Comparative Evaluation of Conventional and Accelerated Castings on Marginal Fit and Surface Roughness.

AIMS: The aim of this study was to evaluate the marginal fit and surface roughness of complete cast crowns made by a conventional and an accelerated casting technique. SETTINGS AND
DESIGN: This study was divided into three parts. In Part I, the marginal fit of full metal crowns made by both casting techniques in the vertical direction was checked, in Part II, the fit of sectional metal crowns in the horizontal direction made by both casting techniques was checked, and in Part III, the surface roughness of disc-shaped metal plate specimens made by both casting techniques was checked.
MATERIALS AND METHODS: A conventional technique was compared with an accelerated technique. In Part I of the study, the marginal fit of the full metal crowns as well as in Part II, the horizontal fit of sectional metal crowns made by both casting techniques was determined, and in Part III, the surface roughness of castings made with the same techniques was compared. STATISTICAL ANALYSIS USED: The results of the t-test and independent sample test do not indicate statistically significant differences in the marginal discrepancy detected between the two casting techniques.
RESULTS: For the marginal discrepancy and surface roughness, crowns fabricated with the accelerated technique were significantly different from those fabricated with the conventional technique.
CONCLUSIONS: Accelerated casting technique showed quite satisfactory results, but the conventional technique was superior in terms of marginal fit and surface roughness.

Introduction

The fixed prosthesis has been placed long before the birth of “Christ,” over 2000 years.[1] Treating a patient by fixed prosthodontics requires a combination of many aspects of dental treatment.[2] Conventional casting requires 2–3 h for its completion. An alternative to the conventional casting has now become a need.[34] In accelerated casting technique, within 20–30 min, the operator completes the casting procedure.[567891011] Literature suggests that there have been few studies on the accelerated casting technique. Thus, the present study was conducted to evaluate the marginal fit and surface roughness of crowns made by conventional and accelerated techniques.

Materials and Methods

This study was planned and carried out using the following materials and methods. Elastomeric impression material, die stone with high-strength Type IV, inlay wax Type II, soft wax, phosphate-bonded investment material (Biosint® Extra for accelerated casting technique and Bellasun–Bego for conventional casting technique with special investment liquid Biosol E), and base metal alloys including nickel–chrome and cobalt–chrome Type V were the materials utilized in this study.

Methods

The present study was carried out in the Department of Prosthetic Dentistry. This study was divided into three parts. In Part I, the marginal fit of full metal crowns made by conventional and accelerated casting in the vertical direction was checked, in Part II, the fit of sectional metal crowns in the horizontal direction made by both casting techniques was checked, and in Part III, the surface roughness of disc-shaped metal plate specimens made by both casting techniques was checked and compared.

Procedures employed in Part I of this study were as follows:

Standardized stainless steel master die fabrication

A stainless steel master die for making of full crowns was fabricated, which includes preparation of die for full crown, stainless steel ring spacer, die former, and a base. Preparation for full metal coping on master die had 8° axial wall taper, the cervico-occlusal length was 6 mm, the diameter of the occlusal surface was 6 mm, and the finish line was of 90° shoulder having 1 mm width. Two occlusal grooves were prepared in buccolingual and mesiodistal direction having 0.5 mm depth to serve the purpose of seating grooves for crowns. A stainless steel ring spacer of 1 mm height and 1 mm width all over was fabricated to fit accurately around the prepared shoulder. A metal die former was fabricated in such a way that when it was positioned accurately over the metal die, 1 mm uniform thickness of space was left in all directions around the prepared master die to achieve standardized wax pattern.

Making of stone dies

Custom trays were fabricated for making stone dies. In this part of study, twenty stone dies were made for making of wax pattern specimens. The prepared twenty stone dies were divided into two groups: Group I and Group II. Group I consisted of ten stone dies for full crown wax patterns and used for conventional casting technique, and then, these ten stone dies were further divided into two subgroups A and B. Group A consisted of five prepared stone dies used for conventional casting using nickel–chrome casting alloy. Similarly, Group B consisted of five stone dies for conventional casting using cobalt–chrome casting alloy. Likely in Group II, ten stone dies were used for accelerated casting. Again Group II was divided into two subgroups C and D with five specimens in each subgroup which were utilized for nickel–chrome and cobalt–chrome casting alloys, respectively.

Making of wax pattern specimens

Two coats of the die spacer were applied over each stone die which was 2 mm short of margin. Inlay casting wax was softened and melted with Bunsen burner, and once the inlay wax gets the adequate flow, it was flown in to the standardized 1 mm cavity obtained in between the stone die with a ring spacer and stainless steel die former, and then, the base was positioned over it to have uniform thickness of 1 mm of wax coping. The die assembly was held together for 1 min and immersed in room temperature water for 2–3 min. The uniform thicknesses of wax pattern specimens were verified, and completed specimens were utilized for the measurement of marginal discrepancy before casting.

A total of 20 wax pattern specimens for full metal copings were fabricated at a constant room temperature of 24–28°C throughout the procedure.

