Dr. Naisargi Shah1, Dr. Anshul Khanna2, Dr. Adhithi Prabhu3, Dr. Prakruti Shah4, Dr. Ashutosh Pai2, Dr. Rahul Malu3, Dr. Bhavna Ahuja3

1Professor and Head of Department

2Associate Professor,

3Assistant Professor

4Post Graduate student

Department of Prosthodontics, Crown & Bridge.,

T.P.C.T’s Terna Dental College, Navi Mumbai

ABSTRACT

Context: Marginal fit of FDP frameworks depends on fabrication methods and production steps, including porcelain firing, which deforms the framework and alters marginal fit. Although, it’s uncertain whether coping thicknesses have an effect on marginal fit of PFM restorations after repeated firing cycles.

Aim: To comparatively evaluate the effect of three different DMLS coping thickness on the marginal fit of metal ceramic restorations after repeated firing cycles.

Settings and Design: In vitro pilot study.

Methods and Material: An extracted tooth was prepared to receive a PFM crown which was scanned and digitally sectioned, saved as an STL file and printed with DMLS technology. Fifteen Co-Cr metal copings of three different thicknesses namely, 0.3 (group A), 0.5 (group B) and 0.7mm (group C) having an internal space of 20 μm were virtually designed and fabricated. Copings were layered with ceramic and horizontal marginal discrepancy was evaluated under stereomicroscope with x20 magnification after every firing cycle. Data was collected as per readings obtained from TS view software.

Difference between three groups were analysed statistically using Kruskal Wallis ANOVA followed by post hoc analysis using Bonferroni corrections.

Results: Significant differences were observed in the marginal gaps when group A and C were compared. Statistically significant differences were obtained before and after porcelain firing. However, all marginal gap values were well within clinically acceptable range. (30-50 μm)

Conclusions: Within the limitations of the study, it was concluded that a correlation exists between the thickness of metal coping and marginal adaptation of DMLS copings of metal ceramic restorations after repeated firing cycles.

Key-words

direct metal laser sintering, coping thickness, Co-Cr alloy, marginal fit, firing cycles.

Citations

Shah N, Khanna A, Prabhu A, Shah P, Pai A, Malu R et al. The Effect of Three Different Direct Metal Laser Sintered Coping Thickness on The Marginal Fit of Metal Ceramic Restorations After Repeated Firing Cycles: An In Vitro Pilot Study. J Prosthodont Dent Mater 2022;3(1):50-61.

I) INTRODUCTION

Metal ceramic restorations are considered one of the most commonly used restorative materials for fixed dental prostheses (FDPs). They are considered the gold standard in fixed prosthodontics since they possess superior mechanical properties and good aesthetics. As we are aware that metal ceramic restorations consist of a metal substructure and an overlying ceramic layer, its fabrication requires specific properties such as, the alloy and porcelain should have a closely matching coefficient of thermal expansion in which the metal should have a slightly higher value, to produce the required compressive stresses in the porcelain.1

For long-term clinical success and longevity of any fixed dental prosthesis, factors such as marginal fit of the restoration deems necessary. The marginal and internal fit of any restoration is generally influenced by clinical, laboratory as well as dental material factors. Increased marginal discrepancies can lead to biological complications like increased plaque accumulation which in turn increases the probability of secondary caries, ultimately leading to the failure of prosthesis.2 It can also lead to mechanical complications like veneer chipping.3

The success of any coping fabrication technique is to achieve the best possible fit. In recent times, both subtractive as well as additive manufacturing technologies have been used for the fabrication of Co-Cr frameworks. Among the additive manufacturing technology, selective laser melting (SLM) technology which includes direct metal laser sintering (DMLS) has been popularly used in the fabrication of Co-Cr alloy frameworks which are more biocompatible and exhibit better mechanical properties in terms of its rigidity, especially for long span FDPs, when compared with Ni-Cr alloys.4,5,6,7,8,10,11 SLM systems are based on powder-bed technology which are used to fabricate metal components from a 3D computer assisted device (CAD) by using a laser beam to melt thin layers of Co-Cr metal powder.11 These systems have a high degree of precision, which reduces the risk of casting shrinkage of the alloy that occurs during conventional casting procedures like lost wax technique. Furthermore, this technology allows the metal powders to consolidate, thus giving an excellent detailed reproduction of the coping.

