Dental Impression Technique for Indirect Restorations

Dr. Laura Tam
Authored by:
Dr. Laura Tam

Introduction

An accurate impression of crown and inlay preparations is essential for the fabrication of these indirect restorations. Important factors include proper material selection, tray design and tissue management. The objective of this article is to outline basic principles and issues related to dental impression techniques.

Background

Materials

Impression materials can be categorized as reversible hydrocolloids (agar), irreversible hydrocolloids (alginate), polysulfide elastomers (rubber base), polyether elastomers and silicone elastomers (type 1 condensation-type and type 2 addition-type).

The most widely used impression material for crown and bridge is the addition-type silicone, or polyvinyl siloxane (PVS), impression material. PVS materials do not produce a volatile by-product during polymerization and therefore, minimal dimensional changes occur during setting.1 Dimensional accuracy is a key property and PVS materials have superior accuracy and dimensional stability. Additionally, PVS materials have excellent flow, flexibility and elastic recovery properties when used correctly. They have a relatively short setting time, moderate to high tear resistance and only minimal dimensional changes after multiple pours. They do not have an objectionable odour or taste.2, 3 Some PVS materials have added surfactants which improve the wettability of the set impression material. This improves the quality (reduced incidence of voids/bubbles and improved detail) of the stone casts that are poured up from these impressions.4, 5 Although these PVS materials are marketed as “hydrophilic”, they are not truly hydrophilic and cannot compensate for poor moisture control and a wet environment during impression-taking.

PVS materials are available in a range of viscosities. The low viscosity materials have excellent flow and ability to capture the fine detail of the prepared tooth, including the margins. However, they should not be used in bulk because they undergo greater polymerization shrinkage during the setting reaction. The bulk of the material within the impression tray is a higher viscosity PVS material. Medium, heavy and putty consistency PVS materials have less polymerization shrinkage and forcibly adapt the low-viscosity materials to the preparation and surrounding tissues. However, putty viscosity materials do not meet the ADA and British specifications that elastomeric impression materials must be able to reproduce fine detail of 25um and 20 um respectively6, 7and should therefore not be used directly to capture the fine detail of the prepared tooth.

Some latex gloves contain sulfur compounds which may inhibit the polymerization of PVS impression materials. Therefore, vinyl gloves should be used for mixing PVS putty materials. Similarly, handling of retraction cords with latex gloves should be minimized and if a rubber dam was used, the preparation should be washed well prior to impression taking.

Tray Design

The selection of impression tray is important for impression accuracy as well as patient comfort. Impression trays should be rigid to resist deformation when taking an impression. The use of custom tray is ideal because they are rigid, custom-fitted and allow control of the thickness of impression material. The use of a small volume of impression material will reduce the overall amount of polymerization shrinkage that will occur and hence, will improve the accuracy of the impression. Properly constructed custom trays maintain a relatively uniform 2-4 mm cross section of impression material thus ensuring uniform polymerization shrinkage and optimal impression accuracy.8 Therefore, custom trays are recommended for routine use in impression procedures.

When a custom tray is not available, stock trays can be used with PVS materials using a putty/wash technique using a one- (twin mix) or two-stage technique. In the two-stage technique, an initial putty impression within a rigid stock tray is made intraorally over an arch or over a study model covered with a polyethylene relief material (first-stage). Failure to provide adequate relief for the low viscosity wash material may cause hydrostatic pressure and deformation of the set putty during second-stage impression. Then, when the impression is removed, the putty recoils and the resulting dies are will be narrower than the preparations. Further relief may be provided by moving the tray bucco-lingually during the first-stage impression or by cutting back the set putty to provide adequate escape for the wash material. The final (second-stage) impression is then made by syringing low-viscosity PVS material around the prepared tooth and into the tray, and seating the first-stage putty impression over the syringed low-viscosity material. Bonding of the low-viscosity material to the set putty material will be impaired if there has been contamination of the putty surface.

In the one-stage technique, the stock tray is loaded with freshly-mixed putty material while the low-viscosity material is syringed over the prepared tooth. The tray loaded with putty is then seated over the syringed material so that both putty and low-viscosity materials set together. Difficulties with this technique, however, include early setting of the syringed material resulting in surface defects and poor blending between the two materials (“dragging”, ridges), and displacement of the low-viscosity material by the putty-viscosity material resulting in aspects of the tooth preparation captured by the less accurate putty material.

