Crowns and bridges

What crowns and bridges are

A crown, sometimes called a cap, covers the whole visible part of a tooth down to the gum. It is made to fit a tooth that has first been shaped, or prepared, and is fixed in place with dental cement or bonded to the tooth. It cannot be taken out at home.

A bridge fills a gap left by one or more missing teeth. The false tooth is called a pontic, and the teeth that hold it are called abutment teeth. A bridge is also fixed in place, and is cleaned in the mouth like natural teeth.

When a crown is used

A crown may be advised for a tooth that:

• has a large filling and too little of its own structure left to hold a new filling;
• is cracked or broken;
• has had root canal treatment, especially a back tooth, which can become more brittle;
• is badly worn;
• is badly discoloured or misshapen, where simpler options would not work.

Crowns also hold bridges, and cover implants. When little of a root-filled tooth is left above the gum but the root is sound, a post may be placed in the root to help hold the crown. Because shaping a tooth for a crown removes tooth tissue all round, a smaller repair such as a filling or an onlay, which covers only part of the tooth, is often considered first.

Bridges, and the other ways to fill a gap

There are two main kinds of bridge:

• a conventional bridge, where the teeth on either side of the gap are prepared for crowns and the false tooth is joined between them. Sometimes a bridge is held from one side only;
• a resin-bonded bridge, also called an adhesive bridge, where a thin wing is bonded to the back of a neighbouring tooth. It needs little or no drilling and is often used for front teeth, where biting forces are lighter.

A bridge suits a gap when the neighbouring teeth are strong and the gums and bone around them are healthy. The alternatives are an implant-supported crown, which leaves the neighbouring teeth untouched but needs surgery and enough bone; a removable partial denture; or leaving the gap. A gap is sometimes reasonable to leave, particularly towards the back of the mouth, though neighbouring teeth can drift or tilt into it over the years.

Materials

Crowns and bridges are made in a dental laboratory, or sometimes milled chairside from a digital design. The main materials are:

• all-ceramic, including glass-ceramics and zirconia: tooth-coloured, with no metal. Zirconia is especially strong;
• metal-ceramic, where porcelain is fused over a metal base: strong, though a thin dark line can show at the edge if the gum later recedes;
• gold alloy: very durable and needs less tooth removed, but its colour limits it mostly to back teeth.

The choice depends on where the tooth is, how much of it is left, the bite, any grinding and how it will look.

How treatment usually proceeds

1. Assessment. An examination and X-rays to check the tooth, its pulp, the gums and the bone. Decay, gum disease or an infected pulp is treated first, and a weak tooth may need its core rebuilt.

2. Preparation. Under local anaesthetic, a layer of the tooth is shaped away all round to make room for the crown. For a conventional bridge, both supporting teeth are prepared.

3. Impression or scan. A mould or a digital scan records the prepared teeth and the bite, and the shade is chosen.

4. Temporary crown or bridge. A temporary one protects the tooth while the final one is made. It can come loose, so sticky and hard foods are best avoided on that side.

5. Fitting. The dentist checks the fit, the bite and the colour, then cements or bonds the crown or bridge in place. Some ceramic crowns are designed, made and fitted at a single visit.

Risks and limits

• Sensitivity after preparation is common and usually settles.
• Shaping a tooth can irritate the pulp. A prepared tooth sometimes needs root canal treatment later, even years afterwards, and this is more likely in a tooth that already had a deep filling.
• The cement can wash out, so that a crown or bridge comes loose. Decay can then start under it unnoticed. A resin-bonded bridge can come unstuck and often be re-bonded.
• Decay and gum disease can develop where the crown meets the tooth, and around the teeth that support a bridge.
• Porcelain can chip, and the tooth under a crown can fracture. Grinding raises this risk.
• If one supporting tooth of a bridge fails, the whole bridge may need to be replaced.

How long a crown or bridge lasts differs from person to person. Cleaning, diet, grinding and regular check-ups all play a large part.

Looking after crowns and bridges

Brush twice a day with a fluoride toothpaste, paying attention to the gum line around the crown. Clean between the teeth every day. Under the false tooth of a bridge, ordinary floss cannot pass from above, so a floss threader, a thicker "super floss" or a small interdental brush is used to clean the space beneath it. Avoid biting on ice, pens or other hard objects, and ask about a night guard if you grind your teeth. A loose crown, a bad taste around a bridge or new pain should be checked by the dentist.

A back tooth that has had root canal treatment is often protected with a crown. Where a tooth is missing, an implant is another way to fill the gap.