|Year : 2014 | Volume
| Issue : 5 | Page : 641-647
|Evaluation of the marginal fit of metal copings fabricated on three different marginal designs using conventional and accelerated casting techniques: An in vitro study
Sharad Vaidya1, Hari Parkash2, Akshay Bhargava2, Sharad Gupta2
1 Department of Prosthodontics and Implantology, Bhojia Dental College and Hospital, Baddi, Solan, Himachal Pradesh, India
2 Department of Prosthodontics, I.T.S Centre for Dental Studies and Research, Ghaziabad, Uttar Pradesh, India
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|Date of Submission||20-Nov-2013|
|Date of Decision||25-Sep-2014|
|Date of Acceptance||28-Oct-2014|
|Date of Web Publication||16-Dec-2014|
| Abstract|| |
Introduction: Abundant resources and techniques have been used for complete coverage crown fabrication. Conventional investing and casting procedures for phosphate-bonded investments require a 2- to 4-h procedure before completion. Accelerated casting techniques have been used, but may not result in castings with matching marginal accuracy.
Aims and Objectives: The study measured the marginal gap and determined the clinical acceptability of single cast copings invested in a phosphate-bonded investment with the use of conventional and accelerated methods.
Materials and Methods: One hundred and twenty cast coping samples were fabricated using conventional and accelerated methods, with three finish lines: Chamfer, shoulder and shoulder with bevel. Sixty copings were prepared with each technique. Each coping was examined with a stereomicroscope at four predetermined sites and measurements of marginal gaps were documented for each.
Statistical Analysis: A master chart was prepared for all the data and was analyzed using Statistical Package for the Social Sciences version. Evidence of marginal gap was then evaluated by t-test. Analysis of variance and Post-hoc analysis were used to compare two groups as well as to make comparisons between three subgroups .
Results: Measurements recorded showed no statistically significant difference between conventional and accelerated groups.
Conclusion: Among the three marginal designs studied, shoulder with bevel showed the best marginal fit with conventional as well as accelerated casting techniques. Accelerated casting technique could be a vital alternative to the time-consuming conventional casting technique. The marginal fit between the two casting techniques showed no statistical difference.
Keywords: Accelerated casting technique, conventional casting technique, finish line, marginal gap
|How to cite this article:|
Vaidya S, Parkash H, Bhargava A, Gupta S. Evaluation of the marginal fit of metal copings fabricated on three different marginal designs using conventional and accelerated casting techniques: An in vitro study. Indian J Dent Res 2014;25:641-7
One of the most important factors in crown casting has always been "marginal fit". It is a direct indicator of the precision of the casting technique. Marginal fit of casting is one factor that can lead directly or indirectly to the development of secondary caries, adverse pulpal reactions and periodontal disease. Internal surface roughness or even a small nodule can indirectly result in a marginal discrepancy in the form of a gap, limiting the uniform space for luting cement.
|How to cite this URL:|
Vaidya S, Parkash H, Bhargava A, Gupta S. Evaluation of the marginal fit of metal copings fabricated on three different marginal designs using conventional and accelerated casting techniques: An in vitro study. Indian J Dent Res [serial online] 2014 [cited 2020 Jun 6];25:641-7. Available from: http://www.ijdr.in/text.asp?2014/25/5/641/147114
Casting is one of the extremely demanding processes/procedures in dentistry. Castings are routinely fabricated using adaptation of lost wax technique.  The majority of these efforts deal with the so-called "conventional" investing and casting techniques, which usually require at least 1 h of bench set for the investment followed by a one- or two-stage wax elimination procedure before casting. The whole process is time-consuming and requires approximately 2-4 h for completion.
Accelerated, non-standard, casting techniques have been reported in an effort to achieve similar quality results in significantly less time (30-40 min). ,,,,,,,,, Several researchers have investigated, analyzed, and compared the accelerated technique with traditional methods. In a recent study Konstantoulakis et al.  measured marginal discrepancies with an accelerated casting technique. However, inadequate sample size in the study decreased the statistical significance and power of the final data.
