The irreversible hydrocolloid that we commonly refer to as alginate is extensively used for study casts, master casts, and working models for the fabrication of intraoral appliances.1 Alginate is the most common impression material used for creating the opposing model for crown and bridge, but it has many shortcomings as an ideal impression material. The ideal impression material should reproduce oral detail accurately, have high tear strength, have reasonable working and setting time, be biocompatible, be dimensionally stable, allow multiple pours, and not be affected in its accuracy by disinfection.2
|Figure 1. The dehydration of the flanges of this alginate impression is noticeable 2 minutes after taking the impression.||Figure 2. Amount of powder in the measuring cup can be highly variable.|
|Figure 3. Each and every mix of traditional alginate will have different thixotropy and working/setting time due to variables in water/powder ratio and temperature of the water.||Figure 4. Model derived from alginate impression without the use of an adhesive. Note that the lower left quadrant has pulled away from the tray.|
|Figure 5. Alginate substitute (COUNTER-FIT [CLINICIAN’S CHOICE]) being injected into a Borderlock (CLINICIAN’S CHOICE) tray lined with a PVS (Affinity Tray Adhesive [CLINICIAN’S CHOICE]) adhesive.||Figure 6. Internal detail of a COUNTER-FIT impression that has been repoured 4 times. (An alginate should never be poured a second time.)|
BACKGROUND ON ALGINATES
Due to its hydrophilic nature, alginate takes a good impression in a moist environment.3 However, this very feature, which is so important to clinicians, creates one of the most severe limitations for its use. This very property creates an impression that is not dimensionally stable due to environmental humidity and temperature.4-7 This feature of hydrophilicity also affects the stability of polyethers which can change dimensionally (larger by 30 to 240 µm; or smaller by 20 to 120 µm) depending on humidity.8 Polyether impressions and alginate impressions wrapped in moist towels should never be shipped to the laboratory together in the same polybag since the polyether impression can absorb water from the alginate impression or its wet wrap. Alginate impressions should be poured in dental stone immediately for maximum accuracy because of the material’s ability to imbibe or lose moisture. After 10 minutes of storage time, alginate begins to distort, and after one to 3 hours cannot be used for many clinical purposes, especially fixed prosthodontics (Figure 1).9
The stability of alginates is also affected by the reaction of syneresis, in which fibril cross-linking continues, creating a contraction with time and an exudation of water. This reaction therefore necessitates immediate pour up of the alginate. Irreversible hydrocolloids such as alginate can only be poured up once because of hydrophilicity.
Hydrophilicity also creates a monumental problem with disinfection of irreversible hydrocolloid with hundreds of research articles looking at types of disinfectants in various combinations with a multitude of alginates. These investigations show that disinfection can create severe effects on dimensional accuracy with immersion creating the greatest inaccuracy, as well as an effect on the surface quality of the casts produced.10-14 Alginate has a low tear strength and tears easily especially in deep undercuts and pontic areas. Alginate has a tendency to stick to teeth, with possible alginate tears on removal of the alginate, and polishing or prophylaxis of the teeth exacerbates the problem, preventing the alginate material from wetting the teeth and reproducing detail.15 If the teeth are dried from taking one impression, retaking a good detailed second impression of the same arch is impossible.2
If one adds the clinical variability of the inaccuracy of the amount of powder used in the mix (Figure 2); the amount and temperature of the water both which affect thixotropy and working time (Figure 3); the effects of mixing methods on the mechanical properties of alginates16; the effect of type and design of tray on clinical accuracy17,18; the effects of using adhesives19,20 (Figure 4); and the exposure to hazardous dust,1,21 it is not surprising that alginate substitutes are becoming more popular (Table)(Figure 5).
