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Impression Techniques
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IMPRESSION TECHNIQUES:
       The simplest technique with the most consistently useable results is the open-bite, one-step, heavy/light technique in which a light-bodied material is injected around the tooth and a heavy bodied material is placed in the tray. This method is appropriate for addition-reaction silicones, and polyethers. It is also ideal for all closed-bite impressions. Although more difficult, the open-bite, two-step, putty wash technique (used with addition-reaction silicones) can be more accurate—if it is performed properly. The open-bite, one-step, putty wash and the closed-bite, double-arch techniques are also popular and are presented here. Many of the principles discussed apply to most other impression techniques.

Open-Bite, One-Step, Heavy/Light Technique

The one-step, heavy/light technique is popular because it is more rapid than the one-step, putty-wash technique and more accurate than the one-step, light-bodied technique. It is appropriate for single or multiple individual teeth without bridge abutments. It is one of the most accurate techniques for taking impressions of undercuts.
        Select a Tray. Most trays, even slightly flexible ones, provide acceptable results. The size of the tray is more critical than its rigidity. The tray should fit the teeth as closely and uniformly as possible.
        Select a Material. One of the most common causes of distorted impressions is the use of fast-setting materials. If impression material starts to set prior to placement, the material’s "memory" prompts distortion after removal from the mouth. Colder impression materials set more slowly. To gain additional working time, refrigerate impression materials before use. This is especially helpful with multiple-unit impressions.
       Mix the Impression Material. Begin mixing the impression material when the preparation is complete, the retracted gingival sulcus can be washed and dried without hemorrhage, and the margins are clearly visible. While the assistant is mixing, the dentist can dry the mouth and place dry angles and cotton rolls for moisture control. Avoid touching the teeth with latex gloves.
       The assistant mixes the light-bodied injection material by hand or with an automixer, loads it into a syringe, and hands it to the dentist. While the dentist is injecting this material around the prepared teeth and into the pits and fissures of all teeth in the impression, the assistant mixes the heavy bodied material, fills the tray two-thirds full (avoiding the palate), and hands the tray to the dentist. Alternatively, the dentist can use a gun-type automixer to fill the syringe while the assistant begins mixing tray material. Electric automixers work well with heavy bodied materials since the force required to extrude these materials is difficult for many assistants.
       Every sulcus has moisture. It is important the dentist not displace this fluid when injecting around the teeth. Otherwise, when the injected material comes around to meet itself, it has a wave of fluid in front of it. Such a wave often results in a fin at the margin where the excess fluid escaped. With deep margins, some clinicians like to gently blow the impression material to the bottom of the sulcus and reinject around the prepared tooth. This works well when the retraction is minimal and the sulcus is very dry.
        Seat the Tray. Place the tray in the mouth without touching the teeth (a gentle touch is usually okay). With upper trays, seat the tray from back to front; with lower trays, seat from front to back. Many patients can hold a tray in place by gently biting on two cotton rolls on each side of the tray. Hold the tray still until it is absolutely set, and then leave it in place a minute longer. Tray removal prior to complete setting invites distortion.
        Remove the Impression. First, apply firm pressure on the borders of the tray in a gingival direction to break the seal and reduce the likelihood of tearing. Second, use a finger to release the anterior seal of the tray. Then grasp the handle and very rapidly remove the impression from the anterior teeth first. Next, release the impression from the posterior segment on one side and rotate it out of the mouth. Do this quickly or the material is more likely to tear and distort.
        Inspect the Impression. Inspect for defects in critical areas. The preparation margin is usually in syringe material while the body of the preparation is usually of heavy bodied material. Since the materials set together, heavy bodied material showing through is not a problem. Retake the impression if the impression tray shows through over a prepared tooth.

Open-Bite, Two-Step, Putty Wash Technique

The putty wash technique is advised for use with bridges and splinted teeth. The technique reduces the large dimensional change that occurs when any single material is used in a full-arch tray. Done properly, this technique is highly accurate because the impression is taken in two steps and the putty, due to its high filler loading, has substantially less polymerization shrinkage and thermal contraction than the wash material.
       This technique involves placing a plastic covering over the putty before taking the impression. Many manufacturers provide this covering, which blocks out proximal undercuts and creates vertical stops in the putty impression. The plastic covering also prevents saliva from contaminating the putty, which could significantly reduce the bond of the injection material and cause the wash material to separate and distort. Plastic sandwich bags are an excellent alternative: Simply load the impression tray with putty, place it inside a thin plastic sandwich bag, and seat the tray.

