| Abstract|| |
Context: Polyurethane resin is a possible alternative to type IV dental stone for fabrication of indirect restorations however its dimensional accuracy is questionable.
Aim: The aim was to investigate the dimensional accuracy of silica filled polyurethane resin die material by evaluating the marginal fit and adaptation of indirect gold castings.
Settings and Design: Experimental, in vitro study.
Materials and Methods: Totally 40 copper plated replicas of a nickel chrome master die analogous to a veneer gold crown preparation were made and impressions recorded using polyvinylsiloxane material. Twenty impressions were poured in type IV dental stone (control group (Vel-mix, Kerr, UK) and the remaining (n = 20) in silica filled polyurethane die material (test group) (Alpha Die MF, CA, USA). Gold castings were fabricated for each die using standardized techniques. The castings were seated on their respective copper plated dies, embedded in resin and sectioned. The specimens were analyzed by measuring marginal opening and the area beneath the casting at a ×63 magnification and using image analysis software.
Statistical Analysis Used: Data were analyzed using a Student's t-test.
Results: No significant difference was observed between the experimental groups (P > 0.05). The mean marginal opening for type IV, dental stone and polyurethane resin, was 57 ± 22.6 μm and 63.47 ± 27.1 μm, respectively. Stone displayed a smaller area beneath the casting (31581 ± 16297 μm 2 ) as compared to polyurethane resin (35003 ± 23039 μm 2 ).
Conclusions: The fit and adaptation of indirect gold castings made on polyurethane and type IV dental stone dies were comparable.
Keywords: Crown, die material, indirect casting, marginal fit, polyurethane resin
|How to cite this article:|
Lillywhite GR, Vohra F. Influence of polyurethane resin dies on the fit and adaptation of full veneer crowns. Indian J Dent Res 2015;26:72-6
Fit is most commonly expressed, as a measurement of the marginal opening that exists between the casting and preparation finish line, and 25-40 μm is considered as desired marginal gap.  Failure to achieve an optimum fit results in plaque accumulation, periodontal disease, microleakage and recurrent caries thereby compromising the longevity of indirect restorations.  Therefore, long-term success of indirect restorations has been intimately associated with good marginal fit.
|How to cite this URL:|
Lillywhite GR, Vohra F. Influence of polyurethane resin dies on the fit and adaptation of full veneer crowns. Indian J Dent Res [serial online] 2015 [cited 2019 Nov 14];26:72-6. Available from: http://www.ijdr.in/text.asp?2015/26/1/72/156814
Restoration of teeth using indirect castings made from lost wax technique has traditionally been used in prosthetic dentistry. A series of procedures are performed to attain a well-fitting casting, each of which individually influences the fit. Accuracy of indirect castings is associated with the dimensional stability of die materials, as a die must be able to reproduce the dimensions and detail of the tooth preparation while resisting damage during removal from impression and cast manufacturing procedures.  Die materials currently available include improved dental stone, resin reinforced stone, epoxy resin, metals (silver and copper plated) and polyurethane resin. 
International Standards Organization type IV and V dental stone are among the most widely used die materials due to their excellent dimensional accuracy, low cost and ease of use. ,,,,, However, dental stones have less than desired strength, poor detail reproduction and low abrasion resistance for complex prosthodontic procedures.  Alternatives exist in the form of epoxy resin; polyurethane resin reinforced stone and electroformed dies. Resin die materials (epoxy and polyurethane) offer superior physical properties when compared to dental stones but are limited by dimensional changes and working difficulties during fabrication. ,,,,, Studies investigating the accuracy of resin dies have discouraged their clinical use due to the significant dimensional shrinkage as compared to stone dies. ,,,
Polyurethane resin die, and cast material has been available for a number of years and is being remarketed in newer presentations with altered silica filler proportions (61%) (AlphaDie MF, CA, USA). In an attempt to improve the dimensional accuracy of polyurethane dies, Derrien and Sturtz  investigated the impact of physically altering the amount of silica filler by weight (20%, 40% and 60%) in the resin and its impact on the dimensional stability and transverse strength using calibration plate technique. The authors reported a significant reduction in polymerization shrinkage claiming to compensate for the dimensional change of elastomeric impression material resulting in dies near identical to prepared tooth.  However, the study was unable to find investigations probing the impact of higher silica content in a commercially available polyurethane resin die material in a clinically simulating set-up.  It is therefore tempting to evaluate that influence of polyurethane dies with higher silica filler on the fit of indirect full veneer crowns. The null hypothesis is that crowns fabricated on silica filled polyurethane resin dies have adequate accuracy of fit and adaptation when investigated in a clinically resembling set up, as compared to crowns made on type IV dental stone dies (control).
