An investigation on fatigue, fracture resistance, and color properties of aesthetic CAD/CAM monolithic ceramics

Abstract

Objectives: To evaluate and compare fracture resistance, translucency, and color reproducibility, as well as the effect of aging on the fracture load and color stability of novel monolithic CAD/CAM ceramics.

Materials and methods: One hundred crowns of uniform thickness were milled from five ceramic blocks (n = 20): partially crystallized lithium disilicate (PLD) and fully crystallized lithium disilicate (FLD), lithium metasilicate (LMS), 4Y-TZP (SMZ), and 5Y-TZP (UMZ) monolithic zirconia. PLD crowns were glazed, LMS was fired, and FLD was polished. SMZ and UMZ crowns were sintered and polished. Crowns were adhesively cemented to epoxy dies. Half of the crowns (n = 10) were subjected to 1.200.000 load cycles with thermal cycling. Color space values L, a, b defined by the Commission Internationale de l´Eclairage (CIELAB) were measured before and after aging, and (∆E) was calculated. Both aged and non-aged specimens were loaded until fracture in a universal testing machine and the fracture load was recorded. X-ray diffraction (XRD) and scanning electron microscope (SEM) fractographic analysis were carried out on fractured fragments of representative samples. For translucency and color reproducibility, 50 rectangular-shaped specimens were fabricated and processed as described previously. Color values were measured over black and white backgrounds, and the translucency parameter (TP) was computed. Using the shade verification mode, (∆E) to shade A3 was calculated. Data were statistically analyzed using one-way and two-way ANOVA, and t-test.

Results: Aging did not affect fracture resistance significantly (p > 0.05). The highest mean fracture load was obtained for the SMZ and UMZ. A significant color change was observed after aging in all groups. The highest TP was noted for FLD. SMZ and UMZ had the best shade match.

Conclusions: Zirconia showed higher fracture resistance and color stability than lithium silicate ceramics. Lithium silicate ceramics were more translucent. The experimental FLD demonstrated high translucency.

Clinical relevance: Tested ceramics showed sufficient stability to withstand masticatory forces. Characterization of final restorations might be mandatory for better color match.

Keywords: Aging; CAD/CAM; Color; Dental ceramics; Dental materials; Fracture resistance; Lithium disilicate; Monolithic; Prosthodontics; Translucency; Translucent zirconia.

Fig. 1

Load to fracture test using universal testing machine. The piston applied perpendicular to the occlusal surface and driven at a crosshead speed of 1 mm/min

Fig. 2

Load to fracture results in newton before and after aging. PLD, partially crystalized lithium disilicate; FLD, fully crystallized lithium disilicate; LMS, lithium metasilicate; SMZ, super-translucent monolithic zirconia; UMZ, ultra-translucent monolithic zirconia. The capital letters show significant difference among groups with no aging. The small letters show the significant difference among groups after aging. Means that do not share the same letter are significantly different. Two-way ANOVA followed by Tukey's multiple comparisons test. Significance level at p < 0.05

Fig. 3

Mean and standard deviations of the ∆E for ceramic crowns after thermomechanical aging. PLD, partially crystalized lithium disilicate; FLD, fully crystallized lithium disilicate; LMS, lithium metasilicate; SMZ, super-translucent monolithic zirconia; UMZ, ultra-translucent monolithic zirconia. The letters show the significant difference among groups. Means that do not share the same letter are significantly different. One-way ANOVA followed by Tukey’s multiple comparisons test. Significance level at p < 0.05

Fig. 4

SEM images of selected fractured segments after fracture loading test showing the fracture patterns. PLD, partially crystallized lithium disilicate; FLD, fully crystallized lithium disilicate; LMS, lithium metasilicate; SMZ, super-translucent monolithic zirconia; UMZ, ultra-translucent monolithic zirconia. PLD showed crescent-shaped fracture (dotted arrow). White arrows point to fracture origins starting from tensile regions. FLD and LMS demonstrated cervical fractures 1 mm below the cervical margin (black arrows)

Fig. 5

XRD patterns of tested groups. PLD, partially crystalized lithium disilicate; FLD, fully crystallized lithium disilicate; LMS, lithium metasilicate; SMZ, super-translucent monolithic zirconia; UMZ, ultra-translucent monolithic zirconia. A = before aging, B = after aging, t = tetragonal zirconium oxide, c = cubic zirconium oxide

Fig. 6

(a) Translucency parameter values and (b) difference in color (∆E) from selected shade A3 of the ceramic specimens. PLD, partially crystalized lithium disilicate; FLD, fully crystallized lithium disilicate; LMS, lithium metasilicate; SMZ, super-translucent monolithic zirconia; UMZ, ultra-translucent monolithic zirconia. The letters show the significant difference among groups. Means that do not share the same letter are significantly different. One-way ANOVA followed by Tukey’s multiple comparisons test. Significance level at P < 0.05