Structural integrity of dental crowns.

Sammanfattning: There is currently no standardized method for testing dentures made of dental ceramic materials. This creates problems for comparisons between studies concerning strength in materials and geometry. The geometry of the maxillary first premolar was used for the analysis. The modeled crown has two underlying layers, a thin cement layer closest to the crown that attaches the crown to dentin. Simplifications are made so that dentin geometry is modeled in CAD as a cuboid and the cement layer above follow its geometry. For the actual analysis, a FE-analysis has been made of different loading positions and areas to clearly show how important it is to be consistent in the selection of these. The load has been placed on two surfaces with 150 N each to resemble a real occlusal maximum load. The standard area used in other analysis was set to be 5.5 mm2 on each cusp. A comparative analysis has been made of various ceramic materials to see what impact it has on the cement layer strength. Similarly the cement material properties were modified to see how it affects the ceramic crowns strength. In real applications, one usually tries to achieve as thin cement layer as possible but here the analysis is of how much influence the cement layer thickness has on the overall crown strength. Two cylindrically shaped simplified models were created to see how much the anatomical geometry effects on the maximum stresses. One was modeled with a flat top and the other with an angled top. The load placement and the magnitude of its area turned out to be very important for the resulting maximum stresses. Surface area ranged from 0.07 - 26 mm2, and the resulting von Mises stresses for these ranged from 1120 to 34 MPa. Generally speaking, varying cement materials available on the market does not give huge impact on ceramic bearing stress. Panavia F 2.0 was found to be the strongest cementing layer that caused the highest failure load of the existing materials on the market. Super Bond B&D gave the lowest failure load. However, you could see that it was a bit more important to be careful for the crown material. The made up ceramic materials (Hypothetical Ceramic and Experimental Ceramic) proved to cause relatively close stresses for the cement to fail. It was also clear that a thicker cement layer reduces the expected failure load and the structure becomes weaker. The simplified models seemed to give a substantial exaggeration of strength. The expected failure load was nearly twice as large for these (slightly lower for the angled model) than for the anatomical model. When comparing with the test data from another study it proved that it does not match with the simulations. In the study different cementing materials had been tested and Super Bond B&D turned out to be the strongest material. With this in hand it can be said that a stable test setup with consistently identical geometries is necessary in order to evaluate these kinds of structures.