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An Impression Technique for Repeated Success
Predictability of the outcome is a critical factor in the technique of choice by the individual clinician.
By Jacinthe M. Paquette, DDS | Cherilyn G. Sheets, DDS
One of the more critical aspects of providing patients indirect restorations is the ability to capture an accurate representation of the tooth preparation. Through the years, numerous techniques and impression materials have been developed and promoted by the profession and by manufacturers working individually and in tandem. The key issues for success involve moisture control, atraumatic tissue retraction, cleanliness of the tooth preparation and associated tissues, avoidance of air entrapment, avoidance of material tearing or distortion upon removal, and adequate inspection of the impression after removal to ensure complete accuracy.1 Whether the treatment is simple (such as a single crown), or complex (with numerous preparations to be impressed), the impression-making process can be challenging. New and evolving digital impression technologies may someday minimize some of these challenges, but it may still be some time before traditional impressions are no longer necessary.
Through a patient example, this article will outline the requirements necessary to achieve accuracy in fixed prosthodontic impressions, examine a variety of materials available, and describe a technique that, once mastered, will provide repeated success.
The patient initially sought treatment because of an active periodontal infection surrounding a newly designed implant provisional prosthesis. She was dissatisfied with the esthetics of her existing restorations, and knew that she had multiple dental restorations that needed replacement. The patient had the maxillary arch addressed first, and was now ready to proceed with the treatment of the mandibular arch (Figure 1). Several of her mandibular teeth had been provisionally restored as older restorations had failed over the last several years. Many of the remaining mandibular restorations exhibited structural breakdown, marginal leakage, poorly contoured anatomy, and ill-matching shades.
The maxillary arch rehabilitation provided an idealized restored dentition and occlusion to oppose the mandibular arch. The mandibular arch was readied for restorative treatment through several preliminary therapies, which included clear orthodontic aligners for minor tooth movement (Invisalign, Align Technology, Inc., www.invisalign.com) and tooth-whitening procedures to re-establish an esthetic shade of the natural dentition (Venus White® Pro, Heraeus Kulzer, www.heraeus-dental-us.com). The patient simultaneously went through a periodontal therapy regimen with a strong emphasis on effective plaque control instruction to idealize tissue health prior to the commencement of the restorative procedures.
The restorative treatment objectives for the patient’s mandibular arch were to protect the structural integrity of each tooth with conservative, strong restorations appropriately designed to compensate for the individual defects that were present2,3 (Figure 2). More specifically, the treatment plan included several porcelain-bonded restorations, as well as full-coverage crowns where indicated. A diagnostic wax-up confirmed that the treatment objectives could be met, and the patient approved the treatment plan as presented.
During the preparation appointment, appropriate tooth preparation objectives were achieved: adequate space was provided for restorative materials, periodontal health was preserved with careful preparation techniques, and the esthetic requirements were refined through the evaluation of the diagnostic wax-up.
The “Double Cord” Impression Technique
There are certainly many popular impression techniques, but predictability of the outcome is a critical factor in the technique of choice by the individual clinician. The “double-cord” tissue retraction technique has been described by others and has proven to provide a reliable component of the overall technique to achieve the detail required at the margin area.4
As previously emphasized, periodontal tissue management is an important part of the impression procedure. Once the tissues are healthy, care must be taken at the time of active treatment to minimize any tissue trauma during both the preparation and impression phases of tooth restoration. This goal is accomplished in part through careful placement of either equigingival or slightly subgingival preparation margins that follow the undulations of the gingival contours where possible.5 The careful treatment of the gingiva ensures a more favorable tissue environment at the time of the impression by minimizing tissue hemorrhaging. All of these factors help to ensure that the restorative margin levels established at the time of the preparation more predictably remain healthy and stable throughout the lifetime of the restoration. The overall technique for this patient followed the treatment steps described below.
The teeth to be impressed were cleaned with water spray in combination with a preparation cleansing solution (PrepQuick™, Ultradent Products, Inc., www.ultradent.com). Additionally, a hemostatic solution was applied to control and minimize bleeding and/or prevent sulcular fluids (ViscoStat®, Ultradent Products). The preparations were then washed for a second time and dried thoroughly.
