Depth of Preparation Line — When Subgingival, When Supragingival

One of the biggest causes of laboratory errors is an imprecise preparation line on the impression. The laboratory receives an image where the margin of the preparation disappears into the soft tissues — no clear boundary line that the technician could reproduce in CAD. The result: a crown margin that doesn't fit or is too loose. The decision about margin depth is not arbitrary — it results from biological limits, the clinical zone, and aesthetics. deltalabs. knows exactly which marginal line guarantees a repeatable result.

Main Problem — Blurred Line on Impression Causes Return to Lab

When you place a margin deep subgingivally without retraction, the soft tissue obscures the preparation margin. The impression shows a blurry boundary — the lab technician is unsure where exactly the tooth preparation ends. Consequences of a blurred line:

• Preparation line unclear in CAD software
• Technician forced to rely on intuition, not precision
• Crown margin too loose (allows bacteria) or too tight (irritates)
• Feedback from the lab — "please send an impression with a clear line"
• Delay in order fulfillment

Rule: Clear preparation line = fast production. Blurred line = problems.

Principles of Depth Depending on Clinical Zone

Aesthetic Zone (anterior teeth)

In the aesthetic zone, the margin must be hidden. You work subgingivally, but always with retraction — the cord temporarily irritates, creating space for line clarity.

Non-aesthetic Zone (posterior teeth)

The patient doesn't see the margin — you work above the gingival line. Minimize irritation, maximize comfort.

Biological Width — What it is and Why Violation Costs the Patient

Biological width — the space between the apical extent of the prepared margin and the crest of the alveolar bone. Average dimensions: 2.0–3.0 mm (approx. 2.5 mm for most). This is not a suggestion — it's a biological limit. If you place the margin too deep, the periodontal tissue must "recede." Consequences of violating biological width:

• Periodontal inflammation — patient feels discomfort
• Gingival recession — margin recedes, black triangles
• Asymmetry of the smile line — aesthetic change
• Infectious effect — inflammation also affects adjacent teeth

Rule: Radiographically measure the distance between the planned preparation line and the alveolar crest. If \\< 2.0 mm — work supragingivally.

Retraction Techniques — Cord vs. Electrosurgery

Retraction Cord (standard)

Procedure:

1. Place epinephrine-impregnated cord in the gingival sulcus
2. Wait 3–5 minutes
3. Remove the cord — gingiva is expanded
4. Immediately — before it returns — take the impression (A-silicone)

Advantages: reliable, inexpensive, controlled. Disadvantages: patient discomfort, requires time.

Electrosurgery (for bleeding)

If bleeding occurs despite the cord:

1. Electrosurgical tip 2–3 mm
2. Pulse 1–2 seconds per spot
3. Hemostasis
4. Impression immediately (A-silicone)

Advantages: rapid hemostasis, patient feels no pain. Disadvantages: requires equipment, risk of burns. Best practice: Cord first. If it doesn't work — electrosurgery.

Decision Table for the Clinician