A fractographic and numerical assessment of dental zirconia implant fractures.

Introduction: Zirconia is a promising alternative to titanium dental implants, offering good osseointegration and aesthetics, but early clinical use has shown notable fracture rates linked to processing flaws, design issues, and biomechanical overloading. The aim of this study was to complement a prospective clinical trial reporting a 24.2% five-year failure rate due to implant fractures, and to systematically identify the reasons for fracture using fractographic and numerical approaches.Materials & Methods: Four retrieved, horizontally fractured zirconia implants (ZV3 implants) were examined using SEM fractography, surface profilometry, and micro-CT to characterize fracture origins, surface roughness, and porosity. Finite element analysis modelled stress distribution under horizontal loading, varying bone support levels and occlusal loading angles in relationship to axial offset.Results: All implants fractured palatal-to-buccal, originating at the root of the first endosseous thread. SEM revealed extensive pressing-related volume defects and severe surface grinding damage due to the subtractive milling and sandblasting procedures. Surface roughness (Rz) was 15 ± 7 µm; micro-CT revealed a defect size frequency peaking at 30 µm, although reaching up to 50 - 70 µm. FEA showed peak stress of 554 MPa at the fracture site under a 600 N applied loading, rising to 1376 MPa with 3 mm simulated bone loss. A simulated lateral crown alignment with the implant axis of 1 mm, further reduced the maximum principal stress at the fracture site by about 100 MPa.Discussion: Fracture mechanics estimations have shown that the measured critical defects could have reduced fracture strength from a theoretical ~ 1200 MPa to 800 - 430 MPa, which could have been sufficient to trigger clinical failures. Stress concentration in the root of the first thread, combined with bone loss and off-axis loading led to increased risk for fracture. Conclusions: Implant fracture resulted from pressing and surface machining defects interacting with a stress-concentrating thread geometry, aggravated by bone loss and off-axis loading. These findings highlight the importance of material quality, implant design, and optimal surgical and prosthetic planning.