Why Is Biological Width Important for a Dental Crown?
Some crowns look perfectly acceptable and behave perfectly badly. The gum around them is permanently red, it bleeds every time it is touched, and no amount of improved cleaning makes any difference. Patients are often told they need to floss more carefully. Frequently that is not the problem at all.
What is usually happening is that the edge of the crown has been placed too far below the gum, into a zone of tissue the body treats as non-negotiable. The gum will not settle, because something is occupying space that biology has reserved.
This is one of the most common causes of persistent problems around otherwise well-made crowns, and it is entirely preventable at the planning stage.
What Is Biological Width?
What is the space being talked about?
Between the top of the bone crest and the bottom of the gum crevice sits a band of soft tissue attachment: a layer of connective tissue fibres attached to the root surface, and above it a junctional epithelium attached to the tooth. Together these occupy a fairly consistent dimension of roughly two millimetres, though it varies between individuals and between sites. Add the depth of the gum crevice above it, and you have around three millimetres from bone crest to gum margin. That attachment zone is what is meant by biological width, and it is the tooth's seal against the outside world.
Understanding the Anatomy
Working upwards from the bone:
1. Connective tissue attachment — collagen fibres inserting into the cementum on the root surface, running perpendicular to it. Typically around one millimetre
2. Junctional epithelium — specialised epithelium attached to the tooth surface above it. Also typically around one millimetre
3. Gingival sulcus — the shallow crevice between gum and tooth, usually one to three millimetres deep in health
The first two together form the attachment. This is the biological seal that prevents bacteria reaching bone, and it is the reason a healthy tooth can sit permanently in a bacterially contaminated environment without the bone around it dissolving.
Around an implant the arrangement differs — the fibres run parallel rather than inserting — which is why implants have a weaker seal, as covered in our article on biological changes after implant placement.
Why It Matters for Crowns
A crown needs a margin — the line where the restoration ends and the tooth begins. Where that margin sits relative to the gum is a deliberate decision.
Above the gum (supragingival). Easiest to clean, easiest to inspect, easiest to take an accurate impression of, and kindest to the tissue. It is also visible, which is why it is often avoided at the front.
At the gum margin (equigingival). A compromise.
Below the gum (subgingival). Used for aesthetics, or where decay or a fracture extends below the gum. Acceptable provided the margin stays within the sulcus and does not encroach on the attachment beneath it.
The problem arises when a margin is placed deeper than the sulcus, into the attachment zone. The body responds in one of two ways, and neither is desirable:
Chronic inflammation. The tissue remains permanently inflamed, bleeding readily and never settling, because a foreign surface is sitting where the attachment should be.
Bone recession. The bone crest resorbs downwards to recreate the required attachment dimension below the margin. The gum recedes with it, and the crown margin ends up exposed — often with a visible line, particularly on a metal-backed crown.
Which of the two happens depends partly on gum thickness. Thin, scalloped tissue tends to recede; thick, flat tissue tends to become chronically inflamed.
Consequences of Getting It Wrong
• Persistent bleeding gums around the crown that do not respond to improved cleaning
• Swollen gums and redness confined to one or two teeth
• Discomfort when brushing or flossing that area
• Progressive bone loss localised around the crowned tooth
• Receding gums, exposing the margin and spoiling the appearance
• Decay at the exposed margin, which is often on root surface and therefore progresses quickly
• Eventual need to remake the crown, sometimes with additional surgical treatment first
A useful clinical clue: inflammation localised to one or two crowned teeth while the rest of the mouth is healthy points towards a local cause rather than a hygiene failure. Generalised inflammation across the mouth is a different matter and suggests gum disease.
Assessment and Planning
Before crown preparation, this should be evaluated:
Bone sounding. Under local anaesthetic, a probe is passed through the attachment to the bone crest to measure the distance from the proposed margin to bone. This is the definitive way to know whether there is adequate space.
Radiographs to assess bone levels, though they show interproximal bone better than the surfaces facing the cheek and tongue.
Assessment of gum thickness (biotype), because thin tissue behaves differently from thick.
Where the margin actually needs to be. If aesthetics do not demand it, a margin at or above the gum is usually the better clinical choice.
Whether crown lengthening is needed. Where decay or fracture extends too far below the gum, the position of the bone can be altered surgically to re-establish adequate space above it. Healing then takes some weeks before the definitive crown is made — proceeding too early risks recession afterwards.
Whether orthodontic extrusion is preferable. Gently moving the tooth coronally brings the margin above bone level without removing bone from neighbouring teeth. It takes longer but is more conservative in the right case.
Whether the tooth is restorable at all. Sometimes the honest answer is that it is not, and extraction with implant or bridge replacement is the more sensible plan than a crown with a poor prognosis.
