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| Year : 2013 | Volume
: 24
| Issue : 1 | Page : 147-148 |
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| Evaluation of marginal gap of Ni-Cr copings made with conventional and accelerated casting techniques |
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Pavan Kumar Tannamala1, Nagarasampatti Sivaprakasam Azhagarasan2, K Chitra Shankar2
1 Department of Prosthodontics, Narayana Dental College, Nellore, Andhra Pradesh, India
2 Department of Prosthodontics, Ragas Dental College and Hospital, Chennai, Tamil Nadu, India
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| Date of Submission | 22-Jul-2011 |
| Date of Decision | 27-Jan-2012 |
| Date of Acceptance | 13-Sep-2012 |
| Date of Web Publication | 12-Jul-2013 |
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 Abstract | | |
Context: Conventional casting techniques following the manufacturers' recommendations are time consuming. Accelerated casting techniques have been reported, but their accuracy with base metal alloys has not been adequately studied.
Aim: We measured the vertical marginal gap of nickel-chromium copings made by conventional and accelerated casting techniques and determined the clinical acceptability of the cast copings in this study.
Settings and Design: Experimental design, in vitro study, lab settings.
Materials and Methods: Ten copings each were cast by conventional and accelerated casting techniques. All copings were identical, only their mold preparation schedules differed. Microscopic measurements were recorded at ×80 magnification on the perpendicular to the axial wall at four predetermined sites. The marginal gap values were evaluated by paired t test.
Results: The mean marginal gap by conventional technique (34.02 μm) is approximately 10 μm lesser than that of accelerated casting technique (44.62 μm). As the P value is less than 0.0001, there is highly significant difference between the two techniques with regard to vertical marginal gap.
Conclusion: The accelerated casting technique is time saving and the marginal gap measured was within the clinically acceptable limits and could be an alternative to time-consuming conventional techniques. Keywords: Accelerated casting technique, conventional casting technique, marginal gap
How to cite this article:
Tannamala PK, Azhagarasan NS, Shankar K C. Evaluation of marginal gap of Ni-Cr copings made with conventional and accelerated casting techniques. Indian J Dent Res 2013;24:147-8 |
How to cite this URL:
Tannamala PK, Azhagarasan NS, Shankar K C. Evaluation of marginal gap of Ni-Cr copings made with conventional and accelerated casting techniques. Indian J Dent Res [serial online] 2013 [cited 2023 Mar 31];24:147-8. Available from: https://www.ijdr.in/text.asp?2013/24/1/147/114940 |
The success of any cast restoration depends upon the fit of casting to the underlying tooth structure. [1] A deficient margin leads to plaque retention resulting in gingival inflammation, marginal leakage which can lead to secondary caries, sensitivity, gingival recession, cement dissolution, and debonding of the restoration. [1],[2],[3] The marginal discrepancies of cast restorations are inevitable, in spite of careful attention to waxing, investing, and casting procedures. It is one of the tasks of luting cements to close these discrepancies. But cement will dissolve under the margins if the gap is too large. [2] The accuracy of casting is subjected to volumetric changes occurring due to shrinkage of wax and alloys. This shrinkage can be compensated by setting expansion, hygroscopic expansion, or thermal expansion of the investment. [4]
There have been numerous reports, on attempts to perfect the casting procedures by improving investment materials and techniques. [4],[5],[6],[7],[8] The majority of these efforts deal with so-called "conventional" investing and casting techniques, which usually require at least 1 h bench set for the investment, followed by a one-, two-, or three-stage wax elimination cycle as recommended by the manufacturer before the casting procedure. [6],[8],[9],[10] The whole process is time consuming and requires approximately 2-4 h for completion. [6],[8]
The accelerated casting techniques have been reported in an effort to achieve similar quality results in significantly less time. [6],[8] Though studies [6],[8] have reported that marginal discrepancies by accelerated casting technique are within the clinically acceptable limits, some studies [5],[6],[8] have reported that this procedure is technique-sensitive. Most of these studies have reported the effect of accelerated casting procedures on the fit of noble alloy castings. However, the effect of accelerated procedures on the marginal fit of base metal alloy restorations has not been adequately studied.
