Previously in dental technology the crown margin was regarded as the area on dental technical restorations subjected to least loading. Different scientific studies into the fracture behaviour of dental restorations, however, arrive at the opposite result.
The main conclusion of these studies is that the origin of fractures in all-ceramic restorations can be found in the marginal area in the majority of cases. The reason for this is that the occlusal and incisal masticatory forces [see green arrows in picture 1 & 2] are transmitted from the surface of the restoration to the marginal area and result in splitting of the crown exposed to loading [see orange arrows in picture 1 & 2], particularly with thin-walled geometries. Tensile stresses critical for ceramic materials are thus created in the marginal area of the restoration [see red arrows in picture 1 & 2], which could be the cause of fractures. The risk of fracture is also increased by rough margins, inhomogeneous surfaces and larger gaps for the luting material.
Particular attention should therefore be given to the following points when designing the crown margin to ensure permanently reliable all-ceramic restorations:
1. Adequate material thickness, including in the marginal area
(shoulderless preparations are therefore contraindicated for all-ceramic restorations)
2. Smooth margins without chipping
3. Perfect surfaces (polish)
4. Adequate, uniformly thin cement gap thickness