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Table of Contents   
ORIGINAL RESEARCH  
Year : 2020  |  Volume : 31  |  Issue : 4  |  Page : 537-545
Comparative evaluation of wear resistance of CAD-CAM zirconia and cast cobalt chromium alloy for indirect restorations against human enamel - An In Vitro study


Department of Prosthodontics and Implantology, Ragas Dental College and Hospital, Chennai, Tamil Nadu, India

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Date of Submission17-Oct-2018
Date of Acceptance02-Jun-2019
Date of Web Publication16-Oct-2020
 

   Abstract 


Purpose of the Study: To comparatively evaluate the wear resistance of two different posterior indirect restorative materials against human enamel. Materials and Methods: Two different posterior indirect restorative materials of ten in each Group-I (Monolithic zirconia) (n = 10) and Group-II (Cast cobalt chromium) (n = 10) were formed into disc and used as a substrate for the wear test. Freshly extracted mandibular first premolars were used as a pin (antagonist) (n = 20). Pin-on-disc wear testing machine was used to simulate the masticatory parameters and evaluation of the wear parameters was done after 1,20,000 cycles, with load of 40N for specific duration. Data related to wear testing procedure were tabulated and evaluated. Results were statistically analyzed using Mann–Whitney test and Wilcoxon signed rank test. Results: Mean surface roughness value (Ra value) obtained for Group-I disc test samples showed no statistical significance (P value < 0.05). Mean wear rate value of test disc samples was statistically significant between Group-I and Group-II (P value < 0.05). Mean surface roughness value was statistically significant between Groups–I and II before and after wear test (P value < 0.05). Conclusion: From the results obtained, it was found that the Group-I (Monolithic zirconia) exhibited higher wear resistance than Group-II (cast cobalt chromium) and causes less wear to the opposing enamel antagonist.

Keywords: Cobalt chromium alloy, masticatory cycles, monolithic CAD-CAM zirconia, surface roughness, wear rate

How to cite this article:
Kumar N A, Sampathkumar J, Ramakrishnan H, Mahadevan V. Comparative evaluation of wear resistance of CAD-CAM zirconia and cast cobalt chromium alloy for indirect restorations against human enamel - An In Vitro study. Indian J Dent Res 2020;31:537-45

How to cite this URL:
Kumar N A, Sampathkumar J, Ramakrishnan H, Mahadevan V. Comparative evaluation of wear resistance of CAD-CAM zirconia and cast cobalt chromium alloy for indirect restorations against human enamel - An In Vitro study. Indian J Dent Res [serial online] 2020 [cited 2020 Oct 30];31:537-45. Available from: https://www.ijdr.in/text.asp?2020/31/4/537/298419



   Introduction Top


The functional ability to salvage and rehabilitate mutilated or missing teeth, so as to improve patient's oral function, comfort and aesthetics is one of the paramount service provided by fixed prosthodontics. Innovations in digital technology and improvements in material science have made the treatment outcome more predictable. Fixed replacements for missing teeth are more sought after than removable options for several reasons, such as strength, fixed mode of retention, and psychological comfort. Biomaterials used in the construction of such prostheses can be regarded as the foremost important requisite which influences the clinical outcome and the success of the treatment provided.

The methods of fabrication of fixed dental prostheses have undergone tremendous change from the time Dr. William H. Taggart introduced the lost-wax technique for cast metal restoration in 1907.[1] Noble metal casting alloys by virtue of their biocompatibility and superior marginal adaptation could no longer be considered for dental use due to increase in the price of gold and this resulted in the usage of nonprecious alloys.[2]

The favorable mechanical properties of nonprecious alloys allows for restorations with lesser thickness and more rigidity. The rigid characteristic of these alloys is mainly related to their high elastic moduli.[3]

Base metal alloys especially nickel–chromium were used mainly as metal framework for fixed dental prostheses. The allergenic nature of nickel and beryllium which are the main components has prompted the usage of other base metals, such as titanium and cobalt chromium.[4],[5],[6],[7]

Cobalt chromium alloy has the innate nature of higher melting temperature and this makes the casting and finishing procedures more technique sensitive than noble metal alloys. However, these limitations can be reduced due to advancements in casting methods, improved material properties, and design of the metal substructure. Cobalt chromium-based alloys are being used commonly nowadays as metal substructure for porcelain fused metal restorations or as all-metal restorations in the posterior region, as they have excellent marginal integrity and minimal adverse reactions.[3],[7],[8]

