Veneer and Onlay Preparation — When Less Means More

A veneer is not a small crown. An onlay is not an intermediate solution between a filling and a crown. Each of these restorations requires a different approach to preparation — less invasiveness, but greater precision. Errors in tooth preparation lead to material fractures and restarting the problem.

Why Veneer Preparation Differs from Crown Preparation

A veneer covers only the labial aspect of the tooth (and part of the incisal surface). It does not encircle the entire structure. This means one thing: you don't reduce as much. Facial reduction: 0.5–0.7 mm — significantly less than for crowns (1–1.5 mm). Incisal margin: Do NOT create a "shoulder" or "window prep." The margin can be:

On enamel (ideally) — with no visible line under the microscope
At the cemental border (acceptable) — but clearly defined

Gingival margin: A gradual, distinct margin for the technician, but gentle for the periodontium. The patient should not feel the edge with their tongue. Why this difference matters: Less reduction = a more conservative approach. A veneer saves tooth structure, so aggressive milling is not required.

Reduction and Margin — Minimalist Principles

Parameter	Veneer	Crown
Facial Reduction	0.5–0.7 mm	1.0–1.5 mm
Incisal Margin	On enamel (or cement)	Always clearly defined
Gingival Margin	Chamfer	Chamfer or Chamfer with a wall
Angle Rounding	Mandatory	Recommended

Incisal Margin: If you must extend into dentin, do so subtly and always inform the laboratory: "Margin partially on dentin, be mindful of sensitivity." Rounding: Sharp internal angles guarantee fracture. All transitions must be rounded (R ≥ 0.5 mm).

Onlay — Rounded Angles and Wings

An onlay is a hybrid between a filling and a crown. It covers working cusps (usually at least one) and can extend onto mesial and distal surfaces. Preparation geometry:

No vertical walls — rather gentle transitions at an angle of 6–10°.
Rounded internal angles (very important — these are stress concentration points).
Depth: 1.5–2.0 mm in the cusp zone, 0.5–1.0 mm at transitions.

Wings: Wings are extensions of the onlay onto the cusp ridges, where the onlay "embraces" the functional surface. This should not be a wide slab — it should be a delicate, rounded connection. Wings reduce the risk of chipping and improve retention. Not all onlays require them — it depends on the height of the cusps and the tooth's geometry. Geometry errors:

Sharp internal angles = ceramic fractures
Too much reduction = unnecessarily invasive and less retentive
Enamel margin in unprepared areas = leakage

Most Common Errors and How to Avoid Them

Error	Consequence	How to Avoid
Treating a veneer like a small crown — excessive reduction	Unnecessary destruction of tooth structure, risk of pulp exposure	Remember: 0.5–0.7 mm is sufficient
Sharp internal angles (especially in onlays)	Guaranteed ceramic fracture under occlusal load	Round all transitions (R ≥ 0.5 mm)
Lack of clear margin definition	Technician does not know where the preparation ends	Use a marker and describe clearly in the prescription
Gingival margin in dentin without prior notification	Post-preparation sensitivity, aesthetic issues	If extending into dentin, write: "Margin on dentin"
Window prep for veneers	Risk of wall fractures, unaesthetic	You don't need a step — the margin can be on enamel

Communication with the laboratory — what to write in the Rx

Clear communication reduces the risk of misunderstandings: For veneers:

"Ceramic veneer, enamel margin"
"Bruxism patient — consider increased thickness"
"Margin extends into dentin — be mindful of sensitivity"

For onlays:

"Onlay, wings on unmodified cusps (e.g., 1, 2, 3)"
"Patient has bruxism — increased thickness"
"High cusps — wings mandatory"

Universal:

Always specify: "Veneer" or "Onlay" — not "small crown"
If the patient is a bruxer — always mention it