Purpose: To assess the effect of different thicknesses of porcelain at the gingival of pontics, on the fracture load of zirconia-based, all-ceramic fixed dental prostheses (FDPs), anchored by inlays.
Methods: Box inlay cavities were prepared on mandibular molars and premolars. 40 FDPs with yttrium-stabilized zirconia frameworks of identical dimensions were manufactured using a CAD/CAM system and veneered with a press ceramic. The FDPs, replacing a premolar and a molar, were divided into four groups. In Group FR, the framework was all around unveneered. The next three groups received a 1 mm ceramic veneer on the buccal, occlusal and lingual side, but differed in the thickness of the ceramic veneer in the gingival, tensile zone of the pontics. In Group B-0, the gingival veneering was 0 mm, in Group B 1 mm and in Group B-2, 2 mm of gingival porcelain. A group of inlay-retained metal-ceramic FDPs (mc) served as control. All FDPs were subjected to thermal cycling and 600,000 cycles of mechanical load of 50 N. The load to fracture (N) was measured and fracture sites were evaluated macroscopically. A single-factor Analysis of Variance was used to analyze the data.
Results: None of the FDPs debonded after thermal cycling or mechanical loading and no signs of fractures or other defects were observed. The mean fracture loads and standard deviations in parentheses were: 647 N (123) for Group B-0, 716 N (102) for Group FR, 812 N (48) for Group B-1, 934 N (129) for Group B-2 and 1005 N (SD 81) for Group MC. Means for Groups B-0 and FR were not shown to differ, and the same for mean fracture strength of Groups B-2 and MC.