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 Table of Contents  
ORIGINAL HYPOTHESIS
Year : 2020  |  Volume : 12  |  Issue : 7  |  Page : 47-52

Surface roughness of two different monolithic materials after chewing simulation


1 Masters of Fixed Prosthodontics, Faculty of Dentistry, Alfarabi Private College for Dentistry and Nursing, Jeddah, KSA, Future University in Egypt, Cairo, Egypt
2 Professor of Fixed Prosthodontics, Faculty of Dentistry, Cairo University, Cairo, Egypt
3 Assistant professor of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Future University in Egypt, Cairo, Egypt
4 Lecturer of Fixed Prosthodontics, Faculty of Oral and Dental Medicine, Future University in Egypt, Cairo, Egypt
5 General Dentist, Ministry of Health, Jeddah, KSA
6 General Dentist, Hani Ragaban Clinics, Jeddah, KSA
7 General Dentist, King Fahad General Hospital, Jeddah, KSA

Date of Web Publication17-Jan-2020

Correspondence Address:
Dr. Wafaa Mahmoud Hamed
Masters of Fixed Prosthodontics, BDS, MSD, faculty of dentistry, Alfarabi Private College for Dentistry and Nursing- Jeddah-KSA. Future University in Egypt
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jioh.jioh_272_19

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  Abstract 

Aims and Objectives: The aim of this study was to evaluate two-body wear and surface roughness of two different monolithic ceramics: polymer-infiltrated ceramic (PICN) and lithium disilicate. Materials and Methods: Chewing simulator was used to investigate two-body wear (75,000 cycles, 49N, and 60 cycle/min). The tested samples were divided into two groups according to their materials; each group comprised 14 ceramic discs. Subtractive weight loss was used to statistically analyze the wear of all samples before and after chewing simulation test. Surface roughness was measured before and after chewing simulation test using three-dimensional optical profilometry. Data were collected and analyzed using analysis of variance test, and then verified by unpaired t-test. Results: Statistically significant differences were found for two-body wear, with higher mean weight loss in PICN than lithium disilicate after chewing simulation. PICN had higher mean surface roughness value than lithium disilicate after chewing simulation. Conclusion: PICN showed higher wear regarding weight loss and surface roughness changes than lithium disilicate.

Keywords: Dental Material, Lithium Disilicate, Occlusal Wear, VITA ENAMIC


How to cite this article:
Hamed WM, Anwar E, Adel R, Aboushahba M, Abdeen MF, Dagal RS, Rizq MH. Surface roughness of two different monolithic materials after chewing simulation. J Int Oral Health 2020;12, Suppl S1:47-52

How to cite this URL:
Hamed WM, Anwar E, Adel R, Aboushahba M, Abdeen MF, Dagal RS, Rizq MH. Surface roughness of two different monolithic materials after chewing simulation. J Int Oral Health [serial online] 2020 [cited 2020 Sep 26];12, Suppl S1:47-52. Available from: http://www.jioh.org/text.asp?2020/12/7/47/276087


  Introduction Top


Proper selection of restorative material to maintain occlusal harmony and normal chewing function is an essential factor in successful restoration,[1] in addition to proper choosing of each dental materials for typical clinical condition and masticatory load.[2] Keeping in mind the above fact, the occlusal wear of teeth and restorative materials are a complex and multifactorial phenomenon with biological, mechanical, and chemical factors.[3] Ideally, the wear of restorative material should be similar or in a close approximation to the wear of the natural enamel.[1]

