An onlay is one of those restorations that most patients have never heard of until it is recommended to them, and then they want to understand why it is not a crown and whether that matters. It does matter, and the distinction is worth explaining clearly.
The coverage question
A crown covers the entire tooth above the gum line: all four walls and the biting surface. The crown replaces the outer shape of the tooth completely, which means a substantial amount of the original tooth structure has to be removed to make room for it.
An onlay covers part of the tooth. Specifically, the biting surface and one or more of the outer walls, but not all four walls. The portions of the tooth that are still sound stay intact. The onlay is bonded over what remains.
An inlay covers only the biting surface within the outer walls, which is effectively a large shaped filling placed in a single solid piece rather than built up layer by layer.
These are not separate treatment philosophies. They are different restorations for different levels of damage. The choice between them depends on how much sound tooth structure is left and where the damage is.
Why preserving tooth structure matters
The guiding principle here is the one that shapes most of the restorative decisions at Whitehorse Dental: recommend the simplest solution that actually solves the problem. A crown is not better than an onlay just because it is a more complete restoration. For a tooth that does not need a crown, a crown removes tooth structure that did not need to be removed.
Two things follow from that.
Tooth removal is irreversible. The reduction for a crown takes about one to two millimetres of tooth on all surfaces, which is a significant amount for a tooth that is already compromised. An onlay preparation removes only the damaged or unsupported material, and leaves the sound enamel walls that are intact. Enamel that is intact is better than a ceramic wall doing the same job, because it is the original material and it is already bonded to the dentine beneath it.
The adhesive bond is stronger when there is more tooth to bond to. An onlay bonds adhesively to the tooth, and the bond works because the fitting surface is in contact with real tooth structure. More bonding surface means a stronger restoration.
When an onlay is the right choice
The clinical picture is usually a back tooth, typically a molar or premolar, with one of the following:
- A large existing filling that has fractured, leaked, or developed decay around its margins, where the outer walls of the tooth are still standing.
- A cracked tooth where the crack has not extended below the gum line, and where the outer walls are sound enough to retain the restoration.
- Extensive decay that has hollowed out the biting surface, where a direct composite filling would be too large to place reliably.
What disqualifies an onlay in favour of a crown is when the outer walls have gone: either fractured below the gum line, thinned to the point where they would not survive the biting forces, or so undermined by decay that leaving them creates a weak point. At that stage, capping the tooth completely is the more durable decision.
The material: e.max for almost all of them
Onlays here are made in e.max (lithium disilicate) in the vast majority of cases. E.max bonds adhesively, and the adhesive bond is particularly important for an onlay because the onlay does not have the full circumferential grip that a crown has. The bond between the ceramic and the tooth is doing more structural work than it does in a crown. E.max is also translucent in a way that matches enamel, so the restoration looks like part of the tooth.
Gold is an alternative for a lower back tooth where the appearance is not a priority, and it has real advantages: it is the most dimensionally accurate material and its marginal fit is excellent. Where e.max is not indicated and the patient is open to gold, we discuss it.
The appointments: two, same as a crown
The process is similar to a crown.
At the first appointment, the damaged material, failing filling, and any decay are removed, and the remaining tooth is shaped for the onlay preparation. We use rubber dam throughout the preparation, because the adhesive bond that holds the onlay depends on a dry field. We take a shade and scan with the Medit intraoral scanner rather than taking an impression. A temporary restoration goes in while the laboratory makes the final piece.
At the fitting appointment, the temporary comes off, the fitting surface of the onlay and the tooth preparation are both air-abraded to remove the smear layer and maximise the bonding surface, and the onlay is adhesively cemented. The bite is checked in lateral and protrusive movements as well as straight biting.
The two-appointment approach, rather than same-day milling, gives us a laboratory-quality fit. Same-day milled restorations have improved substantially over the years but still fall short on the contact point accuracy and marginal fit that a dental laboratory produces on a well-scanned case.
Gums before restorations
If the tooth has gum inflammation around it, we address that before touching the tooth. Inflamed gum tissue bleeds, contaminates the bonding field, and shifts position as it heals, which changes where the margin of the restoration sits. Perio first, then restoration, always in that order. For most teeth it is a matter of better home care and a professional clean before the preparation appointment. Where there is established bone loss it may involve a periodontal course of treatment first. We are explicit about this timeline at the assessment rather than discovering it partway through a procedure.
The honest question about cost
A crown costs more than a direct composite filling. An onlay typically costs in a similar range to a crown, because the laboratory work and the two appointments are the same. What patients are paying for in an onlay rather than a direct composite is a harder, better-fitting, longer-lasting restoration that sits outside the tooth rather than one built inside it.
The decision to do an onlay rather than a large composite filling is a clinical one based on how much tooth is remaining, not an upsell. And the decision to do an onlay rather than a crown is a decision to preserve more of the original tooth, not to provide a lesser restoration.
For an existing restoration that is showing signs of failure, it is worth getting it looked at before it actually breaks. A tooth that loses an outer wall suddenly requires a crown. The same tooth assessed before the wall goes might well have kept an onlay for ten years.
Booking
If you have been told a restoration is failing, or if you have a tooth that has been sensitive to biting pressure, it is worth a proper look. We are at 129A Canterbury Road, Blackburn, with on-site parking and Saturday morning appointments available. Consultations are available in Mandarin, Cantonese, Teochew and Shanghainese.
Call (03) 8838 8820 or text 0435 025 318.
Related reading: getting a crown, appointment by appointment covers the full crown process including the same rubber dam and air abrasion protocol. Do you really need a crown? is the decision framework for teeth where the choice is more genuinely open. The crowns and bridges service page covers inlays, onlays and crowns together.