Evaluation of Evaporation Effect on the Film Thickness of Two Commercially Available Die Spacers on Three Different Surfaces

INTRODUCTION

Success of a cast restoration is dependent on a number of factors. A proper fit of the casting on to the prepared tooth and its cementation is an important aspect that determines its long-term clinical success. Casting fit is a function of many variables, related to changes in size and shape, introduced by the materials used in fabrication of a restoration. Even if all the variables are controlled carefully to ensure a perfect fit, the restoration still will not fit because of insufficient space for a luting agent.

The main requirement of a full-cast crown is that it should seat completely on the prepared tooth without ant hydraulic pressure from the luting agent. Failure to completely seat the crown will result in supraocclusion, loss of proximal contact, food impaction, plaque retention, secondary caries, and periodontal breakdown.

The most popular and accurate method^1–3 to achieve adequate internal relief in full veneer cast restoration is the application of die spacer. Die spacer is a solution that is painted on to the die before fabrication of a wax pattern. The use of die spacer provides uniform space for the luting agent, reduces elevation of the cast restorations, improves outflow of excess cement, and lowers seating forces. Ideal thickness for luting cement ranges from 25 to 40 μm, which is provided by application of die spacer. Die spacer thickness should be large enough to allow proper seating of the casting yet not so large as to cause excessive cement thickness.

Basic composition of a die spacer includes an organic solvent like ketone with fine metal oxide particles dispersed in the medium. The metal oxide powder must remain dispersed in the organic solvent; any change in the volume of solvent due to evaporation will lead to thickening of the die spacer. This will result in alteration of the film thickness of the die spacer.

The evaporation of organic matrix liquid is unavoidable when the bottle stands open for some time while the die spacer is being applied on a die. An open bottle will probably result in higher concentration of metal oxide particles in die spacer solution resulting in an increased spacer thickness for each coating.

The clinical consequence of larger die spacer thickness resulting from open bottle and evaporation of the organic matrix may be excess luting cement film thickness. In order to provide ideal cement film thickness, it is mandatory that thickness of die spacer should be controlled to 25–40 μm.

Grajower and Zuberi⁴ (1989) found 140% increase in film thickness of an old Tru Fit die spacer. Gegauff and Rosenstiel⁵ (1989) found 148% increase in film thickness in old vs. new Tru Fit bottles.

The present study was undertaken:

• To evaluate the effect of evaporation of die spacer components on die spacer thickness.
• To compare the die spacer thickness on a glass slide, on a die stone slab without hardener, and on a die stone slab coated with a hardener.
• To evaluate the percentage increase in die spacer film thickness after the bottle is left open for a progressively increasing time interval.
• To ascertain the number of applications of die spacer that are required to achieve an optimal film thickness, after the bottle is left open for incremental time intervals.

MATERIALS AND METHODS

This in vitro study was undertaken to evaluate the effect of evaporation of die spacer components on die spacer film thickness. In this study, two die spacers Nice Fit (Shofu Inc., Japan) and Durolan (DFS, Germany) commonly available in the market were used. Both the die spacers were applied on three surfaces: (1) glass slide, (2) die stone slab without hardener, (3) die stone slab coated with die hardener after the die spacer bottles were left open for 0, 2, 4, 8, and 12 hours. The film thickness for each application of die spacer was measured with the help of SEM.

RESULTS

Table 1 shows percentage increase in film thickness for all the three subgroups for various time intervals for Nice Fit die spacer. Table 2 shows percentage thickness for Durolan die spacer for different intervals of time. The one-way analysis of variance for film thickness measurement for different slides and for different intervals of time was found statistically significant (p %3C; 0.05) for both the die spacers.

To compare film thickness of die spacer on three different types of slide, a Student “t-” test was performed for Nice Fit and Durolan. Comparison between film thickness on glass slide (subgroup A) and die stone slab coated with hardener (subgroup C) was not statistically significant (p %3E; 0.05) but comparison of thickness of subgroup B with subgroups A and C was statistically significant (Table 3 ).

DISCUSSION

An optimal space of 25–40 μm should exist between the internal surface of the casting and the prepared surface of the tooth everywhere except a small area immediately adjacent to the margins.³ The space provides room for the luting agent and allows complete seating of the restoration during cementation.

Conversely, if the cement space is too wide, the casting will be loose on the tooth, resistance form will be reduced, and the position of the casting will be difficult to maintain during evaluation and occlusal adjustment. In addition, the risk of the crown loosening during function increases considerably, and its longevity is adversely affected.³

Die spacer is a solution that is painted on the die before the fabrication of the pattern. The die spacer consists of metal oxide powders and adhesives dispensed in an organic liquid such as ketone. Painting of the layers of die spacer on the die provides space for the cement, helps in easy seating of the crown, reduces the hydraulic pressure, and helps to reduce the force applied to seat a crown.^6,7

For optimal results, die spacer thickness should be uniform ranging from 25 to 40 μm. The amount of space required between a prepared tooth and casting can be controlled by varying the number of coats of die spacer. To maintain optimal clinical effectiveness of the die spacer, its composition must be held constant because change in composition will lead to a change in the film thickness of the die spacer. If the bottle is open, the organic matrix liquid tends to evaporate because of its high vapor pressure, thereby changing the die spacer composition. The overall time that a bottle is open during clinical use can affect the die spacer thickness and therefore long-term clinical success of a restoration.

Most traditional die spacers are somewhat similar in composition; they comprise of metal oxide, adhesives, and a volatile organic matrix liquid. Differences in the evaporation rates and the effects of evaporation, if any, are expected with die spacers from different manufacturers. Therefore, two die spacers from different manufacturers, i.e., Nice Fit (Shofu Inc, Kyoto, Japan) and Durolan (DFS, Riedenburg, Germany) were selected to enhance the direct clinical significance of the present study to rule out the variations occurring in only one material.

