Crown, Onlay or Filling?

Crown, Onlay or Filling: What Decides It?

The short answer is how much sound tooth is left, and where it is. A filling replaces a missing piece of tooth from the inside and relies on the walls around it for strength. An onlay is a precisely made piece of ceramic bonded over the top of the tooth to cover and protect one or more of its cusps. A crown covers the whole visible part of the tooth.

The more tooth that has been lost to decay, old fillings or fracture, the more the remaining walls flex when you chew, and the more a restoration needs to hold the tooth together rather than simply fill a gap.

Three other things can shift the decision:

• Cracks, which can make a tooth with a modest cavity behave like a weak one
• Root canal treatment, which changes how a back tooth should be protected
• Your bite, particularly if you clench or grind

At Comely Bank Dental we check all of these before recommending anything, and explain our reasoning.

How Do the Three Options Compare?

Two teeth with the same size of cavity can still need different treatment because of a crack, a heavy bite or the health of the nerve, so the decision is always made tooth by tooth.

How Much Tooth Is Left?

Back teeth have raised points on the biting surface called cusps, supported by the walls of the tooth. An intact tooth behaves like a solid block. Once a cavity is cleaned out, the cusps either side become free-standing walls, and the wider and deeper the cavity, the thinner those walls become.

When we examine a damaged tooth, we look at how wide the cavity is, how thick the remaining walls are, whether any cusp is undermined and whether the damage runs down towards the gum.

As a rough guide, a cavity that takes up less than about a third of the width of the biting surface, with sturdy walls, usually does well with a filling. Once it spreads across more than half, or a wall is thin enough to flex, covering that cusp tends to be the safer, longer-lasting choice. A crown comes in when there isn't enough tooth left to bond an onlay to reliably, or when the damage goes all the way round.

We often can't be certain until an old filling is out. If the plan might change once we can see the tooth properly, we'll tell you beforehand, and we won't do anything different without your agreement.

Why Does a Crack Change the Plan?

Cracks are common in teeth that have carried big fillings for years or take heavy force from clenching. Many are harmless surface lines in the enamel that need nothing. The ones that matter run into the dentine underneath.

Typical signs of a cracked tooth include:

• A sharp, brief pain when you bite on something, sometimes only as you let go
• Sensitivity to cold that settles quickly
• Difficulty pinning down exactly which tooth it is

A filling sits inside the tooth, so it can't stop the two sides of a crack flexing apart. An onlay or crown wraps over the cusps and holds them together, which is why we usually recommend cusp coverage for a cracked tooth even when the cavity itself is small.

There are limits. If a crack has reached the nerve, the tooth may also need root canal treatment. If it runs down into the root, the tooth often can't be saved. Catching cracks early gives the best chance of keeping the tooth.

Do Root-Filled Teeth Always Need a Crown?

Not always, but back teeth usually need their cusps covered. You may have heard that root canal treatment makes a tooth brittle. In reality, the weakness comes from what has been lost along the way: the decay or fracture that led to the infection, the old fillings, and the opening made to reach the canals. A root-filled molar or premolar has often lost much of its internal support.

Research has found that root-filled back teeth last longer when their cusps are covered, so we normally plan an onlay or crown within a few months of the root filling being finished. Until then, a well-sealed filling protects it, but don't leave it too long.

Front teeth are different. They take less biting force, so a root-filled front tooth with most of its structure intact can often be restored with a white filling alone. A crown is only needed when a lot of the tooth is missing.

Where very little tooth remains above the gum, we may first place a fibre post (£200) in the root and a composite build-up (£350) to give the crown something solid to hold on to.

Why We Start with the Smallest Option That Will Last

Every time a tooth is restored, it loses a little more of itself. A filling replaced several times gets bigger each time, until the tooth needs an onlay, then a crown, and sometimes root canal treatment or extraction. Good dentistry aims to slow that cycle down.

So we work upwards from the most conservative option:

1. A filling, if the tooth is strong enough to support one for years
2. An onlay, if cusps need protecting but plenty of healthy tooth remains around them
3. A crown, only when an onlay can't do the job

An onlay is bonded to the tooth rather than relying on a tapered shape for grip, so the preparation follows the damage and keeps the healthy walls. A full crown needs the tooth reduced evenly all the way round, which removes noticeably more healthy tissue. Crown preparation also carries a small long-term risk to the nerve, so on a living tooth with a reasonable amount left, an onlay is often the kinder choice.

Sometimes a crown is clearly right, and our dental crowns page explains when. But you shouldn't be offered one simply because it's the default.

Which Materials Do We Use?

Composite resin is the tooth-coloured material used for white fillings. It's bonded to the tooth, built up in layers and set with a curing light, all in one visit. It's excellent for small and medium cavities. In very large cavities in back teeth it's more likely to wear, chip or leak at the edges over time, which is one reason bigger restorations are often better made as onlays.

Ceramic, made in a dental laboratory, is used for onlays and crowns. Modern glass-ceramics combine strength with a natural, slightly translucent look, and they bond well to the tooth. Where the bite is heavy, such as a back molar in someone who grinds, a stronger zirconia-based ceramic may be the better choice.

If you grind, we may suggest a night guard to protect the new work, because even the best ceramic can chip under constant clenching.

Do I Need Putty Impressions for a Crown or Onlay?

No. At our Comely Bank practice we use an iTero intraoral scanner instead of trays of impression putty. A small wand is moved around your teeth and builds an accurate 3D model on screen in a few minutes. There's no tray of material to gag on, and you can see the tooth and its preparation for yourself.

An onlay or crown usually takes two visits:

1. First visit: the tooth is numbed, decay and old filling material are removed, the tooth is shaped and scanned, and a temporary is fitted.
2. Laboratory stage: the onlay or crown is made from the digital scan, usually within a week or two.
3. Second visit: the temporary comes off, and the new restoration is tried in, checked against your bite and bonded or cemented in place.

A white filling is completed in one appointment. Our technology page has more on the scanner.

What Does Each Option Cost in Edinburgh?

Our guide fees at Comely Bank Dental are:

• Medium white filling: £220
• Large white filling: £250
• Ceramic onlay: £800
• Ceramic crown, back tooth: from £760
• Ceramic crown, front tooth: from £800

A post or build-up, if needed, is listed separately. An onlay costs more than a large filling, but where a large filling would be likely to fail sooner, the onlay can work out better over the life of the tooth. We'll explain that trade-off honestly for your tooth.

Every fee is a guide price, confirmed in a written treatment plan before treatment starts, and nothing is added without your agreement. Members of our dental plan save 10% on most private treatment.

Questions

This guide is general information, not a diagnosis. A dentist needs to examine you to say what is right for your teeth.