Impressions

Soft Tissue Control Guidelines

  • A thorough periodontal lead-up should be performed on and around the abutment tooth prior to the preparation and impression stages and appropriate hygiene instruction
  • Delivery of 0.12% chlorhexidine gluconate for two weeks preoperatively, for two weeks during provisionalisation and two weeks postoperatively
  • Utilise a quality minimal thickness retraction cord packer at all times

Gingival Retraction 

Margin Placement

The final cervical finish line placement in the crevice should not damage the soft tissues. The most atraumatic way to avoid soft tissue injury is by use of rotary instruments (Tissue Protection End-Cutting Burs).

Retraction

Be gentle. Cord should be moistened with water prior to removal from the sulcus.

Single-cord Technique (commonly used but often insufficient)

Use when preparing margins at or above tissue height when gingival tissues are healthy and no bleeding occurs when the cord is packed. A retraction cord with an appropriate diameter is placed into the sulcus for 8-10 minutes.

Double-cord Technique

Best used when subgingival margins are required and/or if the tissue health is not ideal.

Capturing Margins with Digital Dentistry  

  • A dual-cord retraction technique is advisable. After the preparation, a thin cord is placed and an initial scan is performed. Any areas on the preparation that need to be modified are noted and the preparation is refined
  • Digital impressioning will only capture visible data. The dentist is compelled to provide the necessary retraction. Current digital systems do not scan through soft tissues
  • Digital impressions are sensitive to moisture contamination. Blood and saliva obscure the surface to the tooth or margin from the camera and prevent an accurate recording
  • Isolate the margins of all of the tooth preparations and ensure that they are visible to the eye before making a digital impression
  • Any sharp preparation areas will be distorted by scanning. The preparation should be smoothed and then re-scanned
  • A digital scan should capture the entire restorative margin as well as about 0.5 mm of the tooth/root surface apical to the margin for fabrication of the correct emergence profile

Impression Tray Selection  

  • Full-arch, perforated metal, rigid plastic or custom trays are recommended for fixed or removable prosthetic restorations involving three or more units (veneers and long-span bridges)
  • Shillingburg HT. et al (2012) stated the dual-arch technique is best used for patients with:
      • intact, mutually protected Angle Class I occlusion
      • single tooth restorations with intact adjacent & opposing teeth with no arc of  closure interference into maximum interdigitation