To compare experimentally the failure modes of endodontically treated teeth restored with custom-made polyetheretherketone (PEEK) post-core (milled and pressed) to those restored with the standard technique of customized fiberglass posts with composite cores, and to analyze numerically stress distribution patterns in each system using the finite element analysis (FEA). Experimentally, 21 mandibular premolars were endodontically treated, prepared for post-restoration, mounted in resin mold, decapitated, and assigned to three groups: M (milled PEEK), P (pressed PEEK), and C (control group
ready-made fiberglass posts customized with resin composite). All post-core restorations were cemented and subjected to thermocycling. Failure modes were visually evaluated after fracture by means of a universal testing machine. Numerically, stress analysis was assessed using FEA, based on digital models designed to replicate the experimental setup. Milled PEEK posts showed a uniformly favorable failure mode across all specimens (100%), whereas pressed PEEK and customized fiberglass posts displayed more variable failure modes, yet with a higher proportion of favorable outcomes. Numerically, the highest VME stress values were in the cervical half of the post area (up to 62.7 MPa), with minimal differences between milled and pressed PEEK posts. PEEK posts had greater stress concentrations in the cervical area of the post area, while fiberglass posts showed slightly higher stress in the middle and apical areas. Custom-made milled and pressed PEEK posts are effective for restoring endodontically treated teeth, with milled PEEK posts showing the most favorable failure mode and stress distribution pattern.