What Makes a Dental Crown Biologically Compatible With the Gums?

When someone asks whether a crown will be kind to their gums, they are usually asking about the material. Is zirconia better than metal? Does porcelain irritate tissue?

Material is part of the answer, but it is rarely the decisive part. Gum tissue around a crown responds to four separate things, and in clinical practice the material itself is usually the one that causes the fewest problems. The other three — shape, surface finish and what gets left behind during cementation — account for most of the inflammation people experience.

The single most frequent cause of unhappy gum tissue around a crown is a crown that is too fat where it emerges from the gum.

Every tooth has an emergence profile — the shape it takes as it comes up out of the tissue. A natural tooth emerges relatively flat for the first millimetre or so before flaring outward to its full width. The gum sits against that surface and is supported by it without being pushed.

A crown that bulges outward too early does two things at once. It pushes the tissue away from the tooth, opening the space at the gum margin. And it creates an overhanging ledge underneath which a toothbrush cannot reach. The result is a sheltered plaque trap sitting directly against tissue that is already under mechanical pressure. Inflammation is the predictable outcome.

Over-contouring happens for understandable reasons. It can be a laboratory compensating for a preparation that removed too little tooth, leaving insufficient room for material of adequate thickness. It can be an attempt to close a space or improve appearance. Either way, the tissue registers it.

Under-contouring is also possible but far less troublesome. A slightly flat crown leaves a small space that is easier to clean; it may collect food but it does not inflame tissue in the same way. If a compromise has to be made, flat beats fat.

This is closely related to the question of where the edge of the crown sits, which we cover separately in our articles on crown margin placement and gum health and margin placement in healthy gum tissue.

Plaque does not adhere equally to all surfaces. There is a reasonably well-established roughness threshold, often quoted as around 0.2 micrometres of average surface roughness, below which further smoothing produces no additional benefit and above which bacterial accumulation rises sharply.

Well-glazed porcelain and highly polished zirconia both sit comfortably below that threshold. Gold, polished properly, also does. The problem arises when a glazed surface is adjusted at the chair — a bite contact ground down, a contour reduced — and then not repolished. A ground ceramic surface is considerably rougher than a glazed one, and if that adjusted area is near the gum margin it becomes a retention site.

This is why chairside adjustment should always be followed by polishing with a ceramic polishing sequence, and why a crown returned to the laboratory for re-glazing after significant adjustment is often the better answer. It is a small step that has a disproportionate effect on how the tissue behaves over the following years.

This is the factor most likely to cause trouble and the least likely to be discussed.

When a crown is cemented, excess material is squeezed out at the margin. If the margin sits above the gum, that excess is visible and straightforward to clean off. If the margin sits beneath the gum — which it often does for aesthetic reasons at front teeth — the excess is pushed down into the space between crown and tissue, where it cannot be seen.

Set cement has a rough surface and sits in exactly the location where it will hold plaque permanently. Residual cement is a recognised cause of persistent localised inflammation that does not respond to improved cleaning, because the irritant is physically embedded below the gum line.

The practical responses are to keep margins as shallow as the aesthetic situation allows, to use cements that can be cleaned away at a controlled setting stage, and to check radiographically where there is any doubt. Where inflammation persists around one crown despite good hygiene, retained cement is worth considering before assuming the patient's cleaning is at fault.

Materials do differ, but less dramatically than people expect.

Zirconia is chemically inert, takes an excellent polish and shows low plaque affinity in laboratory studies. It is generally well tolerated by tissue.

Lithium disilicate and feldspathic porcelain are similarly inert when properly glazed and have a long track record against gum tissue.

Gold and high-noble alloys are extremely well tolerated. Their reputation for gum compatibility is deserved; the objection to them is appearance, not biology.

Base metal alloys, particularly older nickel-containing formulations, are the outlier. Nickel sensitivity is not rare, and it is more common in women than men. Where a metal-ceramic crown has a base-metal substructure with an exposed collar at the margin, some patients show a localised reaction. The characteristic grey line at the gum with older metal-based crowns is usually a combination of metal showing through thin tissue and slight recession, rather than a reaction as such — a topic covered in our article on grey lines at the gum with older crowns.

The broader comparison of how contemporary ceramics behave against tissue is set out in our piece on modern ceramic crowns and gum health.

• Over-contoured crowns are the most common cause of gum inflammation around a restoration.

• Surface roughness above roughly 0.2 micrometres markedly increases plaque retention; adjusted ceramic must be repolished.

• Residual cement below the gum line causes localised inflammation that cleaning alone will not resolve.

• Most contemporary crown materials are well tolerated; base metal alloys are the main exception.

• Respecting the tissue attachment zone above the bone underpins all of the above.

There is a zone of soft tissue attachment immediately above the bone — connective tissue fibres and junctional epithelium together occupying roughly two millimetres, with a gum sulcus above that. This arrangement is consistent and the body defends it.

If a crown margin is placed into that zone, the tissue responds either by inflaming persistently or by receding until the required space is re-established. Neither outcome is desirable, and neither can be corrected by better brushing.

Where decay or fracture extends deep enough that a margin would otherwise violate this zone, the answer is either surgical crown lengthening to reposition the bone level, or orthodontic extrusion to bring the tooth up. The topic is covered in detail in our article on biological width and dental crowns, and its implant equivalent in managing biological width around implants.

A crown that the gum tolerates well usually has a margin that follows the natural scallop of the tissue rather than cutting straight across it, sits at or just below the gum crest where appearance demands it and above the crest where it does not, emerges from the tissue with a flat profile before flaring, has a smooth glazed or polished surface throughout including any adjusted areas, has clean contact points with adjacent teeth that allow floss or an interdental brush to pass without shredding, and has been cemented with all excess removed.

Signs that something is not right include tissue that stays red and puffy around one crown while the rest of the mouth is healthy, bleeding localised to that tooth, floss shredding at the contact, or a persistent odour from that site. These warrant assessment rather than more vigorous brushing. Our article on poorly designed crowns and gum inflammation covers what can be done.

If a crown is causing persistent gum irritation, or you are planning crown treatment and want to understand how the design will affect your tissue, arrange an assessment through our contact page.

You can read more on our dental crowns page, our gum disease treatment page, our receding gums page and our dental hygiene page.

This article is for general information only and does not constitute dental or medical advice. The cause of gum irritation around any particular restoration can only be determined by clinical examination and, where appropriate, radiographs. Always consult a registered dental professional about your own circumstances.

Next review due: 6 August 2027