Preparing teeth properly for indirect restorations is critical for creating an acceptable outcome that satisfies the Esthetic, Function, Structure, Biology (EFSB) requirements. Obtaining accurate and predictable impressions is an integral part of the restorative process. In today's world, impressions may be obtained either through digital technology or a more traditional analog technique.
Whatever the method, the marginal fit is a key parameter for a successful outcome. The digital world has advanced tremendously regarding its application in restorative dentistry. Scanning prepared teeth and milling restorations for same-day delivery to the patient provides an entirely new opportunity for dentists to help their patients receive dental care.
The digital approach is certainly efficient and effective but just like the analog technique, success depends upon obtaining clear preparation margins. This article is aimed at providing a few “impression pearls” to help bring predictability to the capture of restoratively prepared tooth structure.
Pearl 1 – Cotton wisps
Whether impressions are obtained through digital or analog means, clear capture of the preparation margins is imperative for restoration success. There is more forgiveness in the analog capture world than in a digital scan.
Digitally speaking, if the scanner recording tip (or eye) cannot “see” the marginal area (with approximately 0.5 mm separation between tooth structure and tissue), significant inaccuracy and unreadability are expected. All tissue interference must be removed to digitally capture margins precisely. If tissue overhangs the marginal area, or if crevicular fluid or hemorrhage is present, a clear capture is improbable.
This is not as much an issue in the analog world. In using a polyvinyl or silicone impression technique, the injection and flow of the material may deflect the fluid. Additionally, if the impeding interferences are not diverted, the impinging tissue will be captured within the impression. This tissue obstacle will be trimmed away from the margin at the time of master die stone manufacture. No matter which technique (i.e., analog or digital) is applied, tissue and fluid control lead to greater impression predictability.
Tissue retraction cord is mandatory with subgingival margins. The most common approach for most dentists is to use a two-cord technique. Using this method, the tooth is prepared for the gingival margin. The first cord is placed to retract the tissue and allow for sub-gingival tooth preparation.
Subsequently, a second cord is placed on top of the first to control fluid and physically deflect the tissue. The superior cord is removed just before the impression process. Although the technique yields predictable margin capture, the two-cord placement may injure or insult the tissue biologic width dimensions and result in potential tissue recession.
As an alternative to placing two cords, cotton wisps dipped in Hemodent (a buffered aluminum chloride liquid from Premier Dental) may be gently placed over the first cord. The cotton wisps offer the opportunity to use a varied thickness dimension. Pulling cotton fibers from a cotton roll results in an amalgamation of fiber size. One end of the wisp may be only a few fibers in thickness. The opposite end may include greater consistency.
By using gentle pressure in placing the wisp, and utilizing the chemical action of the Hemodent, tissue retraction becomes adequate and less traumatic. The results are improved marginal visualization and attainment.
Pearl 2 – Micro-tip impression syringe
Analog silicone/polyvinyl impression materials are packaged in two-part cartridges – a heavy and light body, respectively. The gun-like device mixes the base and catalyst materials as it is expressed through the swirly mixing tips. The system tip is designed to adequately mix the material for direct application to the prepared tooth structure.
As efficient as this apparatus is at delivering a properly mixed impression material to the preparation site, the size of the contrivance can be challenging for individuals with small- or medium-sized hands. As many dentists utilize their dental assistants in this impression phase, it can be very unwieldy for many to properly place and extrude the substance. Capturing an air bubble or void within the impression is a frequent and disconcerting result.
As an alternative to this large and cumbersome process, the 3M Intra-Oral Syringe is a fantastic device that is easy to manipulate and creates a greater opportunity for accuracy and precision of polyvinyl/silicone impressions. It is designed to provide ease and efficiency in the mixing and application of the light body impression material.
The imprint syringe is easily loaded directly from the light body cartridge by engaging the two material exit ports. As the trigger of the large cartridge is depressed, impression material flows into the imprint syringe.
The imprint syringe is then detached from the cartridge and the plunger is inserted into the rear ports of the microsyringe. As pressure is applied to the piston, the appropriately mixed impression material is expressed easily and accurately over the tooth preparation and margins. There is minimal possibility of bubble or void formation. Each syringe will hold enough material for 4-6 teeth.
In more extensive cases, I instruct my assistant to load two syringes, before loading the heavy-body tray material. These implant syringes are disposable, so clean-up is insignificant. This product can be ordered from your dental supply dealer. Our office orders these from Henry Schein. The syringes cost about $80 for a package of 50. I have found these are invaluable in achieving predictable impression results.
Pearl 3 – Light body wash impression technique
The light body wash impression technique is not like the more standard putty-wash technique. This is a simple concept that adds extra insurance against trapping air bubbles or obtaining a material pull in the final impression outcome.
The standard putty-wash technique is utilized by many dentists. This technique uses a putty initial impression of the unprepared tooth or teeth. Once the putty is set or cured, it is removed and set aside until the tooth preparation(s) is complete.
When the dentist is satisfied with margin placement and refinement, a light body injection material is placed over the tooth preparations. The previously obtained putty impression is replaced over this slurry of light body material. The concept is the putty impression acts like a customized tray to push the light body material to capture a definitive replica of the preparations.
Conceptually this makes sense, but, in my opinion, there are consequential risks associated with this technique that limit predictability. The possibility of not accurately re-seating the putty impression is a significant deterrent. Additionally, there is the possibility that as the light body material is setting, there may be some distortion of the previously set putty material due to a hydraulic effect. For these two reasons (and a couple of others), my preference is to not utilize this impression technique.
The two-stage approach of placing the heavy body material into a tray and syringing light body material around the tooth preparations is my method of choice. But I incorporate one additional step in this technique. I refer to it as a “light body wash.”
This procedure involves injecting some remaining light body material directly on the heavy body material loaded in the tray. This “wash” is placed in the vicinity of the prepared tooth structure before seating the tray intraorally. This “wash” mixes directly with the light body material previously placed over the tooth preparations. It works to avoid air entrapment and creates a seamless melding of materials for an improved impression outcome.
While it is very simple, it provides improved confidence and accuracy in the impression process. In using this strategy, it is very rare to realize voids, pulls, or bubbles.
Our indirect restorative dentistry is dependent upon refined tooth preparation and contour. Capturing definitive and precise margins is critical for restoration success.
I hope these three simple pearls of wisdom provide some strategies for improving your impression outcomes.
Jeffrey Bonk, D.D.S., is a member of Spear Resident Faculty.