A small non-impregnated retraction cord (GingiKnit+ size 0a, Dux Dental, www.duxdental.com) was moistened in an aluminum-chloride hemostatic agent and gently placed in a circle with no overlaps around the circumference of the tooth preparation. After the first cord was placed, a second, larger cord (Ultrapak size 0, Ultradent Products), also saturated with the same hemostatic agent, was placed in the sulcus directly over the first retraction cord (Figure 3). The purpose of the first cord is to gently expose the marginal area of the preparation. The purpose of the second cord is to further displace the tissue above the first cord to allow sufficient space for the impression material to flow around the preparation and fill the sulcular area. An adequate thickness of sulcular impression material maximizes the tear strength and minimizes distortion due to thin, flexible impression walls.
The non-impregnated impression retraction cord is recommended for two primary reasons. First, epinephrine-impregnated cord has been shown to introduce undue high and uncontrolled levels of systemic epinephrine to the patient, which can lead to elevated blood pressure and provoke an unfavorable response in the patient.6 This is especially hazardous in the cardiac patient. Secondly, impregnated cord can introduce further trauma to the tissues, which can promote undesirable premature recession or other changes in the tissue architecture.
After both cords were placed, the preparation was examined for any debris or hemorrhagic byproducts around the preparation margins. Residual debris was removed with a water- or astringent-moistened pellet. The cords need to remain undisturbed for approximately 4 minutes. Once ready, the top cord was removed, leaving the residual sulcular cord in place to maintain the exposure of the margin area to be captured in the impression (Figure 4). Once again, the preparation was re-examined to quickly verify that the preparation was clean, dry, and all margins were visible prior to dispensing the material.
An assortment of polyvinylsilaxane (PVS) impression materials is available today that offer numerous advantages, including:
- Flexible setting times and short intraoral setting times.
- Excellent dimensional stability, which allows multiple pours to be made in the same impression with consistent accuracy.
- Good flowability and wettability with an improved gypsum castability compared to that of the polyether materials.7,8
- High tear strength,9 and the best elastic recovery of all available materials.
- Extremely accurate detail reproduction that meets American Dental Association specifications for impression materials capable of recording fine detail of 25 μm or less.
- Resistance to slumping.
- Compatibility with various die materials, such as gypsum.
- Ability to work well with electroplating, and are preferred when using refractory materials.
The new generation of PVS materials includes reliable products with unique features that allow the clinician to meet challenging clinical needs. Used in conjunction with adequate preparations, good tissue management and good technique, these products can prove to be an important adjunct to success with final impressions.
Taking the Final Impression
Maximizing the accuracy of impressions requires an impression tray that fits accurately and passively in the mouth. It must be rigid and ideally allow for a 1-mm to 3-mm bulk of material. Custom impression trays improve accuracy. The custom impression tray should be fabricated at least 24 hours before use to ensure complete curing of the tray material.10 Retention of the impression material in either stock or custom trays is achieved by mechanical and/or adhesive retention. The elastomeric adhesives are material specific and, therefore, must be matched to the impression material used. The adhesive should be applied to the tray 15 minutes before the impression procedure to allow for complete drying and effectiveness of the material.
Once the tissue retraction had sufficiently exposed the preparations to be impressed, a light-viscosity injection material (Flexitime® Xtreme, Correct Flow, Heraeus Kulzer) was syringed around the preparation margins. During this process, the mixing tip was kept buried in the material while it was being expressed to avoid air entrapment (Figure 5). The wash material was further directed into the sulcular area by using a gentle stream of dry air from the syringe.
The air pressure places an initial coating of material over the preparation and exposes areas on the preparation where moisture contamination may still be present. If moisture is found, the preparation needs to be cleaned and dried before proceeding with the process. The thixotropic, non-slumping nature of the Flexitime system enables it to stay where it is applied and to flow nicely into the sulcular area. Additional material was placed over the preparation, followed by insertion of a tray filled with a heavy-body material (Flexitime® Xtreme Heavy Body, Heraeus Kulzer) (Figure 6).
