Introduction
Inlays and onlays are conservative indirect restorations that preserve healthy tooth structure while restoring function, form, and aesthetics. Unlike direct restorations placed chairside, inlays and onlays are fabricated outside the mouth on a working model created by pouring the impression in die stone or by generating a digital model through an intraoral scanner. In either workflow, the impression must capture the complete preparation geometry, all margins, and the surrounding tooth structure with high precision. The success of these restorations depends heavily on the accuracy of the impression, which serves as the primary link between the prepared tooth and the final prosthesis fabricated in the laboratory.
Any error at the impression stage carries through to the final restoration, resulting in marginal discrepancies, open contacts, or occlusal interferences.
This article presents a step by step guide to the impression procedure for inlays and onlays, covering material selection, tissue management, the syringe wash technique, quality evaluation, and provisional restoration. A comprehensive flowchart at the end summarises the entire clinical sequence.
What Are Inlays and Onlays?
Inlay
An inlay is an indirect restoration that fits within the confines of the cusps of a posterior tooth.
It replaces the damaged or carious portion of the tooth without covering or reinforcing any cusp tip.
Inlays are indicated when the cavity preparation is too large for a direct composite restoration but does not involve cusp replacement, for example large proximal surface restorations.
They are fabricated from ceramic (such as lithium disilicate or leucite reinforced glass ceramic), composite resin, or gold alloy, and are bonded or cemented into the preparation.
Onlay
An onlay extends beyond the cusp tips, covering one or more cusps of the tooth.
It is indicated when one or more cusps are weakened, undermined, or fractured, requiring protection and reinforcement.
An onlay provides greater coverage and structural support compared to an inlay while still being more conservative than a full coverage crown.
The same material options (ceramic, composite, gold) apply. Onlay preparations require additional reduction over the cusps being covered, typically 1.5 to 2.0 mm for ceramic materials.
Key Distinction: An inlay sits inside the cusps. An onlay covers one or more cusps. Both are indirect restorations requiring a precise impression for laboratory fabrication.
Impression basics
For fixed prosthodontic procedures including inlays, onlays, crowns, and bridges, elastomeric impression materials are the standard of care. Polyvinyl siloxane (PVS) and polyether are most commonly used due to their accuracy (detail reproduction below 20 µm), elastic recovery greater than 99%, and dimensional stability.
The dual viscosity technique (putty wash or heavy body/light body) is the most widely employed method: a high viscosity material provides structural support while a low viscosity wash flows into preparation details and around the margins.
Steps in Impression Making for Inlays and Onlays
Step 1: Case Assessment and Treatment Planning
Before any clinical procedure begins, a thorough assessment is essential. This includes clinical examination of the tooth (caries extent, crack lines, remaining tooth structure), periapical and bitewing radiographs, evaluation of the occlusion (centric contacts, excursive movements), and determination of the appropriate restorative material. For onlays, assess which cusps need coverage based on the extent of structural compromise.
Step 2: Tooth Preparation
Remove all caries and defective restorations. Create slightly divergent walls with a taper of 6 to 10 degrees for a clear path of insertion. Round all internal line angles to reduce stress concentration and facilitate impression material flow. The pulpal floor should be flat, the gingival floor smooth, and the cavosurface margin clearly defined. Minimum depth is 1.5 to 2.0 mm for adequate material thickness. The box walls should diverge in an occlusal direction. For onlays, reduce the cusps to be covered by at least 1.5 mm (ceramic) or 1.0 mm (gold), creating a defined shoulder margin at the cusp overlay junction.
Step 3: Gingival Retraction (When Required)
If any margin extends subgingivally, gingival retraction is mandatory. Place a retraction cord (size #00 or #0) into the sulcus using a cord packing instrument. The cord may be impregnated with aluminium chloride or ferric sulfate for hemostasis. Allow 5 to 10 minutes of dwell time. For a dual cord technique, place a thinner cord first followed by a thicker cord on top. Remove the outermost cord immediately before injecting the impression material, leaving the thinner cord to maintain tissue deflection and control fluid seepage.
