Resources / Clinical Technique

Retraction Cord vs. Paste: Science of Margin Capture

The physics of intraoral scanners — why digital impressions require physical tissue displacement, what the retraction literature actually shows about cords, hemostatic pastes, and lasers, and how to protocol a predictable digital scan.

By GIE Dental LaboratoryUpdated July 202611 min read7 peer-reviewed studies + 2 manufacturer sources cited

About this guide This guide summarizes independent, peer-reviewed research and manufacturer product documentation. It is an educational resource — not clinical advice. Every citation was individually verified against its original source before publishing. Every source is listed and linked in the references.

01 — Scanner Physics

The optical limits of intraoral scanners

When taking a traditional impression with polyvinyl siloxane (PVS) or polyether, the heavy-body material acts as a piston: as the tray seats, hydraulic pressure forces the elastomer into the sulcus, displacing saliva and pushing back the gingival cuff. The material can flow around a margin even when it's slightly subgingival.¹

Intraoral scanners have no hydraulic pressure. They're cameras recording light bouncing off a solid surface. If a margin is covered by even a thin film of tissue, blood, or crevicular fluid, the scanner can't capture it — the software either guesses at a line or leaves a void, producing an open margin or an over-contoured restoration. A 2021 randomized controlled pilot trial specifically evaluating subgingival vertical margins with intraoral optical scanning confirmed that reproducibility depends heavily on how well the margin is exposed before scanning.³

In practice, this means achieving real gingival displacement and hemostasis before scanning — the tissue needs to be dry and physically held clear of the prep edge, not just free of visible blood.

02 — Evidence

What the displacement literature actually shows

A classic pair of studies from the 1990s established much of what's still used to reason about gingival retraction timing today. Baharav and colleagues found that the length of time a displacement cord remains seated affects how wide the resulting crevice is — displacement isn't instantaneous, and cutting the seating time short reduces how much lateral opening you get.² A companion study by the same group also tracked how quickly the gingival crevice closes back up after the cord is removed¹ — which is the practical reason "scan immediately" is standard advice, digital or not: the retracted tissue doesn't stay open indefinitely once the cord comes out.

More broadly, a critical review of gingival displacement methods in prosthodontics lays out the same core requirement digital workflows inherited from conventional ones: whatever method you choose has to achieve real lateral displacement, not just dry the field, or the margin won't be usable downstream.⁴

03 — Comparison

Comparing retraction methods: cord, paste, and laser

Clinicians have several ways to displace tissue, and their measured effectiveness varies:

Double-Cord

Most Predictable Lateral Displacement

A controlled comparison found aluminum chloride-impregnated cord produced a mean gingival displacement of roughly 825 µm, compared with about 482 µm for an aluminum chloride paste (Expasyl) and 215 µm with no retraction at all.⁵ A thinner first cord manages fluid while a larger second cord stretches the gingival collar laterally.

Retraction Paste

Best for Hemostasis, Less Trauma

A systematic review comparing cord and paste found both effective, but paste is generally gentler on the tissue — the trade-off is that it produces meaningfully less lateral displacement than cord in head-to-head testing.⁶⁵

Laser Troughing

Fastest, Best Bleeding Control

A 2025 systematic review found laser troughing achieved the highest pooled mean horizontal displacement (about 0.53 mm) among all methods studied, with better bleeding control and shorter chair time than conventional techniques — though it's more technique-sensitive.⁷

04 — Protocol

Step-by-step double-cord digital protocol

For predictable digital margin capture on subgingival preparations, a double-cord technique — the same one described in Ultradent's own ViscoStat Clear clinical protocol — works well:⁸

First cord (#000 or #00): Pack a thin, knitted cord soaked in an aluminum chloride hemostatic (such as ViscoStat Clear, 25% aluminum chloride) into the base of the sulcus, ends cut flush so they don't overlap. Leave this cord in place during scanning.
Second cord (#0 or #1): Pack a larger cord over the first, visibly pushing the gingival margin laterally. Leave in place for several minutes — per the displacement-time research above, this isn't a step to rush.²
Moisturize and remove: Wet the second cord slightly before removal to avoid tearing the sulcular epithelium and triggering fresh bleeding. Gently pull it out.
Dry and scan immediately: Air-dry the sulcus and scan right away — the retracted tissue collar starts to close relatively quickly once the second cord is out, so delay works against you.¹

05 — FAQ

Frequently asked questions

Can I scan with the second cord still in the sulcus?

