurred in M2 with 84.37 MPa under oblique loading, yet M2 provided the best protection of peri-implant tissues. Under oblique loading, prosthesis displacement was 270 μm in M1 and 210 μm in M2. Maximum micromovements of 27 μm were well below the 150 μm threshold, being highest in M1 and lowest in M2. More rigid frameworks better protect implants and peri-implant bone by concentrating forces within the prosthesis. However, the glass fiber framework also exhibited the highest internal stress, which may influence its long-term durability and fatigue life. Therefore, the balance between tissue protection and framework durability must be carefully considered in immediately loaded All-on-4 rehabilitations.