Margin Placement for Healthy Gum Tissue Around Crowns

Where the edge of a crown finishes relative to the gum is decided before the tooth is prepared, and once the crown is made it cannot be changed without remaking it. It is one of the less visible decisions in restorative dentistry and one of the most consequential.

There are three options, and each involves a genuine trade-off between cleanability and appearance. Understanding the trade-off explains a good deal about why some crowned teeth have persistently irritated gums and others do not.

For the broader relationship between crowns and gum health, our article on how crown margin placement affects gum health covers the general picture, and marginal fit covers the closely related question of how accurately the margin adapts.

Supragingival — above the gum. The margin finishes on tooth structure that is entirely visible and above the gum line.

Equigingival — level with the gum. The margin sits at the gum crest.

Subgingival — beneath the gum. The margin finishes inside the gum crevice, hidden from view.

From a purely biological standpoint, supragingival is the preferred position, and by a considerable margin.

You can clean it. A margin you can see and reach with a brush and floss is a margin that stays free of plaque. A subgingival margin is, by definition, in a place you cannot reach.

The dentist can see it. During preparation, during impression taking, during cementation, and at every subsequent check-up. Excess cement can be seen and removed. Recurrent decay can be detected early. None of this is reliably true beneath the gum.

It is easier to record accurately. Impression material and intraoral scanners both need to see the margin. Where it is subgingival, the tissue must be displaced — with cord, paste or occasionally a small amount of soft tissue removal — to expose it, and every one of those steps introduces the possibility of error and of trauma to the tissue.

It is nowhere near the biological width. This is the crucial point. Around a natural tooth there is a band of soft tissue attachment — roughly two millimetres — between the base of the gum crevice and the crest of the bone. The body defends this dimension. A restoration margin that encroaches on it produces persistent inflammation and, over time, the bone recedes until the required space is re-established. The result is chronic redness, bleeding, a gum that never quite settles, and sometimes recession that exposes the margin anyway. Our article on biological width and crowns explains this in detail.

It preserves tooth structure. Preparing further down the tooth removes more of it.

If supragingival is better, why is it not always used? Because there are situations where it cannot be.

Aesthetics. On a front tooth, the junction between crown and tooth is visible if it sits above the gum. Even a well-matched ceramic has an edge, and where the underlying tooth is dark or a metal substructure is used, a visible margin can produce a grey line. Our article on grey lines at the gum with older crowns covers this.

Existing decay or a previous restoration extending subgingivally. The margin has to be placed on sound tooth beyond the defect. There is no choice here.

Retention. A short clinical crown provides limited surface for the cement to grip. Extending the preparation apically increases the available height.

Root sensitivity or existing exposed root surface, where covering the area is part of the plan.

The reasonable compromise, widely used, is to place margins subgingivally only where they show — the front surfaces of visible teeth — and keep them at or above the gum everywhere else, including the palatal and interproximal surfaces of the same tooth. A margin does not have to sit at one level all the way round.

Where a margin must go below the gum, the depth matters enormously. The working principle is that the margin should stay within the gum crevice and not intrude on the attachment beneath it — generally no more than about half a millimetre into the crevice, and crucially remaining a safe distance from the bone crest.

When there is not enough tooth above the bone to allow this, forcing the margin deeper is the wrong answer. The alternatives are:

Crown lengthening, a surgical procedure removing a small amount of gum and bone to expose more tooth and re-establish the required dimension. It adds a healing period of several weeks before the crown can be made, which is why it is planned rather than improvised.

Orthodontic extrusion, gently moving the tooth coronally to bring the margin into a better position.

Reconsidering whether the tooth is restorable. Sometimes the honest answer. Crown versus extraction covers that decision.

Margin position gets the attention, but the shape of the crown as it emerges from the gum matters just as much.

An over-contoured crown — bulging outwards near the gum — creates a ledge that traps plaque and holds the gum away from the tooth. This is a common cause of persistent inflammation around an otherwise well-made crown, and it is not fixed by better brushing, because the shape prevents effective cleaning.

An under-contoured crown leaves the gum unsupported, which can result in food packing and a loss of papilla shape.

An overhang — crown material extending beyond the prepared tooth at the margin — is worse than either, functioning as a permanent plaque trap and a reliable cause of localised gum disease and bone loss.

Our article on veneer edge design and gum health covers the same principle in a different context, and gum irritation from veneers is closely related.

Excess cement that escapes into the gum crevice and is not removed acts as a foreign body and a plaque retainer. It is a well-documented cause of localised inflammation and bone loss.

Where the margin is visible, excess cement is seen and removed. Where it is deep, it can be missed, and the resulting inflammation may not be attributed to the right cause for years. This is a substantial part of the argument for keeping margins as shallow as the situation allows.

Around a well-made and well-placed crown margin, the gum should look and behave exactly like the gum around your other teeth — firm, pale pink, not bleeding when cleaned.

Worth reporting:

• Bleeding when you brush or floss around one crowned tooth in particular.

• Gum that stays red and puffy around a crown while the rest is healthy.

• Floss that shreds or catches at the margin.

• Food consistently packing at that tooth.

• A dark line appearing at the gum, which may indicate recession exposing the margin.

• Persistent tenderness or a bad taste localised to that tooth.

• Recession that is progressing.

Localised inflammation around a single crowned tooth is not something to manage with more brushing. It usually has a structural cause, and that cause is worth identifying.

Our articles on why gums bleed when brushing and on how modern ceramic crowns interact with gum tissue cover related ground.

Is it bad if my crown margin is under the gum?

Not necessarily. A margin placed shallowly within the crevice, well clear of the attachment, and with a well-contoured crown over it, can be perfectly healthy. Problems arise when it is placed too deep or the contour is wrong.

Why do my gums bleed around one crown but nowhere else?

Localised inflammation around a single restored tooth usually has a local cause — an overhang, an over-contoured crown, retained cement, a margin encroaching on the attachment, or recurrent decay. It is worth assessing rather than attributing to technique.

Can a crown margin be corrected without replacing the crown?

Sometimes. A small overhang can occasionally be smoothed, and contour can be adjusted marginally. A margin that is fundamentally in the wrong position generally requires the crown to be remade.

What is crown lengthening and why might I need it?

It is a minor surgical procedure that exposes more tooth by reducing gum and bone slightly, used where there is not enough sound tooth above the bone to place a margin without encroaching on the attachment.

Will a dark line always appear at the gum eventually?

No. Dark lines are most associated with older metal-ceramic crowns combined with gum recession. All-ceramic crowns do not produce the same effect, though recession can still expose a margin.

If the gum around a crowned tooth has been persistently red, tender or prone to bleeding, that is worth assessing — the cause is usually structural rather than a matter of cleaning harder.

You can contact our team at our Wimpole Street practice. Our dental crowns page explains how crown treatment is planned here.

This article provides general information about crown margin placement and does not constitute individual dental advice. The cause of gum inflammation around a restoration can only be determined by clinical examination and appropriate radiographs. Wimpole Dental is regulated by the Care Quality Commission, and our clinicians are registered with the General Dental Council.

Next review due: 5 September 2027