Composite resin is a tooth-colored filling material used to repair cavities, chips, and other damage to teeth. It consists of a plastic resin matrix mixed with fine glass or ceramic particles, and it’s hardened in place using a special curing light. Since the 1960s, composite resin has steadily replaced silver amalgam as the go-to material for dental restorations, largely because it can be matched to the color of your natural teeth.
What Composite Resin Is Made Of
Composite resin has two main components: a polymer resin that acts as the “glue” and inorganic filler particles that provide strength. The resin base in most modern composites is built on a molecule called Bis-GMA, first developed over 40 years ago, or one of its chemical relatives. Some composites use urethane-based resins instead. On its own, this resin would be too soft and prone to wear. That’s where the fillers come in.
The filler particles are tiny pieces of materials like silica, zirconia, or barium aluminum silicate glass. These particles make up the bulk of the material by weight and give it the hardness needed to withstand chewing forces. The amount and size of filler particles directly affect how strong, polishable, and translucent the final restoration looks. A bonding agent (called a coupling agent) holds the resin and filler together at a microscopic level, and a light-sensitive chemical triggers the hardening process.
How Your Dentist Applies It
Getting a composite filling is straightforward and typically takes one appointment. Your dentist first removes any decay and roughens the tooth surface, then applies a liquid bonding agent that helps the composite grip the tooth. The composite itself has a putty-like consistency, so it can be sculpted directly onto the tooth and shaped to match your bite.
Once the shape looks right, your dentist points a blue curing light at the material. This light activates a chemical called camphorquinone inside the resin, which triggers a chain reaction that hardens the composite in seconds. Traditional composites need to be applied in thin layers of about 2 millimeters at a time, with each layer cured separately. This layering approach ensures the light penetrates fully and reduces internal stress as the material shrinks slightly during hardening.
Newer “bulk-fill” composites can be placed in layers of 4 to 5 millimeters at once. They contain modified fillers and more efficient light-sensitive chemicals that allow light to penetrate deeper, cutting down on chair time considerably.
Types of Composite Resin
Composites are classified by the size of their filler particles, and each type has trade-offs between strength and appearance.
- Macrofilled composites were the first generation, with relatively large filler particles (10 to 50 micrometers). They’re strong enough for high-stress areas like molars but difficult to polish to a smooth, natural-looking finish.
- Microfilled composites use much smaller particles (0.01 to 0.1 micrometers) and polish beautifully, but they’re weaker. They work best in low-stress spots like the edges of front teeth.
- Hybrid composites combine large and small particles to get the best of both worlds: better strength than microfills and better polish than macrofills. They’re a common choice for moderate-stress restorations.
- Nanofilled composites are the newest option, with particles as small as 5 nanometers. They offer the highest compressive strength of any type (around 460 MPa, compared to 250 to 300 MPa for macrofills) and can achieve an excellent surface finish. They’re used on both front and back teeth.
Most general-purpose composites sold today are nanofilled or nano-hybrid formulations, since they handle the widest range of situations well.
How Color Matching Works
One of composite resin’s biggest advantages is that it mimics the look of natural teeth. Your dentist selects a shade using a standardized guide, choosing colors that match the hue, brightness, and saturation of your surrounding teeth. But color matching goes deeper than picking the right shade off a chart.
Natural teeth aren’t a single uniform color. The inner layer (dentin) is more opaque and yellow, while the outer layer (enamel) is more translucent and bluish-white. For highly visible restorations, dentists use a layering technique with different shades of composite to replicate this structure. A more opaque, warmer layer goes down first to mimic dentin, followed by a more translucent layer to mimic enamel. The thickness of each layer matters: even small changes in layer thickness can significantly alter the final shade, translucency, and overall appearance of the restoration. The filler content in the composite also affects how light passes through it, influencing how lifelike the result looks.
How Long Composite Fillings Last
Composite resin fillings are durable, though they generally don’t last quite as long as silver amalgam in the back teeth. Research puts the median survival time for composite fillings in the range of 8 to 17 years, depending on the study and conditions. One long-term study in a general dental practice found a median survival time of 16 years for posterior composites, with a low failure rate of just 5% at ten years. After that point, failures accelerated more quickly.
Amalgam fillings tend to edge ahead in longevity studies, with median survival times often reported between 12 and 22 years. But the gap has been narrowing as composite materials improve. Some studies now report five-year and ten-year survival rates for composites that match or slightly exceed amalgam: one study found composites survived at 91.7% after five years and 82.2% after ten, compared to 89.6% and 79.2% for amalgam over the same period.
How long your filling lasts depends heavily on its size, its location in your mouth, your bite forces, and your oral hygiene habits. Larger fillings in back teeth that bear heavy chewing loads fail sooner than small fillings in front teeth.
Why Composite Fillings Fail
The primary reason composite restorations eventually need replacement is secondary decay, meaning new cavities forming at the edges where the filling meets the tooth. In the first five years, failures are more likely related to the technique used during placement or material selection. After five years, secondary decay becomes the dominant issue.
The material itself can also break down over time. Years of chewing, temperature changes from hot and cold foods, and moisture exposure gradually degrade the bond between the resin and filler particles. This can lead to microcracking, surface wear, and eventually bulk fracture. Studies have attributed anywhere from very low percentages to about 32% of failures to outright fracture of the composite, depending on the type of restoration and how long it was tracked. Tiny voids trapped during placement and the internal stress from polymerization shrinkage (the slight contraction that happens when the material hardens) can serve as starting points for these cracks.
Safety and BPA Concerns
Because the Bis-GMA resin is chemically related to bisphenol A (BPA), some patients worry about BPA exposure from composite fillings. Pure BPA is not actually found in dental composites. Trace amounts can sometimes be present as an impurity from the manufacturing process, but research consistently shows that any BPA released from composite fillings is minimal.
In one study that measured BPA in saliva and blood after composite placement, the highest level found in saliva was 0.52 nanograms per milliliter, and the highest in blood was 1.66 nanograms per milliliter. Both values fell well below thresholds considered harmful. BPA levels did not significantly increase over time after placement, and no measurable effect on hormone levels was detected. The consensus across studies is that BPA released from dental composites is not at a level that poses a health risk.
What Composite Fillings Cost
Without insurance, a composite resin filling typically costs between $200 and $335 per tooth. The price varies based on the size of the cavity, how many surfaces of the tooth are involved, and where you live. A small, single-surface filling on a front tooth costs less than a large, multi-surface filling on a molar. Most dental insurance plans cover composite fillings, though some older plans may only cover composite at the same rate they’d pay for amalgam, leaving you to pay the difference.