Background <p>The aim of this study was to investigate the mechanical behaviour and stress distribution characteristics of different prosthetic materials (Co-Cr/porcelain; Zirconia/porcelain; Fibre- reinforced composite/Composite) used in mandibular posterior implant-supported fixed prostheses using finite element analysis (FEA), with particular emphasis on the influence of material stiffness under different loading conditions.</p> Methods <p>Three-dimensional finite element models of a mandibular first premolar and first molar were constructed. Implant-supported fixed prosthetic restorations were simulated using three different restorative materials: metal-supported porcelain (MP), zirconia-supported ceramic (ZP), and fibre-reinforced composite (Trinia-composite, TC). Vertical and oblique occlusal loads were applied to the models. Maximum principal stress, minimum principal stress, and Von Mises stress values were calculated to evaluate stress distribution within the restorative materials and supporting structures.</p> Results <p>Across all models, oblique loading resulted in higher stress concentrations compared with axial loading. The first premolar models exhibited higher stress levels than the first molar models under identical loading conditions. The mechanical behaviour and stress distribution in the fixed prostheses were influenced by the prosthetic material used. Restorations fabricated with MP and ZP demonstrated higher stress values than those restored with TC. The fibre-reinforced composite material showed a more homogeneous stress distribution, indicating a more favourable load-transfer behaviour.</p> Conclusions <p>All tested materials generated stress values within physiological limits. The fibre-reinforced polymer-based restoration exhibited improved stress redistribution, most likely due to its elastic compatibility peri-implant structures. These findings suggest that fibre-reinforced composites may offer biomechanical advantages in posterior implant-supported restorations.</p>