Role of Margin Design on Marginal Adaptation and Fracture Strength

Objectives: To evaluate the impact of margin design on the marginal adaptation and fracture strength of crowns fabricated with polymer-infiltrated ceramic network (PICN) and zirconia-reinforced lithium disilicate (ZRLD).
Methods: Sixty crowns (n=10) were milled from ZRLD (Vita Suprinity) and PICN (Vita Enamic) CAD/CAM blocks. Each crown was designed from 3D-printed teeth prepared with one of the three following margin designs: light chamfer, heavy chamfer, or modified shoulder. Dual-cure resin cement was used to cement the crowns. Margins were assessed in six standardized areas at baseline with digital scanner (Omnicam, Sirona) and microscope. A score was assigned to each segment based on marginal adaptation (0=closed, 1=open 0.01-0.50mm, 2=open 0.51mm-1.0mm, 3=open >1.0mm). Crowns were then subjected to 500,000 mechanical load cycles to simulate masticatory forces. After cycling, each marginal segment was re-evaluated and rescored. Crowns were then loaded until catastrophic failure to obtain fracture strength (N). Three-way ANOVA was used to statistically analyze marginal adaptation whereas two-way ANOVA was used for fracture strength. After, both were analyzed with Tukey’s test (α=5%).
Results: All crowns survived the mechanical cycling. After cycling, all margins were significantly more open than before cycling (p<0.001). There was no difference between PICN and ZRLD materials (p=0.49) on marginal adaptation. For PICN, more open margins were attained with heavy chamfer than with light chamfer (p=0.03). PICN showed no difference (p=0.883) in fracture strength among margin designs. Light chamfer in ZRLD crowns provided significantly lower fracture strength compared to other margin designs (p<0.001). Light chamfer attained similar fracture strength for both ceramics (p=0.589). However, ZRLD was superior to PICN for the other two margin designs (p<0.001).
Conclusions: Within the limitations of this study, we can conclude that light chamfer finishing margins are preferable to PICN rather than heavy chamfer. Nevertheless, light chamfer may provide lower fracture strength for ZRLD ceramics.