If you know what to look for when evaluating your impression, you can predict the fit and function of your definitive restoration.
The purpose of this paper is to discus the difference between a quality impression and the “clinically acceptable” impression.
The laboratory technician, the impression material, the type of quadrant tray or the technique, who or what is responsible for the infinite waste of time known as the chair side adjustment or remake?
There are many articles and publications dealing with colour communication and a fair number that deal with implant and cosmetic dentistry. However all this information, communication and fabrication of the perfect esthetic restoration is worthless without an accurate quality impression.
It is unfortunate that “quality dentistry” has been replaced with the phrase “clinically acceptable.” Ask yourself this, what type of dentistry do you practice?
Yes, most clinicians do take the time to properly prepare a tooth for an indirect restoration. Most use quality hemostatic agents such as a ferric sulfate gel (ViscoStat, Ultradent Products) or an aluminum sulfate gel (Tissue Goo, Clinicians Choice) to ensure profound hemostasis and sulcular fluid control. Some take it one step further by scouring and disinfecting the preparation with a chlorhexadine disinfectant (Consepsis Scrub, Ultradent Products Inc.) or will use mild acid of 10% EDTA (Detail, Clinicians Choice) to decrease surface tension and remove residual organic debris.
The use of products such as Detail (Clinicians Choice) should become common practice in every PVS impression procedure. 10% EDTA removes residue such as the sulfides from hemostatic agents,1 proteins from latex gloves and contamination from the cement of provisional restorations, all of which have been known to inhibit the set of PVS impression materials.2 Where the inconsistencies lie, is understanding what to look for when evaluating the final impression. Clinicians routinely look for a clearly visible 360 degree margin that is free of bubbles, voids or pulls prior to calling the lab, but is that enough?
The impression is the most important tool a technician has to reproduce a functional and naturally esthetic restoration. Although it is vital to have a clearly visible margin there are other factors fundamental in the fit, function and insertion of the final restoration.
Clinical indications of compromised impressions include:
1. Consistently having to adjust heavy occlusal contacts.
2. Consistently having to adjust tight interproximal contacts.
3. Consistently having open contacts.
4. The requirement of several layers of die spacer on the working die.3
5. The need to apply a foil spacer to opposing arch to prevent occlusal adjustments.
What do we need to look for when evaluating the impression?
– Is the entire preparation covered in light body (Fig. 1)?
– Putty is accurate to 75m.*
– Heavy body is accurate to 50m.*
– Light body / wash material is accurate to 20m*.
Have the heavy hydraulics of your Putty or Heavy Body PVS material displaced the light body away from the preparation site (Fig. 2)? If so, you are compromising the overall fit and retention of the definitive restoration.
Why? Technicians apply 50 microns of die spacer to working dies to allow room for cementation materials which are designed to work in very thin layers. If the model / die reproduction was made from and inaccurate impression the result is a space too thick resulting in fracture of the cement causing debonding.
Is there a seamless blend between your heavy body and light body/wash material?
Are you using a two step technique? These techniques have a potential flaw which can result in both occlusal, and interproximal contact adjustments:
A raised or open seam separating the heavy body and light body wash material (Fig. 3) indicates that when the impression was relined and reseated, that it was not seated completely. This seam is a clear sign that occlusal adjustments will be necessary.
Delamination of the light body indicates inadequate fusion between the two viscosity materials. Two step techniques require the isolation of the preparation and the adjacent teeth with a wrap or foil to prevent saliva contamination.4 Contamination from blood or sulcular fluid will prevent the two viscosities from adhering to one another (Fig. 4).
If the heavy body has not been properly relieved and vented prior to relining with your wash material the result is lateral distortion and expansion of the impression. The expansion of the buccolingual walls will result in mesiodistal stress in the impression materials which can result in tight interproximal contacts that prevent proper seating and require chairside adjustments.
Is there burn through of your impression tray, are the sidewalls visible?
Narrow plastic dual arch trays often do not allow for an adequate amount of impression material to accurately capture detail without impinging upon tissue.
Can you see the tray through the impression material? This may indicate a narrow tray leading to distortion from clenching pressure causing distortion due to deflection and rebound (Fig. 5).
The high side walls of plastic trays tend to expand laterally under load and rebound upon removal. This elastic rebound distortion is one of the causes for tight fitting crowns and interproximal contact adjustments
Impression material can separate from the tray upon removal of die stone which can lead to distortion of the antagonist (Fig. 6).