Over the last 30 years, various types of all ceramic systems have been developed in a tendency to shift toward metal-free restorations to meet the increased demands for highly aesthetic, biocompatible, and long-lasting restorations of both patients and dentists.[4] A monolithic approach was introduced using lithium-disilicate glass ceramic. This glass ceramic is comprised of 70% prismatic lithium-disilicate crystals (0.5–5 µm long) dispersed in a glassy matrix. Lithium-disilicate microstructure has numerous small interlocking plate-like randomly oriented crystals. This crystal size and orientation causes cracks to deflect, branch, or blunt, which can account for the increase in flexural strength and fracture toughness as compared to lucite-reinforced ceramics.[5] Another ceramic that can be used as a monolithic restoration is hybrid ceramic (polymer-infiltrated ceramic [PICN]), which has a dual-network structure and combines the positive characteristics of composite and ceramic. The dominant ceramic network in this dental material is strengthened by a polymer network, whereby both networks penetrate fully.[6] Lower brittleness, higher flexural strength with rigidity, high strength, and easy milling in computer-aided design/computer-aided manufacturing (CAD/CAM) system are all properties that are supposed to be found in PICN.[7] Dental ceramics have not only many advantages but also some disadvantages in its properties; among them is the wear that affects both material and opposing dentition. Wear results in a slow, progressive loss of material and tooth substance. Initially, this process shows as a flattening of the occlusal cusp tips and incisal edge.[7] Wear can be regarded as pathological condition when the biological, functional, and esthetic condition of the masticatory system is effectively compromised.[8] Another problem of ceramic surface property is roughness, which is one of important factors of the restorative material that determines the clinical success of the dental restorations. Surface roughness of restorative materials can influence it by resulting in superficial staining, gingival inflammation, and secondary caries that eventually affect the clinical performance of restorations.[7] This study was conducted to show the effect of wear on surface roughness, which will help in proper selection of restorative material leading to maintenance of occlusal harmony and normal chewing function.

The aim of this study was to evaluate the wear (represented by weight loss) by using a chewing simulator on surface roughness of two different monolithic ceramics: hybrid ceramic (PICN) and lithium disilicate.


  Materials and Methods Top


This in vitro study was conducted in Cairo, Egypt during the period between September 2017 and May 2018. Samples were subjected to CAD/CAM and microtome at DDA (Digital Dental Academy, Nasr city, Cairo, Egypt). Whereas chewing simulation, weight loss and surface roughness were tested at Dr. Mohammed Abbas Laboratory, Heliopolis, Cairo, Egypt).

Sample criteria

Judgment sampling is also known as purposive, selective, or subjective sampling. In this study, the materials were selected for their known mechanical and physical characteristics which are in a close approximate to the natural enamel,[15],[16] as well as, their wide use in the dental practice.

Materials

Six blocks of PICN for CAD/CAM technology (dimension 12 mm × 14 mm × 18 mm) (VITA Zahnfabrik, Bad Säckingen, Germany) and six blocks of lithium disiliacte glass ceramic (IPS e.max CAD) (dimension 12 mm × 14 mm × 18 mm) (IPS e.max, Ivoclar Vivadent AG, Schaan, Liechtenstein) were used to fabricate all the specimens.

Methods

Samples preparation

Six blocks of lithium disilicate (e.max CAD) and six blocks of PICN (VITA ENAMIC, Bad Säckingen, Germany) were grounded to cylinders (10 mm × 10 mm) using a CAD/CAM machine (CAM 5-S1 Impression, Ammerbuch, Germany). Each cylinder was sectioned into discs using microtome (IsoMet 4000 Linear Precision Saw, Buehler, Illinois). The top and bottom of each cylinder were excluded. Fourteen discs from each material were selected with a total of 28 discs for all samples. Each disc was measured for thickness of 2 ± 0.12 mm using the digital caliber (Mitutoyo 500‑ 197‑20/30 Absolute Digital Digimatic Vernier Caliper 200 mm/8ʺ, Aurora, Illinois, USA).

Lithium disilicate crystallization

All lithium disilicate discs were crystallized in furnace at 850°C (1562°F) for 35min following the manufacturer’s instructions (P500, Ivoclar Vivadent AG, Schaan, Liechtenstein).

