To Evaluate the Marginal Adaptation of Porcelain Fused to Metal Crown with Different Base Metal Alloys.

INTRODUCTION: The success of any restoration depends on the marginal seal. The adaptation of castings, luting cement, and the surface structures of the margins are all important factors in achieving marginal seal. AIM: The aim of this study was to evaluate the vertical marginal discrepancy of cast copings obtained by employing conventional casting technique with two different base metal alloys with two different finish lines before and after porcelain firing.
MATERIAL AND METHODS: A total of forty wax copings were fabricated with stainless steel die assembly and divided into four groups with ten specimens for each metal and each finish line. Measurements were recorded from coping margin to the stainless steel die margin for vertical marginal gap recordings. Each metal coping was finished, and porcelain application was completed. Copings with porcelain were placed on their respective dies, again subjected to the same measuring microscope for checking the vertical marginal discrepancy by the same operator and results.
RESULTS: The results of the present study showed that the mean vertical marginal gaps of all the cast copings obtained in each group (G1-G8) were within clinically acceptable limits. The mean vertical marginal gap of G1 was 135.36 ± 2.30 μm, G2 was 67.22 ± 4.25 μm, G3: 39.47 ± 2.98 μm, G4: 71.00 ± 3.97 μm, G5: 109.57 ± 2.98 μm, G6: 109.57 ± 2.98 μm, and G8: 114.58 ± 2.40 μm.
CONCLUSION: The difference in the vertical marginal gap of cast copings obtained in different groups was statistically highly significant at 0.005 level, while the difference in the vertical marginal gap of cast copings obtained at different points was statistically nonsignificant. Copyright:

INTRODUCTION

The success of any restoration depends on the marginal seal. The adaptation of castings, luting cement, and the surface structures of the margins are all important factors in achieving marginal seal. A thin line of cementing medium is exposed to the action of saliva, foreign bodies, and oral fluids if the fit of the cemented restoration is insufficient due to weak marginal seal. This inadequacy of marginal fit is a source of marginal leakage, which promotes plaque and bacterial deposition, which contributes to secondary caries and periodontal disease, as well as concomitant deterioration of the restorations.[12]

With the introduction of the metal-ceramic crown, it became the most popular restoration in dentistry as it derives its esthetics from porcelain and strength from underlying metal substructure and also marginal integration for good marginal seal. The accurate fit of the casting to the underlying tooth structure is crucial to the success of any cast restoration as it allows for less plaque accumulation at the marginal area, better mechanical properties, less cement space, and a better esthetic outcome.[34]

Despite diligent waxing, investing, and casting practices, marginal discrepancies in cast restorations are unavoidable. Even though published data on clinically acceptable gaps vary from 30 μm to 200 μm, standard textbook reference on cast restorations mentions a marginal gap of up to 74 μm as clinically acceptable. The rate of luting cement dissolution has been related empirically to the degree of marginal opening. If the marginal differences are larger than 150 μm, saliva has a greater effect on cement dissolution.[5]

Keeping this in mind, many materials and methods have been suggested by various authors to improve the fit and marginal accuracy of the casting. The nature of the preparation, the impression, the working cast, the quality of the wax used in the lost wax technique, and the precision of the casting all affect the fit.

Attempts to perfect casting procedures by developing investment materials and techniques have been recorded in various studies. The aim of these studies are based on what are considered as “traditional” investing and casting techniques. The entire procedure takes about 2–4 h to complete. Therefore, accelerated casting techniques have been reported in an effort to achieve similar quality results in significantly less time. In a cost-effective and time-saving manner, the pattern is invested, cast, and delivered. This combination provides several benefits to the patient, dentist, and laboratory technicians and has gained popularity as a means of increasing productivity. Less studies have been done related to the effect of these accelerated procedures on the marginal fit of base metal alloys.[6]

Therefore, the aim of this study was to evaluate the vertical marginal discrepancy of cast copings obtained by employing conventional casting technique with two different base metal alloys with two different finish lines before and after porcelain firing.

MATERIALS AND METHODS

The present study was conducted in the department of Buddha Institute of Dental Sciences and Hospital, Patna.

A total of forty wax copings were fabricated with stainless steel die assembly and divided into four groups with ten specimens for each metal and each finish line.

Methodology

For the wax pattern fabrication, the custom-made stainless steel die assembly was used. The coping pattern of forty copings were made and randomly divided into four groups: G1, G2, G3, and G4, and ten specimens were used for each group of the study. Next preformed wax sprue of 2.5 cm lengths and 2.5 mm diameter 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. 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. A phosphate-bonded investment was used to invest all wax patterns in a metal ring with ceramic ring liner. The invested pattern was then allowed to bench set for 20 min. The investing procedure was the same for all the patterns of the three test groups. After that, the set investment mold was placed in burnout furnace along with the casting crucible at room temperature. Burnout of the wax pattern was done using a programmed preheating technique. The investment mold was averted later near the end of the burnout cycle with the sprue hole facing upward to enable the escape of the wax pattern and allow mold expansion. The same procedure was followed for each of the ten samples of this test group.

