14.4 Impression Materials, Cements & Other Materials
Key Takeaways
- Elastomeric impression materials — addition silicones (polyvinyl siloxane), polyethers, and condensation silicones — give high accuracy and elastic recovery; hydrocolloids (agar, alginate) are less accurate but cheap and useful for study models.
- Addition silicone (PVS) is the most dimensionally stable and accurate, hydrophobic (needs surfactant), and has the best elastic recovery; polyether is hydrophilic and accurate but absorbs water and is stiffer to remove.
- GIC chemically bonds to tooth and releases fluoride but is weak and moisture-sensitive; RMGIC adds light-cure resin for command set; compomers are polyacid-modified composites needing bonding.
- Zinc phosphate cement is strong but non-adhesive and acidic to the pulp; zinc polycarboxylate adheres to tooth; resin luting cements bond both to tooth (adhesive) and restoration (silane/MDP) for ceramic restorations.
- Dimensional accuracy is set by handling: single-stage vs two-stage putty-wash, mixing technique, syringing light-body, and pouring within the stability window.
Impression Materials
A dental impression records negative detail of the prepared tooth and surrounding structures; the cast poured from it must be dimensionally accurate to produce a well-fitting indirect restoration.
Hydrocolloids
| Material | Reversible? | Accuracy | Use |
|---|---|---|---|
| Alginate (irreversible hydrocolloid) | No (chemical set) | Moderate — limited detail, dimensional instability | Study models, orthodontic models, primary impressions |
| Agar (reversible hydrocolloid) | Yes (thermoreversible gel–sol) | Good accuracy, hydrophilic | Rarely used now; needs water-cooled trays |
Alginate must be poured promptly (within the hour) or stored in 100% humidity because it shrinks by syneresis (water loss) and expands by imbibition (water uptake).
Elastomers
| Property | Addition silicone (PVS) | Polyether | Condensation silicone | Polysulphide |
|---|---|---|---|---|
| Accuracy | Excellent | Excellent | Good (less stable) | Good |
| Dimensional stability | Excellent (can pour days later) | Good (if dry) | Poor (shrinkage on set) | Moderate |
| Elastic recovery | Best | Excellent | Moderate | Good |
| Hydrophilicity | Hydrophobic (needs surfactant) | Hydrophilic | Hydrophobic | Moderate |
| Stiffness on set | Low–medium (easy removal) | High (stiff; harder to remove, atraumatic risk) | Low | Low |
| Taste/smell | Neutral | Pleasant | Neutral | Unpleasant (sulphur) |
| Hydrogen release | None (addition) | None | Yes (can inhibit addition silicone) | — |
Technique — Putty-Wash vs Single-Stage
- Two-stage putty-wash — a putty preliminary impression is taken, relieved, then a light-body wash captures detail. Accurate but the putty can lock undercuts; careful relief prevents distortion.
- Single-stage (simultaneous) — putty and wash mixed and seated together; faster, technique-dependent on even wash distribution.
Hydrogen Inhibition
Condensation silicones release hydrogen gas as they set, which can inhibit the surface set of an addition silicone used in the same impression. Avoid latex gloves with addition silicone (sulphur inhibits platinum catalyst); use nitrile or vinyl. Use a surfactant to wet the moist tooth before PVS.
Cements (Luting Agents)
| Cement | Strength | Adhesion | Notes / use |
|---|---|---|---|
| Zinc phosphate | High compressive, low tensile | None (mechanical) | Classic luting agent; acidic initial pH irritates pulp; needs liner |
| Zinc polycarboxylate | Moderate | Adheres to tooth (polyacrylic acid to Ca²⁺) | Pulp-friendly; lower strength; for less retentive prep needs mechanical retention |
| Glass ionomer (GIC) | Moderate | Chemical bond to tooth, releases fluoride | Soluble early; film thickness critical |
| Resin-modified GIC (RMGIC) | Moderate–high | Chemical + resin | Command set portion; fluoride release; common for crowns |
| Resin cement | High | Adhesive (tooth via adhesive, ceramic via silane/MDP) | Highest retention; technique-sensitive; for ceramic and bonded restorations |
Selection: resin cement for all-ceramic and bonded restorations; GIC/RMGIC for metal crowns and where moisture control is harder; polycarboxylate for sensitive pulps; phosphate as a reliable traditional option where good retention form exists.
