Crown margins explained: where the crown meets your tooth, and why it matters

Look closely at a crowned tooth and you might just make out a fine line near the gum. That is the margin, the boundary where the crown stops and your own tooth carries on. Patients rarely ask about it, yet dentists and technicians spend a lot of their effort on it, because most long-term crown problems begin there. Gum inflammation and decay under a crown both tend to start at the edge. This article explains what margins are, the main designs, where they can sit against the gum, and what makes a good one.

What the margin is

To fit a crown, the dentist reduces the tooth all the way round so the crown can sit over it without looking bulky. Near the gum, the preparation ends in a defined edge called the finish line. The crown is made to end exactly on that line, and the junction between the two is the margin.

Three things meet there: the crown, a thin layer of cement, and natural tooth. If the crown fits closely and the cement layer is thin and sealed, the junction is smooth and easy to clean. If there is a gap, a step or an overhang, it becomes a ledge for plaque.

Margin designs

The shape of the finish line determines how thick the crown can be at its very edge. The common designs are:

• Chamfer: a curved, scooped edge. Widely used for all-ceramic and metal-ceramic crowns.
• Shoulder: a flat ledge at roughly a right angle to the tooth, usually with a rounded inner corner. It gives the most bulk at the edge, which suits some ceramics.
• Knife-edge or feather-edge: little or no ledge, keeping more tooth but leaving very little room for material at the edge.

Which is "best"? Less than you might think rides on this. Goodacre's 2001 review of tooth preparation concluded that finish lines should be chosen on the type of crown, aesthetics, ease of forming and experience, and that claims of better fit with particular designs could not be confirmed by research. A 2015 literature review in Clinical Oral Investigations reached a similar view: no finish line design had been proven superior for marginal accuracy, and good visibility of the margin to the technician or the intraoral scanner seemed to matter most. A 2020 systematic review of zirconia crowns found shoulder preparations gave slightly better marginal fit than chamfers, a small difference rather than a decisive one.

The material sets the minimum. Strong zirconia can be made thinner at the edge than glass-ceramic or a metal-ceramic crown that has to hide its metal. That is one reason the choice between zirconia, E-max and metal-ceramic affects how much tooth needs removing, explained on what happens to your teeth.

Where the margin sits against the gum

There are three positions:

• Supragingival: above the gumline, on visible tooth.
• Equigingival: level with the gum edge.
• Subgingival: tucked slightly under the gum.

Why above the gum is preferred when possible

The strongest long-term evidence comes from a Scandinavian study of 160 men followed for 26 years, published in 2001. Researchers compared tooth surfaces that stayed free of restoration edges near the gum with surfaces that gained a filling margin below the gum during the study. The results confirmed what dentists had long suspected: restorations placed below the gumline were detrimental to gum and periodontal health. The extra loss of attachment started slowly and could be detected one to three years after the restorations were placed.

Goodacre's review stated that a supragingival location is more acceptable, and the 2015 review recommended placing the margin above the gum whenever possible. Above the gum, you can clean the edge, your dentist can see it and check it, and the crown is less likely to irritate the gum.

When a margin goes below the gum

There are good reasons to hide a margin. On a front tooth, a visible edge can spoil the look, especially if the tooth underneath is dark. The edge may also need to go lower to cover old decay, a fracture or a large filling. In those cases the margin is usually placed only slightly under the gum, not deep.

How deep is too deep? Between the bottom of the gum pocket and the bone, the body keeps a band of soft-tissue attachment, often called the biologic width. A 2003 review by Padbury and colleagues discussed how restoration margins that invade this zone are linked to inflammation, and why surgical crown lengthening is sometimes used to move the gum and bone back before a crown is made. Our article on crown lengthening covers that procedure.

What makes a good margin

A close fit

No crown fits with zero gap; there is always a thin layer of cement. The question is how thin. A much-quoted 1971 study by McLean and von Fraunhofer in the British Dental Journal is often cited as the source of a clinically acceptable marginal gap of around 120 micrometres, a little over a tenth of a millimetre. A 2013 systematic review of ceramic crowns found that 94.9% of the marginal gaps measured across studies fell within the range of clinical acceptability, and identified four things that affect it: the finish line design, the cement space built into the design, the veneering process and cementation itself.

