Does a Crown Go Below the Gum Line?

A dental crown can extend below the gum line, and in many cases it does, but whether it should depends on why the crown is being placed and how much healthy tooth structure remains above the gums. Dentists call the lower edge of a crown its “margin,” and that margin can sit above the gum line (supragingival), right at the gum line (equigingival), or tucked underneath it (subgingival). The research consistently shows that subgingival margins carry more risk to your gum health than margins placed at or above the gum line, yet certain clinical situations leave your dentist with no choice but to go below.

Why a Crown Might Need to Go Below the Gum Line

In a perfect world, your dentist would keep every crown margin visible and accessible, sitting right at or above your gum tissue. That makes the crown easier to fabricate, easier to fit precisely, easier to clean, and gentler on your gums. A literature review on tooth preparation for full-coverage restorations concluded that a supragingival position should be preferred whenever possible, precisely because the gum tissues can be affected by the restoration margin.

But teeth rarely break or decay in convenient places. Several situations push the margin below the gum line:

• Deep decay or fracture: If the tooth is damaged well below the gum line, the crown margin has to reach down far enough to sit on solid, healthy tooth structure. There is simply nothing sound to grab onto higher up.
• Old fillings: A large existing restoration that already extends subgingivally may force the new crown to follow suit.
• Cosmetics: On front teeth, a visible line where the crown meets the tooth can look unnatural. Placing the margin just under the gum tissue hides that junction.
• Retention: When a tooth is short or heavily worn, dipping below the gum provides extra surface area for the crown to hold onto.

The cosmetic reason is probably the most common one patients encounter. If you are getting a crown on a front tooth made of porcelain or ceramic, your dentist may tuck the margin about half a millimeter below the gum to keep the transition invisible when you smile.

How Deep Is Safe

There is a critical zone of soft tissue and bone attachment that surrounds every tooth, sometimes called the biological width. This is the body’s natural seal between the tooth root and the outside world, consisting of the tissue that attaches the gum to the tooth and the connective tissue fibers anchoring down to the bone. Violating this zone by pushing a crown margin too deep triggers the body’s defense response, and that response is not subtle: it can cause chronic gum inflammation, loss of attachment, and even bone loss around the tooth.

A comprehensive review of the biological width found that a crown’s edge should not be placed any closer than about 2.5 millimeters to the underlying bone. In patients with a healthy, normal gum crest, a margin positioned about half a millimeter below the gum line tends to be well tolerated by the gum tissues and can remain stable over time.¹ Go much deeper than that without accounting for the biological width, and you are asking for trouble.²

This is why your dentist may measure the distance from the gum line to the bone, sometimes by gently probing the area while you are numb. That measurement tells them how much room they have to work with before they encroach on the biological width. If the math does not add up, a different approach is needed.

What the Research Shows About Gum Health

The evidence on subgingival crown margins and gum health is fairly consistent, and it is not flattering to subgingival placement. A systematic review and meta-analysis comparing subgingival to supragingival margins found that subgingival margins were associated with deeper probing depths (about 0.57 mm deeper on average) and significantly higher gingival inflammation scores, even though plaque levels were not meaningfully different between the two groups.³ In other words, the gums around subgingival margins were more inflamed and the pockets around them were deeper, regardless of how well patients cleaned them.

An earlier study looking specifically at patients who received regular preventive dental care found the same pattern. Even among people who showed up faithfully for their cleanings, subgingival cast restoration margins were associated with significantly greater gingival inflammation and deeper probing depths on nearly every tooth surface examined.⁴ That finding matters because it undercuts the assumption that good hygiene alone can compensate for a subgingival margin. It helps, certainly, but it does not eliminate the added risk.

