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

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 microm 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. .     

Determining the accuracy of stock and custom tray impression/casts

SUMMARY A study was conducted to evaluate the accuracy of casts made from stock tray and custom tray impressions using polysiloxane impression material. The results indicate that all casts distort but that impressions made from custom trays were more accurate and consistent in reproduction than were stock tray impressions.     

The effect of tray selection on the accuracy of elastomeric impression materials

This study evaluated the accuracy of reproduction of stone casts made from impressions using different tray and impression materials. The tray materials used were an acrylic resin, a thermoplastic, and a plastic. The impression materials used were an additional silicone, a polyether, and a polysulfide. Impressions were made of a stainless steel master die that simulated crown preparations for a fixed partial denture and an acrylic resin model with cross-arch and anteroposterior landmarks in stainless steel that typify clinical intra-arch distances. Impressions of the fixed partial denture simulation were made with all three impression materials and all three tray types. Impressions of the cross-arch and anteroposterior landmarks were made by using all three tray types with only the addition reaction silicone impression material. Impressions were poured at 1 hour with a type IV dental stone. Data were analyzed by using ANOVA with a sample size of five. Results indicated that custom-made trays of acrylic resin and the thermoplastic material performed similarly regarding die accuracy and produced clinically acceptable casts. The stock plastic tray consistently produced casts with greater dimensional change than the two custom trays.     

A predictable and accurate technique with elastomeric impression materials

A method for obtaining more predictable and accurate final impressions with polyvinylsiloxane impression materials in conjunction with stock trays is proposed and tested. Heavy impression material is used in advance for construction of a modified custom tray, while extra-light material is used for obtaining a more accurate final impression.     

Accuracy of open tray implant impressions: an in vitro comparison of stock versus custom trays

STATEMENT OF PROBLEM: The accuracy of an implant fixture-level impression is affected by the type of impression tray used. PURPOSE: The purpose of this in vitro study was to investigate the accuracy of open tray implant impressions comparing polycarbonate stock impression trays and rigid custom-made impression trays to make implant fixture-level impressions. MATERIAL AND METHODS: Gold cylinder pairs, splinted by gold bars (reference frameworks) were constructed on an aluminum typodont. Polyether impressions were made of 2 pairs of Br?nemark 3.75-mm diameter fixtures mounted in an aluminium typodont, with 3 stock impression trays, 3 close-fit custom trays, and 3 spaced custom impression trays, by use of an open tray technique. The casts produced were assessed for accuracy by attaching the reference frameworks with alternate single screws and measuring the vertical fit discrepancy of these reference frameworks to the analogs within the working cast using a traveling microscope. Comparison of gap dimensions by tray type was performed with a nonparametric Kruskal-Wallis analysis of variance (ANOVA) followed by pair-wise Mann-Whitney U tests. To adjust for multiple comparisons in the post-ANOVA contrasts (P<.02). Comparison of gap dimensions between anterior and posterior regions were performed with Mann-Whitney U tests (P <.05). RESULTS: The results showed that the mean fit accuracy, as measured by vertical fit discrepancy, of casts from the stock trays (23 +/- 20 microm) were statistically significantly less (P<.001) than the spaced custom trays (12 +/- 10 microm) or close fit custom trays (11 +/- 10 microm). The difference in median gap size for analogs with a 20-mm separation was 10 microm. CONCLUSION: Within the limits of this in vitro study, rigid custom trays produced significantly more accurate impressions than the polycarbonate stock trays. The stock trays used in this study could not produce accurate impressions consistently. For analogs with a 20-mm separation, there was a difference in medians of 10 microm in accuracy between the stock and custom trays.     

Effect of impression tray design and impression technique upon the accuracy of stone casts produced from a putty-wash polyvinyl siloxane impression material.

