A composite filling is made of two main ingredients: a plastic resin and finely ground glass particles. These two components are blended together with a coupling agent, a light-sensitive activator, and pigments to create a paste that your dentist sculpts into a cavity, then hardens with a blue light. The result is a tooth-colored restoration that bonds directly to your tooth structure.
What makes composite interesting is that neither ingredient works well alone. The resin by itself would be too soft and wear down quickly. The glass particles alone would crumble. Combined, they produce a material strong enough to handle the forces of chewing while closely mimicking the look of natural enamel.
The Resin Matrix
The plastic portion of a composite filling is a blend of synthetic resins. The primary one is a molecule called Bis-GMA, a thick, honey-like substance that forms the structural backbone of the filling. Because Bis-GMA is so viscous on its own, manufacturers mix in thinner resins to make the material workable. One common thinner is TEGDMA, which makes the paste flow more easily but also causes more shrinkage as the filling hardens. Another option, UDMA, offers a middle ground: it reduces shrinkage compared to TEGDMA while keeping the material manageable for your dentist to place.
The ratio of these resins matters. More of the thinner resin means the composite flows into the cavity’s nooks and corners more easily, but it also shrinks more as it sets. Manufacturers carefully balance these proportions to minimize shrinkage while keeping the paste smooth enough to work with.
The Glass Filler Particles
Filler particles make up the bulk of a composite filling, typically 50% to 70% of the material by volume. These are microscopic pieces of glass or ceramic, most commonly barium silicate glass, quartz, or zirconium silicate. Many composites also include a small percentage (5% to 10% by weight) of ultra-fine silica particles just 0.04 micrometers across, far smaller than a red blood cell.
The size of these particles is one of the biggest differences between composite types. Microhybrid composites use a mix of particle sizes, with larger ones averaging around 0.6 micrometers. Nanofilled composites use particles as small as 5 to 20 nanometers. Smaller particles produce a smoother surface that polishes to a higher shine, which is why nanofilled composites tend to look more lifelike on front teeth. Microhybrid composites, with their range of particle sizes, pack more densely and test slightly harder, making them a reliable choice for back teeth that take heavy chewing forces.
Barium-containing glass particles serve a second purpose: they show up on X-rays. This lets your dentist distinguish the filling from your natural tooth and spot any decay forming underneath it.
The Coupling Agent That Holds It Together
Glass and plastic don’t naturally stick to each other. To solve this, the glass particles are coated with a silane coupling agent before being mixed into the resin. Silane is a molecule with one end that bonds to glass and another end that bonds to plastic, acting as a chemical bridge between the two materials. Without this bridge, the filler particles would pull away from the resin under stress, and the filling would break down quickly.
How the Filling Hardens
Composite fillings start as a soft paste and harden only when exposed to a specific wavelength of light. The material contains a tiny amount of a light-sensitive compound, most commonly one called camphorquinone. This compound absorbs blue light in the 400 to 490 nanometer range, with peak absorption at 468 nanometers. When your dentist holds a bright blue curing light over the filling, camphorquinone absorbs that energy and triggers a chain reaction that links the resin molecules into a rigid, cross-linked network.
This is why your dentist builds up a composite filling in layers, curing each one for several seconds before adding the next. The blue light can only penetrate so deep, so thinner layers ensure the material hardens completely throughout.
Why Shrinkage Matters
Every composite filling shrinks slightly as it hardens, typically between 0.1% and 2% in any given direction. This shrinkage creates stress at the bond between the filling and your tooth, which can lead to tiny gaps, sensitivity, or eventual failure if not managed well. Manufacturers reduce this problem in two ways: increasing the proportion of glass filler (glass doesn’t shrink, so more filler means less resin to contract) and adjusting the resin chemistry to slow down how fast the material stiffens during curing, giving the composite time to flow and relieve internal stress before it fully locks into place.
Your dentist also manages shrinkage by placing the filling in small increments rather than one big lump. Each thin layer shrinks a little, but the total stress on the tooth stays low.
How Composites Match Your Tooth Color
The natural, tooth-like appearance of a composite filling comes down to a clever optical trick. Manufacturers select glass fillers whose light-bending properties (refractive index) closely match the resin they’re suspended in. When filler and resin bend light the same way, light passes through the material smoothly instead of scattering at every particle boundary. The ideal refractive index for the fillers is between 1.47 and 1.52, matched to a cured resin matrix around 1.52.
When this match is precise, the composite can actually blend visually with the surrounding tooth structure, a phenomenon dentists call the “chameleon effect.” The filling picks up color cues from the tooth around it, making it nearly invisible. Pigments and opacifiers are added in small amounts to fine-tune the shade, allowing dentists to choose from dozens of color options to match your specific teeth.
Flowable vs. Standard Composites
Not all composite fillings have the same consistency. Standard composites have a putty-like texture that your dentist packs into larger cavities and sculpts to match the shape of your tooth. Flowable composites contain less filler, around 37% to 53% by volume compared to the usual 50% to 70%, which gives them a runnier, more fluid consistency. They’re often used as a thin liner at the bottom of a cavity because they adapt tightly to the walls and irregular surfaces of the prepared tooth. Your dentist may place a layer of flowable composite first, then build up the rest of the filling with the sturdier standard material on top.
BPA Safety
Because Bis-GMA is derived from bisphenol A (BPA), some people wonder whether composite fillings release this chemical into the body. Research measuring BPA levels in both saliva and blood after composite placement has found that any release is extremely small and not statistically significant compared to baseline levels. In one study tracking patients over five weeks, the highest saliva concentration measured was 0.52 nanograms per milliliter, a trace amount that did not rise meaningfully above pre-filling levels. Blood concentrations followed the same pattern, showing no significant increase over time. The consensus from safety research is that the BPA exposure from composite fillings is negligible.
How Long Composites Last
A large meta-analysis of posterior composite fillings found an average annual failure rate of 1.8% at five years and 2.4% at ten years. In practical terms, that means the vast majority of composite fillings on back teeth are still intact after a decade. The two biggest factors that shorten a filling’s life are high cavity risk (people who develop new cavities frequently tend to see fillings fail sooner) and the number of surfaces the filling covers. A small filling on one surface of a tooth lasts longer than a large filling wrapping around multiple surfaces, simply because there’s less material under less stress.
Front teeth generally put less force on fillings, so anterior composites can last even longer, though they may need replacement sooner for cosmetic reasons like staining or color changes over time.