| Materials and methods|| |
A maxillary right first molar typodont tooth (KaVo Dental, Fruehauf Drive, Charlotte, NC) was prepared for a full veneer gold crown with an axial taper of 6°, occlusal reduction of 1 mm (mm) and a 0.5 mm wide circumferential shoulder using a milling machine. Margins were planned with a chisel, and the preparation was smoothed with silicon-impregnated burs. A nickel-chrome replica of the prepared tooth was casted using lost wax technique. After polishing, the casting was mounted on an acrylic resin base, and a circle was scribed into the base to aid accurate seating of impression vials [Figure 1].
|Figure 1: Study method. (a) Master nickel-chrome replica of full veneer preparation on typodont tooth. (b) Die with wax pattern, stone die, polyurethane die and die with sprue (left to right). (c) Specimen encased in resin and divided mesiodistally into mesial and distal sides. (d) Magnified specimen ×63 with 200 ìm measuring grid superimposed|
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Impressions of the nickel-chrome casting were made using polyvinylsiloxane (PVS) impression material (President, Coltene Whaledent, USA). Impressions were copper-plated using electroplating in a solution of acidic copper sulfate at an initial current of 100 mA for 9 h and then at a current of 200 mA for a further 18 h. Auto polymerizing acrylic resin was then built up incrementally within the copper plated die with a brush. The dies were removed and mounted on bases of type III dental stone. These bases had a circle scribed into them concentrically around the die to allow accurate seating of subsequent impressions. A total of 40 dies were randomly allocated to two groups of 20 each. For all the copper plated dies, impressions were again recorded using PVS material in a temporary crown. Each impression was examined under ×25 magnification for flaws.
Half of the impressions were poured with type IV die stone (control group) (Vel-mix, Kerr, UK) after it was mixed according to manufacturer's instructions (100 g in 23 ml distilled water) under a vacuum at 400 rpm for 30 s. The material was allowed to set for 2 h and the dies were pinned after removal. For resin die fabrication (test group) (Alpha Die MF, Schultz Dental Group, Sylmar, CA, USA), 5 ml of the base liquid was mixed with 11.5 g of silica filler and later 2.5 ml of hardener was added with continued mixing for 30 s. The resin was then poured under gentle vibration into the impression. The impression was transferred to a dry pressure vessel and allowed to set under 2-bar pressure for 2 h before they were pinned.
Wax up for crown fabrication was carried out by a single operator using molten dipping wax and carving under magnification, at the margin molten type 2 inlay wax (Kerr, UK) was used. A preformed, pre-adjusted plastic sprue with an integral reservoir was attached to the mesiobuccal cusp tip and was marked for identification [Figure 1]. The patterns were invested in groups of five according to manufacturers instructions in phosphate-bonded investment material (Dievest, Degussa, Germany). Castings were made in an induction-casting machine under vacuum using type III gold alloy. Castings were inspected under ×10 magnification. The casting was accepted when it fit the die along the entire margin length.
The copper replica was matched to its respective casting. Cyanoacrylate was used around the margin to retain the casting on the copper replica, both of which were then embedded in polyester resin (Bondaglas, Kent, UK). When cured the blocks were sectioned (Isomet 100, Buehler, USA) axially (mesio-distal) using a diamond coated blade (Series 15HC, Buehler, USA) at 800 rpm under 500 g load. Sectioning yielded two halves of the specimen, one designated A, the other B. Each half had a mesial and a distal site [Figure 1], there were a total of 40 measurements made for each group.