Stone can lock around exposed tray and penetrate through mesh making it difficult remove with out causing damage to the model or distortion to the impression, this eliminates the possibility of a second pour or check model (Fig. 7).
The entire tray should be encompassed in rigid supporting heavy body material (Fig. 8).
Is there burn through of the tray mesh? Is the mesh visible through the impression material (Fig. 9)?
a. Indicates too much bite pressure, thus creating compression distortion resulting in tight interproximal contacts caused by the intruding and / or tipping of the adjacent dentition.
b. Could cause the impression to tear upon removal from the model creating a distorted inaccurate opposing model.
c. Eliminates the ability to pour an accurate duplicate or check model.
Are you using a retraction cord?
Many clinicians today leave retraction cord in the sulcus and pick it up with their impression material (Fig. 10). This is done with the best intentions and the assumption that it will make it easier for the technician to read the margins. However, this technique can create significant challenges.
The majority of retraction cords on the market today are made of cotton which means:
If left wet when the master die model is poured it will not set completely and can create a margin with a paste like consistency.
If left dry when the master die model is poured, it will absorb water from the die stone creating a margin with a brittle chalk like surface.
If contaminated with a hemostatic agent, it could prevent the set of PVS materials all together.
Removal of retraction cord if drawn with the impression:
The cord should be cut out by the clinician with a scalpel to reduce the risk of tearing the impression at the margin.
If not removed prior to pouring the cord will bond to the die stone and tear the impression upon removal from the model, thus eliminating the opportunity for the lab to pour an accurate duplicate or check model.
2ND GENERATION PVS IMPRESSION MATERIALS
The addition of surfactants
In order to create a more user friendly PVS impression material, most manufacturers have added surfactants to their formulation in an effort to make them more hydrophilic. In order to accomplish this there had to be a compromise. Second generation PVS ma
terials are based on linear polymer technology which does not allow for the chemical integration of the surfactant, thus reducing the chemical conversion rate from 100% to approximately 94%. The compromise is a 25-60% reduction in tear strength5 depending on individual product chemistry. If you have ever left impression material in the sulcus upon the removal of the impression you aware of this fact. This reduction in tear strength is also realized in the laboratory when tearing the impression upon removal from the master model occurs. This eliminates the opportunity to pour an accurate duplicate or check model.
Heavy Hydraulics
Another characteristic commonly associated with a 2nd generation material is its reliance on heavy hydraulics to push and adapt the light body / wash materials to the margin. This has a clear disadvantage. The hydraulic pressure which forces the material to capture a clean margin will displace the wash away from the rest of the prepared tooth surface. The result you will observe is the margin in one colour (light body) but the rest of the preparation is captured in a different colour (Heavy Body)?
It is a fact that light body / wash materials are accurate to 20 um, Putty to 75m and Heavy Body to 50m. Although this is considered clinically acceptable it is necessary to remember crown and bridge cements were designed to work best in thin layers and the variable thickness from the heavy body impression material can create undue stress to the cement leading to possible fracture or debonding.
Fluid Viscosity
For the most part 2nd generation PVS materials will bond to each other regardless of viscosity, but have you ever noticed a seam between the heavy body and wash material? Have you ever suggested that the technician apply a foil spacer to the opposing arch (Fig. 11) because you find yourself constantly adjusting occlusion. This is not the fault of the clinician nor is it the fault of the technician, but an inherent flaw in the 2nd generation materials. In an effort to increase the rigidity of the heavy body material when set, manufacturers have added a significant amount of filler which does not flow as readily and creates an excess of hydraulic pressure which over displaces the light body or wash materials.
3RD GENERATION PVS IMPRESSION MATERIALS
The addition of surfactants.
Third generation materials are based on Branched Resin Technology. This chemistry is highly reactive providing fast independent intra oral set time utilizing grafted surfactants that are evenly distributed throughout to increase the hydrophilic performance and ensuring a stable 100% cure / conversion rate, which resists ripping or tearing clinically as well as in the laboratory. The hydrophile is grafted to the branched resin which creates a heat activated chemical reaction to create a shorter intra oral set time increasing patient compliance and reducing valuable chair time. This chemistry allows for independent working and set times rather than having to add the two to determine final set time.
Passive Hydraulics
Third generation materials flow exceptionally well due to the resin base capturing the exact detail of the entire preparation with light passive bite pressure without over displacing the wash material, an everyday occurrence associated with second generation materials. This prevents the displacement of the light body from around the preparation to ensure a more accurate fitting restoration.