PICN finishing

PICN finishing and polishing was performed using 3M ESPE Sof-Lex medium-grained polishing discs (Leicestershine, Germany) followed by high-gloss polishing with gray diamond-coated polisher (VITA ENAMIC polishing set, VITA Zahnfabrik, Bad Säckingen, Germany) without water coolant.[9]

Teeth preparation

Twenty-eight freshly extracted and free from any carious lesions human permanent upper first premolars for treatment of diabetic patients were collected from different general public hospitals. The extracted teeth were cleaned to remove any calculus and soft-tissue remnants using ultrasonic scaler equipped with PIEZO Scaler Tip 201 (PIEZOsoft Ultrasonic Scaler, KaVo Dental, Biberach an der Riss, Germany). Then, teeth were polished with non-fluoridated polishing paste (Proxyt, Vivadent, Liechtenstein) and stored in saline solution (0.9% 500 mL R.C., Almottahedoon Pharma, Elsharkeya, Egypt).[10] The antagonist samples (n = 28) were prepared from the palatal cusp of upper maxillary premolar. Each premolar was sectioned mesiodistally using slow-speed diamond disc (Diatech, Goltène AG, Altstätten Switzerland) with copious water coolant[11] to obtain crack free cusps. Finally, premolars were stored in saline solution, which was changed every 2 days to avoid dehydration of the enamel specimens.[11]

Testing procedures

Weighing the samples

Fourteen discs of each material group (Group 1: PICN and Group 2: lithium disilicate) were weighed by an electronic analytical balance (Sartorius AG, Germany) before and after chewing simulation.

Chewing simulation––placing samples in holders

Samples of both groups (Group 1: PICN and Group 2: lithium disilicate) were placed on plastic holders with a circular hole (10 mm×2 mm), in which the specimen can be embedded. The antagonist enamel cusps were individually and directly fixed in a metallic holder (Jackob’s chuck) that can be tightened with a screw.

Chewing simulator

The two-body wear testing was performed using programmable logic controlled equipment ROBOTA (Model ACH‑09075DC‑T, AdTech Technology Co. Ltd., Neu-Isenburg, Germany), a chewing simulator integrated with thermocyclic protocol operated on servo-motor. ROBOTA chewing simulator has four chambers simulating the vertical and horizontal movements simultaneously in the thermodynamic condition. Each chamber consists of an upper Jackob’s chuck tooth antagonist holder that can be tightened with a screw and a lower plastic sample holder in which the specimen can be embedded. This chewing simulator is composed of two articulating parts: movable upper parts in which the antagonist enamel cusps were fixed and the lower fixed part in which the ceramic samples were fixed by a special plastic holder. Contact point geometry was established between the movable antagonist cusp specimens and fixed flat ceramic specimens. Accompanying water cycling (5°C/ 55°C) was used as a lubricating medium.

Wear by weight loss

The substance loss of the samples after loading was measured by weighing in the electronic analytical balance with an accuracy of 0.0001mg to observe the difference in weight before and after chewing simulation. This electronic balance has a fully automated calibration technology and a micro-weighing scale; values of all the samples were accurately measured. Each sample was cleaned and dried with tissue paper before weighing.

To ensure accuracy, the balance was kept on a freestanding table at all times––away from vibrations––and then the samples were weighed with the glass doors of the balance closed to avoid the effect of air drafts.

Surface roughness methodology

The optical profilometry tends to fulfill the need for qualitative characterization of surface topography with and without contact.[12] All samples were checked for surface roughness (Ra) before and after the wear cycles using a three-dimensional (3D) optical profilometry surface analyzer system. The Ra value was measured with a 3D optical profilometry surface analyzer system (USB Digital Surface Profile Gauge, Scope Capture Digital Microscope, Guangdong, China), and data were collected using the roughness tester supplier software (ElcoMaster version 2.0, Elcometer Instruments, Manchester, England). For every reading made, the mean Ra value (measured in μm) Was measured by the difference between the peaks and valleys registered after the needle of the profilometer scanned a stretch of 2 mm in length, with a cut‑off of 0.25 mm. Each surface was scanned three times, starting from three different points and always ending with the needle scanning the center of the specimen. The mean value of the three readings yielded the mean value of the roughness of each specimen.

