TL;DR
An onlay restores a molar or premolar whose cusps, the raised points of the chewing surface, are cracked, broken or weakened by a large filling. It’s made in porcelain by a dental laboratory, bonded on at a second visit, and covers only the parts of the tooth that need it. Compared with a crown, it keeps more healthy enamel, and its edges usually sit above the gum where they’re easy to clean. Dr. Carleigh Prane weighs your bite and any grinding before choosing an onlay over a crown.
What is an onlay, and how is it different from an inlay?
An onlay is a custom porcelain restoration that sits on top of a back tooth and replaces one or more of its cusps. An inlay fits inside the cusps and leaves them alone. An onlay reaches over them, so it can rebuild a corner that has broken away or shield a cusp that’s close to cracking.
Dentists sometimes call an onlay a partial crown, and that’s a fair description. It protects the tooth where it’s weak and stops short of covering it all the way around. On a molar, that often means the outer walls facing your cheek and tongue stay exactly as they grew.
Why choose an onlay over a crown?
An onlay keeps more of your natural tooth. A crown needs the tooth reduced on every side so the crown can fit over it, including walls that may be perfectly healthy. An onlay only asks for room where the damage is, and the untouched enamel goes on doing its job.
Its edges usually sit on the chewing part of the tooth, above the gum. That makes them easy to see on an X-ray, easy to keep clean and gentler on the gum than a margin tucked beneath it. Because porcelain is fused to the tooth with adhesive rather than simply cemented over it, a well-fitted onlay also helps hold the remaining tooth together.
Which teeth are good candidates for an onlay?
An onlay works best on a back tooth with damage concentrated in one area and solid walls elsewhere. We often recommend one in situations like these:
It’s a poorer choice when decay or cracks run all the way around the tooth, when the tooth is very short, or when heavy grinding would load the porcelain more than it can take. In those cases a crown is the safer repair.
- A cusp that has broken off, leaving the rest of the tooth intact
- A large old filling that has left one or two cusps thin and undermined
- A cracked cusp found early, before the crack reaches the root
- Some back teeth after root canal treatment, where the walls left are still thick
What happens at the two onlay appointments?
An onlay takes two visits. At the first, the tooth is numbed, the old filling and any decay are removed, and the weakened cusps are trimmed back to sound tooth. We take an impression, agree on a shade, and cover the tooth with a temporary so it stays protected and comfortable.
A dental laboratory shapes the porcelain to fit. At the second appointment we try it in, check the edges and contacts, and bond it using a resin cement that grips both porcelain and tooth. Then we check how it meets the opposite teeth as you bite and slide, and polish it smooth.
How does Dr. Prane decide between an onlay and a crown?
The decision rests on how much sound tooth is left and what forces your bite puts on it. Dr. Prane’s training with the Dawson Academy centered on how the teeth, muscles and jaw joints work together, so she looks at where the tooth takes its load before choosing a repair. A cusp that carries heavy force when you slide your jaw may need more coverage than one that barely touches.
Cracks get particular attention. A crack that stays in one cusp can often be covered by an onlay, while one that runs across the tooth usually calls for a crown. If you’ve had sharp pain when biting, our page on a cracked tooth explains how we find the cause.