An inlay repairs damage within the cusps of a back tooth — essentially a lab-made, custom-fitted restoration that sits in the central area of the chewing surface. An onlay covers one or more of those cusps, extending further across the tooth. Both are stronger and longer-lasting than direct fillings. The difference is the size and location of the damage.
When a cavity is small, a filling handles it in one visit. When a tooth is severely broken or decayed, a crown covers the whole structure. But there’s a meaningful middle ground — when damage is too large for a filling but doesn’t justify a full crown. That’s where inlays and onlays come in.
Key Takeaways
- An inlay restores damage within the cusps of a back tooth — a custom, lab-fabricated alternative to a direct filling
- An onlay covers one or more cusps, extending onto the chewing surface — sometimes called a partial crown
- Both are made in a dental lab and bonded permanently to the tooth
- Inlays and onlays typically last 10 to 30 years — significantly longer than direct composite fillings
Inlay vs. Onlay: What Each One Covers
Inlay. Sits within the pits and grooves of the tooth’s chewing surface, between the cusps. Picture the raised bumps on a molar as peaks and the grooves between them as valleys. An inlay fills the valley without touching the peaks. It’s the right option when decay or damage is contained within that central area.
Onlay. Covers one or more of the cusps themselves, along with any damaged area between them. When decay or a fracture reaches one of those raised peaks, an onlay is needed. It extends further across the chewing surface and sometimes covers the entire top of the tooth. This is why onlays are sometimes called partial crowns.
The determining factor is where the damage is. If it’s between the cusps: inlay. If it involves the cusps: onlay. The procedure for both is essentially identical.
How Inlays and Onlays Compare to Fillings and Crowns
Here’s where each restoration fits in the spectrum of tooth repair:
| Direct filling | Inlay | Onlay | Crown | |
|---|---|---|---|---|
| What it covers | Central area within tooth | Between the cusps | One or more cusps + surrounding area | Entire tooth above gumline |
| How it’s made | Applied chairside, one visit | Lab-fabricated, 2 visits | Lab-fabricated, 2 visits | Lab-fabricated, 2 visits |
| Lifespan | 5 to 10 years | 10 to 30 years | 10 to 30 years | 10 to 15 years |
| Best for | Small cavities | Larger cavities within the cusps | Cusp damage, larger decay | Severely damaged or weakened tooth |
One key advantage inlays and onlays have over crowns: they preserve more of the natural tooth. Crowns require significant shaping of the tooth structure to make room for the cap. Inlays and onlays remove only what’s damaged.
What Are They Made Of?
Porcelain (ceramic): the most common material. Tooth-colored, natural appearance, excellent durability. Most patients choose porcelain.
Composite resin: also tooth-colored. Less expensive than porcelain but shorter lifespan. Suitable for patients prioritizing lower upfront cost.
Gold: the longest-lasting dental material. Some providers prefer gold for back molars where durability matters most and appearance is less of a concern.
The Two-Visit Process
At the first visit, the area is numbed, decay is removed, and the tooth is shaped. An impression is taken and sent to the lab. A temporary restoration protects the tooth in the meantime. The lab fabrication typically takes 1 to 2 weeks.
At the second visit, the temporary is removed. The permanent inlay or onlay is tried in, adjusted if needed, then bonded permanently to the tooth.
The lab process is what gives inlays and onlays their precision. The restoration is fabricated outside the mouth under ideal conditions, custom-fitted before bonding. This produces a better fit, a stronger seal, and a longer-lasting result than what can be done directly in the chair. If you have a tooth that may need more than a filling, schedule your appointment at our Edmonds office and we’ll take a look at what the right option is for your situation.
