Abstract
Aim:
(1) The aim of this study was to evaluate the effect of a ceramic bonder at the metal ceramic interface in sintered and casted cobalt–chromium (Co–Cr) alloy. (2) To compare the shear bond strength between the different manufacturing techniques: Casting and direct metal laser sintering (DMLS).
Setting and Design:
In vitro comparative study.
Materials and Methods:
For the casting group, 40 clear acrylic patterns with dimensions of 20 mm × 10 mm × 2.5 mm were designed in a software and casted with Co–Cr alloy. For DMLS samples, a standard tessellation language file with the abovementioned dimensions was created through a software to fabricate 40 samples. All the samples were equally distributed into the following four groups:
Group A – Casted samples with the application of Cerambond (n = 20)
Group B – Casted samples without application of Cerambond (n = 20)
Group C – DMLS samples with application of Cerambond (n = 20)
Group D – DMLS without application of Cerambond (n = 20).
Ceramic buildup was done on all 80 samples, followed by firing up to a temperature of 920°C in a ceramic furnace. SBS was evaluated using a universal testing machine and failure modes were examined under the electron microscope.
Statistical Analysis Used:
ANOVA test and Tukey’s honestly significance difference post hoc test for multiple comparisons.
Results:
One-way analysis of variance test revealed that the shear load and bond strength values of all four groups were statistically different with P < 0.001. Post hoc Tukey’s test showed statistically significant difference among the four groups. The mean shear strength of Group C was significantly greater when compared to other groups, respectively.
Conclusions:
Within the limitations of this study, the application of Cerambond to both casted and sintered samples showed significantly increased SBS values and it was also observed that sintered samples had higher strength than casted samples. Altogether, the results indicate that the use of Cerambond increased the shear strength between cobalt Cr alloy and ceramics, thereby prolonging the longevity of the restorations.
Keywords: Cobalt–chromium, coefficient of thermal expansion, porcelain-fused metals, shear bond strength
INTRODUCTION
Metal-ceramic restorations are considered the “gold standard” in prosthodontics despite many advances in the evolution of newer and stronger metal substructure fabrication materials and also methods, namely direct metal laser sintering (DMLS) and milling. Porcelain-fused metal crowns combine the biomechanical strength of metals with the esthetics of ceramic materials. However, frequent chipping of layered ceramic is a concerning situation in the metal-ceramic restorations. The debonding of ceramic occurring at the metal-ceramic interface requiring either repair or replacement of the crowns or fixed partial dentures.[1,2] It seems that this weakness in layered porcelain is caused by the gap of bonding between the veneer material and the core alloy. For the longevity of restorations, this weakness has to be critically considered.
The parameters which influence the porcelain’s capacity to resist fracture during clinical use include the nature and strength of the bond, thermal conductivity, and coefficient of thermal expansion (CTE) between metal and ceramic.[3,4,5] The most determining parameter among all is the difference in CTEs of porcelain and metal. Hence, to compensate for this difference, a CTE modifier (Cerambond) has been introduced to improve the bonding of layering ceramic by making the CTE compatible to the metal.
The study objective was to assess the manufacturer’s claim that the use of Cerambond at the metal-ceramic interface can improve the shear bond strength (SBS) in both casted and sintered cobalt–chromium (Co–Cr) alloys. With the knowledge gathered from the literature, it was hypothesized that the use of this CTE modifier (Ceram bond) can increase the bonding between the metal substructure and the layering ceramic.[6,7]
MATERIALS AND METHODS
A total of 80 Co–Cr alloy specimens were fabricated with dimensions of 20 mm in length, 10 mm in width, and 2.5 mm in thickness by conventional casting (n = 40) and DMLS technique (n = 40). The sample size was determined by using software (NMaster 2.0, Informer technologies, Europe). The sample size was calculated at n = 15 for each subgroup, but for standardization, 20 samples were fabricated for each subgroup.
