How a composite filling works, and how it differs from amalgam
Amalgam does not stick to teeth. It is packed into a cavity and held by the shape of that cavity, which means cutting undercuts so it cannot come out. Some of the tooth removed for an amalgam was never decayed. It went for retention.
Composite is held by adhesion. Enamel is etched so the surface becomes microscopically rough, bonding resin flows in and is set hard with a light, and the composite joins chemically to that resin. Because the bond holds it, the cavity only needs to be as big as the decay plus enough access to reach it.
None of this makes amalgam a bad material. It tolerates a damp field that composite cannot, and in very large restorations it still has an advantage in bulk strength.
Composite or amalgam
The question is not which material is better, but which suits the tooth in front of you.
| Consideration | Composite | Amalgam |
|---|---|---|
| Appearance | Matched to your tooth shade | Clearly visible, and it greys the tooth around it over time |
| How it is held | Bonded to enamel and dentine | Held by the shape of the cavity |
| Tooth removed | The decay, plus access to reach it | The decay, plus sound tooth cut away to retain it |
| Moisture | Bonding fails if the tooth cannot be kept dry | Far more tolerant of a damp field |
| Where it struggles | Large cavities, heavy grinding, deep margins | Anywhere appearance matters |
What happens at the appointment
It is made and finished in the mouth in one visit, with no laboratory stage and no temporary.
-
Shade first, then numbing and isolation
The shade is taken before anything else, because teeth dehydrate and lighten within minutes of the mouth being held open. Then local anesthetic and a rubber dam, because saliva reaching the prepared surface ruins the bond.
-
Removing the decay
Only the decayed tissue and enough sound tooth to reach the edges. Sometimes decay runs deeper than the x-ray suggested and the plan changes, and you are told before we carry on.
-
Etching, bonding and layering
Acid gel, rinsed thoroughly, then bonding resin cured with a light. Composite shrinks slightly as it sets, so it goes in increments, cured layer by layer, to limit stress on the bond.
-
Shaping, bite and polish
Anatomy contoured, contact with the neighbor checked with floss, bite adjusted, surface polished. A rough margin collects plaque and a high bite causes sensitivity.
Where composite is not the right answer
- Very large restorations. Once most of the biting surface and a cusp have gone, composite is being asked to be structural rather than restorative, and that tooth needs an onlay or a crown.
- Heavy grinding. Composite wears and chips under sustained load. It can still be right for a grinder, alongside a night guard.
- Margins deep under the gum. Adhesion needs a dry field, and a margin where fluid seeps continuously is the enemy of bonding.
How long they last, and why margins stain
A small composite in a front tooth, or a modest one on the biting surface of a molar, often gives ten years or more. A large multi-surface composite in the mouth of a grinder may give five to seven. Size, position, the bite, diet and how dry the tooth was kept drive it.
Staining at the margin is what people notice first on a front tooth. The junction takes up color from coffee, tea and tobacco, and a thin brown line after several years is common. On its own that is cosmetic rather than structural. It matters when there is a gap behind it, and telling those apart is what the x-ray is for.
Composite bonds to composite, so a chipped or stained restoration can often be repaired rather than remade. Every replacement cycle takes a little more tooth, so the rule is simple: if it can be repaired, repair it. That is the logic behind preventive care.
Composite filling questions we get asked
How much does a tooth-colored filling cost?
It depends on how many surfaces are involved and how large the cavity is. Plans often set the benefit for a back tooth at the amalgam equivalent and leave the difference to you, though every policy differs. See insurance.
Why is my tooth sensitive after a filling?
Some sensitivity for a few days to a couple of weeks is normal, particularly with a deep filling. Pain on biting usually means the bite is a fraction high, and adjusting it takes a couple of minutes. Pain that lingers or wakes you at night needs looking at.
Are composite fillings as strong as silver ones?
For normal chewing in small and moderate cavities, modern composites handle the load well. In very large restorations amalgam still has an edge in bulk strength, which is why the size of the cavity matters more than the material.
Do white fillings whiten?
No. Composite holds the shade it was made in. If you plan to lighten your teeth, do the whitening first, let the shade settle, then have front fillings matched to it.