Despite the introduction of several high aesthetic all ceramic systems, such as lithium di-silicate glass, alumina, and spinel for the fabrication of metal-free restoration, they were found inferior to porcelain fused metal restorations with regards to mechanical properties like flexural strength, Young's modulus etc., This prevented the use of these restoration materials for posterior teeth and long span replacement. This led to the emergence of zirconia oxide material which can fulfill aesthetics and functional requirements.[9],[10],[11]

Zirconia oxide has been used in medicine and orthopedics several decades ago, but was introduced in restorative dentistry as a core material for crown and bridge restorations only recently.[12] Pure zirconia has a monoclinic crystal structure (m) at room temperature which is stable at 1170°c and transforms into cubic (c) and tetragonal (t) phase at 2370°c. Upon cooling, the metastable tetragonal zirconia is transformed into stable monoclinic zirconia. The tetragonal to monoclinic t-m phase transformation is associated with a 3–5% volume expansion resulting in cracks and flaws leading to spontaneous failure of pure zirconia at room temperature. Hence, stabilizing oxides like CaO, MgO, and Y2O3 are added to pure zirconia in order to stabilize it by allowing more stable tetragonal form to exist at room temperature even after sintering. This form of zirconia with oxides added to it is known as partially stabilized zirconia (PSZ). Compared to other core ceramics yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramic demonstrates superior properties such as high flexural strength, fracture toughness, high hardness, and excellent chemical resistance.[13],[14]

Zirconia has mechanical properties similar to stainless steel and was even referred as, “Ceramic steel” by Garvi et al. (1975).[12],[15] The average load sustaining capability of zirconia restoration as studied by Luthy et al. is in the range of 755N which can be compared to metal ceramic systems and more than alumina and lithium di-silicate ceramic systems. Due to its opacity and insufficient translucency, Zirconia requires a veneering porcelain to achieve acceptable aesthetics, but more often cracking or chipping of porcelain veneer has been reported to be a major drawback.[16]

With the increase in the translucency of Zicronia, fully contoured monolithic zirconia restorations without veneering porcelain have gained popularity due to the advances in computer-aided design and computer-aided milling (CAD-CAM) technology.[15],[17] Monolithic zirconia has been indicated in the posterior region mainly for single unit restoration in order to eliminate veneer chipping and it has also been suggested in situ ations with reduced interocclusal space because of its ability to resist high load with only 0.5 mm occlusal thickness.[18],[19],[20]

Zirconia-based restoration can be fabricated according to two different CAD-CAM techniques namely soft and hard machining. Soft machining technique involves milling of pre-sintered blanks in an enlarged dimension which are then sintered to obtained a framework of desired dimensions following 25% of shrinkage. In hard machining, fully sintered blank is machined to a framework of accurate dimensions.[13]

One of the most important physical properties of restorative material is wear resistance. Wear of the material is influenced by numerous factors including contact, surface roughness, velocity, load, temperature, and lubrication.

Occlusal wear results in alteration of the surface texture of the substrate material. Increased wear leads to loss of masticatory efficiency, faulty tooth relationship, and increased horizontal stress and associated sequelae.[21],[22],[23],[24],[25],[26],[27]

Hardness is considered to be related to wear and is the most commonly examined mechanical property indicator for restorative material.[28],[29]

Surface texture analysis using 3D surface profilometry aids in qualitative assessment as well as quantitative measure of the surface roughness (represented as Ra vale) and helps to extrapolate the results with those obtained from three-body wear tests and qualitative scanning electron microscope (SEM) analysis.[18],[30],[31]

A sensitive method to study differences in the structural integrity of dental restorative materials is by determination of wear using three-body wear test. Three-body abrasion results in surfaces that are rubbed away by an intervening slurry of abrasive particles. In the mouth, these conditions predominantly occur during masticatory tooth movement.[8],[10],[23],[24],[32],[33],xs[34]

There are several testing methods capable of analyzing three-body wear patterns between biomaterial – slurry – tooth enamel, such as pin-on-disc, reciprocating, twin disc, etc., Several authors have reported that pin-on-disc is the most reliable method to evaluate wear. These in vitro wear tests are conducted to simulate a particular duration of masticatory cycle by setting a fixed number of wear cycles in the test equipment.[2],[20],[30],[35],[36],[37]

