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ORIGINAL RESEARCH
Year : 2014  |  Volume : 6  |  Issue : 2  |  Page : 98-102

Vertical marginal discrepancies of metal castings obtained using different pattern materials: A scanning electron microscope study


Department of Prosthodontics, Srinivas Institute of Dental Sciences, Mukka, Surathkal, Mangalore, Karnataka, India

Date of Web Publication28-Oct-2014

Correspondence Address:
Anand Farias
Department of Prosthodontics, Srinivas Institute of Dental Sciences, Mukka, Surathkal, Mangalore - 574 146, Karnataka
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2231-0754.143492

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   Abstract 

Background and Objectives: Dental Casting involves various stages of processing, out of which any may affect the dimensional accuracy. The fit of a casting depends not only on the method of fabrication employed but also on the type of materials utilized. One important variable in the casting process is the type of pattern material employed. This study was carried out to determine and compare the effect of different pattern materials on the vertical marginal accuracy of complete cast crowns. Materials and Methods: A standardized metal master die simulating a prepared crown was used to prepare 60 models on which patterns were fabricated using Inlay Pattern Wax; Auto-polymerized Pattern Resin and Light Cured Modeling Resin and cast immediately. Castings of the patterns were subjected to analysis of marginal fit using a scanning electron microscope (SEM). Results: One-way ANOVA result showed a significant difference in the gap observed between the castings fabricated using the three types of pattern materials (P < 0.001). Post-hoc Bonferroni tests showed significant difference between the castings fabricated using Inlay Type B pattern wax and Auto-polymerized pattern resin as well as between Inlay Type B pattern wax and Light-cured modeling resin (P < 0.01). Significant difference exists between Auto-polymerized pattern resin and Light-cured modeling resin (P > 0.05). Conclusion: With strict adherence to the principles of pattern fabrication and immediate casting, Inlay wax can still be the pattern material of choice to produce a casting with minimal marginal discrepancy with added advantages of being user friendly and cost effective.

Keywords: Dental casting, dental pattern resin, inlay casting wax, light cured modeling material, vertical marginal accuracy


How to cite this article:
Sushma R, Farias A, Soni R. Vertical marginal discrepancies of metal castings obtained using different pattern materials: A scanning electron microscope study. J Int Clin Dent Res Organ 2014;6:98-102

How to cite this URL:
Sushma R, Farias A, Soni R. Vertical marginal discrepancies of metal castings obtained using different pattern materials: A scanning electron microscope study. J Int Clin Dent Res Organ [serial online] 2014 [cited 2019 Oct 22];6:98-102. Available from: http://www.jicdro.org/text.asp?2014/6/2/98/143492


   Introduction Top


Dental casting is one of the most widely used methods for the fabrication of an indirect dental restoration. The ability to fabricate precise castings and reproduce them repeatedly has been the principle objective of many investigators. [1] Taggart's lost wax technique [2] is the most preferred method for the fabrication of cast restorations till date.

The ultimate assessment of the cast restoration lies in the accuracy of the casting and duplication of tooth morphology. [3] The final fit of the casting is the result of a controlled expansion of the mold, which is the cumulative result of variables such as the composition of the investment material, water-powder ratio, manipulation technique, temperature of the room and liquid, and type of the liner. [4],[5],[6],[7] The fabrication of an acceptable pattern and the type of the pattern material used is an important variable that will affect the marginal fit through-out the casting procedure.

From the early use of bees wax for making impressions to the commonly used techniques such as lost-wax technique for dental castings or interocclusal records, waxes have always been among the most popular and useful dental material. Nevertheless, use of a dental wax is associated with the control of certain critical characteristics that affect its applicability such as the thermal expansion and contraction with other major reasons for distortion.

