The answer first
Short answer: Give each material the room it needs: typically about 1.5–2.0 mm occlusal for e.max, about 1.0–1.5 mm for full-contour and multilayer zirconia (some high-strength brands allow less), an extra 1–2 mm for layered zirconia, and about 0.3–0.9 mm, mostly in enamel, for veneers. Use a smooth, continuous chamfer or rounded shoulder with no sharp internal angles. Then send a scan or impression where every millimeter of margin is visible.
Most remakes we see aren't material failures. They start at the prep or the record. Here's what we look for when your case reaches the bench.
How much reduction does each material need?
These are typical ranges from published manufacturer recommendations. Product-specific minimums differ, so treat them as a starting point and check the IFU for the exact material on your Rx. For diagrams and printable PDFs, see our prep guides.
| Restoration | Occlusal / incisal | Axial | Margin |
|---|---|---|---|
| IPS e.max crown | ~1.5–2.0 mm | ~1.2–1.5 mm anterior, ~1.5 mm posterior | ≥1.0 mm rounded shoulder or deep chamfer |
| Full-contour (monolithic) zirconia | ~1.0–1.5 mm (some high-strength 3Y brands allow 0.5 mm where space is limited) | ~0.8–1.0 mm anterior, ~1.0–1.5 mm posterior | Chamfer or rounded shoulder |
| Multilayer zirconia | ~1.0–1.5 mm (min. ~0.8 mm anterior, ~1.0 mm posterior) | ~0.8–1.0 mm anterior, ~1.0–1.5 mm posterior | ~0.8–1.0 mm chamfer or rounded shoulder |
| Layered (veneered) zirconia | Monolithic figures plus ~1–2 mm for the porcelain | ~1.5 mm or more | ~1.0 mm chamfer or rounded shoulder |
| Porcelain veneer (e.max Press or feldspathic) | ~1.0–1.5 mm incisal, if the edge is covered | ~0.3–0.5 mm cervical, ~0.5–0.7 mm middle, ~0.7–0.9 mm incisal third (e.max: at least 0.6 mm cervical) | Light chamfer, in enamel |
Follow the manufacturer's instructions for use for the specific material. They set the minimum wall thickness, connector size and cementation requirements.
e.max crowns
Lithium disilicate gets its clinical strength from thickness and bonding. A typical prep has:
- uniform reduction that follows the anatomy, not a flat table
- rounded line angles
- a total taper of roughly 4–8° when you cement conventionally
- enough clearance to keep the crown at full thickness everywhere, including the central fossa and functional cusps
Full-contour zirconia
This is the material for limited clearance, but "minimal" isn't "none." High-translucency (4Y/5Y) zirconia needs more thickness than high-strength 3Y: typically at least 0.8 mm anterior and 1.0 mm posterior. If you want the esthetics of a translucent disc, give it the room of a translucent disc. Some brands allow very thin or feather-edge margins. A defined light chamfer still gives us a clearer finish line to design to.
Layered zirconia
Prep it like a PFM. The core needs its minimum, and the porcelain needs about 1–2 mm on top of that. Under-reduction here is what causes chipping and bulky, over-contoured crowns.
Veneers
Let the final result drive the reduction:
- Start from a wax-up or mock-up. Prep through it with depth-cutting burs, so you remove only what the final contour needs rather than measuring off a tooth you're about to change.
- Keep the prep in enamel wherever possible. Bond strength to enamel is the foundation of veneer longevity.
- Choose an incisal design that fits the case: a butt joint or incisal overlap when you're lengthening or covering the edge, a window prep when you aren't.
- Decide on contacts deliberately. Break them when you're changing shade or closing space, so the margin isn't visible from the side.
What margin design should you use?
- e.max: a rounded shoulder or deep chamfer of at least 1.0 mm. No bevels or feather edges.
- Zirconia: a chamfer or rounded shoulder of about 0.8–1.0 mm. A defined finish line beats a vague one. (Our Zerapure™ multilayer zirconia also suits knife-edge and shoulder-bevel preps.)
- Veneers: a light chamfer in enamel.
Whatever the material:
- Make it smooth and continuous. Ditching, steps and ragged enamel are what the software and the die have to guess at.
- Use rounded internal angles. Sharp internal line angles concentrate stress in ceramics and are hard to mill accurately.
- Avoid lips, J-margins and unsupported enamel at the edge of a shoulder. They fracture, and the crown then looks open.
- Keep margins supragingival or equigingival where esthetics allow, especially for bonded restorations, where isolation determines bond quality.
- Make margins capturable. A subgingival margin you can't retract and scan clearly is a margin we have to estimate.
Which prep mistakes cause the most remakes?
These are some of the most common causes of fit and esthetic problems, in no particular order:
- Insufficient occlusal clearance, especially on functional cusps and in excursions, not just in centric. Check clearance in the bite scan before you send.
- An unreadable margin: blood, crevicular fluid, tissue collapse or a scan that stops short. We can't design to what we can't see.
- Undercuts or over-taper. Undercuts block seating. Excess taper on a short prep loses retention, especially with conventional cement.
- Sharp line angles and an uneven floor, which force thin spots into the restoration.
- Bite problems: a distorted or open bite registration, a missing opposing arch, or a partial bite scan that doesn't capture both sides.
- No stump shade on e.max cases. With a translucent material, the stump is part of the final color.
- A material and prep mismatch, such as e.max on Rx with zirconia-level reduction.
- No reference for anterior esthetics: missing photos, shade tab or pre-op model, or no note on what the patient wants changed.
What should you check before sending a scan or impression?
Digital scans
- Prep scanned after hemostasis and retraction, with the margin visible 360°
- Opposing arch scanned completely
- Bite scan on both sides, in maximum intercuspation, not propped open by retraction cord or cheeks
- Clearance checked in your scanner software before you send
- Adjacent teeth and contacts fully captured
- Pre-op scan or wax-up/mock-up scan included for anterior and veneer cases
- Stump shade noted for e.max and translucent zirconia
- Shade photo with the tab in the same plane as the tooth, plus a polarized photo if you have one
- Rx complete: tooth numbers, material, shade, margin notes, requested date
- Sent through your scanner's lab connection. We accept iTero (Lab ID 4364), 3Shape, Medit, Shining 3D, Straumann AXS, Dexis and DS Core; other systems can link to us through info@adlny.com.
Conventional impressions
- Margin captured in full with no voids, drags or tears at the finish line
- Tray rigid and fully seated, with no tray contact on the preps
- Impression disinfected per your protocol and labeled
- Opposing model and bite registration that seats without rocking
- Rx form and shade information included. Download our Rx form.
How can the lab help before you prep?
For complex anterior cases, talk to us first. We make diagnostic wax-ups and mock-ups, which turn "how much do I take off" into a measurable answer. For difficult shades, our technicians can take a custom shade at our Manhattan lab, or at your office in Manhattan. We can also review a design with you remotely before we mill.
FAQ
How much occlusal reduction does an e.max crown need?
Can I use a feather-edge margin for zirconia?
How thin can a porcelain veneer be?
What's the most common reason a crown doesn't fit?
Should I send a stump shade?
Want a second set of eyes on a prep? Send the scan before you dismiss the patient. We offer same-day review of prep scans. If there's a clearance or margin issue, it's much easier to fix that day than at the seat appointment. Send a case, or read more about IPS e.max, zirconia crowns and veneers, or open our prep guides.
Written by Advanced Dental Laboratory, 274 Madison Avenue, New York. General guidance for dental professionals; always follow the manufacturer’s instructions for use.