The purpose of this study was to measure and compare the effect of conventional and accelerated casting techniques on the vertical marginal fit of base metal alloy cast copings.
| Materials and Methods | |  |
Wax pattern fabrication
The custom-made stainless steel die assembly was used to fabricate standardized wax patterns. The stainless steel master die and stainless steel former used in this study were custom made, based on the model used by Konstantoulakis et al. [6] and Schilling et al. [8] in their studies. This assembly essentially is of two parts, namely, the stainless steel master die and the stainless steel former which fits over the die [Figure 1]. The stainless steel master die simulated a crown preparation with a 10° total axial wall taper. The height of the die and its occlusal diameter is 6 mm. The occlusal surface had occlusal cross hairs (or grooves) to aid in repositioning of the pattern/casting. The base has a height of 3 cm and a diameter of 2 cm. The base is sectioned along its circumference which divides it into an upper one third part and a lower two third part. The upper one third can be moved up and down from the lower two third of the base. This aids in easy removal of wax pattern/cast coping. Four markings present on the base of the die, separated by 90°, each serve as standard reference points for measurement of the vertical marginal gap of all the cast copings. The finish line was a 90° shoulder with a width of 0.5 mm.
A custom-made stainless steel former was fabricated, such that it could be accurately positioned over the stainless steel die. The stainless steel former was larger than the die in all dimensions by 0.5 mm uniformly. This was done to maintain a space of 0.5 mm throughout between the die and the former. This space helped obtain the wax patterns with a uniform thickness of 0.5 mm and a 90° shoulder margin [Figure 2]. | Figure 2: Dimensions of custom-made stainless steel master die. (a) Line diagram of custom-made stainless steel master die. (b) Line diagram of the occlusal view of custom-made stainless steel master die showing occlusal cross hairs and four standardized reference points on the base for recording the measurements. (c) Line diagram of custom-made stainless steel former
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A fine coating of die lubricant/separator (Die Lube Wax Sep, Dentecon, Los Angeles, CA, USA) was applied onto the die and the fitting surface of the stainless steel former using small paint brush. This liquid film wets the die surface allowing closer contact of the wax, ensuring greater accuracy of the pattern. It also allows easy removal of the wax pattern from the die and prevents the pattern from adhering to the stainless steel former. The stainless steel former was filled with molten inlay wax (GC, Taisei Dental Mfg. Co., Ltd., Osaka, Japan) and was pressed on the stainless steel die. The die-former assembly was held together for 1 min with finger pressure. The die was then separated from the former, and the wax pattern obtained [Figure 3]. The excess wax was trimmed using a PKT no. 4 carver. A uniform thickness of 0.5 mm was obtained throughout the coping. The coping pattern was checked for even thickness of wax using a wax caliper. In this manner, a total of 20 wax copings were made and randomly divided into two groups, namely, conventional casting technique (M1) and accelerated casting technique (M2), and 10 specimens were used for each group of the study.
Attachment of sprue former
Preformed wax sprue (Bego, Bremen, Germany) of 2.5 cm length and 2.5 mm was attached to the wax pattern with a reservoir 3 mm from the end of the sprue. To minimize distortion, the wax pattern was sprued while it was seated on the stainless steel die. One end of the sprue was attached to the pattern at an angle of 45°. The point of attachment was flared and not constricted to decrease porosity and increase mold filling. The wax pattern was removed from the die and the other end of the sprue was attached to the crucible former. The wax pattern was sprayed with a wax surfactant spray (Aurofilm, Bego, Bremen, Germany) to obtain a clean pattern and to reduce surface tension of all wax surfaces, and thereby improve wettability.
