12 posterior single tooth crowns made from monolithic zirconium-reinforced glass ceramics(ZRGC). Restorations were evaluated prosthetic and periodontal criteria at baseline, after 6 and 12 months. While prosthetic evaluation was performed according to Modified US Public Health Service criteria; probing depth(PD), clinical attachment level(CAL), gingival bleeding time index(GBTI), gingival(GI) and periodontal index(PI) were used in periodontal evaluation.

Group 2: 12 posterior single tooth crowns made from monolithic lithium disilicate glass ceramics(LGC). Restorations were evaluated prosthetic and periodontal criteria at baseline, after 6 and 12 months. While prosthetic evaluation was performed according to Modified US Public Health Service criteria; probing depth(PD), clinical attachment level(CAL), gingival bleeding time index(GBTI), gingival(GI) and periodontal index(PI) were used in periodontal evaluation.

12 posterior single tooth crowns made from monolithic resin infiltrated glass ceramics(RIGC). Restorations were evaluated prosthetic and periodontal criteria at baseline, after 6 and 12 months. While prosthetic evaluation was performed according to Modified US Public Health Service criteria; probing depth(PD), clinical attachment level(CAL), gingival bleeding time index(GBTI), gingival(GI) and periodontal index(PI) were used in periodontal evaluation.

12 crowns were made from each group of three different ceramics: ZRGC, LDGC and RIGC. Thereafter, the restorations was evaluated in terms of any failure or loss of the restoration at any follow-up (6th and 12th months) periods. The results were obtained proportionally in percent (%).

As recorded at the baseline (0), each restoration was assessed according to the modified the United States Public Health Service (USPHS) Ryge Criteria for prosthetic evaluation by a calibrated prosthodontics.USPHS criteria is based on the following ratings: Color match (Alpha,Bravo, Charlie), cavosurface marginal discoloration (Alpha,Bravo, Charlie), secondary caries (Alpha,Bravo), anatomic contour (Alpha,Bravo, Charlie), marginal integrity (Alpha,Bravo, Charlie), surface texture (Alpha,Bravo, Charlie) and gross fracture (Alpha,Bravo, Charlie).

Probing depth(PD): Probing depth is defined as the distance from the free gingival margin to the bottom of the periodontal pocket (mm).

Clinical attachment level(CAL): The loss of CAL were measured and calculated by measuring recession and subtracting the periodontal probing depth (mm).

Gingival bleeding time index(GBTI): The probe were swiped at apical portion of the gingival crevice from line angle interproximally and after 10 seconds assessed if there was bleeding or no bleeding on the mesial and distal (unit/tooth).

Plaque index(PI, according to Silness and Löe, 1964): The total mean PI is the sum of the individual scores divided by the number of investigated sites. The thickness and extension of plaque were assessed (unit/tooth).

Gingival index(GI, according to Löe, 1967): The total mean GI is the sum of the individual scores divided by the number of investigated sites. The GI for each tooth is the sum of the 4 individual scores divided by 4 (unit/tooth).