No-prep veneers, and how much enamel there actually is

Twenty extracted human upper central and lateral incisors were embedded, sectioned through the midline at the incisal, middle and cervical thirds of the labial surface, and measured under a reflected stereomicroscope. On the central incisors the thinnest enamel measured about 345 micrometres and on the laterals about 235 micrometres, and in both cases that thinnest region was the cervical third, the band nearest the gum. The thickest enamel on the same teeth, at the incisal third, measured up to about 1260 and 1220 micrometres respectively.^[1]

Put those two facts side by side and the standard figure stops being reassuring. A preparation taken to a uniform 0.5 mm removes under half the enamel available at the biting edge and passes straight through the 0.345 mm available at the gum. The same study tested this directly: a chamfer preparation, which carries a defined depth to the margin, tended to expose dentine, while a knife-edge preparation at the gingival region exposed none, and the difference was statistically significant. The authors concluded plainly that enamel thickness in the gingival area does not permit a chamfer preparation.^[1]

Seventy extracted upper central incisors were scanned, prepared under digital guidance to leave defined percentages of enamel on the bonding surface, and tested for shear bond strength. Bonded to a fully intact enamel surface, the veneers held at 19.93 MPa. Bonded to a surface with no enamel left at all, the same veneers held at 9.63 MPa. Between 40 and 100 per cent preserved enamel there was no statistically significant difference, and below that the strength dropped away, with the 20 per cent group at 11.32 MPa. The authors identified 40 per cent preserved enamel as the minimum acceptable value for reliable bonding.^[2]

This is why the conversation about veneers is really a conversation about margins rather than about thickness. A review in the Journal of the American Dental Association set the same boundary from the clinical side: a successful preparation may have dentine exposed within the body of the preparation, but most of it must remain in enamel and all of the margins must end in enamel, because the dentine bond is weaker and depends on a multi-step, technique-sensitive process. The same review is blunt about the wider question, concluding that aligning a healthy tooth with a veneer is not a conservative procedure, and that orthodontics, bleaching, direct bonding and enamelplasty should be offered first.^[3]

A no-prep veneer adds material without removing any, so it only suits a tooth that has room for the addition. That means teeth sitting slightly back from where you want the final surface to be, small teeth, narrow teeth, or arches with spacing to close. Laboratory work on very thin ceramics supports the material side of this: laminates finished under 0.5 mm thick on human upper central incisors reached mean fracture loads of 431.8 N for lithium disilicate and 454.4 N for feldspathic porcelain, with no significant difference between the materials, though their failure modes differed.^[4] Thin ceramic bonded to sound enamel is not fragile.

Where a no-prep design does not work is equally definite, and it is worth hearing before treatment rather than after. Teeth that already sit forward will be pushed further forward by adding material to the front of them, which reads as bulky and makes the gum margin harder to clean. A large shade change needs opacity, and opacity needs thickness, so a very dark tooth and a very thin veneer are working against each other. Where the enamel has already been lost to wear or erosion, there may not be a sound surface to bond to at all, and the honest answer there is to treat the wear rather than to cover it. In those cases a minimal preparation confined to enamel is the more conservative choice, not the less, because it produces a restoration that fits the tooth and lasts.