Veneers and crowns are similar in that they change the appearance of a tooth by replacing a thin outer layer of the tooth with a ceramic-based replacement. Both act to improve the aesthetics of teeth, however, only the crown also serves to protect weak teeth from breaking.
A veneer provides partial coverage of teeth to improve their appearance. A thin layer on the front of the teeth is removed from the gum-line to the biting edge, and replaced with a cemented ceramic veneer. Veneers can be used to improve the shape and contour of teeth, to brighten and correct stained or discolored teeth, and to change the alignment of teeth without the use of braces. In some cases, veneers can be used when a front tooth has been broken and a veneer is a more aesthetic restorative option than a standard white filling.
However, if a tooth has had a more extensive break, has had a root canal, or had a large filling, a crown may be a better option. Crowns are a full coverage option that provides the same aesthetic corrections as a veneer but replaces the outer layer of the teeth all around. A crown acts as a cap on the tooth, completely encasing the tooth front and back from gum-line to biting tip, not just replacing the front surface as a veneer does. With a crown on, any forces on the tooth act to hold the tooth together. Whereas with a veneer, in the case of a large filling, the forces on the tooth can cause remaining walls to break off.
Teeth that have had root canals are especially vulnerable to breaking. This is due to the compromised remaining structural integrity of the tooth after a root canal has been done. During this treatment, a large amount of core structure is removed leaving the outer walls of the tooth vulnerable to fracture under biting force. As a result, crowns are routinely recommended to all teeth that have had root canal therapy.
Therefore, when deciding between a veneer and a crown, it comes down to the structural integrity of the tooth. An unfilled tooth in good condition is better suited for a veneer, which requires minimal tooth structure removal. Whereas a tooth with large fillings, a previous break, or root canal therapy would last longer with the full coverage protection of a crown.