November 1, 2014
by Leendert (Len) Boksman DDS, BSc, FADI, FICD; Cornelis H. Pameijer, DMD, DSc, PhD; Dr. Gildo Coelho Santos Jr., DDS, MSc, PhD
Crown and bridge impressions can either be described as an all-in-one impression or a two-step impression technique. The most commonly used impression technique, as seen in our dental laboratories, is the all-in-one, which utilizes a full or dual-arch impression tray, with a combination of heavy body tray material and a flowable wash (Fig. 1). A one step putty wash impression technique using a vinyl polysiloxane can also be very successful.1 The two-step impression technique can use a putty, heavy body, or medium body impression material to make a preliminary impression. After removal from the mouth alight body wash material is added and the tray reseated in the mouth. The preliminary impression can utilize a spacer, can be relieved (Figs. 2A and B), vented, can have two holes placed to direct the wash, or not relieved at all. The move towards the two step impression technique has been driven by the concept that it is easier to capture the margins of the preparation due to hydraulic pressure rather than using a retraction cord or paste system. However, the scientific literature proves that there is a greater probability of errors introduced by the two-step technique over the one-step non-relief impression.2,3
FIGURE 1. A typical all-in-one full arch impression utilizing a heavy body tray material (Affinity Heavy Body Clinical Research Dental), with a light body wash material (Affinity Light Body) in a rigid Heatwave Tray (Clinical Research Dental).
FIGURE 2A. & 2B. The putty impression is relieved with a scalpel around all the teeth removing undercuts to allow seating. The final shows an even distribution of the wash material.
The hydraulic and hydrophobic impression (H&H) technique has been recommended as a method to capture crown preparation margins without the need for gingival retraction. This technique utilizes a preliminary impression of the prepared unit, usually with a vinyl polysiloxane (VPS), and usually with a putty, although many units have been taken with a heavy body VPS as well. The preliminary impression is not relieved in any fashion, a spacer is not used, and the preliminary impression is lined with a light body, either regular or high flow VPS. The patient is then asked to bite back into the impression. The resultant pressure of the light body material being extruded from the preliminary impression, since there is no space for it, creates hydraulic pressure into the sulcus, laterally and occlusally throughout the entire impression.
Dental laboratories often see this technique used in a dual-arch impression, with various impression materials and trays, and with seemingly good capture of the margins of the preparation. Why therefore, do some laboratories experience up to a 50 percent remake when clinicians are utilizing this technique?4
In a previous article “Clinical Predictability with Dual –arch Impressions – Plastic Trays are not the answer”5, the authors summarized the technique and variables based on the dental scientific literature. They were as follows:
1. Casts made from dual arch impressions provide more accurate maximal intercuspal relationships6 (Figs. 3 and 4)
FIGURE 3. A typical mounting result when using an alginate driven opposing model, a VPS bite registration material, and a VPS final crown and bridge impression. The alginate driven model does not fit into the bite registration because the bite registration is too accurate.
2. Plastic dual arch impression trays compromise the results with consistent discrepancies because the plastic trays distort. This can be due to impingement of the tray on the hard tissues of the mouth, patients biting on the buccal-lingual connector if it does not have sufficient space in the retro-molar area, the act of repeated swallowing moving the lingual rim of the tray, rheologic pressure from the impression material moving the borders of the tray.
3. The elastic memory of plastic dual arch impression trays cause the tray to flex back into its original position, and depending on the impression material used, distort the final impression.7
4. Metal impression trays such as the Quad Tray series (Clinical Research Dental) (Figs. 5A and B, and Figs. 6A and B) eliminate this distortion.8
FIGURE 5A. & 5B. The Quad-Tray XL (Clinical Research Dental) is fabricated from aluminum, so that there is no elastic memory and has been modified with higher side walls to give more impression material support. Elongation of the tray allows for a better capture of the occlusion. InFlex has been specially formulated to be very stiff, to fight flex and distortion.
