Inlays & onlays.

Inlay or onlay: what each one covers

Back teeth have raised points on their biting surface called cusps, with grooves and a hollow between them. The difference between an inlay and an onlay comes down to which of these parts the restoration replaces.

Both are made outside the mouth from a mould or scan of your prepared tooth, then bonded in. That is the key difference from a filling, which is shaped directly in the tooth while you are in the chair.

Sometimes the choice is only confirmed once the old filling is out and the remaining walls can be seen. A cusp found to be thin or cracked at that point is usually covered for protection, which turns a planned inlay into an onlay.

• Inlay: sits inside the tooth, filling the hollow between the cusps, much like a filling would
• Onlay: replaces the hollow and also covers one or more cusps, so it wraps over part of the biting surface
• Crown: covers every surface of the tooth down to the gum, which an onlay does not

When they are chosen over a large filling or a crown

A direct filling works well for small and medium cavities. When a cavity becomes very wide, a filling has more of the chewing load to carry on its own, and recreating a tight contact with the neighbouring tooth gets harder. Composite also shrinks slightly as it sets, and in a big cavity that shrinkage can stress the tooth or open a small gap at the edge.

Because an inlay or onlay is made and hardened before it goes in, that shrinkage happens outside your mouth, and the fit and shape of the contacts can be controlled more closely. Compared with a crown, an onlay usually means less healthy tooth is removed, and its edges can often sit above the gum where they are easier to clean and check.

Your dentist may suggest one for:

• An old, large filling that is cracking or leaking
• Decay that has left the cavity too wide for a filling to be strong
• A cusp that is cracked, undermined or has broken away
• A molar after root canal treatment, where covering the cusps helps protect it

Composite or porcelain

We make inlays and onlays in composite resin and in porcelain. Both are tooth-coloured and both are bonded to the tooth, but they behave a little differently over the years.

Porcelain holds its colour and gloss well and resists staining, although it can chip under a heavy bite and is harder to repair in the mouth. Composite is softer, which can make it kinder to the opposing tooth, and small chips can often be patched with new composite. It can, however, wear and pick up stain more readily than porcelain.

Your dentist will recommend a material based on the size of the repair, how much force that tooth takes and whether you grind, and will explain the reasons.

When an inlay or onlay may not suit

These restorations need enough sound tooth around them to bond to. If most of the tooth has broken down, or a crack runs deep below the gum, a crown may protect it better, and some cracked teeth can’t be saved at all.

Heavy grinding can chip porcelain, so a composite onlay and a night splint may be suggested instead. If decay is very active or the gums are inflamed, your dentist will usually want to get that under control first, because new decay at the edges is one of the common reasons any restoration fails. Inlays and onlays are mainly used on back teeth; front teeth are more often repaired with fillings, veneers or crowns.

Settling in and possible problems

Some sensitivity to cold or biting is common for a few days after the bonding visit, and usually eases on its own. A restoration that feels high can be adjusted.

Less often, porcelain can chip, an inlay or onlay can come unstuck, or decay can start at the edge where it meets the tooth. Removing a large old filling sometimes reveals deeper decay or cracks than the x‑ray showed, and in a small number of deep teeth the nerve becomes inflamed after treatment and the tooth goes on to need root canal treatment. Your dentist will mention any risks that apply to your tooth before you decide.

Looking after it, and how long it may last

An inlay or onlay is cleaned like the rest of your teeth. The place to pay most attention is the join between the restoration and the tooth, so floss or use an interdental brush there every day and brush along the gum line.

How long one lasts depends on the material, the size of the repair, your bite and whether decay is kept away from the edges. Many last for years, and we check the margins at each check‑up. If you clench or grind, wearing a night splint can reduce the load on the tooth, and it helps to avoid biting on ice, pen lids or very hard lollies.

What affects the cost

Inlays and onlays usually cost more than a filling because they are made in a dental laboratory and take two appointments, and usually less than a crown on the same tooth. The material, the number of tooth surfaces involved and whether cusps are covered all change the fee.

Many funds group inlays and onlays with major dental rather than general dental, which can affect both your rebate and any waiting period, so it is worth asking your fund about the specific item numbers before you book.