17.6 Impression Materials and Digital Scanning
Key Takeaways
- Definitive impressions must capture detail below 20 micrometres and remain dimensionally stable until pouring.
- Polyvinyl siloxane has excellent dimensional stability, under 0.05% change at 24 hours, and produces no volatile byproducts.
- Sulphur in latex gloves poisons the platinum catalyst of addition silicone, so nitrile or vinyl gloves must be used.
- Polyether is intrinsically hydrophilic and captures detail in moist fields but is rigid and difficult to remove past undercuts.
- Intraoral scanners avoid tray distortion and impression disinfection but struggle with deep subgingival margins and fluid contamination.
2. Impression Materials in Fixed Prosthodontics
Definitive impressions must capture fine detail (< 20 µm) and maintain dimensional stability through pouring and stone crystallization.
Elastomer Polymerization Chemistry & Byproducts
Addition Silicone (PVS):
Vinyl Siloxane + Silane Prepolymer ──[Platinum Catalyst]──> Crosslinked Silicone
**Byproducts:** NONE (Except trace H2 gas; Pd scavengers added)
**Dimensional Change:** < 0.05% (Highest Stability)
Polyether:
Aziridine Rings ──[Aromatic Sulfonate Ester]──> Crosslinked Polyether
**Byproducts:** NONE
**Dimensional Change:** < 0.15% (Inherently Hydrophilic, High Rigidity)
Condensation Silicone:
Silanol + Ethyl Silicate ──[Tin Octoate Catalyst]──> Silicone + **ETHANOL (Volatile)**
**Dimensional Change:** > 0.5% Shrinkage within hours
Polysulphide:
Polysulphide Polymer + Lead Dioxide ──> Crosslinked Rubber + **WATER (Evaporates)**
**Dimensional Change:** > 0.4% Shrinkage within hours
Polyvinyl Siloxane (PVS / Addition Silicone)
- Polymer Chemistry: Hydrosilylation reaction between vinyl-terminated polymethylsiloxane prepolymers and silane-containing cross-linking agents, catalyzed by chloroplatinic acid (platinum catalyst).
- Dimensional Stability: The gold standard. Zero volatile reaction byproducts are generated, yielding the lowest polymerization shrinkage of any elastomer (< 0.05% at 24 hours) and an elastic recovery exceeding 99.5%. Pores and models can be repoured multiple times over several weeks.
- Hydrophobicity: PVS is inherently hydrophobic (high contact angle with water). Manufacturers add non-ionic surfactants to lower surface tension, but an absolutely dry field is mandatory for capturing marginal detail.
- Latex Glove Inhibition (The Sulphur Trap): Sulphur compounds, specifically dithiocarbamate vulcanization accelerators found on the surface of powdered and unpowdered latex gloves, poison the platinum catalyst. Touching the tooth preparation, retraction cords, putty, or mixing tips with latex gloves halts addition polymerization, leaving a sticky, unset layer of silicone on the critical margins. Clinicians must wear nitrile or vinyl gloves during all phases of PVS impression taking.
Polyether (e.g., Impregum)
- Polymer Chemistry: Cationic ring-opening addition polymerization of difunctional polyether prepolymers containing terminal reactive aziridine rings, initiated by an alkyl-aromatic sulfonate ester initiator.
- Intrinsic Hydrophilicity: Polyether contains polar ether groups, imparting natural hydrophilicity with a low water contact angle without requiring added surfactants. It excels at capturing subgingival margins in the presence of trace crevicular moisture.
- Mechanical Rigidity: Polyethers exhibit the highest stiffness (elastic modulus / Shore hardness) among impression materials. If severe interdental undercuts, open embrasures, or bridge spans are not meticulously blocked out with utility wax, the set tray can lock permanently into the patient's mouth or fracture stone dies during laboratory retrieval.
