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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 4  |  Page : 446-450
Comparison of marginal accuracy of castings fabricated by conventional casting technique and accelerated casting technique: An in vitro study


1 Department of Prosthodontics, A.C.P.M. Dental College, Sakri Road, Dhule, Maharashtra, India
2 Department of Operative Dentistry and Endodontics, A.C.P.M. Dental College, Sakri Road, Dhule, Maharashtra, India

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Date of Submission25-Oct-2012
Date of Decision18-Jan-2013
Date of Acceptance18-Feb-2013
Date of Web Publication19-Sep-2013
 

   Abstract 

Background: Conventional casting technique is time consuming when compared to accelerated casting technique. In this study, marginal accuracy of castings fabricated using accelerated and conventional casting technique was compared.
Materials and Methods: 20 wax patterns were fabricated and the marginal discrepancy between the die and patterns were measured using Optical stereomicroscope. Ten wax patterns were used for Conventional casting and the rest for Accelerated casting. A Nickel-Chromium alloy was used for the casting. The castings were measured for marginal discrepancies and compared.
Results: Castings fabricated using Conventional casting technique showed less vertical marginal discrepancy than the castings fabricated by Accelerated casting technique. The values were statistically highly significant.
Conclusion: Conventional casting technique produced better marginal accuracy when compared to Accelerated casting. The vertical marginal discrepancy produced by the Accelerated casting technique was well within the maximum clinical tolerance limits.
Clinical Implication: Accelerated casting technique can be used to save lab time to fabricate clinical crowns with acceptable vertical marginal discrepancy.

Keywords: Accelerated casting, conventional casting, marginal accuracy, phosphate-bonded investment

How to cite this article:
Reddy S S, Revathi K, Reddy S K. Comparison of marginal accuracy of castings fabricated by conventional casting technique and accelerated casting technique: An in vitro study. Indian J Dent Res 2013;24:446-50

How to cite this URL:
Reddy S S, Revathi K, Reddy S K. Comparison of marginal accuracy of castings fabricated by conventional casting technique and accelerated casting technique: An in vitro study. Indian J Dent Res [serial online] 2013 [cited 2019 Aug 22];24:446-50. Available from: http://www.ijdr.in/text.asp?2013/24/4/446/118390
Although, the "Lost wax" technique has been used since ancient times, it has become a common practice in dentistry after it was introduced by William H. Taggart in 1907. [1]

Conventional casting technique which is routinely used in dentistry usually requires at least 1 h for the investment to set, followed by a one or two stage wax elimination procedure before casting is done. This procedure is time consuming and requires approximately 2-4 h for completion. [1],[2],[3]

Accelerated casting technique has been reported to achieve similar quality results in significantly less time, namely in 30-40 min for the fabrication of High noble alloy crowns. [1],[2],[3]

A published attempt to accelerate the lost wax technique with the use of phosphate bonded investment for a complete crown was made in 1988 by Marzouk and Kerby. [1],[2]

This study evaluated the marginal accuracy of full coverage single crowns made with an accelerated casting technique that uses phosphate bonded investment material and a Nickel-Chromium (Ni-Cr) alloy.


   Materials and Methods Top


The study was conducted in Department of Prosthodontics, including Crown and Bridge and Implantology, College of Dental Sciences in assistance with Department of Oral Pathology, College of Dental Sciences, Davangere.

A precisely machined brass master die [Figure 1] was designed to simulate a complete molar crown preparation. The die measured 6 mm from the occlusal surface to finish line with a 6° taper towards the occlusal surface from the finish line. The metal die was mounted on a cylindrical base of 20 mm length and a diameter of 10 mm. A shallow axial groove was given for orientation of casting during seating. Reference marks were scribed one each on the buccal, lingual, mesial and distal areas on the root stump near the cervical margin around the circumference of the die. These marks were used later as reference marks for the measurements.
Figure 1: Master die with counter die

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A counter die [Figure 1] with dimensions 1 mm larger than the master die was made in order to make wax patterns of uniform dimensions. [4]

patterns (Bego, Germany) were fabricated on the master die by closing the counter die until the demarcated mark over the die to obtain a wax pattern of uniform thickness. [4] The margins were readapted and divided into 2 groups with 10 wax patterns in each group.