During the polymerization/setting phase, the impression tray must be kept in an undisturbed, steady position until the material is completely polymerized. Any household timer can be used to ensure the appropriate setting time. To prevent distortion, the impression tray is removed from the mouth in a quick snap motion. The impression should then be examined with oculars or a microscope, if available. Circumferential margin detail should be present with impression material extending beyond the preparation margins (Figure 7). A fully extended impression provides the laboratory technician with an accurate recording of the emergence profile of the tooth, which will result in the best contours for the final restoration.
Inspection of the impression for any voids, distortions, or other defects is critical. If any inaccuracies are noted, a new impression should be made immediately to take advantage of the tissue displacement. An immediate second impression can be taken with or without the removal of the deeper/sulcular cord. Sometimes the second cord will be removed with the impression. As long as the cord is beneath the margin of the tooth, the impression will not be compromised (Figure 8). The key to a successful impression is proper preparation and systematic protocols. When the impression procedure was completed, the patient was provisionalized to await the final restorations (Figure 9).
More Extensive Treatment
The challenges of moisture control, bleeding control, and working time become increasingly critical as the complexity of the treatment increases. A very effective means of moisture control is the use of antisialogogues. Saliva inhibition medications can be prescribed to the patient as a premedication when moisture control is expected to be difficult. Propantheline, given 30 minutes before the impression appointment, remains effective for up to 3 hours. Contraindications to prescribing propantheline include glaucoma, prostrate hypertrophy, and pregnancy. Patients who wear contact lenses should be instructed not to wear them while taking propantheline.
Some clinicians recommend prescribing the use of 0.12% chlorhexidine gluconate for 2 weeks before the preparation phase of treatment, which has been shown to significantly reduce plaque levels and associated gingivitis. This protocol improves gingival health and results in reduced bleeding during preparation and the impression procedure.11,12
Another great aid in moisture control for single- or quadrant-die impressions is the use of an isolating aid such as the Isolite™ (Isolite Systems, www.isolitesystems.com). Products like this allow the patient to have the support system of a bite block, a flap to isolate the tongue and cheeks, and numerous suction devices to remove moisture at the working field. Additionally, some devices have accessory lights that bring greater illumination to further increase visibility.
With extensive restorative treatments (ie, a full-mouth or full-arch rehabilitation), there is often a need for increased working time during the final impression procedure. When numerous teeth are being impressed, impression materials can potentially begin to set before tray insertion. Cooling the impression material before use can prevent premature setting. Supercooling the impression materials can significantly increase the working time, allowing the operator sufficient time to manipulate the materials.13 Even though most PVS impression materials share many positive features, there are often unique features of individual brands that the clinician can use when advantageous. For instance, a feature of the impression material chosen for the full-arch restoration in this article is that the set has been calibrated to allow the operator the maximum working time prior to initiating a quick-setting response. This helps to prevent the earliest injected impression material from starting to set prior to the impression tray being seated, eliminating or greatly reducing impression distortion.
A good, reliable technique combined with a quality product can ensure accuracy in the critical impression phase of treatment. The technique described here provides a predictable method to meet all impression needs that are critical to creating accurate laboratory models for the fabrication of the final restorations (Figure 10). It is important that the clinician thoroughly understand the capabilities of the materials and the steps necessary to achieve an ideal outcome. Thorough knowledge combined with good clinical technique can provide the accuracy required to achieve ideally fitting restorations and preserve periodontal health. The true measure of overall success is the longevity of the restoration and the maintenance of tissue health. For this, restorations need to have marginal precision achieved through the avenue of excellent impressions (Figure 11 and Figure 12).
Drs. Paquette and Sheets received honoraria from Heraeus for this article.
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About the Authors
Jacinthe M. Paquette, DDS
Newport Beach, California
Cherilyn G. Sheets, DDS
Newport Beach, California