What margin design is preferred for all-ceramic crowns?

The preferred margin design for all-ceramic crowns is the heavy chamfer. This design provides a smooth transition between the preparation and the soft tissue, which is essential for achieving an optimal aesthetic result with all-ceramic materials that provide excellent translucency and coloration.

A heavy chamfer margin is characterized by its rounded edges, which help reduce the chances of porcelain fracture during shaping and finishing. This margin design also allows for adequate thickness of the ceramic material, which is crucial to ensure both the mechanical strength and the lifelike appearance of the restoration. The heavy chamfer supports the material's structural integrity and can help enhance the bonding between the tooth structure and the ceramic, contributing to better overall retention of the crown.

In addition, the heavy chamfer design facilitates more straightforward impressions and can help minimize the risk of marginal discrepancies, thereby ensuring a better fit and reducing the potential for future complications. This margin does not create sharp angles that might lead to stress concentrations in the ceramic material, which is particularly important given the brittleness of some all-ceramic materials.

The other options do not provide the same level of benefits in terms of aesthetics, strength, and practical application for all-ceramic crowns.