Specimens were renumbered to carry out blind measurements using image analysis software (DP Soft, Olympus, UK). Both halves were examined at a magnification of ×63, and an image was captured into the software of the marginal areas of each specimen. Prior to measurement, the operator was calibrated, and all measurements were made using the software viewed on a 17" cathode ray monitor at ×2 digital enlargement of the original ×63 optical magnification.
For marginal opening measurements, the image was rotated on the screen until the axial surface of the copper replica was aligned with the y-axis of a grid of 200 μm 2 that was superimposed on the image. The grid was then repositioned until an intersection was cited directly over the point at which the replica margin crossed the y-axis [Figure 1]. A line was then traced from this point to a point along the y-axis until it crossed the casting margin. This distance was considered as the marginal opening and was calculated automatically by the software (DP Soft, Olympus, UK) and expressed in μm. A marginal opening of below 40 μm was considered excellent, between 40 μm and 100 μm was acceptable and more than100 μm was regarded as unacceptable. For measurements beneath the castings, software was set to area measurement mode. The cursor was used to trace the area between castings and die. The line drawn to represent marginal opening demarcated the outer border. The inner limit was set at the point where the interface met the third grid line on the y-axis up from the replica margin. The software (DP Soft, Olympus, UK) automatically calculated the area and expressed it in μm 2 . All the data were entered into a database and analyzed statistically using a t-test.
Data were entered into an electronic database (Excel, Microsoft, USA) and statistical analysis performed blind using statistical package for the social sciences (SPSS 11.0, Inc., Chicago, IL, USA).
| Results|| |
Of the originally fabricated 40 castings, 3 (2 for type IV stone and 1 for polyurethane resin) had to be remade due to casting error. All 40 subsequent specimens were sectioned, and images were captured successfully.
There was no observed difference in the amount of adjustment required to seat castings between the two groups. The mean values for measurements of marginal opening and area beneath the casting, together with their respective standard deviations are presented in [Table 1]. An independent variables t-test was applied to the values for mesial, distal and aggregated measurement sites. No significant difference was shown for marginal opening and area beneath the casting between the two experimental groups [Table 2] and [Table 3]. Furthermore, a correlation analysis did not reveal any relationship between measurements for marginal opening and area beneath the casting.
|Table 1: Means and standard deviations of marginal opening and area beneath the castings for polyurethane resin and type IV stone dies |
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|Table 2: Comparison of means (t-test) for marginal opening between polyurethane resin and type IV stone dies |
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|Table 3: Comparison of means (t-test) for the area beneath casting between polyurethane resin and type IV stone |
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[Figure 2] provides a comparison between the numbers of excellent, acceptable and unacceptable mesial-distal margins for the two groups. Castings fabricated on polyurethane resin dies showed considerably greater number of unacceptable margins (27.5%) as compared to type IV stone dies (7.5%).
| Discussion|| |
Dental stone dies are commonly used in the fabrication of indirect restorations for many reasons, including, compatibility, workability, availability and low cost. However, for complex prosthodontic procedures, such as tall and thin tooth preparations and fabrication of pick-up copings for impressions of multiple teeth, the poor transverse strength and brittle nature of stone dies is a considerable disadvantage. Polyurethane resin with 60% silica filler has been reported to combine optimum transverse strength with desired dimensional accuracy in a laboratory setting.  Therefore this study was conducted to test the hypothesis that crowns fabricated on silica filled (60%) polyurethane resin dies (test group) have adequate accuracy of fit and adaptation when investigated in a clinically resembling set up, as compared to crowns made on type IV dental stone dies (control group). Although the means of marginal openings and area beneath the casting for crowns made on filled polyurethane resin dies were higher than the crowns for stone dies, there was no statistical significant difference between the two groups. In addition, 72.5% of mesial-distal crown margins in the resin group were found acceptable in comparison to 92.7% of acceptable crown margins in stone group.
