Influence of loading site and ceramic strength on the failure of teeth restored with ceramic inlays

Abstract

The purpose of this study was to evaluate the influence of ceramic material strength and loading site on the failure of teeth restored with ceramic inlays using a universal testing machine. Ninety intact, caries-free human maxillary premolars were divided randomly into six groups (n=15): Group 1, Class II mesio-occluso-distal (MOD) cavity preparation and restoration with IPS Empress CAD and compressive force loaded on inlay; Group 2, MOD cavity preparation and restoration with IPS e.max CAD and compressive force loaded on inlay; Group 3, MOD cavity preparation and restoration with IPS Empress CAD and compressive force loaded on tooth; Group 4, MOD cavity preparation and restoration with IPS e.max CAD and compressive force loaded on tooth; Group 5, MOD cavity preparation and no restoration and compressive force loaded on tooth; and Group 6, intact teeth and compressive force loaded on tooth. Class II MOD preparation was as follows. The pulpal floor was formed at a depth of 2 mm from the occlusal cavosurface margin of the preparation, and the isthmus was half of the intercuspal distance. The proximal box width was 1.5 mm, and the axial wall was 2 mm in height. Margins were prepared with 90-degree cavosurface angles. Specimens were tested in a universal testing machine at 1.0 mm/min using two steel sphere plungers (3 mm and 6 mm in diameter). Peak load to fracture (N) was measured for each specimen. Means were calculated and analyzed with a one-way ANOVA and Tukey’s test (α=0.05). The mean peak fracture loads (N, mean ± S.D.) were as follows: Group 1 - 689.4 ± 138.3, Group 2 - 813.6 ± 151.8, Group 3 - 1236.0 ± 292.4, Group 4 - 1244.1 ± 379.4, Group 5 - 822.3 ± 258.6, Group 6 - 1363.0 ± 342.5. Based on the results obtained under in vitro experimental conditions, the following conclusions were drawn. The ceramic strength did not statistically influence fracture resistance. However, the mean fracture load of the IPS e.max CAD inlay was superior to that of the IPS Empress CAD inlay when the force was loaded on the ceramic restoration. The main fracture type differed according to the loading site. When force was loaded on the ceramic inlay, the proximal box fracture was the main type of fracture. When force was loaded on teeth, the cuspal fracture was the main type of fracture. Future studies on the failure of teeth restored with inlays should consider the loading site.