  • Group1: Representing castings made following conventional casting technique.
  • Group 2: Representing castings made following accelerated casting technique.
The marginal discrepancy between the metal die and the wax pattern was measured on an optical stereomicroscope [Figure 2]. Measurements were made at predetermined areas that were marked on the metal die.
Figure 2: Measurement of the marginal gap of wax pattern using optical stereomicroscope

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The wax patterns were spurred. Group 1 and Group 2 wax patterns were invested individually with individual casting rings lined with a ceramic ring liner.

They were invested with Bellasun (Bego, Germany) phosphate-bonded investment (60 g of powder to 16 ml of 100% mixing liquid). [5]

Group 1: The investment was allowed to set for 2-3 h. Then the casting ring was placed in a burnout furnace at room temperature and the temperature was raised to 250°C and maintained for 60 min. Thereafter the temperature was raised to 950°C and held for 30 min. [6],[7]

Group 2: The investment was allowed to set for 13-17 min, immediately the casting ring was placed in a burnout furnace at a pre-heated temperature of 815°C for 15 min. [1],[2],[3],[7]

After completion of the burnout, the casting procedure was carried out in an induction-casting machine using Ni-Cr alloy (Heranium. S Heraeus Kulzer, Germany).

The castings were recovered [Figure 3], burs were used to remove the investment from the inner surface of the casting such as a thin layer of investment was left behind. Sandblasting was done to remove the residual investment and oxide layer.
Figure 3: Recovered Group 1 and Group 2 castings

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The completed castings were seated on the metal die under finger pressure [Figure 4]. The marginal discrepancy between the metal die and the castings were measured [Table 1] on an optical stereo microscope at predetermined points using Image Pro-Plus software [Figure 5] and [Figure 6].
Figure 4: Castings on the master die

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Figure 5: Measurement of marginal gap of castings using optical stereomicroscope

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Figure 6: Measurement of marginal gap using Image Pro-Plus Software

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All measurements were executed by a single operator and the readings were tabulated and used for the statistical analysis (student's unpaired t-test was used). The marginal accuracy of wax patterns were also compared with the Conventional and Accelerated castings [Table 2] and [Table 3].
Table 1: The comparison of the vertical marginal discrepancy between Group 1 and Group 2 castings, and die at buccal, mesial, lingual and distal areas

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Table 2: The comparison of the marginal accuracy between the wax patterns and conventional castings

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Table 3: The comparison of the mean marginal accuracy between the wax patterns and accelerated castings

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   Results Top


The castings fabricated using conventional casting technique showed less vertical marginal discrepancy (48.26 ± 4.34μ) than the castings fabricated by accelerated casting technique (92.41 ± 12.48μ). The values were statistically highly significant (P < 0.001). [Also refer [Table 2] and [Table 3].


   Discussion Top


Fixed Prosthodontics has become a major part of current restorative dentistry. [8] Various alloys and techniques have been introduced for the casting of the fixed partial dentures. [2] Ever escalating cost of gold has made a paradigm shift to the use of base metal alloys ever since their introduction to the profession over 50 years.

Of all the base metal alloy systems, the most successful are Ni-Cr alloys because of their mechanical properties. Ni and Cr form the major component and the balance of the composition includes Mo, Si, Fe, and Ce. The properties of this alloy are: Density that is one-half that of gold alloy, modulus of elasticity that is 2-21/2 times more than that of the gold alloy, sag resistance that is 9 times greater, yield strength that is approximately 20,000 p.s.i. greater, elongation values between 3 and 22 per cent, and cost that is one-fifth that of the gold alloy.

