Deformation, shrinkage and cumulative errors: why the conventional impression chain compromises aligner precision

Polymerisation shrinkage, plaster expansion, pouring and storage errors: comprehensive physicochemical analysis of dimensional error sources in the conventional impression chain.

The conventional impression chain — from intraoral taking to model delivery at the laboratory — involves a succession of physicochemical transformations, each introducing its own dimensional error. These errors, individually acceptable in certain prosthetic contexts, become clinically significant in the demanding context of orthodontic aligner fabrication. This article analyses each error source with its quantitative data and underlying mechanisms.

1. Physical chemistry of impression materials: deformation mechanisms

Understanding deformations requires descending to the molecular mechanism level. The two major families of elastomeric impression materials (A-silicone and polyether) exhibit distinct deformation mechanisms:

• Polyvinylsiloxane (A-silicone): the addition reaction between vinyl groups and silicon hydrides, catalysed by chloroplatinic acid, generates no volatile by-products. Volumetric shrinkage is therefore minimal (0.05–0.15%) and essentially thermal. Elastic deformation on removal is 0.2–0.4%, recovered to over 99% in 30 seconds if the material is correctly polymerised.
• Polyether: cationic polymerisation generates significant internal stresses. The material is intrinsically hydrophilic, giving it good wettability but leading to water absorption of up to 0.5% by weight after 24 h exposure to ambient humidity, inducing progressive dimensional expansion.
• Alginate (irreversible hydrocolloid): gelation of alginate salt with calcium sulphate releases bound water that evaporates after setting. Synaeresis (contraction by water expulsion) can reach 3 to 5% by volume if the impression is exposed to dry air, while imbibition (expansion by water absorption) can reach 2 to 4% in a humid environment. These two opposing processes make alginate dimensional stability fundamentally unpredictable.

2. Deformation on removal: an inevitable error

Removing the impression from the oral cavity imposes temporary mechanical deformation in undercut areas (areas where anatomical geometry creates mechanical blocking during extraction). For VPS — the most widely used material in precision impressions — deformation on removal follows the Kelvin-Voigt viscoelastic behaviour law: instantaneous elastic deformation (recovered immediately) and deferred viscoelastic deformation (recovered in 10 to 60 minutes depending on the material's Shore hardness).

In clinical practice, pouring must be delayed by at least 30 minutes after removal to allow viscoelastic recovery to complete. This temporal constraint is often ignored in practice, generating models systematically deformed in undercut areas — precisely the cervical and interproximal areas most important for aligner fit.

3. Plaster thermal expansion: a systematic bias

Type IV or V dental stone used for model pouring undergoes setting expansion during the rehydration reaction of calcium sulphate hemihydrate. Depending on formulation and water/plaster ratio:

For a 120 mm arch, a setting expansion of 0.2% (type IV, best conditions) translates to an absolute error of 0.24 mm = 240 µm. This error is systematic and proportionally affects all dimensions of the model. It adds, not algebraically but vectorially, to all other error sources in the chain.

4. Air bubbles: point defects with high clinical impact

Air bubbles trapped during stone pouring create artefact tubercles on the model — small surface elevations where a bubble was present in the impression. These tubercles, even 0.1 to 0.3 mm in height, create parasitic contact points on the thermoformed aligner that modify force distribution in an unplanned way. Prevention involves pouring under a vibrator and using a wetting agent (surfactant) on the impression before pouring.

5. Transport and storage errors

The logistics chain between practice and laboratory introduces additional documented but rarely quantified error sources:

• Thermal variations in transit: VPS impressions stored in vehicles or postal packages can undergo thermal cycles of ±30°C. The thermal expansion coefficient of VPS is 200–300 µm/m/°C — on a 120 mm arch, a 20°C variation produces a dimensional error of 48–72 µm.
• Mechanical deformation of impression under its own weight: complete impressions, if not correctly positioned in their container, can undergo deformation by sagging of unsupported areas.
• Handling errors during model demoulding: microcracks in the plaster model, visually imperceptible, create missing or erroneous data areas during laboratory scanning.
• Cross-contamination and chemical disinfection: disinfectant solutions (1% hypochlorite, glutaraldehyde, ortho-phthalaldehyde) can cause micro-surface expansions in alginate (+ 0.5% in 10 min in 1% hypochlorite) and slight surface deformation in polyethers.

6. Cumulative error analysis: the uncertainty propagation rule

When independent errors combine in a measurement chain, their combined uncertainty is expressed according to the uncertainty propagation rule (JCGM 100:2008 — Guide to the Expression of Uncertainty in Measurement). For independent, uncorrelated errors, the combined uncertainty is the square root of the sum of squares of individual uncertainties. For the alginate-type IV plaster chain under non-optimal conditions:

For comparison, the VPS chain (type IV, optimal protocol, pouring delay > 30 min) presents an estimated cumulative error of 80–150 µm. And the intraoral scan chain (confocal microscopy, optimised protocol) stands at 20–45 µm. The gap between the alginate chain and the intraoral scanner represents a factor of 5 to 10 in terms of dimensional error.

7. Clinical relevance threshold: when error becomes a problem

To contextualise these errors, it is useful to compare them with the functional parameters of aligners:

• Standard aligner thickness: 700 to 900 µm. A 200 µm error represents 22 to 28% of total thickness — sufficient to significantly modify contact stresses.
• Amplitude of an elementary movement per stage: 100 to 250 µm. A 200 µm error represents 80 to 200% of a movement — meaning an entire stage can be "absorbed" by the model's dimensional error.
• Standard rectangular attachment height: 1.5 to 2.0 mm. A 200 µm error represents 10 to 13% of attachment height — significant modification of the transmitted force vector.
• Average inter-bracket space (if coexisting with braces): 0.5 to 3.0 mm. Errors > 100 µm in these spaces directly compromise correct expression of the mechanics.

Conclusion

The conventional impression chain accumulates inevitable physicochemical error sources that, under real clinical conditions (pouring delays, storage conditions, stone quality), result in models whose fidelity is insufficient for optimal precision aligner fabrication. The transition to a complete digital chain — direct intraoral scan, STL data transmission, fabrication on validated printed model — is not merely a technological modernisation but a clinical necessity to guarantee the predictability of aligner orthodontic treatments.