This study examined the accuracy of stone casts produced from impressions taken in stock polycarbonate trays, some of which had been strengthened with autopolymerizing polymethyl methacrylate resin. Three techniques were used to make the impression of an acrylic master model of the mandibular arch on which two extracoronal preparations for bridgework and one intracoronal inlay preparation had been carried out. Each preparation had been indented with a reference point for later measurement. The impression material was a putty-wash polyvinyl siloxane material. Five impressions were taken for each type of tray for each impression technique and these were cast in die-stone after 24 h. The distances between the points were measured with a reflex microscope and the means determined for each design of tray. The mean difference between casts produced from the various tray designs and the acrylic master model were determined for each of the distances between the three measuring points for the various impression techniques. Statistical analysis showed that, with the polycarbonate stock trays, there were significant differences between some of the modifications and between them and the acrylic model, for the three distances (P less than 0.05). These differences were limited to one measurement for one design of tray for each of the two-stage impression methods. With the one-stage technique the unreinforced tray and those reinforced with acrylic, over the heels and anteriorly, and the barred design were statistically significantly different from the acrylic model for measurement A-B.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparing effects of tray treatment on the accuracy of dies

The effects of tray treatments on the accuracy of dies from addition silicone impressions were investigated. Tray treatments included custom acrylic resin tray with adhesive, perforated custom acrylic resin tray without adhesive, and perforated custom acrylic resin tray with adhesive. No appreciable differences were found in the complete crowns among the three tray treatments on the first pours. Significant statistical differences observed in the MOD and occlusal inlays were nonetheless of questionable clinical significance. Adhesives are advisable if the impressions are poured repeatedly, however, to minimize accidental separation of the impression from the tray. The second casts were less accurate with complete crowns and MOD inlays when perforated trays were used without adhesives.     

An alternative putty/wash impression technique

Conventionally, custom trays have been made in the laboratory on casts constructed from preliminary impressions. A spaced tray made in this way allows a uniform thickness of impression material of approximately 2 mm which is said to enhance the accuracy of the impression-taking procedure. A technique is described to fulfill this criterion without the need for a primary impression, by constructing a spaced putty custom tray inside the mouth.     

Evaluation of Polycaprolaitone Custom Tray Material

Custom impression trays are necessary for accurate impressions in fixed prosthodontics when using polysulfide impression materials. These custom impression trays are needed to provide for a uniform thickness of impression material to minimize distortion. Custom impression trays have historically been made from acrylic resin. New materials have been developed as alternatives to acrylic resin, and this study evaluates one product currently available that is composed of polycaprolaitone. This study evaluates the modulus of elasticity of polycaprolaitone as compared with acrylic resin, and also evaluates the amount of permanent deformation of the new material when placed under a load. The elastic modulus of polycaprolaitone was tested using three-point bending of specimens placed in a universal mechanical test system. The elastic modulus of the new material was found to be significantly lower than that of acrylic resin. The next test evaluated the amount of permanent deformation of the polycaprolaitone material when specimens approximating the shape and size of custom impression trays were placed under a load. These trays were placed in a universal mechanical test system and differing loads were placed on them. Results showed that although the trays deformed at relatively low loads, all deformation was fully recovered within a clinically insignificant period of time (less than 30 seconds). From these experiments it was concluded that although the polycaprolaitone material has a lower modulus of elasticity and will deform under relatively low loads, the material recovers completely, and thus the deformation would not interfere with the clinical performance of polycaprolaitone as a custom tray material.     

Dimensional change of impressions on sterilization

The volume of impressions, before and after disinfection in a gluteraldehyde solution for longer than ten hours (that is, sterilization), was measured indirectly from dies produced from the impressions. Three elastomeric impression materials were used in both acrylic resin and poly-vinyl-chloride 'trays' to form the impressions. One impression material appeared to be practically superior to the others with respect to variability of volume. The effect of tray material on change in volume (calculated as after disinfection minus before disinfection) was significant for one impression material; acrylic trays produced the greatest effect. The change in volume was significant for two impression materials; acrylic trays were associated with greater increase in volume. It was concluded that if impressions are to be sterilized, a tray material having minimal potential to absorb disinfectant should be used.     

The effect of a custom edentulous impression tray on lingual sulcus depth

An acrylic resin custom edentulous impression tray and standardized irreversible hydrocolloid impression technique were used. Ten impressions were made, after which the lingual border of the tray was reduced by 1.5 mm before the next series of impressions was made. This was done three times. Impressions were cast in a miter box. The plaster blocks so obtained were sectioned in the (a) retromylohyoid region, (b) 10 mm anterior, and (c) in the region of the mental foramen. In the retromylohyoid region there was a significant difference between the flange length after each reduction of the custom tray (p less than 0.01). Ten millimeters anteriorly the differences after each reduction except between the first and second reduction were less significant. In the region of the mental foramen there was a significant difference after the first reduction, but there was no significant difference between the flange lengths obtained thereafter.     

