Seven simple steps to save time and money for your dental practice
Any dentist can make a perfect dental impression every time with only slight modications in technique and materials. Creating a flawless impression is not easy–even for doctors who’ve performed thousands of impressions in their practices. However, do-overs are costly, wasting both precious chair time and supplies. These unnecessary costs can add up considerably over time.
So we offer seven simple steps you can take to ensure you’re getting accurate, high-quality VPS (Vinyl Polysiloxane) impressions the first time. And now is the perfect time to think about the materials and techniques you use every day.
This article focuses on a heavy body/light body wash technique in which light-body impression material is applied first, immediately followed by a tray llned with heavy-body material, so that both materials are allowed to polymerize together.
The First Step: Understand the Causes of Common Impression Issues
First, let’s look at the most common dental impression issues and their causes.
Voids are gaps and spaces left by air pockets in the impression material. They’re typically caused by:
- Not applying enough material
- Improper wetting or intimate contact of prep with light body material
- Too much moisture on prep on in sulcus
- Improper retraction (in which the sulcus is not wide enough for light body to flow into it, especially in the line angles)
Thin walls are areas of the impression where there are gaps. These errors are caused by insufficient retraction, resulting in the following problems:
- The marginal tissue is not retracted enough to prevent collapse over the sulcus
- There’s inadequate sulcular width for light body material to flow
- Insufficient proximal reduction or failure to re-contour adjacent tooth anatomy
Tears are breaks in the impression, often caused by:
- The light-body VPS impression material not setting fully before removal
- Insufficient tissue retraction, particularly where the sulcus is thin and deep
- Presence of deep undercuts in the prep or adjacent tooth
Pulls are drag mark distortions caused by:
- Inserting a tray of heavy-body material past its working time
- Waiting past working time of the light-body material before inserting the heavy-body tray
- Removing the heavy-body tray before the material is fully set
- Too much moisture on the prep, wicking away the impression material
Step-by-Step Best Practices
Knowing the causes of the most common impression problems is half the battle. The other half comes down to technique. Even if you’ve been making impressions for decades, small technique adjustments may save your practice significant time and money.
The following methods have been developed over 40 years of clinical practice and from tips shared by other leading dentists…as well as insights acquired via exhaustive research and trial in development of impression products and materials.
Step 1: Use a 2-Cord Retraction Technique
To make a perfect dental impression requires excellent tissue management.
After prepping the tooth, apply a hemostatic agent to limit moisture/blood as much as possible.
First, Choose a Knitted Retraction Cord
The key is to use the most absorbent gingival retraction cord you can find. Knitted cords (as opposed to twisted or braided) are the best choice. Knitted cords are less likely to come apart during packing. Ultradent’s cords have a tight knit and are less absorbent of hemostatic agents. VacuCords are knitted a little less tightly to allow better absorption.
Then, use a 2-cord retraction technique: Apply a small cord first, followed by a larger second cord on top.
Single-cord applications are more likely to result in a margin that’s too thin, causing distortions during pouring. Using two cords, especially in a deep sulcus, allows for broader tissue displacement, more space for the light body wash to set, and better margin, all resulting in a better final impression. Leave the first cord in place during impression-taking for moisture control.
But there will be times when absolute moisture control is needed. In these instances, tear away a thin strip of cotton from a cotton roll, soak in a hemostatic solution, such as aluminum chloride, and place it over the first cord. Then add the second cord. This extra step will guarantee perfect moisture control as well as the desired amount of retraction.
Tips for Applying the Retraction Cords
Select your cord size according to the amount of retraction required (e.g., size #00 for a shallow sulcus, thicker for a deeper sulcus, etc.). Soak your retraction cord in the hemostatic agent.
Apply the small cord (e.g., #00) first. It will stay in place during the impression to help ensure a well-dened margin. As needed, place a narrow strip of aluminum chloride-soaked cotton on top of the first cord.
Pack a thicker second cord (e.g., #02) directly over the top of the first. The second cord should be larger because the bottom of the sulcus is a “V” shape, narrower at the bottom.
Allow approximately 3-5 minutes (or more, depending on the agent you use; follow the manufacturer’s recommendations) for the hemostatic agent to work. Note: If you placed a thin strip of cotton on top of the first cord before placing the second cord, then no waiting time is required.
During this time, the cord may dry out, increasing the chance of it sticking to the gingival tissue and causing additional bleeding upon removal. To avoid this, re-wet the larger cord prior to removal.
Once you’ve removed the larger cord successfully (and determined there is no additional seepage or bleeding at the sulcus), dry the surface and sulcus using compressed air. Then ready the light body impression material (wash).