Fluid Viscosity
Third generation materials such as Affinity (Clinicians Choice) have multiple technique specific viscosities. Each of which exhibits the same fluid characteristic providing a seamless blend between Heavy Body or Inflex — the only High Durometer impression material designed specifically for dual arch impressions, Clinician’s Choice. In addition to the seamless interface, these materials will not delaminate nor do they rely on heavy hydraulics which will displace wash materials away from the preparation which could compromise the fit of the final restoration.
Dual arch quadrant impression trays
In an effort to reduce the amount of chair time and material costs required for crown and bridge impressions, manufactures have introduced the dual arch impression tray. There are two types of quadrant trays available, plastic trays such as the Triple Tray (Premier Dental) and the dead soft aluminum trays such as the Quad Tray Extreme (Clinicians Choice) both of which exhibit different properties and degrees of success. Although most laboratories prefer full arch impressions for group function and a stable centric occlusion the dual arch trays can work exceptionally well if used properly.6 They are ideally suited for one to three unit restorations but require both an anterior and posterior stop with a separate bite registration to ensure proper articulation in centric occlusion.
There should be no controversy over which type tray will deliver a more accurate impression. The benefit to the plastic tray is that it is cost effective per tray, but not if you consider the higher probability of chairside adjustments or remakes. The challenge with plastic trays is that plastic is elastic and has a memory. When using a plastic dual arch tray with high side walls (Fig. 12) be aware that the impression material when under heavy load will force the impression tray sidewalls to flex laterally7 and due to the elastic nature will try to spring back upon removal, which can torque an impression creating tight interproximal contacts. Distortion related to plastic trays is not visible but it is obvious during insertion of the restoration.
The rigid yet dead soft aluminum impression tray can be customized to any shape if necessary to accommodate mal-aligned dentition in any arch form and can be adjusted to prevent the impingement of a predominant lingual mandibular tori. The pliability of the aluminum tray can distort under load as well but the dead soft metal will stay distorted therefore it will not torque, rebound or distort the impression upon removal.
CLINICALLY PROVEN IMPRESSION MATERIALS
3rd generation impression materials
– Exhibit higher tear strength.
– Resin based chemistry allowing for a seamless blend between heavy and light body / wash materials.
– Have a 100% conversion rate for added strength and stability.
– Grafted surfactants are evenly distributed throughout material.
Dead soft aluminum trays
– Do not have a memory.
– Can be customized as needed chairside.
– Are wide enough to prevent burn through.
– Minimal side walls to prevent lateral distortion.
– High durometer inflexible heavy body to add stability to tray/ impressions.
Success factors for a quality dual arch impression:
– Complete hemostasis and sulcular fluid must be controlled.
– Cord must be removed prior to impression even with double cord technique to ensure contaminants from hemostatic agents do not inhibit the set of impression materials.
– Preparation must be thoroughly cleaned to remove hemostatic or latex residue.
– Margins must be visible and clearly defined.
– Impression must be completely free of pulls, voids, bubbles or tears.
– Complete fusion or seamless blend of light and heavy body materials.
– Tray is encompassed in an Inflexible high durometer heavy body material.
– Tray must not be visible through impression material (burn through).
– Mesh must not be visible through impression material.
– Light body / wash material must completely cover all surfaces of preparation.
Dual arch trays, such as Quad Trays Xtreme, (Clinicians Choice) or Triple Trays (Premier) were developed to capture both working and opposing arches in the impression material while at the same time providing a bite registration. Using a quadrant type, dual arch impression has bee
n proven to be a more accurate bite registration than the use of both maxillary and mandibular full arch models, when the proper technique is used.6
The clinical challenge is communicating passive bite pressure to the patient and within the working time of the impression material. To prevent burn through or exposure of the impression tray mesh the patient should not clench but rather close passively into the impression tray. Clinicians should, for every case provide a separate rigid, inflexible and accurate bite registration such as Quick Bite (Clinicians Choice) which has a fast intra oral set time of 45 seconds.
Wax bite registrations are problematic due to the pliability and the ease of distortion caused by both handling and temperature and should not be used for working or die models. PVS bite registration materials should not be used to articulate a working / die model with an alginate produced antagonist due to the mismatch in accuracy.9
CLINICAL TIP: Prevent the patient from biting through the impression material and exposing the dividing mesh: while the patient is biting into the impression, the clinician’s hands should be on the patient’s cheeks. As soon as the Masseter begins to flex, the patient should be instructed to stop and hold that position.
