Polymerisation Shrinkage
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Direct answer
Every composite contracts as it cures: dimethacrylate monomers like Bis-GMA and the diluent TEGDMA draw closer as carbon double bonds convert to polymer chains, producing volumetric shrinkage of roughly 2-3 per cent in conventional composites — 3-5 per cent in lightly filled flowables, and below 1 per cent in ring-opening silorane chemistry. Whether that contraction harms the tooth depends on shrinkage stress, governed by the configuration factor, the bonded-to-free surface ratio: a Class I cavity (C-factor near 5) converts nearly all shrinkage into wall stress, while a low C-factor flat surface vents it. The clinical consequences — microleakage, cuspal deflection, enamel microcracks, post-operative sensitivity and debonding — are managed by incremental oblique layering, stress-modified curing and low-shrink chemistry rather than by any single material choice.
What you must remember
- Numbers to quote: conventional paste composites shrink about 2-3 volumetric per cent, flowables 3-5 (lower filler loading), packables and high-fill hybrids at the low end, silorane-based materials under 1.
- C-factor is the ratio of bonded to unbonded surfaces: Class I roughly 5 is the worst, Class V roughly 1 the friendliest — a question that appears in nearly every paper.
- Polymerisation runs pre-gel and post-gel: before the gel point, viscous flow compensates contraction; after it, stress transmits to the walls, which is why curing rate manipulation (soft-start, ramped, pulse-delay curing) lowers stress even though total shrinkage is unchanged.
- Flowables' higher shrinkage does not disqualify them as thin liners — the "elastic cavity liner" concept trades a small stressed volume for stress absorption — but thick flowable builds invite leakage.
- Incremental oblique placement in increments of 2 mm or less reduces the effective C-factor of each portion and doubles wall adaptation; bulk-fill composites are formulated for 4-5 mm single increments through raised translucency and modified initiator chemistry.
- Post-gel stress deflects cusps measurably in wide MOD cavities and can crack marginal enamel — the mechanism behind white margin lines and early sensitivity.
- Degree of conversion in dental composites is partial (commonly around half to two-thirds of double bonds); oxygen-inhibited surface layers are the reason incremental bonding works.
- Hygroscopic expansion over months partially offsets shrinkage gap — a comfort, not a strategy.
Stress-managing a Class I restoration
A deep occlusal cavity in a first molar, margins in enamel, floor close to the pulp. Before the adhesive, think geometry: this is a C-factor near 5, the configuration most likely to convert shrinkage into debonding at the pulpal floor. Line the deepest dentine thinly for pulp protection, bond, then build in a stress-aware sequence: first a thin lateral wall portion (an oblique wedge against one axial wall, its opposite face free), light-cured, then successive wedges from alternating sides, each converting the cavity into a lower effective C-factor triangle, none thicker than 2 mm. Use a soft-start or ramped curing profile for the early increments — the lower initial irradiance extends the pre-gel flow window — and cure each increment fully before the next. Finish with an anatomically carved final layer, checking contacts and occlusion before the patient closes on fresh composite. Had the cavity been wider with undermined cusps, the stress plan would expand: consider an indirect bonded inlay or onlay, which moves the shrinkage problem into the laboratory and replaces it with a thin, polymerised-in-advance lute line. What the patient feels as "sensitivity after a filling" is, more often than exam candidates admit, this physics.
Where students slip
The first slip is conflating shrinkage with stress: the exam asks which cavity shrinks the composite most and which stresses the bond most — high C-factor answers the second, not the first. The second is over-crediting soft-start curing: it reduces stress magnitude and rate, never the total volumetric shrinkage, a distinction tested as a true-false almost every year. The third is bulk-fill overreach: accepting 4-5 mm increments as permission to ignore C-factor entirely, when the evidence supports bulk-fill in indicated depths with adequate cure and not as a universal philosophy. And when asked to name a low-shrink chemistry, the specific answer is silorane's ring-opening polymerisation — generic "newer materials" earns nothing.
Frequently asked questions
What is the C-factor and its typical Class I value?
The bonded-to-unbonded surface ratio; Class I approaches 5, the highest among common preparations and the greatest shrinkage stress.
How much do composites shrink volumetrically?
About 2-3 per cent for pastes, 3-5 for flowables, under 1 per cent for silorane ring-opening materials.
Why does incremental layering reduce stress?
Each oblique 2 mm portion bonds limited wall area with a free surface to flow toward, lowering the effective configuration factor.
Does soft-start curing reduce shrinkage?
No — it reduces stress by extending the pre-gel flow phase; total volumetric contraction is essentially unchanged.
Why do flowables shrink more than packables?
Lower filler content raises the resin fraction, converting monomer contraction directly into greater volumetric shrinkage.
What signs indicate shrinkage stress damage?
Marginal staining, enamel microcracks, cuspal deflection, post-operative sensitivity, and eventual gap with secondary caries.