14.3 Composite Resin & Adhesive Dentistry
Key Takeaways
- Composites are a matrix of dimethacrylate resin (e.g. Bis-GMA/TEGDMA) filled with inorganic particles (glass/silica); filler load and particle size govern mechanical properties, wear, polish, and shrinkage.
- Polymerisation shrinkage (1.5–5% by volume) generates contraction stress and marginal gaps; placement is controlled by incremental layering, bulk-fill materials with stress modulators, and good light-curing technique.
- The acid-etch bond to enamel is micromechanical resin tag infiltration of etched enamel prisms; dentine bonding uses a hybrid layer of infiltrated collagen and a smear layer managed by total-etch or self-etch adhesive strategies.
- Generation classifications simplify the strategy: total-etch (etch-and-rinse) vs self-etch (one-step or two-step); universal adhesives can be used in either mode and bond to multiple substrates.
- Light-curing variables — irradiance, wavelength (460–480 nm for camphorquinone), distance, time, and resin depth — determine degree of conversion and depth of cure; inadequate curing underlies sensitivity and failure.
Composition and Classification
A dental composite has three components: an organic resin matrix (typically Bis-GMA blended with lower-viscosity diluents like TEGDMA), inorganic filler particles (barium glass, silica, zirconia), and a coupling agent (silane) that bonds filler to matrix. The photoinitiator is usually camphorquinone (absorbs blue light around 460–480 nm) with a tertiary amine co-initiator.
Filler and Composite Types
| Type | Filler size | Use |
|---|---|---|
| Macrofilled | 10–50 µm | Strong but unpolishable, rough surface |
| Microfilled | 0.01–0.1 µm | Polishable, aesthetic, low strength — Class III/V |
| Hybrid / nanohybrid | mixed sub-micron + nano | Current universal materials — balance of strength and polish |
| Bulk-fill | modified with stress modulators | 4–5 mm increments, lower shrinkage |
Filler loading (by weight/volume) raises modulus and wear resistance and lowers polymerisation shrinkage and water sorption. Heavily filled composites are stronger and shrink less but harder to polish.
Polymerisation Shrinkage and Stress
The setting reaction converts monomer to polymer with a 1.5–5% volume contraction. This contraction produces polymerisation stress at the bonded interface; if stress exceeds bond strength, the restoration debonds, opening a marginal gap that drives microleakage, postoperative sensitivity, and recurrent caries.
Strategies to Manage Shrinkage
- Incremental layering — place 2 mm increments (oblique layers) so each shrink toward the cavity walls; reduces C-factor (ratio of bonded to unbonded surfaces) stress.
- Bulk-fill composites — formulated with stress modulators and increased translucency for depth of cure; place up to 4–5 mm in one increment.
- Wet-bonding technique (total-etch) — keep dentine moist to keep collagen expanded for hybrid layer infiltration.
- Good light-curing — adequate irradiance and time.
The C-factor (configuration factor) = bonded surface area / unbonded surface area. A Class I has a high C-factor (5 walls bonded) → high shrinkage stress; Class IV is low C-factor → less stress. Incremental placement lowers the effective C-factor.
Adhesive Dentistry — Enamel and Dentine Bonding
Enamel Bonding (the original acid-etch)
- Etch enamel with 35–37% phosphoric acid for 15–30 s → microporous surface of dissolved prism peripheries and cores.
- Rinse and dry; low-viscosity resin flows into the etched microporosities and polymerises, forming resin tags — a micromechanical bond (20–30 MPa).
Enamel bonding is reliable; dry etched enamel gives a frosty-white appearance.
Dentine Bonding
Dentine is harder to bond: it is wet, vital, and covered by a smear layer. The bond depends on forming a hybrid layer — resin infiltrating the demineralised collagen network.
| Strategy | Steps | Notes |
|---|---|---|
| Total-etch (etch-and-rinse) | Etch enamel + dentine 15 s → rinse → leave dentine moist → apply primer + adhesive (2- or 3-bottle) | Most predictable bond on dry enamel; risk of over-drying dentine (collagen collapse) or over-wetting |
| Self-etch | Acidic primer etches and infiltrates simultaneously → adhesive | Skips separate rinse; less technique-sensitive; bond slightly lower on enamel |
| Universal (multi-mode) | Can be total-etch, selective-etch, or self-etch; MDP functional monomer bonds to multiple substrates | Current standard; versatile |
Generations (simplified)
- 3rd generation — etch dentine only, first dentine bond; weak.
- 4th/5th — total-etch, multi-bottle; 4th = separate primer + bond, 5th = combined.
- 6th/7th — self-etch, one-step or two-step; simplified but more hydrolysis risk.
- 8th (universal) — can be used in any mode; the modern choice.
Key bonding chemistry: the functional monomer 10-MDP (10-methacryloyloxydecyl dihydrogen phosphate) forms a stable chemical bond to hydroxyapatite and to metals — the basis of universal adhesives.
Light-Curing Technique
Adequate cure is non-negotiable; inadequate cure causes sensitivity, recurrent caries, and restoration failure.
| Factor | Requirement |
|---|---|
| Wavelength | Match initiator — camphorquinone peak 460–480 nm (blue) |
| Irradiance | ≥400 mW/cm² at the surface; modern LED lights 800–1500 mW/cm² |
| Time | 10–20 s for 2 mm conventional; bulk-fill may need 20–40 s |
| Distance | Light tip as close as possible; each 1 mm distance halves irradiance |
| Depth of cure | ~2 mm conventional; 4–5 mm bulk-fill |
The Marus '10 second rule' is a teaching aid — verify output regularly with a radiometer. Overcuring generates heat (pulp risk); undercuring is the commoner error. Curing lights must be checked for output and the tip kept clean.
Clinical Workflow Summary
- Isolate (rubber dam), clean cavity.
- Bevel enamel margins where aesthetic (Class IV); butt margins where load-bearing.
- Etch enamel 30 s, dentine 15 s (total-etch) OR apply self-etch/universal as directed.
- Rinse, keep dentine moist, apply primer/adhesive, light-cure.
- Place composite in increments, light-cure each.
- Finish and polish.
Pitfalls
- Contamination with saliva/blood after etching — re-etch.
- Over-drying dentine (total-etch) — collagen collapses, hybrid layer fails.
- Under-cured increments — place within depth of cure.
- Bulk-curing too deep a conventional composite — depth of cure insufficient.
What is the mechanism by which composite resin bonds to etched enamel?
Which clinical factor most increases polymerisation shrinkage stress at the composite–tooth interface?
A dentist using a total-etch (etch-and-rinse) adhesive accidentally over-dries the dentine after rinsing. What is the likely consequence and how should it be corrected?
Camphorquinone, the photoinitiator in most dental composites, is activated by light in which wavelength band?