The primary concern is for healthy tooth structure. In the interest of maximizing bond strength, all the buccal enamel should be retained. Where some minimal preparation is necessary, however, the preparation must be left in the enamel layer if at all possible.

A, All the buccal enamel has been retained.
B, Preparation for the veneer kept within enamel.

Almost any preparation penetrates to the dentin at some point, usually toward the gingival portion of the tooth where the enamel thickness flares down to a knife-edge at the cemento-enamel junction. 

Although the preparation in part A is entirely within enamel, most clinical preparations may enter the dentin as seen in part B.

The initial signs of subveneer staining were followed by percolation, then fracture of the gingival segment of the laminate. Even with the best of the newer dentin adhesive agents, the porcelain bond to enamel is still significantly greater than the adhesion to dentin, so it is advisable to minimize dentin exposure during the preparation.

Initial subveneer staining followed by percolation, fracture and repair of the veneer.

The margins of the porcelain veneer should be placed where the patient can readily access them for routine home maintenance. In particular, the gingival margin should be located supragingivally. The microenvironment of the veneer margin consists of a knife-edge silanated porcelain bonded onto enamel by a composite resin cement. 

The supragingival margin (A) causes no gingival irritation and is maintainable (B)

Home maintenance of the supragingival margin.

The enamel and porcelain are both biocompatible with the gingiva and well-tolerated. The composite, however, tends to cause irritation if it comes into intimate contact with the gingiva. When the margin is placed subgingivally, this luting material is in direct contact with the free gingival margin, eventually causing irritation and recession. With a supragingival margin, the composite is reasonably away from periodontal structures and is unlikely to cause tissue irritation.

The porcelain subgingival margin (A) and the luting cement irritates the gingival margin and is not readily maintainable (B)

Limited home maintenance of the subgingival margin.

All visually accessible areas of the tooth should be covered by porcelain. 

Crown and visible root (left)
Veneer is slightly larger than facial surface of tooth (right).

The area most often overlooked in this respect is the gingival portion of the proximo-facial line angle. With the gingival recession, these areas of slight concavity are uncovered. The dark underlying tooth structure is visibly unesthetic when the patient is viewed from the side.

Proximofacial line angle not covered by porcelain.

A slight reduction of the proximo-facial line angle is usually all that is needed to permit an acceptable path of insertion for the proximally extended porcelain laminate.

A to C, The veneer allows show-through of darker dentin at proximogingival area
D, Dentin is slightly reduced at the proximogingival area with a diamond bur
E, The slight reduction of the proximofacial line angle eliminates dark show-through.


Contemporary Esthetic Dentistry, George Freedman.

Related topics:

Ceramic veneers preparartion .. Part 1

Ceramic veneers preparartion .. Part 2 " Conventional preparation "

Ceramic veneers preparartion .. Part 3 " Guided prep. "

Ceramic veneers preparation .. Part 4 "Special cases"

Adhesive cementation " emax cementation protocol “