Fresh Ni-Cr alloy shows the greatest porcelain adherence. There is no significant change in bond strength of ceramic to alloy with up to 75% of used Ni-Cr alloy. Subjecting the Ni-Cr alloy for recasting will not bring any significant change in the castability of the alloy. That re-casted alloy can be used at least once again.

The majority of the fixed partial dentures are fabricated using "conventional" investing and casting techniques, which approximately require 2-4 h. [1],[2],[6],[7]

A modified technique called accelerated casting technique has been reported with comparable results. [1],[2],[3],[8],[9]

This study was conducted to compare the marginal accuracy of full coverage single crowns fabricated following conventional casting technique and accelerated casting technique that uses phosphate bonded investment material and a Ni-Cr alloy.

In this study, the mean vertical marginal discrepancy of Group 1 and Group 2 wax patterns ranged from 1.93-13.62 μ to 1.93-15.53 μ respectively. This discrepancy can be attributed to the following reason.

Wax distortion is probably the most serious problem that can occur during fabrication and removal of the pattern from the die. This distortion results from thermal changes and the relaxation of stresses that are caused by contraction on cooling, occluded air in the wax, molding, carving, removal, and the time and temperature of storage. [6]

To minimize the distortion of the wax pattern it is advised to use low storage temperature and it should be invested immediately after fabrication. [6],[7],[10]

Statistically significant differences were found between the mean vertical marginal discrepancies of Conventional and Accelerated castings (P < 0.001).

Clinical tolerance limits for the fit and marginal adaptation of a cast restoration are actually not known. However, several investigations reported that marginal gaps up to 74 μ, 104 μ, or 120 μ are considered to be clinically acceptable. [1] The accuracy of fit of the cast restoration is essential for its longevity and clinical success. The accuracy of fit is affected by the quality of the preparation, the impression, the working cast, the quality of the wax and the accuracy of the castings. [4]

In the present study, mean vertical marginal opening was 48.26 ± 4.34 μ and 92.41 ± 12.48 μ for Group 1 and Group 2 castings respectively, these values are well within the clinical tolerance limits. This could be due to an array of factors such as setting expansion, thermal expansion of investment, shrinkage of wax and alloy. All these above factors influence both Group 1 and Group 2 castings as the same procedures have been followed for both.

The phosphate investment binder chemistry is fairly complex and results in a very energetic reaction. Phosphate bonded investment reacts with water and exhibits shrinkage essentially within the same temperature range as gypsum-bonded investments. This contraction is practically eliminated when colloidal silica solution replaces water. [6]

Various authors have confirmed the elevated thermal expansion with the use of special liquid for mixing investment. The expansion can be variedby the proportions of silica solution and water. Phosphate-bonded investment mixed with 100% special liquid resulted in higher heat and higher settingexpansion. [5] In the present study 100% special liquid was used for the investment.

Accelerated schedules may take advantage of the characteristic exothermal setting reaction of Phosphate bonded investment materials. Heat-enhanced setting expansion continues uninterrupted as the mold is transferred into a pre-heated furnace for thermal expansion. Thermal expansion decreases with time after mixing and the bench-set period might be an important variable for obtaining optimal expansion. [3]

The importance of introducing the mold into the pre-heated oven when the investment has reached its peak temperature was first emphasized by Marzouk and Kerby. When the investment reaches its maximum exothermic setting reaction temperature, most of the chemical reactions and most of the setting expansion are considered to have been completed and the investment has sufficient strength to withstand the thermal shock. [1]

Gypsum bonded investment exhibits maximum exothermic setting reaction temperature lower than those of phosphate bonded investments and lower than the softening temperature of most dental waxes as well. However, because of their low strength and decomposition that occurs at temperatures above 700°C (1292°F), they are not as suitable with accelerated casting techniques such as the phosphate bonded investment material. [1]

The marginal discrepancy of both the conventional and accelerated castings obtained in this study was higher than the previously conducted studies which used a high noble alloy. The main reason for this variation may be the difference in the casting shrinkage of the gold based (1.42-1.56%) and Ni-Cr alloys (2.30%). [6]