Step 4: Tray Selection and Adhesive Application
Select an appropriately sized impression tray. Options include a rigid metal stock tray, a custom acrylic tray, or a triple tray (for single unit restorations capturing preparation, opposing arch, and bite simultaneously). The tray must be rigid to prevent flexure during seating, which would distort the impression. Apply the manufacturer recommended tray adhesive to all internal surfaces of the tray and allow it to dry completely (typically 5 to 10 minutes). The adhesive creates a chemical bond between the tray and the impression material, preventing separation during removal.
Step 5: Impression Material Preparation
Using an automixing cartridge system, dispense the heavy body or putty material into the tray. Simultaneously, load the light body material into a syringe with an intraoral tip. Automixing systems ensure consistent base to catalyst ratios and minimise air bubble incorporation compared to hand mixing. Verify the expiry date of all materials and ensure cartridges have been stored at the recommended temperature.
Step 6: Light Body Syringe Application
This is the most critical step. Inject the light body (wash) material directly around the entire preparation, starting at the deepest part of the preparation (the pulpal or gingival floor) and working outward. Cover all margins, internal line angles, and the cavosurface junction with a continuous, unbroken bead of material. Avoid trapping air by keeping the syringe tip embedded in the material as you dispense. Also apply light body over adjacent teeth to capture contact areas. For onlay preparations, ensure complete coverage of the reduced cusp surfaces and the buccal and lingual margins.
Clinical Tip: Use a gentle air stream to thin the light body wash over the preparation just before tray seating. This creates a more uniform film and helps displace moisture from the sulcus.
Step 7: Tray Seating and Holding
Seat the heavy body loaded tray over the syringe material with a firm, even, and controlled motion. Centre the tray over the arch and press down steadily. Do not rock or shift the tray once seated. Maintain consistent finger pressure throughout the entire setting time recommended by the manufacturer (typically 4 to 6 minutes for PVS, 6 to 8 minutes for polyether, measured from start of mix). Instruct the patient to remain still and avoid biting, swallowing forcefully, or moving the tongue.
Step 8: Removal and Quality Inspection
Once the material has fully set, remove the impression with a single, quick, firm snap along the path of insertion. Immediately inspect under good lighting, preferably with magnification. Verify that all preparation margins are clearly and continuously captured, internal surfaces are void free with sharp anatomical detail, there are no tears or distortions (especially at the gingival margin), and adjacent teeth and contacts are fully recorded. If any defects are found, discard the impression and repeat the procedure.
Step 9: Opposing Arch and Bite Registration
Record the opposing arch impression using alginate (for cost efficiency) or PVS (for greater accuracy). Take a bite registration using a rigid bite registration material (PVS based or polyether based) in centric occlusion. Ensure the registration is thin and captures the cusp tips of both arches without perforations. The bite registration allows the laboratory to mount the casts in accurate occlusal relationship.
Step 10: Provisional Restoration
Fabricate a provisional inlay or onlay using a pre operative putty matrix and bis acryl composite or a prefabricated provisional material. Cement with a temporary cement (eugenol free if a resin bonded ceramic restoration is planned). Verify occlusion in centric and excursive movements and check proximal contacts. Instruct the patient on temporary care.
Step 11: Disinfection and Laboratory Communication
Rinse the impression thoroughly under running water to remove saliva and debris. Disinfect according to the impression material manufacturer’s protocol (immersion or spray with an approved disinfectant such as 2% glutaraldehyde or 1:10 sodium hypochlorite). Package the impression in a sealed bag with the laboratory prescription specifying the tooth number, material choice, shade (using a shade guide or digital shade measurement), and any special instructions. Dispatch to the laboratory promptly.
Clinical Flowchart
The following flowchart summarises the complete step by step impression protocol for inlays and onlays, from initial assessment through laboratory dispatch.
