Color stability and Microstructure Changes of Two Pressable Ceramics after Repeated Firing protocols

Statement of the problem: it is important that researchers gather more information regarding the effect of repeated firing on optical properties and microstructure of all ceramic restoration
Purpose: The aim of this in vitro study was to investigate the effect of repeated firing on color stability and microstructure changes using X-ray diffraction, EDAX and SEM of two pressable ceramics.
Methods: A total number of forty eight freshly extracted maxillary central incisors were collected. The selected teeth were cleaned and disinfected in 0.5% sodium hypochlorite solution, then stored in distilled water for maximum two weeks until the testing began. The roots of the selected teeth were serrated with a disc for retention. The teeth were mounted vertically into auto-polymerizing acrylic resin material. Full coverage all-ceramic preparation was performed for all teeth. The prepared teeth were randomly divided into two equal test groups (n=24) according to the all-ceramic materials used for crown fabrication as follows: Group I: IPS e-max Press (LD): Twenty four prepared teeth were restored with pressable lithium di-silicate glass ceramics (IPS e.max press, Ivoclar, Vivadent AG, Schann, Lieshtenstein). Group II: Celtra Press (ZL): Twenty four prepared teeth were restored with pressable zirconia-reinforced lithium di-silicate glass ceramics (Celtra Press, Dentsply, Sirona.). Then samples were subjected to repeated firing cycles up to five firing cycles. Samples were divided into four equal subgroups (n= 6), according to the number of firing cycles performed for each sample. Subgroup (A) : Control samples, subgroup (B):1st firing cycle ,subgroup (C): 3rd firing cycle, Subgroup (D): 5th firing cycle. All ceramic crowns fabrication was done according to manufacturer instructions for each material. For each crown the L *, a* and b* was measured using spectrophotometer (Vita Easy shade) by placing the probe tip on the central part of the labial surface of the crown, the colorimetric values of ΔL*, Δ a* and Δb* were measured from differences in the respective L*, a*and b* values. The total color difference ΔE* were measured at each firing cycles subgroups for each group of ceramic materials. Data were collected, tabulated and statistically analyzed. Microstructural analysis for the two tested all ceramic materials was examined by X-ray diffractometer, scanning electron microscopy, the elemental chemical composition as well as quantitative analysis was measured by energy dispersive X -ray spectroscopy (EDAX) .This analysis was done after the control firing and the 5th firing cycle.
Results: Regardless of the ceramic material, statistically significant color differences were resulted by repeated firing as follows ; perceptible but yet clinically acceptable for lithium di-silicate glass ceramics (LD), whereas, perceptible and clinically unacceptable for zirconia reinforced lithium di-silicate(ZL). The microstructure of the two pressable ceramic materials (LD, ZL) turned out to be unstable after repeated firing cycles for both ceramic materials.
Conclusions: Color stability is affected by repeated firing for both tested materials. Repeated firing is not recommended for ziconia reinforced lithium di-silicate because color changes (ΔE = 4.18) are exceeding the clinical acceptability.Repeated firings might result in microstructral changes within the ceramic materials.Microstructure analysis through SEM, EDAX and XRD is a reliable analytical approach.