File-splitting multilayer vs monolithic Y-TZP: Fatigue flexural strength and loading stresses by finite element analysis.

OBJECTIVE

To compare file-splitting multilayer (fused and cemented) with monolithic Y-TZP on the fatigue flexural strength and finite element analysis (FEA) stresses. Additionally, to verify the effect of the material under tension in multilayer Y-TZP.

METHODS

Disc-shaped (diameter: 14.4mm; thickness: 1.4mm) monolithic Y-TZP (IPS e.max ZirCAD - Ivoclar Vivadent) and trilayer specimens with Y-TZP framework (IPS e.max ZirCAD), intermediate layer of fusion ceramic (IPS e.max CAD Crystall./Connect) or resin cement (Multilink Automix) and lithium disilicate veneer (IPS e.max CAD) were divided into five groups (n=20): monolithic Y-TZP (M), fused file-splitting with framework under tension (F-FT), cemented file-splitting with framework under tension (C-FT), fused file-splitting with veneer under tension (F-VT) and cemented file-splitting with veneer under tension (C-VT). Fatigue flexural strength was determined (piston-on-three ball) by the staircase approach (750,000 cycles; 20Hz). Mean and confidence intervals (CI) were calculated. FEA was evaluated under the application of the experimental mean fatigue load.

RESULTS

The fatigue strength was statistically different for all groups. Means and CI (MPa) were: M - 405.92 (CI 397.58-414.26), F-FT - 377.73 (CI 374.59-380.88), C-FT - 346.54 (CI 340.62-352.46), F-VT - 154.79 (CI 151.86-157.72) and C-VT - 100.34 (CI 97.42-103.26). FEA tensile stresses were similar to the mean experimental values (up to ≅10MPa of variation), with the most discrepant calculated stresses for C-FT (≅20MPa higher than experimental result).

SIGNIFICANCE

Monolithic specimens showed the highest flexural fatigue strength and fused file-splitting resulted in higher fatigue strength than cemented file-splitting. Groups with the framework under tension exhibited higher flexural fatigue strength.