Pin-on-disc wear testing is a quantitative method for evaluating the wear rate, volume loss of substrate material and the antagonist. This tribological test can simulate multiple modes of wear including unidirectional, bidirectional, and omnidirectional forces.[37]

Dental interest in CAD-CAM zirconia restoration has increased due to its higher fracture toughness, lower abrasion to the antagonist enamel, availability in different shaded blanks. Previous studies evaluated the tribological behavior for metal ceramic systems and nickel chrome alloys. Studies evaluating wear of CAD-CAM zirconia and cobalt chromium alloys as posterior indirect restorative materials are very few.[15],[30],[31],[38] The application of monolithic zirconia as a material for crown and bridge are still in its early stages and studies pertaining to its wear are sparse.

Scanning electron microscopy is used as additional tool for qualitative interpretation of the in vitro test. SEM images helps in improving our understanding of the effect of various test parameter on the test materials. SEM has been used in previous tribological studies to achieve this purpose.[29],[31],[39]

The null hypothesis in this study was there are no significant differences in the wear resistance between CAD monolithic zirconia and CO-Cr alloy against human enamel.


   Materials and Methods Top


Twenty extracted human mandibular first premolars [Figure 1]a and [Figure 1]b free of dental caries which were soaked in hydrogen peroxide in order to remove and clean debris were used for this study. For wear testing in a pin-on-disc machine, the natural tooth, which would serve as the pin was fabricated with the help of silicone putty matrix obtained from cylindrical metal insert of dimension 10 mm diameter × 32 mm length. Upon polymerization of putty silicone, the metal insert was removed and autopolymerizing Polymethylmethacrylate (PMMA) resin poured into the channel. Care was taken to ensure that the tooth was vertical and at right angles to the PMMA holder [Figure 2] Total of 20 mandibular first premolar were embedded in PMMA resin to be used as a pin.
Figure 1: (a) Natural teeth specimens of extracted mandibular first premolars. (a) Group1 (ten) (b) Group 2 (ten)

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Figure 2: Schematic diagram of placing tooth in the acrylic resin

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Out of the 20 natural teeth, first 10 teeth specimen were weighed separately and the resultant measurements were noted. These teeth were designated to be used as an antagonist (pin) against monolithic zirconia disc samples [Figure 3]a. Similarly, the next 10 natural teeth specimens were weighed separately and were designated to be used as an antagonist against cobalt chromium disc samples [Figure 3]b.
Figure 3: (a) Antagonist teeth (Pin samples) specimens. (b) Antagonist teeth (Pin samples) specimens

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The stainless-steel metal disc of dimensions 20 mm × 5 mm was employed in the production of monolithic zirconia (Ceramill CAD-CAM material-Ceramill Zolid-AmannGirrbach, Austria) discs were made using a Ceramill CAD and CAM system and finishing was carried out using fine diamond abrasive and silicone rubber wheels. Cast cobalt chromium (Wironit-Bego, Germany) discs were made using conventional casting of wax pattern made from silicone putty (Aquasil, Dentsply) mould of same stainless steel metal disc (20 mm × 5 mm).

A total of 20 (n = 20) test samples from two definitive restorative materials were used for the present study. Milled monolithic zirconia test samples were listed as Group I (n = 10) [Figure 4]. Cast cobalt chromium test samples were listed as Group II (n = 10) [Figure 5].
Figure 4: Group I- Monolithic zirconia

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Figure 5: Group II- Cast cobalt chromium

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Quantitative analysis of disc test samples before wear test

Weighing on a microbalance

The disc samples of both the groups were weighed individually before wear testing procedure using a microbalance weighing machine. The weight of each sample was calibrated in grams and these values were used to calculate the wear rate of the test material.

3D surface profilometry-Surface roughness (Ra) value

The test disc samples of both groups were subjected individually to 3D surface profile scanning before wear testing. 3D surface roughness was measured using a 3D noncontact profilometer. The average surface roughness (Ra) value of each disc sample was obtained at the magnification of the optical lens which was standardized at 50 × for all the samples. The resultant pictograph was viewed as 3D and advanced 3D views using Advanced Aspheric Analysis Software.

