Why We Need Impressions
Every indirect dental restoration — whether a crown, bridge, inlay, onlay, denture, orthodontic appliance, or implant prosthesis — is fabricated outside the mouth on a replica model. The impression is the critical link between the patient’s oral anatomy and the laboratory. Its accuracy directly determines the marginal fit, occlusal harmony, and long-term success of the final prosthesis.
Specific reasons impressions are indispensable include: recording the prepared tooth geometry and finish-line for fixed restorations; capturing the edentulous ridge form, palatal contours, and functional borders for removable dentures; documenting the three-dimensional position of implant fixtures through scan bodies or impression copings; obtaining baseline study models for orthodontic diagnosis and treatment planning; fabricating surgical guides, stents, bleaching trays, mouthguards, and night guards; and monitoring changes in tooth position, wear, or soft-tissue dimensions over time through serial models.
Even with the rapid adoption of intraoral scanning, the objective remains the same — an accurate three-dimensional record of the clinical situation. The method of capture (material-based versus optical) is evolving, but the fundamental requirement for precision and completeness has not changed.
Different Armamentarium for Impressions
The impression armamentarium can be divided into seven functional categories: trays, impression materials, mixing and dispensing devices, adhesives and spacers, tissue management instruments, disinfection supplies, and digital scanning equipment. Each category plays a defined role in ensuring the impression procedure yields clinically acceptable results. The table below provides a comprehensive inventory.
Category | Items | Purpose |
|---|---|---|
Impression Trays | Stock metal/plastic trays (perforated & non-perforated), custom acrylic trays, sectional trays, triple trays | Support and confine impression material during setting; custom trays improve accuracy for final impressions |
Impression Materials | Alginate, PVS (addition silicone), polyether, polysulfide, ZOE paste, agar, impression compound | Record negative replica of oral tissues for cast fabrication |
Mixing & Dispensing | Rubber mixing bowl, alginate spatula, automixing gun with cartridges, mixing pads, dispensing tips | Ensure homogeneous, void-free material mix; automixing reduces bubbles and ensures consistent ratio |
Adhesives & Spacers | Tray adhesive (material-specific), baseplate wax spacer, tray adhesive solvent | Bond material to tray and create relief space in custom trays |
Retraction & Tissue Management | Retraction cords (#000 to #3), retraction paste, hemostatic agents, electrosurgery unit | Expose subgingival finish lines and control moisture/bleeding |
Disinfection | Surface disinfectant spray, immersion solutions (glutaraldehyde, sodium hypochlorite) | Decontaminate impressions before dispatch to laboratory |
Digital Scanning | Intraoral scanner, scan bodies, scanning powder (if needed), calibration tools | Capture optical digital impressions; eliminate material-based errors |
Tray selection is the first decision.
Stock trays (metal or plastic, perforated or rim-lock) are used for preliminary impressions,
Custom trays fabricated on primary casts are essential for final impressions in complete denture and precision fixed prosthodontics.
Triple trays capture the preparation, opposing arch, and bite registration simultaneously and are popular for single-unit restorations.
Material handling equipment has advanced significantly.
Automixing cartridge systems for PVS and polyether have largely replaced hand mixing, providing consistent base-to-catalyst ratios, reduced void incorporation, and extended working time.
For alginate, mechanical vacuum mixers are available in high-volume clinics, though manual bowl-and-spatula mixing remains standard in most practices.
We need adhesives and spacers to make impressions accurate and stable:
Tray adhesive (material-specific):Bonds the impression material to the tray to prevent pull-away, bubbles, and distortion on removal. Use the adhesive made for the same material (e.g., alginate, PVS, polyether) and let it dry before loading.
Baseplate wax spacer:Creates uniform material thickness and space for wash/detail, improving accuracy and reducing internal stress/pressure spots. Commonly adapted as an even layer, stopping short of borders as needed.
Tray adhesive solvent:Removes old adhesive from trays (and helps clean before reapplication) so new adhesive bonds reliably. Use with ventilation and follow the product instructions; clean and dry the tray afterward.
Tissue management deserves emphasis. Retracting the gingiva to expose subgingival margins is essential for an accurate impression of the finish line.
Retraction cords impregnated with hemostatic agents (aluminium chloride, ferric sulfate) remain the most common method.
Retraction pastes (e.g., Expasyl, Traxodent) offer a cordless alternative that is less technique-sensitive.
Electrosurgery and laser-assisted retraction are used in select cases.
Dental impression disinfection is important because impressions can carry saliva, blood, and germs from the patient to your team and the dental lab. Disinfecting them helps prevent cross-contamination and protects patients, staff, and lab technicians.
Rinse immediatelyunder cool water to remove saliva/blood/debris.
Disinfectusing anapprovedproduct that’scompatible with the impression material(spray or immersion) andtime the full contact time.
Rinse again, shake off excess water (don’t over-dry), andpackage in a clean bag/container.
Label for the lab:“Disinfected,” product used, time completed, and initials.
Intraoral Scanners: Types and Specifications
Intraoral scanners (IOS) represent the digital evolution of impression making. These handheld devices project a light source (laser or structured light) onto oral tissues, capture the reflected pattern using imaging sensors, and reconstruct a three-dimensional surface model through proprietary software algorithms. The output is a digital file (typically STL, PLY, or DCM format) that can be transmitted electronically to the laboratory or used directly for chairside CAD/CAM milling.
Modern scanners operate on one of several optical principles: confocal microscopy (CEREC Omnicam), active wavefront sampling, triangulation (Planmeca Emerald), parallel confocal imaging (iTero), ultrafast optical sectioning (3Shape TRIOS), or video-based 3D-in-motion capture (Medit). Each approach has implications for scan speed, depth of field, accuracy, and sensitivity to ambient light or moisture. Regardless of the underlying technology, current-generation IOS devices consistently achieve clinically acceptable accuracy for most indications, with deviations typically within 50 µm for full-arch scans and below 20 µm for single-unit preparations.
Comparative Specifications of Leading Intraoral Scanners
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