All-ceramic partial coverage restorations on natural molars. Masticatory fatigue loading and fracture resistance

Abstract

Purpose: To determine the reliability and fracture resistance of adhesively luted all-ceramic partial coverage restorations (PCR) on natural upper molars after masticatory fatigue loading. Methods: 64 maxillary molars were divided into four groups of 16 specimens each. Control Group NP remained unprepared. Teeth in Groups EM, EX and PC were prepared equally according to standardized guidelines and restored with the following PCR: Group EM= IPS Empress; Group EX=IPS e.max Press; Group PC=ProCAD/Cerec 3. The 48 PCR were all bonded adhesively with dual polymerizing composite (Variolink II). All specimens were subjected to cyclic mechanical loading (1.2 million cycles, 1.6 Hz, invariable occlusal load 49 N) with thermal cycling (5°C/55°C, dwell time 60 seconds, 5500 cycles) in a mastication simulator. Failure was defined by bulk fracture of a specimen. Subcritical crack patterns were observed. Surviving specimens were loaded until fracture in a universal testing machine. The load to fracture values (N) (1.5 mm/minute crosshead speed) were automatically recorded by controlling software. Statistical analysis of the data was performed by Kruskal Wallis test (ANOVA) (α= 0.05) and pair-wise Wilcoxon rank sum test (α= 0.05). Results: All specimens survived mastication simulation. The following median fracture resistance values (N) (IQR= x.25-x.75) were recorded: Group-NP 1960 (1481-2228), Group-EM 1400 (1043-1722), Group-EX 1489 (1114-1751), Group-PC 2134 (2052-2391). The load-to-fracture values of Groups NP and PC in particular were significantly higher (P< 0.05) than those of Groups EM and EX. ProCAD restorations demonstrated the best predictability of failure (SD<). Within the limitations of this study, maxillary molars restored with ProCAD PCR showed a similar fracture resistance when compared to natural teeth. Since the majority of IPS e.max Press and ProCad restorations survived loads within the range of physiological mastication forces, both materials appeared to be suitable for the predictable use of posterior partial crowns.