The disc samples of both the groups were subjected individually to Vickers hardness test before the wear testing procedure. Vickers hardness value was measured using Wilson Wolpert hardness testing machine.

Wear testing procedure

The pin-on-disc wear testing machine (DUCOM, Bangalore), [Figure 6] is designed and developed in accordance with ASTM G-99-04 was employed in wear testing. The pin specimen revolves on the disc in a circular motion making multiple wear passes on the same track. The device has a load cell which measures the magnitude of frictional forces that allows the coefficient of friction to be determined. To simulate the oral condition in wear test, equal amount of rice flour and saline was taken and mixed to a slurry consistency was used as third body abrasive which acts as a food substance.
Figure 6: (a) Pin-on-disc wear testing machine. (b) Graphic values monitoring devices

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A provision has been made to hold and prevent the dislodgement of the test sample during the revolution of the pin-on-disc machine with the help of an ejection screw, which fastens the test material from below the circular disc.

The test sample was positioned in customized metal mould disc and was tightened to the revolving platform of the wear testing machine (pin-on-disc machine). The tooth was inserted into the pin holder and adjusted to maintain a point contact with the disc sample. Each test sample was subjected to 120,000 wear cycles at 800 Rpm. The above-mentioned parameters (wear cycles, Rpm, time duration and the load = 40N) were set in the control panel of the wear testing machine, and the wear testing was conducted individually for all the 20 test samples (test samples of Group I and Group II). The entire procedure of wear testing was carried out under the constant flow of third body abrasive medium (rice flour in slurry form) was introduced into the wear testing machine using infusion tube [Figure 7].
Figure 7: Wear testing under abrasive medium

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Quantitative analysis of antagonist tooth and disc samples of Group I and Group II after wear testing

After wear test, the teeth samples were cleaned thoroughly of debris and abrasive medium residue left on its surfaces. All the teeth samples of respective groups were weighed using micro balance to determine the weight loss after wear test. Weight loss of teeth sample was recorded and the difference was calculated. Likewise, the test disc samples of the two groups were cleaned to remove any debris and residues of the abrasive medium and weighed individually after wear testing procedure using a microbalance and these values were used to calculate the wear rate of the test sample materials.

The test disc samples of both groups were subjected individually to 3D surface profile scanning after wear testing. The average surface roughness (Ra) value of the wear track of each test disc sample was obtained under same standardized magnification (50×).

Measurement of wear rate

After the completion of the wear testing procedure of each test disc sample, it was removed from the testing apparatus. The difference in the loss of weight for test disc sample material was obtained from the difference between the initial and final weight before and after wear test for Group I and Group II test samples using micro balance weighing machine in grams. Wear rate of the test disc sample was obtained using wear rate formula.

Statistical analysis

The data were subjected to statistical analysis using SPSS software for Windows 10.0.5 (SPSS Software Corp., Munich, Germany). Nonparametric Mann–Whitney U test was used for statistical analysis to compare mean wear rate and mean surface roughness (Ra value) between Group I (monolithic zirconia) and Group II (cobalt chromium) test samples. Wilcoxon Signed Rank test was performed to test the surface roughness within the tested group samples.


   Results Top


Mean surface roughness value (Ra value) calculated within the groups was not statistically significant in Group-I test sample (P value < 0.005). Mean wear rate value of test disc samples was statistically significant between Group-I and Group-II (P value < 0.005). Mean surface roughness value was statistically significant between Groups – I and II before and after wear test (P value < 0.005).


   Discussion Top


In the present study, the tooth was made to contact at a single point through the buccal cusp tip alone. A total of 20 test samples were prepared and were grouped as Group-I and Group-II with each group containing ten test samples respectively. Group-I test samples consisted of monolithic zirconia and Group-II test sample consisted of cast cobalt chromium. A total of (n = 20) mandibular first premolars were selected as antagonist pin against the test material used in the study.

