Maii I. M. Khalil, Cherif A. Mohsen And Shams W. A. Ali,

ABSTRACT
Statement Of Problem: Color Stability Is An Important Factor To Ensure The Long-term Clinical Success Of Ceramic Restorations. The Aim Of This Study Was To Evaluate The Effect Of Repeated Firing Cycles On Color Stability Of Three Shades Of Zirconia Ceramic System. Materials& Methods: Seventy-five Ceramic Discs Were Fabricated. Discs Were Divided Into Three Main Groups According To Their Shades. Group(1) Shade A1, Group(2) Shade A3 & Group(3) Shade A4(n=25). Then Each Group Was Subdivided Into Five Subgroups According To The Number Of Firing Cycles Done (n=15) : SG(I)No Firing Cycles (control), SG(II) 3 Firing Cycles, SG(III) 5 Firing Cycles, SG(IV) 7 Firing Cycles And SG(V) 9 Firing Cycles. Color Readings Were Obtained With A Spectrophotometer Before And After Firings And L*, A*, And B* Coordinates And Total Color Variation (?E) Were Statistically Analyzed Using IBM SPSS Software.

Results: Results Revealed That For The 3 Shades (A1, A3 &A4), Color Change (DE) Had A Statistically Significant Difference Among The Four Different Firing Cycles (3, 5, 7 And 9) As Its P=0.01.Also, The Change In Color Parameters DL, Da And Db, Among The Four Different Firing Cycles Were A Statistically Significant With P=0.01. Conclusion: The L*a*b* Values Of The Ceramic Systems Were Affected By The Number Of Firings (3, 5, 7, Or 9 Firings) And Veneering Porcelain Shade (A1, A3 Or A4). L* Value Of All Specimens (at Different Shades) Was Increased Which Means That Specimens Become Lighter After Increasing The Number Of Firings. While, A* And B* Values Were Variables. The Mean Color Differences (?E) Caused By Repeated Firings Were Sometimes Below The Perceivable Level (?E<3) And Others Above This Level (?E>3) And Represent Visually Detectable Color Differences With The Change In Ceramic Shades And Firing Cycles. Clinical Recommendations: Zirconia Ceramic Restorations Should Not Be Exposed To Repeated Number Of Firings To Not Produce Detectable Color Changes.