Effect of distal cantilever and anterior implant diameter on framework and cortical bone stress in a PEEK-based All-on-Four mandibular prosthesis

Background: Polyetheretherketone (PEEK) has been proposed as an alternative framework material for full-arch implant-supported prostheses due to its lower elastic modulus relative to metallic frameworks. However, quantitative data regarding the combined biomechanical influence of distal cantilever extension and anterior implant diameter in PEEK-based mandibular All-on-Four rehabilitations remain limited. Purpose: To quantify the effects of distal cantilever presence and anterior implant diameter (3.5 mm vs. 4.5 mm) on framework stress and peri-implant cortical bone response in a PEEK-based mandibular All-on-Four prosthesis using three-dimensional finite element analysis. Methods: Four implant configurations were modeled: Standard-NC (4.5 mm anterior, no cantilever), Mixed-NC (3.5/4.5 mm anterior, no cantilever), Standard-C (4.5 mm anterior, with cantilever), and Mixed-C (3.5/4.5 mm anterior, with cantilever). A 300 N static load was applied at premolar and molar positions under vertical (0°) and 30° oblique loading. Peak von Mises stress in the PEEK framework and crestal cortical bone, along with maximum principal cortical strain, were recorded. Results: Distal cantilever extension increased peak framework stress by up to 59% under premolar vertical loading (222.24 MPa vs. 139.60 MPa) and increased crestal cortical stress by approximately 49% (18.36 MPa vs. 12.31 MPa) compared with the non-cantilevered baseline configuration. Under molar vertical loading, maximum principal strain increased by approximately 105% in the Mixed-C model compared to the Standard-NC configuration (0.0080 vs. 0.0039). Reduction of anterior implant diameter increased cortical strain by approximately 28-31% in non-cantilevered models and further amplified strain when combined with cantilever extension. Peak framework stresses reached 222.24 MPa in localized regions. Conclusion: Distal cantilever extension was the primary determinant of biomechanical amplification in a PEEK-based mandibular All-on-Four prosthesis, while reduced anterior implant diameter exerted a secondary but additive effect. These findings highlight the mechanical sensitivity of low-modulus framework systems to cantilever geometry and implant diameter in full-arch rehabilitations.