DENTAL TECHNIQUE
Technique to verify the accuracy of a definitive cast before the fabrication of a fixed dental prosthesis Ra’fat I. Farah, BDS, MSc, Jor Boarda and Abdullah M. Alshabi, BDSb An accurate definitive cast that ABSTRACT is nearly an exact duplicate of This report describes a straightforward technique for verifying the accuracy of a definitive cast by the 3-dimensional geometry of using a maximal intercuspation record fabricated from polyvinyl siloxane occlusal registration the entire preparation and material. This precise verification method detects inaccurate casts before the dental prosthesis is surrounding intraoral tissue in fabricated, thus saving chairside and laboratory time while reducing the number of costly prosthesis the patient’s mouth is essential remakes. (J Prosthet Dent 2016;116:325-327) for a precisely fitting a dental the undetectable detachment of the impression material prosthesis. from the tray lead to similar inaccurate results in the This accurate definitive cast can be obtained only from resultant definitive cast.6,7 The presence of a straightforan adequate, undistorted definitive impression made ward and precise method to verify the accuracy of the using appropriate materials and techniques. Unfortudefinitive cast will preserve clinical and laboratory time nately, inadequate impressions are frequently encounand reduce the number of costly remakes. tered in dental laboratories.1,2 In general, the errors seen This report describes a straightforward technique that in such impressions can be divided into 2 categories. The verifies the accuracy of a definitive cast before fabrication first category includes detectable errors such as voids, of the fixed dental prosthesis. The technique uses an tears, or air bubbles and either inadequate tray pressure interocclusal record in maximum intercuspation fabrion the soft tissue or flow problems, most of which can be cated with polyvinyl siloxane (PVS) registration material detected and rectified by the clinician before sending the 3 as a verification index. impression to the dental laboratory ; the second category consists of undetectable (invisible) errors that go unTECHNIQUE identified until the clinician attempts to seat the prosthesis. These errors may be caused by a slight flexing of 1. After preparing the teeth (Fig. 1), inject the PVS the tray, the loss of adhesion between the impression registration material (CharmFlex Bite; DentKist, Inc) material and the tray, shrinkage caused by polymerizaon the prepared and adjacent teeth with an autotion of the impression material, or the presence of a semixing gun (Garant Dispenser; 3M ESPE) (Fig. 2). vere undercut, leading to a distorted impression and an 2. When the coverage of the prepared teeth and at inaccurate cast.4,5 least 1 tooth mesial and distal to the preparation is Unfortunately, the probability of encountering an uncomplete, instruct the patient to close firmly until all detectable distorted impression is high because the most posterior teeth are in contact normally in the widely used impression trays are plastic stock trays.1,3 maximal intercuspal position. Even slight flexing of the tray will distort the impression, 3. Evaluate registration to determine whether teeth are resulting in an inaccurate definitive cast. Moreover, fully seated. Instruct the patient to keep the teeth unblocked severe undercuts, polymerization shrinkage, or
a
Assistant Professor, Department of Prosthodontics, College of Dentistry, Qassim University, Qassim, Saudi Arabia. Intern, College of Dentistry, Qassim University, Qassim, Saudi Arabia.
b
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Figure 1. Buccal view of prepared teeth.
Figure 2. Dispensing polyvinyl siloxane occlusal registration material from automixing gun onto prepared teeth.
Figure 3. Patient instructed to keep teeth together until material completely polymerizes.
Figure 4. Polyvinyl siloxane registration record before trimming.
