Delayed Pour of a Contaminated Long-Span VPS Impression
A 62-year-old woman requires a six-unit metal-ceramic fixed partial denture from the maxillary right first premolar to the maxillary left second premolar (FDI 14–25). The clinician used a one-step vinyl polysiloxane (VPS) impression in a rigid stock tray after gingival retraction with aluminum chloride-impregnated cord. Blood and crevicular fluid were visible at the finish lines of 14 and 25. The impression reached the laboratory four days later and had been stored in a closed plastic bag inside a car during a 38°C summer day. A technician pours it immediately with Type IV dental stone (linear setting expansion 0.28%), vacuum-mixed for 30 seconds. After separation, the dies show voids at the finish line on 14 and 25 and slight flattening of the palatal contours. The clinician asks the laboratory to pour it again because the impression is still good. The laboratory must decide whether to proceed, request a new impression, or adjust the die preparation.
1. Identify the likely causes of the die defects and explain why re-pouring the same VPS impression cannot be considered a reliable correction. Include the dimensional consequence of the 0.28% stone expansion over a 35 mm span.
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Likely causes
- Blood and crevicular fluid are incompatible with VPS, which is hydrophobic. Moisture and protein at the finish line prevent complete wetting, producing voids and rounded margins.
- Delayed pouring after four days, combined with heat exposure in a closed car, allows continued polymerization and stress relaxation in the impression. The impression can distort even though VPS has good short-term dimensional stability.
- A stock tray may flex during removal, especially in a long span, adding unrecovered strain.
- The voids are localized; the palatal flattening suggests overall distortion, not just a stone mixing error.
Why re-pouring will not correct the problem
Re-pouring can improve stone reproduction only if the impression itself is accurate. Here the impression has both surface contamination and possible bulk distortion. A second pour reproduces the same contaminated finish line and the same distorted palatal shape. It may look better because bubbles are random, but it does not restore missing detail.
Expansion calculation
A 0.28% linear expansion over 35 mm is 35 × 0.0028 = 0.098 mm, almost 0.1 mm. That may be intentional to compensate for casting and porcelain shrinkage, but it cannot compensate for a marginal void or a nonuniform impression. Over a six-unit span, the error accumulates and can affect proximal contacts and occlusion.
Judgement
Reject the impression. Request a new impression with hemostasis, a rigid custom or stock tray, and prompt pouring, or use a material and technique validated for delayed pour. Do not adjust the preparation to fit a defective die.
2. What clinical and laboratory steps would you require before fabricating this six-unit framework, and how would you control expansion and distortion?
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Clinical steps
- Re-evaluate tissue health. Aluminum chloride alone may not control bleeding; use a hemostatic agent appropriate for the retraction technique and verify a dry field before impressioning.
- Use a rigid tray and a verified one-step or two-step VPS technique. Ensure the tray does not flex and that the impression is poured without delay.
- Record the opposing arch, interocclusal record, and shade. Confirm the preparation finish line is visible and supragingival where possible.
Laboratory steps
- Pour a single, vacuum-mixed Type IV die stone with controlled water-to-powder ratio. Record the batch and expansion certificate.
- Measure the cast with a micrometer or caliper at fixed reference points. If the 0.28% expansion is higher than the alloy/porcelain compensation requires, use a lower-expansion die stone or a validated compensation protocol rather than arbitrary scraping.
- Section the dies, use die spacer, and articulate against the opposing cast. Check the 35 mm span for uniform clearance.
- Wax the framework with adequate connectors and verify the fit on the sectioned cast before casting.
Control of distortion
Expansion is not 'error' if it is uniform and planned; it becomes error when it is nonuniform. Keep the same stone batch, mixing time, and pouring direction. Do not re-pour to chase a defect. If CAD/CAM is used, scan a verified cast and inspect the digital margin.
Decision rule
Do not proceed with any die that has a void within the marginal zone. A remake impression is cheaper than a failed six-unit framework.
3. The clinician insists on proceeding. What written rejection criteria would you use, and how would you document the decision?
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Rejection criteria
- Any void, tear, or contamination within 0.5 mm of the finish line, or any finish line that cannot be traced continuously.
- Visible distortion of palatal or ridge contours when compared with the opposing cast or a verified pre-impression record.
- A cast that cannot be articulated without rocking or a cast with a broken die.
- An impression that was stored outside validated conditions and shows contamination, syneresis, or separation.
Documentation
- Photograph the impression under magnification and the dies at the defective margins.
- Record the impression material, tray type, retraction agent, storage time and temperature, stone batch, water-to-powder ratio, and measured expansion.
- State the rejection in writing to the clinician, with the specific criterion and the requested corrective action.
Reasoning
Proceeding would transfer an unverified error into a six-unit framework. The laboratory cannot know whether the void caused a 0.1 mm marginal opening or a larger internal discrepancy. A written rejection protects the patient and the laboratory and creates a quality-assurance record. If the clinician still insists, the laboratory should decline to guarantee fit and should not adjust the die to disguise the defect.