Casted samples (n = 40)
A rectangular-shaped pattern of the abovementioned dimensions was designed in software (CorelDraw Graphics Suite Version 22, Corel corporation, Ottawa) to fabricate 40 acrylic blocks. Phosphate-bonded investment material (Brevest Eco speed) was used for the investment of all the samples and wax was eliminated. Casting was done in a centrifugal casting machine (Mini Casting Machine) using Co–Cr alloy pellets (Denchrome C - Co- 58%, Cr-24.75%, W-8.65%, Mo-1.24%, Fe-1.4%, Si-0.99%, and the CTE was 14.0 × 10 −6°C.). Each sample was examined for any casting defect and the desired ones were selected. Before ceramic layering, each sample was sandblasted with 110 µm aluminum oxide powder (Korox 110) at 45° angle and 20 cm distance from the sample at two bar pressure. They were cleansed with a steam jet for 10 s. These sample dimensions were meticulously measured using a vernier caliper.
These samples have been randomly segregated into two groups.
Group A – Casted samples with Cerambond application (n = 20)
Group B – Casted samples without Cerambond application (n = 20).
Laser sintered samples (n = 40)
Materialize magic version 24 software (Materialize company, Belgium) was used to create a standard tessellation language file with the abovementioned dimensions to fabricate 40 samples. The machine (SLM 125) was equipped with a continuous Nd: YAG laser, and the metal powder was melted in 0.03 mm thick layers until the samples of desired dimensions were achieved. The samples were sandblasted with 110 µm aluminum oxide (Korox 110) particles and steam jet for 10 s. These samples were randomly segregated into two groups.
Group C – DMLS samples with Cerambond application (n = 20)
Group D – DMLS samples without Cerambond application (n = 20).
Application of Cerambond
Cerambond (Bredent) was applied on Group A and C samples with a paintbrush in a unidirectional manner at one end of one coat to achieve a uniform layer thickness. The samples were placed in a ceramic furnace, which was preheated to 650°C to dry the Cerambond for 1 min and the firing was continued up to 980°C under vacuum with a temperature increase rate at 55°C/min following the instructions in the manufacturer manual. Once the process of firing was completed, they exhibited a beige to golden-yellow color.
Application of dental porcelain
Feldspathic porcelain (Ivoclar IPS Classic) buildup was done in increments on all the samples to achieve dimensions of 10 mm × 10 mm × 1.5 mm that included wash opaque, opaque, and dentin layers and subjected to a firing cycle in a calibrated porcelain furnace (Progamat P310). The manufacturer’s firing schedule [Table 1] was followed for the firing protocol. That is, with the aid of a paintbrush, the opaque layer was coated on the metal substrates by painting a viscous aqueous suspension of the opaque powder onto the metal substrate and placed into a preheated furnace at 403°C for 6 min. Then, the temperature was increased to 910°C with a heating rate of 80 K/min for 1 min. Similarly, the second and third layers of dentin were applied and fired (each layer separately and sequentially) at 920°C.
Table 1.
Firing temperatures
| Preheating (°C) | Drying (min) | Increasing temperature (°C/min) | Vacuum final temperature | Final temperature | Holding time (min) | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ceram bond | 650 | 3 | 55 | 975 | 980 | 1 | ||||||
| Wash opaque - 1st firing | 403 | 6 | 80 | 919 | 920 | 1 | ||||||
| Paste opaque - 2nd firing | 403 | 6 | 80 | 909 | 910 | 1 | ||||||
| Dentin body - 1st firing | 403 | 4 | 60 | 919 | 920 | 1 | ||||||
| Dentin body - 2nd firing | 403 | 4 | 60 | 909 | 910 | 1 |
For standardization of feldspathic porcelain thickness, a customized guide was fabricated using a clear acrylic sheet of thickness 1.5 mm. Within this sheet, a square window (10 mm × 10 mm) was cut with a laser and this guide was placed on each sample after buildup and checked whether the upper surface of the ceramic merged with the adjacent acrylic guide. Adjustments were made accordingly and the samples were again measured in multiple locations using Vernier caliper to ensure the ceramic thickness was 1.5 mm [Figure 1]. These samples were stored according to their respective groups in four individual airtight containers. Using a universal testing machine, SBS was determined.
Figure 1.