To our knowledge from indexed literature, there is a scarcity of studies that have assessed the fit of indirect casting using filled polyurethane resin dies in a clinically simulated study design. Derrien and Sturtz  demonstrated a reduction in polymerization shrinkage at the expense of transverse strength of polyurethane resin by increasing the silica fillers. However, a 60% filler load resulted in lower polymerization shrinkage of 0.025% and still yielded a material that was 2.5 times stronger than improved stone in 3-point testing.  The material evaluated in the present study (AlphaDie MF, Schultz Dental Group, Sylmar, CA, USA) had 61% silica filler load. Other workers have reported a combination of expansion and contraction  for polyurethane resin, however, in one study  the dimensional changes for polyurethane resin in comparison with other die materials were statistically insignificant. In addition the desired abrasion resistance  of polyurethane dies results in a dense and non-absorbent surface, facilitating easy lift off of patterns, which was also noted in the present study.
Image analysis to measure crown fit is not a new concept, but its suitability and accuracy was unknown. A simple calibration test confirmed the accuracy and reproducibility of the system, by measuring a known distance between two points and calculating means and standard deviations. Using Dahlberg's error calculation, a high degree of repeatability was recorded. However, there is a debate within the literature with reference to the most appropriate method of recording marginal opening. ,, The use of sectioned specimens to measure casting fit and adaptation has been widely reported. Investigators have reported the observation, that magnified images of margins appear rounded and complicate measurement.  To overcome this problem, Sorenson  reported a technique similar to the one utilized in the present study. Using overlays on photographs of sectioned specimens, the authors demonstrated good reproducibility. White et al.  also utilized a similar method and reported high levels of reproducibility.
The results reported are broadly in line with those studies reporting marginal opening using sectioned specimens. Plekavich and Joncas  reported openings ranging from 49 μm- 161 μm and Gelbard et al.  reported mean values ranging from 59 μm to 128 μm for castings made on a similar range of materials including those in the present study. The reported similarities in the means of the study groups and the statistical analysis suggest that the polyurethane resin may be a suitable substitute for improved stone dies used for the fabrication of complete veneer castings. Both materials produce castings with clinically acceptable margins but inspection of the raw data revealed that there was greater variation and larger means for marginal openings and area beneath casting for polyurethane resin [Table 1].
Polyurethane resin, while capable of high accuracy, showed a greater potential to produce castings with poor marginal openings (27.5% unacceptable margins [Figure 2]). This is assumed to be associated with the reported variability in dimensions of dies fabricated from this material. , Therefore, the dental operator must decide whether the benefits of increased strength, good detail reproduction and superior abrasion resistance of filled polyurethane resin outweigh the potential for error caused by the dimensional variation. This is clinically applicable when impressions of tall and thin tooth preparations are to be poured for die fabrication. The stresses produced during the removal of cast from rigid elastomeric impression materials are likely to fracture dies of improved dental stone but not dies of polyurethane resin. A further application for filled polyurethane dies is direct build-up of porcelain on tooth preparation margins. The compact, non-absorbent resin surface allows the porcelain to lift away cleanly. This is demanding to achieve on stone dies without surface treatment, which may alter the accuracy of the die. ,
|Figure 2: Comparison of number of mesial and distal crown margin quality fabricated on type IV stone, and polyurethane resin dies|
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This study confirms the potential for filled polyurethane resin when used to fabricate single dies within the limitation of laboratory settings. However, further work is required to evaluate the utilization of filled polyurethane resin for whole arch casts fabrication, especially in the case of indirect construction of implant-retained restorations.
| Conclusions|| |
Within the limitations of the study, the following conclusion may be drawn:
- Castings made using filled polyurethane resin had statistically similar marginal openings and volumes beneath the casting as compared to those produced on type IV dental stone
- Both groups produced marginal openings that are clinically acceptable (polyurethane resin 72.5% and type IV stone 92.5%)
- The mean values for marginal opening and area beneath the casting showed greater variations for the polyurethane resin than type IV stone.
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Department of Prosthetic Dental Science, College of Dentistry, King Saud University, Riyadh 11545
Kingdom of Saudi Arabia
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]