|Figure 7. Alginate derived stone models on a PVS bite registration material showing an obvious open-bite relationship.||Figure 8. Alginate driven stone models on Quickbite (CLINICIAN’S CHOICE) PVS bite registration material showing inaccurate fit.|
|Figure 9. Mounted case from Figure 8 showing mismounted open bite.||Figure 10. COUNTER-FIT driven stone model on Quickbite PVS bite registration material shows good interdigitation of the 3 components.|
|Figure 11. Precise mounting of the opposing casts is obvious when using matching accuracies in the impression and bite registration materials. This creates clinical predictability.|
Alginate substitutes are low cost polyvinyl siloxanes (PVS) and, as PVS, they have the same characteristics of the higher priced PVS materials that are used for final impressioning in fixed prosthodontics. Christensen22 describes these alginate substitutes as accurate, clean to use (no bowl to clean, no dust), with no taste (some are flavored which increases salivation and is not a good thing), no unpleasant odor, and the ability to delay pouring or to make additional pours of the same impression (Figure 6).22 Multiple studies have demonstrated that PVS is the most dimensionally stable impression material23,24 and has a higher tear strength than alginate. Unlike alginates they are not affected by disinfection techniques.25
The ability to pour these alginate substitutes at any time provides a whole new window of opportunity4, and now allows the clinician to send these opposing impressions along with the final crown and bridge impression to the laboratory, totally eliminating the need to pour these up, separate them, and trim them in the dental office. Not only is this a tremendous time- and cost-savings in the office, but now this process is a billable procedure as part of the laboratory fee for the crown and bridge case. Currently most dentists absorb the cost of fabricating the opposing model and do not bill it as a separate procedure. Since the PVS has better surface replication of the opposing dentition, and since the variables around mixing the dental stone are more closely monitored in the laboratory, the opposing model will be much more detailed and accurate.
One of the main driving forces in a change in direction from opposing models produced from alginate to opposing models derived from alginate substitutes in fixed prosthodontics, is the severe mismatch of accuracy between modern bite registration materials and alginate driven stone models. Boksman26 looked at PVS bite registration materials and the need for extensive occlusal adjustments on crown and bridge prostheses, showing the severe open-bite mounting that can result when using a highly accurate PVS bite registration with a poorly detailed alginate driven stone model (Figure 7). Using PVS for the final impression, for the bite registration, and for the opposing models eliminates the dimensional mismatch between materials, resulting in more accurate mounting of the case and less clinical necessity for occlusal adjustments (Figures 8 to 11).
The surface replication of the casts is also much smoother than models derived from alginate impressions. Even though the cost of the PVS alginate substitutes is higher than the irreversible hydrocolloid alginate materials, the impression is easier to take due to the thixotropic nature of these materials, the time and cost of pouring up the opposing model can be eliminated, the impression can be repoured if the model is broken or chipped, there is increased office efficiency, the opposing model is more accurate, the bite registration actually fits, and the time and frustration of adjusting the final prosthesis is minimized. These many benefits more than compensate for the additional cost. In addition, the clinician can now (in some cases) bill this as part of the laboratory procedure.
- Pace SL. Polyvinyl impression materials vs alginate impression materials. Contemp Dental Assisting. Feb 2006:20-23.
- Rubel BS. Impression materials: a comparative review of impression materials most commonly used in restorative dentistry. Dent Clin North Am. 2007;51:629-642.
- Cohen BI, Pagnillo M, Deutsch AS, et al. Dimensional accuracy of three different alginate impression materials. J Prosthodont. 1995 ;4:195-199.
- Perry R. Using polyvinyl impressions for study models: a case report. Dent Today. Oct 2004; 23:106-107.
- Chen SY, Liang WM, Chen FN. Factors affecting the accuracy of elastometric impression materials. J Dent. 2004;32:603-609.
- Straw J, Iuorno F, Lindauer S. Dimensional stability of Kromopan, an irreversible hydrocolloid impression material. Presented at: 32nd Annual Meeting and Exhibition of the ADR; March 12-15, 2003; San Antonio, TX. Abstract 0290. http://iadr.confex.com/iadr/2003SanAnton/techprogram/abstract_26049.htm. Accessed February 23, 2009.
- Bayindir F, Yanikoglu N, Duymus Z. Thermal and pH changes, and dimensional stability in irreversible hydrocolloid impression material during setting. Dent Mater J. 2002;21:200-209.
- Kanehira M, Finger WJ, Endo T. Volatization of components from and water absorption of polyether impressions. J Dent. 2006;34:134-138.
