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

Dr. Naisargi Shah1
, Dr. Anshul Khanna2
, Dr. Adhithi Prabhu3
, Dr. Prakruti Shah4
, Dr. Ashutosh Pai
2
,
Dr. Rahul Malu3
, Dr. Bhavna Ahuja3
1
Professor and Head of Department
2
Associate Professor, 3
Assistant Professor
4
Post Graduate student
Department of Prosthodontics, Crown & Bridge.,
T.P.C.T’s Terna Dental College, Navi Mumbai
5
Reader, Department of Public Health Dentistry,
T P C T’s Terna Dental College, Navi Mumbai.
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 51
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.
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.
However, the marginal fit of metal frameworks not only depend on fabrication methods but also on
subsequent production steps, including porcelain firing, which could deform the framework and alter
the marginal fit
12
. Porcelain firing cycles need a variety of high- temperature treatments that may lead
to distortion of copings, resulting in a marginal discrepancy. There have been various studies13,14,15
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 52
stating that the marginal accuracy is influenced by repeated firing cycles. But many studies that were
evaluated, considered a single standard coping thickness in their methodology.
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.
SUBJECTS AND METHODS
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.

Fig. 1 Fabrication of the master model
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
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 53
Fig. 2 Fabrication of the metal copings
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.
Fig 3: A – Application of Opaquer
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 54
Figure 3B – Ceramic layering, C – Putty index for standardization of ceramic layering
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.
Fig 4: A- Placement of coping on Master die, B- Horizontal marginal discrepancy measured
perpendicular to the path of draw of the coping.
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 55
Figure 5: Stereomicroscope
Figure 6: Horizontal marginal discrepancy measured after A- First firing cycle, B- Second firing cycle,
C- Third firing cycle, D- Fourth firing cycle
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.
Table 1. Mean, standard deviation and p values of all groups
Mean Std. Deviation Std. Error P value
Group A1 Group A 29.5080 5.48628 2.45354 0.032
Group B 28.3740 2.76692 1.23740
Group C 22.7720 2.04438 .91427
Group A2 Group A 39.2000 12.59271 5.63163 0.044
Group B 28.3720 4.04622 1.80953
Group C 25.3860 4.29616 1.92130
Group A3 Group A 45.3220 8.96766 4.01046 0.008
Group B 36.2140 5.21439 2.33195
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 56
Group C 28.3800 6.14865 2.74976
Group A4 Group A 50.7540 6.40472 2.86428 0.001
Group B 41.4380 3.82699 1.71148
Group C 33.3340 5.17644 2.31497
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.
Table 2. Level of significance of all groups after first firing cycle.
After first firing
cycle
Group A(0.3) Group B (0.5) Group C (0.7)
Group A(0.3) 1.00 0.044
Group B (0.5) 1.00 0.106
Group C (0.7) 0.044 0.106
Table 3. Level of significance of all groups after second firing cycle.
After second
firing cycle
Group A(0.3) Group B (0.5) Group C (0.7)
Group A(0.3) _ 0.163 0.056
Group B (0.5) 0.163 _ 1.00
Group C (0.7) 0.056 1.00 _
Table 4. Level of significance of all groups after third firing cycle
After third firing
cycle
Group A(0.3) Group B (0.5) Group C (0.7)
Group A(0.3) 0.183 0.007
Group B (0.5) 0.183 0.302
Group C (0.7) 0.007 0.302
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 57
Table 5. Level of significance of all groups after fourth firing cycle
After fourth firing
cycle
Group A(0.3) Group B (0.5) Group C (0.7)
Group A(0.3) 0.047 0.001
Group B (0.5) 0.047 0.093
Group C (0.7) 0.001 0.093
Fig 7: Graph depicting the difference in marginal discrepancy of the three different coping thicknesses
after every firing cycle.
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 al
21
, 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
Shah N. et al: Effect of Different DMLS Coping Thickness on Fit of Metal Ceramic Restorations
The JPDM I Vol 3 I Issue 1 I 2022 58
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 A
13
, Önöral Ö
14
, Bajaj G
15
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 B
26
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 al
27
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 Huang
23
, Gaikwad
24
and Zheng
25
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 an