Since there are very limited studies that compare the different coping thicknesses and consecutive firing cycles on the marginal fit of metal ceramic restorations, the aim of this in vitro pilot study was to evaluate the effect of three different direct metal laser sintered coping thicknesses on the marginal fit of metal ceramic restorations after repeated firing cycles.

II) SUBJECTS AND METHODS

II) Fabrication of the master model (Fig 1):

A freshly extracted maxillary second premolar was prepared for a porcelain fused to metal crown. A 360-degree, 1.0-mm chamfer margin finish line with an axial taper of 6 degrees, and an occlusal reduction of 1.5 mm was prepared, which was then scanned for the fabrication of master model/die. In order to evaluate the marginal fit of the copings and reduce manual errors that occur during sectioning of the die, the scanned master die was digitally sectioned in the buccolingual direction and was saved as a STL file which was then fabricated and printed by direct metal laser sintering technology.

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Fabrication of the metal copings (Fig 2):

Fifteen Co-Cr metal copings of three different thicknesses namely, 0.3mm (group A), 0.5mm (group B) and 0.7mm (group C) having an internal space of 20 μm were virtually designed over the master die. The copings were then fabricated from Co-Cr alloy powder by direct metal laser sintering (DMLS) technology

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Layering of DMLS copings (Fig 3):

The DMLS copings were subjected to four firing cycles, in which the copings were oxidized in the first cycle, opaquer layer was applied using a recommended primer (C- bond) in the second cycle after which ceramic layering was done according to the manufacturer’s instructions (with dentin and enamel layering of ceramic) which was subjected to third and final firing cycle respectively. A putty index was moulded according to the wax pattern made over the master die that measured around 1.5mm in thickness corresponding to the final restoration thickness of the metal ceramic crown. This index was used accordingly to maintain the uniformity of the overlying layering ceramic.

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Marginal gap measurement:

The marginal fit was evaluated using a single point of reference (horizontal marginal discrepancy measured perpendicular to the path of draw of the coping on the proximal wall) (Fig 4a, 4b). A stereomicroscope (Fig 5) with its corresponding software (TS View digital imaging software, version 6.2.2.2, by Tuscen imaging technology Co. Limited), was the equipment of choice to evaluate the marginal discrepancy. The measurement for every DMLS coping was obtained before and after consecutive porcelain firing. After every firing cycle the copings were inserted on the master die and then placed under the stereomicroscope with x20 magnification to check for the horizontal marginal discrepancy. Copings were thoroughly cleaned after every firing cycle and digital photographs were taken for each group accordingly (Fig 6 a, b, c, d). The data was collected as per the readings obtained from the TS view digital imaging software. The horizontal marginal discrepancy measured in micrometers (μm) was considered as the statistical unit.

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RESULTS:

Statistical analysis::

The findings were tabulated, and the three groups were analysed statistically using ANOVA test/ Kruskal Wallis ANOVA followed by post hoc analysis using Bonferroni corrections. The significance level was set at p≤0.05. Statistical analysis was performed using SPSS® software.

The mean ± standard deviation (SD) after first firing cycle for group A (0.3mm) was 29.50 ± 5.48 μm, group B(0.5mm) 28.37 ± 2.76 μm and for group C (0.7mm) 22.77 ± 2.04 μm. For the second firing cycle, the values were 39.20 ± 12.59 μm, 28.37 ± 4.04 μm, 25.38 ± 4.29 μm and for the third firing cycle the values were 45.32 ± 8.96 μm, 36.21 ± 5.21 μm and 28.38 ± 6.14 μm.

Consequently, the mean ± SD after the final firing cycle were around 50.75 ± 6.40 μm, 41.43 ± 3.82 μm, 33.33 ± 5.17 μm respectively. Table 1 represents the mean and standard deviation of the groups after every firing cycle with its respective p values.

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Table 2, 3, 4 and 5 represents the level of significance (p values) of all three groups after every firing cycle.

The graph (fig 7) represents the mean of all three-coping thicknesses after every firing cycle where all three groups were compared with different color-coding. It was observed that the marginal discrepancy in 0.3mm of coping thickness increased to a significant extent after repeated firing cycles as compared to groups having 0.5mm and 0.7mm thickness respectively.

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DISCUSSION

Marginal fit or adaptability is one of the crucial factors that is considered essential in the longevity of indirect full coverage restorations.16 The presence of marginal discrepancies can lead to increased plaque accumulation, alter the distribution of microflora and can thus contribute to a higher risk of caries in the abutment teeth.17,18,19,20 According to Holmes et al21, the fit of a casting can be best defined in terms of the “misfit” that can be measured at various points between the casting surface and the tooth. However, the size of a clinically acceptable marginal discrepancy is still controversial. A marginal discrepancy of less than 120 μm has been reported to be acceptable.3,6,17,22 But what constitutes an adequate fit has still not been satisfactorily resolved in the literature.