Statistical analysis

Numerical data were explored for normality by checking the distribution of data and using test of normality (Kolmogorov–Smirnov and Shapiro–Wilk tests). All data showed parametric distribution. Parametric data were presented as mean, standard deviation (SD), and 95% confidence interval (CI) for the mean values. Two-way analysis of variance (ANOVA) was used to study the effect of wear and its interaction on mean weight loss and surface roughness. Unpaired t-test was used for pairwise comparison when ANOVA test was significant. The significance level was set at P ≤ 0.05. Statistical analysis was performed with Statistical Package for the Social Sciences software, version 20.0 for Windows (IBM, New York).


  Results Top


Part I: Weight loss test

Comparison between the mean weight loss (mg) in both the PICN and lithium disilicate groups before and after chewing simulation using two-way ANOVA test showed that there was a significant difference in both groups at P < 0.001. However, no significant difference was found in Group 1 at P = 0.156 and at P = 0.708 in Group 2 before and after chewing simulation [Table 1].
Table 1: Comparison of weight values (mg) among the two groups––Group 1: polymer-infiltrated ceramic (PICN) and Group 2: lithium disilicate before and after chewing simulation

Click here to view


Comparison between percent (%) of change in weight in both groups made using unpaired t-test revealed that the difference was statistically significant (P < 0.001). PICN had higher mean percent decrease (2.27 ± 0.55) than lithium disilicate (0.522 ± 0.182) [Table 2].
Table 2: Comparison between percent of change in weight among the two groups––Group 1: polymer-infiltrated ceramic (PICN) and Group 2: lithium disilicate

Click here to view


Part II: Surface roughness test

Comparison between the mean surface roughness (µm) before and after chewing simulation using two-way ANOVA test revealed that there was significant difference in both groups at P = 0.014 and P < 0.001). No significant difference (P = 0.055) was found in PICN before and after chewing simulation and in lithium disilicate (P = 0.0744) before and after chewing simulation [Table 3].
Table 3: Comparison of surface roughness values (µm) among the two groups––Group 1: polymer-infiltrated ceramic (PICN) and Group 2: lithium disilicate groups before and after chewing simulation

Click here to view


Comparison between percent of change in surface roughness in both groups made using unpaired t-test revealed that the difference was statistically significant (P < 0.001). A percent change in surface roughness (%) in both the groups was noted. PICN had higher mean percent increase (0.864 ± 0.364) than lithium disilicate (0.158 ± 0.045). Unpaired t-test revealed that the difference was statistically significant (P < 0.001) [Table 4].
Table 4: Comparison between percent of change in surface roughness among the two groups––Group 1: polymer-infiltrated ceramic (PICN) and Group 2: lithium disilicate

Click here to view



  Discussion Top


Many dental materials struggle to survive, while many others showed their proficiency in both function and aesthetic.[4]

Wear of restorations is considered one of the main concerns in dentistry especially in the posterior regions.[13] Wear can affect the occlusal thickness of these restorations, which can hinder aesthetic appearance, lead to occlusion changes and functional limitations, which limit their clinical success, especially on the posterior teeth.[13],[14],[15] Therefore, this study was conducted to evaluate the surface roughness of PICN and lithium disilicate after wear. The importance and clinical implication of this study appeared by proper understanding of wear mechanism and its effect in surface roughness of dental restorative materials, which will eventually lead to a successful selection process.[1],[2]

In vivo quantification of enamel and material wear is difficult, time consuming and with relatively high standard deviation results due to the biological spread between studied individuals.[11] Wear is a complex process that depends on both intrinsic and extrinsic factors such as thickness and hardness of both material and enamel, masticatory function, tooth form and type, position of teeth in relation to the arch, and the type and pH of the food.[16],[17] Consequently, chewing simulators are commonly used to conduct in vitro studies in order to simulate oral wear and test its effect on dental ceramic properties.