The marginal discrepancies of various restorations fabricated through different casting techniques with different marginal configurations have not been studied extensively in the literature. Different finish-line designs have been advocated for tooth preparations for several reasons. These finish lines have been studied with no accurate conclusions. ,, However, due to the limited and contradictory theoretical, laboratory, and clinical evidence available, it is not clear which finish line design, if any, may offer the greatest advantage with a particular casting technique. Therefore, a prospective study was designed to evaluate and compare the marginal fit of metal copings fabricated on three different marginal designs using conventional and accelerated casting techniques.
| Materials and methods|| |
The study was conducted in the Department of Prosthodontics from November 2011 to June 2012. To conduct this study, a total of 120 cast coping samples were fabricated. The samples were divided into two groups Group I (conventional casting technique) and Group II (accelerated casting technique) [Table 1] having 60 samples in each group. Based on the type of finish line that is, chamfer (a), shoulder (b) and shoulder with bevel (c), the samples were further subdivided into three subgroups viz subgroup Ia, Ib, Ic and IIa, IIb, IIc. The marginal fit was evaluated by using a Stereomicroscope (Los Angeles, CA 90034 U.S.A) (Labomed-Zoomar [Lawrence and Mayo] type - 022545) under × 50 magnification [Table 2].
|Table 1: Comparison between conventional and accelerated casting technique |
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Stainless steel die fabrication
Three stainless steel dies were machined to simulate the shape and dimension of a prepared maxillary first molar. , The upper section of the die simulated the tooth preparation for fabrication of wax pattern and seating of finished copings. The lower section of the master die was rectangular in shape containing reference lines for measurement of marginal integrity of the copings. The upper section was made with a vertical length of 6 mm from the gingivoaxial line angle to the occlusoaxial line angle. The diameter of the die at the cavosurface line angle was 10 mm. The chamfer width was 1 mm. The convergence angle was kept at 6º/axial wall that resulted in a diameter of 7.0 mm at the occlusal end of the die. The rectangular columnar section measured 15 mm in height and 10 mm in width. A U-shaped orientation notch was placed on one surface of the finish line for exact positioning of the metal copings for measurement [Figure 1], [Figure 2], [Figure 3]. ,
|Figure 1: AutoCAD generated schematic representation of die with chamfer margin|
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|Figure 2: AutoCAD generated schematic representation of die with shoulder margin|
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|Figure 3: AutoCAD generated schematic representation of die with shoulder with bevel margin|
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Wax pattern fabrication
Die lubricant (Isocera, Bego, Germany) was applied to the metal die and machined split assembly [Figure 4] so as to facilitate easy removal of the wax patterns. The molten Inlay wax (Bego, Germany) was placed in an electrically controlled wax bath (Bego, Germany) and kept at the recommended temperature of 160°F. To ensure proper marginal adaptation, all margins of the patterns were readapted and redefined using sealing wax.
|Figure 4: Machined split assembly containing master die with 0.5 mm space for wax patterns of uniform size (arrows showing the space)|
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Investment of wax pattern and casting
Powder: Liquid ratio
Investing was done following the manufacturer's recommendations using Deguvest Impact (Degudent, Dentsply, USA) phosphate-bonded investment material for Group I and Group II. Group I: Investment material powder was mixed with liquid according to manufacturer's recommendation (150 g powder with 27 ml mixing liquid and 8 ml distilled water) Group II: Investment material powder was mixed with special liquid according to manufacturer's recommendation (150 g powder with 35 ml mixing liquid).
Mixing and investing
Liquid was mixed with the powder, hand spatulated for 20 s and then vacuum mixed for 60 s. For group I samples a two-stage program burnout process was followed - the investment was allowed to bench set for 1 h and then placed in the burnout furnace. The temperature was brought to 300°C at the rate of temperature increase of 14°C/min and held for 15 min. The temperature was then slowly increased to 600°C at a rate of 25°C/min and held for 30 min and finally temperature was raised up to 900°C and kept for a holding time of 1 h. The casting was carried out in induction casting machine (Ducatron, UGIN, France). For group II samples, a single stage burnout process was followed; that is the investment allowed to set for 15 min and then placed directly in a preheated furnace for 30 min at 900°C.
After 1 h of bench cooling the casting was divested, and the residual surface investment was removed by sandblasting with 110 µm aluminum oxide particles. Copings were separated from their respective sprues with carborundum discs. After washing, the intaglio surfaces of the castings were inspected for nodules. Castings having more than 1 internal nodule or with nodules on the margins were rejected. Single nodule of 0.25 µm diameter or less were removed with a round bur and castings with nodules >0.25 µm diameter were rejected.