The casting procedure was performed quickly to prevent heat loss resulting in the thermal contraction of the mold. The casting was done in an induction casting machine in two different nickel–chromium alloys. The nickel–chromium alloy 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. The casting procedure followed was the same for all the test samples. A total of forty castings were thus made to obtain cast copings for the evaluation in this study.

Following casting, the hot casting ring was bench cooled at room temperature, and the cylinder of investment containing the casting was pressed out from the ring. The obtained copings were 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 four test groups.

Measurements of marginal gap

Each casting was seated on the stainless steel die with finger pressure. Microscopic measurements were recorded at ×50 magnification perpendicular to the axial wall with a measuring microscope. Measurements were recorded from the 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 and the ten test samples belonging to the four test groups. The measurements thus obtained were tabulated and statistically analyzed.

Each metal coping was finished, and porcelain application was completed. Copings with porcelain were placed on their respective dies, again subjected to the same measuring microscope for checking the vertical marginal discrepancy by the same operator, and results were tabulated and compared for statistical significance.

Statistical analysis

The results were tabulated and means of different groups were compared using Student's t-test, and P < 0.05 was considered as statistically significant. ANOVA was also performed to compare the amount of marginal fit before and after firing of porcelain within study groups. Statistical analyses were performed using SPSS version 21.0 (SPSS Inc., Chicago, USA).

RESULTS

The results of the present study as tabulated in Table 1 show the observed vertical marginal gap of different study groups before and after porcelain firing at various points of different dies included.

Table 1

Mean value and standard deviation of observed vertical marginal gap at various points of different die

Group	Mean±SD
	Point 1	Point 2	Point 3	Point 4
G1. ME alloy (metal coping) shoulder	35.00±3.52	35.55±2.71	35.40±3.24	35.50±3.25
G2. ME alloy (porcelain) shoulder	66.40±5.68	66.35±3.81	67.90±4.51	68.25±4.12
G3. HERAENIUM-S alloy (metal coping) shoulder	39.50±2.48	39.30±3.39	38.20±2.93	40.90±4.30
G4. HARAENIUM-S alloy (porcelain) shoulder	70.85±4.23	71.10±4.29	70.30±4.98	71.75±3.58
G5. ME alloy (metal copings) chamfer	77.40±2.01	78.15±2.33	78.45±2.37	78.45±2.70
G6. ME alloy (porcelain) chamfer	110.20±2.85	109.70±2.96	110.45±4.54	107.95±3.98
G7. HERAENIUM-S alloy (metal copings) chamfer	82.90±2.32	82.65±2.27	82.85±2.24	80.80±1.85
G8. HERAENIUM-S alloy (porcelain) chamfer	114.60±3.50	115.35±2.72	114.65±2.18	113.75±3.49

SD: Standard deviation

Table 2 shows that the vertical marginal gap of the cast copings on shoulder margin obtained by ME alloy before porcelain firing showed a mean value of 35.36 ± 2.30 μm (G1). The vertical marginal gap of cast copings on shoulder margin obtained by ME alloy after porcelain firing showed a mean value of 67.22 ± 4.25 μm (G2) [Figure 1].

Table 2

Overall mean value of observed vertical marginal gap different die

Group	Mean±SD
G1. ME alloy (metal copings) shoulder	35.36±2.30
G2. ME alloy (porcelain) shoulder	67.22±4.25
G3. HERAENIUM-S alloy (metal copings) shoulder	39.47±2.98
G4. HERAENIUM-S alloy (porcelain) shoulder	71.00±3.97
G5. ME alloy (metal copings) chamfer	78.11±1.86
G6. ME alloy (porcelain) chamfer	109.57±2.98
G7. HERAENIUM-S alloy (meta copings) chamfer	82.30±1.78
G8. HERAENIUM-S alloy (porcelain) chamfer	114.59±2.40

SD: Standard deviation

Figure 1

Graph showing the mean value marginal gap of ME alloy (metal coping) at various points of different finish lines

The vertical marginal gap of cast copings on shoulder margin obtained by HERAENIUM-S alloy before porcelain firing showed a mean value of 39.47 ± 2.98 μm (G3). The vertical marginal gap of cast copings on shoulder margin obtained by HERAENIUM-S alloy after porcelain firing showed a mean value of 71.00 ± 3.97 μm (G4) [Figure 2].

Figure 2

Graph showing the mean value marginal gap of ME alloy (porcelain firing) at various points of different finish lines

The vertical marginal gap of the cast copings on chamfer margin obtained by ME alloy before porcelain firing showed a mean value of 109.57 ± 2.98 μm (G5). The vertical marginal gap of the cast copings on chamfer margin obtained by ME alloy after porcelain firing showed a mean value of 109.57 ± 2.98 μm (G6) [Figure 3]. The vertical marginal gap of cast copings on chamfer margin obtained by HERAENIUM-S alloy after porcelain firing showed a mean value of 114.58 ± 2.40 μm (G8) [Figure 4].