In this study, samples were subjected to wear testing at a frequency of 60 cycle/min for 75,000 cycles, which represents 6 months in service.[13] The load of 5kg (49N) used in this study is equal to the normal posterior teeth load during function.[18] Palatal cusps of upper maxillary premolars were used as natural antagonists. Flat enamel planes prepared from labial, mesial, or distal surface of tooth were recommended by many authors.[19] However, the enamel cusp was found to be much stronger under compression than that found on tooth side.[10] Consequently, using cuspal enamel samples were highly clinically relevant. The same parameters were performed in studies.[10],[20]

This study revealed a non-statistically significant difference with a decrease in PICN weight values after chewing simulation as compared to PICN before chewing simulation. These results were in agreement with those obtained by Elhomaimy et al.[10] Similarly, a non-statistically significant difference was found with decrease in lithium disilicate weight values after chewing simulation as compared to lithium disilicate before chewing simulation. These findings were in agreement with those obtained by Hassan and Gad[11] [Table 1]. This may be due to the fact that during the wear process, a certain amount of force is applied to erode the surface molecules and this loss of the material surface layer is the direct cause of weight decrease.[21] A statistically significant difference in percent of change in weight values was found in both groups, where PICN showed higher weight loss as compared to lithium disilicate) [Table 2]. This can be explained depending on the microstructural composition of these materials.[22] Sonmez et al.[23] found that chewing simulation process seems to affect PICN materials more than glass ceramics regarding their weight loss.

Surface roughness is an important property of material; the rougher the material, the more wear of the material itself, as well as the opposing (tooth/material). For that purpose, dental materials with less surface roughness are desired and preferred.[24] A statistically nonsignificant difference was noted in surface roughness values of PICN before and after chewing simulation, with a higher value after chewing simulation [Table 3]. These results were in agreement with those obtained by Naumova et al.[25] This may be due to the fact that PICN is damage tolerant; the reaction of PICN to repeated wear impacts showed some degree of elastic deformation and less degree of surface roughness under load.[26],[27] In addition, a nonsignificant difference in surface roughness was observed in both values before and after chewing simulation of lithium disilicate [Table 3]. These results were in agreement with those obtained by Hassan and Gad.[11] This might be attributed to the fact that glass ceramic materials do not show any significant changes in the surface mechanical properties as these materials are too hard and need longer wear to create any changes.[28]

PICN shows a significant difference with a higher surface roughness value before chewing simulation as compared with lithium disilicate) [Table 3]. This might be due to the increase in surface porosity of PICN caused by interpenetrating ceramic/polymer networks.[29] Comparing surface roughness values after chewing simulation of PICN and lithium disilicate revealed a statistically significant difference with higher values in PICN. Also, a significant difference was found in the percent change of surface roughness between both groups (PICN and lithium disilicate) with a significant increase in surface roughness in PICN [Table 4].

Limitations of current study include; fewer experimental groups (PICN and Lithium disilicate) where in fact, there are other materials that could be compared and studied. Another limitation was the number of wear cycles used, which were equally to 6-month oral environmental wear. However, increasing the number of cycle might give more information about the materials behavior and characters.


  Conclusion Top


Within the limitation of this in-vitro study, we conclude that both materials, PICN and lithium disilicate, lose weight by wear. PICN loses weight more than lithium disilicate after subjecting them to the same wear parameters. PICN showed a higher surface roughness than lithium disilicate before and after subjecting both materials to the same wear parameters. Based on the results of this study, future studies may be conducted for the use of new materials with high performance to guarantee lifetime of these materials and long-term clinical success.

Acknowledgement

We would like to thank Dr. Ahmed Abdelfattah Hamdi for helping during all process of this paper.

Financial support and sponsorship

This study was funded by the first author.

Conflicts of interest

There are no conflicts of interest.

 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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