The other probable reasons for these results might be the combined effects of [1] the marginal discrepancy of the wax patterns. [2] No die spacer was used, therefore, the castings bind more on the axial walls of the metal die [3] No adjustments were made on the internal surfaces of the castings. [11],[12]

The furnace burnout is thought to provide mold expansion and wax elimination. In case of Accelerated casting technique, a shorter burnout time yields inadequate mold expansion leading to under-sized castings. [3]

In this study, the internal surface adjustments of the castings were not performed and120 μ was considered as the maximum clinically acceptable marginal gap for the castings. [1]


   Conclusion Top


Under the conditions of this study the following conclusions were drawn:

The full coverage single crowns fabricated by accelerated casting technique showed higher marginal discrepancy compared to those fabricated by conventional casting technique. However, clinically acceptable complete castings can be obtained with the accelerated casting technique using Ni-Cr alloy.

 
   References Top

1.Konstantoulakis E, Nakajima H, Woody RD, Miller AW. Marginal fit and surface roughness of crowns made with an accelerated casting technique. J Prosthet Dent 1998;80:337-45.  Back to cited text no. 1
[PUBMED]    
2.Schilling ER, Miller BH, Woody RD, Miller AW 3 rd , Nunn ME. Marginal gap of crowns made with a phosphate-bonded investment and accelerated casting method. J Prosthet Dent 1999;81:129-34.  Back to cited text no. 2
    
3.Blackman RB. Evaluation of the dimensional changes and surface roughness of gold crowns cast with rapidly prepared phosphate-bonded investment: A pilot study. J Prosthet Dent 2000;83:187-93.  Back to cited text no. 3
[PUBMED]    
4.Gill SP, Datta K. Analysis of the marginal gap of complete crowns made by using wet and dry ceramic ring liners-An in vitro study. J Indian Prosthodont Soc 2005;5:152-4.  Back to cited text no. 4
    
5.Ito M, Kuroiwa A, Nagasawa S, Yoshida T, Yagasaki H, Oshida Y. Effect of wax melting range and investment liquid concentration on the accuracy of a three-quarter crown casting. J Prosthet Dent 2002;87:57-61.  Back to cited text no. 5
[PUBMED]    
6.Anusavice KJ, Phillips RW. Dental Waxes, Casting investments and procedure. Science of Dental Materials. 11 th ed. Philadelphia: W.B. Saunders; 2003. p. 283-350.  Back to cited text no. 6
    
7.Rosenstiel SF, Land MF, Fujimoto J. Wax patterns, Investing and casting. Contemporary Fixed Prosthodontics. 3 rd ed. St Louis: Mosby; 2002. p. 457-87, 567-91.  Back to cited text no. 7
    
8.Schneider RL. A one-appointment procedure for cast post and core restorations. J Prosthet Dent 1994;71:420-2.  Back to cited text no. 8
    
9.Campagni WV, Majchrowicz M. An accelerated technique for casting post and core restorations. J Prosthet Dent 1991;66:155-6.  Back to cited text no. 9
    
10.Morey EF. Dimensional accuracy of small gold alloy castings. Part 1. A brief history and the behaviour of inlay waxes. Aust Dent J 1991;36:302-9.  Back to cited text no. 10
    
11.Lombardas P, Carbunaru A, McAlarney ME, Toothaker RW. Dimensional accuracy of castings produced with ringless and metal ring investment systems. J Prosthet Dent 2000;84:27-31.  Back to cited text no. 11
    
12.Craig RG, Powers JM. Waxes, Casting and Soldering procedures. Restorative Dental Materials. 11 th ed. St Louis: Mosby; 2002. p. 424-48, 516-42.  Back to cited text no. 12
    

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Correspondence Address:
S Srikanth Reddy
Department of Prosthodontics, A.C.P.M. Dental College, Sakri Road, Dhule, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.118390

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    Figures

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