Cord is meant to move tissue laterally, away from the tooth, not necessarily apically. However, during this lateral movement, the tissue typically also moves apically. The technique, therefore, is not to jam the cord down deep into the sulcus, but rather to gently place it without much force. One way to refine your cord packing skills is to perform this procedure on a patient who has not been anesthetized. It is not the goal of cord placement to search for the epithelial attachment. The goal is to expose tooth structure for access during tooth preparation and to expose margins when taking an impression.
There are different types of cord based on the weave as well as the impregnation of medicaments to aid with hemostasis. The weave is largely a matter of personal preference, although some seem to facilitate placement of the cord as opposed to making it more difficult.
The impregnated medicament is a different story. Although racemic epinephrine is the more or less acknowledged gold standard, its well-known cardiac effects can be quite sobering if you have a patient who has experienced an episode as you are packing cord. Because of this potential, many clinicians steer clear of using cord with epinephrine and use those types impregnated with aluminum sulfate or use just plain cord, which would be the best choice assuming again the tissue is healthy and there is no bleeding when it is being packed.
Another issue with medicaments in cord is their potential effect on the setting mechanism of various types of impression materials. To be on the safe side, it is usually prudent to thoroughly wash the cord with a fairly vigorous air-water spray once it is placed AND any bleeding has been eliminated. You could also wash the impression site after removing the cord, but you are typically trying to get the syringe impression material injected into the sulcus as quickly as possible and rinsing the site is usually not done at this point.
There are also various cord placement techniques using one or two cords, with the first one being smaller and remaining in the sulcus during the impression taking session. This is, again, a matter of personal preference and experience. If the first cord is small enough in diameter to be placed completely apical to your margin, then there is no reason not to leave in place. This means selecting a brand of cord that has the very small sizes.
When it comes to placement, a single strand of cord is packed (placed is probably a better word) gently into the sulcus. Its length should conform to the circumference of the tooth. For most teeth, 35mm will be more than adequate to go around the tooth one time. For molars, 40mm should suffice. After placing the cord completely around the tooth one time, cut off any excess with scissors so that neither end is sticking out where a diamond could snag it.
Once the cord has exposed the subgingival tooth structure, it can be prepared without traumatizing the tissue. You can remove this cord to give you a better view of the preparation, especially when checking for irregularities on butt joint shoulder margins, or it can be left in place. If the tooth needs additional preparation, it is done now.
If you removed the cord during the previous step, repack at this time. The size (diameter) and length of cord are dependent on how far your preparation goes subgingivally and the firmness of the tissue. In addition, the tissue investing anterior teeth is usually more delicate compared to posteriors. Therefore, in general, smaller cords would be used for anterior teeth. For some teeth, the cord will only be placed around the tooth once while other teeth require twice around for proper retraction. For most teeth, 75mm will be sufficient to go around twice. For molars, use 85mm. Any excess can be cut off.
Dip the cord in a hemostatic agent (if necessary) and lightly blot the excess. Place one end of the cord in the sulcus on one side of the tooth and begin packing. Be sure to use an instrument thin enough to get into the sulcus but not too thin so it goes right through the cord. As you pack forward, it may be necessary to backtrack occasionally to ensure previously packed cord has not been pulled out of the sulcus. Do not force the cord as it should be placed gently.
Once you have encircled the tooth one time, you need to decide if going around again is necessary. If you can see your entire margin and the cord is fully visible around the entire tooth, you are finished. Otherwise, go around the tooth again. Cut off most of the excess but leave a small end to grab when it is time to remove the cord and take the impression.
With the double cord technique, the first cord is left in the sulcus and a second cord is placed on top of the first. When it comes time to take the impression, only the top cord is removed. As mentioned previously, assuming you used a hemostatic agent on the cord, be sure to rinse it thoroughly after it is placed with the air-water syringe to dilute the medicament.
The dwell time for cord depends on the depth of the margin and the condition of the tissue. For absolute certainty that retraction will be effective, leave the cord in place for a full 10 minutes. Despite the warnings from some manufacturers of cord that five minutes is the maximum dwell time, we have not experienced any untoward effects from leaving the cord in place for a full 10 minutes. However, if the preparation is just slightly subgingival and there was minimal or no bleeding during the preparation phase, five minutes may be adequate.
When it is time to remove the cord, grab the exposed end and remove it carefully. Be sure the cord is slightly moist before removing it or you could denude the sulcus and cause bleeding. If all the previous steps have been completed successfully, your margins should be fully exposed once the cord is removed. For most impression materials, a dry, but not desiccated, preparation will give you the best results. However, most current impression materials are either inherently hydrophilic or are compatible with some moisture on the teeth. Therefore, absolute dryness is usually not a requirement to take a successful impression.