- Nichols PV. An investigation of the dimensional stability of dental alginates. Sydney, Australia: Faculty of Dentistry, University of Sydney; 2006. http://hdl.handle.net/2123/1270. Accessed February 23, 2009.
- Taylor RL, Wright PS, Maryan C. Disinfection procedures: their effect on the dimensional accuracy and surface quality of irreversible hydrocolloid impression materials and gypsum casts. Dental Mater. 2002;18:103-110.
- Jagger DC, Al Jabra O, Harrison A, et al. The effect of a range of disinfectants on the dimensional accuracy of some impression materials. Eur J Prosthodont Restor Dent. 2004;12:154-160.
- Machado C, Johnston W, Coste A, et al. Simulated clinical compatibility of disinfectant solutions with alginate impression materials. Presented at: IADR General Session & Exhibition; June 28-July 1, 2006; Brisbane, Australia. Abstract 2467. http://iadr.confex.com/ iadr/2006Brisb/techprogram/abstract_82984.htm. Accessed February 23, 2009.
- Muller-Bolla M, Lupi-Pegurier L, Velly AM, et al. A survey of disinfection of irreversible hydrocolloid and silicone impressions in European Union dental schools: epidemiologic study. Int J Prosthodont. 2004;17:165-171.
- Lu JX, Zhang FM, Chen YM, et al. The effect of disinfection on dimension stability of impressions [in Chinese]. Shanghai Kou Qiang Yi Xue. 2004;13:290-292.
- Phoenix RD, Cagna DR, DeFreest CE. Stewart’s Clinical Removable Partial Prosthodontics. 3rd ed. Chicago, IL: Quintessence Publishing; 2003: 162-167.
- Frey G, Lu H, Powers J. Effect of mixing methods on mechanical properties of alginate impression materials. J Prosthodont. 2005;14: 221-225.
- Mendez AJ. The influence of impression trays on the accuracy of stone casts poured from irreversible hydrocolloid impressions. J Prosthet Dent. 1985;54:383-388.
- Gordon GE, Johnson GH, Drennon DG. The effect of tray selection on the accuracy of elastomeric impression materials. J Prosthet Dent. 1990;63:12-15.
- Leung KC, Chow TW, Woo EC, et al. Effect of adhesive drying time on the bond strength of irreversible hydrocolloid to stainless steel. J Prosthet Dent. 1999;81:586-590.
- Smith SJ, McCord JF, Macfarlane TV. Factors that affect the adhesion of two irreversible hydrocolloid materials to two custom tray materials. J Prosthet Dent. 2002;88:423-430.
- Craig RG. Review of dental impression materials. Adv Dent Res. 1988;2:51-64.
- Christensen GJ. Ask Dr. Christensen. Dent Econ. March 2008;98:66.
- Karthikeyan K, Annapurni H. Comparative evaluation of dimensional stability of three types of interocclusal recording materials: an in vitro study. J Indian Prosthodont Soc. 2007;7:24-27.
- Boksman L. Eliminating variables in impression-taking. Ontario Dentist. Dec 2005:22-25.
- Waranowicz MT, O’Keefe KL. Alginates and alginate substitutes. The Dental Advisor. 2007;24:1-7.
- Boksman L. Point of care: how do I minimize the amount of occlusal adjustment necessary for a crown? J Can Dent Assoc. 2005;71:494-495.
Dr. Boksman practices in London, Ontario, Canada. He is adjunct clinical professor at the Schulich School of Medicine and Dentistry, University of Western Ontario. He is a Fellow in the Academy of Dentistry International and in the International College of Dentists. He has published more than 100 articles and various chapters in dental texts and lectures nationally and internationally on restorative dentistry. He can be reached at (519) 641-3066 x 292 or via e-mail at lboksman@clinicalresearch dental.com.
Disclosure: Dr. Boksman holds a paid part-time consulting position with Clinicians Choice and Clinical Research Dental, with the title of director of clinical affairs.
Disclosure: Mr. Tousignant is currently serving as technical support manager for Clinical Research Dental where he provides technical support and hands-on courses consistent with the company’s philosophy, teaching better dentistry.