A testing procedure which simulates the process of wear that occurs in the mouth was developed. The number of masticatory cycles per day ranges approximately from 800 to 1400 cycles.[10] In this study, the chewing simulator was set to simulate 1, 20, 000 masticatory cycles,[40],[41] i.e. (6 months of intraoral use). This was in accordance with previous studies by Chong et al.,[42] D'Arcangelo et al.,[22] Beuer et al.[17] The wear testing procedure for the 20 test samples was standardized at 40N load at 800 rpm for 1, 20, 000 cycles. The entire procedure lasted for two hours and thirty minutes. Rice flour in a slurry consistency was used as a third-body abrasive medium during the procedure.[23] All the test samples and antagonist tooth specimens were weighed before and after the testing procedure and the values were noted and tabulated. [Figure 8]a, [Figure 8]b.
Figure 8: (a) Postwear test- Group I Monolithic zirconia. (b) Postwear teat- Group II Cast cobalt chromium

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The difference between the initial and the final weight of the test samples and the tooth specimens were calculated. On comparing the weight loss of the antagonist tooth specimen, the loss of weight was significantly higher in Group-II (cast cobalt chromium) than that of Group-I (monolithic zirconia). This difference can be attributed to the increased surface roughness value of cobalt chromium sample tested. [Table 1], [Table 2], [Table 3]
Table 1: Mean average value of all parameters of Group I (monolithic zirconia) and Group II (cast cobalt chromium)

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Table 2: Comparison of mean weight loss between the antagonist teeth samples of Group-I Monolithic zirconia and Group-II Cast cobalt chromium after wear test

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Table 3: Comparison of mean weight loss of Group I monolithic zirconia and Group II cast cobalt chromium test samples before and after wear test

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A 3D noncontact laser surface profilometry was used in the present study to analyze the surface texture of the test samples (Groups I and II) [Figure 9]a and [Figure 9]b [Figure 10]a and b]. The mean surface roughness (Ra) value for Group I and Group II before wear test was found to be 0.2142 and 0.4232 μm, respectively. The mean surface roughness value after wear testing for Group I and Group II was found to be 0.2132 and 0.4514 μm, respectively [Table 1]. Statistically significant difference was observed when mean surface roughness value (Ra) was compared between cast cobalt chromium (Group II) and monolithic zirconia before and after wear test [Table 4] and [Table 5].
Figure 9: (a) Advanced 3D view of surface topography of Group I- Before wear test. (b) Advanced 3D view of surface topography of Group I- After wear test

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Figure 10: (a) Advanced 3D view of surface topography of Group II- Before wear test. (b) Advanced 3D view of surface topography of Group II- After wear test

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Table 4: Comparison of mean microhardness (HV) value between Group I – Monolithic zirconia and Group II – Cast cobalt chromium test samples

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Table 5: Comparison of mean wear rate between Group I – Monolithic zirconia and Group II – cast cobalt chromium disc test sample

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The findings show that there is an increase in the Ra values for the Group II test samples after wear test. There was not much difference in the Ra values of Group I test samples before and after the wear test.

Highest mean surface roughness value after wear test was exhibited by cast cobalt chromium (Group II). The lower mean surface roughness value was shown by monolithic zirconia (Group I) [Table 1]. There was a statistically significant difference in the mean surface roughness value of cast cobalt chromium (Group II) test samples before and after wear test [Table 6]. In contrast, there was no statistically significant difference was seen in the mean surface roughness value of monolithic zirconia (Group I) before and after wear test [Table 7].
Table 6: Comparison of mean surface roughness of Group-I Monolithic zirconia and Group-II Cast cobalt chromium disc samples before wear test

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Table 7: Comparison of mean surface roughness of Group-I Monolithic zirconia and Group-II Cast cobalt chromium disc samples after wear test

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Increased in the amount of surface roughness value observed for cast cobalt chromium alloy could be attributed to the grain size and metal microstructure as well as phase structure of the alloys.[7] Multiple-phase alloys have areas of different composition in its microstructure which influences the change in the surface roughness. Method of fabrication was also found to influence the abrasive characteristic of cobalt chromium restoration.[7],[31] This was evidenced by well-defined peaks and valleys under examination by 3D imaging following wear test. This increased surface value of cast cobalt chromium alloy can be cited as an explanation for the increased amount of weight loss of the antagonist teeth sample after wear test.

