الفهرس 
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Abstract
Statement of problem: Studies have compared the clinical performance of ceramics; however, the color compatibility of ceramic systems when constructing minimally invasive laminate veneers using different preparation techniques and chemical structures is unknown. In addition, establishing the correct match with the desired shade is still difficult.Aim of the study: To assess both the effect of veneer preparation technique (No-preparation versus 0.3mm Minimal-preparation) on the final color of two different machinable ceramic veneer systems (IPS e.max CAD Lithium disilicate-based glass ceramic and recently introduced Vita Enamic hybrid ceramic), and the effect of thermal cycling aging on their color stability. Materials and Methods: Twenty-eight intact human maxillary central incisors with homogenous dimensions and morphology were selected and, based on their color measurements, classified in to 2 main groups (n=14) according to ceramic veneer material; (V) for Vita Enamic and (E) for IPS e.max CAD. Each main group was further subdivided in to 2 subgroups (n=7) relative to preparation technique; (І) with no preparation and (ІІ) with 0.3mm preparation. Color measurements were performed with a 0°/diffuse reflectance spectrophotometer using the CIE L*a*b* system, at baseline before preparation, after veneer cementation, and after thermal cycling. Color differences were calculated with a ΔE* value < 3.3 was considered clinically acceptable. The data were analyzed using Paired t-test, One-way ANOVA, LSD Post Hoc test, and finally with Two-way ANOVA at p-value ≤ 0.05. Results: Between baseline and after veneering, no statistical significant differences were found between the different groups considering for mean ∆L*, ∆a*, ∆b*, and ∆E* values (p > 0.05) except ∆E* values between VⅠ and EⅠ groups. Between veneering and preselected target shade, there were statistical significant differences between both veneer systems whatever technique was, and no statistically significant differences resulted for mean ∆L*, ∆a*, ∆b*, and ∆E* values between both preparation techniques whatever veneer material used. After thermal cycling, statistical significant differences for all three coordinates, for all groups were recorded (p = 0.000) with decreased L* values (darker) and increased a* and b* values (more reddish and yellowish). Also, It was revealed that statistical significant differences were recorded between all groups in terms of the mean ∆L*, ∆a*, ∆b*, and ∆E* values, except between VⅠ and VⅠⅠ groups where no statistical significant differences detected.Conclusions: 1) The preparation technique did not have a significant effect either on the final color after veneer cementation or in establishing the desired preselected target shade, while the veneer material had a significant effect on both. 2) After thermal cycling, Vita Enamic veneers expressed high unacceptable color change compared to the clinically acceptable change of IPS e.max CAD veneers. 3) Both preparation technique and veneer material had a significant effect on the color change occurred after thermal cycling aging.