| With features of both metal strength and porcelain's artistic beauty, metal-ceramic crown has been widely applied in clinic. Multiple firing of metal-ceramic crown is possible due to color adjustment for color matching and decoration as well as shape modification. Strength of metal-ceramic crown is possiblely affected by multiple firing.Objectives: To observe surface structure of metal-ceramic crowns with different firing number, to observe changes of its surface structure after loading, to observe porcelain's fracture sections by scanning electron microscope (SEM) and through destruction tests on metal-ceramic crowns, effects of the number of firing on metal-ceramic crown microstructure and strength are studied, which provide experimental basis for technical manufacture and clinical application of metal-ceramic crown.Methods: 1. Models preparation: a metal die of simulating crown core is produced with precision machine; Totally 20 (in 5 groups, A to E) metal-ceramic crown models, with thickness of 1.8mm, are conventionally produced; The number of firing for each model from group A to E are 4, 6, 8, 10, 12 respectively. 2. Structure of surface cracks, poresand crystals of each model are observed with SEM. 3. With Instron testing machine, vertical force of SOON is loaded on occlusal surface of each model for 5 minutes. 4. Changes of surface structure after loading are observed with SEM. 5. With Instron testing machine, destruction test on each model is performed to test fracture force values. 6. Statistical analysis of those values are performed. 7. Each model's structure in fracture section is observed with SEM.Results: 1. Metal-ceramic crown surface microstructure: in group A, no cracks, small pores and crystals evenly scattered; in group B, few small shallow and narrow cracks and crystals of different size scattered, pores more dense than in group A; in group C, many shallow and broad cracks widely distributed, partly peelings shown, crystals with different size scattered, small pore densely distributed with occasional larger pores distributed in clusters; in group D, surface uneven, small deep and narrow cracks spreading connected, medium-sized crystals scattered, large quantity of small pores densely distributed as vortex; in group E, surface undulated, few deep and narrow cracks distributed in same direction, crystals with different size scattered, small pores distributed in strings, medium-sized in curls, large occasionally shown. 2. The surface microstructure after loading: in group A, few small shallow cracks occurred, large cracks occasionally shown, few small pores scattered; in group B, many small shallow cracks and few large cracksoccurred, many small pores scattered, few irregular pores connected with cracks; in group C, shallow cracks densely distributed, many irregular small peelings, many pores densely distributed; in group D, long and shallow cracks densely distributed, deep cracks occasionally shown, pores with different size occurred; in group E, irregular large shallow peelings occured, large quantity of pores densely distributed. 3. Even values of strength obtained from destruction test are as follows (unit: kg): group A-197.5, group B-181.25, group C-130, group D-120, group E-109. 4. Analysis of variance (ANOVA) results indicate that there are significant difference of destruction test values on models with different number of firing (PO.01), multiple comparison results indicate that significant difference in destruction test values exist between group A and C, A and D, A and E, B and C, B and D, B and E (P<0.01); no significant difference between any other two groups (P>0.05). 5. The fracture sections structure: in group A, fracture sections even , few pores and short saw-toothed cracks scattered, uniform glassy structure shown between cracks; in group B, fracture sections even, few pores and short bending cracks scattered, uniform glassy structure shown between cracks; in group C, fracture sections uneven, pores of different size scattered, many long irregular cracks int... |
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