Casting techniques that involve the burn-out of a pattern traditionally use wax or acrylic resin for the development of the patterns. [8] Auto-polymerized acrylic resins (introduced in 1950) have been used for castings requiring greater dimensional stability. Auto-polymerization reduces the amount of time available for manipulation, but the rigidity and hardness of the auto-polymerized resin allows contouring to be performed with abrasive instruments. [9],[10] The major disadvantage of acrylic resin is its high polymerization shrinkage. [11]

To overcome the drawbacks of these materials, Light-cured pattern resins were introduced which have better fit and stability after polymerization. Literature states that the advantages of these light-curing resins are low polymerization shrinkage, adequate dimensional stability, ease of manipulation, reduced chair side time, and absence of residue on burn-out. [12]

Taking into considerations the behavior, the limitations and the scope of pattern wax, acrylic pattern resin and the light-cured resins, a study was designed to evaluate and compare the effect of different pattern materials on the vertical marginal accuracy of complete cast crowns.


   Materials and methods Top


Fabrication of the master die

  1. A standardized metal master die simulating a prepared crown was prepared with an occluso-gingival height of 5 mm, a taper of 6° and 1.5 mm shoulder all around. The overall diameter of the die at the base was 10mm and the height of the base was3.5 mm [Figure 1].
  2. An orientation notch was made on the occluso-axial margin of the master die for the correct seating of the casting.
  3. A cylindrical stainless steel waxing sleeve was fabricated to approximate all surfaces of the die with an internal gap of 1.5 mm. The internal diameter of the sleeve was10 mm and height 10 mm.
Figure 1: Metal master die and sleeve

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Fabrication of the specimens

Impression of the metal master die was made using addition silicone impression material (Aquasil, Dentsply India Pvt. Ltd.) using a putty-wash technique. To prepare the master die, Type IV die stone (Kalrock, KalabhaiKarson, India) was vacuum mixed and poured. Thirty impressions were made using the same technique and 30 individual stone dies were fabricated. Die spacer (Picofit, Renfert, Germany) was applied on each of these dies leaving 0.5 mm of margin in the cervical area on all the dies. Pico-sep (Renfert, Germany) was used as the die lubricant onto the stone dies for the easy retrieval of the patterns.

The specimens were divided into three groups of 10 each:

Group 1: 10 dies on which the patterns were prepared using Inlay Type B patternwax (Crown wax, Bego, Germany).

Group 2: 10 dies on which the patterns were prepared using Auto-polymerized Pattern resin (Duralay, Reliance Dental Mfg. Co, Worth, IL).

Group 3: 10 dies on which the patterns were prepared using Light-cured modeling resin (Triad VLC Burnout Paste, Dentsply International Inc., York, PA).

Preparation of the patterns

Inlay Type B pattern wax


An electrically controlled wax bath (Renfert, Germany) was used to melt at 100°C. The metal waxing sleeve was lubricated with petroleum jelly. The sleeve was fitted on the stone die. Fluid wax was poured into the gap between the stone die and the metal sleeve and the molten wax was allowed to cool down to room temperature. After the wax had cooled to room temperature, the margins were finished using P. K. Thomas carvers.

Auto-polymerized pattern resin

The metal waxing sleeve was lubricated with petroleum jelly. The acrylic material was applied by wetting a fine brush with monomer and dipping it in the powder to produce a bead of acrylic material and thus applied incrementally. The acrylic resin pattern was trimmed carefully using a fine grit metal trimming laboratory bur to exact dimensions.

Light-curing modeling material

Light-curing modeling material onto the gypsum stone dies. The sleeve was lubricated with petroleum jelly. The pattern material was layered and polymerized in the light curing unit (triad 2000 VLC unit, Dentsply, India Pvt Ltd) in increments and each increment was cured for 90 seconds constantly maintaining the length and width of the pattern using the sleeve.

Spruing of the patterns

Once all the patterns were fabricated 4mm sprues were attached vertically at the center of the occlusal surface of each pattern. The patterns were sprued away from the heat center of the ring to prevent casting defects. The patterns were carefully separated with minimal distortion and invested immediately.