Custom and stock trays should be painted with a thin layer of adhesive to ensure that the impression shrinks towards the tray as it polymerizes. This will result in a slightly larger die, which is preferable to a smaller die. It is important to give the adhesive time to dry or it will act as a lubricant rather than as an adhesive.

Tissue management

The best tissue management is to avoid tissue injury during tooth preparation and to place supragingival margins wherever possible. However, esthetic demands, short teeth, decay, fracture and previously placed restorations will often necessitate the placement of subgingival margins in the tooth preparation. A dental impression must not only record the prepared tooth surface and margins, but it must also record the root surface beyond the margins. Reproduction of the root surface beyond the margins is essential for demarcation of the margins and for the assessment of the emergence profile for the crown.

Gingival “retraction” procedures are used to laterally (not vertically!) displace the gingiva so that a bulk of low-viscosity impression can enter the widened sulcus and capture the root surface and marginal details. A greater bulk of material at the margin is less susceptible to distort or tear during impression removal. For elastomeric impression materials, the minimal dimension of the sulcus for impression accuracy is 0.2-0.3mm.9

The use of gingival retraction cord for crown and bridge impressions is well-established. It is used to keep the gingival sulcus free of fluids by soaking crevicular fluids or by holding hemostatic agents, and to temporarily displace the gingival tissues away for the prepared tooth without permanent tissue damage. Cords are twisted, knitted or braided. Twisted cords have a tendency to unravel so the latter two types of cord are preferable. Ultrapak, a braided cord, is available in a range of sizes (smallest to largest diameter: 000, 00, 0, 1, 2, 3).

In the single cord technique, the retraction cord is left in place for at least 4 minutes and then removed prior to the impression. It has been suggested that a time of 4 minutes is needed to adequately open the gingival sulcus and that longer times are not significantly better.10 The single cord technique may be used when there is excellent gingival health (3mm pocket or less). The largest diameter cord that can be inserted gently into the sulcus should be used. Excessive forces could cause trauma leading to gingival recession. In a deep sulcus, the tissue can collapse over the single cord thus preventing proper tissue reflection. If there is gingival inflammation, removal of the single cord can elicit gingival hemorrhage.

The double-cord technique is the preferred technique for routine dental impressions. In the double-cord technique, a small diameter cord is placed into the sulcus and a second cord is place on top to further seat the first cord and to provide greater gingival displacement. After a minimum of 4 minutes, it is removed while the first small diameter cord is left in place during the impression to prevent sulcular seepage and hemorrhage, and for continued gingival displacement during the impression procedure.

Retraction cords are available as medicated (with a hemostatic agent) or plain. If gingival hemorrhage is an issue, plain retraction cord may be soaked with a hemostatic medicament prior to use. Because the use of epinephrine with retraction cords has the potential to cause systemic side effects, alternative astringents and hemostatic agents, such as a 25% aluminum chloride solution (Hemodent), are more commonly used for hemostasis. However, they should be used sparingly, ie. excess solution should be blotted prior to cord placement. The acidic pH of hemostatic agents could potentially contribute to postoperative sensitivity if it was injudiciously applied over the tooth preparation.11

A ferric sulfate medicament (Viscostat) can also be used for hemostasis. When ferric sulfate is used, the solution is actively infused or burnished into the sulcus prior to cord placement. Ferric sulfate is very effective but can also temporarily stain the gingival black for 24-48 hours.

Procedure

Tray fabrication

  1. Prepare 2 full arch custom trays with handles on the study model.
  2. Custom trays can be fabricated using self-cured (polymethylmethacrylate) or photo-cured (Triad) materials. Self-cured (polymethylmethacrylate) trays undergo prolonged setting shrinkage and therefore need to be fabricated at least 24 hours prior to use.
  3. Use two layers of baseplate wax and cover with tinfoil. Do not include the palate.
  4. Carve 3 widely-spaced occlusal stops into the wax, ideally located over non-functioning cusp tips to guide tray-seating.
  5. Remove the tinfoil and wax spacer.
  6. Paint adhesive on the tray and allow adhesive to dry for at least 10 minutes.

