Objective: Despite the high fracture strength of modern dental core ceramics, fracture is a common cause of failure in clinical trials. Fracture origins are typically in the crown margin in the appoximal region where the margin curves upwards. The scientific evidence for optimal marginal design of preparation is limited. The aim was to use a clinically relevant test method to evaluate the effect on strength of the cervical margin’s curvature of ceramic crowns.
Method: Thirty zirconia (Zerion HT, Straumann) and 30 glass ceramic (IPS e.max, Ivoclar Vivadent) full-contour crowns were produced for three different premolar preparations. The preparations were made with a chamfer of 0.6 mm with variation in the approximal crown margin curvature; low curvature (r=12 mm), moderate curvature (r=2.5 mm), and high curvature (r=1.5). The crowns were loaded until fracture with a method that mimics clinical fracture modes.
Result: There were statistically significant differences both among margin design and material groups (Table 1, p<0.001 Mann Whitney U test). A moderate to low curvature of the approximal crown margins was associated with higher strength compared with a high curvature. Table 1. The mean load at fracture in Newton (standard deviation) for the different test groups and materials. Low curvature Moderate curvature High curvature Glass-ceramic 1202 (155) 1006 (147) 658 (74) Zirconia 2294 (310) 2014 (309) 1614 (300) Conclusion: The curvature of the cervical crown margin affects the load at fracture for all-ceramic crowns.

Ceramic Crown Margin Curvatures’ Effect on Fracture Strength
Øilo M, Kvam K, Gjerdet NR
Conference Paper · September 2014 Conference: 2014 IADR/PER Congress