Investing the wax pattern
All wax patterns were invested individually using graphite-free, phosphate-bonded investment material. Each wax pattern was immediately invested after marginal refinement to minimize distortion. Casting rings were lined with one nonoverlapping layer of dry ceramic ring liner (FlexVest liner, Ivoclair Vivadent, Liechtenstein, Germany), which was maintained 3 mm below the top of the ring. A 6mm distance was provided between the margin of coping and top of the ring. The liquid is prepared by mixing the colloidal silica and distilled water in a ratio of 75:25 to achieve optimum expansion; therefore, 750 ml of colloidal silica liquid (Investment BS Liquid 1, Heraeus Kulzer GmbH, Gruner Weg, Germany) was mixed with 250 ml of distilled water (Metro Labs, Pondicherry, India) to obtain the above-mentioned ratio. Then 60 g of investment powder phosphate-bonded investment (Moldavest exact, Heraeus Kulzer GmbH, Gruner Weg, Germany) was mixed with 13 ml of premixed liquid using a spatula to stir the investment material by hand until the entire material was wetted thoroughly. Then mechanical mixing was done under vacuum using vacuum mixer (Whip Mix. Inc. Co., Louisville, USA). The mix was paddled under vacuum for 60 s, and care was taken to ensure adequate vacuum. Once the investment was mixed, the entire pattern was painted with a thin layer of investment using a small paint brush. The casting ring (Whip Mix Inc. Co., LouisvilleUSA) was positioned on the crucible former, and the reminder of the investment was vibrated slowly into the ring. The invested pattern was allowed to bench set for 20 min. The investing procedure was same for all the patterns of the two test groups.
Wax pattern elimination
The wax elimination procedure was different for each of the two test groups (M1, M2) as described below.
M1 technique (conventional casting technique): After a 20-min bench-set time, the set investment mold was placed in the burnout furnace (Technico, Technico Laboratory Products Pvt. Ltd., Chennai, India) along with the casting crucible. The wax burnout was done using a programmed preheating technique, i.e. the ring was kept in the furnace at room temperature and was heated up to 270°C at a rate of 8°C/min and was held at this temperature for 30 min; then heated from 270°C to 580°C at a rate of 8°C/min and was held at this temperature for 30 min; and further heated from 580°C to 950°C at a rate of 8°C/min and was held at this final temperature for 30 min. The same procedure was followed for each of the 10 samples of this test group.
M2 technique (accelerated casting technique): After a 20-min benchset time, the set investment mold was placed directly in a preheated burnout furnace (Technico, Technico laboratory products Pvt. Ltd.) maintained at 950°C and held for 30 min at this temperature to ensure complete burnout of the wax pattern. The same procedure was followed for each of the 10 samples of this test group.
Casting
The casting procedure was performed quickly to prevent heat loss from the ring resulting in the thermal contraction of the mold. The preheated casting crucible and the investment mold were taken out of the furnace and were placed in the casting machine. The casting was done in an induction casting machine (Fornax GEU, Bego, Germany). The nickel-chromium alloy (Heraenium-S, Heraus Kulzer GmbH, Germany) was heated sufficiently till the alloy ingot turned to a molten state, and the crucible was released and centrifugal force ensured the completion of the casting procedure. Investment was allowed to cool down to room temperature. The casting procedure followed was the same for all the test samples. A total of 20 castings were made to obtain cast copings for the evaluation in this study. Among the 20 castings, 10 were obtained by M1 technique and 10 by M2 technique.
Divesting and finishing the cast coping
Following casting, the hot casting ring was bench cooled to room temperature, and then the cylinder of investment containing the casting was pressed out from the ring. The investment cylinder was cleaved along its long axis, and the casting was lifted free. Adherent investment was removed from the casting by sandblasting with 50 μm alumina at 80 psi pressure. The sprue was removed with an ultra thin abrasive disc (Dentorium, New York, USA). The copings were steam cleansed and checked visually. The internal surface was inspected and relieved of all nodules with a round carbide bur. This procedure was followed for each of the ten samples of the two test groups.
