Composite Restorations
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Direct answer
Filler particle size decides what a composite can do: macrofilled materials carry strong but rough 8–12 µm quartz particles, microfills use 0.04 µm silica for polish and aesthetics at lower strength, hybrids blend both, and nanofilled resins now deliver strength with a glass-like finish. All are dimethacrylate resins (Bis-GMA or UDMA base) cured to 2–3% volumetric shrinkage, which is the material's central problem: shrinkage stress on a Class I box (the highest C-factor cavity) pulls margins open unless placement is incremental, oblique and properly light-cured. With bonding, moisture control and layered technique, composites serve Class I to VI cavities, diastema closures and veneers.
What you must remember
- Classification by filler: macrofilled (traditional, 8–12 µm quartz), microfilled (0.04 µm colloidal silica, best polish, weakest), hybrid and microhybrid (strength plus polish), nanofilled (nanomers and nanoclusters, both together).
- Filler loading runs roughly 60–80% by weight; higher loading stiffens the material and lowers shrinkage but reduces polish in traditional systems.
- Consistency families: flowable (lower filler, adapts liner-like into narrow cavities but shrinks more), packable or condensable for posterior, and bulk-fill types placed in 4–5 mm increments with high-power curing.
- Resin matrix: Bis-GMA or UDMA diluted with TEGDMA; camphorquinone photoinitiator activated by blue light around 470 nm.
- Polymerisation shrinkage of 2–3 volume % creates stress proportional to the C-factor (bonded to unbonded surface ratio); Class I cavities score highest, around 5, so they leak worst.
- Incremental oblique layering in increments no thicker than 2 mm reduces the C-factor and colour mismatch, and each layer is cured before the next.
- Enamel margins may be bevelled (0.25–0.5 mm) to expose rod ends and blend the composite; dentine is bonded with a suitable adhesive, never bevelled.
- Shades (dentine and body, enamel and translucent effects, plus opaquers for discolouration) are chosen hydrated, before isolation, against the natural tooth.
- Contraindications: inability to isolate, heavy bruxism, deep subgingival margins, and allergy to resin monomers.
Building a Class IV incisal edge, stepwise
A teenager has fractured the right central incisor's mesial incisal angle. Choose the shade with the tooth wet and before rubber dam placement, because desiccation lightens enamel and misleads the eye. Isolate, bevel the enamel margin 0.25–0.5 mm around the fracture to increase etchable surface and hide the interface in a gradual transition, and protect the exposed dentine. A matrix strip is loosely adapted — a Class IV is built freehand, the strip only shaping the proximal surface. Etch, rinse, keep dentine moist, prime and bond, cure. Now build in oblique increments: dentine shade replacing the fractured dentine core, then body shade in angled layers that never span the whole cavity at once — each increment touches fewer opposing walls, so shrinkage stress stays low and the enamel–composite seam is not pulled apart. Cure each layer 20–40 seconds with the tip 1–2 mm away, finish with fine diamonds or 12-fluted carbides and aluminium oxide discs from coarse to superfine, and check excursive guidance so the fresh edge is not carried into premature contact.
Where students slip
The exam favourite is post-operative sensitivity blamed on "the material" when it is technique: shrinkage pulling on a high C-factor floor, over-etching with desiccation, or curing one bulk increment. The expected viva chain is C-factor, incremental cure, and a low-shrink or lined approach. The second slip is aesthetics-order: choosing shade after isolation and under surgery light, which produces mismatched restorations; the correct sequence is pre-isolation, hydrated, natural-light shade selection with body and incisal translucency mapped. Third is the bevel: students bevel dentine (wrong — it weakens the cavity and adhesion) or skip the enamel bevel entirely, leaving a visible butt seam in the incisal edge. Finally, remembering that flowable composites adapt well but shrink more — they are liners and small-cavity materials, not bulk substitutes for hybrid builds.
Frequently asked questions
What is the C-factor and why does it matter?
The ratio of bonded to unbonded cavity surfaces; the higher it is (Class I is worst), the more polymerisation shrinkage stress pulls at margins, so incremental oblique placement is used to reduce it.
Which composite polishes best for an anterior restoration?
Microfilled and nanofilled composites, whose sub-micron or nano particles leave the smoothest surface; hybrids trade some polish for strength.
Why must increments be no thicker than 2 mm?
Blue light attenuates through composite; beyond about 2 mm the deeper resin undercures, leaving a soft base that later wears or discolours (bulk-fills are the engineered exception at 4–5 mm).
Why is a bevel placed on enamel margins?
It exposes more enamel rod ends for etching, increases the bonded area and turns the margin into a gradual blend that is far less visible incisally.
When are composites contraindicated?
When isolation is impossible, the patient bruxes heavily, margins extend deep subgingivally into moisture, or monomer allergy exists — amalgam or indirect alternatives are then chosen.