Figure 3

Graph showing the mean value marginal gap of HERAENIUM-S alloy (metal copings) at various points of different finish lines

Figure 4

Graph showing the mean value marginal gap of HERAENIUM-S alloy (porcelain firing) at various points of different finish lines

The order of discrepancy values of vertical marginal gap of the cast copings in this study showed that ME alloy before porcelain firing on shoulder margin had a gap mean value of 35.36 μm while HERAENIUM-S alloy before porcelain firing on shoulder margin had a mean gap value of 39.47 μm. ME alloy after porcelain firing on shoulder margin showed a mean value of 67.22 μm while HARAENIUM-S alloy before porcelain firing on shoulder margin showed a mean value of 71.00 μm.

ME alloy before porcelain firing on chamfer margin showed a mean value of 78.11 μm while HARAENIUM-S alloy before porcelain firing on chamfer margin had a mean value of 82.30 μm. ME alloy after porcelain firing on chamfer margin showed a mean value of 109.57 μm while HARAENIUM-S alloy after porcelain firing on chamfer margin had a mean value of 114.58 μm.

DISCUSSION

For the durability, longevity, and clinical success of any cast restoration, the accuracy of its marginal fit is critical. A lack of proper fit may be harmful to both the tooth and the periodontal tissues. Marginal misfit calculated at different points between the casting surface and the tooth is the best way to describe a casting's fit. Points around the internal surface, at the margin, or on the exterior surface of the casting may be used to determine the distance between the casting and the tooth.[7]

This study has been done to evaluate and compare the vertical marginal gaps of cast copings made by two different metal alloys before and after porcelain firing. The techniques used in the study were conventional casting technique with wax elimination and porcelain application and porcelain firing as per manufacturer's recommendations. In this present study of the marginal discrepancy at different finish lines, the test specimens were subjected to identical casting procedures and identical number of firing cycles. Marginal discrepancy was evaluated with the help of measuring microscope.

A total of forty wax copings were fabricated with stainless steel die assembly and divided into four groups with ten specimens for each metal and each finish line. The marginal discrepancy in the metal coping before and after porcelain firing was measured evaluated and subjected to statistical analysis. The mean marginal gap in G1, G3, G5, and G7 was quite less as compared to G2, G4, G6, and G8. Hence, Groups G2, G4, G6, and G8 were difficult to compensate for the porcelain shrinkage.

The results of the present study showed that the difference in the vertical marginal gap of cast copings obtained in different groups was statistically highly significant at 0.005 level, while the difference in the vertical marginal gap of cast copings obtained at different points was statistically nonsignificant. The mean vertical marginal gaps of all the cast copings obtained in each group (G1–G8) were within clinically acceptable limits.

Various theories have been proposed for the marginal fit discrepancy. One such theory proposes that the marginal fit difference may be due to the type of metal used, as in high-strength alloys, the metal oxide that forms on the inside of the metal castings during the different firing cycles is very thick.

Another such theory states that due to difference in the rate of contraction between the metal substructure and the adhering porcelain during the cooling portion of the firing period, an interfacial stress is created. These stresses will cause deformation, resulting in a marginal fit discrepancy. It has been also reported earlier that during the process of porcelain firing, the porcelain shrinks by around 15% of its original size, and the metal contracts along with it, resulting in a marginal fit discrepancy. If inadvertently the internal parts of the castings become contaminated due to some low-fusing spot melts, they can hereby infest the grain structure and cause enlargement leading to shrinkage of coping.[8910]

In a similar study by Vojdani et al.[11] with an aim to evaluate the marginal fit of zirconia computer-aided design/computer-aided manufacture ceramic crowns before and after porcelain firing and also to observe the influence of finish line configuration on the marginal fit was done and it was concluded that significant differences were present between marginal fit of chamfer and shoulder finish line groups before and after porcelain firing (P = 0.014 and P = 0.000, respectively). The marginal gap of copings with shoulder finish line was significantly smaller than those with chamfer configuration (P = 0.000), but there were no significant differences between the two marginal designs after porcelain firing (P = 0.341). They thus concluded both margin configurations showed marginal gaps that were within a reported clinically acceptable range of marginal discrepancy.

Limitations of the study were as follows:

The mean of the marginal discrepancies measured at four different markings was considered as the discrepancy of that surface, but these markings were not discriminated

Factors such as wax pattern distortions which occur before investing, location of pattern, and nonuniform mold expansion are beyond the control of the operator and inherent in all castings

The present study was conducted on metal dies, but the actual tooth anatomy is different

Die spacers were not used in the present study

Sample size was small to evaluate the marginal discrepancies.

CONCLUSION

Hence, we conclude that there was a statistically highly significant difference in the vertical marginal gap of cast copings obtained in different groups, while the difference in the vertical marginal gap of cast copings obtained at different points was statistically nonsignificant. The mean vertical marginal gaps of all the cast copings obtained in each group (G1–G8) were within clinically acceptable limits.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.