Two Impressions and a Closed Lab
At 4:40 on a Friday afternoon, the dentist takes a maxillary alginate impression for orthodontic study models and a mandibular vinyl polysiloxane impression for a crown. The courier collected at noon and the lab is closed until Monday; the office is closed until Monday as well. The receptionist has set both impressions, unbagged, on the front counter. At 4:55 the assistant notices beads of fluid forming at the palatal vault of the alginate and a distinct void in the rugae area, while the VPS impression looks clean and intact. The dentist has already left and the assistant is due to clock out.
1. How should each impression be handled for the weekend, and what is the reasoning behind the difference?
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The two materials fail in completely different ways.
- Alginate is a hydrocolloid and is dimensionally unstable in both directions: it loses water (syneresis - the beads of fluid she is seeing) and it swells if it takes on water (imbibition). Both distort. The ideal is to pour within about 10-12 minutes. A short delay of a few hours in a sealed bag with a damp paper towel is generally acceptable; a weekend is not. The defensible options are to pour the study model now in orthodontic stone in the office, or to retake on Monday. Wrapping it and hoping is not an option.
- Do not refrigerate alginate or store it in water. Both add distortion rather than preventing it.
- VPS is dimensionally stable and tolerates a weekend without change. Disinfect it, dry it, bag it, and it can go to the lab Monday.
- Label both bags with patient name, date, tooth, and material, and keep them off a sunny counter.
- Escalate rather than clock out silently: a phone call or note to the dentist about the alginate plan is part of the job, because the retake is an appointment that has to be made.
2. Both impressions are contaminated. What disinfection is appropriate for each, and what can go wrong?
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Disinfect by label, and watch contact time - especially on the hydrocolloid.
- The impression is a contaminated item. Rinse away visible blood and saliva first, then disinfect before it leaves the operatory - not at the front desk.
- VPS tolerates spray or immersion in an accepted intermediate-level disinfectant; follow the labelled contact time, then rinse. Immersion is not a problem for addition silicone.
- Alginate is the risk: it can imbibe the disinfectant solution. Use a spray, or immersion only if the disinfectant label specifically states compatibility with hydrocolloids, and do not exceed the labelled time. A long soak in a glutaraldehyde or hypochlorite bath will distort it even before the weekend does.
- Do not pool liquid in the palatal vault - invert and drain, and blot rather than soak.
- Record the disinfectant, concentration, and contact time on the lab slip. The lab, and any subsequent question about the case, depends on that entry.
- Gloves and eye protection apply here just as they do chairside; the impression is the most contaminated object in the operatory at that moment.
3. The void in the rugae area: does the alginate get poured or not? How do you decide and what do you check for next time?
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Decide by what the model is for, and never by what is convenient at 5:00 p.m.
- Study models are for arch form and occlusion. A localized void in the rugae may not affect their purpose at all, so the casts can still be diagnostic. But the assistant does not make that call alone - show the dentist and document the defect and the decision.
- A crown impression is a different standard. Any void, drag, or distortion at or near the finish line means a retake. A faulty cast produces an open margin, and the patient pays for it in a remake.
- Never send a defective impression hoping the lab will sort it out. The lab can pour it; the lab cannot fix the fit. Sending it also transfers the discovery to the patient's seat appointment.
- Look at causes for next time: tray size and depth, alginate mixed too thick or with the wrong water temperature, tray removed before the material set, or the tray seated with the palate under-filled. The rugae void in particular points to insufficient material in the vault or a tray that did not seat fully.
- Close the loop: note the retake needed, tell the front desk, and put the reason on the lab slip so the same thing is not repeated on the next set.