        Select a Tray. The over-impression should be taken in a rigid plastic or metal tray with adhesive. A non-rigid tray could flex and result in a distorted impression.
        Mix the Putty. Putties must be mixed by hand to avoid contact with latex gloves or latex glove powder. Isolation with a bag limits the potential for patient exposure to contamination.
       Knead the putty to a uniform color and place only enough in the impression tray to cover the teeth of concern. Overfilling increases the chance of tray distortion. The palate on the maxillary arch is included only if needed for a prosthetic device. Isolation with a bag reduces this concern.
        Seat the Tray. Place the plastic covering over the putty filled impression tray and seat it in the mouth. After it has set, remove the tray, peel off the plastic covering, and trim the excess material. Inspect the impression and remove any loose or interproximal material with a laboratory knife. Place the tray back in the plastic bag and reinsert. It should fit passively without binding and achieve a positive stop. Because putty is elastic, areas that bind can rebound and cause distortion.
       Using a Gauze Spacer. With partial coverage restorations, use a damp gauze pad to gain clearance for the wash material. Place the damp pad over the tooth to be prepared prior to placing the unset putty filled tray. Use of a gauze spacer is also recommended if the preparation was cut before taking the over-impression. Some clinicians use a spacer routinely to increase impression thickness at the margins and avoid putty show-through.
        Mix the Impression Material. As previously described.
        Inject the Impression Material. The assistant fills the syringe (using half the impression material) and hands it to the dentist. The dentist injects material around each tooth and into the central grooves of all the teeth in the impression.
       Only the margins of the preparation must be covered. Excess material in the sulcus past the margin can result in increased tearing and distortion. While the dentist injects, the assistant puts the remaining impression material into the putty over-impression (removing the plastic covering first).
        Seat the Tray. Allow the filled tray to slowly settle into place, posterior first, by applying light pressure. Give a slight pull on the lips to encourage excess material to flow behind them. When taking a lower impression, have the patient lift his/her tongue over the tray to reduce the likelihood of the tray being dislodged during swallowing. The seated tray must fit passively with no compression from placement or biting forces. Since putty is elastic, pressure points, including from firm biting, will relapse when the impression is removed and distort the impression. Once the tray is in place, hold the tray still until it is absolutely set.
        Remove the Impression. As previously described.
        Inspect the Impression. Inspect for defects. Injection impression material should constitute the entire preparation. If impression putty shows through in the prepared areas, retake the impression.

Open-Bite, One-Step Putty-Wash Technique

In the one-step putty-wash technique, both the putty and wash set in the mouth at the same time. Accuracy is compromised because a large mass of material must polymerize at once. This technique is only recommended for addition-reaction silicones. It should not be used with condensation-reaction silicones or in impressions with undercuts. It is best reserved for teeth with supragingival margins.
        Select a Tray. Perforated metal trays are preferred for their rigidity. The viscous impression putty may expand a flexible tray, distorting it on removal.

        Mix the Impression Material. Dispense adequate amounts of putty and syringe material. Begin mixing both when ready to take the impression.
       Knead the putty to a uniform color and place only enough in the impression tray to cover the teeth of concern. Overfilling increases the chance of tray distortion. The palate on the maxillary arch is included only if needed for a prosthetic device.
        Seat the Impression. Seat as described for the open-bite, two-step technique.
        Remove the Impression. As previously described.
        Inspect the Impression.Inspect for defects. Injection impression material should constitute the entire preparation. Since the putty and syringe materials set together, putty show-through is not a problem.

Closed-Bite, Double-Arch Technique

       The double-arch impression technique in which the final impression of the prepared teeth and the opposing teeth is taken in a single quadrant has been widely used in the last three decades. It is popular because it is rapid, saves impression material, reduces the likelihood of gagging, and is more comfortable for the patient. Done properly it can overcome errors in mounting casts in centric occlusion or errors from mandible deformation on opening.
       Although popular, this technique is overused. Since it provides only centric occlusion it gives the laboratory technician no balancing or working guidance. Few impression techniques have been abused as much as this one. A good rule: If in doubt, don't use this technique.
       Although the closed-bite, double-arch technique can be used with any material, a heavy viscosity or a heavy/light single mix technique is recommended.
        Select a Tray. Select either a totally flexible plastic with gauze tray or a rigid metal tray with gauze insert. A disposable flexible tray sold as a bite registration tray is a good choice. Metal trays should be rigid in all dimensions and have replacement gauze forms.
       Try the tray in the mouth to check for proper fit and tissue impingement in the retromolar area. Have the patient practice biting in centric occlusion with the tray in place and identify teeth across the arch that contact. Place a dry angle over the partoid duct to deflect the tissue and contain the impression material.
        Mix the Impression Material. Dispense equal amounts of heavy bodied and syringe impression material before removing the retraction cord. Begin mixing when ready to take the impression.
       The assistant mixes the syringe material, fills the syringe, and hands it to the dentist to inject material around each tooth and into the central grooves of all teeth in the impression quadrant. Meanwhile, the assistant mixes the heavy bodied impression material and places it in excess on both sides of the tray.
        Seat the Tray. Gently place the tray. Have the patient close into centric and verify the cross-arch occlusion. For restorations with occlusal margins, place gauze between the teeth on the opposite side to open the bite. Although this does not permit replication of centric occlusion, it reduces the chances of a distorted impression. Have the patient hold still until the material is set, using their hand to support their chin if necessary for stability.
        Remove the Impression. Have the patient open. The impression will generally separate first from the untreated arch. Next, compress the impression against the remaining arch to break the seal. Then grasp the handle and very rapidly remove the impression from the prepared teeth.
       Inspect the Impression. Inspect for defects. Show-through of heavy bodied material is not a problem. The impression must be retaken if the impression tray or gauze shows through. The impression should be somewhat rigid to prevent flex when poured in stone.



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