Marginal discrepancy of wax pattern specimens checked in the vertical direction

A reference mark (a small straight line in the vertical direction) was scribed on the margin of each wax pattern and on the external margin of its respective stone die at four areas such as buccal, lingual, mesial, and distal [Figure 1]. The distance between the lower margin of the wax pattern and the upper margin of the stone die was utilized for calculation using traveling stereomicroscope (Pisco, P-351) [Figure 2]. Twenty full crown wax pattern specimens were tested for the measurement of marginal discrepancy in the vertical direction.

Figure 1

Reference points were utilized for the measurement of horizontal fit/adaptation

Figure 2

Measurements were made under a traveling stereomicroscope

Casting technique

The following procedure was carried out for making of crowns including spruing, investing, wax elimination or burnout, and casting.

Spruing

The spruing procedure was the same for conventional as well as for accelerated casting techniques. To minimize the distortion, each wax pattern was invested immediately after completion of all measurements. Two wax pattern specimens were invested in each casting ring.[1]

Investing

Bellasun phosphate-bonded investment material was used for conventional casting. All the investments were mixed according to the manufacturer's instructions. The special liquid to distilled water ratio offered adequate expansion for the investment mold. Total liquid to powder ratio was taken by the manufacturer's instructions according to the amount of expansion required. All phosphate-bonded investment materials were used in ambient room temperature at 22°C–24°C throughout the study. All phosphate-bonded investments were initially hand-mixed for 10 s and then vacuum mixed over vibrator for 60 s.[1] Then, with the completion of the investing procedure, the optimum bench set time was provided for conventional casting technique, and it was 1–1½ h.

Investing procedure for accelerated casting

The investment material utilized for this technique was Biosint Extra phosphate-bonded investment. The two phosphate-bonded investments are different because of its contents such as 74.1% silica filler (61.8% quartz and 12.3% cristobalite), 10.1% MgO, and 15% monoammonium phosphate. Adequate compressive strength (i.e., 4 MPa) has been achieved using a special liquid and water in the ratio of 3:1 (75% special liquid and 25% distilled water). The average time to reach maximum temperature for Biosint Extra phosphate-bonded investment material was recorded, and it was 12.25 min, which was utilized for the accelerated casting technique.

Wax elimination or burnout

For Conventional Casting Technique

A two-stage wax elimination cycle was used in this study in accordance with the manufacturer's recommendations. The temperature of the furnace was raised at the rate of 8°C/min up to 427°C for the first cycle and then at the rate of 14°C/min up to 815°C for the second cycle. The usual burnout temperature for phosphate-bonded investment has been ranging from 750°C to 1030°C.

For Accelerated Casting Technique

After 12.25 min of the bench set time of Biosint Extra phosphate-bonded investment, the investment mold was placed in a preheated furnace/oven at 815°C for 15 min. It was found that 15 min at 815°C was adequate time for the complete elimination of the wax from the investment mold.

Casting technique

After completion of burnout and the thermal treatment of the investment up to the required temperature, both alloys were melted and castings were made using an induction casting machine.

Evaluation of the castings and measurements

The casting rings were bench cooled and castings were removed. The castings were cleaned from investment with plastic hand instruments. Castings were placed in a hydrofluoric acid substitute in an ultrasonic cleaner for 12 min. Sand blasting is avoided as it would alter the degree of the surface roughness. The surface roughness was examined with scanning electron microscope (SEM) under × 250 magnifications [Figures 3 and 4]. The castings were seated on stone die without the stainless steel ring spacer. The distance between wax pattern margins and shoulder of the stone die, which was 1 mm as maintained by the stainless steel ring, was subtracted from the respective distance measured between the casting and shoulder of the stone die. The measurements were made in four different areas randomly by a traveling stereomicroscope. The resultant difference was the net marginal discrepancy.

Figure 3

Scanning electron microscope pictures for comparison of surface roughness under × 250 magnification - For conventional technique

Figure 4

Scanning electron microscope pictures for comparison of surface roughness under × 250 magnification - For accelerated technique

In Part II, the fit of sectional metal crowns in the horizontal direction made by both casting techniques was checked. The procedures employed for making of sectional metal crowns were the same as in Part I of the study. The only difference lies in the master stainless steel die which was sectional. There was no stainless steel ring spacer as such for making sectional metal crowns. Twenty wax pattern specimens were made, and they were divided into two main groups, one for conventional and one for accelerated casting. The main groups were again subdivided into two subgroups, one for nickel–chrome and one for cobalt–chrome casting alloys. Castings were made by a similar procedure as mentioned in Part I of the study. Similarly, the horizontal fit of the sectional metal crown was measured by a traveling stereomicroscope [Figure 2].

In this sectional view, the fit of the internal surface of the sectional crown and the external surface of the stone die was successfully assessed. After adjustment of the sectional crown over its respective stone dies, three points on the stone die that were A, B, and C and the corresponding three points on metal coping that were D, E, and F were taken for the measurement of horizontal discrepancy using a traveling stereomicroscope. Distance from A to D and from C to F was used for horizontal discrepancy at the cervical region.