Tissue management. Double-cord technique

  1. Ensure a dry and clean operating field with the use of cotton rolls, dri-angles and the saliva ejector as necessary.
  2. Select a cord of a diameter to approximately fill one-half of the sulcus: eg size 000 (smaller sulcus) or 0 (larger sulcus) cord.
  3. Cut cord length to approximate the circumference of the prepared tooth.
  4. Soak the cord in water, or if necessary, Hemodent, and then blot. The cord must be moist, not wet, during placement.
  5. Use cotton pliers or a hemostat to loop the cord over the tooth.
  6. Insert the cord into the sulcus using a Hollenback carver with controlled pressure in a direction that is parallel to the outer root surface. It is usually easier to start the cord placement at a line angle. Once initial placement has been secured, step the retraction instrument along the cord. Insert the cord using approximately 1 mm increments, while exerting pressure towards the previously placed increment. A straight vertical pressure or pressure away from the previously-placed increment will tend to pull out the previously-placed increment of cord. Ensure that the cord is being pushed into the sulcus and not against the gingival or preparation margin.
  7. When the cord placement has encompassed the entire tooth circumference, the excess cord is cut with scissors. Ensure that the cut ends are flush and tucked into the sulcus.
  8. Place a second, same or larger diameter, cord on top of the first cord and leave in place for at least 4 minutes.
  9. It is critical to inspect the arch for significant undercuts, eg. bridges, wide interdental gingival embrasures, that will impede tray removal. These undercuts must be blocked using rope wax prior to taking the impression.
  10. When you are ready to take the impression, wash the impression site and dry to remove visible moisture. Remove the second (outer) retraction cord. The cord should be moist during withdrawal to prevent tearing of the sulcular epithelium.
  11. Inspect to see that the entire circumference of the margin of the prepared tooth is clearly visible, clean and dry. If there is slight hemorrhage, place a second fresh retraction cord and leave 5 minutes. If there is significant hemorrhage, a reassessment is necessary to determine the need for auxilliary periodontal or extended temporization procedures. If the margins are not clearly visible, clean and dry, there is no point to attempt to take the impression!

Impression

  1. After verifying that the margins are clearly visible, ask the assistant to load the low-viscosity impression material into the impression syringe. Do not try to inject the material directly from the mixing gun. Syringe this material carefully around the tooth margins by keeping the tip of the syringe approximating the margins in one slow circumferential movement.  Avoid withdrawing the syringe tip which can result in voids inclusions. Continue this circumferential movement into an upward spiral to completely cover the occlusal aspect of the prepared tooth.
  2. Immediately seat the tray, loaded with heavy-body impression material, posteriorly first, then over incisors. Hold passively for 5 minutes until full set. Do not permit the patient to hold the impression tray while setting. Active pressure may cause a rebound effect when the impression is removed from the mouth after setting.
  3. Remove the tray with a snap removal of the posterior segment followed by the anterior segment.
  4. Wash and dry the impression. Inspect the impression for reproduction of the entire tooth preparation with special attention to the margins. Errors on the margins of the impression include: incomplete capture of margins (caused by inadequate displacement of gingival tissues, or when the circumferential application is incomplete), bubbles (caused by the entrapment of air when the syringe is lifted away from the margin of the tooth during circumferential application, or by blowing air at the impression material), defects of impression material (caused by saliva or blood contamination), and tearing of impression material (caused by too thin section of impression material or slow removal of impression tray).12 The impression may still be acceptable if a small defect is in a noncritical area (eg. away from the margin).
  5. Remember to remove the retraction cord from the gingival sulcus after the impression. A contact time beyond 30 minutes could produce injury to the sulcular epithelium, with regeneration occurring after more than 10 days.13 Rinse the site to remove traces of the residual chemical agents.
  6. Disinfect the impression using Lab X, a 70% ethyl alcohol with 0.1% phenylphenol.

Clinical Implications

The range of available impression materials and techniques should allow dentists to make very accurate impressions for crown and bridge. It is the responsibility of the dentist, NOT the lab technician, to decide whether or not an impression is acceptable. When there is uncertainty, the dentist could choose to send the impression to the lab for a pour up of the die. A careful inspection of the returned die will often show details that are not visible upon clinical inspection of the tooth or impression. Some deficiencies may result from other aspects of the restorative procedure, such as poorly-defined margins in the tooth preparation, and not from the impression procedure alone.

There is a concern that there are many inadequate impressions that are sent to commercial laboratories for restoration fabrication. Inadequate impressions lead to inaccurate restorations, resulting in frustrating misfits and need for adjustments. Greater attention to detail and an understanding of the various impression material and technique factors will result in a more accurate record of the prepared tooth and hence, an optimal fit of indirectly-fabricated restorations.

