Abstract: BACKGROUND: Some researchers have pointed out that the porcelain edge design can well solve the problem of “gingival black line” of metal ceramic crown in anterior tooth restoration, but the mechanical research methods used to test its strength cannot simulate the real strength of metal ceramic crown under the influence of humid environment and fatigue in oral environment.  
OBJECTIVE: To discuss clinical suitability of porcelain crowns with porcelain edge design by comparing the fatigue life and strength of cobalt-chromium metal crowns with porcelain edge design in simulated oral environment.  
METHODS: Sixty cobalt-chromium metal crowns were prepared. The cobalt-chromium metal porcelain crowns with the metal abutment edge ending at the middle of the shoulder (conventional design) were considered as group A (n=20); the metal abutment edge ending at the junction of the shoulder and the axial wall was considered as group B (n=20); and the metal abutment edge ending 1 mm above the junction of the shoulder and the axial wall was considered as group C (n=20). All samples were immersed in artificial saliva and placed at a constant temperature of 37 ℃ for 24 hours. Subsequently, the samples were subjected to fatigue cycles on an electromagnetic fatigue tester with a loading force of 50 N. The samples were removed after 0.75 million and 1.5 million cycles respectively, to observe the surface condition. The samples without porcelain breaking were transferred to the universal material testing machine for static loading until porcelain breaking and the load force borne by each sample at the time of porcelain breaking was recorded.  
RESULTS AND CONCLUSION: (1) After 0.75 million and 1.5 million cycles, porcelain breaking was not observed in all specimens and no cracks were observed on the surface of all porcelain crowns. (2) The average load force value of the specimens in group A was the largest, about 1 785 N; the average load force value of the specimens in group B was the second, about 1 426 N; the average load force value of the specimens in group C was the smallest, about 1 134 N. There was a significant difference in the load force value among the three groups (P < 0.05). (3) The fatigue life of the three kinds of ceramic crowns with edge design is more than ten years, and their strength can also meet the needs of normal occlusion.

Key words: metal-ceramic crowns, cobalt-chromium alloy, fatigue investigation, static loading, porcelain marginal, gingival black line