Abstract: Objective To compare the effect of 3 different reinforcement designs in the metal frameworks on the strength of veneering porcelain of 4-unit metal-ceramic fixed bridges by means of load-to-failure method. Methods 4-unit metal-ceramic fixed bridges with contiguous missing of C5 and C6, with C4 and C7 as abutments
were fabricated, with 3 different reinforcement designs of the metal frameworks: Non-reinforcement, lingual punctuate metal reinforcement, and lingual linear metal reinforcement. After cementation of the bridge, vertical loads were then applied on the occlusal surface at the center of the pontics on a testing machine（at a crosshead speed of 0.5 mm·min-1） until the first sign of porcelain crack appeared. Load-to-failure forces and modes of porcelain failure were recorded. The load-to-failure forces among the three different reinforcement design groups were statistically analyzed with SPSS 11.0 software, using one-way ANOVA and SNK comparison（α=0.05）, and the modes of porcelain failure were also compared. Results The load-to-failure forces of the bridges in the three reinforcement design groups were（0.87 ±0.06）kN（non -reinforcement）, （1.03 ±0.15）kN（lingual punctate metal reinforcement）, and（0.93±0.10）kN（lingual linear metal reinforcement） respectively. Significant difference was found between non-reinforcement and lingual punctate metal reinforcement group（P <0.05）, while no significant difference was found between the other groups（P >0.05）. The modes of porcelain fracture were about the same in the three
groups. Conclusion The bridges with punctuate reinforcement demonstrated the maximal load-to-failure forces.

Key words: fixed partial denture, framework design, porcelain failure, strength