Translational Research on Clinically Failed Zirconia Implants

Objectives: To provide fractographic analysis of clinically fractured zirconia implants recovered with their cemented crown. To calculate bending moments, corresponding stress and crack onset location on the implant's fracture surface using a mathematical model integrating spatial coordinates of the crown-implant part and occlusal loading obtained from 2D and 3D images.

Methods: 15 fractured zirconia implants parts (11 posterior and 4 anterior) with their all- ceramic crowns still cemented on it were recovered. The implants were first generations from four manufacturers (AXIS Biodental, Z-Systems, Straumann, Swiss Dental Solutions). The time-to-failure varied between 2weeks and 9years. Fractography was performed identifying the failure origin and characteristic surface crack features. From 2D and 3D digital images of the crown-implant part, spatial coordinates anchoring the crown's occlusal contacts with the implant's central axis and reference plane were integrated in a mathematical model spreadsheet. Loads of 500 N in total were selectively distributed over identified occlusal contacts from wear patterns. The resultant bending and torsion moments, corresponding shear, tensile, maximum principal stress and von Mises stress were calculated. The fracture crack onset location on the implant's fracture surface was given by an angular position with respect to an occlusal reference and compared with the location of the fracture origin identified from fractographic analysis.

Results: Implants fractured from the periphery of the smaller inner diameter between two threads at the bone-entrance level except for one implant which failed half-way within the bone. The porous coating (AXIS Biodental) and the large grit alumina sandblasting (Z-System) created surface defects directly related to the fracture origin. The model spreadsheet showed how occlusal loading with respect to the implant's central axis affects bending moments and crack onset. Dominant loads distributed on contacts with important wear pattern provided a calculated crack onset location in good agreement with the fractographic findings of the fracture origin.

Significance: Recovered broken zirconia implant parts with their restorative crowns can provide not only information regarding the failure origin using fractography but also knowledge regarding occlusal crown loading with respect to the implant's axis. The mathematical model was helpful in showing how occlusal loading affects the location of the fracture initiation site on clinical zirconia implant fracture cases.