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occluded for 2 minutes while the material undergoes polymerization (Fig. 3). Remove the occlusal registration from the mouth and rinse the record under running tap water. Inspect it to ensure that all of the necessary details have been captured (Fig. 4). Trim the record with a scalpel with a no. 15 blade to remove all areas of the record that contact soft tissues and remove the facial aspect of the registration by cutting along a line through the buccal cusp tips to facilitate and verify the complete seating of the record. Fit the trimmed record inside the patient’s mouth on the prepared teeth to confirm the record is completely seated and stable and that it can be replaced reproducibly. Then disinfect the record. Make the definitive impression and fabricate the definitive cast (Hera Moldastone CN; Heraeus Kulzer GmbH). Place the trimmed PVS interocclusal record on the definitive solid cast and evaluate fit. If the record
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Figure 5. Polyvinyl siloxane registration record not fully seated, confirming inaccuracy of stone cast.
does not fully seat, the probability that the definitive cast is inaccurate is high (Fig. 5), indicating that the impression should be remade. However, if the record seats passively with no gap or rocking (Fig. 6), Farah and Alshabi
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impression by verifying the accuracy of the resultant definitive cast. SUMMARY
Figure 6. Polyvinyl siloxane registration record seated passively, confirming accuracy of stone cast.
This technique is a straightforward, precise verification technique that does not require extra clinical or laboratory steps. Moreover, PVS is considered one of the most accurate interocclusal recording materials; and most commercially available PVS occlusal registration material comply with American National Standards Institute/ American Dental Association specification 19-2004.8 This material, which is sufficiently stiff, has good detail reproduction and can reproduce a 20 mm-wide line. It also has high (>96%) recovery and long-term dimensional stability; it exhibits a linear dimensional change of less than 0.5% after 2 weeks, even after undergoing disinfection.9 Additionally, PVS can be easily trimmed and is acceptable to the patient because it polymerizes quickly and comes in pleasant flavors.10 These characteristics make the PVS interocclusal registration record a suitable verification index. REFERENCES
Figure 7. Postoperative photograph showing cemented definitive fixed dental prosthesis.
the laboratory can proceed with the definitive prosthesis with a high probability that an accurately fitting prosthesis can be fabricated (Fig. 7). DISCUSSION A technique to verify the accuracy of a definitive cast in fixed prosthodontics using a PVS interocclusal registration record is described. This verification index, if recorded and trimmed accurately, can capture the 3-dimensional orientation and spatial relationships of the prepared teeth and those between prepared teeth and adjacent teeth. The index can then be used to verify that these relationships are duplicated in the resultant definitive cast and to facilitate the detection of errors in making the master
Farah and Alshabi
1. Winstanley RB, Carrotte PV, Johnson A. The quality of impressions for crowns and bridges received at commercial dental laboratories. Br Dent J 1997;183:209-13. 2. Storey D, Coward TJ. The quality of impressions for crowns and bridges: an assessment of the work received at three commercial dental laboratories. Assessing the quality of the impressions of prepared teeth. Eur J Prosthodont Restor Dent 2013;21:53-7. 3. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent 2005;94:112-7. 4. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 5th ed. St. Louis, MO: Mosby/Elsevier; 2016. p. 383-4. 5. Wassell RW, Barker D, Walls AW. Crowns and other extra-coronal restorations: impression materials and technique. Br Dent J 2002;192:679-90. 6. Albers HF. Impressions A text for technique and material selection. 1st ed. Santa Rosa, CA: Alto Books; 1990. p. 55-9. 7. Thongthammachat S, Moore BK, Barco MT II, Hovijitra S, Brown DT, Andres CJ. Dimensional accuracy of dental casts: influence of tray material, impression material, and time. J Prosthodont 2002;11:98-108. 8. American National Standards Institute and American Dental Association. ANSI/ADA Specification no. 19-2004: dental elastomeric impression materials. New York: ANSI; 2004. 9. Megremis S, Tiba A, Vogt K. An evaluation of eight elastomeric occlusal registration materials. J Am Dent Assoc 2012;143:1358-60. 10. Von Fraunhofer JA. Dental materials at a glance. 2nd ed. Oxford: WileyBlackwell; 2013. p. 33. Corresponding author: Dr Ra’fat I. Farah Qassim University College of Dentistry P.O. Box: 6700 Al-Mulaydah, Qassim, Ksa 51452 Saudi Arabia Email:
[email protected] Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.
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