The present study evaluated the effect of different DMLS coping thicknesses on the marginal fit of metal ceramic restorations after repeated firing cycles. Many studies13,14,15 have proved that there is a relation of repeated firing cycles on the marginal fit of metal copings. This is because the creep of the alloys under high temperatures can cause deformation of the coping, and with an increase in the temperature as well as the number of firing cycles, it will lead to marginal distortions.25 It also depends on the sag resistance of the dental alloy to resist the flow under its own weight during porcelain firing and soldering.9 This phenomenon is particularly important in long-span bridges, where due to increased weight of the prosthesis, the porcelain- firing temperatures may cause the unsupported alloy substructure to deform permanently which results in an ill-fitting restoration. Co-Cr copings were therefore selected as the metallic substructure for the present study due to its superior mechanical properties and rigidity that it encompasses. The findings of this study also showed statistically significant differences between groups A (0.3mm) and C (0.7mm) after every firing cycle showing continuous increase in the marginal discrepancies which was in accordance with studies conducted by Di Fiore A13, Önöral Ö14, Bajaj G15 stating a relation of repeated firing cycles on the marginal fit of metal copings. The present study not only demonstrated this effect but also co-related it with the various thicknesses of the copings. It was also observed that the copings with the least thickness had the greatest marginal distortion at the end of all the firing cycles as compared to the copings of comparatively greater thicknesses.

An in vitro study conducted by Yildirim B26 compared the marginal fit of metal ceramic restorations fabricated by using CAD/CAM milling and DMLS technology before and after ceramic firing and cementation. He reported that the marginal gap did increase between the crown and abutment. A similar study was also conducted by Hong et al27 where they evaluated the marginal fit of metal ceramic restorations with casting and SLM processes before and after porcelain firing. They reported that the the marginal gap values increased in the casting as well as SLM groups after porcelain firing. In the present study, like the previous studies, porcelain firing increased the marginal discrepancy values after every firing cycle.

Studies conducted by Huang23, Gaikwad24 and Zheng25 have mentioned a better marginal accuracy of DMLS crowns as compared to other CAD/CAM technologies and conventional copings fabricated by lost wax technique. Therefore, a total of fifteen coping samples in the current pilot study were printed by direct metal laser sintering technology in an attempt to make the samples as accurate as possible. Horizontal marginal discrepancy was evaluated and defined as the horizontal marginal misfit measured perpendicular to the path of draw of the coping after every firing cycle and the results showed a mean marginal discrepancy of 50.7 μm (group A), 41.4 μm (group B), 33.3 μm (group C) respectively after the final firing cycle. Thus, all the readings that were observed were in the clinically acceptable range of around 30-50 μm which were way below the acceptable and suggested values. These values showed the possibility that a lower clinically acceptable range (of 30-50 μm) can be estimated for DMLS copings and that even with a 0.3mm coping thickness showing maximum marginal discrepancy, did not exceed more than 60 μm.

The current pilot study did have some limitations which include:

  • A two-dimensional value was achieved in a three-dimensional volume between the crown and the master die, which could not be assessed.

  • Since the current study was a pilot study, the stereomicroscope was used for measuring the marginal discrepancy. Furthermore, the study can be carried out on scanning electron microscope for better accuracy of the results.

  • Only the horizontal marginal discrepancy was evaluated. For more detailed results, vertical as well as absolute marginal discrepancy needs to be evaluated. Further research needs to be done to compare the effect of marginal fit using additive with subtractive manufacturing technologies using different coping thicknesses.

    • CONCLUSION

    Within the limitations of the current study, it can be concluded that

  • There is a definite effect of three different direct metal laser sintered coping thicknesses on the marginal fit of metal ceramic restorations after repeated firing cycles.
  • 0.7mm coping thickness showed the least marginal discrepancy whereas 0.3mm coping thickness showed the maximum marginal discrepancy.
  • With respect to the firing cycles, the first firing cycle for all three coping thicknesses showed the least discrepancy whereas the final firing cycle showed the maximum discrepancy.
  • A clinically acceptable marginal discrepancy of around 30-50 μm can be considered even for a 0.3mm thickness DMLS copings showing maximum distortion.

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