Investing and casting

The patterns were coated with a surface tension reducing agent (Picosilk, Renfert, Germany), dried and invested in the ringless casting system (Bego, Germany). The Phosphate bonded investment material (Bellasun, Bego, Germany) was vacuumed mixed (EasyMix Vacuum Mixer, Bego) with 70% colloidal silica suspension (Begosol, Bego, Germany) and 30% distilled water. Metal was cast at 990°C in an electronic induction casting machine (Degutron, Degussa AG) using Ni-Cr alloy (Wiron, Bego Germany).

Seating of the castings on the stone dies

The castings were retrieved using a deflasking unit (Wassermann, Germany) and checked for surface distortions, nodules and irregularities visually using a stereomicroscope (2.5Χ, Lawrence and Mayo). They were eliminated using a round carbide bur under magnification taking care only to grind away the nodules and irregularities. Defective castings were excluded from the study and such castings were refabricated. Trimmed castings were then seated on the dies without any pressure. Marginal fit was visually assessed. Specimens with high marginal discrepancy were discarded and casting procedure was reemployed to obtain fresh castings.

Image analysis

The specimens were gold coated using the Gold Sputtering Device (Anvela RF Magnetron Sputtering Unit, Model SPF-332H) for conduction and subsequently scanned using the Scanning Electron Microscope (ESEM Quanta 200, FEI). The marginal discrepancies of the castings fabricated using Inlay Pattern wax [Figure 2], Auto-polymerized pattern resin [Figure 3], Light-cured modeling resin [Figure 4] were measured with the help of image analyzer software Sigma Scan Pro. Three readings showing the marginal discrepancies at two points for each die were recorded in microns. The results were subjected to statistical analysis.
Figure 2: SEM Photograph showing the vertical marginal discrepancy of the castings fabricated using Inlay Type B pattern wax

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Figure 3: SEM Photograph showing the vertical marginal discrepancy of the castings fabricated using Light-curing modeling material

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Figure 4: SEM Photograph showing the vertical marginal discrepancy of the castings fabricated using Auto-Polymerized pattern resin

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   Results Top


Statistical analysis was performed using One-Way ANOVA and Post-Hoc Bonferroni Tests. Statistical analysis showed a mean marginal discrepancy of 165.62 microns for the castings fabricated using Inlay Type B pattern wax. Mean marginal discrepancy of 177.17 microns was observed for the castings fabricated using Light-cured modeling resin. For castings fabricated using Auto-polymerized pattern resin it was observed to be 184.43 microns [Graph 1]. [Additional file 1]

One-way ANOVA result concludes that there was a significant difference in the gap observed between the castings fabricated using the 3 types of pattern materials (P < 0.001) [Table 1].
Table 1: Results of one way ANOVA comparing the mean marginal gaps of the castings obtained

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Post-hoc Bonferroni test shows significant difference between the castings fabricated using Inlay Type B pattern wax and Auto-polymerized pattern resin as well as between Inlay Type B pattern wax and Light-cured modeling resin (P < 0.01) [Table 2]. Significant difference exists between Auto-polymerized pattern resin and Light-cured modeling resin (P > 0.05).
Table 2: Results of multiple comparisons based on Bonferroni's test

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   Discussion Top


The fabrication of acceptable pattern is an important variable which will affect marginal fit throughout the casting procedure. Marginal fit has been identified as one of the reasons for failure of cast restorations. The fabrication of cast restorations involves numerous technique sensitive steps to obtain restorations with accurate fit.

In an investigation on the behavior of Inlay wax patterns and concluded that if the wax is effectively manipulated, the resulting pattern shows less distortion. They stated that the wax patterns had been unjustly accused of being a major cause of dimensional error and that Inlay waxes were comparatively stable materials if invested immediately not mistreated during pattern fabrication. [13] Wax patterns were not constant in their dimensions under varying thermal conditions. Fusayama, [14] observed that the wax molded into the cavity exhibited shrinkage during solidification.