Evaluation of accuracy of the wash impression technique to rebase and replace the resilient part of a soft-lined denture

The dimensional inaccuracies that might be introduced during a wash impression technique within a heat-cured acrylic resin tray lined with a silicone resilient denture base material were investigated with the use of an electronic digitizer. Casts were poured from impressions of a machined metal die using a light- and heavy-bodied addition silicone in the soft-lined tray and compared with casts poured from impressions made in a hard-based tray and with the master die itself. The dimensions of the casts poured from the heavy-bodied impression in the soft-lined trays were significantly different from those poured from impressions in the hard-based tray when compared with the metal die. Those produced from the light-bodied material showed no statistically significant dimensional difference.     

Dimensional stability of elastomeric impression materials in custom-made and stock trays

Elastomeric impression materials for fixed prosthodontics are considered most stable when they have an even thickness of 2 to 4 mm. To obtain this, a custom-made impression tray is recommended. The purpose of the present study was to compare the stability of impressions made in custom trays and trays made of chromium-plated brass. The impression materials chosen were polyether and silicone.Two master models of the upper jaw were made of metal. The canines and first molars represented abutment teeth with flat occlusal surfaces. An engraved cross on each surface made it possible to measure in a microscope the distances between the abutment teeth on the models and in the impressions. The accuracy of the method was within ±8 μm. Twelve standardized impressions were made with each impression material in the two types of trays. The distances between the abutment teeth were measured immediately on removal of the impression, and after 1 and 24 hours. Although ample amount of impression material (2 to 9 mm) was allowed, the linear dimensional stability of the impressions made in stock trays was not inferior to the stability of impressions made in custom-made trays.     

The accuracy of polyvinyl siloxane impressions made with standard and reinforced stock trays

A laboratory investigation was done to assess the influence of proprietary stock trays on the accuracy of impressions recorded with heavy light-body (HL) and putty light-body (PL) wash impression techniques. Two brands of trays were tested and the same trays were reinforced with acrylic resin. Individual die accuracy and overall distortion of the resultant casts were assessed. PL impressions in both stock trays gave undersized buccolingual dimensions at the preparation finishing lines whereas reinforcing the trays reduced this distortion. Significant inaccuracy at the second molar was found for all trays when PL impressions were made. These distortions would have resulted in clinically unacceptable dies. HL impressions, regardless of tray type, produced highly accurate dies at this critical site. Resultant overall cast distortion was reduced, but not eliminated, by using reinforced trays with either PL or HL techniques.     

Comparison of vertical movement occurring during loading of distal-extension removable partial denture bases made by three impression techniques.

Although the altered-cast impression technique attains stability and controlled tissue support for distal-extension removable partial dentures, the procedure is time-consuming and technique-sensitive. This study clinically compared the vertical displacement of distal-extension removable partial dentures made from different impression techniques. The impressions studied were the altered-cast impression, an impression made from a border-molded custom tray, and a stock tray irreversible hydrocolloid impression that served as a control. A post hoc analysis, using a Tukey Q-test, exhibited significance (p less than 0.01) between the two impression techniques after intraoral loading was performed. Although statistically significant, the 0.19 mm difference between the impression techniques may or may not be clinically relevant. Additional research is required in this area.     

A clinical pilot study of the dimensional accuracy of double-arch and complete-arch impressions

STATEMENT OF PROBLEM: It has been suggested that articulated casts fabricated with the double-arch impression technique may have superior occlusal accuracy than those fabricated with a single complete-arch impression. However, lack of tray rigidity may lead to flexure of the impression/tray complex, resulting in inaccurate dies in the bucco-lingual dimension. PURPOSE: This clinical pilot study compared the dimensions of dies fabricated with 3 types of double-arch impressions to dies fabricated with the conventional complete-arch, custom tray method. MATERIAL AND METHODS: Thirty-five addition silicone impressions were made of cast metal copings cemented onto natural teeth prepared as complete-crown abutments. Four combinations of tray types and impression material viscosity were used: (1) complete-arch, custom acrylic trays loaded with heavy-bodied material; (2) double-arch, disposable plastic trays loaded with heavy-bodied material; (3) double-arch, disposable plastic trays loaded with putty material; and (4) double-arch, reusable brass metal trays loaded with heavy-bodied material. Immediately prior to tray insertion, light-bodied impression material was syringed over all copings as a wash. The 4 copings were fabricated from cast gold and simulated metal-ceramic complete-crown thimbles with polished collars and had "projections" on the occlusal surfaces. The impressions were poured in type IV die-stone. Bucco-lingual and inter-abutment dimensions were measured. The differences between the stone dimensions and cast metal control dimensions were calculated and converted to percent dimensional change. The data were analyzed with 1-way analysis of variance, Student t tests, and Mann-Whitney tests (P<.05). RESULTS: The plastic double-arch tray loaded with heavy-viscosity addition silicone and a low-viscosity wash produced the least accurate combination inter- and intra-abutment dimensions. For this protocol, 1.17% mean dimensional change was recorded. This result was significantly different than that obtained for the other 3 impression methods tested. No significant differences were found between the complete-arch method and protocols in which putty was loaded in a plastic or metal tray. CONCLUSION: Within the limitations of this pilot study, the more rigid tray/impression material combinations more accurately replicated stone dies.     