Step 2. Use Compressed Air to Evenly Distribute Material
Have your dental assistant fill the tray with heavy-body material while you’re applying the light-body impression material to the prepared tooth. First apply a thin rope of light body into the sulcus followed by a quick blast of compressed air to spread out the material, making sure that the margin is properly coated, all the moisture is removed from the sulcus, and no air bubble is trapped between the light body material and the margins.
How to Apply Light Body Impression Material:
When applying light-body material, keep the tip in contact with the sulcus in one continuous motion to avoid accidentally introducing air pockets into the material. Start at the sulcus and work over the prepared tooth and surrounding area.
Figure 1: Apply Light Body Material.
Once this first layer of light-body material is in place, use air to blow it down into the sulcus. Air will distribute the material evenly and likely displace enough material that you’ll be able to see the margin. Add more material to the area for complete coverage.
Figure 2: Use compressed air to blow the light body impression material down into the sulcus. Add more material to completely cover the margin.
Figure 3: Apply Light Body Material to the Opposing Arch.
Next, apply light-body material to the opposing arch. This ensures that you capture the details of the opposing tooth anatomy, yielding better occlusal contact.
Step 3. Choose Rigid Trays for Heavy-Body Material
Your choice of impression trays can make a significant difference in terms of the accuracy and cost-effectiveness of impression-making.
Dual arch trays require less material than full arch trays, saving your practice time and money in the long run. Consider using dual arch trays for single (and even certain multiple) units. With one tray, you can obtain an impression of both the tooth and the opposing arch.
Plastic trays are less expensive than metal trays. And, since they’re disposable, they eliminate the possibility of cross-contamination. However, plastic is inherently less rigid than metal, introducing the chance of distortion in the final impression.
Thus, when it comes to applying heavy-body impression material over the light-body wash, choose the most rigid disposable tray you can.
Extremely rigid plastic trays are available that reduce the potential for distortion in the final impression:
- Harmony Posterior Dual Arch Trays are available with a 2mm distal steel bar, combining the performance benefits of metal trays with the disposable nature and cost savings of plastic.
- Harmony Anterior Dual Arch Trays feature rigid sidewalls to ensure an accurate bite as well as an optional facial midline tool to give the technician a guide as to where to set the anterior restorations such that both the embrasures and incisal edge positions are in harmony with the face.
Step 4. Choose the Right VPS Impression Material
It’s best to use heavy-body material from the same brand as your light-body material to ensure optimal compatibility and complementary set times. For optimal results, aim to have the heavy-body and light-body material set simultaneously.
At this step, be generous with the amount of heavy-body material you use to ensure proper coverage, clear margins, and no gaps or air pockets.
Using a Facial Midline Tool with your anterior tray will help ensure proper bite registration that’s in proper alignment with the face. Ho Dental Harmony anterior dual arch trays come with a facial midline tool. Or you can purchase Facial Midline Tools separately.
Step 5: Allow Ample Set Time
Pulls or tears in the impression are often the results of the impression material not being wholly set before removal. Allow the tray to stay in place for the impression material manufacturer’s recommended set time. Then, add an extra minute to ensure the material is fully set prior to removal.
Step 6: Remove the Tray in the Direction of the Axis
For best results, always remove the tray in the direction of the tooth axis of the prepared tooth. Ho Dental VPS material has high elastic strength, which allows it to compress and retain its shape upon removal.
Step 7: Touch Up with Remaining Material for a Perfect Dental Impression
In an ideal world, every impression would come out flawlessly after following the steps described. Occasionally, that won’t be the case. There are still a couple of things you can do to touch up an imperfect impression and avoid a costly re-make.
For one, you can fill marginal voids by using the remaining impression material in your syringe. Remove the mixing tip from the body of the syringe and squeeze out a small amount of unmixed material onto a pad. Mix the material using a spatula or the wooden end of a Q-tip and gather the material into a small mound. Dip the sharp end of an explorer into the mound and carry the tiny bit of material into the small marginal void and quickly blend it so that it conforms with the adjacent finish line of the prep.
You can also apply extra material to build up thin areas. Doing this will make the impression more stable and resistant to distortion when poured.
Usually, only a tiny amount of material is required to make fixes like these. Using a fresh syringe to apply it can be wasteful because you have to throw away the rest of the unfinished syringe. But not if you purchase replacement mixing tips.
Keep extra syringe tips handy for occasions when you need to apply more than just a tiny amount of material, such as reinforcing a wide area of thin impression material near the margin.
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