Fatigue resistance of monolithic strength-gradient zirconia materials.

PURPOSE Evaluation of the effect of three different dynamic fatigue protocols on the fracture resistance of two monolithic strength-gradient zirconia materials. MATERIALS AND METHODS A total of 240 specimens (3 × 4 × 45 mm) was milled from two different layers (incisal and middle) of two types of strength-gradient zirconia blanks (IPS e. max ZirCAD MT Multi A2 vs. IPS e. max ZirCAD Prime A2), resulting in 60 specimens per material and layer group (IPS e. max ZirCAD MT Multi A2: incisal (MI), middle (MM); IPS e. max ZirCAD Prime A2: incisal (PI), middle (PM)). Each group was divided into one static (n = 15) and three dynamic fatigue protocols (N = 45, n = 15): i. 50 N increase every 5000 cycles ii. Increase by 5% of static fracture load every 5000 cycles iii. 10 N increase every 1000 cycles until facture. All specimens were loaded until facture in CeraTest 2 k. Kaplan-Meier, Log-Rank and Chi-squared-test as well as Weibull statistics were performed. A fractographic analysis was performed. The specimens were classified according to the number of crack origins and evaluated using the Ciba-Geigy table. RESULTS With regard to the fracture load, in the static loading MI and PI showed a higher fracture load and in dynamic fatigue protocol 2 PI showed a lower fracture load. The number of cycles until fracture only differed within three groups: MM and MI survived a higher number of cycles in dynamic fatigue protocol 2; PI survived a higher number of cycles in dynamic fatigue protocol 2 than in protocol 1. Within dynamic fatigue protocols, PM resisted the highest number of cycles in protocol 1 and 3 and MI in protocol 2. Comparing groups, Weibull modulus differed only within the static loading, with PI showing lower values than MM and MI. Within the material groups, MI showed higher values in static loading than in dynamic fatigue protocol 1 and 2, and PI showed higher values in the dynamic fatigue protocol 3 than in static loading. With regard to fracture patterns, no differences were found between the groups. CONCLUSIONS Dynamic fatigue protocols provide clinically relevant information on the long-term stability and reliability of monolithic strength-gradient zirconia materials. However, no definitive instructions for dynamic testing can be provided from this investigation