- Storage Requirement: Polyethers absorb water and swell in humid conditions. Impressions must be stored dry at room temperature and must never be submerged in water or disinfectant baths for extended periods.
| Elastomeric Property | Polyvinyl Siloxane (PVS) | Polyether | Condensation Silicone | Polysulphide |
|---|---|---|---|---|
| Polymerization Type | Addition (Hydrosilylation) | Addition (Ring-opening) | Condensation | Condensation |
| Volatile Byproduct | None (Trace H₂ gas) | None | Ethanol (Ethyl alcohol) | Water (H₂O) |
| Dimensional Shrinkage (24h) | < 0.05% | < 0.15% | 0.4–0.6% | 0.3–0.5% |
| Elastic Recovery | > 99.5% | ~98.5% | ~98.0% | ~97.0% |
| Inherent Hydrophilicity | Hydrophobic (Surfactant modified) | Intrinsic Hydrophilic | Hydrophobic | Moderately Hydrophilic |
| Stiffness / Rigidity | Moderate to High | Very High | Moderate | Low (Very Flexible) |
| Major Clinical Limitation | Latex glove sulphur inhibition | Traumatic lock-in undercuts | Immediate pour required | Foul odour; long set (10 min) |
3. Digital Impressions: Intraoral Optical Scanning (IOS)
Modern digital impression systems replace elastomeric trays with intraoral optical wands utilizing structured light projection, active wavefront sampling, or confocal microscopy.
- Operating Principles: The scanner captures thousands of two-dimensional optical slices per second. Algorithms stitch overlapping point clouds into a 3D polygonal surface mesh (typically formatted as STL, OBJ, or PLY files).
- Clinical Advantages:
- Completely eliminates patient gagging, discomfort, and tray distortion;
- Eliminates stone expansion and gypsum die abrasion errors;
- Real-time feedback: Preparation taper, undercuts, and clearance can be checked immediately, allowing immediate re-prep if needed;
- Rapid digital transmission to dental laboratories.
- Clinical Limitations & Physics Constraints:
- Line of Sight: Optical scanners cannot penetrate blood, saliva, or soft tissue. Subgingival margins obscured by bleeding or tissue collapse cannot be captured. Dual-cord retraction or surgical troughing is just as vital for IOS as for elastomeric impressions.
- Stitching Error Drift: Scanning long edentulous spans or full arches is susceptible to cumulative stitching drift, which can introduce minor rotational errors in full-arch implant frameworks.
Choosing and Handling Impression Materials
The examinable comparisons are dimensional accuracy, hydrophilicity, tear strength and handling. Addition silicone has excellent accuracy and dimensional stability, allowing delayed pouring, but it is intrinsically hydrophobic and requires a dry field; its set is inhibited by latex gloves and by some retraction cord chemicals, which is why non-latex gloves are specified. Polyether is more hydrophilic, so it copes better with a slightly moist sulcus and gives excellent surface detail, but it is stiff on removal — a problem with undercuts and periodontally compromised teeth — and it absorbs water, so it must not be stored in disinfectant for prolonged periods. Alginate is cheap and hydrophilic but dimensionally unstable through syneresis and imbibition and must be cast within the manufacturer's stated window, conventionally within an hour and kept in a sealed humid bag meanwhile. Impression compound and zinc oxide eugenol retain a role in complete denture technique.
Disinfection and Laboratory Communication
Every impression leaving the surgery must be rinsed under running water to remove saliva and blood and then disinfected according to the manufacturer's instructions, typically by immersion in a validated disinfectant for a specified time, before being sent to the laboratory. This is both an infection control requirement and a quality requirement, because prolonged immersion distorts alginate and polyether. The accompanying laboratory prescription must identify the patient and clinician, state the work required, the materials and shade, and the date required, and the clinician retains responsibility for the prescription's accuracy. Under UK medical devices regulation, a custom-made device requires a statement of manufacture from the laboratory, which must be retained and offered to the patient.