FLOWCHART: Steps in Impression Making for Inlays and Onlays | ||
|---|---|---|
Step | Stage | Key Actions |
1 | Case Assessment and Treatment Planning | Clinical and radiographic evaluation, caries extent, occlusal analysis, material selection (ceramic, composite, gold) |
2 | Tooth Preparation | Caries removal, divergent walls (6–10° taper), rounded internal angles, flat pulpal/gingival floor, 1.5–2.0 mm depth, isthmus width ≥1/3 intercuspal distance |
3 | Gingival Retraction (if subgingival) | Retraction cord placement (#00 or #0), hemostatic agent application, 5–10 min dwell time, cord removal just before impression |
4 | Tray Selection and Adhesive Application | Select rigid custom or triple tray, apply material specific tray adhesive, allow 5–10 min drying time |
5 | Impression Material Preparation | Dispense PVS or polyether via automixing gun, load heavy body into tray, load light body into syringe tip |
6 | Light Body Syringe Application | Inject light body around entire preparation, cover all margins and internal line angles, syringe from deepest area outward, maintain continuous bead |
7 | Tray Seating and Setting | Seat loaded tray over the syringe material, apply firm and even pressure, hold steady during full setting time (typically 4–6 min for PVS) |
8 | Impression Removal and Inspection | Remove with single firm snap, inspect for margin capture, check for voids, tears, pulls, and bubbles, re-impression if defects found |
9 | Opposing Arch and Bite Registration | Take opposing arch impression (alginate or PVS), record centric occlusion with bite registration material |
10 | Provisional Restoration Placement | Fabricate and cement provisional inlay/onlay, verify occlusion and contacts, instruct patient on temporary care |
11 | Disinfection and Laboratory Dispatch | Rinse impression under water, disinfect per manufacturer protocol, package with prescription and shade, send to laboratory |
Conclusion
The impression is the single most important step linking chairside tooth preparation to the final laboratory fabricated inlay or onlay. Every stage of the impression protocol, from tissue retraction to syringe technique to quality inspection, contributes to the accuracy of the working model and, ultimately, to the marginal fit, occlusal harmony, and longevity of the restoration.
Polyvinyl siloxane and polyether remain the materials of choice for conventional impression techniques, with the dual viscosity (heavy body plus light body) approach offering the best combination of structural support and fine detail capture. Intraoral scanners are increasingly viable for inlay and onlay cases, though mastery of the conventional technique remains essential for all clinical scenarios.
By following a systematic, step by step protocol and applying rigorous quality checks at every stage, clinicians can consistently produce impressions of the highest accuracy, leading to well fitting indirect restorations that serve patients for years to come.
Conventional versus digital impression quality control
Both workflows must communicate the complete preparation, a continuous finish line, proximal relationships, adjacent contacts and occlusion. A technically impressive scan or impression is still unacceptable when a clinically important margin cannot be read.
| Checkpoint | Conventional workflow | Digital workflow |
|---|---|---|
| Moisture control | Prevent contamination during material setting | Maintain a dry, visible scanning field |
| Margin capture | Inspect for tears, pulls and voids | Inspect the 3D model for missing or stitched data |
| Proximal area | Confirm tray and material extension | Ensure complete interproximal acquisition |
| Occlusion | Use a stable interocclusal record when needed | Verify bite alignment and scan accuracy |
Use the crown and bridge checklist for broader fixed-prosthodontic planning. Compare material choices in the PEEK dentistry review. For direct alternatives, see the Class III composite case.
Frequently asked questions
When should an impression or scan be repeated?
Repeat it whenever the finish line or other information essential to fabrication is incomplete, distorted or ambiguous.
Is a digital impression always more accurate?
No. Accuracy depends on preparation, isolation, scanner, scanning strategy, span and operator technique.
Why capture the opposing arch and bite carefully?
The laboratory needs accurate interarch information to design functional occlusion and minimise chairside adjustment.