A TECHNIQUE FOR TAKING IMPRESSIONS

Just as any procedure starts with the first step, so does the procedure of a cast restoration begin with a properly prepared smooth preparation. After the preparation the next important step is an accurate, clean impression which reproduces the fine internal detail of the preparation as well as the delicate cavosurface margins.

All too often the operator spends a great deal of time and energy preparing a nice preparation, only to compromise the remaining steps of the cast procedure with an inadequate impression. The operator may wonder why later the casting fits the die but not the tooth. Impression taking and sulcular packing is often left in the hands of an improperly trained assistant. The patient may also be asked to hold the impression tray while the material is setting and the operator and assistant are doing other things. This time-saving step, may later end up to be more costly, when the casting and impression need to be redone because adherence to detail was not followed.

When each step in a step-by-step procedure is completed before the next step is undertaken, a good accurate, detailed impression will consistently result with very few retakes. This not only saves operating time, but also materials which translates into a cost savings.

The following is a detailed Technique used by the Academy of Richard V. Tucker Cast Gold Study Clubs which consistently produces an accurate impression and the resultant stone working die.

Step 1: After the preparation is completed, a two-, or three-ply soft cotton cord, wetted with a hemostatic agent (25% aluminum chloride), is gently teased into the gingival sulcus in those areas where the preparation ends at or below the free gingival crest. A flat-ended packing instrument is held at about 45 degrees to the tooth while the cord is being teased into the sulcus. Care must be taken not to traumatize the gingival sulcus and the attachment apparatus. Extra pieces of wetted cord can be placed interproximally to expand the tissue outwardly at the gingival crest. This packing of the sulcus may be done under the dam after the inter-septal rubber has been cut in the area of the preparation. The dam offers retraction and isolation of the preparation, while the cord is being placed and minimizes the spread of the hemostatic agent to the rest of the oral cavity and its associated offensive taste.

Step 2: After the packed cord has been in place for 3 - 7 minutes, depending on the health of the sulcular tissue; the preparation is cleansed of debris with water irrigation, and dried. A maxillary area can be isolated with cotton rolls or dri-angles while a mandibular area is isolated with cotton rolls placed in a cotton roll holder. Additional suction may be useful to control saliva buildup.

Step 3: The cord is now withdrawn by the operator as the assistant gently blows air into the sulcus. The air will prevent sulcular seepage and the collapse of the free gingival crest over the margin. Air is continually blown gently by the assistant while the preparation is being examined visually for the adequacy of exposed margins and no sulcular bleeding. Any small changes to the preparation may usually be made at this time without repacking the sulcus. Be sure that adequate tooth surface is retracted beyond the gingival margin of the preparation. The area must be now kept dry from the time of cord removal through the insertion of the impression tray.

Step 4: If the sulcus is still bleeding or the tissue is inadequately packed away from the margin, the sulcus needs to be repacked with the appropriate number of wetted cords. Steps 2 and 3 need to be repeated before proceeding on to step 5.

Step 5: When the sulcus is free of blood and debris and the retraction is adequate, the operator continues to blow air lightly into the sulcus. Meanwhile the assistant lays out the impression material on the pad and begins mixing. The syringe is loaded and passed to the operator to be inserted into and around the gingival sulcus of the preparation. The assistant aids in retraction and keeping the area dry, while the operator is moving the syringe back and forth to avoid trapping air in the line and point angles and the gingival sulcus. A 25 or 27 gauge needle with the point removed may be placed in the depth of the retention holes to eliminate air entrapment while the impression material is being injected.

Step 6: The operator passes the syringe back to the assistant and maintains retraction while the assistant loads the impression tray with the remaining mixture, which has started to thicken. The loaded tray is passed back to the operator and the assistant aids in retraction of the tongue or cheek as the tray is being inserted into the mouth. The tray should be held by the operator or assistant steadily until the material has set completely. This step is very important in preventing distortion of the impression while the material is setting. Allow a minute or two more in the mouth to ensure that removal is not premature.

Step 7: Remove the tray and check for bubbles, voids, and adequate amount of impression past the gingival margins. If the sulcular tissue is kept dry by the assistant while the impression is being examined, an additional impression can often be taken without repacking the tissue, If the impression material does not extend 1 mm below the margin, then steps 2 through 7 need to be repeated. The retaking of an impression should rarely be necessary, if the tissues are properly packed and the sulcus carefully examined prior to the mixing of the impression material.

Step 8: The impression is taken immediately to the laboratory to be poured in a minimal expanding, fast setting die stone (Fuji Rock-golden brown). This step minimizes the possibility of an inaccurate die from dimensional changes in the impression material. A surfactant is placed in the impression and blown dry. The die stone, which can be pre weighed or weighed at the time of pouring, is mixed with the proper amount of distilled water to achieve the optimum water/powder ratio as recommended by the manufacturer. The impression is poured and dowel pins or systems of choice may be used. After the stone has set, the models may be separated and mounted on an articulator of choice.

If this step-by-step procedure is followed meticulously, impression need not be a fearful or unpredictable step in cast restorative procedures. Much pleasure and satisfaction is gained by the operator at the seating appointment when the casting fits the tooth exactly as it fits the die.

Other procedures and materials have and can be used to take impressions but the verifiability for accuracy and the replication of detail, as well as repeatability of accurate stone dies, can not be substantiated by the author. Many years of clinical experience by many excellent clinician have proven this technique and this impression material to produce repeated accurate impressions and resultant stone dies.