In fixed prosthodontics we use many impression methods to capture the detail of our tooth preparations, from traditional impression techniques to digital scanning of the tooth preparation. Either way, one step in the process is often crucial: gingival retraction.
Gingival retraction is the reversible displacement of the soft tissues to expose the finish line of the tooth preparation. It is absolutely essential to ensure that this area is captured accurately and reproduced in a cast to allow fabrication of an accurately fitting restoration.
The gingival tissues can be retracted mechanically, and indeed mechano-chemically. The most common techniques used clinically are retraction cords and retraction agents. Although techniques vary depending on individual manufacturers’ instructions, a number of steps can be carried out to optimise gingival retraction.
It is always best to achieve good gingival health before preparations, gingival retraction and impressions are attempted (Figures 1, 2 and 3). Tooth preparation should be carried out carefully to avoid unnecessary trauma to the sulcular and periodontal attachment (Figures 4, 5 and 6).
If the soft tissues are bleeding profusely after tooth preparation, it may be best to delay the impression stage to another day to allow healing. The key to good soft tissue healing before the impression appointment is the provision of a well-fitting, highly polished provisional restoration. This will maintain the tissue stability from preparation through to final restorations (Figures 7, 8 and 9).
Chemical agents can be used to help control bleeding prior to impression making. Many retraction cords are impregnated with haemostatic agents to control bleeding during gingival retraction: common astringents include aluminium potassium sulphate, aluminium sulphate, aluminium chloride and ferric sulphate. Separate haemostatic agents can be used, such as ferric sulphate and aluminium chloride, to soak cords or use topically on bleeding spots. The use of cords impregnated with vasoconstrictors such as adrenaline/epinephrine has become less popular due to both local and systemic side effects. Haemostatic agents may affect the polymerisation of additional silicone impression materials if they are not adequately washed away. They should be rinsed for at least 10 seconds with water spray.
Retraction cords are still the most popular method used. Newer cords place more emphasis on cord material and design. Some cords are tightly braided to aid rigidity and grip, e.g., StayPut. Others are loosely knitted to reduce pressure during insertion and removal, and to soak up gingival crevicular fluid, e.g., Ultrapak (Table 1).
Retraction agents and pastes are made from various materials, but can be categorised into two types: non-medicated silicone materials, which are purely mechanical, e.g., Magic foam cord; and, kaolin-based materials impregnated with aluminium chloride, e.g., Expasyl.
The retraction technique selected depends on the position of the finish line and the gingival biotype. In patients with thin biotypes it is best to use less traumatic techniques to minimise the risk of gingival recession, e.g., single thin-knitted cord or retraction paste. Patients with thicker tissues and deeper margins may require more displacement, so techniques such as the double cord technique may be used. In the double cord technique one thin cord is placed apically and a thicker cord is placed coronally for horizontal and vertical retraction. The most superficial cord is removed immediately prior to impression making.
- Cut the cord to the appropriate length. Place the cord circumferentially into the sulcus, gently inserting it with a thin tipped cord-packing instrument. If you intend to remove the cord, leave excess exposed (Figure 10).
- The decision to leave a cord in place or to remove it depends on biotype and what is visible after cord insertion. If the cord itself is obstructing access to the finish line it should be removed (Figures 11 and 12). If the cord is below the finish line and all of the desired surfaces are accessible to impression material then it can be left in place (Figures 13 and 14).
- In a two-cord technique, the apical thin cord is left in place, and the thicker coronal cord is removed.
- Always wet the cord prior to removal to avoid tearing of the gingival tissues, then dry gently after removal prior to placing impression material. Cord should not be left in the sulcus for more than five to 10 minutes.
Each system varies, so follow the manufacturer’s instructions:
- paste should be carefully released into the sulcus lateral to the prepared tooth (Figures 15 and 16);
- follow the manufacturer’s instructions with regard to time left in situ (usually three to six minutes); and,
- with air and water combined, flush away any remnants of paste, then dry carefully; pastes containing haemostatic agents must be thoroughly washed away as they may inhibit the set of some impression materials.
Good light and magnification can help with either procedure. Accurate retraction leads to an accurate impression and, subsequently, a well-fitting final restoration (Figures 17, 18 and 19).
Dr Rebecca Carville
BA BDentSc (TCD) DCh Dent (TCD) FFD RCSI