Grouping of samples. DMLS: Direct metal laser sintering
Shear bond test
These samples were tightened in the lower compartment of a computer-controlled universal testing machine (MCS) [Figure 2]. A mono-beveled chisel-shaped metallic rod was attached to the upper movable compartment of the universal testing machine. The surface of the sample with porcelain over it was placed downward and the two ends of each rod were placed on the supporting fulcrums with 0.9 mm diameter and 20 mm distance from each other. The load was applied at the center of each sample by a crosshead speed of 1 mm/min. The load (N) that caused the debonding of the ceramic was recorded. The SBS values obtained were computer-generated in the units of megapascals (Mpa).
Figure 2.
Placement of sample in a universal testing machine
A scanning electron microscope (SEM) (HITACHI S-3400N) was used to observe the fractured surfaces to identify the failure modes (adhesive or cohesive or mixed).
The results obtained were systematically tabulated in the form of tables and graphs. One-way analysis of variance test was performed to compare the SBS of all four groups. Tukey’s post hoc analyses with pairwise comparisons were made to compare the mean difference among four groups (Group A, B, C, and D) with confidence intervals of 95%, and P < 0.05 was considered statistically significant.
RESULTS
The SBS and ultimate shear load values obtained were subjected to statistical analysis using SPSS software v2.5 (IBM Private Ltd., New York) and tabulated [Table 2 and Graph 1]. In the casted group, results revealed that the mean and standard deviation values for both shear load and SBS were higher in samples with Cerambond (n = 20) than samples without Cerambond (n = 20) (Group A – 57.94 > Group B – 50.64 Mpa). Whereas in the DMLS group, the bond strength values were again higher in samples with Cerambond (n = 20) than without Cerambond (n = 20) (Group C – 68.62> Group D – 54.92 Mpa).
Table 2.
Mean and standard deviation of shear load and shear bond strength values of all four groups
| Groups | Shear load values |
Shear bond strength (Mpa) |
P | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Mean | SD | |||||||
| A | 869.60 | 71.51 | 57.94 | 4.78 | <0.001 | |||||
| B | 760.01 | 56.77 | 50.64 | 3.77 | ||||||
| C | 1029.65 | 46.35 | 68.62 | 3.09 | ||||||
| D | 824.25 | 63.10 | 54.92 | 4.20 | ||||||
SD: Standard deviation
Graph 1.
Graphical representation showing shear bond strength values of the four groups
When compared in relation to the method of fabrication, samples in the DMLS group (n = 20) showed higher bond strength values than the casted group (n = 20) (Group D – 54.92 > Group B – 50.64 Mpa). Meanwhile, the DMLS samples with Cerambond (n = 20) had higher strength values than casted samples with Cerambond (n = 20) (Group C – 68.62 > Group A – 57.94 Mpa).
For observation of fractured surfaces, five samples were chosen randomly from each group and examined at ×200 using SEM. Out of 10 samples from Group A and Group C [Figures 3 and 4], eight samples showed cohesive failures and two samples exhibited mixed failures. From 10 samples of Group B and D [Figures 5 and 6], five samples showed adhesive failure and five samples exhibited mixed failure modes.
Figure 3.
Scanning electron microscope image of Group A specimen – shows cohesive failure within porcelain. CF: Cohesive failure
Figure 4.
Scanning electron microscope image of Group C specimen - cohesive failure within the porcelain. CF: Cohesive failure
Figure 5.
Scanning electron microscope image of Group B specimen-mixed failure with remnants of opaque layer on the metal surface. OL: Opaque layer, MS: Metal surface
Figure 6.
Scanning electron microscope image of Group D specimen-mixed failure with remnants of opaque layer on the metal surface. OL: Opaque layer, MS: Metal surface
DISCUSSION
CTE compatibility between ceramic and metal is required for successful PFM prostheses, as CTE mismatches lead to bond failure.[8]
In this study, Cerambond was used which may have compensated these differences in the expansion coefficients between metal and ceramic by blocking the escaping metal oxides which in turn increased the bond strength.
The standard requirement for the bond strength of dental ceramic materials (ISO 9693-1:2012) and metal-ceramic systems for dental restorations (DIN EN ISO 9693: 2012/ISO 9693: 2019) is 25 MPa.[9,10,11] All the results obtained from the current study have exceeded the limit.