The present study showed that the mean of the marginal discrepancy for the castings fabricated using Inlay Type B pattern wax was less when compared with Light-cured modeling resin and Auto-polymerized pattern resin which supports the work done by Hollenback and Rhodes. [13] The Inlay wax when effectively handled and invested immediately leads to less dimensional error than the Light-cured modeling resin and the Auto-polymerized pattern resin. This could be attributed to the elastic memory of the Inlay wax which shows a tendency to revert to its original form after being worked or changed in shape as shown by Smyd. [14]

Resins are alternative materials which are used as pattern materials for casting. Auto polymerizing methyl methacrylate resins have offered improved dimensional stability even if investing immediately was not possible. They allow easy manipulation with rotary instruments without fear of distorting the pattern. [15] Improved dimensional stability and ease of manipulation have made Auto-Polymerized pattern materials popular with clinicians for direct pattern fabrication although they have never challenged wax as an indirect pattern material. This is mainly because the major disadvantage of pattern resin is its polymerization shrinkage which is much more than inlay pattern wax and eventually leads to discrepancies in the castings.

Light-curing modeling materials on the other hand, rely on the entry of light of sufficient intensity to initiate polymerization. [16] Light-curing materials are popular in dentistry, and have superseded chemically activated materials in a variety of clinical and laboratory applications. They provide many benefits, including faster and more complete curing; reduced porosity as mixing is generally not required; almost instant finishing; adequate working time for complex procedures; and material economy. Light intensity is greater at the surface of a material specimen, but at deeper levels it is attenuated by absorption and scatter, which limits the depth of cure which could be achieved. Important limiting factors include the intensity of the incident light, the duration of exposure, material color and the nature and volume of filler. In addition to the inherent draw-backs of the material, the cost and the equipment associated with light-cure resin makes it secondary to inlay wax.

Iglesias et al., [11] and Whitworth et al., [12] compared the marginal fit of MOD inlay and full-crown patterns. In their study, patterns were fabricated from Inlay wax; Auto-polymerized pattern resin, and Light-curing diacrylate resin pattern materials. They showed that the polymerization shrinkage was least for inlay wax and maximum for auto-polymerized pattern resin. The results obtained in the present study can be correlated to the results of the above mentioned study although the results of the present study may have been affected by various fabrication and manipulative variables.

A study conducted by Danesh G et al., [17] on the various polymerization properties of the Light-curing and Auto-polymerized pattern resins reflected that the volumetric shrinkage of the Light-curing resins are similar to the Auto-polymerized pattern resin, which may be correlated to the results of the present study.

The results obtained in this study can be attributed to the different types of pattern materials used in the study and also the various factors involved in the casting procedure itself, like the distortion of the patterns during their removal from the die, expansion of the patterns during the setting of the investment and expansion of the investment. Retrieval of the patterns from the die may also have affected the final result.


   Conclusion Top


The clinical success of a cast restoration depends on the dimensional accuracy of the casting. This demands the highest technical standards at each stage of restoration fabrication and knowledge of the materials involved to optimize their performance. The factors must be thoroughly understood to obtain an accurate restoration, which is in harmony with the stomatognathic system.

Within the limits of the present study, it may be concluded that with strict adherence to the principles of pattern fabrication, Inlay Pattern wax if invested immediately can still lead the charts by being the material of choice for fabrication of a casting with minimal marginal discrepancy which is economical and less technique sensitive.

 
   References Top

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Hollenback GM, Rhoads JE. A study of the behaviour of pattern wax. Part II. J South Calif Dent Assoc 1960;28:6-13.  Back to cited text no. 13
    
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Nomoto R, Asada M, McCabe JF, Hirano S. Light exposure required for optimum conversion of light activated resin systems. Dent Mater 2006;22:1135-42.  Back to cited text no. 16
    
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2]



 

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