Measurements of marginal gap
Each casting was seated on the stainless steel die with finger pressure [Figure 4]. Microscopic measurements were recorded at ×80 magnification on the perpendicular to the axial wall with a photo microscope (Reichert Polyvar 2 met photo microscope, Reichert, Wien, Austria). Measurements were recorded from coping margin to the stainless steel die margin for vertical marginal gap recordings. Marginal gaps were measured to the nearest micron on each casting at the four predetermined sites on the base of the stainless steel die separated by 90°. The same procedure was followed to record the vertical marginal gap for each of the 10 test samples belonging to the two test groups. The measurements thus obtained were tabulated and statistically analyzed.
| Results | | |
[Table 1] shows the mean vertical marginal gap of each sample, and mean and standard deviation of M1 and M2 techniques. The mean marginal gap by conventional technique (34.02 μm) was approximately 10 μm lesser than accelerated casting technique (44.62 μm). As the P value is less than 0.0001, there is highly significant difference between the two techniques with regard to vertical marginal gap. | Table 1: Comparison of the vertical marginal gap obtained by M1 and M2 techniques
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| Discussion | | |
In dentistry, lost wax process of casting metals became common practice after it was introduced by Taggart in 1907. [6] Castings made by Taggart were generally too small and did not fit the cavities properly. [7],[11] The casting process used in dentistry based on the lost wax technique has been receiving continuous investigations exploring the behavior of the materials involved. [12] The majority of the efforts deal with the so-called conventional casting technique which is more time consuming.
Accelerated casting techniques have been reported in an effort to achieve similar quality results in significantly less time. These techniques have the ability to shorten the investing and casting process, thereby improving productivity. The first published attempt to accelerate the lost wax technique with the use of phosphate-bonded investment for complete crown was made in 1988 by Marzouk and Kerby [13] who recognized the importance of investment temperature. Their study revealed no statistical circumferential difference between investment groups introduced in a 1350°F preheated oven after 15-min bench set and the conventional technique. [8] Campagni et al. [14] tested the fit of dowel and cores made of noble alloy by an accelerated casting technique, and similar studies were subsequently conducted by Bailey and Sherrard [15] and Schneider. [16] All these investigations concluded that the use of a predetermined bench-set time, reduced investment weakness and that standardized accelerated procedures for all types of investments were inadvisable. [8]
Blackman [5] measured marginal sharpness and diameter changes for crowns cast with type III gold alloy by using phosphate-bonded investment and rapid burnout techniques, and concluded that rapid mold preparation resulted in loss of marginal fineness. Murakami et al. [17] studied the rapid burnout technique with gypsum-bonded investment to examine surface aspects and fit of complete crowns and it was noted that the setting process was considered in progress at 30 min when the mold was placed in preheated oven and stated that because of this step, less expansion had occurred and was probably responsible for increased gaps. Konstantoulakis [6] and Schilling et al. [8] evaluated the marginal fit and surface roughness of complete cast crowns made with a conventional and accelerated casting technique and reported that crowns fabricated with the accelerated casting technique were not significantly different from those fabricated with conventional technique. The accuracy of base metal alloy castings obtained by different investing and burnout procedures with phosphate-bonded investments was not adequately studied. Though studies [6],[8] have reported that marginal discrepancies by accelerated casting technique are within the clinically acceptable limits, some studies [5],[6],[8] have reported that this procedure is technique sensitive.
The introduction of ceramometal technology required the use of higher melting range alloys to withstand the firing cycle of porcelain without noticeable distortion. Base metal alloys are one of the alloys that are routinely used for obtaining ceramometal restorations. The alloy used in this study was nickel-chromium alloy used for ceramometal restorations. An investment that can resist higher temperatures and higher stress during casting [7] is required. A phosphate-bonded investment fulfills these requirements, and therefore has been used in this study.
A custom-made stainless steel master die was used to fabricate the standardized wax patterns. The master die was based on the models used in similar previous studies. [6],[8] This standardized stainless steel die facilitated standardizing the dimensions of the test wax patterns. Four markings present on the base of the die, separated by 90°, served as standard reference points for measurement of the vertical gap of all the cast copings. Each pattern was immediately invested to minimize distortion. [4],[7],[8] Different ratios of special liquid to distilled water have been recommended to obtain the required mold expansion. In this study, the ratio of special liquid to distilled water of approximately 75:25 in volume was used. This special liquid to distilled water ratio has been shown to offer adequate expansion for complete crown castings, as determined in a previous study. [6] The liquid to powder ratio was as recommended by the manufacturer, i.e., 60 g powder: 13 ml liquid. Vacuum mixing was done and investing of all the samples was done as recommended by the manufacturer.