Smooth contours

A crown that bulges out past the tooth at the margin (an overhang) traps plaque. One that stops short leaves a step of exposed, cut tooth. Both make cleaning harder. The aim is a crown that flows into the tooth with no ledge you can catch with floss.

Clean cement

Excess cement squeezed out at the margin must be removed, particularly below the gum where it is harder to see. How crowns are fixed, and why the cement differs by material, is covered in how crowns are cemented.

Something you can keep clean

Even a perfect margin relies on you. Brushing along the gumline and cleaning between the teeth keep plaque off the junction. Our aftercare and longevity page has the routine, and regular check-ups are part of the conditions of our 5-year written guarantee.

Things that move a margin after the design

Even a well-designed margin can change between the screen and the mouth. The 2020 zirconia fit review found that veneering a zirconia framework with porcelain and the cementation step itself both increased marginal and internal gaps, while a larger cement space in the design improved how the crown seated. The 2013 ceramic review listed the same group of factors. So a crown that seats perfectly at the try-in can sit very slightly higher once cement is under it, and the cement space has to be planned to allow for that.

Gums also change. After a tooth is prepared, the gum around it can shrink back a little as it heals, and over years most people's gums recede to some degree. A margin placed just at the gum on a front tooth can become visible later. This is one reason dentists often wait for the gum to settle before final scans in the smile zone, and why it is worth knowing in advance whether an edge is likely to show on your visible teeth.

Margins and your treatment

At our clinic teeth are digitally scanned, zirconia crowns are designed on screen and milled, and fit, bite and shade are checked at a try-in before permanent fixing. The try-in is where a margin that does not seat properly gets caught. If you want to understand how your own teeth would be prepared and where the edges would sit, ask for a free written plan through our contact page.

Common questions

Why can I feel a ridge at the edge of my crown?

A slight ledge, a gap or an overhang at the margin can often be felt with a fingernail or floss. Some are harmless, but a catch that traps floss or food is worth showing your dentist, because it is harder to keep clean and can be where decay starts.

Is it better for the crown edge to be hidden under the gum?

Only when appearance or the tooth's condition requires it. Margins below the gum are harder to clean and harder to check, and long-term research links them to more gum problems. On back teeth and on less visible surfaces, edges at or above the gum are usually preferred.

Which margin shape is best?

No single design has been proven to give a better fit. A 2015 review found that how clearly the margin can be seen by the technician or scanner mattered more than the exact shape. The material decides how much thickness is needed at the edge.

Can decay start at the crown margin?

Yes. The margin is where the crown, the cement and your own tooth meet, so it is the usual place for decay under a crown to begin. Our article on decay under a crown explains why and how to prevent it.

How will I know my margins fit well?

Your dentist checks the seat of each crown at the try-in, by eye, with a probe and on X-ray where useful. At our clinic, fit, bite and shade are checked at the try-in before anything is fixed for good.

Sources

1. Goodacre CJ, Campagni WV, Aquilino SA (2001). Tooth preparations for complete crowns: an art form based on scientific principles. J Prosthet Dent
2. Podhorsky A, Rehmann P, Wöstmann B (2015). Tooth preparation for full-coverage restorations: a literature review. Clin Oral Investig
3. Schätzle M, Land NP, Anerud A et al. (2001). The influence of margins of restorations of the periodontal tissues over 26 years. J Clin Periodontol
4. Padbury A Jr, Eber R, Wang HL (2003). Interactions between the gingiva and the margin of restorations. J Clin Periodontol
5. Contrepois M, Soenen A, Bartala M, Laviole O (2013). Marginal adaptation of ceramic crowns: a systematic review. J Prosthet Dent
6. Ahmed WM, Shariati B, Gazzaz AZ, Sayed ME, Carvalho RM (2020). Fit of tooth-supported zirconia single crowns: a systematic review of the literature. Clin Exp Dent Res
7. McLean JW, von Fraunhofer JA (1971). The estimation of cement film thickness by an in vivo technique. Br Dent J

Checked October 2026. Where we give our own view as a clinic, we say so.