Additional considerations
Although the dual arch impression certainly has its benefits you must also be aware of its limitations. Many laboratories use disposable plastic quadrant articulators (Fig. 13) which as we know will flex under pressure due to the elastic properties. These elastic properties can pose a significant challenge when restoring the terminal tooth in the arch.
If an anterior and posterior stop are not present in the impression / model it could potentially cause a rocking or a cantilever effect when adjusting the occlusion leading to open occlusal contacts.8 When restoring the terminal tooth in the arch (Fig. 14) it is recommended to use a rigid metal articulator to prevent the flexure commonly associated with plastic disposable articulators.
2nd vs. 3rd generation bite registration material
Third generation bite registration materials such as Quick Bite (Clinicians Choice) are resin filled materials with the high durometer required for an accurate articulation yet highly resistant to fracture.
Second generation bite registration materials are highly filled with silica (sand) which is extremely brittle and crumbles or breaks with little effort.
The addition of flavours and / or scents
With the exception of alginate / hydrocolloid impression materials, the ideal condition for taking an impression is a clean dry field free of blood, saliva or sulcular fluid. How many times have you chased these contaminants around the sulcus with your impression tip only to encounter a bubble, void or contaminated impression? Having to retake impressions or worse having to remake a restoration is costly and unnecessary.
PVS impression materials do not have the bad taste commonly associated with polyesthers. In fact they don’t have a taste to them at all! The introduction of a flavour or scent does not increase patient acceptance but instead stimulates the salivary glands, which is a contraindication for achieving an accurate impression.
Dental materials are constantly evolving to create more accurate user friendly products that enable us the ability to provide better dentistry for our patients.
The evidence has proven that the latest 3rd generation impression materials when combined with an aluminum dead soft quadrant trays offer the most reliable, consistent and accurate impressions and bite registration available today.
Gregg Tousignant, CDT, Certified Dental Technician since 1994 from the National Board For Certification in the USA. He has worked for 12 years as a dental technician, was certified as an instructor for Heraeus Kulzer where he provided a number of hands on programs for indirect composites and denture injections systems.
Gregg currently provides lectures and hands on courses for both the general/cosmetic dentist as well as dental and hygiene schools across Canada. Course topics include Tooth Whitening, Impressioning, Temporization, Direct Veneer Colour Change including the invisible Class IV restoration and the Sensitivity Free Direct Posterior Composite.
He is currently serving as the technical support manager for Clinical Research Dental where he provides technical support and hands on courses consistent with the Company Philosophy, Teach Better Dentistry.
Photographs courtesy of Dr. Robert Cowie, Dr. Len Boksman.
Oral Health welcomes this original article.
REFERENCES
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3.Ceyhan JA, Johnson GH, Lepe X ,School of Dentistry, University of Washington, Seattle, USA., The effect of tray selection, viscosity of impression material, and sequence of pour on the accuracy of dies made from dual-arch impressions. J Prosthet Dent. 2003 Aug;90(2):143-9.
4.Purk JH, Hung SH, Chappell RP, Casper RL, Eick JD, Department of Operative Dentistry, University of Missouri-Kansas City, The effect of time on the adhesion of light-body to heavy-body Express in the two-step reline polyvinylsiloxane impression technique. Am J Dent. 1990 Dec;3(6):249-52.
5.A. Boghosian, E.P. Lautenschlager, Northwestern University Feinberg School of Medicine, Chicago, Il, USA, Tear strength of low viscosity elastomeric impression materials, iadr, 2003 abstract 0137.
6.Cox JR., A clinical study comparing marginal and occlusal accuracy of crowns fabricated from double-arch and complete-arch impressions. Aust Dent J. 2005 Jun;50(2):90-4.
7.Larson TD, Nielsen MA, Brackett WW., College of Dentistry, University of Nebraska Medical Center, Lincoln, 68583-0750, USA. The accuracy of dual-arch impressions: a pilot study / J Prosthet Dent. 2002 Jun;87(6):625-7.
8.Linda J. Thornton, DDS, MS, A Survey on the Utilization of Disposable Quadrant Articulators, / Journal of AGD,Jan/Feb 2002.
9.Dr. Len Boksman, How can I minimize this side effect? JCDA: Dec 2005 – Jan 2006.
* Micron accuracy of viscosity is a result of ADA/ANSI reproduction tests and line reproduction.
