Distinct Structural Determinants of Failure in Morse Taper and Internal Hex Implant Systems

Objectives: To evaluate the influence of implant–abutment connection design, implant diameter, and simulated marginal bone loss on fracture resistance and failure patterns of dental implant systems. Materials and Methods: A total of 180 implant–abutment assemblies were tested, including Morse taper (MT) and internal hex (IH) connections with diameters of 3.5, 4.0, and 5.0 mm. Implants were embedded at two simulated bone levels (0 and 3 mm) and loaded at 30° until failure, in accordance with ISO 14801:2015. Fracture resistance (N) was analyzed using three-way ANOVA. Results: Connection type, implant diameter, bone level, and their interactions significantly affected fracture resistance (p < 0.001). Internal hex implants showed a marked diameter-dependent increase in resistance and greater reduction under simulated bone loss, particularly in reduced diameters. In contrast, Morse taper implants demonstrated similar resistance values regardless of implant diameter or bone level. Failure patterns also differed between systems: internal hex implants exhibited cervical implant fractures in reduced diameters, whereas Morse taper implants showed progressive abutment deformation and/or abutment fracture without implant body fracture. Conclusions: Implant fracture resistance is strongly influenced by implant–abutment connection geometry. Within the limitations of this static in vitro study, MT systems demonstrated diameter-independent mechanical stability and prosthetic-controlled failure patterns, whereas IH systems were highly sensitive to diameter reduction and simulated bone loss.