The Double-Cord Technique for Making Final Impressions
Gerard J. Chiche, DDS
To obtain an accurate impression, the packing of gingival tissues with retraction cord must be delicate and atraumatic as possible. Excessive pressure may result in gingival bleeding, tissue laceration, and, occasionally, permanent gingival recession. In cases with anterior preparations or highly inflamed gingival tissues, it is generally advisable to delay the final impression two to three weeks in order to allow healing of the tissues so they can better withstand cord packing. The potential for permanent gingival recession is greater when a cord is left more than 15 minutes in a thin, facial creviceor if two cords are packed on top of one another with uncontrolled pressure within a shallow crevice. Consequently, the facial crevice in the anterior region should be packed delicately with one cord whenever possible.
The single-cord technique is the simplest and least traumatic packing technique. This approach is indicated when gingival tissues appear healthy and do not bleed as the string is packed. Favorable results can be obtained through the use of plain, braided cords soaked in buffered aluminum-chloride solution. For optimal results, the cord should remain in place for approximately 10 minutes. In many clinical situations and, despite careful precautions, spontaneous bleeding of the gingival crevice may occur during impression making and is often caused by laceration laterally or apically during tooth preparation, or associated with plaque accumulation in the marginal defects of provisional restorations, or with deep margins. Practically, the likelihood of gingival bleeding during the impression procedure must be ascertained prior to cord packing. After the provisional restoration is removed or as the temporary cement is eliminated, spontaneous bleeding of the free gingival margin may occur. If the gingival sulcus appears erythematous or spontaneously bleeds, the retraction sequence should be modified.
The double-cord technique is indicated when the clinician suspects spontaneous bleeding may occur during impression making. In such instances, packing of a second cord becomes a safety precaution. Typically, the interproximal or lingual aspects of the crevice are more prone to localized inflammation, whereas the facial sulcus may remain relatively healthier. Therefore, in a typical double-cord technique, prepacking of the facial crevice is usually avoided in order to minimize the potential for trauma. A braid of extra-thin cord impregnated with buffered aluminum chloride is prepacked and confined into the inflamed portion of the crevice only. The excess braid should be sectioned and the excess seepage and coagulum should be wiped with a cotton pellet. A thin, impregnated, braided cord is then packed circumferentially into the crevice. Before the impression material is injected, the thin cord should be removed, but the extra-thin braid is left in place to ensure hemostasis. It may be packed in the impression, but should be left undisturbed when pouring the impression.
Peripheral Double-String Technique
In other situations, the double-string technique is modified to a peripheral packing of each string. It may be indicated when greater hemorrhage control is required due to highly inflamed gingival tissues (prepack with thin cord) or when the whole gingival crevice is slightly inflamed or ulcerated on the periphery and could be prone to bleeding during tissue manipulation (prepack with thin cord). In both situations, the thin cord is left in place during impression making, while the regular string is pulled prior to injection of the light-bodied material in the crevice. Additionally, if gingival bleeding still occurs, as would happen with exposed connective tissue, the gingival tissues may need to be injected with 2% lidocaine solution with 1/50000 epinephrine to cause local vasoconstriction. The crevice also may be rubbed very delicately with aluminum chloride, ferric sulfate, or a hydrogen peroxide solution.
There is no single approach that will provide consistent results in every clinical situation. Cord size selection is dictated by the gingival thickness, the sulcus depth, and the degree of lateral deflection desired. The health of the gingival tissues dictates the mode of packing:
• Healthy tissues: Single-string technique
• Partially inflamed tissues: Selective single-string technique
• Highly inflamed tissues: Peripheral double-string technique.
Consequently, the clinician must be flexible when selecting his or her cord-packing technique.