Each casting was seated on its respective die and subjected to a constant load of 400 g for 5 min. The weight was removed, and a spring-loaded caliper was used to maintain a constant seating pressure between the casting and steel die during microscopic measurements [Figure 5]. The measurement was done with the help of a stereomicroscope [Labomed-Zoomar, Lawrence and Mayo Type - 022545] at ×50 magnification [Figure 6]. The measurement was done at midpoint of each surface of the die. Two readings were taken at each surface of the die. The mean of each surface was calculated as the final reading for that surface. Data obtained was then computer processed using software Microstructure Analyzer (Banbros, India) [Figure 7].
| Results|| |
A master chart was prepared for all the data and was analyzed using Statistical Package for the Social Sciences version 16. A t-test was used to determine significant differences among groups. One investigator completed all phases of the study, and alternated procedures between the two groups to limit technique variables and bias. Analysis of variance and Post-hoc analysis were used to compare two groups as well as to make comparisons between the three subgroups. Log of transformation was applied. P < 0.05 was considered significant.
On comparison, it was found that marginal gaps for castings made with an accelerated technique (42.25 ± 18.02) showed no statistically significant difference (P = 0.07) when compared with a conventional casting group (38.17 ± 14.92) for all four surfaces. On evaluation of the marginal fit of copings fabricated on three different marginal designs via conventional (Group I: a, b, c) and accelerated (Group II: a, b, c) casting techniques, it was observed that amongst the three marginal configuration designs studied shoulder with bevel (Group Ic) showed the best marginal fit with conventional and accelerated casting technique [Graphs I and II, [Table 3] and [Table 4] on mesial, lingual and distal surfaces, whereas, on buccal surface chamfer margin showed a better marginal fit, the results were statistically significant (P = 0.02). This might be inherent to the technique and the material used.
|Table 3: Comparison of marginal fit in between the three margins of Group I showing the mean, SD and their significance values |
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|Table 4: Comparison of marginal fit in between the three margins of Group II showing the mean, SD and their significance values |
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| Discussion|| |
There are very few published studies that evaluate the dimensional accuracy of complete crowns made with accelerated casting techniques. Insignificant difference was evident in earlier studies regarding marginal discrepancy when tested with the conventional and accelerated techniques. , However, marginal gap measurements were not performed with adequate numbers of specimens to determine any statistical difference between the two groups. A post-hoc power analysis revealed that this study had more than 90% power to detect a clinically significant difference in gap measurements. The literature till date lacks sufficient evidence in determining whether accelerated casting technique provides predictable marginal gaps and clinically acceptable crowns in comparison to conventional casting techniques with different finish line designs. An accelerated casting technique reduces the overall time to 30-40 min.  This technique was suggested by Campagni et al. (1991)  to make cast post-and-core restorations in a single visit For the purpose of standardization and to minimize errors, three stainless steel dies with different marginal designs (chamfer, shoulder and shoulder with bevel) were fabricated. By using standardized steel dies of volumetric size similar to that of teeth, rather than prepared natural teeth, castings of duplicate size and shape were produced. To homogenize the samples, all 120 wax patterns were directly fabricated on the metal dies.
On comparison of the marginal fit of copings fabricated on three different marginal designs via conventional (Group I: a, b, c) and accelerated (Group II: a, b, c) casting techniques [Table 3] and [Table 4], it was observed that among the three marginal configuration designs studied shoulder with bevel (Ic) showed the best marginal fit with conventional casting technique on the lingual (Ia - 36.43 ± 14.78 µm, Ib - 36.43 ± 14.78 µm, Ic - 33.75 ± 4.92 µm), mesial (Ia - 48.09 ± 19.77 µm, Ib - 41.10 ± 18.15 µm, Ic - 37.00 ± 6.49 µm) and distal (Ia - 39.74 ± 17.55 µm, Ib - 39.43 ± 20.86 µm, Ic - 34.67 ± 5.45 µm) surface, whereas, chamfer margin showed a better marginal fit on buccal (Ia - 34.71 ± 13.43 µm, Ib - 43.70 ± 21.72 µm, Ic - 35.19 ± 5.11 µm) surface, but the results were statistically insignificant (P > 0.05) [Table 3].
