Study On The Marginal Fit And The Fracture Resistance Of The Non-precious Metal-ceramic Crowns Of The Different Marginal Types

Objective:To compare the marginal fit and the fracture resistance of the non-precious metal-ceramic crowns of the four different marginal types.Methods:Four groups of the non-precious metal-ceramic crowns were fabricated according to the four different marginal types.Group 1:the metal framework extended to the labial margin of the shoulder.Group 2:the metal framework extended to the middle of the shoulder .Group 3:the metal framework gradually shifted to the axiogingival line angle of the shoulder from 1mm coronal to the axiogingival line angle,with the shoulder-porcelain.Group 4:the metal framework extended to the 1mm coronal to the axiogingival line angle of the shoulder,with the shoulder-porcelain.There were ten specimens each group.The marginal fit was investigated in the microcrope(40X) of the XTB-B type before and after cementation.And the specimens cemented were kept in a bath of the artificial saliva at 37°C for 24 hours,then were subjected to the loading parallel to the axis of the specimen until fracture with Instron 1342 testing machine at a speed of 1mm/min.The loads at fracture were recorded.Results:Before the cementation,the labial marginal fit of the Group 3 and the Group 4 was better(p<0.05),the lingual marginal fit of the Group 1 and the Group 2 was better(p<0.05),and the Group 4 was the worst in the marginal fit around the crown(p<0.05).After the cementation,the labial marginal fit of the Group 1 and the Group 4 was better(p<0.05),and the lingual marginal fit of the Group 1 and the Group 2 was better(p<0.05);the Group 1 was the best at the marginal fit around the crown,the Group 2 was the second,and the Group 3 and the Group 4 were the worst(p<0.05).It was statistically significantly different between the cementation before and after(p<0.05).The fracture load of the Group 2 was the largest(p<0.05);the fracture load of the Group 4 was the smallest(p<0.05);and there was no statistically significant difference between the Group 1 and the Group 3(p>0.05).Conclusions:Before the cementation,the labial marginal fit of the two groups with the shoulder porcelain is better,and after the cementation,the labial marginal fit of the metal-ceramic crowns whose metal framework extends to the labial margin of the shoulder and the other extends to the 1mm coronal to the axiogingival line angle of the shoulder,with the shoulder porcelain ,is better;the lingual marginal fit of the two groups with the common marginal types is better before and after the cementation;the metal-ceramic crown whose metal framework extends to the labial margin of the shoulder is the best at the marginal fit around the crown.The marginal discrepancy of the metal-ceramic crowns of the four different marginal types increases obviously after the cementation.The fracture load of the metal-ceramic crown with the metal framework extending to the middle of the shoulder is the largest; the fracture load of the metal-ceramic crown with the metal framework extending to the 1mm coronal to the axiogingival line angle of the shoulder is the smallest; the fracture resistance of the non-precious metal-ceramic crowns of the four different marginal types in the study all can support the normal biting forces.