Liners, Bases, and Varnishes
- Liner (thin, <0.5 mm) — protects the pulp or medicates it: calcium hydroxide (Dycal) for indirect/ direct pulp caps, stimulating reparative dentine; RMGIC liner for pulpal floor.
- Base (thicker, 1–2 mm) — provides thermal insulation and stress distribution under amalgam: GIC, ZOE, or RMGIC.
- Varnish — thin resin to seal dentinal tubules and reduce microleakage under amalgam (largely superseded by dentine bonding agents).
Calcium Hydroxide and MTA
- Calcium hydroxide — high pH (~12) causes mild necrosis that triggers reparative dentine bridge formation; used for direct and indirect pulp caps.
- MTA / bioceramic — now preferred for direct pulp capping and perforation repair; sets in moisture, biocompatible, seals well; slower set (replaced by faster bioceramics in many uses).
Selection by Situation — a Quick Map
| Situation | Material of choice |
|---|---|
| Class II amalgam, deep floor | Base with GIC/RMGIC; cavity varnish or bonding agent at margins |
| All-ceramic crown | Resin cement (adhesive to tooth + silane to ceramic) |
| Metal crown, good retention | GIC or RMGIC; phosphate acceptable |
| Direct pulp cap | Calcium hydroxide or bioceramic (MTA) |
| Final impression for crowns | Addition silicone (PVS) two-stage putty-wash, or polyether |
| Study model | Alginate, poured within the hour |
Pitfalls
- Storing an alginate impression dry → syneresis shrinkage and distorted cast.
- Latex gloves inhibiting addition silicone set at the impression surface.
- Mixing condensation silicone (hydrogen) with addition silicone → surface inhibition.
- Pouring polyether wet → water absorption and swelling distortion.
- Using a resin cement without proper tooth adhesive or ceramic silane → low bond and early failure.
Disinfection of Impressions
All impressions must be disinfected before handling or pouring to control cross-infection. The method depends on the material's dimensional stability and water sensitivity.
| Material | Recommended disinfection | Stability after disinfection |
|---|---|---|
| Addition silicone (PVS) | Immersion in glutaraldehyde or sodium hypochlorite | Excellent — tolerates immersion, can be poured days later |
| Polyether | Spray disinfection only — AVOID immersion | Absorbs water and swells if immersed; pour within 24 hours |
| Alginate | Brief immersion (~10 minutes) or spray, then pour promptly | Dimensionally unstable — pour within the hour |
| Condensation silicone | Brief immersion or spray | Poor stability — pour as soon as possible |
Working and Setting Times
Mix thoroughly within the working time so the material is homogeneous before it begins to polymerise; seat the tray before the onset of set to avoid distortion and capture fine detail. Once removed, pour within the material's dimensional-stability window: alginate within the hour (or store at 100% humidity), addition silicone can wait days, and polyether should be poured promptly and kept dry.
Digital Impressions (Intraoral Scanning)
Intraoral scanning captures the preparation and margins directly as a digital file, removing the need for impression material, disinfection, and a pour. Advantages include improved patient tolerance, no material-related distortion, and a direct digital workflow for crowns, bridges, and implant frameworks. For single units, accuracy is comparable to PVS; the scan captures the margin and can be reviewed on screen before the patient leaves. However, deep subgingival margins and bleeding sulci can still challenge scan accuracy, and a retraction cord or paste may be needed to expose it.
Which impression material offers the best dimensional stability and elastic recovery, allowing the cast to be poured several days later, but requires a surfactant because it is hydrophobic?
Which luting cement is most appropriate for cementing an all-ceramic crown to maximise adhesion to both the tooth and the restoration?
A latex glove inhibits the setting of which impression material at the contact surface, and what is the mechanism?
Which combination correctly pairs the material with its primary role in deep cavity management?