Similarly, on comparison of marginal fit of copings produced by accelerated casting technique (IIa, IIb, IIc), it was observed that amongst the three marginal configuration designs studied, shoulder with bevel (IIc) showed the best marginal fit on the lingual (IIa - 42.45 ± 23.22 µm, IIb - 44.33 ± 15.05 µm, IIc - 39.97 ± 5.54 µm, mesial (IIa - 50.04 ± 29.68 µm, IIb - 44.95 ± 13.88 µm, IIc - 40.40 ± 10.04 µm) and distal surface (IIa - 42.57 ± 23.61 µm, IIb - 51.33 ± 29.84 µm, IIc - 38.30 ± 9.99 µm). On the buccal surface chamfer margin showed a better marginal fit and the results were statistically significant (P < 0.05). This might be inherent to the technique and the material used in the study [Table 4].
Similar results were demonstrated by Hasti et al.,  and Konstantoulakis et al.  It was concluded that the accelerated casting procedure produced castings with accuracy similar to the conventional method. There is no explanation for the precise fit of copings fabricated through accelerated casting technique other than difference in the composition of investment material, which is a trade secret (not disclosed by manufacturer). The higher content of quartz versus cristobalite usually makes the investment more resistant to fracture when heated rapidly. Quartz expands at higher temperature and its rate of expansion and total expansion are lower, thus resulting in less chances of investment mold damage. Another reason can be that accelerated schedules may take advantage of characteristic exothermal setting reaction of phosphate bonded investment. Heat enhanced setting expansion continues uninterrupted as the mold is transferred into a preheated furnace environment for thermal expansion.
The procedure of fabrication of the castings by alternating between methods proved to be important. Observation of data revealed that the operator's technique sensitivity improved noticeably as more experience was gained throughout the course of the study.
On evaluation of the marginal designs, the overall results showed that the three margins-chamfer, shoulder and shoulder with bevel have insignificant difference in the marginal fit, although based on the mean value, shoulder with bevel margin showed the best marginal fit. The results are in accordance to those found by Amini and Abaslo  and Shillinburg et al. 
Different finish-line designs have been advocated for tooth preparations for several reasons, and there are very few studies to justify the best finish line in terms of marginal adaptation. The longevity of restoration depends upon the marginal fit so obtaining least opening is the primary requisite of the restoration Shillinburg et al.  and Amini and Abaslo.  Published clinical reports by Cogolludo et al.  and Mclean et al. suggested a marginal discrepancy of approximately 100 μm to be clinically acceptable for the successful longevity of restorations. Shillinburg et al. and Buchanan et al.  noted an increase in marginal openings of ceramometal restoration substructures after porcelain application but the values are within the clinically acceptable range and would not alter the clinical implications of this study. Contrary results were found by Dykema et al.,  Gavelis et al.,  Yadav  whereas, Jahangiri et al.,  Rastogi and Kamble  showed that marginal seating is not influenced by either margin design (chamfer, shoulder, shoulder with bevel) or type of luting cement. Possible explanations for the conflicting results may be different testing methods and different materials used for die fabrication.
Obtaining a good marginal seal is one of the most important factors in determining the long-term success of cast restorations. , Because of deficiencies inherent in the dental casting technique, a gap of varying width is likely to occur between the casting and tooth, both internally and at the margin. So influence of the marginal design of a full crown on the occlusal seat and marginal seal of a cemented full crown restoration is of utmost importance. Laboratory methods of investing and casting that require 2-4 h can now be accomplished within 30 min without sacrificing marginal integrity. This rapid method provides the dentist, patient, and dental laboratory technician with a timesaving, cost-effective technique for the fabrication of single-unit castings for metal/ceramic crowns.
| Conclusion|| |
Within the limitations of the study, the following conclusions were drawn:
- Marginal gaps for castings made with an accelerated technique showed no statistical difference when compared with a conventional casting group (w.r.t to all the four surfaces). The accelerated casting technique offers a cost effective and time-saving method by which single-unit castings for metal/metal-ceramic crowns can be fabricated
- The methods used for accelerating the casting process are technique sensitive. Repeated use of the accelerated technique can provide the dental laboratory technician with predictable, clinically acceptable castings for metal/metal-ceramic crowns
- Shoulder with bevel margin presented with the best marginal fit in comparison to shoulder and chamfer.
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Department of Prosthodontics and Implantology, Bhojia Dental College and Hospital, Baddi, Solan, Himachal Pradesh
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3], [Table 4]
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