Purpose This study evaluated the influence on dimensional accuracy of dental casts made with different types of trays and impression materials and poured at different and multiple times.

Materials and Methods Two types of stock trays (plastic stock tray, perforated metal stock tray) and 4 types of custom tray materials (autopolymerizing acrylic resin, thermoplastic resin, and 2 types of light-polymerized acrylic resins) were used with 2 types of impression materials (addition polymerizing silicone and polyether), to make impressions of a metal master model. Each tray and impression material was used to make 5 impressions. Casts were made by multiple pourings at 30 minutes, 6 hours, 24 hours, and 30 days after impression making. Using a measuring microscope, 12 distances were calculated based on measurements of 8 reference points. The absolute value of the difference of each measurement was calculated, as was the corresponding measurement on the master model. A Bayesian model using a simple noninformative prior was used to analyze these data.

Results Statistical differences within 6 μm were found only with thermoplastic resin tray material for addition silicone, and for thermoplastic resin tray material and 1 type of light-polymerized acrylic resin for polyether. Neither stock trays nor custom trays contributed to the differences in accuracy of the casts. All deviations in casts made with silicone impression material were within a clinically acceptable range. For the polyether, distortions occurred that were clinically unacceptable. Impressions made from polyether distorted over time. Silicone impression material has dimensional stability up to 30 days.

Conclusion Accurate casts can be made with either stock trays or custom trays. An impression made from polyether should be poured only once and within 24 hours after impression making, because of the distortion of the material over time. Silicone impression material has better dimensional stability than polyether.

Dimensional accuracy of dental casts: influence of tray material, impression material, and time.

Statement of problem Numerous investigations have been done to determine the most accurate method to reproduce a dental arch. Investigations have used different definitive cast configurations, materials, and measuring techniques to evaluate dimensional change, leaving clinicians undecided as to the most accurate method to reproduce a dental arch. Purpose of study The purpose of this investigation was to evaluate the effect of impression tray selection on accuracy of reproductions of a dental arch using a 3-dimensional optical digitizer. Materials and methods Impressions were made of a stone cast of a dental arch using vinyl polysiloxane impression material in custom and stock impression trays (n=12). Custom trays, designed with full palatal coverage, were fabricated on a separate standard stone cast using 2 layers of pink baseplate wax as the spacer and tissue stops on 1 anterior tooth and both first molar teeth. Impressions were poured with type IV gypsum and allowed to set with the tray inverted (n=5) or not inverted (n=5). The standard cast and resulting casts were digitized using an optical digitizer. Custom software was used to align and process the 3-dimensional images created by the digitizer. Each image was superimposed onto the standard cast image, curves were created of the percent of points less than a defined distance between the 2 superimposed images, and the area under the curve was calculated. A weighted average area was calculated for each cast, and these values were analyzed using a 2-way ANOVA (α=.05). Results The main effect of tray type, laboratory setting technique, and the interaction between these 2 characteristics was analyzed. None of these effects was significant. Conclusion The differences in accuracy of reproductions of a dental arch, regardless of the impression and laboratory techniques, were not significant when evaluated using a 3-dimensional optical digitizer.

Evaluation of dental arch reproduction using three-dimensional optical digitization.

Statement of problem Fabrication of indirect complete crowns that are in occlusal harmony upon insertion remains a problem in restorative dentistry, and dental stone cast expansion may play a role. Purpose This 3-part investigation compared occlusal contacts in a simulated patient with a harmonious occlusion with centric occlusion equal to maximum intercuspation (CO=MI) and an inharmonious occlusion (CO≠MI) with mounted stone casts, and compared the occlusal contacts after fabrication of a posterior complete crown fabricated on equilibrated and nonequilibrated dental stone casts. Material and methods A dentoform mounted in a semi-adjustable articulator served as the simulated patient and control. In part 1, a single set of maxillary and mandibular ADA type IV and V derived dental stone casts were fabricated and mounted (CO≠MI), and occlusal contacts/near contacts were compared. In part 2, 10 type IV and 10 type V cast mountings (CO=MI) were compared. In part 3, 10 type IV cast mountings were fabricated for adjustment (experimental) and 10 for no adjustment (control). A mandibular ADA type IV gold alloy complete crown was fabricated and adjusted on each set of casts and then returned to the dentoform. Vinyl polysiloxane interocclusal records of all mountings were scanned for optical density, and contacts were used to quantify occlusal contacts as exhibiting contact or near contact. Data were analyzed with Kruskal-Wallis ANOVA and Mann-Whitney U tests (α=.05). Results The cast adjustment protocol (intervention) was successful in eliminating the majority of the occlusal disharmony in the casts believed to be caused by the effects of stone expansion. Actual and near contact areas for cast mountings of the equilibrated simulated patient were significantly different from those of the simulated patient ( P P =.001, respectively). Actual and near contact areas for inserted crowns fabricated from adjusted casts were significantly different from those of the simulated patient ( P P =.007, respectively), but actual contact areas were not different from those of the simulated patient with no crown inserted. Conclusions In this study, occlusal contacts of a simulated patient (dentoform) could not be accurately replicated with mounted dental stone casts. A cast adjustment procedure can aid in fabrication of a crown with a more accurate occlusion.