The conventional burnout procedures usually recommend burnout temperatures for phosphate-bonded investments in a range of 750°-1030°C. A three-stage burnout procedure was used for obtaining the cast copings for the first test group in this study as recommended by the manufacturer. Accelerated techniques are time saving, and hence offer advantage to commercial laboratories. The pattern is invested, cast, and delivered in a cost-effective, time-saving manner. Accelerated casting technique was used for obtaining the cast copings for the second test group in this study.
The manufacturer of the phosphate-bonded investment used in this study recommends that the investment can be used for an accelerated as well as for the conventional casting technique with three-stage wax elimination. Hence, this material was chosen for investing the wax patterns for all the test groups in this study. Also, using a single investment material for all the test groups helps to eliminate any variability in the test results.
The casting procedure was performed by using an induction casting machine. All castings, one at a time, were seated on the stainless steel die with finger pressure and the vertical marginal gap was measured on four predetermined areas that were marked on the metal die using a photo microscope (Reichert Polyvar 2 met photo microscope, Reichert) at a magnification of ×80. The results of this study were subjected to statistical analysis by paired t test.
The basic data show a mean value of 34.02 μm for conventional casting technique and 44.62 μm for accelerated casting technique. The statistical analysis by paired t test indicated that the difference in the marginal gap measurements for the two techniques showed the P < 0.0001. This denotes significance at 1% level. Higher values of vertical marginal gap were found with accelerated casting technique when compared to the conventional casting technique with three-stage wax elimination and this difference was statistically significant.
The results indicate that within the conditions of the study, the castings produced by conventional casting technique with three-stage wax elimination technique fit better than the castings produced using accelerated casting technique. Papadopoulos and Axelsson [18] reported a superior fit of crowns on dies if phosphate-bonded investment molds were prepared with longer burnout schedules; marginal gaps were 5 times greater with short schedules. [5] Though the marginal gap due to accelerated technique is significantly larger than that due to conventional techniques in the study, the mean marginal gap of 44.62 μm by accelerated technique in this study is within the clinically acceptable limits. [9] Accelerated technique may take advantage of characteristic exothermal setting reaction of phosphate-bonded investments. Heat-enhanced setting expansion continues uninterrupted as the mold is transferred into a preheated furnace for thermal expansion. [8] This may probably be the reason for the marginal gap of cast copings by accelerated technique to be within the clinically acceptable limit.
The results of this study encourage further research with accelerated technique and reinforce the need to identify the factors that facilitate better marginal fit of cast restorations. Only one single wax pattern was investigated per casting ring in this study. The performance of the described accelerated casting technique in the fabrication of fixed partial denture frameworks requires further investigation.
| Conclusion | | |
The following conclusions were drawn from the data obtained in this study:
- A vertical marginal gap was present with all the cast copings obtained by the two techniques used in this study.
- The vertical marginal gap of cast copings obtained by conventional casting technique showed a mean value of 34.02 μm.
- The vertical marginal gap of cast copings obtained by accelerated technique showed a mean value of 44.62 μm.
- The vertical marginal gap of cast copings obtained by accelerated casting technique showed a significant difference from those obtained by the conventional casting techniques.
- The mean vertical marginal gaps of all the cast copings obtained by the two techniques were within the clinically acceptable limit. [9]
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Correspondence Address:
Pavan Kumar Tannamala
Department of Prosthodontics, Narayana Dental College, Nellore, Andhra Pradesh
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.114940
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1] |
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evaluation of marginal gap of ni-cr copings made with conventional and accelerated casting techniques |
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| tannamala, p.k. and azhagarasan, n.s. and shankar, k.c. | | indian journal of dental research. 2013; 24(1): 147-148 | | [Pubmed] | |
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