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Prosthodontics & Occlusion · patient case with 1 question

Impression taking for a full-coverage crown

Patient: Female, 48 years old

Chief Complaint: Attending for the impression stage of a crown on tooth #19.

Background: No systemic disease, no medications, no allergies. Healthy periodontium.

Vital Signs: BP 120/76 mmHg · HR 72 bpm

Findings: Preparation of #19 complete with a chamfer finish line placed 0.5 mm subgingivally on the mesial. Sulcus depth 2 mm, no bleeding on probing, adequate attached gingiva. A retraction cord is to be placed before the definitive impression.

Question 1

What does the retraction cord achieve at the gingival margin?

  1. LATERAL displacement of the gingival tissue away from the preparation, opening the sulcus so impression material can flow around the finish line
  2. Chemical cauterisation of the sulcular epithelium
  3. Permanent apical repositioning of the gingival margin to lengthen the clinical crown
  4. Coronal displacement of the tissue to protect the finish line during impression taking
Show answer and explanation

Correct answer: A. LATERAL displacement of the gingival tissue away from the preparation, opening the sulcus so impression material can flow around the finish line

  • A. CORRECT. Retraction cord displaces the free gingiva LATERALLY — horizontally, away from the tooth — and to a small extent apically, opening the sulcus so that light-body impression material can flow past the finish line and record it plus a band of unprepared tooth beyond it. It also controls sulcular fluid and haemorrhage, particularly when impregnated with aluminium chloride or ferric sulfate. The displacement is TEMPORARY: tissue rebounds within minutes, which is why the impression is taken immediately after cord removal.
  • B. Cord does not cauterise. Chemical tissue removal is what electrosurgery or a diode laser does, and those are alternatives to cord rather than descriptions of it.
  • C. Permanent apical repositioning is CROWN LENGTHENING — a surgical procedure. Retraction cord produces a transient displacement, and if it causes permanent recession that is an injury, not the intended effect.
  • D. Coronal displacement is the opposite of what is needed — it would bury the margin further.

Note: Two practical points. Pack cord gently and never force it beyond about 0.5 mm below the finish line, or the junctional epithelium is damaged and recession follows. And judge the impression before dismissing the patient: it must record the ENTIRE margin plus unprepared tooth beyond it, with no voids, drags or blood. A margin you cannot see on the impression is a margin the technician will guess at.

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