FIGURE 6A. & 6B. The Anterior Quad-Tray X2 has been modified to provide an ergonomic handle, higher sidewalls and provide better impressioning of the adjacent teeth. InFlex has a strain in compression of 1.3 making it the stiffest PVS material that can be used with this technique.
5. The American Dental Association recommends a specific heavy body impression material be used with a strain in compression of less than two percent such as InFlex (Clinical Research Dental, London, Ontario) and Aquasil Ultra Rigid (Dentsply/Caulk Milford PA). These are extremely stiff inflexible materials to add stability and eliminate distortion.9
6. When burn-through of the plastic tray is seen in the dual arch the impression it is distorted.
7. Dual-arch impressions taken with an inflexible quadrant metal tray are as accurate as full arch impressions taken with custom trays.10,11
In a 2007 article, Cowie et al.12 discussed the two step impression technique. Challenges with the two-step impression techniques were identified as follows:
1. Reseating the impression in the mouth often results in a “step” in the impression (Fig. 7).
FIGURE 7. Incomplete reseating of the relined impression often results in a step or ledge in the occlusion. There is wash material over the first molar but not the premolar resulting in an inaccurate occlusal table.
2. This technique more often results in occlusal inaccuracies13 (Fig. 8).
3. There is an increased risk of the wash not completely bonding to the set putty material due to salivary contamination14 (Fig. 9).
4. Second pours can often not be made as the wash separates when the working model is removed from the impression (Fig. 10).
5. Hydrostatic pressure can cause a “rebound effect”.
6. Any active force (biting into the impression) causes an increase in elastic recoil resulting in a reduction in the size of the prosthesis.15
7. For this technique to work, if the preliminary impression is not relieved, an increase in degree of preparation taper is required, but this negatively affects the clinical performance/retention of the crown and bridge prosthesis.16
8. Increased layers of die spacer are needed for this technique, the amount of which is arbitrarily chosen, to allow for the restoration to fit, which also compromises the performance of cements or bonding systems used. Cements work best in thin layers of 40 microns or less.
It has often been said that two materials cannot occupy the same space at the same time, and this is especially true when taking a two-step H&H impression. The lack of understanding of the flexibility of many plastic tray systems, elastic recoil of impression materials used, and the hydraulic pressure resulting in unseen distortion due to the elastic recoil or rebound, creates the variability and inaccuracy in the clinical end product, when trying to seat the crown and bridge prosthesis.
Scientific documentation from the literature includes these direct quotes from the authors’ publications:
“The disadvantages of this two-stage technique include the additional time of having to wait for two materials to set, contamination of the putty with saliva which may prevent light body adhering to it, and difficulty in reseating the set putty in the mouth. The most accurate impression is usually achieved using heavy body and light body addition silicone in conjunction with a rigid tray.”17
“Based on in vitro studies, 1 stage impressions showed predominantly superior accuracy compared to 2 stage impressions.” In this study, even though the preliminary impression was relieved to make room for the wash ” the benefit of 2-stage putty was impressions with regard to a more complete rendering of subgingival finishing lines should be questioned in light of this study’s results.”18
“The two-step putty wash technique has its problems, the most common of which is invisible when the impression is recorded only becoming apparent when the restoration is tried in and fails to seat satisfactorily. The problem that causes invisible, but sometimes-gross distortions, is recoil. Considerable forces are needed to seat putty impressions, which can result either in outward flexion of the tray wall or the incorporation of residual stresses within the material. On removing the tray from the mouth the tray walls rebound resulting in dies, which are undersized bucco-lingually.”19