Contentious Issues/Other Materials and Techniques

Application of air. Some operators feel that an application of air from the air-water syringe will help push the low-viscosity material towards the margins and into the sulcus during the impression-taking procedure. The risk of this technique is the inclusion of air (voids) caused by the impression material folding over itself when excessive air pressure is applied. In most instances, the additional second stage heavy-body impression material is sufficient to adapt the light-body material towards the preparation margins.

Dual-arch impressions. Full-arch trays are recommended to facilitate proper articulation of the casts during mounting procedures. The use of a segmental or quadrant impression, such as the dual-arch impression technique, is discouraged because it is difficult to verify mounting when there is no recording of the contralateral arch.

The dual-arch impression technique is a sectional impression where the impression of the prepared tooth is taken as the patient closes to simultaneously record the opposing teeth in centric occlusion. This technique is contraindicated when there are more than 2 crowns being made simultaneously, when the prepared tooth (teeth) are not bounded by intact and opposed teeth, if the patient cannot close directly into a stable and reproducible intercuspal position, and if the tray interferes with patient closure. Accuracy of the dual-arch technique is compromised if the patient does not close fully into maximum intercuspation or if a flexible tray is used. In most instances, the perceived simplicity of this technique does not outweigh the sensitivity of the dual-arch impression technique to errors.

Monophase impressions. The advantages of using a single medium-body viscosity impression material are simplification in technique and elimination of disparate setting times and viscosities when a putty/wash technique is used. However, accuracy will be sacrificed due to greater polymerization shrinkage and reduced ability to capture fine detail.

Electrosurgery. Electrosurgery may be used to remove the cut excessive gingival tissue or the lining of the gingival sulcus. The electrosurgery cauterizes cut tissue and assists with hemostasis. However, care must be used to prevent excessive temperature rise by keeping the electrosurgery tip moving against the gingival tissues, and avoiding touching teeth, bone or metal restorations to avoid pulp damage or bone necrosis. Electrosurgery should be avoided where the gingiva is thin and friable, especially in esthetic regions where gingival recession would pose a significant problem.

Expasyl. Expasyl is a paste of kaolin with the astringent aluminum chloride. It is a new material marketed to be used as an alternative to retraction cord to temporarily displace the gingival tissues without trauma. However, little is known about its clinical effectiveness, particularly its ability to physically displace tightly closed or thick gingival tissues.

 

References: 

Craig RG, Restorative Dental Materials. 11th ed. St. Louis: Mosby, 2002.

Donovan TE, Chee WWL. A review of contemporary impression materials and techniques. Dent Clin N Am 2004;(48):445-470.

Stewardson D. Trends in indirect dentistry: 5. impression materials and techniques. Dent Update 2005;(32):374-393.

Boening K, Walter M, Schuette U. Clinical significance of surface activation of silicone impression materials. J Dent 1998;(26):447-452.

Takahashi H, Finger W. Dentin surface reproduction with hydrophilic and hydrophobic impression materials. Dent Mater 1991;(7):197-201.

ANSI/ADA specification #19. Non-aqueous elastomeric dental impression materials. American Dental Association, 1982.

BS EN ISO 4823. Dentistry- elastic impression materials. British Standards Organization, 2001.

Eames WB, Sieweke JC, Wallace SW, Rogers LB. Elastocmeric impresssion materials: effect of bulk on accuracy. J Prosthet Dent 1979;(27):304-307.

Laufer B, Ganor Y, Baharav H, Cardash HS. The effect of marginal thickness on the distortion of different impression materials. J Prosthet Dent 1996;(76):466-471.

Baharav H, Laufer B, Langer Y, Cardash HS. The effect of displacement time on gingival crevice width. Int J Prostho 1997;(10):248-253.

Woody RD, Miller A, Staffanou RS. Review of the pH of hemostatic agents used in tissue displacement. J Prosthet Dent 193;(7):191-192.

Jaeipour G. Improving decision-making in restorations: evaluation of impressions and working casts. Calif Dent Assoc J 2007;(35):637-640.

Harrison JD. Effect of retraction materials on the gingival sulcus epithelium. J Prosthet Dent 1961;(11):514-521.

Article Reviewed By

Dr. James Brown & Dr. Dorothy McComb