When Professional Dental Assessment May Be Needed
Arrange an assessment if:
• The gum around a crown bleeds persistently despite good cleaning
• Tissue around one or two teeth is red and swollen while the rest is healthy
• The gum has receded around a crown, exposing a margin or a dark line
• A crowned tooth is tender to clean around
• You have a lost crown or filling
• A crown is being planned and decay extends below the gum
• You are unhappy with the appearance of an existing crown at the gum line
If a crown is being planned for you, it is entirely reasonable to ask where the margin will sit and why.
Maintaining Crowned Teeth
• Brush twice daily with a soft brush, angling bristles towards the gum line
• Clean between the teeth daily with floss or correctly sized interdental brushes
• Attend hygiene appointments at the recommended interval so pocket depths and bleeding are monitored
• Keep routine check-ups with radiographs so bone levels around crowns are compared over time
• Report persistent bleeding around a single crown rather than assuming it is your technique
• Wear a night guard if you grind
• Ask your hygienist to check specifically around each crown margin
Our dental crowns page explains what crown treatment involves.
Key Points to Remember
• Biological width is the soft tissue attachment between bone crest and gum crevice
• It occupies a fairly consistent dimension that the body defends
• A crown margin encroaching on it causes chronic inflammation or bone recession
• Which of the two occurs depends partly on gum thickness
• Margins above or at the gum are kinder and easier to maintain than deep ones
• Bone sounding before preparation establishes whether there is adequate space
• Crown lengthening or orthodontic extrusion can create space where it is lacking
• Persistent bleeding around a single crown is a clinical sign, not a cleaning failure
The NHS page on crowns explains what crowns are used for and what treatment involves.
Frequently Asked Questions
1. How do I know if my crown has violated the biological width?
You cannot tell for certain yourself, but the pattern is suggestive: persistent redness, swelling and bleeding confined to one or two crowned teeth, unresponsive to improved cleaning, sometimes with recession or discomfort. Diagnosis requires a clinical examination including probing and often bone sounding, along with radiographs to assess bone levels.
2. Can it be corrected once a crown is in place?
Often, yes, though it usually means remaking the crown. The options are surgical crown lengthening to reposition the bone, orthodontic extrusion to move the tooth, or in some cases simply remaking the crown with a shallower margin. Which is appropriate depends on how much space is lacking, how much tooth structure remains and the effect on neighbouring teeth.
3. What is crown lengthening?
A surgical procedure that repositions the gum and bone apically to expose more tooth structure and re-establish adequate space above the bone. It is performed under local anaesthetic, and healing takes some weeks before the definitive crown is made. It affects the gum contour of the treated tooth and sometimes its neighbours, so aesthetic implications are discussed beforehand.
4. Should crown margins always be above the gum?
Ideally, from a biological standpoint — they are easier to clean, inspect and record accurately. In practice, aesthetics at the front and decay or fractures extending below the gum often make a subgingival margin necessary. What matters is that it stays within the sulcus rather than encroaching on the attachment beneath.
5. Why does my gum bleed around one crown but nowhere else?
Localised inflammation around a single restored tooth points to a local cause rather than a general hygiene problem. Possibilities include a margin encroaching on the attachment, an overhang of excess material, a poorly contoured crown creating a plaque trap, an open contact packing food, or decay at the margin. It should be examined rather than attributed to cleaning.
6. Does this apply to veneers and fillings too?
Yes. The principle applies to any restoration margin. Deep subgingival fillings, veneer margins placed too far below the gum, and overhanging restorations can all cause the same response. It is most often discussed in relation to crowns because crown margins are more frequently placed subgingivally for aesthetic reasons.
Conclusion
If the gum around one crown has never settled, the problem may not be how you clean. It may be where the crown ends.
This is diagnosable and usually correctable, but it requires someone to consider it rather than repeating hygiene advice. If you have a crown that has been inflamed for years, that is worth asking about specifically.
To have a crown assessed, arrange an appointment at 22 Wimpole St, London W1G 8GQ, or call 020 7183 0692.
Dental symptoms and treatment options should always be assessed individually during a clinical examination.
Dental Disclaimer
This article is for general educational information only and is not professional dental advice, diagnosis, or a treatment recommendation. Information here is general and cannot replace an in-person assessment by a qualified, GDC-registered dental professional. Symptoms, suitability, fees, timelines, and outcomes vary according to individual clinical circumstances. If you have pain, swelling, or other concerning symptoms, seek prompt professional dental care. Always request a written treatment plan and cost estimate before proceeding with treatment.
Written Date: 3 September 2026 Next Review Date: 3 September 2027
Written by Dr Narges Ameri · reviewed by Dr Narges Ameri, GDC 325081
This article is general information, not personal clinical advice. For a diagnosis and a plan tailored to you, book a consultation with a GDC-registered dentist.