Accuracy of complete dental arch impressions and stone casts using a three-dimensional measurement system. Effects on accuracy of rubber impression materials and trays

The purpose of this study was to investigate and compare the accuracy of complete dental arch impressions and stone casts made with two kinds of impression materials (addition-type silicone and polysulfide rubber) and trays (custom tray and modified custom tray). In addition, the effect of the quantity of stone was examined. Impressions were made from a metallic model of a simplified maxillary dentition. Impressions and stone casts were measured respectively with a three-dimensional measuring microscope. The results were as follows: 1. Distortions of impressions were so small that the reproducibilities of impressions were superior three-dimensionally. These kinds of impressions and trays did not influence the accuracy of impressions. 2. The setting expansion of the stone in the impression occurred in the outward direction and was affected by the kinds of impressions and trays. 3. The arch widths and lengths of the stone casts tended to increase in number. 4. Stone casts made with addition-type silicone impression material and a custom tray were the most accurate because the combination of the impression material and tray effectively suppressed the setting expansion of stone. 5. The accuracy of stone casts could be improved by controlling the quantity of stone.     

A laboratory study of dimensional changes for three elastomeric impression materials using custom and stock trays

Clinical success of fixed prosthodontic procedures is dependent in part upon the dimensional accuracy of elastomeric impression materials and impression procedures. Three elastomeric impression materials were used in custom and stock trays to determine the accuracy of impressions taken from an experimental stainless steel model representing premolar and molar bridge abutment preparations. Horizontal and vertical individual abutment and interabutment dimensions were measured on die stone replicas, and the measurements compared with those obtained from stainless steel master models. The results of this study demonstrate polysulphide is the least accurate impression material for both vertical and horizontal individual abutment dimensions. However, for interabutment horizontal dimensions, no statistical differences were noted between impression material types when using a custom tray. Stock trays produced unreliable results for all the materials tested.     

2种托盘制取松动基牙模型的精度比较

目的：评价成品托盘和个别托盘使用海藻酸盐印模材料对松动牙取模时模型的精度的影响。方法：用成品托盘和个别托盘对同一松动牙模型取模,各制得20个超硬人造石模型。使用电子游标卡尺测量取得的人造石模型上标各记点间距离,测量精度为0.001mm,将测得数据进行统计分析。结果：采用2种托盘对松动牙取模精度无显著性差异,2种托盘对于松动牙取模精度均已超出临床所允许范围。结论：当使用海藻酸盐作为印模材料时,成品托盘和按照常规方法制作的个别托盘不适于松动牙的取模,及对于松动牙,应尽可能避免使用海藻酸盐印模材料。     

The influence of impression trays on the accuracy of stone casts poured from irreversible hydrocolloid impressions

An investigation was conducted to determine which of four commonly used impression trays yields the best results when making irreversible hydrocolloid impressions. This objective was achieved by evaluating the comparative accuracy of stone casts obtained from irreversible hydrocolloid impressions made with trays of different characteristics. The following conclusions are drawn from this study. Some kind of distortion can be expected in irreversible hydrocolloid impressions with the use of any of the tested impression trays. The perforated trays (B and D) reproduce more accurately the distances along the length and the width of the arch than the nonperforated trays (A and C). The depth of the palatal vault (distance f-g) is reproduced most accurately by the Rim-lock nonperforated tray (A) followed closely by the stock perforated (B) and the custom nonperforated (C) trays. Under the conditions of this study, all the impressions had a tendency to be oversized except the impressions made with the stock perforated tray (B). Those impressions were slightly undersized for all but two measured distances. Clinically significant inaccuracies produced by any of the four tested impression trays were not found in this study.