A recent study performed at NIOM showed that a modified design of the crown margin gave stronger crowns without jeopardizing the aesthetics.
The traditional way of using zirconia in dental crowns is to cover the entire core with veneering ceramics. This requires a substantial removal of tooth substance with the subsequent risk of pulp injury in order to avoid buccally bulging crowns. Furthermore, these crowns have been prone to chipping off of the outer veneer. The more recent alternative has been to produce monolithic zirconia crowns, i.e. crowns without veneering ceramic to full anatomical contour. These crowns exhibit increased strength and less chipping, but often lack the satisfactory similarity to adjacent teeth.
The fact that the monolithic crowns fractured at a statistically significant higher load and presented more complicated fracture modes than the core–veneer crowns indicates that the thickness of the crown wall affects the fracture strength of zirconia crowns. Since most fractures are initiated in the cervical part of the ceramic crown, it was suggested that modifying the cervical region could improve the crown strength.
The present study showed that by producing the crown core with a cervical collar of un-veneered zirconia, the strength increased without jeopardizing the aesthetics (Figure 1). The cervical collar requires less tooth substance removal and yet a favourable crown contour can be made, similar to an old gold band for metal–acrylic restorations. However, monolithic zirconia crowns made with full anatomic contour were the strongest of the all-ceramic crown types.
In cases with high aesthetic demands, the use of a core–veneer design with a cervical collar of zirconia is recommended to improve the crown strength.
Øilo M, Kvam K, Gjerdet NR. Load at fracture of monolithic and bilayered zirconia crowns with and without a cervical zirconia collar. J Prosthet Dent, e-pub ahead of print (http://www.ncbi.nlm.nih.gov/pubmed/26775244).