No. The second cord pushes tissue out but sits directly against the prep margin — if left in during scanning, the scanner records the cord's fibers instead of the tooth, and the restoration won't seat. The second cord comes out immediately before scanning; the thin first cord can stay.

Why do retraction pastes sometimes cause scans to fail?

Pastes like Traxodent (15% aluminum chloride in a clay base) leave a chalky residue if not rinsed thoroughly, and that residue can act as a spacer that distorts the scan. Rinse and dry aggressively before scanning — the manufacturer's own instructions flag this exact failure mode.

Which hemostatic agent is best for digital dentistry?

Aluminum chloride agents (like ViscoStat Clear) are often preferred over ferric sulfate for esthetic cases. A narrative review on ferric sulfate notes it can discolor dentin by precipitating ferric sulfide in an anaerobic environment — a concern under translucent restorations like e.max — though the same review notes gingival discoloration is transient and rinses away with about 10 seconds of water irrigation.⁹

References

Laufer BZ, Baharav H, Langer Y, Cardash HS. The closure of the gingival crevice following gingival retraction for impression making. J Oral Rehabil. 1997;24(9):629-635. doi:10.1046/j.1365-2842.1997.00558.x · PubMed 9357742peer-reviewed
Baharav H, Laufer BZ, Langer Y, Cardash HS. The effect of displacement time on gingival crevice width. Int J Prosthodont. 1997;10(3):248-253. PubMed 9484057peer-reviewed
Ferrari Cagidiaco E, Zarone F, Discepoli N, Joda T, Ferrari M. Analysis of the reproducibility of subgingival vertical margins using intraoral optical scanning (IOS): A randomized controlled pilot trial. J Clin Med. 2021;10(5):941. doi:10.3390/jcm10050941 · PubMed 33804358peer-reviewed
Prasad KD, Hegde C, Agrawal G, Shetty M. Gingival displacement in prosthodontics: A critical review of existing methods. J Interdiscip Dentistry. 2011;1(2):80-86. doi:10.4103/2229-5194.85023peer-reviewed
Kavita K, Sinha RI, Singh R, Singh R, Reddy KRP, Kulkarni G. Assessment of aluminum chloride retraction cords, Expasyl, and tetrahydrozoline-soaked retraction systems in gingival retraction. J Pharm Bioallied Sci. 2020;12(Suppl 1):S440-S443. doi:10.4103/jpbs.JPBS_131_20 · PubMed 33149502peer-reviewed
Bennani V, Chuang Y-S, Aarts JM, Brunton P. Evaluation of effectiveness and adverse effects of retraction cord vs retraction paste: A systematic review. Int J Prosthodont Restor Dent. 2021;11(4):183-190. ijoprd.compeer-reviewed, systematic review
Tamim H, Usumez A, Franzen R. Effectiveness of laser-assisted gingival troughing and conventional gingival displacement methods in fixed prosthodontics: A systematic review. J Prosthet Dent. 2025 (epub 2024). PubMed 38212156peer-reviewed, systematic review
Ultradent Products, Inc. ViscoStat Clear (25% Aluminum Chloride) — product information and clinical technique. ultradent.commanufacturer documentation
Bandi M, Mallineni SK, Nuvvula S. Clinical applications of ferric sulfate in dentistry: A narrative review. J Conserv Dent. 2017;20(4):278-281. doi:10.4103/JCD.JCD_259_16 · PubMed 29259368peer-reviewed