Another study focusing on posterior teeth found that the risk of bleeding at subgingival crown margins was roughly double the risk at supragingival margins. Poor oral hygiene before treatment and plaque accumulation also predicted bleeding, and the tongue side of the tooth was more vulnerable than the cheek side.⁵

The practical takeaway is straightforward: if your dentist can avoid going below the gum line, the evidence favors staying above it. But when they cannot, a shallow subgingival margin (roughly half a millimeter deep) with a well-designed contour is the safer compromise.

The Shape of the Margin Matters Too

How the dentist shapes the lower edge of the crown preparation makes a difference in how the gums respond over time. The most common margin designs are the chamfer (a curved ledge), the shoulder (a flat shelf), and the feather edge (a very thin, tapered finish). A 12-month clinical trial comparing feather-edge and chamfer designs in the subgingival zone found a statistically significant difference between the two in terms of bleeding on probing and gum recession. The preparation geometry affects how the crown emerges from the gum tissue, how smoothly it transitions from tooth to restoration, and how easy it is to keep clean.

Clinical investigations with follow-ups of two to three years showed that subgingival chamfer or shoulder margins were associated with higher plaque scores and bleeding on probing rates above 25 percent. In contrast, a newer technique called BOPT (biologically oriented preparation technique), which creates a smooth, cuff-like transition without a sharp ledge, showed significantly lower bleeding scores (under 10 percent) and reduced plaque accumulation over similar time periods.⁶ This is still an evolving area, and BOPT requires skill and experience, but it suggests that the old default of a chamfer finish line is not necessarily the final word.

When the Tooth Is Too Short for a Crown

Sometimes the problem is not just that decay goes below the gum line, but that there is not enough tooth left above it to support a crown at all. This is where crown lengthening comes in. Crown lengthening is a surgical procedure that exposes more tooth by repositioning the gum tissue further down the root, with or without removing some of the supporting bone.⁷ The goal is to create enough exposed tooth for the crown to attach to while respecting the biological width.

In crown lengthening, bone reduction is often necessary to provide adequate distance from the bone crest to where the crown margin will sit.⁸ This means moving the whole attachment apparatus further down the root so the dentist can place the crown margin in a healthy zone. It is not a minor procedure, and it requires healing time before the final crown can be placed, often several weeks to a few months. But for teeth that would otherwise be extracted, crown lengthening can save the day.

An alternative for certain situations is deep margin elevation, a technique where the dentist uses a bonded restoration (typically composite resin) to raise a very deep cavity margin upward, closer to the gum line or above it, before placing the final crown. This converts a deep subgingival margin into one that is easier to manage.⁹ It avoids surgery, which appeals to both patients and clinicians, though it requires careful technique and the right conditions to work reliably.

The Scanning and Impression Challenge

A subgingival margin creates a practical headache beyond gum health: capturing it accurately. Whether your dentist is taking a traditional impression with putty-like material or using a digital intraoral scanner, the margin needs to be visible and accessible to get a good recording of the tooth’s shape. If the scanner or impression material cannot reach the margin clearly, the lab cannot build a crown that fits precisely.

To temporarily push the gum tissue away from a subgingival margin, dentists use retraction methods, which include mechanical techniques like placing a thin cord into the gum pocket, chemical agents that shrink the tissue, or combinations of both.¹⁰ This step is not just a convenience; without it, the crown may have a poor fit at the margin, leaving a gap where bacteria and debris can accumulate.

Digital scanners, which are increasingly common, struggle as the margin goes deeper. A study comparing two popular intraoral scanners at different subgingival depths found that accuracy dropped significantly as the margin went deeper. Without a retraction cord, both scanners exceeded 100 micrometers of error once the margin was deeper than 0.5 millimeters, and their worst accuracy occurred at 1 millimeter of depth, with errors of roughly 228 and 256 micrometers respectively.¹¹ For context, the best-fitting crowns have margins within about 50 to 100 micrometers, so errors at that level can meaningfully affect the outcome. The deeper the margin, the harder it is to get an accurate crown, which is yet another reason to keep it shallow when possible.