In a study looking at the H&H technique: “During reseating of the tray, the wash induces tension on the high-viscosity material, thus inducing deformation on the already set impression material. After setting and on removal, the high-consistency material is likely to exhibit elastic recovery, returning to its original position, thus resulting in smaller dies. Although the elevated hardness of the high-viscosity material indicates little flexibility and high degree of rigidity, it was not capable to avoid the dimensional alteration of the vinyl polysiloxane in the 2-step technique without relief.”20
“The impression technique with a spacer was more accurate in all dimensions as compared to rewash technique.”21
“An increase in scattering was noted for all disposable stock plastic trays groups, indicating that impression taking is less reliable with this type of tray. The viscosity of the wash material significantly affected the impression’s accuracy. A possible explanation is the elastic deformation of the set putty material and tray caused by reduced flow of the higher viscosity wash material during the second stage of impression taking. The use of metal trays is superior regarding the dimensional accuracy and reliability of impression taking and should therefore be preferred.”22
Note that these results documenting the variability and lack of accuracy of the H&H technique are supported by numerous other scientific documentation.23–27
In Samet’s study, “A clinical evaluation of fixed partial denture impressions”, published in the Journal of Prost
hetic Dentistry,28 he looked at the quality of impressions sent to commercial laboratories for the fabrication of fixed prostheses, and found that of the impressions 89.1 percent had one or more observable errors. Unfortunately the errors introduced by tray flexure, material distortion and elastic rebound cannot be identified by sight alone, but are patently obvious when the crown and bridge unit does not fit.
It is easy for the clinician to blame the laboratory if a unit does not fit, but it is often the clinician’s lack of understanding of material science, and scientifically sound proper clinical protocol, that affects the final crown and bridge outcome. Suggested clinical protocol should be evidence based in the scientific literature, and not based on hearsay or empirical personal experiences.
Dr. Len Boksman is a part-time consultant to Clinical Research Dental and Clinician’s Choice.
Dr. Pameijer has no disclosures.
Dr. Gildo C. Santos Jr. is a part time consultant (Research and Development) for Clinical Research Dental and Clinician’s Choice OH
Dr. Len Boksman is a former tenured Associate Professor of Operative Dentistry at the Schulich School of Medicine and Dentistry, London, Ontario, Canada. He has recently retired from private practice and volunteers his time as Adjunct Professor in the University of Technology Dental School, Jamaica. He can be reached at email@example.com
Dr. Cornelis H. Pameijer is Professor Emeritus, University of Connecticut School of Dental Medicine, Farmington, Connecticut, USA.
Dr. Gildo Coelho Santos Jr. is an Assistant Professor and Chair of the Department of Restorative Dentistry, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada.
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2. Siemer A, Balkenhol M, Trost M, Ferger P, Woestmann B. Accuracy of one-step vs. two-step putty-wash impressions. J Dent Res AADR Abstract 82nd General session March 10-13, 2004
3. Mandios M. Polyvinylsiloxane impression materials: An update on clinical use. Austral Dent J 1998;43(6):428-34
4. Personal communication with various dental laboratories in Canada and the US.
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6. Cox JR. A clinical study comparing marginal and occlusal accuracy of crowns fabricated from double arch and complete arch impressions. Austral Dent J 2005;50:90-94
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9. ADA Professional Product Review. Elastomeric Impression Materials 2012;Vol 7 Issue 1
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21. Amini P, Tavallaei M. Effects of Rewash on the Accuracy of Stone Dies Produced by Putty-Wash Technique. J of Kerman U of Med Sci. 2013;20(2)
22. Balkenhol M, Ferger BP, Wostmann B. Dimensional Accuracy of 2-Stage Putty Wash Impressions: Influence of Impression Trays and Viscosity. Int J Prosthodont 2007;20(6):573-575
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24. Hung SH, Purk JH, Tira DE, Eick JD. Accuracy of one-step versus two-step putty wash addition silicone impression technique. J Prosthet Dent 1992;67(5):583-589
25. Johnson GH. Accuracy of addition silicones as a function of technique. J Prosthet Dent 1986;55(2):197-203
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28. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. Journal Prosthet Dent 2005;94(2):112-117