In Part III, a standardized stainless steel master plate with central space having dimensions of 2.5 cm × 2.5 cm length and thickness of 0.5 mm was made. The purpose of making a master plate was to prepare wax specimens for casting of those wax specimens. The surface roughness of disc-shaped castings was evaluated and compared in both the conventional and accelerated techniques. From the main master plate, twenty wax specimens were fabricated using inlay wax. Out of the twenty specimens, ten were used for conventional casting technique and ten for accelerated casting technique. Out of the ten specimens, five specimens were used for nickel–chrome and five for cobalt–chrome castings. After casting, all the disc-shaped plates were cleaned and placed in an ultrasonic unit for 10–12 min. These casted specimens were checked for surface roughness under SEM (JEOL, Japan) with × 250 magnification [Figures 3 and 4].

Results

The present study was divided into three parts; accordingly, the following results were obtained. In Part I of the study, the marginal fit of full metal crowns made by conventional and accelerated casting in the vertical direction was checked. “Vertical marginal discrepancy of crowns made by accelerated casting was more than conventional casting technique.” The results obtained from Part I of the study are presented in Table 1. The mean marginal discrepancy of complete crown castings and the range between minimum and maximum values are presented in Table 1. Statistical differences in vertical marginal discrepancy were detected between the accelerated and conventional casting techniques (P < 0.05) [Graph 1].

Table 1

Comparison of vertical marginal discrepancy of Ni-Cr and Co-Cr alloys in both casting technique

Graph 1

Comparison of vertical marginal discrepancy of Ni-Cr and Co-Cr alloys in both casting technique

In Part II, the fit of sectional metal crowns in the horizontal direction made by both casting techniques was checked. “Internal fit of crowns in horizontal direction made by accelerated casting and conventional casting technique was same.” The results obtained from Part II of the study are presented in Table 2. The mean horizontal discrepancy of sectional crown castings and the range between minimum and maximum values are presented in Table 2. Nonsignificant results were found in the horizontal discrepancy of crowns made by accelerated and conventional techniques and was almost the same (P < 0.05) [Graph 2].

Table 2

Comparison of horizontal discrepancy of Ni-Cr and Co-Cr alloys in both casting technique

Graph 2

Comparison of horizontal discrepancy of Ni-Cr and Co-Cr alloys in both casting technique

In Part III, the surface roughness of the disc-shaped metal plate specimens made by both casting techniques was checked and compared. The results of surface roughness test were carried out under SEM. “Accelerated casting showed more surface roughness than conventional casting technique.” Evaluation of the SEM microphotographs revealed apparent differences in the grain structure [Figures 3 and 4] between the castings made by conventional technique and those made by accelerated technique. Differences found in the surface roughness were demonstrated in specimens made by accelerated technique.

Discussion

The laboratory procedure for making of castings by conventional technique requires about 2–3 h for its completion. Some researchers tried accelerated casting technique by which the time required to complete the casting was reduced to 30 min. Previous studies suggest that the results of accelerated casting technique were as comparable to the results of the conventional casting technique.[789] Until 1998, to the best of our knowledge, only three studies were found in the literature on extracoronal retainers as mentioned by Konstantoulakis et al.[6813]

There are many variables that control and affect the dimensional changes that occur in the interim stages between the die and the final casting. All the steps involved in this study were standardized and executed accordingly. To avoid discrepancies, suitable investment and alloys were used. In accelerated casting, the importance of introducing the investment ring into the preheated furnace when the investment has reached its peak temperature was first emphasized by Marzouk and Kerby. Blackman used 15 and 30 min for bench set and after burnout cycles with the investment material (complete-cp) and he found 30% mold damage in the 15 min group and no mold damage in 30 min group. In the earliest study conducted by Marzouk and Kerby in 1988 and Blackman in 1993, there were found no significant differences in the circumference or in the external diameter at the margin level of crowns.[68]

In this study, the mean values for vertical marginal discrepancy of casting with the conventional casting technique for Part I were 1017 μm and those for the accelerated casting technique were 1087 μm. This represents that the difference between two techniques was 69 μm. It has been suggested that the clinical acceptance of the marginal discrepancy in the vertical direction is up to 120 μm, but in comparison, conventional casting technique is more precise, accurate, and perfect than accelerated casting technique in terms of dimensions. In Part II of the study, no significant differences were found in horizontal discrepancy. Thus, considering the values obtained from both the marginal fit and the surface roughness, conventional casting technique was superior to the accelerated casting technique.

Conclusions

To reduce the laboratory working time, an attempt was made in this study to use the accelerated casting technique for making of full veneer crowns using a base metal alloy. In conclusion, the accelerated casting technique showed quite satisfactory results, but the conventional casting was superior in terms of marginal fit and surface roughness. It has been suggested that accelerated casting technique also saves considerable time for the patient, technician, and the dentist without affecting the dimensions of the castings but that is up to some extent. Accelerated casting technique can be suggested as a practical alternative to the time-consuming conventional casting procedure. However, further study is needed to evaluate the marginal accuracy and surface roughness with a large number of study samples.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.