In the present study, three different surfaces were used for die spacer application: (1) glass slide, (2) die stone slab without hardener, and (3) die stone slab with hardener. Glass slide being a nonabsorbable smooth surface was used in this study to confirm the thickness of a single uniform stroke of application of die spacer. Two other surfaces, viz., die stone slabs with and without hardener, were used to mimic the clinical practice of fabrication of dies. The effect of die hardener on the film thickness of a single application of die spacer was also evaluated.

According to a study by Oliva et al.,² measurement of the film thickness varied significantly when measured at seven locations on the 100 dies. The cusp tips and occlusoaxial line angles had the least thickness of the die spacer when compared to other locations on the die. To eliminate the variable of different film thicknesses at different location on a die, flat, smooth, and uniform surface with little or no intersample surface variation was required. Therefore, die stone slabs of the size 6 cm × 8 mm × 2 mm were fabricated by vacuum mixing die stone. The surface of the die stone slab, which had set in contact with the glass slab, was absolutely flat, smooth, and uniform and, therefore, was used for the application of die spacer.

Thus, variables due to surface roughness of stone dies and variables due to different film thickness at different locations on a prepared tooth die were eliminated and selection of glass microscopic slide and die stone slab for application of die spacer was deemed appropriate.

To maintain uniformity of application, the die spacer was applied between the L1 and L5 marks on each slide because thickness may vary from the point of initial contact of brush to the point of removal of brush. The film thickness was measured at three points (L2, L3, L4) at a uniform distance from each other to avoid variations among samples.

The difference in samples of die stone without hardener (subgroup B) could be attributed to the porous nature of the die stone, absorbing more of organic matrix and due to the wetting properties of die spacer liquid on the nonporous smooth surface compared to the porous surface of die stone. The film thickness of die stone with hardener was almost similar to glass slide; this may have been achieved by blocking of die stone pores with die hardener providing a smooth nonporous surface.

The increase in film thickness of die spacer with open bottle time probably is caused by the subsequent higher concentration of metal oxide particles in the die spacer solution with more metal oxide being applied per brush stroke. The higher concentration of adhesive during evaporation also may contribute to increased thickness.

Another study be Psillakis et al.⁸ showed increase in die spacer thickness by 1, 142, 105, and 329% when the bottles were left open for 1, 4, 8, and 24 hours, respectively, compared with bottle open for 0 hour. The results of the present study were similar to the above findings.

As it is evident from Table 1 that the increase in film thickness of Nice Fit die spacer occurs as the time interval increases. This can help us in determining of number of coats that can be curtailed to achieve an ideal die spacer film thickness in the range of 25–40 μm. After an interval of 2 hours, Nice Fit die spacer coat should be reduced from four coats at 0 hour to three coats. After 4 hours interval, the coat should be reduced to two to achieve ideal film thickness. But after 8 hours, single uniform coat of die spacer gave a film thickness of 21 μm, which is not sufficient, and two coats of die spacer will give film thickness of more than 40 μm, leading to excessive relief under casting. Therefore, thinner has to be added to achieve the ideal range of die spacer film thickness but a study is needed to ascertain the amount of thinner that needs to be added.

Similarly, single coat of Durolan die spacer (Table 2 ) gives 13 μm of average film thickness to achieve ideal thickness of 25–40 μm, three coats must be applied at 0 hour. At 2 hours interval of time, the film thickness of die spacer increased to 20 μm approximately, so the number of coats must be reduced to two. At 4 hours interval of time, the film thickness of die spacer increased to 33 μm, so single coat is sufficient. But after 8 hours, excess relief may be provided due to increased thickness of the single coat of die spacer, so thinner has to be added to achieve the ideal range of die spacer film thickness.

SUMMARY AND CONCLUSION

The present study was undertaken to evaluate the effect of evaporation on die spacer thickness of the two commercially available die spacer materials. Thickness of the die spacer was evaluated on the glass slides, on die stone slabs without hardener, and on die stone slabs coated with hardener. The bottles of Nice fit and Durolan die spacers were left open for 0, 2, 4, 8, and 12 hours at 22°C. One uniform brush stroke of spacer was applied to clean glass slides, die stone slabs, and die stone slabs coated with hardener, for each interval of time. The thickness of die spacer was measured and subjected to the statistical analysis.

Study showed that:

• Average film thickness for single uniform coat of die spacer was 8.1 μm for Nice Fit and 13.4 μm for Durolan die spacer on glass slide.
• The die spacer thickness increased with open bottle time for both the die spacers tested.
• Increase in film thickness of both the die spacers was more or less comparable to each other for all the time intervals.
• Thickness of die spacer applied on the die stone slab without hardener was more as compared to that on the glass slide and on the die stone slab coated with hardener.
• The thickness of the die spacer on the glass slide and die stone slab coated with hardener was found to be almost similar because coating of die hardener produced smooth and nonabsorbable surface.
• As the thickness of die spacer increases with time, the number of coats applied to the die should be reduced to achieve ideal film thickness of 25–40 μm. Therefore, for Nice Fit die spacer four coats are required from fresh bottle, three coats when a bottle is left open for 2 hours, and two coats when bottle is left open for 4 hours. Similarly, for Durolan three coats are needed at 0 hour, two coats after 2 hour, and one coat after 4 hours interval of time. After 8 hours, a single coat of die spacer would result in a film thickness beyond the ideal range. Hence, thinner liquid supplied by the manufacturer would have to be added to reduce the thickness. However, further studies are required to ascertain the amount of thinner, which is required to be added and the effect this would have on the thickness of the die spacer.

REFERENCES

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