Low temperature degradation and reliability of one-piece ceramic oral implants with a porous surface

Low temperature degradation of zirconia (3Y-TZP) oral implants and its effect on fatigue reliability is poorly documented. Objective: The aim of this investigation was to follow the aging process occurring at the surface of implants exhibiting a porous coating and to assess its influence on their mechanical (fatigue) properties. Methods: Tetragonal to monoclinic transformation (t-m) was evaluated during accelerated aging tests up to 100 h in autoclave (134°C, 2 bars) by X-ray diffraction (XRD) and focused ion beam (FIB). A series of implants were steam-aged for 20 h before fatigue testing. Such temperature-time conditions would correspond roughly to 40 years in vivo. The aged specimens and a non-aged control group were step-stress fatigued until failure or survival. Results: The evolution of XRD surface monoclinic content was slow, i.e. 16% and 35% for 20 and 100 h respectively. However, FIB revealed a significant transformation, initiated at the interface between the porous layer and the bulk, preferentially growing towards the bulk. FIB is therefore better indicated than XRD to follow aging in such implants. Higher average fatigue strength (aged 1235 N versus non-aged 826 N) and reliability levels were observed for the 20 h aged group. Significance: After aging for durations compatible with clinical use, 3Y-TZP with porous surface presented higher fatigue performance. This is in contrast to previous studies where loss of strength due to aging was often reported. Generalizations must therefore be avoided when considering aging of zirconia dental products and every new material/process combination should be tested before drawing conclusions.