Let’s be honest, there’s hardly anything more frustrating than pouring your effort into a crown or bridge case, meticulously following every step, only for the restoration to arrive from the lab tight-fitting and just refusing to seat properly. You end up spending valuable time and effort trying to adjust it, and often, you might even have to redo the entire case from scratch.
Many of us are quick to point fingers at the lab, but the truth is, quite frequently, the issue originates right with us, stemming from a seemingly simple yet critical step: the silicone impression.
The two-stage impression technique, commonly known as the Double Impression or Two-Step Putty/Wash Technique, is undeniably the most accurate method, especially crucial for zirconia work and other precision restorations. Unfortunately, it harbors a subtle pitfall that many of us unknowingly fall into: hydraulic pressure.
In this article, we’re going to walk you through why this problem occurs, and then we’ll share an incredibly simple trick that will help you completely avoid it, consistently achieving precise, perfect impressions every single time.
What is “Hydraulic Pressure” and Why Does It Spoil Your Impression?
When we take the initial impression using the heavy body (Putty), this material solidifies, effectively creating a rigid “table” or foundation. Then, when we proceed to take the second impression, we apply the light body (Wash) material over the putty and onto the prepared teeth, re-inserting the impression tray into the patient’s mouth.
This is where the problem creeps in. Any excess light body needs somewhere to escape. And because the putty is rigid and there isn’t sufficient space, immense hydraulic pressure builds up. This pressure can cause two major disasters:
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Distorts the Light Body Layer: It forces the light body to be incredibly thin in some areas and excessively thick in others, thereby altering the true dimensions of the prepared tooth.
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Compresses the Teeth Themselves: It can cause a very slight but significant movement of the teeth within the impression.
The final outcome? A distorted, inaccurate impression. The lab then pours this flawed impression, producing a model with incorrect dimensions, which inevitably leads to a tight-fitting, ill-fitting restoration (1).
The Brilliant Solution: The Simple Spacer Trick
The entire solution boils down to one single concept: “Space.” We absolutely must create a uniform, consistent space between the putty and the teeth, giving the light body enough room to flow freely without any pressure.
There are various methods to achieve this space (such as meticulously scraping the putty with a bur), but the technique we’re about to share is by far the easiest, most accurate, and quickest.
Here’s a Step-by-Step Breakdown:
1. Prepare Your Putty and Place the Spacer:
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Mix your putty and load it into the impression tray, just as you normally would.
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Before you insert the tray into the patient’s mouth, grab a thin “spacer” and completely cover the entire surface of the putty.
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What kind of spacer? The simplest options are a thin over-glove or even a clean piece of plastic bag (like a sandwich bag). Some companies also sell specialized plastic sheets designed precisely for this purpose, known as Impression Spacers.
2. Take Your First Impression (with the Spacer):
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Insert the tray into the patient’s mouth and take your primary impression as usual.
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Wait patiently until the putty has completely set.
3. Remove the Spacer and Prepare for the Wash:
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Carefully remove the impression tray from the patient’s mouth.
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Now, gently peel off the plastic layer or the glove you had placed.
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Take a look at your impression. You’ll notice that the spacer has created a subtle, uniform “gap” around all the teeth. This specific gap is the entire secret to a successful impression!
4. Take Your Second Impression (The Wash Impression):
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You are now fully prepared for the final impression.
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Thoroughly dry the prepared teeth.
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Inject the light body meticulously around your finish line, ensuring excellent coverage. Also, apply a thin layer of it into the putty impression.
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Confidently and smoothly re-seat the tray into the patient’s mouth.
Why This Trick Works and Delivers Perfect Results
When you consistently use a spacer, you achieve three fantastic objectives in one straightforward step:
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You Completely Eliminate Hydraulic Pressure: The light body finds ample, comfortable space to move within, allowing it to register details without any pressure-induced distortion or alteration (2).
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You Get a Uniform Light Body Thickness: This makes your impression far more dimensionally stable and significantly less prone to error.
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You Capture the Finest Details: Because the light body isn’t “suffocated” or compressed, it gains the remarkable ability to flow into and register every minute detail of the finish line with absolute clarity.
The final outcome? An incredibly accurate and passive impression. The lab will then be able to pour a precise model, and your final restoration will seat effortlessly into place the very first time.
The Takeaway: Don’t Overcomplicate It, Just Create “Space”
The entire field of fixed prosthodontics hinges on precision. And this deceptively simple trick is one of the most crucial secrets to achieving that precision in your silicone impressions. Rather than painstakingly trimming the putty or leaving things to chance, just use any simple, readily available spacer in your clinic.
This seemingly minor extra step, which won’t even take you 10 seconds, could potentially save you hours of adjustments in the clinic, or even the entire cost of redoing the impression and lab work. Give it a try, and witness the remarkable difference in the quality of your restorations yourself.
