Accurate Impressions: A Crucial Link to Prosthodontic Success
David Avery, CDT
The successful delivery of dental prosthetics is dependent on many steps in the operatory and the laboratory. The most important process is impression taking, which provides the resulting impression and master cast. Failure to record the appropriate areas of the oral cavity accurately leads to excessive adjustments, lost chair time and, sometimes, starting over. With so much at stake, it is surprising to find that the majority of impressions received in dental laboratories are deficient in some way.
The technical aspects of impression material use are well documented in the directions for each system manufactured, and the clinician taking the impression needs to follow them carefully. This article focuses on the most common omissions and mistakes that occur in the impression-taking process and the negative effect(s) each mistake has on the clinical delivery of the prescribed restoration.
Fixed Prosthetic PVS Impressions
Inadequate Material in the Occlusal Areas Adjacent to the Posterior Crown or Bridge Preparation(s)
This omission can lead to a misarticulated cast, which results in inaccurate occlusion at delivery.
Use of a Posterior Quadrant Impression Tray for the Capture of an Incisor Restoration
Use of the incorrect tray often produces an impression that failed to capture the contralateral tooth, which leaves the technician “in the dark” regarding the contour and position needed for the crown, bridge, or veneer to match the existing dentition.
Impression Tray Size is Too Small
Use of an inadequately sized tray can create exposure of the tray wall through the impression material, which limits the needed material in potentially crucial margin areas and, in the case of double-arch quadrant trays, increases the potential for distortion (Figure 1).
Incorrect Tray Adhesive Technique
Tray adhesive is critical to accuracy when using vinyl polysiloxane (VPS) impression material. VPS materials shrink during setting. The tray adhesive ensures the retention of the material to the tray, controlling shrinkage. It is extremely important to use the correct adhesive for the specific brand of impression material used. Also, failure to precisely follow the instructions for the adhesive regarding setting time can lead to distortion and an ill-fitting crown, bridge, or veneer (Figure 2).
"Relining" an Impression that has Set
The addition of material into a previously set impression to capture an area of deficiency (void) is very risky. The elastic nature of these materials allows “distortion” of the previously set material when replaced into the mouth and rebound on removal. This can result in an ill-fitting restoration at delivery (Figure 3).
Inadequate Time Management
Time management is critical in impression taking. Each product's instructions explain the available working time, as well as the correct set time. The use of a timer is strongly recommended to ensure against early removal of the impression from the mouth. Errors created by early removable typically are not visibly detectable and, therefore, not caught until the attempted, unsuccessful delivery of the restoration.
Removable Prosthetic Impressions
When taking PVS impressions for removable complete or partial dentures, the clinician needs to be attentive to the concerns previously discussed as well as additional concerns. For these impressions, there is greater emphasis on including the required soft-tissue anatomy.
Proper Alginate Impression Technique
Irreversible hydrocolloid (alginate) is still widely recognized as the most accurate choice for removable partial dentures and occlusal splints. Unfortunately, the additional time and attention required to use alginate properly is leading many clinicians to choose PVS. Alginate material also should be mixed according to the specific manufacturer’s directions. Application of material to the occlusal (posterior) and lingual (anterior) surfaces of the teeth before placing the loaded tray eliminates the trapping of air in these areas. This prevents "positives" from appearing on the resulting cast. These irregularities can dramatically affect the accuracy of the resulting fit of an appliance or the occlusion if the intended use was as an opposing cast.
Proper Stone Cast Development
Conventional irreversible hydrocolloid (alginate) should be poured immediately on removal from the mouth. Because the material is mostly water, it begins to shrink almost immediately when exposed to air. If you cannot pour the impression right away, disinfect, rinse thoroughly, and place it in a sealed environment such as a plastic food storage container or an airtight zipper-type plastic bag with a few drops of water. Failure to comply with this requirement will lead to problems at delivery of the involved prosthesis, such as tight-fitting removable partial dentures, occlusal splints, and nightguards. If the impression was taken as an opposing cast, this error can lead to excessive occlusal adjustments of crowns, bridges, removable partial and complete dentures, and occlusal splints.
The more recently developed irreversible hydrocolloid substitute materials reportedly allow pouring the cast later without distortion. Numerous alginate “substitutes” are being introduced to the dental marketplace, which merit investigation to determine if they provide a solution to this problem.
Gypsum material must be weighed properly and measured to the manufacturer's specifications to ensure accuracy of the resulting stone cast. These materials are available in premeasured packaging, but at an increased cost. Many offices use a food scale to weigh stone in 100-g portions and store these portions in sealable plastic bags for maximum use of space. A simple milliliter graduate should be used for water measurement. This often overlooked detail can aid significantly in the production of consistently well-fitting appliances.
The stone cast must be removed from the alginate at the appropriate time, upon setting. The two types of stone are regular-set (45-minute set time) and quick-set (12-minute set time). After the cast has gone through the “heat of reaction” and cools down, it starts to absorb the water from the alginate impression. If left for an extensive period, this will compromise the surface of the cast. The test for this occurrence is simply to rub a finger over the surface of the cast. If dust from the surface of the cast comes off on the finger, the cast is inaccurate. This seemingly minor detail will lead to the same problems previously mentioned in the section on timely pouring of the stone cast. The use of a timer is recommended to ensure timely removal of the cast. Avoid pouring up alginates at the end of the day, unless someone will be available to remove the cast after the correct set time. It is better to store the impression overnight, as previously described, and pour up the following morning.
Digital Impression Technology
No current discussion of dental impression materials and techniques is complete without a mention of the digital technologies that are quickly developing. These systems allow the digital capture of the required areas for crown- and-bridge construction. The convenience, accuracy, and patient comfort made possible by these systems are great benefits for the dental team and many experts predict their imminent success. The two dedicated systems currently in use are the iTero™ digital impression system (Cadent, Carlstadt, NJ) and the Lava™ Chairside Oral Scanner (C.O.S.) (3M ESPE, St. Paul, MN). CEREC® 3D (Sirona Dental Systems LLC, Charlotte, NC) and E4D Dentist™ (D4D Technologies LLC, Dallas, TX) use digital impression technology as part of their computer-aided design/computer-aided manufacture (CAD/CAM) systems.
The wide variety of restorative options now available requires significantly more consideration from the clinician than in the past. Fixed prosthodontics options include ceramometal (traditional alloys; Captek™, Captek, a division of Precious Chemicals Company Inc, Altamonte Springs, FL; unique tri-metal), cast gold and nonmetal cementable (Procera®, Nobel Biocare USA, LLC, Yorba Linda, CA; Lava™, 3M ESPE, St. Paul, MN; Cercon®, DENTSPLY Ceramco, Burlington, NJ; ZENO®, Wieland Dental + Technik GmbH and Co KG, Pforzheim, Germany), nonmetal etched and bonded (IPS Empress®, Ivoclar Vivadent Inc, Amherst, NY), and stacked feldspathic porcelain (Enamel Art i.b.v., Drake Precision Dental Laboratory, Charlotte, NC; composite resin). RPD material choices include: Vitallium® cast high-heat alloy (DENTSPLY Austenal,York, PA); type IV gold; titanium alloy; and thermoplastic resins.
The Restorative Team
As a crucial member of the restorative team, dental assistants are responsible for ensuring that every step of every procedure is done correctly, including impression taking. All members of the restorative team are bound together in an attempt to meet the patient’s esthetic and functional expectations. When attention to detail is the norm rather than the exception, we all succeed.
About the Author
David Avery, CDT
Director of Training and Education
Drake Precision Dental Laboratory
Charlotte, North Carolina