The samples without Cerambond from DMLS (Group D) showed higher strength than casting (Group B). The probable reason may be that, in the DMLS technique, there will be the addition of sintered powder particles on to the surface to make them roughened. It is possible to say that this surface roughness of DMLS samples contributes to their ability to expand the porcelain’s contact area with the framework. Vafaee et al.[12] and Zhang et al.[13] have also proved that these surface porosities improve the bond strength and offer strong micromechanical retention for porcelain, which was in conformity with our study.
The mean SBS for casting samples with Cerambond (Group A) showed lesser strength when compared to the DMLS group with Cerambond (Group C). The possible reason might be for an already roughened surface of DMLS samples the application of Cerambond had further increased the bonding between the metal structure and layering ceramic.[7]
In comparison between casting (Groups A and B) and DMLS (Groups C and D), the samples with Cerambond showed comparatively higher bond strength values. In support of our present study, Minesaki et al.[14] and Tholey et al.[15] have stated that Si particles in Cerambond react with metal oxides to create a new interface that seals the alloy surface and keeps it from oxidizing further. This prevents the formation of a thick layer of oxidation and ensures that opaque porcelain fuses well.[16,17]
Furthermore, Yoo et al.,[8] Lee et al.,[18] and Al Bakkar et al.[6] have assessed the failure modes at the metal-ceramic interface after debonding under SEM. They determined whether the fracture was cohesive, adhesive, or mixed.[19,20]
In our study, the analysis of fracture type revealed that, out of 10 samples from Group A and C, eight samples showed cohesive failures exhibiting ceramic on the surface of the metal and two samples showed mixed failures exhibiting both opaque layer and ceramic on the surface of the metal. Out of 10 samples from Groups B and D, five samples showed adhesive failure, fractures occurred at the metal-ceramic interface and five samples exhibited mixed failures. Adhesive failure is considered suboptimal as it indicates a lower bond strength between metal and ceramic. Whereas in cohesive failure, the samples exhibited ceramic on the surface of the metal, which means higher bond strength, indicating that the use of the CTE modifier (Cerambond) increased the bond strength between ceramic and metal.
Therefore, in accordance with the manufacturer, the Cerambond used in our study compensated for these disparities between the CTEs of porcelain and metal, thereby increasing the bond strength between metal and ceramic. Moreover, it has also been proved that the DMLS fabrication technique showed superior strength qualities than the conventional casting technique.
Hence, we assumed that using Cerambond would be advantageous in implant prosthesis cases as they cannot absorb excessive loads due to lack of periodontal ligament and also in long-span prosthesis which sustain heavy internal stresses.
Therefore, Cerambond application would significantly minimize the frequency of chipping or delamination of ceramic, enhancing the shear strength between ceramic and Co–Cr alloy.
Limitations of the study
The study had some limitations which cannot reproduce all clinical parameters. Hence, longitudinal clinical trials are still needed to be evaluated to overcome the following limitations:
This study was performed in in vitro conditions; therefore, there would be no influence of saliva, temperature, and pH changes. Hence, more studies have to be done in in vivo environment to evaluate the restoration longevity in clinical use
Furthermore, tests based on thermal and mechanical cycling (i.e. TMC) procedures, by simulating the oral conditions before evaluating its strength at the metal-porcelain interface, would provide more valid information for clinical purposes
Only one product (Cerambond) was evaluated in this study. However, further research is needed to compare its efficacy with various other available products
In addition, after the application of Cerambond at the junction of metal-ceramic, the elemental composition, morphology, and thickness of the oxide layer formed were not analyzed.
CONCLUSION
Within the limitations of the present study, the following conclusions were drawn:
Cerambond application had improved the shear strength in both casted and DMLS samples, indicating its use in all single crowns, long-span bridges, and implant prosthesis
Bond strength values were comparatively higher in samples fabricated by DMLS than in the casting technique
All of the samples with Cerambond showed cohesive and mixed failures, indicating higher bond strength
The use of cerambond is highly recommended for long-span prostheses fabricated by either casting or DMLS techniques, which could provide long-term sustainability.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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