The effect of equilibrating mounted dental stone casts on the occlusal harmony of cast metal complete crowns

Objectives. The impression-taking technique is one of the most critical factors that not only prevents the shrinkage caused by polymerization but also enhances the accuracy of implant impressions. Also, choosing the right time of taking impressions after splinting implants is one of the important criteria that affects the impression-taking technique. Accordingly, the present study aimed to evaluate the accuracy of different splint methods for implant impressions made at different times. Methods. In this in vitro study, a two-piece metallic index was prepared, and the patient’s jaw was simulated by placing self-cured acrylic resin in the lower part of the index. Then, two holes were made in the acrylic resin at a specific distance from each other, and the analogs were placed in these holes. Splinting of impression copings was carried out with autopolymerized acrylic resin (GC Pattern resin LS, GC America Inc., USA), and an open tray impression approach was performed. Thirty-six casts in three groups (n = 12) were fabricated from the acrylic model. After scanning the casts, the impression accuracy was compared between the three study groups by measuring the distance between the outer portions of the scan bodies screw-retained on implant analogs inside the cast using the Exocad software (2015.07 version). Group 1: splinting impression copings with autopolymerized acrylic resin and impression making immediately after the setting time (4 minutes); group 2: splinting and impression procedure after 17 minutes with splint sectioning and reconnection; group 3: splinting and impression procedure after 24 hours with splint sectioning and reconnection. The data were analyzed using SPSS 17 using the Kruskal–Wallis test. Results. The mean distance measured in group 1 was 19.14 ± 0.029 mm, which was significantly lower than the main model. The distances were 19.15 ± 0.039 and 19.159 ± 0.33 mm in groups 2 and 3, respectively. These two groups were not significantly different from the main model. Moreover, the mean distance measured in the three impression techniques was similar. Conclusions. There was no significant difference in the measurements between group 2, group 3, and the main model. Therefore, dentists can make an impression after 17 minutes to reduce chair time.

/pdf/comparison-of-the-accuracy-of-fixture-level-implant-1o8jp0uoex.pdf

Comparison of the Accuracy of Fixture-Level Implant Impression Making with Different Splinting Techniques.

................................................................................................................................... ii Preface ..................................................................................................................................... iv Table of Contents .................................................................................................................... v List of Tables ......................................................................................................................... vii List of Figures ....................................................................................................................... viii Glossary .................................................................................................................................. ix Acknowledgments ................................................................................................................... x Dedication .............................................................................................................................. xii Chapter 1: Introduction ........................................................................................................ 1 1.1	   Tooth Preparation ......................................................................................................... 1 1.1.1	   Assessment of Fit .................................................................................................. 3 1.2	   The Conventional Method ........................................................................................... 4 1.2.1	   Impression ............................................................................................................. 5 1.2.1.1	   Conventional Impression Materials ............................................................... 5 1.2.1.2	   Tray Selection and Adhesives ...................................................................... 11 1.2.1.3	   Disinfection of the Impression ..................................................................... 14 1.2.1.4	   Transportation of the Impression to the Dental Laboratory ......................... 15 1.2.2	   Pouring Dental Impressions to create Casts ........................................................ 17 1.2.2.1	   Gypsum Stone .............................................................................................. 18 1.2.3	   Crown Fabrication .............................................................................................. 19 1.3	   The Digital Method .................................................................................................... 20

A comparison of crown marginal fit fabricated using digital and conventional methods

Purpose The accuracy of master cast reproduction by a polyvinylsiloxane impression material using two visible-light-curing resin and an autopolymerizing polymethyl methacrylate resin custom tray materials was investigated.



Materials and Methods Custom trays were fabricated from a master cast that had three index points marked on both the inner and outer vestibules. Impressions were made of the master cast using Extrude and then poured in Die Keen Green stone. The distances between the reproduced index points were measured to ±0.01 mm with a traveling microscope and the algebraic norms calculated for each tray material.



Results No differences (p > .05) were found in the dimensions of the inner index points, while the separations of the outer index points indicated that there were differences in the accuracy of reproduction (p > .01) by the three tray materials. The index points reproduced on the casts, compared with the master cast, were closer together for the Triad Blue trays than for the TruTray and Ontray impression trays.



Conclusions All three tray materials produced acceptable casts, but the greatest accuracy was achieved with TruTray, followed by Ontray. Triad Blue produced casts that were slightly smaller than the master. In practice, the small measured differences in cast dimensions may not have clinical significance.

The Effects of Custom Tray Material on the Accuracy of Master Casts

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The Effects of Custom Tray Material on the Accuracy of Master Casts