Veneer preparations that include incisal reduction require a decision about lingual margin design. The two most common designs are a butt joint or a lingual wrap design. There are pros and cons to each of these designs and one or the other may be appropriate depending on the parameters of the case. Creating a butt joint margin is what I prepare more commonly these days. This design allows us to have adequate thickness of porcelain at the incisal edge to allow the technician artistic freedom to create incisal translucency and dentinal effects. Wrapping over the incisal edge also creates the ability for the technician to have artistic freedom, so what are the differences.
Veneer preparations are designed to draw from the facial instead over down the incisal like a traditional crown preparation. As soon as we wrap over the lingual we now have to create draw between the lingual wall and the facial, specifically the gingival one third. The creation of draw down the incisal has a positive implication, it makes knowing the veneer is seated easier, as it has only one place to be. The lingual wall prep prevents tipping to the facial during bonding, but it also necessitates increased reduction on the facial. This reduction usually means we breaks through the enamel at the gingival one third and increase the amount of dentin bonding for our veneer.
Another consideration is the stress on the marginal interface. Upper anterior teeth are loaded from the lingual creating flex of the teeth in a labial direction. The greatest concentration of flexural stress in an upper anterior tooth is the concavity above the cingulum on the lingual according to research by Pascal Magne. A challenge we face with veneers is the long term integrity of the margins and preventing marginal leakage and breakdown. To this end when we wrap the lingual we place a margin in the area of the tooth with the highest flex and risk of marginal breakdown.
The limitation of a butt joint margin is moving the incisal edge in the final restoration. If we are attempting to reposition the incisal edge labially, or alter a tooth that is rotated the technician may need porcelain down the lingual of the tooth to create running room to create the new tooth position. When this is necessary I often move the lingual margin onto the top of the cingulum and out of the lingual concavity.