The Ra value observed for monolithic zirconia (Group I) sample was comparatively lesser than cast cobalt chromium test sample (Group II). The smaller grain size will make the material much more wear resistance by increasing the energy needed to remove the grain from the ceramic matrix. This might explain why the wear of opposing enamel did not change the surface roughness of monolithic zirconia samples tested. This was observed from the 3D images depicting not well defined, sparsely distributed peaks and valleys. Several earlier studies were conducted to evaluate the effect of various surface modification procedures on surface roughness of different ceramic systems and concluded that these procedures have significant influence on the abrasive nature of the restorative material.[6],[19],[20],[40],[42],[43]

Vickers hardness test was used in this study because it is often easier to use than other hardness test, since the required calculation are independent of the size of the indenter and the intender can be used for all materials irrespective of hardness. The microhardness value of the test samples was found to influence the wear resistance property of the materials. This microhardness value was determined using Vickers hardness testing machine (Wilson Wolpert). The mean microhardness value of Group I and Group II test samples was 1132.04 and 352.75 HV, respectively [Table 1]. On comparison of the mean microhardness value of Group I and Group II samples there was statistically significant difference, this can be explained by the difference in the grain size and the fabrication method of the two test samples used in this study [Table 8].
Table 8: Comparison of mean surface roughness of Group-I Monolithic zirconia disc samples before and after wear test

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The wear rate of the test samples was obtained from the data of weight loss of the test samples before and after wear test. On comparison of the mean wear rate of Group I test sample (0.0080 mg/min) and Group II test sample (0.0854 mg/min) there was a statistically significant difference was found [Table 9].
Table 9: Comparison of mean surface roughness of Group-II Cast cobalt chromium disc samples before and after wear test

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The higher wear rate exhibited by cast cobalt chromium samples can be explained by the increased surface roughness value and decrease in the microhardness value obtained. Similarly, the lower wear rate noted with the monolithic zirconia test samples could be probably explain on the basis that the surface roughness value of the material obtained was comparatively lower and increased micro-hardness value.

From the overall result obtained in the present study, Group I test samples exhibited lower values in surface roughness both before and after wear test and higher microhardness value. This is in correlation with qualitative assessment by SEM analysis. Cast cobalt chromium test samples (Group II) showed increased surface roughness value before and after wear test and its microhardness value was lesser this was in accordance with the qualitative SEM analysis [Figure 11], [Figure 12], [Figure 13], [Figure 14].
Figure 11: (a) SEM photomicrograph of tested Group-I sample under 250 × magnification before wear test. (b) SEM photomicrograph of tested Group-I sample under 500 × magnification before wear test

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Figure 12: (a) SEM photomicrograph of tested Group-I sample under 250 × magnification after wear test. (b) SEM photomicrograph of tested Group-I sample under 500 × magnification after wear test

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Figure 13: (a) SEM photomicrograph of tested Group- II sample under 250 × magnification before wear test. (b) SEM photomicrograph of tested Group- II sample under 500 × magnification before wear test

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Figure 14: (a) SEM photomicrograph of tested Group- II sample under 250 × magnification after wear test. (b) SEM photomicrograph of tested Group- II sample under 500 × magnification after wear test

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   Conclusion Top


Within the limitations of the present study, the following conclusions were made.

Cast cobalt chromium exhibited significantly higher surfaces roughness before and after wear test when compared with monolithic zirconia which was validated by surface profilometry and SEM analysis.

Monolithic zirconia exhibited smoother surface profile after wear test.

Average teeth weight loss for group II cast cobalt chromium showed significantly higher results than Group I monolithic zirconia, indicative of abrasive nature of the cobalt chromium.

Group I monolithic zirconia material showed significantly lower wear rate when compared with Group II cast cobalt chromium material.

Wear resistance was found to be significantly higher in Group I monolithic zirconia than Group II cast cobalt chromium material since the wear rate is lesser for monolithic zirconia.

Therefore, the null hypothesis was rejected because of significant differences in the outcomes of the various testing procedures.


   Clinical Significance Top


Wear resistance of monolithic zirconia is comparatively higher than cast chromium cobalt against human enamel which had been identified by various testing parameters and therefore helps in the preservation of human enamel under masticatory loading.

Limitations

Present study has some limitations. The effect of thermocycling on the properties of the material tested was not evaluated; hence, a proper protocol for thermocycling needs to be employed in the subsequent studies.


   Recommendations Top


Future studies replicating clinical scenarios, different indirect posterior restorative materials, various fabrication techniques, and various wear testing parameters with a larger sample size are recommended to enhance the result with the present study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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Correspondence Address:
Dr. Hariharan Ramakrishnan
Department of Prosthodontics and Implantology, Ragas Dental College and Hospital, Uthandi, Chennai - 600 119
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijdr.IJDR_779_18

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