Crown or Filling? How Dentists Decide
The structural reasoning behind choosing a filling, an onlay, or a full crown — and why the biggest filling is not always the best choice.
February 24, 2026 · 6 min read · Reviewed by Dr. Marianna Zadov, DDS
It is a structural question
A tooth is a hollow structure whose strength comes from its walls and the ridges connecting them. Every restoration is a trade-off: how much of the remaining structure can be preserved while still sealing the tooth and resisting chewing force.
Fillings replace missing structure but do not reinforce what is left. Crowns and onlays wrap over the cusps and hold the tooth together, which is a fundamentally different mechanical job.
When a filling is right
Small to moderate decay with intact surrounding walls, no cracks running under a cusp, and no heavy grinding history. Bonded composite is conservative, done in one visit, and easily repaired.
The limit is width. Once a preparation exceeds roughly a third to a half of the distance between cusps, the remaining walls become flex-prone, and a large filling can act like a wedge that eventually splits the tooth.
When a crown or onlay makes sense
A cracked cusp, a tooth that has had root canal treatment, a fractured wall, repeated failure of large fillings, or a tooth with very little remaining structure. In these cases coverage prevents the catastrophic vertical fracture that ends with extraction.
An onlay is the middle option — it covers the weakened cusps without reducing the entire tooth. When it is viable, it is often the better answer than a full crown.
Cost over time, not cost today
A crown costs more up front than a filling. But replacing a large filling three times over a decade and then needing a crown anyway, possibly with root canal treatment attached, costs considerably more.
We show you the photographs and the X-ray and explain the reasoning rather than simply announcing the plan. Our crowns and bridges page covers materials and the appointment sequence.
Talk it through with us
This article is general education, not personal dental advice. For a recommendation specific to your mouth, book an exam with Dr. Zadov or read more about our preventive care.