Microleakage and the Risk of New Decay

No crown margin seals perfectly. There is always a microscopic gap between the crown and the tooth, and over years of exposure to saliva, food, and bacteria, that gap can allow seepage. Researchers call this microleakage, and it is one of the main reasons crowns eventually fail. A study comparing different materials used to repair or seal the crown-tooth junction found that all restorations showed some degree of microleakage at both the crown-restoration and root-restoration interfaces.¹²

When the margin sits below the gum, any gap or imperfection is harder to detect and harder to clean. You cannot see it, your toothbrush cannot reach it effectively, and even your dentist may have trouble spotting early decay there during a routine exam. This is part of why subgingival margins have historically been associated with plaque retention. As one older but still-cited paper noted, subgingival restorations retain plaque that can fuel destructive periodontal disease, and in many cases interdental brushes are recommended to help manage the problem.¹³

The Extra Risks Around Implant Crowns

Most of this discussion applies to crowns on natural teeth, but crowns on dental implants face an additional concern: excess cement. Implant crowns are often cemented onto an abutment that screws into the implant, and because the margin of that crown frequently sits below the gum line, any cement that squeezes out during placement can become trapped in the gum tissue. Unlike around natural teeth, where the gum attaches to the root in a relatively tight seal, the tissue around an implant is less robust at defending itself against foreign material.

A clinical study using endoscopes to look beneath the gum around implants found that residual cement appeared capable of inducing inflammation and tissue destruction, leading to conditions ranging from mild inflammation to serious peri-implant bone loss.¹⁴ The risks of undetected residual cement around implant crowns have been well documented, and reducing the volume of excess material that gets extruded during cementation is a key part of managing them.¹⁵

This is one reason screw-retained implant crowns, which do not need cement at all, have gained popularity, especially when the margin would sit deep below the gum. If a cemented implant crown is chosen, techniques like cementing the crown onto the abutment outside the mouth and then screwing the assembly in can significantly reduce the amount of cement that ends up where it should not be.

What You Can Do About It

If your dentist recommends a crown, it is worth asking where the margin will be placed and why. Here are a few things to keep in mind:

• Ask about margin placement: If the margin can stay above or at the gum line without compromising the crown’s hold or appearance, that is generally the safer choice for your gums.
• Understand the trade-off for front teeth: On visible teeth, a slightly subgingival margin (about half a millimeter) is a reasonable compromise for cosmetics. Deeper than that should have a clear clinical justification.
• Use interdental brushes or floss threaders: If you have subgingival crown margins, standard flossing may not be enough. Interdental brushes can reach areas that string floss misses, particularly on back teeth where the margin wraps around toward the tongue.
• Keep up with professional cleanings: Even diligent home care does not fully neutralize the extra inflammation risk of subgingival margins. Regular professional cleanings give your hygienist a chance to debride areas you cannot reach and to check for early signs of trouble.
• Watch for warning signs: Persistent bleeding around a crowned tooth, a bad taste or smell coming from the area, or gum that looks redder or puffier around the crown than around neighboring teeth are all worth mentioning at your next appointment.

Teeth That Naturally Sit Below the Gum Line

It is worth distinguishing the clinical margin question from a different scenario that sometimes confuses patients: teeth that are partially or fully trapped below the gum. Unerupted or impacted teeth, like wisdom teeth, sit beneath the gum because they never fully emerged. They do not involve crowns in the restorative sense. When patients hear “the crown is below the gum line,” they sometimes picture the entire visible part of the tooth buried under gum tissue. In dental restoration, the phrase is much more specific. It refers only to the bottom edge of an artificial cap, and typically by less than a millimeter. The rest of the crown sits above the gum and functions like any other visible tooth surface.

Similarly, if you have a tooth that has broken off at the gum line, the remaining root is below the gum, but whether it can receive a crown depends on how much sound structure remains and whether procedures like crown lengthening can expose enough of it. Some roots are simply too short or too compromised, and extraction with an implant or bridge becomes the better path.